D E C AY A N D R U B B L E
There is no new thing under the sun.
—ECCLESIASTES 1:9
When I first learned that we human beings are radioactive, it
shocked me. I was in high school listening to a lecture by the
remarkable polymath and astrophysicist Tommy Gold, who had
done pioneering work in cosmology, pulsars, and lunar science, and
he informed us that the particles that made up most of the mass of
our bodies, neutrons, are unstable, with a mean lifetime of about ten
minutes.
Given, I hope, that you have been reading this book for longer
than ten minutes, this may surprise you too. The resolution of this
seeming paradox is one of the first and most wonderful of the
gorgeous accidents of nature that make our existence possible. As we
continue to explore more deeply the question “Why are we here?,”
this accident will loom large on the horizon. While the neutron may
seem far removed from light, which has been the centerpiece of our
story thus far, we shall see that the two are ultimately deeply
connected. The decay of neutrons—responsible for the “beta decay”
of unstable nuclei—required physicists to move beyond their simple
and elegant theories of light and open up new fundamental areas of
the universe for investigation.
But I am getting ahead of myself.
In 1929, when Dirac first wrote down his theory of electrons and
radiation, it looked as if it might end up being a theory of almost
͟͝͞
everything. Aside from electromagnetism, the only other force in
town was gravity, and Einstein had just made great strides in
understanding it. Elementary particles consisted of electrons,
photons, and protons, together comprising all the objects that
appeared necessary to understand atoms, chemistry, life, and the
universe.
The discovery of antiparticles upset the applecart somewhat, but
since Dirac’s theory had effectively predicted them (even if Dirac
himself had to catch up with the theory), this was more like a speed
bump on the road to reality than a roadblock or detour.
Then came 1932. Up to that time, scientists had presumed that
atoms were composed entirely of protons and electrons. This posed
a bit of a problem, however, because the masses of atoms didn’t
quite add up. In 1911 Rutherford discovered the existence of the
atomic nucleus, containing almost all the mass of atoms in a small
region one hundred thousand times smaller than the size of the
orbits of the electrons. Following that discovery, it became clear that
the mass of heavy nuclei was just a bit more than twice the mass that
could be accounted for if the number of protons in the nucleus
equaled the number of electrons orbiting the nucleus, ensuring that
atoms would be electrically neutral.
The proposed solution to this conundrum was simple. Actually
twice as many protons were in the nucleus as electrons surrounding
it, but just the right number of electrons were trapped inside the
nucleus, so that again the total electric charge of the atom would be
equal to zero.
However, quantum mechanics implied that the electrons couldn’t
be confined within the nucleus. The argument is a bit technical, but
it goes something like this: If elementary particles have a wavelike
character, then if one is going to confine them to a small distance,
the magnitude of their wavelength must be smaller than the
͝͞͠
confinement scale. But the wavelength associated with a particle is,
in quantum mechanics, inversely proportional to the momentum
carried by the particle, and hence also inversely proportional to the
energy carried by the particle. If electrons were confined to a region
the size of an atomic nucleus, the energy they would need to possess
would be about a million times the energy associated with the
characteristic energies released by electrons as they jump between
energy levels in their atomic orbits.
How could they achieve such energies? They couldn’t. For, even if
electrons were tightly bound to protons within nuclei by electronic
forces, the binding energy that would be released in this process as
they “fell” into the nucleus would be more than ten times smaller
than the energy needed to confine the quantum mechanical electron
wave function to a region contained within the nucleus.
Here too the numbers just didn’t add up.
Physicists at the time were aware of the problem, but lived with it.
I suspect that an agnostic approach was deemed prudent, and
physicists were willing to suspend disbelief until they knew more,
because the issues involved the cutting-edge physics of quantum
mechanics and atomic nuclei. Instead of proposing exotic new
theories (there were probably some at the margins that I am not
aware of), the community was eventually driven by experiments to
overcome its natural hesitation to take the logical next step: to
assume nature was more complicated than had thus far been
revealed.
In 1930, about the time that Dirac was coming to grips with the
possibility that his antiparticles weren’t really protons, a series of
experiments provided just the clues that were needed to unravel the
nuclear paradox. The poetry of the discoveries was rivaled only by
the drama in the private lives of the researchers.
͝͞͡
Max Planck had helped pioneer the quantum revolution by
resolving the paradox of the spectrum of radiation emitted by atomic
systems. So it was fitting that Planck should indirectly help resolve
the paradoxical makeup of the nucleus. While he didn’t himself
spearhead the relevant research, he recognized the talents of a young
student of mathematics, physics, chemistry, and music at the
University of Berlin, Walther Bothe, and in 1912 Planck accepted
him as a doctoral student and mentored him throughout the rest of
his career.
Bothe was spectacularly lucky to be mentored by Planck and,
shortly thereafter, by Hans Geiger, of Geiger counter fame. Geiger, in
my mind, is one of the most talented experimental physicists to have
been overlooked for a Nobel Prize. Geiger had begun his career by
doing the experiments, with Ernest Marsden, that Ernest Rutherford
utilized to discover the existence of the atomic nucleus. Geiger had
just returned from England, where he’d worked with Rutherford, to
direct a new laboratory in Berlin, and one of his first acts was to hire
Bothe as an assistant. There Bothe learned to focus on important
experiments, using simple approaches that yielded immediate
results.
After an “involuntary vacation” of five years, as a prisoner of war
in Siberia during the First World War, Bothe returned and built a
remarkable collaboration with Geiger, eventually succeeding him as
director of the laboratory. During their time together they pioneered
the use of “coincidence methods” to explore atomic, and eventually
nuclear, physics. Using different detectors located around a target,
and using careful timing, they could look for simultaneous events,
signaling that the source had to be a single atomic or nuclear decay.
In 1930 Bothe and his assistant Herbert Becker observed
something completely new and unexpected. While bombarding
beryllium nuclei with products of nuclear decay called alpha
͢͝͞
particles (already known to be the nuclei of helium), the two
observed the emission of a completely new form of high-energy
radiation. This radiation had two unique features. It was more
penetrating than the most energetic gamma rays, but like gamma
rays, the radiation was composed of electrically neutral particles so
that it did not ionize atoms as it passed through matter.
News of this surprising discovery made its way to other physics
laboratories throughout Europe. Bothe and Becker had initially
proposed that this radiation was some new sort of gamma ray. In
Paris, Irène Joliot-Curie, the daughter of famed physicist Marie
Curie, and Irène’s husband, Frédéric, replicated Bothe and Becker’s
results and explored the radiation in more detail. In particular, they
found that when it bombarded a paraffin target, it knocked out
protons with incredible energy.
This observation made it clear that the radiation couldn’t be a
gamma ray. Why?
The answer is relatively simple. If you throw a piece of popcorn at
an oncoming truck, you are unlikely to stop the truck or even break
a window. That is because the popcorn, even if you throw it with
great energy, carries little momentum because the popcorn is light.
To stop a truck you have to change its momentum by a large
amount because, even if it is moving slowly, it is heavy. To stop a
truck or knock a heavy object off the truck, you have to throw a big
rock.
Similarly, to knock out a heavy particle such as a proton from
paraffin, a gamma ray, made of massless photons, would have to
carry great energy (so that the momentum carried by the individual
photons was large enough to kick out a heavy proton), and not
enough energy was available, by an order of magnitude at least, in
any known nuclear-decay processes for this.
ͣ͝͞
Surprisingly, the Joliot-Curies (they were modern and both
adopted the same hyphenated last name) were probably loath, like
Dirac, to propose new elementary particles to explain data—since
protons, electrons, and photons were not only familiar, but sufficient
up to that time to explain everything known, including exotic
quantum phenomena associated with atoms. So, Irène and Frédéric
didn’t make the now-obvious proposal that maybe a new neutral
massive particle was being produced in the decays that Bothe and
Becker had discovered. Unfortunately, a similar timidity caused the
Joliot-Curies to fail to claim discovery of the positron—in spite of
having actually observed it in their experiments before Carl
Anderson reported his own discovery somewhat later.
It fell to the physicist James Chadwick to push things further.
Chadwick clearly had a great nose for physics, but his political
acumen was not so sharp. After graduation from the University of
Manchester with a master’s degree in 1913, working with
Rutherford, he obtained a fellowship that would allow him to study
anywhere. So he went to Berlin to work with Geiger. He couldn’t
have picked a better mentor, and he began to do important studies
of radioactive decays. Unfortunately, the First World War broke out
while Chadwick was in Germany, and he spent the next four years in
an internment camp.
Eventually he returned to Cambridge, where Rutherford had since
moved, to complete his PhD under Rutherford’s direction. Following
this Chadwick stayed on to work with Rutherford and help direct the
Cavendish laboratory there. While he was aware of Bothe and
Becker’s results and even reproduced them, only when one of his
students informed him of the Joliot-Curies’ results did Chadwick
become convinced, using the energy argument I mentioned above,
that the radiation that had been observed had to result from a new
neutral particle—of mass comparable to that of the proton—that
ͤ͝͞
might reside in atomic nuclei, an idea he and Rutherford had been
germinating for years.
Chadwick
reproduced
and
extended
the
Joliot-Curies’
experiments, bombarding targets other than paraffin to explore the
outgoing protons. He confirmed not only that the energetics of the
collisions made it impossible for the source to be gamma rays, but
also that the interaction strength of the new particles with nuclei was
far greater than would be predicted for gamma rays.
Chadwick didn’t dawdle. Within two weeks of beginning his
experiments in 1932, he sent a letter to Nature entitled “Possible
Existence of a Neutron” and followed this up with a more detailed
article sent to the Royal Society. The neutron, which we now know
makes up most of the mass of heavier nuclei, and thus most of the
mass in our bodies, had been discovered.
For his discovery he was awarded the Nobel Prize in Physics three
years later, in 1935. In a kind of poetic justice, three of the people
whose experiments had made Chadwick’s results possible—but who
missed out on identifying the neutron—were awarded Nobel Prizes
for other work. Bothe won the Nobel Prize in 1954 for his work on
using coincidences between observed events in different detectors to
explore the detailed nature of nuclear and atomic phenomena. Both
Irène and Frédéric Joliot-Curie, who barely missed out on two other
Nobel Prize–winning discoveries, won the Nobel Prize in Chemistry
in 1935 for their discovery of artificial radioactivity—which was later
an essential ingredient in the development of both nuclear power
and nuclear weapons. Interestingly, only after winning the Nobel
Prize was Irène awarded a professorship in France. With the two
Nobel Prizes for her mother, Marie, the Curie family garnered a total
of five Nobel Prizes, the most that have ever been received by a
single family.
ͥ͝͞
After his discovery Chadwick set out to measure the mass of the
neutron. His first estimate, in 1933, suggested a mass of slightly less
than the sum of the masses of a proton and an electron. This
reinforced the idea that perhaps the neutron was a bound state of
these two particles, and the mass difference, using Einstein’s relation
E = mc2, was due to the energy lost in binding them together.
However, after several conflicting measurements by other groups,
further analysis a year later by Chadwick using a nuclear reaction
induced by gamma rays—which allowed all energies to be measured
with great precision—definitely indicated that the neutron was
heavier than the sum of the proton and electron masses, even if
barely so, with the mass difference being less than 0.1 percent.
It is said tha
t “close” only matters when tossing horseshoes or
hand grenades, but the closeness in mass between the proton and
the neutron matters a great deal. It is one of the key reasons we exist
today.
Henri Becquerel discovered radioactivity in uranium in 1896, and
only three years later Ernest Rutherford discerned that radioactivity
occurred in two different types, which he labeled alpha and beta
rays. A year later gamma rays were discovered, and Rutherford
confirmed them as a new form of radiation in 1903, when he gave
them their name. Becquerel determined in 1900 that the “rays” in
beta decay were actually electrons, which we now know arise from
the decay of the neutron.
In beta decay a neutron splits into a proton and an electron,
which, as I describe below, would not be possible if the neutron
weren’t slightly heavier than protons. What is surprising about this
neutron decay is not that it occurs, but that it takes so long.
Normally the decay of unstable elementary particles occurs in
millionths or billionths of a second. Isolated neutrons live, on
average, more than ten minutes.
͟͜͝
One of the chief reasons that neutrons live so long is that the
mass of the neutron is only slightly more than the sum of the masses
of a proton plus an electron. Thus, there is only barely enough
energy available, via the neutron’s rest mass, to allow it to decay into
these particles and still conserve energy. (The other reason is that a
neutron doesn’t decay into only a proton plus an electron. It decays
into three particles . . . stay tuned!)
While ten minutes may be an eternity on atomic timescales, it is
pretty short compared to a human life or the lifetime of atoms on
Earth. Returning to the puzzle I mentioned at the beginning of this
chapter, what gives? How can we be largely made up of neutrons if
they decay before the first commercial break in a thirty-minute TV
show?
The answer again lies in the extreme closeness of the neutron and
proton masses. A free neutron decays in ten minutes or so. But
consider a neutron bound inside an atomic nucleus. Being bound
means that it takes energy to kick it out of the nucleus. But that
means that it loses energy when it gets bound to the nucleus in the
first place. But, Einstein told us that the total energy of a massive
particle is proportional to its mass, via E = mc2. That means that, if
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 13