Hiding in the Mirror (14 page)

Over the next decade or so Rutherford and his
student James Chadwick, as well as the Polish-French physicist
Marie Curie, demonstrated that gamma rays were energetic forms of
electromagnetic radiation, while beta rays were energetic
electrons, and alpha rays consisted of the nucleus of helium, the
second lightest element, with a weight about four times that of
hydrogen.

At around this time the nature of recently
discovered atomic nuclei such as helium was puzzling. Since atoms
were neutral objects, the charge of an atomic nucleus is precisely
equal and opposite to the total charge of the electrons orbiting
it. But for some reason, nuclei weighed far more than the amount
that could be accounted for if they simply contained protons, the
heavy, positively charged objects Rutherford identified in 1919. In
1920 Rutherford imagined two different possibilities to account for
this discrepancy: First, some of the protons in a nucleus were
paired with electrons inside the nucleus, canceling their charge.
Alternatively, perhaps there were new neutral particles in nature
with a mass almost identical to that of the proton. Neither
possibility had any real evidence in support of it, but the
emerging laws of quantum mechanics began to argue strongly against
the former.

By 1930, after Heisenberg and Schrödinger had
completed their seminal work, it was recognized that to confine an
electron within a region the size of an atomic nucleus would
require an energy far greater than that which was available from
the electric attraction of protons and electrons. Thus there seemed
no way that one could resolve the apparent paradox of nuclear
masses merely by adding proton-electron pairings to nuclei. This
left the possibility of a new neutral particle as the most likely
option, and motivated by this the German physicists Walther Bothe
and his student Herbert Becker began to utilize radioactivity
itself to explore the atomic nucleus. In 1930 they reported that
when they bombarded beryllium atoms with alpha particles emitted by
a radioactive source made from the element polonium (named after
her native Poland by Marie Curie), a strange new type of neutral
radiation was emitted that could penetrate a brass plate several
centimeters thick without slowing down. This was over twenty times
farther than protons with comparable energy can penetrate.
Moreover, this radiation did not efficiently knock electrons out of
atoms in targets, ionizing them, as charged proton beams would do.
The assumption was made that this penetrating neutral radiation was
a type of gamma ray, that is, high-energy electromagnetic
radiation.

A key clue to the true nature of this radiation
was obtained via experiments performed by the daughter of Marie
Curie, Irène Joliot-Curie, and her husband, Frédíric Joliot-Curie,
although this pair actually misinterpreted the data. They placed a
paraffin wax target in the path of this radiation and discovered
that the radiation knocked protons out of the paraffin with a very
high energy. Since a similar process was known to occur in which
high-energy electromagnetic radiation impinging upon atoms could
knock out electrons, Joliot-Curie and her husband interpreted this
new effect as an analogous phenomenon caused by even higher energy
gamma rays.

Because the proton is, however, almost two
thousand times as heavy as the electron, to knock it out of an atom
would require far more energy than appeared to be available in the
original beryllium emission process. Rutherford and Chadwick
recognized this fact, and in February 1932

Chadwick announced the result of a series of
experiments he had performed analogous to those that had been
performed by Joliot-Curie and her husband. By using different
targets he demonstrated convincingly that the mystery particle
could not be a massless photon, but instead had to have a mass
almost identical to that of the proton. Chadwick had discovered the
neutron, one of the major components of all matter, and in so doing
he solved the mystery of what makes up the missing mass in atomic
nuclei. For this achievement he was awarded the Nobel Prize in
1935. (In a happy coincidence, Joliot-Curie and her husband, who
did not share the prize with Chadwick because of their
misinterpretation of their data, won the chemistry prize that year
for their discovery of artificial radioactivity.) Chadwick’s
discovery revealed a whole new world, previously hidden, inside of
every atomic nucleus. It is amazing, when you think about it, that
less than seventy-five years ago the most abundant component in all
matter, including the very atoms in our bodies, was unknown.
Moreover, what is equally remarkable in retrospect is the fact that
the consideration that led Chadwick to discover the neutron is
really a principle that is taught in high school physics. It can be
restated in a perhaps more intuitive way as follows: If I want to
knock the headlight out of an oncoming truck, I could choose to
throw a piece of popcorn at it, but I would have to throw it much
faster than I am likely to be able to in order to cause any damage.
However, if I use a rock, I don’t have to throw it very fast to
achieve my goal. Chadwick used precisely this line of reasoning to
work out the details of his experiment, and to demonstrate that
knocking protons out of nuclei required a massive, rather than a
massless, projectile. Perhaps more than anything else, however,
Chadwick’s discovery of the neutron opened a Pandora’s box of new
mysteries in elementary particle physics. Gone was the simple world
of protons and electrons, gravity and electromagnetism. Suddenly
the nuclei of atoms became complex amalgamations of protons and
neutrons, held together by some unknown new force.

To make matters even stranger, it turned out
that this new, supposedly elementary particle, a fundamental
constituent of all matter, wasn’t even stable. For if you take a
neutron and isolate it within a box, it will, on average, decay
within a paltry ten minutes or so!

How can it be that a particle that comprises
the better part of every element but hydrogen can be so ephemeral,
and yet continue to dominate the mass of everything we can see? A
miracle of Einstein’s famous relativistic connection between mass
and energy saves the day, and as a result makes our lives
possible.

For it works out that a neutron is only very
slightly heavier than a proton—less than one part in a hundred
heavier, to be exact. When neutron decay was first observed, the
decay products included protons and electrons. Originally, in fact,
Chadwick thought that a neutron might be a compound object,
consisting of a tightly bound proton and electron. However,
relativity makes this impossible, because when particles are bound
to one another it takes energy to tear them apart. But, adding
energy to something, according to the precepts of relativity, makes
it heavier. Thus, a bound state of a proton and an electron would
weigh slightly less than would the proton and electron if they were
separated.

If this were the case, and a neutron were such
a bound state and thus lighter than the sum of the proton plus
electron masses, it would be energetically impossible for it to
spontaneously decay into a free proton and an electron. The
observation of neutron decay therefore implied that the mass of the
neutron had to be larger than this sum, and subsequent careful
measurements showed this to be the case, if just barely. However,
by the same reasoning as given above, when a neutron itself is
bound in a nucleus, by forces that were then unknown, its mass
would be less than the mass of a free, unbound neutron. It turns
out, remarkably, that its mass changes by just enough so that it
can no longer decay into a proton plus electron when it is inside a
nucleus. Thus, neutrons inside nuclei are stable. As a result,
complex nuclei can be stable, and we can exist. Getting back to the
neutron itself, if it were not a bound state of a proton and an
electron, how could it decay into these products? All previous
observations of natural radioactivity involved the disintegration
of heavy complex nuclei into smaller nuclear components. Was the
neutron therefore elementary, or wasn’t it? And what new force
could be responsible for converting neutrons into other particles?
Suddenly the strange new world of elementary particle physics
became even stranger, if such a thing was possible.

And if this wasn’t bad enough, the decay of the
neutron produced yet another puzzle. If a neutron spontaneously
decayed into a proton and an electron, the law of conservation of
energy tells us that the proton and electron would each be emitted
with a fixed amount of energy, so that the total energy after the
decay would equal the energy available from the rest mass of the
neutron. However, when the decay of the neutron was observed, it
turned out that the electrons that were emitted were measured to
have not a fixed energy, but a variable energy, ranging over a
continuum from zero energy of motion (i.e., an electron at rest) to
carrying off the total energy available associated with the mass
difference between the initial neutron and the emitted proton.

If energy was to be conserved in this strange
new subatomic world, there was only one solution:
Another
particle—one that would be invisible to the
detectors—had to be emitted in the neutron decay. In this case,
this mystery particle and the electron could share the total
available energy, with the mystery particle carrying off whatever
energy might not be carried off by the electron. The problem with
this explanation, however, was that the mass difference between the
neutron and the sum of the masses of the proton and electron is
very, very small. This means that this hypothetical particle had to
be very nearly massless. Moreover, in order to have escaped
detection, the particle had to have no charge, and have essentially
no other significant interactions with normal matter! The Italian
physicist Enrico Fermi called this proposed particle a “neutrino,”
which, in Italian, means “little neutron.” It took another twenty
years or so for the neutrino to finally be detected, and in the
interim the subatomic particle menagerie had expanded even further.
The neutrino was simply the first of the novel, exotic, and alien
forms of elementary particles that appeared to exist in nature,
associated with seemingly new forces. This particle also appeared
to not exist as a part of the stuff that makes us up and also
everything we see around us. Moreover, as we shall see, the nature
of some of these new forces defied our very notions of how a
commonsense universe should behave. Coming to grips with the
mysterious plethora of new particles and forces would occupy much
of the rest of the century and would ultimately lead to
speculations that even these particles and forces may reflect only
the very edge of reality.

One final observational development, which
actually occurred before the other two I have described thus far,
contributed to the intellectual excitement of the post-1930 world.
Strictly speaking, it actually occurred in 1929, but it was in the
1930s that it was fully confirmed and that its utterly revolutionary
implications began to be fully appreciated by the scientific
community. This was the discovery by Edwin Hubble that the universe
we live in is not, on its largest scales, eternal and unchanging. A
fascinating character, Hubble was sufficiently accomplished to have
garnered lasting recognition even if he had not been an expert at
selfpromotion. A former high school athlete, Rhodes scholar,
lawyer, and high school Spanish teacher, Hubble returned to his
first love, science, when he was twenty-four. A decade later,
following a stint as a major in World War I, Hubble moved to the
Mount Wilson Observatory to use the new hundred-inch telescope that
had just been completed there. In 1924 he made his first great
discovery, which ultimately changed our picture of the universe as
much as anything that had ever been seen before. Observing faint
variable stars in the Andromeda nebula, as it was then called, he
established that these objects existed at a distance of over one
million lightyears away, more than three times farther away than
the most distant objects known to exist within our own galaxy.
Before this time the conventional wisdom—established by the
influential American astronomer Harlow Shapley, who was the first to
determine the size of the Milky Way—held that our galaxy was in
essence an island universe, containing all there is to see.
Suddenly Hubble’s discovery challenged this picture. The Andromeda
nebula turned out to be a neighboring galaxy of comparable size to
our own, and just one of what is now understood to be more than
four hundred billion galaxies in the observable universe. Could the
universe be infinite in all directions, full of galaxies as far as
the eye could see and beyond? Hubble proceeded over the next five
years to attempt to classify the nature of distant galaxies, and in
1929 arrived at an unexpected conclusion that made his previous
startling discovery pale by comparison. In that year he reported
evidence that distant galaxies are, on average, moving away from us
and that, moreover, their speed is proportional to their distance:
Those twice as far away are moving away twice as fast!

One’s first reaction upon hearing this is to
conclude that we are therefore the center of the universe. Needless
to say (as my wife reminds me daily), this is not the case. What it
does imply, however, is that the space between galaxies is actually
uniformly expanding in all directions. Put more simply, the
universe is expanding. (To prove this to yourself, draw a square
grid of dots on a piece of a paper, with the dots regularly spaced.
Then draw a grid with the same number of dots but with a larger
uniform spacing between them. Then, if you overlay one grid over
the other, placing one of the dots in the second grid right over
the corresponding dot in the first grid, you will see that from the
vantage point of that dot, it looks like all the other dots are
moving away from it, with those twice as far away shifting by twice
the amount. This works no matter which dot you do this with.)