The Science Book

DIGM

200 INTRODUCTION

Max Planck describes J. J. Thomson is awarded Thomas Hunt Morgan Werner Heisenberg sets
discrete packets, or the Nobel Prize in Physics introduces the out his uncertainty
principle.
quanta, of energy. for his discovery of chromosome theory
the electron. of inheritance.

1900 1906 1915 1927
1905 1912 1926 1928

Albert Einstein produces Alfred Wegener Erwin Schrödinger Paul Dirac introduces
his paper on special proposes a theory of unleashes wave quantum
relativity. continental drift.
mechanics. electrodynamics.

W hile the 19th century “black box,” which had stubbornly aspects of the same phenomenon,
had seen a fundamental resisted classical equations, by capable of being converted from
change in the way imagining that electromagnetism one to the other, and his equation
scientists view life processes, traveled not in continuous describing their relation—E = mc2
the first half of the 20th would waves, but in discrete packets, —hinted at an enormous potential
prove even more of a shock. The or “quanta.” Five years later, energy locked inside atoms.
old certainties of classical physics, Albert Einstein, a clerk working
largely unchanged since Isaac at the Swiss Patent Office, Wave–particle duality
Newton, were about to be thrown produced his paper on special Worse was to follow for the old
away, and nothing short of a new relativity, asserting that the picture of the universe. At
way to view space, time, and speed of light is constant and Cambridge, English physicist
matter was to replace it. By 1930, independent of the movement of J. J. Thomson discovered the
the old idea of a predictable source or observer. After working electron, showing that it has a
universe had been shattered. through the implications of general negative charge and is at least
relativity, Einstein had found by a thousand times smaller and
A new physics 1916 that notions of an absolute lighter than any atom. Studying
Physicists were finding that the time and space independent of the the properties of the electron
equations of classical mechanics observer had gone, to be replaced was to produce new puzzles.
were producing some nonsensical by a single space-time, which was Not only did light have particle-like
results. It was clear that something warped by the presence of mass properties, but particles had
was fundamentally wrong. In 1900, to produce gravity. Einstein had wavelike properties, too. Austrian
Max Planck solved the puzzle of the further demonstrated that matter Erwin Schrödinger drew up a
spectrum of radiation emitted by a and energy should be considered series of equations that described

A PARADIGM SHIFT 201

Edwin Hubble finds Georges Lemaître Konrad Lorenz Linus Pauling writes
that the universe suggests that the explains the basis of The Nature of the
universe began as a
is expanding. primeval atom. animal instinct. Chemical Bond, which
uses the ideas of

quantum physics to
explain chemistry.

1929 1931 1935 1939
1930 1934 1936 1942

Subrahmanyan Fritz Zwicky Alan Turing J. Robert Oppenheimer
Chandrasekhar proposes the existence describes the Universal takes on the
describes black holes.
of dark matter. Turing Machine, Manhattan Project
a programmable to develop the
atomic bomb.
computer.

the probability of finding a particle Linus Pauling took this new picture universe was suddenly enormously
in a particular place and state. of an atom and used the ideas of bigger than anyone had thought.
His German colleague Werner quantum physics to explain how Hubble further found that the
Heisenberg showed that there atoms bonded to each another. In universe was expanding in all
was an inherent uncertainty to the process, he showed how the directions. Belgian priest and
the values of place and momentum, discipline of chemistry was, in physicist Georges Lemaître
which was initially thought to be a reality, a subsection of physics. By proposed that the universe had
problem of measurement, but later the 1930s, physicists were working expanded from a “primeval atom.”
found to be fundamental to the on ways to unlock the energy in This was to become the Big Bang
structure of the universe. A strange the atom, and in the US, J. Robert theory. A further puzzle was
picture was emerging of a warped, Oppenheimer led the Manhattan uncovered when astronomer Fritz
relative space-time with particles Project, which was to produce the Zwicky coined the term “dark
of matter smeared across it in the first nuclear weapons. matter” to explain why the Coma
form of probability waves. galaxy cluster appeared to contain
The universe expands 400 times as much mass (as seen
Splitting the atom Up to the 1920s, nebulae were from its gravity) as he could explain
New Zealander Ernest Rutherford thought to be clouds of gas or dust from the observable stars. Not only
first showed that an atom is made within our own galaxy, the Milky was matter not quite what it had
mostly of space, with a small, Way, which comprised the entire been thought to be, but much of it
dense nucleus and electrons in known universe. Then American was not even directly detectable.
orbit around it. He explained astronomer Edwin Hubble It was clear that there were
certain forms of radioactivity as the discovered that these nebulae still major holes in scientific
splitting of this nucleus. Chemist were in fact distant galaxies. The understanding. ■

202 IN CONTEXT

OUANTA ARE BRANCH
DISCRETE Physics
PACKETS
OF ENERGY BEFORE
1860 The distribution of
MAX PLANCK (1858–1947) so-called black-body radiation
fails to match predictions
made by theoretical models.

1870s Austrian physicist
Ludwig Boltzmann’s analysis
of entropy (disorder) introduces
a probabilistic interpretation of
quantum mechanics.

AFTER
1905 Albert Einstein proposes
that the quantum is a real
entity, using Planck’s concept
of quantized light to introduce
the idea of the photon.

1924 Louis de Broglie proves
that matter behaves both as a
particle and as a wave.

1926 Erwin Schrödinger
formulates an equation for the
wave behavior of particles.

I n December 1900, the German
theoretical physicist Max
Planck presented a paper
setting out his method for resolving
a long-standing theoretical conflict.
In doing so, he made one of the
most important conceptual leaps
in the history of physics. Planck’s
paper marked the turning point
between the classical mechanics of
Newton and quantum mechanics.
The certainty and precision of
Newtonian mechanics was to give
way to an uncertain, probabilistic
description of the universe.

Quantum theory has its roots in
the study of thermal radiation, the
phenomenon that explains why we
feel heat from a fire, even when the

A PARADIGM SHIFT 203

See also: Ludwig Boltzmann 139 ■ Albert Einstein 214–21 ■ Erwin Schrödinger 226–33

Classical mechanics treats radiation as if it were emitted across a black-body object whose emitted
continuous range. spectrum is almost entirely a result
of its own temperature. Studying
But nonsense results are reached the distribution of a black body’s
for the distribution of black-body radiation, light would show that emission of
radiation depended only on a body’s
assuming a continuous range. temperature, and not its physical
shape or chemical composition.
Kirchhoff’s hypothesis kick-started
a new experimental program
designed to find a theoretical
framework that would describe
black-body radiation.

The problem is solved by treating radiation as if it were Entropy and black bodies
produced in discrete “quanta.” Planck arrived at his new quantum
theory through the failure of
Radiation is not continuous, classical physics to explain the
but is emitted in discrete quanta of energy. experimental results of black-body
radiation distribution. Much of
air in between it and us is cold. reflects radiation, and it is this Planck’s work focussed on the
Every object absorbs and emits reflected light that gives objects second law of thermodynamics,
electromagnetic radiation. If its color even when they do not glow. which he had identified as an
temperature rises, the wavelength “absolute.” This law states that
of the radiation it emits decreases In 1860, German physicist isolated systems move over time
while its frequency increases. For Gustav Kirchhoff thought of an toward a state of thermodynamic
example, a lump of coal at room idealized concept he called a equilibrium (meaning that all
temperature emits energy below “perfect black body.” This is a parts of the system are at the same
the frequency of visible light, in the theoretical surface that, when at temperature). Planck attempted to ❯❯
infrared spectrum. We cannot see thermal equilibrium (not heating
the emission, so the coal appears up or cooling down), absorbs every A new scientific truth
black. Once we set the coal alight, frequency of electromagnetic does not triumph by
however, it emits higher-frequency radiation that falls on it, and does convincing its opponents and
radiation, glowing a dull red as the not itself reflect any radiation. making them see the light, but
emissions break into the visible The spectrum of thermal radiation rather because…a new
spectrum, then white-hot and coming off this body is “pure,” generation grows up that is
finally a brilliant blue. Extremely since it is not mixed with any
hot objects, such as stars, radiate reflections—it will only be familiar with it.
even shorter-wavelength ultraviolet the result of the body’s own Max Planck
light and X-rays, which again we temperature. Kirchhoff believed
cannot see. Meanwhile, in addition that such “black-body radiation” is
to producing radiation, a body also fundamental in nature—the Sun,
for example, comes close to being a

204 MAX PLANCK

explain the thermal radiation Planck reasoned that this principle
pattern of a black body by figuring should be evident in any theoretical
out the entropy of the system. black-body model.
Entropy is a measure of disorder,
though more strictly it is defined The Wien–Planck Law No real-world object is a perfect
as a count of the number of ways By the 1890s, experiments in black body, but the Sun, black velvet,
a system can organize itself. The Berlin came close to Kirchhoff’s and surfaces coated with lampblack,
higher the entropy of a system, perfect black-body, using so-called such as coal tar, come close.
the more ways the system has cavity radiation. A small hole in a
of organizing and producing the box kept at a constant temperature the data—it could accurately
same overall pattern. For instance, is a good approximation of a black describe the short-wavelength
imagine a room where all the body, as any radiation entering the (high-frequency) spectrum of
molecules of air start off bunched box gets trapped inside, and the thermal emission from objects, but
up in the top corner. There are far body’s emissions are purely a result not the long-wavelength (low-
more ways for the molecules to of its temperature. frequency emissions). This is the
organize themselves so that there point at which Planck broke with
is roughly the same number of The experimental results proved his conservatism and resorted to
them in each cubic centimeter of bothersome for Planck’s colleague Ludwig Boltzmann’s probabilistic
the room than there are for them all Wilhelm Wien, since the low- approach to arrive at a new
to remain in the top corner. Over frequency emissions recorded did expression for his radiation law.
time, they distribute themselves not fit his equations for radiation at
equally throughout the room as all. Something had gone wrong. In Boltzmann had formulated a
the entropy of the system rises. 1899, Planck arrived at a revised new way to look at entropy by
A cornerstone of the second law equation—the Wien–Planck law— regarding a system as a large
of thermodynamics is that entropy that attempted a better description collection of independent atoms
works in one direction only. of the spectrum of thermal and molecules. While the second
En route to thermal equilibrium, radiation from a black body.
the entropy of a system always Science cannot solve the
increases or remains constant. Ultraviolet catastrophe ultimate mystery of nature.
A further challenge came a year And that is because, in the
A cavity with a small hole will later, when British physicists last analysis, we ourselves are
trap most of the radiation that enters Lord Rayleigh and Sir James Jeans a part of the mystery that we
through the hole, making it a good showed how classical physics
approximation of an ideal black body. predicts an absurd distribution are trying to solve.
of energy in black-body emission. Max Planck
The Rayleigh–Jeans Law predicted
that, as the frequency of the
radiation increased, the power it
emitted would grow exponentially.
This “ultraviolet catastrophe”
was so radically at odds with
experimental findings that the
classical theory must have been
seriously awry. If it were correct, a
lethal dose of ultraviolet radiation
would be emitted whenever a light
bulb was turned on.

Planck was not too troubled by
the Rayleigh–Jeans Law. He was
more concerned about the Wien–
Planck Law, which, even in its
revised form, was not matching

A PARADIGM SHIFT 205

law of thermodynamics remained Radiation intensity The ultraviolet catastrophe was a
valid, Boltzmann’s reading gave Visible light nonsense result predicted by classical
it a probabilistic, rather than an physics (shown here as the Raleigh–
absolute, truth. Thus, we observe Jeans Law) in which black-body radiation
entropy simply because it is increased exponentially as its wavelength
overwhelmingly more likely than shortened. By quantizing radiation,
the alternative. A plate breaks but Planck produced a formula that fit the
does not remake itself, but there experimental data.
is no absolute law preventing a
plate from putting itself back Planck Rayleigh–
together—it is just exceedingly Radiation Jeans Law
unlikely to happen. Formula

Quantum of action 1000 2000 3000
Planck used Boltzmann’s statistical
interpretation of entropy to arrive at Wavelength of radiation (nm)
a new expression for the radiation
law. Imagining thermal radiation as Introducing “quanta” of energy to come to terms with the
being produced by individual reduced the number of states of consequences of his own work.
“oscillators,” he needed to count the energy available to the system, While he was never in any doubt
ways in which a given energy could and in doing this (although it about the revolutionary impact
be distributed between them. wasn’t his goal), Planck solved the of what he had done, he was—
ultraviolet catastrophe. He thought according to historian James
To do this, he divided the total of his quanta as a mathematical Franck—“a revolutionary against
energy into a finite number of necessity—as a “trick”—rather his own will.” He found the
discrete energy chunks—a process than something that was real. But consequences of his equations
called “quantization.” Planck was a when Albert Einstein used the not to his taste since they often
gifted cellist and pianist and might concept to explain the photoelectric gave descriptions of physical reality
have imagined these “quanta” in effect in 1905, he insisted that that clashed with our everyday
the same way that a fixed number quanta were a real property of light. experience of the world. But for
of harmonics is available to the better or worse, after Max Planck,
vibrating string of an instrument. As with many of the pioneers the world of physics has never
The resulting equation was simple, of quantum mechanics, Planck been the same. ■
and it fit the experimental data. spent the rest of his life struggling

Max Planck Born in Kiel in northern Germany giving birth to their children.
in 1858, Planck was an able pupil During World War II, an Allied
at school and graduated early, bomb destroyed his house in
at 17. He chose to study physics at Berlin and his papers, and in the
the University of Munich, where closing stages of the war, his
he soon became a pioneer of remaining son was caught up in
quantum physics. He received the plot to assassinate Hitler
the Nobel Prize in Physics in 1918 and was executed. Planck
for his discovery of energy quanta, himself died soon after the war.
although he never was able to
satisfactorily describe the Key works
phenomena as a physical reality.
1900 Entropy and Temperature
Planck’s personal life was of Radiant Heat
beset by tragedy. His first wife 1901 On the Law of Distribution
died in 1909, and his eldest of Energy in the Normal
son was killed in World War I. Spectrum
Both of his twin daughters died

NOW I KNOW WHAT THE

ATOM

LOOKS LIKE

ERNEST RUTHERFORD (1871–1937)



208 ERNEST RUTHERFORD

IN CONTEXT T he discovery at the turn theory based on his “law of multiple
of the 20th century that proportions,” which explained
BRANCH the basic constituent of how elements (simple, uncombined
Physics matter—the atom—could be substances) always combine in
broken into smaller fragments simple, whole-number ratios. Dalton
BEFORE was a watershed moment for saw that this meant that a chemical
c.400 BCE Greek philosopher physics. This astonishing reaction between two substances
Democritus envisages atoms breakthrough revolutionized ideas is no more than the fusing of
as solid, indestructible about how matter is constructed individual small components,
building blocks of matter. and the forces that hold it and repeated countless times. This
the universe together. It revealed was the first modern atomic theory.
1805 John Dalton’s atomic an entirely new world at the
theory of matter marries subatomic level—one that required A stable science
chemical processes to physical a new physics to describe its A self-congratulatory mood was
reality and allows him to interactions—and a slew of detectable in physics at the end of
calculate atomic weights. tiny particles that filled this the 19th century. Certain eminent
infinitesimally small domain. physicists made grandstanding,
1896 Nuclear radiation is declarations to the effect that the
discovered by Henri Becquerel, Atomic theories have a long subject was all but finished—that
and is used to reveal the history. The Greek philosopher the principal discoveries had all
internal structure of the atom. Democritus developed the ideas of been made and the program going
earlier thinkers that everything is forward was one of improving the
AFTER composed of atoms. The Greek accuracy of known quantities “to
1938 Otto Hahn, Fritz word átomos, which is credited to the sixth decimal place.” However,
Strassman, and Lise Meitner Democritus, means indivisible and many research physicists of the
split the atomic nucleus. referred to the basic units of matter. time knew better. It was already
Democritus thought that the clear that they were facing an
2014 Firing increasingly materials must reflect the atoms entirely new and strange set of
energetic particles at the they are made of—so atoms of iron phenomena that defied explanation.
nucleus continues to reveal are solid and strong, while those of
a slew of new subatomic water are smooth and slippery. In 1896, Henri Becquerel,
particles and antiparticles. following a lead from Wilhelm
At the turn of the 19th century, Röntgen’s discovery of mysterious
English natural philosopher John “X-rays” the previous year, had
Dalton proposed a new atomic

Alpha particles fired This means that Electrons are
into atoms sometimes travel an atom must have found to have specific
straight through, sometimes are
a small, dense orbits around
deflected, and sometimes central nucleus. the nucleus.
bounce back.

Now I know So, the atom is made of
what the atom a small, massive nucleus

looks like. with electrons orbiting
it in shells.

A PARADIGM SHIFT 209

See also: John Dalton 112–13 ■ August Kekulé 160–65 ■ Wilhelm Röntgen 186–87 ■ Marie Curie 190–95 ■
Max Planck 202–05 ■ Albert Einstein 214–21 ■ Linus Pauling 254–59 ■ Murray Gell-Mann 302–07

found an unexplained radiation. he demonstrated that he could J. J. Thomson is pictured here at
What were these new radiations break lumps out of atoms. While work in his Cambridge laboratory.
and where were they coming from? investigating the “rays” emanating Thomson’s “plum pudding” model
Becquerel correctly surmised from high-voltage cathodes of the atom was the first to include
that this radiation was emanating (negatively charged electrodes), he the newly discovered electron.
from within uranium salts. found that this particular kind of
When Pierre and Marie Curie radiation was made of individual theoretically. If an atom contains
studied the decay of radium, “corpuscles,” since it created charged particles, why shouldn’t
they discovered a constant and momentary, pointlike sparkles of the opposing particles have equal
seemingly inexhaustible source of light on hitting a phosphorescent mass? Previous atomic theories
energy inside radioactive elements. screen; it was negatively charged, held that atoms were solid lumps.
If this were the case, it would since a beam could be deflected As befit their status as the most
break several fundamental laws of by an electric field; and it was basic constituent of matter, they
physics. Whatever these radiations exceedingly light, weighing less were entire, whole, and perfect.
were, it was clear that there were than a thousandth of the lightest But when viewed in the light of
large gaps in current models. atom, hydrogen. Moreover, the Thomson’s discovery, they clearly
weight of the corpuscle was the were divisible. Put together,
Discovery of the electron same, no matter which element these new radiations raised
The following year, the British was used as a source. Thomson the suspicion that science had
physicist Joseph John (J. J.) had discovered the electron. These failed to understand the vital
Thomson caused a sensation when results were totally unanticipated components of matter and energy. ❯❯

210 ERNEST RUTHERFORD

The plum-pudding model All science is either physics Rutherford had demonstrated that
Thomson’s discovery of the or stamp collecting. radioactivity involved one element
electron earned him the Nobel Ernest Rutherford spontaneously changing into
Prize for Physics in 1906. He was another. Their work was to suggest
enough of a theoretician, however, very different visualization of the new ways to probe the inside of
to see that a radical new model internal structure of the basic unit the atom and see what was there.
of the atom was needed to of all elements.
adequately incorporate his Radioactivity
findings. His answer, produced At the Physical Laboratories Although radioactivity was first
in 1904, was the “plum-pudding” at the University of Manchester, encountered by Becquerel and
model. Atoms have no overall Ernest Rutherford devised and the Curies, it was Rutherford who
electric charge and, since the directed an experiment to test identified and named the three
mass of this new electron was Thomson’s plum-pudding model. different types of what we would
small, Thomson postulated that a This charismatic New Zealander now call nuclear radiation. These
larger positively charged sphere was a gifted experimentalist with are slow-moving, heavy, positively
contained most of the atom’s mass, a keen sense of which details to charged “alpha” particles; fast-
and the electrons were embedded pursue. Rutherford had received the moving, negatively charged “beta”
in it like plums in the dough of 1908 Nobel Prize in Physics for his particles; and highly energetic but
a Christmas pudding. With no “Theory of Atomic Disintegration.” uncharged “gamma” radiation
evidence to suggest otherwise, (p.194). Rutherford classified these
it was sensible to assume that The theory proposed that different forms of radiation by
the point charges, like the plums the radiations emanating from their penetrating power, from the
in a pudding, were arbitrarily radioactive elements were the least-penetrating alpha particles,
distributed across the atom. result of their atoms breaking apart. which are blocked by thin paper,
With the chemist Frederick Soddy, to gamma rays that require a
Rutherford revolution thickness of lead to be stopped.
However, the positively charged He was the first to use alpha
parts of the atom steadfastly particles to explore the atomic
refused to reveal themselves, and realm. He was also the first to
the hunt was on to locate the outline the notion of radioactive
missing member of the atomic half-life and discover that “alpha
pair. The quest resulted in a particles” were helium nuclei—
discovery that would produce a atoms stripped of their electrons.

Ernest Rutherford Brought up in rural New Zealand, Rutherford was an accomplished
Ernest Rutherford was working in administrator, too, and during
the fields when the letter from his lifetime he headed up the
J. J. Thomson arrived informing three top physics research
him of a scholarship to Cambridge laboratories. In 1907, he took
University. In 1895, he was made a the chair in physics at the
research fellow at the Cavendish University of Manchester
Laboratories, where he conducted where he discovered the atomic
experiments alongside Thomson nucleus. In 1919, he returned to
that led to the discovery of the the Cavendish as director.
electron. In 1898, at 27 years old,
Rutherford took up a professorial Key works
post at McGill University in
Montreal, Canada. It was there 1902 The Cause and Nature
that he carried out the work on of Radioactivity, I & II
radioactivity that won him the 1909 The Nature of the α Particle
1908 Nobel Prize in Physics. from Radioactive Substances

A PARADIGM SHIFT 211

Scattered Thin gold microscopes and counting the tiny
particles foil flashes of light on the scintillation
screens. Then, acting on a hunch,
Beam of Rutherford instructed them to
particles position screens that would catch
any high-angle deflections as
Source of Circular scintillation well as at the expected low-angle
particles screen scintillations. With these new
screens in place, they discovered
Geiger and Marsden aimed alpha particles from that some of the alpha particles
a radioactive source at an incredibly thin gold leaf. were being deflected by more than
The scintillation screen could be spun around to 90º, and others were rebounding off
detect particles rebounding at any given angle. the foil back the way they came.
Rutherford described the result as
The gold foil experiment through the foil. Most of the like firing a 15-inch shell at tissue
In 1909, Rutherford set out to probe particles would be deflected only paper and having it bounce back.
the structure of matter using alpha slightly by interaction with the
particles. The previous year, along gold atoms and would be scattered The nuclear atom
with the German Hans Geiger, across shallow angles. Halting heavy alpha particles in
he had developed zinc sulphide their tracks or deflecting them by
“scintillation screens,” which Geiger and Marsden spent high angles was possible only if
enabled individual collisions of long hours sat in the darkened the positive charge and mass of
alpha particles to be counted as laboratory, peering down an atom were concentrated in
brief bright flashes, or scintillations. small volume. In light of these
With the help of undergraduate It was quite the most results, in 1911, Rutherford
student Ernest Marsden, Geiger incredible event that has ever published his conception of
would use these screens to happened to me in my life. It the structure of the atom. The
determine whether matter was was almost as incredible as if “Rutherford Model” is a solar
infinitely divisible or whether you fired a 15-inch shell at a system in miniature, with electrons
atoms contained fundamental orbiting a small, dense, positively
building blocks. piece of tissue paper and it charged nucleus. The model’s
came back and hit you. major innovation was the
They fired a beam of alpha Ernest Rutherford infinitesimally small nucleus,
particles from a radium source at which forced the uncomfortable
an extremely thin strip of gold leaf, conclusion that the atom is not at
just a thousand or so atoms thick. all solid. Matter at an atomic scale
If, as the plum-pudding model held, is mostly space, governed by
gold atoms consisted of a diffuse energy and force. This was a
cloud of positive charge with definitive break from the atomic
pointlike negative charges, then the theories of the previous century.
massive, positively charged alpha
particles would plough straight While Thomson’s “plum-
pudding” atom had been an
instant hit, Rutherford’s model was
largely ignored by the scientific
community. Its failings were all too
plain to see. It was well established
that accelerating electric charges
emit energy as electromagnetic
radiation. Thus, as electrons swoop
around the nucleus—experiencing
circular acceleration that keeps ❯❯

212 ERNEST RUTHERFORD

them in their orbits—they now call photons. Bohr sought to If your experiment needs
ought to be continually emitting explain the precise pattern of statistics, you ought to have
electromagnetic radiation. Steadily absorption and emission of light done a better experiment.
losing energy as they orbited, the from atoms. He suggested that
electrons would spiral inexorably each electron is confined to fixed Ernest Rutherford
into the nucleus. According to orbits within atomic “shells,” and
Rutherford’s model, atoms ought to that the energy levels of the orbits falling out of orbit into the nucleus
be unstable, but clearly they are not. are “quantized”—that is, they can was, for electrons, impossible.
only take certain specific values. Bohr’s was a purely theoretical
A quantum atom model of the atom. However, it
Danish physicist Niels Bohr saved In this orbital model, the agreed with experiment and solved
the Rutherford model of the atom energy of any individual electron many associated problems in an
from languishing in obscurity is closely related to its proximity to elegant stroke. The way in which
by applying new ideas about the atom’s nucleus. The closer an electrons would have to fill up
quantization to matter. The electron is to the nucleus, the less empty shells in a strict order,
quantum revolution had begun energy it has, but it can be excited getting progressively farther from
in 1900 when Max Planck had into higher energy levels by the nucleus, matched the march
proposed the quantization of absorbing electromagnetic of the properties of the elements
radiation, but the field was still radiation of a certain wavelength. seen across the periodic table as
in its infancy in 1913—it would Upon absorbing light, an electron atomic number increases. Even
have to wait until the 1920s for a leaps to a “higher,” or outer, orbit.
formalized mathematical framework Upon attaining this higher state,
of quantum mechanics. At the the electron will promptly drop
time Bohr was working on back into the lower-energy orbit,
this problem, quantum theory releasing a quantum of energy that
essentially consisted of no more precisely matches the energy gap
than Einstein’s notion that light between the two orbitals.
comes in tiny “quanta” (discrete
packets of energy) that we Bohr offered no explanation for
what this meant or what it might
look like—he simply stated that

The plum-pudding model of the atom Electron Proton Neutron
with the electrons spread across a diffuse
nucleus was replaced by Rutherford’s
model with electrons in orbit around
a small, dense nucleus. Bohr refined
Rutherford’s model by adding
quantized orbits for the electrons.
Here, a carbon atom is illustrated.

6 protons +
6 neutrons

Plum-pudding model Rutherford model Bohr model

A PARADIGM SHIFT 213

more convincing was the way in James Chadwick discovered the neutron by bombarding
which the theoretical energy levels beryllium with alpha particles from radioactive polonium.
of the shells neatly fit actual The alpha particles knocked neutrons out of the beryllium.
“spectral series”—the frequencies Then the neutrons dislodged protons from a layer of paraffin,
of light absorbed and emitted by and these protons were detected by an ionization chamber.
different atoms. A long sought after
way to marry electromagnetism Alpha Neutrons
and matter had been realized. particles

Going inside the nucleus Protons
Once this picture of the nuclear
atom had been accepted, the next Polonium Beryllium Paraffin Ionization
stage was to ask what lay inside chamber
the nucleus. In experiments
reported in 1919, Rutherford found positive charges crammed into a it feels no repulsion as it passes
that his beams of alpha particles tiny nucleus. Like charges repel through matter. However, with
could generate hydrogen nuclei each other, so he theorized that mass slightly greater than a proton,
from many different elements. there must be another particle it can easily knock protons out
Hydrogen had long been that somehow dissipates the of the nucleus, something that
recognized as the simplest of all charge or binds the jostling protons otherwise only extremely energetic
the elements and thought of as tightly together. There was also electromagnetic radiation can do.
a building block for all other extra mass in elements heavier
elements, so Rutherford proposed than hydrogen, which could be Electron clouds
that the hydrogen nucleus was in accounted for by a third, neutral but The discovery of the neutron
fact its own fundamental particle, equally massive subatomic particle. completed the picture of the
the proton. atom as a massive nucleus
However, the neutron proved with electrons in orbit around
The next development in atomic difficult to detect and it took nearly it. New discoveries in quantum
structure was James Chadwick’s a decade of searching to find it. physics would refine our view of
1932 discovery of the neutron, in Chadwick was working at the electrons in orbit around a nucleus.
which Rutherford once again had Cavendish Laboratory under the Modern models of the atom feature
a hand. Rutherford had postulated supervision of Rutherford. Guided “clouds” of electrons, which
the existence of the neutron in 1920 by his mentor, he was studying a represent only those areas in
as a way to compensate for the new kind of radiation that had been which we are most likely to find
repulsive effect of many point-sized found by the German physicists an electron, according to its
Walther Bothe and Herbert Becker quantum wavefunction (p.256).
The difficulties disappear when they bombarded beryllium
if it be assumed that the with alpha particles. The picture has been further
radiation consists of particles complicated by the discovery
of mass 1 and charge 0, Chadwick duplicated the that neutrons and protons are
Germans’ results and realized not fundamental particles, but
or neutrons. that this penetrating radiation was are made of arrangements of
James Chadwick the neutron Rutherford had been smaller particles called quarks.
looking for. A neutral particle, Questions about the true structure
such as the neutron, is much of the atom are still actively
more penetrating than a charged being researched. ■
particle, such as a proton, because

GRAVITY

IS A DISTORTION IN THE

SPACE-TIME
CONTINUUM

ALBERT EINSTEIN (1879–1955)



216 ALBERT EINSTEIN If the speed of light through a vacuum is
unchanging…
IN CONTEXT
And the laws of physics appear the same
BRANCH to all observers…
Physics
Then there can be no absolute
BEFORE time or space.
17th century Newtonian
physics provides a description Observers in relative motion to each other
of gravity and motion, which experience space and time differently.
is still adequate for most
everyday calculations. Special relativity shows that there
is no absolute simultaneity.
1900 Max Planck first argues
that light can be considered to
consist of individual packets,
or “quanta,” of energy.

AFTER
1917 Einstein uses general
relativity to produce a model
of the universe. Assuming
that the universe is static,
he introduces a factor called
the cosmological constant
to prevent its theoretical
collapse.

1971 Time dilation due
to general relativity is
demonstrated by flying
atomic clocks around the
world in jet aircraft.

I n the year 1905, the German of the nature of light and energy. of general relativity that presented
scientific journal Annalen der A second was an elegant proof a new and deeper understanding
Physik published four papers that a long-observed physical of gravity, space, and time.
by a single author—a little-known effect called Brownian motion
26-year-old physicist named Albert could demonstrate the existence Quantizing light
Einstein, then working at the of atoms. A third showed the The first of Einstein’s 1905 papers
Swiss patent office. Together, these presence of an ultimate speed limit addressed a long-standing problem
papers would lay the foundations to the universe, and considered the with the photoelectric effect. This
for much of modern physics. strange effects thereof, known as phenomenon had been discovered
special relativity, while the fourth by German physicist Heinrich
Einstein resolved some forever changed our understanding Hertz in 1887. It involves metal
fundamental problems that of the nature of matter, showing electrodes producing a flow
had appeared in the scientific that it was interchangeable with of electricity (that is, emitting
understanding of the physical energy. A decade later, Einstein electrons) when illuminated by
world toward the end of the followed up the implications of certain wavelengths of radiation—
19th century. One of the papers of these latter papers with a theory typically ultraviolet light. The
1905 transformed understanding

A PARADIGM SHIFT 217

See also: Christiaan Huygens 50–51 ■ Isaac Newton 62–69 ■ James Clerk Maxwell 180–85 ■ Max Planck 202–05 ■
Erwin Schrödinger 226–33 ■ Edwin Hubble 236–41 ■ Georges Lemaître 242–45

principle behind the emission Light photons Electrons ejected
is fairly easily described in from the surface
modern terms (energy supplied by Electrons are
the radiation is absorbed by the ejected from the Sodium
outermost electrons in the metal’s surface of sodium
surface atoms, allowing them to only by certain that visible light was just one
break free). The puzzle was that the wavelengths of manifestation of a wider spectrum
same materials stubbornly refused light. Einstein of electromagnetic waves, all of
to emit electrons when illuminated showed that this which must move through the
by longer wavelengths, no matter phenomenon can universe at a single speed.
how intense the light source. be explained if light
travels as individual Since light was understood
This was a problem for the quanta, or photons. to be a transverse wave, it was
classical understanding of light, No matter how many assumed that it propagated
which assumed that intensity, of them there are, if through a medium, just as water
above all, governed the amount of the photons are of the waves travel on the surface of
energy being delivered by a light wrong wavelength, a pond. The properties of this
beam. Einstein’s paper, however, they will not eject hypothetical substance, known as
seized on the idea of “quantized electrons. the “luminiferous ether,” would
light” recently developed by Max give rise to the observed properties
Planck. Einstein showed that if the bombard the surface (that is, how of electromagnetic waves, and
beam of light is split into individual intense the light source is)—if none since they could not alter from
“light quanta” (what we would of them carries sufficient energy, place to place, they would provide
today call photons), then the energy the electrons will not break free. an absolute standard of rest.
carried by each quantum depends
only on its wavelength—the shorter Einstein’s idea was rejected by One expected consequence of
the wavelength, the higher the leading figures of the day, including the fixed ether was that the speed
energy. If the photoelectric effect Planck, but his theory was shown of light from distant objects should
relies on interaction between an to be correct by experiments vary depending on the relative
electron and a single photon, then it conducted by American physicist motion of source and observer.
does not matter how many photons Robert Millikan in 1919. For example, the speed of light
from a distant star should vary ❯❯
The grand aim of all science Special relativity
is to cover the greatest Einstein’s greatest legacy was born
in the third and fourth 1905 papers,
number of empirical facts by which also involved an important
logical deduction from the reconceptualization of the true
smallest number of nature of light. Since the late
hypotheses or axioms. 19th century, physicists had
Albert Einstein faced a crisis in their attempts to
understand the speed of light. Its
approximate value had been known
and calculated with increasing
accuracy since the 17th century,
while James Clerk Maxwell’s
equations had demonstrated

218 ALBERT EINSTEIN

significantly depending on whether Mass and energy are both packets of electromagnetic energy,
it was observed from one side of but different manifestations able to travel through the vacuum
Earth’s orbit, as our planet moved of space with particle-like
away from it at 20 miles/s (30 km/s), of the same thing. properties while still maintaining
or on the opposite side, when the Albert Einstein their wavelike characteristics.
observer was moving toward it
at a similar speed. head on. Special relativity, as Accepting these two
his theory became known, was postulates, Einstein considered
Measuring Earth’s motion developed from an acceptance of the consequences for the rest
through the ether became an two simple postulates—that light of physics, and mechanics in
obsession for late 19th-century moves through a vacuum with a particular. In order for the laws of
physicists. Such a measurement fixed speed that is independent physics to behave in the same way
was the only way of confirming of the motion of the source, and in all inertial reference frames, they
the existence of this mysterious that the laws of physics should would necessarily appear to be
substance, and yet the proof appear the same to observers in different when looking from one
remained elusive. However precise all “inertial” frames of reference— frame to another. Only relative
the measuring equipment, light that is, those not subject to external motion mattered, and when the
always seemed to move at the forces such as acceleration. relative motion between two
same speed. In 1887, US physicists Einstein was undoubtedly helped separate frames of reference
Albert Michelson and Edward in accepting the first bold postulate approached the speed of light
Morley devised an ingenious by his previous acceptance of (“relativistic” speeds) strange
experiment to measure the the quantum nature of light— things began to happen.
so-called ether wind with high conceptually, light quanta are often
precision, but once again found imagined as tiny self-contained The Lorentz factor
no evidence for its existence. The Although Einstein’s paper made
negative result of the Michelson- no formal references to other
Morley experiment shook the belief scientific publications, it did
in the ether’s existence, and similar mention the work of a handful
results from attempts to repeat it of other contemporary scientists,
over the following decades only for Einstein was certainly not the
intensified the sense of crisis. only person working toward an
unorthodox solution to the ether
Einstein’s third 1905 paper, On crisis. Perhaps the most significant
the Electrodynamics of Moving of these was Dutch physicist
Bodies, confronted the problem

Albert Einstein Born in the southern German city He continued to explore the
of Ulm in 1879, Einstein had a implications of his earlier work,
somewhat bumpy secondary contributing to innovations in
education, eventually training at quantum theory. In 1933, fearing
Zurich Polytechnic to become a the rise of the Nazi party,
mathematics teacher. After failing Einstein elected not to return
to find teaching work, he took a to Germany from a foreign tour,
job at the Swiss Patent Office in settling eventually at Princeton
Bern, where he had plenty of University in the United States.
spare time to develop the papers
published in 1905. He attributed Key works
his success in this work to the fact
that he had never lost his childlike 1905 On a Heuristic Viewpoint
sense of wonder. Concerning the Production and
Transformation of Light
Following the demonstration 1915 The Field Equations
of general relativity, Einstein of Gravitation
was propelled to global stardom.

A PARADIGM SHIFT 219

In Einstein’s thought experiment, B appears to run more slowly as
for a stationary observer at point M, measured from the observer’s
two lightning flashes at A and B occur reference frame.
simultaneously. However, to an observer
at point M1 on a train moving at high Illustrating relativity
speed away from A and toward B, the Einstein illustrated special
flash at B occurs before the flash at A. relativity by asking us to consider
two frames of reference in motion
Near the speed of light relative to each other: a moving
train and the embankment next
M1 to it. Two flashes of lightning, at
points A and B, appear to occur
A simultaneously to an observer
standing on the embankment at
M a midpoint between them, M.
An observer on the train is at a
Hendrik Lorentz, whose “Lorentz Lorentz’s work had been coolly position M1 in a separate frame
factor” lay at the heart of Einstein’s received, largely because it of reference. When the flashes
description of physics close to the could not be incorporated into occur, M1 may be passing right by
speed of light. It is defined standard ether theories. Einstein M. However, by the time the light
approached the problem from the has reached the observer on the
mathematically as: 1 other direction, showing that train, the train has moved toward
the Lorentz factor arose as an point B and away from point A.
√ 1 –– v2 / c2 inevitable consequence of the As Einstein puts it, the observer is
principle of special relativity and “riding ahead of the beam of light
Lorentz developed this equation to reexamining the true meaning coming from A.” The observer on
describe the changes in time and of measured time and distance the train concludes that lightning
length measurements required in intervals. An important result of strike B occurred before strike A.
order to reconcile the Maxwell this was the realization that events Einstein now insists that: “Unless
equations of electromagnetism that appeared simultaneous for an we are told the reference-body to
with the principle of relativity. observer in one reference frame which the statement of time refers,
It was crucial to Einstein since it were not necessarily so for there is no meaning in a statement
provided a term for transforming someone in a different reference of the time of an event.” Both time
results as seen by one observer to frame (a phenomenon known as and position are relative concepts.
show what they look like to another the relativity of simultaneity).
observer who is in motion relative Einstein also showed how from Mass-energy equivalence
to the first observer. In the term the point of view of a distant The last of Einstein’s 1905 papers
quoted above, v is the speed of one observer, the length of moving was called Does the Inertia of a
observer compared to the other, objects in their direction of travel Body Depend on its Energy
and c is the speed of light. In most became compressed as they Content? Its three brief pages
situations, v will be very small approached the speed of light, in expanded on an idea touched on in
compared to c, so v2/c2 will be close accordance with a simple equation the previous paper—that the mass
to zero, and the Lorentz factor governed by the Lorentz factor. of a body is a measure of its energy.
close to 1, meaning that it makes Even more strangely, time itself Here, Einstein demonstrated that
almost no difference to calculations. if a body radiates away a certain
amount of energy (E) in the form
of electromagnetic radiation, its
mass will diminish by an amount
equivalent to E/c2. This equation
is easily rewritten to show that the
energy of a stationary particle ❯❯

220 ALBERT EINSTEIN

within a particular reference frame Over the next few years, many a person standing in a sealed
is given by the equation E = mc2. scientists reached the conclusion elevator in empty space. The
This principle of “mass-energy that special relativity offered a elevator is being accelerated in one
equivalence” was to become a better description of the universe direction by a rocket. The person
keystone of 20th-century science, than the discredited ether theory, feels a force pushing up from the
with applications that range from and devised experiments that floor, and pushes back against the
cosmology to nuclear physics. demonstrated relativistic effects floor with equal and opposite force
in action. Meanwhile, Einstein following Newton’s Third Law.
Gravitation fields was already moving on to a new Einstein realized that the person in
Although Einstein’s papers in challenge, extending the principles the elevator would feel exactly as
that annus mirabilis seemed too that he had now established in they would if they were standing
obscure at first to make much order to consider “noninertial” still in a gravitational field.
impression beyond the rarefied situations—those involving
world of physics, it propelled him acceleration and deceleration. In an elevator undergoing
to fame within that community. constant acceleration, a beam of
As early as 1907, Einstein had light fired at an angle perpendicular
Our experience of hit upon the idea that a situation to the acceleration would be
gravity is equivalent to of “free fall” under the influence deflected onto a curved path, and
of gravity is equal to an inertial Einstein reasoned that the same
that of being inside a situation—the equivalence would occur in a gravitational field.
constantly accelerating principle. In 1911, he realized that It was this effect of gravity on
a stationary frame of reference light—known as gravitational
frame of reference. influenced by a gravitational field lensing—that would first
is equivalent to one undergoing demonstrate general relativity.
constant acceleration. Einstein
illustrated this idea by imagining Einstein considered what this
said about the nature of gravity.

The acceleration can Real light Real location Apparent
be explained by a trajectory of the star location of
distortion in the the star

space-time manifold.

If objects with mass Apparent light
distort space-time, trajectory
this explains their
Sun
gravitational attraction.

General relativity According to general relativity, Observer
explains gravity as mass creates a “gravitational well” in
a distortion in the space-time. The idea can be visualized
space-time manifold. by representing three-dimensional space
as a two-dimensional plane. The gravitational
well of a massive object such as the Sun
causes light to be deflected onto a curved path,
altering the apparent position of distant stars to
an observer—an effect called gravitational lensing.

A PARADIGM SHIFT 221

Arthur Eddington’s photographs of
a solar eclipse in 1919 provided the first
evidence for general relativity. Stars
around the Sun appeared out of place,
just as Einstein had predicted.

In particular, he predicted that
relativistic effects such as time
dilation should occur in strong
gravitational fields. The closer a
clock is to a source of gravitation,
the more slowly it will tick. This
effect remained purely theoretical
for many years, but has now been
confirmed using atomic clocks.

Space-time manifold to solve a long-standing mystery— a few months after the end of the
Meanwhile, also in 1907, Einstein’s the way in which Mercury’s closest war, Eddington led an expedition
former tutor Hermann Minkowski approach to the Sun (aphelion) to the island of Príncipe, off the
had hit upon another important precesses, or rotates, around the west coast of Africa, in order to
part of the puzzle. Considering the Sun much more quickly than test the theory of general relativity
effective trade-offs between predicted by Newtonian physics. and its prediction of gravitational
the dimensions of space and time General relativity solved the puzzle. lensing in the most spectacular
involved in special relativity, he circumstances. Einstein had
developed the idea of combining Gravitational lensing predicted as early as 1911 that a
the three dimensions of space Einstein published at a time when total solar eclipse would allow the
with one of time in a space-time much of the world was swept up in effects of gravitational lensing to be
manifold. In Minkowski’s World War I, and English-speaking seen, in the form of apparently out-
interpretation, relativistic effects scientists had other things on their of-place stars around the eclipsed
could be described in geometrical minds. General relativity was a disk (a result of their light being
terms by considering distortions in complex theory and might have deflected as it passed through the
the way that observers in relative languished in obscurity for many warped space-time around the
motion observe the manifold in a years had it not been for the Sun). Eddington’s expedition
different frame of reference. interest of Arthur Eddington, a delivered both impressive images
conscientious objector to the war, of the solar eclipse and convincing
In 1915, Einstein published his and, as it happened, Secretary of proof of Einstein’s theory. When
complete theory of general relativity. the Royal Astronomical Society. published the following year, they
In its finished form, it was nothing proved to be a worldwide sensation,
less than a new description of the Eddington became aware of propelling Einstein to global fame
nature of space, time, matter, and Einstein’s work thanks to letters and ensuring that our ideas about
gravity. Adopting Minkowski’s from Dutch physicist Willem the nature of the universe would
ideas, Einstein saw the “stuff of the de Sitter, and soon became its never be the same again. ■
universe” as a space-time manifold chief advocate in Britain. In 1919,
that could be distorted thanks to
relativistic motion, but could also
be warped by the presence of large
masses such as stars and planets
in a way that was experienced as
gravity. The equations that described
the link between mass, distortion,
and gravity were fiendishly complex,
but Einstein used an approximation

222

EARTH’S DRIFTING
CONTINENTS ARE
GIANT PIECES IN AN
EVER-CHANGING JIGSAW

ALFRED WEGENER (1880–1930)

IN CONTEXT I n 1912, German meteorologist eastern coasts of the Americas are
Alfred Wegener combined roughly parallel with the western
BRANCH several strands of evidence to coasts of Europe and Africa. This
Earth science put forward a theory of continental led scientists to speculate that these
drift, which suggested that Earth’s landmasses were once connected,
BEFORE continents were once connected but challenging conventional notions of
1858 Antonio Snider-Pellegrini moved apart over millions of years. a solid, unchanging planet.
makes a map of the Americas Scientists only accepted his theory
connected to Europe and once they had figured out what In 1858, Paris-based geographer
Africa, to account for identical made such vast landmasses move. Antonio Snider-Pellegrini showed
fossils found on opposite sides that similar plant fossils had been
of the Atlantic Ocean. Looking at the first maps of the found on either side of the Atlantic,
New World and Africa, Francis dating back to the Carboniferous
1872 French geographer Bacon had noted, in 1620, that the period, 359–299 million years ago.
Élisée Reclus proposes that
motion of the continents South America’s Similar plant and Matching rock
caused the formation of the east coast fits animal fossils formations
oceans and mountain ranges. are found in
Africa’s west coast are found in South
1885 Eduard Suess suggests like two giant America and South America
the southern continents were jigsaw pieces. in Africa. and in Africa.
once linked by land bridges.
The continents must once have formed a single landmass.
AFTER
1944 British geographer Earth’s drifting continents are giant pieces
Arthur Holmes proposes in an ever-changing jigsaw.
convection currents in Earth’s
mantle as the mechanism that
moves the crust at the surface.

1960 American geologist
Harry Hess proposes that
seafloor spreading pushes the
continents apart.

A PARADIGM SHIFT 223

See also: Francis Bacon 45 ■ Nicholas Steno 55 ■ James Hutton 96–101 ■ Louis Agassiz 128–29 ■ Charles Darwin 142–49

He made maps showing how the ago, and pointed to Africa’s Great destruction of ocean crust that
American and African continents Rift Valley as evidence of ongoing leads to the displacement of
may once have fit together, and continental breakup. continents. This theory not only
attributed their separation to the vindicated Wegener but is now
biblical Flood. When fossils of Search for a mechanism the bedrock of modern geology. ■
Glossopteris ferns were found in Wegener’s theory was criticized by
South America, India, and Africa, geophysicists for not explaining Pangaea, 200 million years ago
Austrian geologist Eduard Suess how continents move. In the
argued that they must have evolved 1950s, however, new geophysical 75 million years ago
on a single landmass. He suggested techniques revealed a wealth of
that the southern continents were new data. Studies of Earth’s past Present day
once linked by land bridges across magnetic field indicated that the Wegener’s supercontinent is just
the sea, forming a supercontinent ancient continents lay in a different one in a long series. Geologists think
that he called Gondwanaland. position relative to the poles. Sonar the continents may be converging
mapping of the seabed revealed again, to form another supercontinent
Wegener found more examples signs of more recent ocean-floor 250 million years from now.
of similar organisms separated by formation. This was found to occur
oceans, but also similar mountain at mid-ocean ridges, as molten rock
ranges and glacial deposits. Instead erupts through cracks in Earth’s
of earlier ideas that portions of a crust and spreads away from the
supercontinent had sunk beneath ridges as new rock erupts.
the waves, he thought perhaps it
had split apart. Between 1912 and In 1960, Harry Hess realized
1929, he expanded on this theory. that seafloor spreading provided
His supercontinent—Pangaea— the mechanism for continental drift,
connected Suess’s Gondwanaland and presented his theory of plate
to the northern continents of North tectonics. Earth’s crust is made up
America and Eurasia. Wegener of giant plates that continually shift
dated the fragmentation of this as convection currents in the
single landmass to the end of the mantle below bring new rock to the
Mesozoic era, 150 million years surface, and it is the formation and

Alfred Wegener Born in Berlin, Alfred Lothar produced revised and expanded
Wegener obtained a doctorate in editions in 1920, 1922, and 1929,
astronomy from the University of but was frustrated by the lack of
Berlin in 1904, but soon became recognition for his work.
more interested in earth science.
Between 1906 and 1930, he made In 1930, Wegener led a fourth
four trips to Greenland as part of expedition to Greenland, hoping
his pioneering meteorological to collect evidence in support of
studies of Arctic air masses. He the drift theory. On November 1,
used weather balloons to track air his 50th birthday, he set out
circulation and took samples from across the ice to get badly
deep within the ice for evidence of needed supplies, but he died
past climates. before reaching the main camp.

In between these expeditions, Key work
Wegener developed his theory
of continental drift in 1912, and 1915 The Origin of Continents
published it in a book in 1915. He and Oceans

224

CHROMOSOMES
PLAY A ROLE IN
HEREDITY

THOMAS HUNT MORGAN (1866–1945)

IN CONTEXT When cells divide, their chromosomes split and
replicate in ways that parallel the emergence
BRANCH
Biology of inherited characteristics.

BEFORE This suggests that genes controlling these
1866 Gregor Mendel describes characteristics occur on the chromosomes.
laws of inheritance, concluding
that inherited characteristics Some characteristics depend Chromosomes
are controlled by discrete on the sex of the organism, play a role
particles, later called genes. so must be controlled by sex- in heredity.

1900 Dutch botanist Hugo de determining chromosomes.
Vries reaffirms Mendel’s laws.
D uring the 19th century, roles of genes and chromosomes
1902 Theodor Boveri and biologists observing cells in inheritance, explaining evolution
Walter Sutton independently divide under a microscope at a molecular level.
conclude that chromosomes noticed the appearance of pairs of
are involved in inheritance. tiny threads in every cell’s nucleus. Particles of inheritance
These threads could be stained by By the early 20th century, scientists
AFTER dyes for observation, and came to had traced the chromosome’s
1913 Morgan’s student Alfred be called chromosomes, meaning precise movements at cell division,
Sturtevant constructs the first “colored bodies.” The biologists and noticed that the number of
genetic “map,” of the fruit fly. soon began to wonder whether chromosomes varied between
chromosomes had something to species, but that the number in the
1930 Barbara McClintock do with heredity. body cells of the same species were
discovers that genes can shift usually the same. In 1902, German
positions on chromosomes. In 1910, experiments conducted biologist Theodor Boveri, having
by American geneticist Thomas studied the fertilization of a sea
1953 James Watson and Hunt Morgan would confirm the
Francis Crick’s double-helix
model of DNA explains how
genetic information is passed
on during reproduction.

A PARADIGM SHIFT 225

See also: Gregor Mendel 166–71 ■ Barbara McClintock 271 ■
James Watson and Francis Crick 276–83 ■ Michael Syvanen 318–19

urchin, concluded that an organism’s white-eyed, and always male. The Thomas Hunt Morgan
chromosomes had to be present in “white gene” must be linked to sex.
a full set for an embryo to develop When other traits linked to sex Born in Kentucky, US, Thomas
properly. Later that same year, an appeared, Morgan concluded that Hunt Morgan trained as a
American student named Walter all these traits must be inherited zoologist before going
Sutton concluded from his work on jointly and the genes responsible on to study the development
grasshoppers that chromosomes for them must all be carried on of embryos. After moving to
might even mirror the theoretical the chromosome that determines Columbia University in New
“particles of inheritance” proposed sex. The females had a pair of X York in 1904, he began to
in 1866 by Gregor Mendel. chromosomes, while males had focus on the mechanism of
an X and a Y. During reproduction, inheritance. Initially sceptical
Mendel had done exhaustive the offspring inherits an X from the of Mendel’s conclusions, and
experiments in the breeding of pea mother, and an X or a Y from the even of Darwin’s, he focused
plants and, in 1866, suggested that father. The “white gene” is carried his efforts on the breeding of
their inherited characteristics were by the X. The Y chromosome has fruit flies to test his ideas
determined by discrete particles. no corresponding gene. about genetics. His success
Four decades later, to test the with fruit flies would lead
link between chromosomes and Further work led Morgan to many researchers to use them
Mendel’s theory, Morgan embarked the notion that specific genes in genetics experiments.
on research that would combine were not only located on specific
breeding experiments with modern chromosomes, but occupied Morgan’s observation of
microscopy, in what came to be particular positions on them. This stable, inherited mutations in
known as the “Fly Room” at opened up the idea that scientists fruit flies eventually led him to
Columbia University, New York. could “map” an organism’s genes. ■ realize that Darwin was right,
and in 1915, he published a
From peas to fruit flies First Generation (F1) work explaining how heredity
Fruit flies (Drosophila) are gnat- functioned according to
sized insects that can be bred in Male Female Mendel’s laws. Morgan
small glass bottles and can produce continued his research at
the next generation—with a great Second Generation (F2) the California Institute of
many offspring—in just 10 days. Technology (Caltech) and,
This made the fruit fly ideal for Crossbreeding fruit flies over two in 1933, he was awarded the
studying inheritance. Morgan’s generations shows how the white-eyed Nobel Prize in Genetics.
team isolated and crossbred flies trait is passed only to some males,
with particular characteristics, and through the sex chromosomes. Key works
then analyzed the proportions of
variations in the offspring—just as 1910 Sex-limited Inheritance
Mendel had done with his peas. in Drosophila
1915 The Mechanism of
Morgan finally corroborated Mendelian Heredity
Mendel’s results after he spotted 1926 The Theory of the Gene
a male with eyes that were white
rather than the normal red. Mating
a white-eyed male with a red-eyed
female produced only red-eyed
offspring, which suggested that
red was a dominant trait and
white was recessive. When those
offspring were crossbred, one in
four of the next generation was

PARTICLES

HAVE WAVELIKE

PROPERTIES

ERWIN SCHRÖDINGER (1887–1961)



228 ERWIN SCHRÖDINGER

IN CONTEXT E rwin Schrödinger was a key wavelengths of light emitted when
figure in the advancement certain elements were heated. By
BRANCH of quantum physics—the modeling the structure of the atom
Physics science that explains the tiniest with electrons orbiting in discrete
levels of subatomic matter. His star “shells” whose distance from the
BEFORE contribution was a famous equation nucleus determined their energy,
1900 A crisis in the that showed how particles moved Bohr could explain the emission
understanding of light in waves. It formed the basis of spectra (distribution of light
inspires Max Planck to find today’s quantum mechanics and wavelengths) of atoms in terms
a theoretical solution that revolutionized the way we perceive of photons of energy given off as
involves treating light as the world. But this revolution did electrons jumped between orbits.
quantized packets of energy. not happen suddenly. The process However, Bohr’s model lacked a
of discovery was a long one, with theoretical explanation, and could
1905 Albert Einstein many pioneers along the way. only predict the emissions from
demonstrates the reality hydrogen, the simplest atom.
of Planck’s quantized light Quantum theory was originally
through his explanation of limited to the understanding of Wavelike atoms?
the photoelectric effect. light. In 1900, as part of an attempt Einstein’s idea had breathed new
to solve a troubling problem in life into the old theory of light as
1913 Niels Bohr’s model of theoretical physics known as streams of particles, even though
the atom uses the idea that the “ultraviolet catastrophe,” the light had also been proved, through
electrons shifting between German physicist Max Planck Thomas Young’s double-slit
energy levels within an atom proposed treating light as though experiment, to behave as a wave.
emit or absorb individual it came in discrete packets, or The puzzle of how light could
quanta of light (photons). quanta, of energy. Albert Einstein possibly be both particle and wave
then took the next step and argued received a new twist in 1924 from
AFTER that light quanta were indeed a real
1930s Schrödinger’s work, physical phenomenon. 1927 saw a gathering of greats at the
along with that of Paul Dirac Solvay Conference of physics in Brussels.
and Werner Heisenberg, forms Danish physicist Niels Bohr Among others are: 1. Schrödinger,
the foundation of modern knew that Einstein’s idea was 2. Pauli, 3. Heisenberg, 4. Dirac,
particle physics. saying something fundamental 5. de Broglie, 6. Born, 7. Bohr, 8. Planck,
about the nature of light and 9. Curie, 10. Lorentz, 11. Einstein.
atoms, and in 1913 used it to solve
an old problem—the precise

1

4 23
56

8 9 10 11 7

A PARADIGM SHIFT 229

See also: Thomas Young 110–11 ■ Albert Einstein 214–21 ■ Werner Heisenberg 234–35 ■ Paul Dirac 246–47
Richard Feynman 272–73 ■ Hugh Everett III 284–85

If you fire electrons one-by-one This means that they are
through two slits onto a screen, they behaving like waves.
will build up an interference pattern

on the screen.

Particles have A wave function provides a way
wavelike properties. of calculating the probability of
detecting an electron at a particular

point in space-time.

a French PhD student, Louis de h divided by its wavelength. also relatively easy to test. By
Broglie, whose suggestion led the However, since he was dealing 1927, scientists in two separate
quantum revolution into a dramatic with particles whose energy and laboratories had conducted
new phase. Not only did de Broglie mass might be affected by motion experiments to show that electrons
demonstrate with a simple at speeds close to that of light, de diffracted and interfered with each
equation how, in the subatomic Broglie incorporated the Lorentz other in exactly the same way as
world, particles could equally be factor (p.219) into his equation. photons of light. De Broglie’s
waves, he also showed how any This produced a more sophisticated hypothesis was proved.
object, of whatever mass, could version that took into account
behave as a wave to some extent. the effects of relativity. Growing significance
In other words, if light waves had In the meantime, a number of
particle-like properties, then particles De Broglie’s idea was radical theoretical physicists were
of matter—such as electrons— and daring, but it soon had sufficiently intrigued by de Broglie’s
must have wavelike properties. influential supporters, including hypothesis to investigate it further.
Einstein. The hypothesis was In particular, they wanted to know
Planck had calculated the how the properties of such matter
energy of a light photon with the Two seemingly incompatible waves could give rise to the pattern
simple equation E = hv, where E conceptions can each represent of specific energy levels among the
is the energy of the electromagnetic electron orbitals of the hydrogen
quanta, v is the wavelength of an aspect of the truth. atom proposed by Bohr’s model of
the radiation involved, and h is Louis de Broglie the atom. De Broglie himself had
a constant, today known as suggested that the pattern arose
the Planck constant. De Broglie because the circumference of each
showed that a light photon also has orbital must accommodate a whole
momentum, something normally number of wavelengths of the
only associated with particles matter wave. Since the electron’s
with mass and given by multiplying energy level depends on its
the particle’s mass with its speed. distance from the atom’s positively
De Broglie showed that a light charged nucleus, this meant that ❯❯
photon had a momentum of

230 ERWIN SCHRÖDINGER

only certain distances, and certain A classic illustration of wave-particle Interference
energy levels, would be stable. duality involves firing electrons from a pattern
However, de Broglie’s solution relied “gun” through a barrier with two slits in
on treating the matter wave as a it. If electrons are allowed to build up over
one-dimensional wave trapped in time, an interference pattern forms, just
orbit around the nucleus—a full as it would for light waves.
description would need to describe
the wave in three dimensions.

The wave equation Narrow slits into the lowest possible energy
In 1925, three German physicists, state, Pauli developed the
Werner Heisenberg, Max Born, and Electrons exclusion principle. Reasoning
Pascual Jordan, tried to explain the that a particle’s overall quantum
quantum jumps that occurred in Gun state could be defined by a certain
Bohr’s model of the atom with a number of properties, each with a
method called matrix mechanics, to interpret it as the density of fixed number of possible discrete
in which the properties of an atom electric charge, but this was not values, his principle stated
were treated as a mathematical entirely successful. It was Max that it was impossible for two
system that could change over Born who eventually suggested particles within the same system
time. However, the method could what it really was—it was a to have the same quantum
not explain what was actually probability amplitude. In other state simultaneously.
happening inside the atom, and words, it expressed the likelihood
its obscure mathematical language of a measurement finding the In order to explain the pattern
did not make it very popular. electron in that particular place. of electron shells that was apparent
Unlike matrix mechanics, the from the periodic table, Pauli
A year later, an Austrian Schrödinger wave equation or calculated that electrons must be
physicist working in Zurich, Erwin “wave function” was embraced by described by four distinct quantum
Schrödinger, hit upon a better physicists, although it threw open numbers. Three of these—the
approach. He took de Broglie’s a whole range of wider questions principal, azimuthal, and magnetic
wave-particle duality a step further about its proper interpretation. quantum numbers—define the
and began to consider whether electron’s precise place within
there was a mathematical equation Pauli’s exclusion principle the available orbital shells and
of wave motion that would describe Another important piece of the subshells, with the values of the
how a subatomic particle might puzzle fell into place in 1925
move. To formulate his wave courtesy of another Austrian,
equation, he began with the laws Wolfgang Pauli. In order to describe
governing energy and momentum why the electrons within an atom
in ordinary mechanics, then did not all automatically fall directly
amended them to include the
Planck constant and de Broglie’s
law connecting the momentum of
a particle to its wavelength.

When he applied the resulting
equation to the hydrogen atom,
it predicted exactly the specific
energy levels for the atom that had
been observed in experiments. The
equation was a success. But one
awkward issue remained, because
no one, not even Schrödinger, knew
exactly what the wave equation
really described. Schrödinger tried

latter pair limited by the value of Another major success for A PARADIGM SHIFT 231
the principal number. The fourth Schrödinger’s approach was
number, with two possible values, that it offered an explanation for Erwin Schrödinger
was needed to explain why two radioactive alpha decay—in which
electrons can exist in each subshell a fully formed alpha particle Born in Vienna, Austria, in
with slightly different energy levels. (consisting of two protons and two 1887, Erwin Schrödinger
Together, the numbers neatly neutrons) escapes from an atomic studied physics at the
explained the existence of atomic nucleus. According to classical University of Vienna, attaining
orbitals that accept 2, 6, 10, and physics, in order to remain intact, an assistant’s post there
14 electrons respectively. the nucleus had to be surrounded before serving in World War I.
by a potential well steep enough to After the war, he moved first
Today, the fourth quantum prevent particles escaping from it. to Germany, and then to
number is known as spin; it is (A potential well is a region in the University of Zurich,
a particle’s intrinsic angular space where the potential energy Switzerland, where he did
momentum (which is created by is lower than its surroundings, his most important work,
its rotation as it orbits), and has meaning that it traps particles.) immersing himself in the
positive or negative values that If the well was not sufficiently steep, emerging field of quantum
are either whole- or half-integer the nucleus would disintegrate physics. In 1927, he returned
numbers. A few years later, Pauli completely. How, then, could the to Germany, and succeeded
would demonstrate that values of intermittent emissions seen in Max Planck at the Humboldt
spin split all particles into two alpha decay happen while allowing University of Berlin.
major groups—fermions such as the remaining nucleus to survive
electrons (with half-integer spins), intact? The wave equations Schrödinger was a vocal
which obey a set of rules known as overcame the problem because opponent of the Nazis, and left
Fermi–Dirac statistics (pp.246–47), they allowed the energy of the Germany for a post at Oxford
and bosons such as photons (with alpha particle within the nucleus University in 1934. It was
zero or whole-number spin), which to vary. Most of the time, its energy there that he learned he had
obey different rules known as would be low enough to keep it been awarded the 1933 Nobel
Bose–Einstein statistics. Only trapped, but occasionally it would Prize in Physics, with Paul
fermions obey the exclusion rise high enough to overcome the ❯❯ Dirac, for the quantum wave
principle, and this has important equation. By 1936, he was
implications for the understanding Schrödinger’s equation, in its most back in Austria, but had to flee
of everything from collapsing stars general form, shows the development again following Germany’s
to the elementary particles that of a quantum system over time. It annexation of the country. He
make up the universe. requires the use of complex numbers. settled in Ireland for the rest
of his career before retiring to
Schrödinger’s success ih⎯ —∂∂t Ψ = Η Ψ Austria in the 1950s.
Combined with Pauli’s exclusion
principle, Schrödinger’s wave Key works
equation allowed a new and deeper
understanding of the orbitals, 1920 Color Measurement
shells, and subshells within an 1926 Quantization as an
atom. Rather than imagining them Eigenvalue Problem
as classical orbits—well-defined
paths on which the electrons circle
the nucleus—the wave equation
shows that they are actually clouds
of probability—doughnut-shaped
and lobe-shaped regions in which
a particular electron with certain
quantum numbers is likely to be
found (p.256).

232 ERWIN SCHRÖDINGER

wall and escape (an effect now
known as quantum tunneling). The
probability predictions of the wave
equation matched the unpredictable
nature of the radioactive decay.

Uncertainty principle particle can never be “localized” to a Dane Niels Bohr (left) collaborated
The great debate that shaped the point in space and at the same time with Werner Heisenberg, to formulate
development of quantum physics have a defined wavelength. The the Copenhagen interpretation of
during the middle years of the 20th more accurately a particle’s position Schrödinger’s wave function.
century (and remains essentially was pinned down, for example,
unresolved today) surrounded what the harder its momentum was to apparatus (subject to the classical
the wave function actually meant measure. Thus, particles defined by laws of physics) that causes the
for reality. In an echo of the Planck/ a quantum wave function existed wave function to “collapse” and
Einstein debate two decades in a general state of uncertainty. a definite outcome to arise. This
previously, de Broglie saw his and interpretation is perhaps the most
Schrödinger’s equations as mere The road to Copenhagen widely (though not universally)
mathematical tools for describing Measuring the properties of a accepted, and appears to be
movement: for de Broglie, the quantum system always revealed borne out by experiments such
electron was still essentially a the particle to be in one location, as electron diffraction and the
particle—just one that had a wave rather than in its wavelike smear. double-slit experiment for light
property governing its motion and On the scale of classical physics waves. It is possible to devise
location. For Schrödinger, however, and everyday life, most situations an experiment that reveals the
the wave equation was far more involved definite measurements wavelike aspects of light or
fundamental—it described the and definite outcomes, rather than electrons, but impossible to record
way in which the properties of the myriad overlapping possibilities. the properties of individual
electron were physically “smeared The challenge of reconciling particles in the same apparatus.
out” across space. Opposition to quantum uncertainty with reality
Schrödinger’s approach inspired is called the measurement problem, However, while the Copenhagen
Werner Heisenberg to develop and various approaches to it interpretation seems reasonable
another of the century’s great have been put forward, known when dealing with small-scale
ideas—the uncertainty principle as interpretations. systems such as particles, its
(pp.234–35). This was a realization implication that nothing is
that the wave function meant that a The most famous of these is determined until it is measured
the Copenhagen interpretation, troubled many physicists. Einstein
God knows I am no friend devised by Niels Bohr and Werner famously commented that “God
of probability theory, I have Heisenberg in 1927. This states does not throw dice,” while
hated it from the first moment simply that it is the very interaction Schrödinger devised a thought
between the quantum system and experiment to illustrate what he
when our dear friend a large-scale, external observer or viewed as a ridiculous situation.
Max Born gave it birth.
Erwin Schrödinger

A PARADIGM SHIFT 233

Schrödinger’s cat universe splits into mutually an attempt at a strictly causal,
Taken to its logical conclusion, the unobservable alternate histories rather than probabilistic,
Copenhagen interpretation resulted for each of the possible outcomes. explanation, and postulates the
in a seemingly absurd paradox. In other words, Schrödinger’s cat existence of a hidden “implicate”
Schrödinger imagined a cat sealed would both live and die. order to the universe. The
in a box that contains a vial of transactional approach involves
poison linked to a radioactive The “Consistent Histories” waves traveling both forward
source. If the source decays and approach addresses the problem and backward in time.
emits a particle of radiation, a in a rather less radical way, using
mechanism will release a hammer complex mathematics to generalize Perhaps the most intriguing
that breaks the vial of poison. the Copenhagen interpretation. possibility of all, however, is one
According to the Copenhagen This avoids the issues around the that verges on the theological.
interpretation, the radioactive collapse of the wave function, but Working in the 1930s, Hungarian-
source remains in its wave function instead allows probabilities to be born mathematician John von
form (as a so-called superposition assigned to various scenarios, or Neumann concluded that the
of two possible outcomes) until it “histories,” on both a quantum measurement problem implied
is observed. But if that is the case, and classical scale. The approach that the entire universe is subject
the same has to be said of the cat. accepts that only one of these to an all-encompassing wave
histories eventually conforms equation known as the universal
New interpretations to reality, but does not allow wave function, and that it is
Dissatisfaction with apparent prediction of which outcome constantly collapsing as we
paradoxes such as Schrödinger’s that will be—instead it simply measure its various aspects.
cat has spurred scientists to describes how quantum physics Von Neumann’s colleague and
develop various alternative can give rise to the universe we see countryman Eugene Wigner took
interpretations of quantum without wave function collapse. the theory and expanded it to
mechanics. One of the best known suggest that it was not simply
is the “Many Worlds Interpretation” The ensemble, or statistical, interaction with large-scale
put forward in 1956 by American approach is a minimalist systems (as in the Copenhagen
physicist Hugh Everett III. This mathematical interpretation that interpretation) that caused the
resolved the paradox by suggesting was favored by Einstein. The wave function to collapse—it
that during any quantum event, the de Broglie–Bohm theory, which was the presence of intelligent
developed from de Broglie’s initial consciousness itself. ■
reaction to the wave equation, is

Radioactive Geiger
material counter

Hammer

Poison If the source decays, it releases We must measure the
poison and the cat dies. system to find out
A cat inside a sealed box remains whether the source has
alive as long as a radioactive source decayed. Until then, we
in the box does not decay. must think of the cat as
both dead and alive.
Schrödinger’s thought experiment produces a situation
in which, according to a strict reading of the Copenhagen
interpretation, a cat is both alive and dead at the same time.

234

UNCERTAINTY
IS INEVITABLE

WERNER HEISENBERG (1901–1976)

IN CONTEXT F ollowing Louis de Broglie’s Working with Danish physicist
suggestion in 1924 that Niels Bohr, Heisenberg built on
BRANCH on the smallest scales of Schrödinger’s work to develop the
Physics matter, subatomic particles could “Copenhagen interpretation” of
display wavelike properties the way that quantum systems,
BEFORE (pp.226–33), a number of physicists governed by the laws of probability,
1913 Niels Bohr uses the turned their attention to the interact with the large-scale world.
concept of quantized light to question of understanding how One key element of this is the
explain the specific energy the complex properties of an atom “uncertainty principle,” which
levels associated with could arise from the interaction limits the accuracy to which we
electrons inside atoms. of “matter waves” associated can determine properties in
with its constituent particles. quantum systems.
1924 Louis de Broglie In 1925, German scientists
proposes that just as light can Werner Heisenberg, Max Born, The uncertainty principle arose
exhibit particle-like properties and Pascual Jordan used “matrix as a mathematical consequence of
so, on the smallest scale, mechanics” to model the hydrogen matrix mechanics. Heisenberg
particles might sometimes atom’s development over time. This realized that his mathematical
show wavelike behavior. approach was later supplanted by method would not allow certain
Erwin Schrödinger’s wave function. pairs of properties to be determined
AFTER simultaneously with absolute
1927 Heisenberg and Bohr put
forward the highly influential Classical picture Energy Quantum tuneling
Copenhagen interpretation Electron barrier is explained by
of the way that quantum-level Heisenberg’s principle.
events affect the large-scale Quantum picture There is a nonzero
(macroscopic) world. chance that an electron
can pass through
1929 Heisenberg and a barrier even if it
Wolfgang Pauli work on the appears to have too
development of quantum field little energy to do so.
theory, whose foundations
have been laid by Paul Dirac. Electron
wave

A PARADIGM SHIFT 235

See also: Albert Einstein 214–21 ■ Erwin Schrödinger 226–33 ■
Paul Dirac 246–47 ■ Richard Feynman 272–73 ■ Hugh Everett III 284–85

Subatomic particles have This means that you cannot
wavelike qualities. accurately measure both
a particle’s position and

its momentum.

Uncertainty is This uncertainty is a Werner Heisenberg
inevitable. property inherent
to the universe. Born in the southern German
town of Würzburg in 1901,
precision. For example, the far stranger—it turns out that Werner Heisenberg studied
more accurately one measures uncertainty is an inherent feature mathematics and physics at
a particle’s position, the less of quantum systems. the universities of Munich and
accurately one can determine Göttingen, where he studied
its momentum, and vice versa. A helpful way of thinking under Max Born and met his
Heisenberg found that for these about the issue is to consider the future collaborator Niels Bohr
two properties in particular, the matter waves associated with for the first time.
relationship could be written as: the particles: in this situation, the
particle’s momentum affects its He is best known for his
x p ≥ h⎯/2 overall energy and therefore its work on the Copenhagen
where x is the uncertainty of wavelength—but the more tightly interpretation and the
position, p the uncertainty of we pin down the particle’s position, uncertainty principle, but
momentum, and h is a modified the less information we have about Heisenberg also made
version of Planck’s constant (p.202). its wave function, and therefore important contributions to
about its wavelength. Conversely, quantum field theory and
An uncertain universe accurately measuring the developed his own theory
The uncertainty principle is often wavelength requires us to consider of antimatter. Awarded the
described as a consequence of a broader region of space, and Nobel Prize in Physics in
quantum-scale measurements—for therefore sacrifices information 1932, he became one of its
example, it is sometimes said that about the particle’s precise youngest recipients, and his
determining a subatomic particle’s location. Such ideas might seem stature enabled him to speak
position involves the application of strangely at odds with those we out against the Nazis after
a force of some sort that means its experience in the large-scale world, they seized power the
kinetic energy and momentum are but they have nevertheless been following year. However,
less well defined. This explanation, proved real by many experiments, he chose to stay in Germany
put forward at first by Heisenberg and form an important foundation and led the country’s nuclear
himself, led various scientists of modern physics. The uncertainty energy program during
including Einstein to spend time principle explains seemingly World War II.
devising thought experiments that strange real-life phenomena such
might obtain a simultaneous and as quantum tunneling, in which Key works
accurate measurement of position a particle can “tunnel” through a
and momentum by some form of barrier even if its energy suggests 1927 Quantum Theoretical
“trickery.” However, the truth is that it should not be able to. ■ Re-interpretation of Kinematic
and Mechanical Relations
1930 The Physical Principles
of the Quantum Theory
1958 Physics and Philosophy

THE UNIVERSE IS BIG…

AND GETTING

BIGGER

EDWIN HUBBLE (1889–1953)



238 EDWIN HUBBLE B y the early 20th century, amorphous cloud of light, including
ideas about the scale of objects that were later found to be
IN CONTEXT the universe divided galaxies beyond the Milky Way.
astronomers into two schools of
BRANCH thought—those who believed that As telescopes improved
Cosmology the Milky Way galaxy was, generally dramatically during the 19th
speaking, its entire extent, and century, some of the objects
BEFORE those who thought that the Milky catalogued as nebulae began to
1543 Nicolaus Copernicus Way could be just one galaxy reveal distinctive spiral features.
concludes that Earth is not among countless others. Edwin At the same time, the development
the center of the universe. Hubble was to solve the puzzle, and of spectroscopy (the study of the
show that the universe is much interaction between matter and
17th century The changing larger than anyone imagined. radiated energy) suggested that
view of stars offered by Earth’s these spirals were in fact made
orbit around the Sun gives rise Key to the debate was the up of countless individual stars,
to the parallax method for nature of “spiral nebulae.” Today, blending seamlessly together.
measuring stellar distances. a nebula is the term used for an
interstellar cloud of dust and gas, The distribution of these
19th century Improvements but at the time of this debate, it nebulae was interesting too—
to telescopes pave the way for was the name used for any unlike other objects that clustered
the study of starlight and the together in the plane of the Milky
rise of astrophysics. There is a simple relation Way, they were more common in
between the brightness of the the dark skies away from the plane.
AFTER As a result, some astronomers
1927 Georges Lemaître variables and their periods. adopted an idea from the German
proposes that the universe Henrietta Leavitt philosopher Immanuel Kant, who
can be traced back to a single in 1755 suggested that nebulae
point of origin. were “island universes”—systems
similar to the Milky Way but vastly
1990s Astronomers discover more distant, and only visible
that the expansion of the where the distribution of material
universe is accelerating, in our galaxy permits clear views
driven by a force known into what we now call intergalactic
as dark energy. space. Those who continued to
believe that the universe was far
Edwin Hubble
Born in Marshfield, Missouri, in “extragalactic nebulae” in
1889, Edwin Powell Hubble had 1924–25, and his proof of cosmic
a fiercely competitive nature that expansion in 1929. In later
manifested itself in his youth as a years, he campaigned for
gifted athlete. Despite his interest astronomy to be recognized by
in astronomy, he followed his the Nobel Prize Committee. The
father’s wishes and studied law, rules were only changed after
but at 25 years old, after his his death in 1953 and so he was
father’s death, he resolved to never awarded the prize himself.
follow his early passion. His
studies were interrupted by Key works
service in World War I, but after
his return to the United States 1925 Cepheid Variables in
he began to work at the Mount Spiral Nebulae
Wilson Observatory. There he 1929 A Relation Between
did his most important work, Distance and Radial Velocity
publishing his study on among Extra-galactic Nebulae

A PARADIGM SHIFT 239

See also: Nicolaus Copernicus 34–39 ■ Christian Doppler 127 ■ Georges Lemaître 242–45

A Cepheid variable If the Cepheid variable
is a star whose brightness is millions of light years
we can know for certain,
away, it must be
which means that in a galaxy far
we can calculate outside our own.
how far away it is.

Henrietta Leavitt received little The light of every distant The light coming
recognition in her lifetime, but her galaxy is redshifted, and from other galaxies may
discoveries relating to Cepheid variable the farther the galaxy, the be blueshifted (moving
stars were the key that allowed greater the redshift. toward us) or redshifted
astronomers to measure the distance
from Earth to faraway galaxies. (moving away from us).

more limited in extent argued that
the spirals might be suns or solar
systems in the process of formation,
in orbit around the Milky Way.

Stars with a pulse The universe is big…
The answers to this long-standing and getting bigger.
puzzle came in several stages, but
perhaps the most important was the of the clouds, she found, contained were an indication of differences in
establishment of an accurate means huge numbers of variable stars, and their “absolute magnitude” (actual
of measuring the distance to stars. by comparing them across many brightness). Publishing her first
The breakthrough came with the different plates, she not only saw results in 1908, Leavitt noted in
work of Henrietta Swan Leavitt, one that their light was varying in a passing that some stars seemed to
of the team of female astronomers regular cycle, she could also figure show a relationship between their
at Harvard University who were out the period of the cycle. variability period and their absolute
analyzing the properties of starlight. magnitude, but it took another four
By concentrating on these small, years for her to figure out what this
Leavitt was intrigued by the faint, isolated star clouds, Leavitt relationship was. It turned out that,
behavior of variable stars. These could safely assume that the stars for a certain type of variable star
were stars whose brightness within them were all at more or known as a Cepheid variable, stars
appeared to fluctuate, or pulse, less the same distance from Earth. with greater luminosity have longer
because they periodically Though she could not know the variability periods.
expanded and contracted as they distance itself, this was still
neared the end of their lives. She enough to assume that differences Leavitt’s “period-luminosity”
began to study photographic plates in the “apparent magnitude” law would prove the key to
of the Magellanic Clouds, two small (observed brightness) of the stars unlocking the scale of the ❯❯
patches of light visible from the
southern sky that look like isolated
“clumps” of the Milky Way. Each

240 EDWIN HUBBLE

We are reaching the universe once and for all. The “island universe” supporters
into space, farther and Respected Princeton astronomer were represented by Heber D. Curtis
farther, until, with the Harlow Shapley spoke for the “small of the University of Pittsburgh’s
faintest nebulae that can universe” side. He had been the Allegheny Observatory. He based
be detected…we arrive first to use Leavitt’s work on his arguments on comparisons
Cepheids to measure the distance between the rates of bright “nova”
at the frontier of the to globular clusters (dense star explosions in distant spirals and in
known universe. clusters in orbit around the Milky our own Milky Way. Novae are very
Edwin Hubble Way), and discovered that they bright star explosions that can
were typically several thousand serve as distance indicators.
universe—if you could figure out light years away. In 1918, he had
the star’s absolute magnitude used RR Lyrae stars (fainter stars Curtis also cited the evidence
from its variability period, then that behave like Cepheids) to of another, crucial factor—the high
the star’s distance from Earth estimate the size of the Milky Way redshift exhibited by many spiral
could be calculated from its and show that the Sun was nebulae. This phenomenon had
apparent magnitude. The first nowhere near its center. His been discovered by Vesto Slipher
step in figuring this out was arguments appealed to public of the Flagstaff Observatory,
to calibrate the scale, which scepticism toward notions of an Arizona, in 1912—apparent through
was done in 1913 by Swedish enormous universe with many distinctive shifts in the pattern of a
astronomer Ejnar Hertzsprung. galaxies, but also cited specific nebula’s spectral lines toward the
He figured out the distances to evidence (later to be proved red end of the spectrum. Slipher,
13 relatively nearby Cepheids inaccurate), such as reports that Curtis, and many others believed
using the parallax method (p.39). over many years some astronomers that they were caused by the
Cepheids were immensely had actually observed the spiral
bright—thousands of times more nebulae rotating. For this to be By measuring the light from Cepheid
luminous than our Sun (in modern true without parts of the nebula variable stars in the Andromeda nebula,
terminology they are “yellow exceeding the speed of light, Hubble established that Andromeda
supergiants”). In theory, then, they they must be relatively small. was 2.5 million light years way—
were an ideal “standard candle”— and was a galaxy in its own right.
stars whose brightness could be
used to measure huge cosmic
distances. But despite the best
efforts of astronomers, Cepheids
within the spiral nebulae
remained stubbornly elusive.

The Great Debate
In 1920, the Smithsonian Museum
in Washington DC hosted a
debate between the two rival
cosmological schools, hoping to
settle the issue of the scale of

A PARADIGM SHIFT 241

Doppler effect (a change in the In 1842, Christian
wavelength of light due to relative Doppler (p.127)
motion between source and showed that if a light
observer), and therefore indicated source is moving
that the nebulae were moving away toward us or away
from us at very high speeds—far from us, the light
too fast for the Milky Way’s gravity waves arrive at
to keep hold of them. different rates. If
the light source is
Measuring the universe correctly called spiral galaxies. moving toward us,
By 1922–23, Edwin Hubble and As if this revolution in the way we see a bluer color
Milton Humason of California’s we see the universe were not as waves bunch
Mount Wilson Observatory were in enough, Hubble then went on to together at the blue
a position to end the mystery once look at how galaxy distances end of the light
and for all. Using the observatory’s related to the redshifts already spectrum; if it is
new 100 in (2.5 m) Hooker Telescope discovered by Slipher—and here moving away, we
(the largest in the world at that he found a remarkable relationship. see a redder color.
time), they set out to identify By plotting the distances for more Hubble guessed that
Cepheid variables shining within than 40 galaxies against their sodium light was
the spiral nebulae, and this redshifts, he showed a roughly the same color in
time they were successful in linear pattern: the farther away a far galaxies as it is
finding Cepheids in many of the galaxy is, on average, the greater on Earth, but the
largest and brightest nebulae. its redshift and therefore the faster Doppler effect meant
it is receding from Earth. Hubble that it would be
Hubble then plotted their immediately realized that this blueshifted or
periods of variability and therefore could not be because our galaxy is redshifted if moving
their absolute magnitude. From uniquely unpopular, but must be toward or away
this, a simple comparison to a from us.
star’s apparent magnitude revealed
its distance, producing figures that the result of a general cosmic
were typically millions of light expansion—in other words, space
years. This proved conclusively itself is expanding and carrying
that the spiral nebulae were really every single galaxy with it. The
huge, independent star systems, far wider the separation between
beyond the Milky Way and rivaling two galaxies, the faster the space
it in size. Spiral nebulae are now between them will expand. The
rate of expansion of space soon
Equipped with his five became known as the “Hubble
senses, man explores the Constant.” It was conclusively
universe around him and calls measured in 2001 by the space
telescope bearing Hubble’s name.
the adventure science.
Edwin Hubble Long before then, Hubble’s
discovery of the expanding
universe had given rise to one
of the most famous ideas in the
history of science—the Big Bang
theory (pp.242–45). ■

242 IN CONTEXT

THE RADIUS BRANCH
OF SPACE Astronomy
BEGAN
AT ZERO BEFORE
1912 US astronomer
GEORGES LEMAÎTRE (1894–1966) Vesto Slipher discovers
the high redshifts of spiral
nebulae, suggesting they
are moving away from Earth
at high speeds.

1923 Edwin Hubble confirms
that the spiral nebulae are
distant, independent galaxies.

AFTER
1980 US physicist Alan Guth
proposes a brief period of
dramatic inflation in the early
universe to produce the
conditions we see today.

1992 The COBE (Cosmic
Background Explorer) satellite
detects tiny ripples in the
cosmic microwave background
radiation (CMBR)—hints of the
first structure that emerged in
the early universe.

T he idea that the universe
began with a Big Bang,
expanding from a tiny,
superdense, and extremely hot
point in space, is the basis of
modern cosmology, and one that is
often said to have originated with
Edwin Hubble’s 1929 discovery
of cosmic expansion. But the
precursors of the theory predate
Hubble’s breakthrough by several
years, and first sprang from
interpretations of Albert Einstein’s
theory of general relativity as it
applied to the universe as a whole.

When formulating his theory,
Einstein drew on the available
evidence of the time to assume
that the universe was static—

A PARADIGM SHIFT 243

See also: Isaac Newton 62–69 ■ Albert Einstein 214–21 ■
Edwin Hubble 236–41 ■ Fred Hoyle 270

Since the Big Bang 13.8 billion years ago, the expansion of the
universe has been through different phases. There was an initial
period of rapid expansion known as inflation. After that, expansion
slowed, then started to speed up once more.

Present

Time Accelerating Georges Lemaître
(~15 billion years) expansion
Born in Charleroi, Belgium, in
Inflation Slowing expansion 1894, Lemaître studied civil
Big Bang engineering at the Catholic
University of Louvain and
Expanding universe served in World War I before
returning to academia, where
neither expanding nor contracting. Famously, Einstein later called the he studied physics and
General relativity indicated that constant his greatest mistake, mathematics as well as
the universe should collapse under but even at the time he proposed theology. From 1923, he
its own gravity, so Einstein fudged it there were some who found it studied astronomy in Britain
his own equations by adding a unsatisfactory. The Dutch physicist and the United States. On his
term known as the cosmological Willem de Sitter and Russian return to Louvain in 1925 as
constant. Einstein’s constant mathematician Alexander a lecturer, Lemaître began
mathematically counteracted the Friedmann independently to develop his theory of an
gravitational contraction to produce suggested a solution to general expanding universe as an
the presumed static universe. relativity in which the universe was explanation for the redshifts
expanding, and, in 1927, Belgian of the extragalactic nebulae.
astronomer and priest Georges
Lemaître reached the same First published in 1927, in
conclusion, two years ahead of a little-read Belgian journal,
Hubble’s observational proof. Lemaître’s ideas took off
after he published an English
The first stages of the Beginning in fire translation with Arthur
expansion consisted of a In an address to the British Eddington. He lived until
rapid expansion determined Association in 1931, Lemaître 1966—long enough to see
by the mass of the initial atom, took the idea of cosmic expansion proof that his ideas were
almost equal to the present to its logical conclusion, suggesting correct with the discovery
that the universe had sprung from of the cosmic microwave
mass of the universe. a single point that he called the background radiation (CMBR).
Georges Lemaître “primeval atom.” The response
to this radical idea was mixed. Key works

The astronomical establishment 1927 A Homogeneous Universe
of the time was attached to the of Constant Mass and
idea of an eternal universe ❯❯ Growing Radius Accounting
for the Radial Velocity of
Extragalactic Nebulae
1931 The Evolution of the
Universe: Discussion

244 GEORGES LEMAÎTRE

General relativity leads Hubble demonstrates away from a steady state universe.
Lemaître to predict that cosmic expansion. This was a 1948 paper written by
the universe is expanding. Ralph Alpher and George Gamow
of the Johns Hopkins University
Lemaître theorizes that the universe began with a in the US. It was called The Origin
“primeval atom,” a theory later dubbed the “Big Bang.” of Chemical Elements, and
described in detail how subatomic
The discovery of cosmic microwave background radiation particles and lightweight chemical
(CMBR) confirms the Big Bang theory. elements could have been produced
from the raw energy of the Big
The radius of space began at zero. Bang, in accordance with
Einstein’s equation E = mc2. But
without end or beginning, and the In 1949, Hoyle scornfully referred this theory, later known as Big
prospect of a distinct point of origin to the rival theory as a “Big Bang.” Bang nucleosynthesis, explained a
(especially when proposed by a The name stuck. process that could form only the
Catholic priest) was seen as four lightest elements—hydrogen,
introducing an unnecessary Making the elements helium, lithium, and beryllium.
religious element into cosmology. By the time Hoyle had inadvertently Only later was it discovered that
named the theory, a persuasive the heavier elements of the
However, Hubble’s observations piece of evidence in favor of universe are the product of stellar
were undeniable, and some kind of Lemaître’s hypothesis had been nucleosynthesis (a process that
model was needed to explain the published, tipping the balance takes place inside stars). Ironically,
expanding universe. Numerous the evidence showing how stellar
theories were put forward in the nucleosynthesis worked was to be
1930s, but by the late 1940s, just developed by Fred Hoyle.
two remained in play—Lemaître’s
primeval atom, and the rival Nevertheless, there was still
“steady state” model, in which no direct observational evidence
matter was continuously created to determine the truth of either
as the universe expanded. British the Big Bang or a steady state
astronomer Fred Hoyle was the universe. Early attempts to test
champion of the steady state idea. the theories were made in the
1950s using a basic radio telescope
known as the Cambridge
Interferometer. These tests relied
on a simple principle: if the steady
state theory was true, then the

Tiny variations have been found in
the cosmic microwave background
radiation—the different colors in this
image show temperature differences
of less than 400 millionths of a Kelvin.

A PARADIGM SHIFT 245

Arno Penzias and Robert Wilson
detected the background radiation by
accident. At first, they thought the
interference had been caused by bird
droppings on their radio antenna.

universe must be essentially and his colleagues at Princeton minute variations in the signal that
uniform in both time and space; University set out to build a radio allow us to study conditions in the
but if it originated 10–20 billion telescope that could detect this faint universe back to 380,000 years
years ago, as the Big Bang theory signal, which they thought would after the Big Bang.
suggested, and evolved throughout take the form of low-energy radio
its history, then distant reaches of waves. However, they were Later developments
the universe, whose radiation had ultimately beaten to the prize by Despite being proved correct in
taken billions of years to reach Arno Penzias and Robert Wilson, principle, the Big Bang theory has
Earth, should appear substantially two engineers working at the undergone many transformations
different. (This cosmic time nearby Bell Telephone Laboratories. since the 1960s to match it to
machine effect, whereby we see Penzias and Wilson had built a radio our growing understanding of
more distant celestial objects as telescope for satellite communication, the universe. Among the most
they were in the distant past, is but found themselves plagued by an significant are the introduction
known as “lookback time.”) By unwanted background signal that of dark matter and dark energy
measuring the number of distant they could not eliminate. Coming to the story, and the addition of a
galaxies emitting radiation above from all over the sky, it corresponded violent growth spurt in the instant
a certain brightness, it should be to microwave emission from a body after creation, known as Inflation.
possible to distinguish between at a temperature of 3.5K—just 6°F The events that triggered the Big
the two scenarios. (3.5°C) above absolute zero. When Bang remain beyond our reach but
Bell Labs contacted Dicke to ask measurements of the rate of cosmic
The first of the Cambridge for help with their problem, Dicke expansion, aided by instruments
experiments delivered a result that realized that they had found the such as the Hubble Space Telescope,
seemed to support the Big Bang. remnants of the Big Bang—now now allow us to pin down the epoch
However, problems were discovered known as the cosmic microwave of cosmic creation with great
with the radio detectors, so the background radiation (CMBR). accuracy—the universe came into
results had to be disregarded. existence 13.798 billion years ago,
Later results proved more equivocal. The discovery that the CMBR give or take 0.037 billion years.
permeates the universe—a Various theories exist about the
Traces of the Big Bang phenomenon for which the steady future of the universe, but many
Fortunately, the question soon state theory had no explanation— think that it is set to continue
resolved itself by other means. decided the case in favor of the Big expanding until it reaches a state
As early as 1948, Alpher and his Bang. Subsequent measurements of thermodynamic equilibrium, or
colleague Robert Herman had have shown that the CMBR’s true “heat death,” in which matter has
predicted that the Big Bang would average temperature is about disintegrated into cold subatomic
have left a residual heating effect 2.73K, and high-precision satellite particles, in around 10100 years’ time.■
throughout the universe. According measurements have revealed
to the theory, when the universe
was about 380,000 years old, it
had cooled enough to become
transparent, allowing light photons
to travel freely through space for the
first time. The photons that existed
at this time had been propagating
through space ever since, growing
longer and redder as space
expanded. In 1964, Robert Dicke

246

EVERY PARTICLE OF
MATTER HAS AN
ANTIMATTER
COUNTERPART

PAUL DIRAC (1902–1984)

IN CONTEXT Dirac corrects E nglish physicist Paul Dirac
Schrödinger’s wave contributed a huge amount
BRANCH equation to take into account to the theoretical framework
Physics relativistic effects. of quantum physics in the 1920s,
but is probably best known today
BEFORE Dirac’s new equation for predicting the existence of
1925 Werner Heisenberg, predicts the existence antiparticles through mathematics.
Max Born, and Pascual Jordan
develop matrix mechanics of antimatter. Dirac was a postgraduate
to describe the wavelike student at Cambridge University
behavior of particles. Antimatter is when he read Werner Heisenberg’s
subsequently discovered, groundbreaking paper on matrix
1926 Erwin Schrödinger confirming Dirac’s prediction. mechanics, which described how
develops a wave function particles jump from one quantum
describing the change in Every particle state to another. Dirac was one
an electron over time. of matter has of the few people capable of grasping
an antimatter the paper’s difficult mathematics,
AFTER counterpart. and noticed parallels between
1932 The existence of the Heisenberg’s equations and parts
positron, the antiparticle to of the classical (pre-quantum)
the electron, is confirmed by theory of particle motion known
Carl Anderson. as Hamiltonian mechanics. This
allowed Dirac to develop a method
1940s Richard Feynman, by which classical systems could
Sin-Itiro Tomonaga, and be understood on a quantum level.
Julian Schwinger develop
quantum electrodynamics— One early result of this work
a mathematical way to was a derivation of the idea of
describe the interaction quantum spin. Dirac formulated a
between light and matter, set of rules now known as “Fermi-
which fully unites quantum Dirac statistics” (since they were
theory with special relativity. also independently found by Enrico
Fermi). Dirac named particles such
as electrons that have a half-integer
spin value “fermions,” after Fermi.
The rules describe how large

A PARADIGM SHIFT 247

See also: James Clerk Maxwell 180–85 ■ Albert Einstein 214–21 ■ Erwin Schrödinger 226–33 ■
Werner Heisenberg 234–35 ■ Richard Feynman 272–73

numbers of fermions interact with Positron Photon When a particle
one another. In 1926, Dirac’s PhD Electron Annihilation and its antiparticle
supervisor Ralph Fowler used his come together,
statistics to calculate the behavior Photon they annihilate.
of a collapsing stellar core and Their mass turns
explain the origin of superdense into photons of
white dwarf stars. electromagnetic
energy in accord
Quantum field theory with the equation
While much of schoolbook physics E = mc2.
focuses on the properties and
dynamics of individual particles the electron (that is, one that could Carl Anderson in 1932, detected
and bodies under the influence of take into account the effects of first in cosmic rays (high-energy
forces, a deeper understanding particles moving close to the speed particles showered into Earth’s
can be gained by developing field of light, and therefore model the atmosphere from deep space),
theories. These describe the way quantum world more accurately and then in certain types of
that forces make their influence felt than Schrödinger’s nonrelativistic radioactive decay. Since then,
across space. The importance of equation). The so-called Dirac antimatter has become a subject
fields as independent entities was equation also predicted the for intense physical research,
first recognized in the mid-19th existence of particles with identical and also beloved of science-fiction
century by James Clerk Maxwell properties to particles of matter but writers (particularly for its
while he was developing his with opposite electric charge. They habit of “annihilating” with a burst
theory of electromagnetic radiation. were dubbed “antimatter” (a term of energy on contact with normal
Einstein’s general relativity is that had been bandied around in matter). Perhaps more importantly,
another example of a field theory. wilder speculations since the late however, Dirac’s quantum
19th century). field theory laid the foundations
Dirac’s new interpretation of for the theory of quantum
the quantum world was a quantum The antielectron particle, electrodynamics brought to fruition
field theory. In 1928, it allowed him or positron, was experimentally by a later generation of physicists. ■
to produce a relativistic version of confirmed by US physicist
Schrödinger’s wave equation for

Paul Dirac Paul Dirac was a mathematical and Copenhagen before
genius who made several key returning to Cambridge, having
contributions to quantum been appointed the Lucasian
physics, sharing the Nobel Chair in Mathematics. Much of
Prize in Physics with Erwin his later career was focused on
Schrödinger in 1933. Born in quantum electrodynamics. He
Bristol, England, to a Swiss father also pursued the idea of unifying
and an English mother, he earned quantum theory with general
degrees in electrical engineering relativity, but this endeavor met
and mathematics at the city’s with limited success.
university, before continuing his
studies at Cambridge, where he Key works
pursued his fascination with
general relativity and quantum 1930 Principles of Quantum
theory. After his groundbreaking Mechanics
advances of the mid-1920s, he 1966 Lectures on Quantum
continued his work at Göttingen Field Theory

248

THERE IS AN UPPER
LIMIT BEYOND WHICH
A COLLAPSING STELLAR
CORE BECOMES UNSTABLE

SUBRAHMANYAN CHANDRASEKHAR (1910–1995)

IN CONTEXT T he development of known as a singularity—forming a
quantum physics in the black hole. This “Chandrasekhar
BRANCH 1920s had implications for Limit” for a collapsing stellar core is
Astrophysics astronomy, where it was applied to now known to be 1.44 solar masses
the understanding of superdense (or 1.44 times the mass of the Sun).
BEFORE stars known as white dwarfs. However, there is a middle stage
19th century White dwarf These are the burned-out cores of between white dwarf and black
stars are discovered when sunlike stars that have exhausted hole—a city-sized neutron star
astronomers identify a star their nuclear fuel and collapsed, stabilized by another quantum
that has far more mass than under their own gravity, to objects effect called “neutron degeneracy
its tiny size would suggest. about the size of Earth. In 1926, pressure.” Black holes are created
physicists Ralph Fowler and Paul only when the neutron star’s core
AFTER Dirac explained that collapse stops exceeds an upper limit somewhere
1934 Fritz Zwicky and Walter at this size due to the “degenerate between 1.5 and 3 solar masses. ■
Baade propose that explosions electron pressure” that arises
known as supernovae mark whenever electrons are packed The black holes of nature are
the deaths of massive stars, together so tightly that the Pauli the most perfect macroscopic
and the collapse of their cores exclusion principle (p.230)—that no
form neutron stars. two particles can occupy the same objects in the Universe.
quantum state—comes into play. Subrahmanyan
1967 British astronomers Chandrasekhar
Jocelyn Bell and Anthony Forming a black hole
Hewish detect rapidly pulsing In 1930, Indian astrophysicist
radio signals from an object Subrahmanyan Chandrasekhar
now known as a “pulsar”—a figured out that there was an upper
rapidly rotating neutron star. limit to the mass of a stellar core
beyond which gravity would
1971 X-ray emissions from a overcome the degenerate electron
source known as Cygnus X-1 pressure. The stellar core would
are found to originate from hot collapse to a single point in space
material spiraling into what is
probably a black hole—the first See also: John Michell 88–89 ■ Albert Einstein 214–21 ■ Paul Dirac 246–47 ■
such object to be confirmed. Fritz Zwicky 250–51 ■ Stephen Hawking 314