electromagnetic and weak interactions got stronger.
It didn’t take a rocket scientist to wonder whether the strength of
the three different interactions might become identical at some
small-distance scale. When they did the calculations, they found
(with the accuracy with which the interactions were then measured)
that such a unification looked possible, but only if the scale of
unification was about fifteen orders of magnitude in scale smaller
than the size of the proton.
This was good news if the unified theory was the one proposed by
Georgi and Glashow—because if all the particles we observe in
nature got unified in this new large-gauge group, then new gauge
bosons would exist that produce transitions between quarks (which
make up protons and neutrons), and electrons and neutrinos. That
ͥ͞͠
would mean protons could decay into other lighter particles. As
Glashow put it, “Diamonds aren’t forever.”
Even then it was known that protons must have an incredibly
long lifetime. Not just because we still exist almost 14 billion years
after the Big Bang, but because we all don’t die of cancer as children.
If protons decayed with an average lifetime smaller than about a
billion billion years, then enough protons would still decay in our
bodies during our childhood to produce enough radiation to kill us.
Remember that in quantum mechanics, processes are probabilistic.
If an average proton lives a billion billion years, then if one has a
billion billion protons, on average one will decay each year. A lot
more than a billion billion protons are in our bodies.
However, with the incredibly small proposed distance scale and
therefore the incredibly large mass scale associated with
spontaneous symmetry breaking in Grand Unification, the new
gauge bosons would get large masses. That would make the
interactions they mediate be so short-range that they would be
unbelievably weak on the scale of protons and neutrons today. As a
result, while protons could decay, they might live, in this scenario,
perhaps a million billion billion billion years before decaying. No
problem.
• • •
With the results of Glashow and Georgi, and Georgi, Quinn, and
Weinberg, the smell of grand synthesis was in the air. After the
success of the electroweak theory, particle physicists were feeling
ambitious and ready for further unification.
How would one know if these ideas were correct, however? There
was no way to build an accelerator to probe an energy scale a million
billion times greater than the rest mass energy of protons. Such a
machine would have to have a circumference of the Moon’s orbit.
ͥ͞͡
Even if it was possible, considering the earlier debacle over the SSC,
no government would ever foot the bill.
Happily, there was another way, using the kind of probability
arguments I just presented that give limits to the proton lifetime. If
the new Grand Unified Theory predicted a proton lifetime of, say, a
thousand billion billion billion years, then if one could put a
thousand billion billion billion protons in a single detector, on
average one of them would decay each year.
Where could one find so many protons? Simple: in about three
thousand tons of water.
So all that was required was to get a tank of, say, three thousand
tons of water, put it in the dark, make sure there were no
radioactivity backgrounds, surround it with sensitive phototubes that
can detect flashes of light in the detector, and then wait for a year to
see a burst of light when a proton decayed. As daunting as this may
seem, at least two large experiments were commissioned and built to
do just this, one deep underground next to Lake Erie in a salt mine,
and one in a mine near Kamioka, Japan. The mines were necessary
to screen out incoming cosmic rays that would otherwise produce a
background that would swamp any proton decay signal.
Both experiments began taking data around 1982–83. Grand
Unification seemed so compelling that the physics community was
confident a signal would soon appear and Grand Unification would
mean the culmination of a decade of amazing change and discovery
in particle physics—not to mention another Nobel Prize for
Glashow and maybe some others.
Unfortunately, nature was not so kind in this instance. No signals
were seen in the first year, the second, or the third. The simplest
elegant model proposed by Glashow and Georgi was soon ruled out.
But once the Grand Unification bug had caught on, it was not easy
to let it go. Other proposals were made for unified theories that
ͥ͢͞
might cause proton decay to be suppressed beyond the limits of the
ongoing experiments.
On February 23, 1987, however, another event occurred that
demonstrates a maxim I have found is almost universal: every time
we open a new window on the universe, we are surprised. On that
day a group of astronomers observed, in photographic plates
obtained during the night, the closest exploding star (a supernova)
seen in almost four hundred years. The star, about 160,000 light-
years away, was in the Large Magellanic Cloud—a small satellite
galaxy of the Milky Way observable in the southern hemisphere.
If our ideas about exploding stars are correct, most of the energy
released should be in the form of neutrinos, despite that the visible
light released is so great that supernovas are the brightest cosmic
fireworks in the sky when they explode (at a rate of about one
explosion per hundred years per galaxy). Rough estimates then
suggested that the huge IMB (Irvine-Michigan-Brookhaven) and
Kamiokande water detectors should see about twenty neutrino
events. When the IMB and Kamiokande experimentalists went back
and reviewed their data for that day, lo and behold IMB displayed
eight candidate events in a ten-second interval, and Kamiokande
displayed eleven such events. In the world of neutrino physics, this
was a flood of data. The field of neutrino astrophysics had suddenly
reached maturity. These nineteen events produced perhaps nineteen
hundred papers by physicists, such as me, who realized that they
provided an unprecedented window into the core of an exploding
star, and a laboratory not just for astrophysics but also for the
physics of neutrinos themselves.
Spurred on by the realization that large proton-decay detectors
might serve a dual purpose as new astrophysical neutrino detectors,
several groups began to build a new generation of such dual-purpose
detectors. The largest one in the world was again built in the
ͥͣ͞
Kamioka mine and was called Super-Kamiokande, and with good
reason. This mammoth fifty-thousand-ton tank of water, surrounded
by 11,800 phototubes, was operated in a working mine, yet the
experiment was maintained with the purity of a laboratory clean
room. This was absolutely necessary because in a detector of this size
one h
ad to worry not only about external cosmic rays, but also about
internal radioactive contaminants in the water that could swamp any
signals being searched for.
Meanwhile, interest in a related astrophysical neutrino signature
also reached a new high during this period. The Sun produces
neutrinos due to the nuclear reactions in its core that power it, and
over twenty years, using a huge underground detector, Ray Davis had
detected solar neutrinos, but had consistently found an event rate
about a factor of three below what was predicted using the best
models of the Sun. A new type of solar neutrino detector was built
inside a deep mine in Sudbury, Canada, which became known as the
Sudbury Neutrino Observatory (SNO).
Super-Kamiokande has now been operating almost continuously,
through various upgrades, for more than twenty years. No proton-
decay signals have been seen, and no new supernovas observed.
However, the precision observations of neutrinos at this huge
detector, combined with complementary observations at SNO,
definitely established that the solar neutrino deficit observed by Ray
Davis is real, and moreover that it is not due to astrophysical effects
in the Sun but rather due to the properties of neutrinos. At least one
of the three known types of neutrinos is not massless—although it
has a small mass indeed, perhaps a hundred million times smaller
than the mass of the next-lightest particle in nature, the electron.
Since the Standard Model does not accommodate neutrinos’ masses,
this was the first definitive observation that some new physics,
ͥͤ͞
beyond the Standard Model and beyond the Higgs, must be
operating in nature.
Soon after this, observations of higher-energy neutrinos that
regularly bombard Earth as high-energy cosmic-ray protons hit the
atmosphere and produce a downward shower of particles, including
neutrinos, demonstrated that yet a second neutrino has mass. This
mass is somewhat larger, but still far smaller than the mass of the
electron. For these results team leaders at SNO and Kamiokande
were awarded the 2015 Nobel Prize in Physics—a week before I
wrote the first draft of these words. To date these tantalizing hints of
new physics are not explained by current theories.
The absence of proton decay, while disappointing, turned out to
be not totally unexpected. Since Grand Unification was first
proposed, the physics landscape had shifted slightly. More precise
measurements of the actual strengths of the three nongravitational
interactions—combined with more sophisticated calculations of the
change in the strength of these interactions with distance—
demonstrated that if the particles of the Standard Model are the only
ones existing in nature, the strength of the three forces will not unify
at a single scale. In order for Grand Unification to take place, some
new physics at energy scales beyond those that have been observed
thus far must exist. The presence of new particles would not only
change the rate at which the three known interactions change with
scale so that they might unify at a single scale of energy, it would also
tend to drive up the Grand Unification scale and thus suppress the
rate of proton decay—leading to predicted lifetimes in excess of a
million billion billion billion years.
As these developments were taking place, theorists were driven
by new mathematical tools to explore a possible new type of
symmetry in nature, which became known as supersymmetry. This
fundamental symmetry is different from any previous known
ͥͥ͞
symmetry, in that it connects the two different types of particles in
nature, fermions (particles with half-integer spins) and bosons
(particles with integer spins). The upshot of this (many other books,
including some by me, explore this idea in detail) is that if this
symmetry exists in nature, then for every known particle in the
Standard Model at least one corresponding new elementary particle
must exist. For every known boson there must exist a new fermion.
For every known fermion there must exist a new boson.
Since we haven’t seen these particles, this symmetry cannot be
manifest in the world at the level we experience it, and it must be
broken, meaning the new particles will all get masses that could be
heavy enough so that they haven’t been seen in any accelerator
constructed thus far.
What could be so attractive about a symmetry that suddenly
doubles all the particles in nature without any evidence of any of the
new particles? In large part the seduction lay in the very fact of
Grand Unification. Because if a Grand Unified Theory exists at a
mass scale of fifteen to sixteen orders of magnitude higher energy
than the rest mass of the proton, this is also about thirteen orders of
magnitude higher than the scale of electroweak symmetry breaking.
The big question is why and how such a huge difference in scales can
exist for the fundamental laws of nature. In particular, if the
Standard Model Higgs is the true last remnant of the Standard
Model, then the question arises, Why is the energy scale of Higgs
symmetry breaking thirteen orders of magnitude smaller-scale than
the scale of symmetry breaking associated with whatever new field
must be introduced to break the GUT symmetry into its separate
component forces?
The problem is a little more severe than it appears. Scalar
particles such as the Higgs have several new quantum mechanical
properties that are unlike those of fermions or spin 1 particles such
͟͜͜
as gauge particles. When one considers the effects of virtual particles,
including particles of arbitrarily large mass, such as the gauge
particles of a presumed Grand Unified Theory, these tend to drive
up the mass and symmetry-breaking scale of the Higgs so that it
essentially becomes close to, or identical to, the heavy GUT scale.
This generates a problem that has become known as the naturalness
problem. It is technically unnatural to have a huge hierarchy
between the scale at which the electroweak symmetry is broken by
the Higgs particle and the scale at which the GUT symmetry is
broken by whatever new heavy scalar field breaks that symmetry.
The brilliant mathematical physicist Edward Witten argued in an
influential paper in 1981 that supersymmetry had a special property.
It could tame the effect that virtual particles of arbitrarily high mass
and energy have on the properties of the world at the scales we can
currently probe. Because virtual fermions and virtual bosons of the
same mass produce quantum corrections that are identical except
for a sign, if every boson is accompanied by a fermion of equal mass,
then the quantum effects of the virtual particles will cancel out. This
means that the effects of virtual particles of arbitrarily high mass and
energy on the physical properties of the universe on scales we can
measure would now be completely removed.
If, however, su
persymmetry is itself broken, then the quantum
corrections will not quite cancel out. Instead they would yield
contributions to masses that are the same order as the
supersymmetry-breaking scale. If it was comparable to the scale of
the electroweak symmetry breaking, then it would explain why the
Higgs mass scale is what it is. And it also means we should expect to
begin to observe a lot of new particles—the supersymmetric partners
of ordinary matter—at the scale currently being probed at the LHC.
This would solve the naturalness problem because it would
protect the Higgs boson masses from possible quantum corrections
͟͜͝
that could drive them up to be as large as the energy scale associated
with Grand Unification. Supersymmetry could allow a “natural”
large hierarchy in energy (and mass) separating the electroweak scale
from the Grand Unified scale.
That supersymmetry could in principle solve the hierarchy
problem, as it has become known, greatly increased its stock with
physicists. It caused theorists to begin to explore realistic models that
incorporated supersymmetry breaking and to explore the other
physical consequences of this idea. When they did so, the stock price
of supersymmetry went through the roof. For if one included the
possibility of spontaneously broken supersymmetry into calculations
of how the three nongravitational forces change with distance, then
suddenly the strength of the three forces would naturally converge at
a single, very small-distance scale. Grand Unification became viable
again!
Models in which supersymmetry is broken have another
attractive feature. It was pointed out, well before the top quark was
discovered, that if the top quark was heavy, then through its
interactions with other supersymmetric partners, it could produce
quantum corrections to the Higgs particle properties that would
cause the Higgs field to condense at its currently measured energy
scale if Grand Unification occurred at a much higher, superheavy
scale. In short, the energy scale of electroweak symmetry breaking
could be generated naturally within a theory in which Grand
Unification occurs at a much higher energy scale. When the top
quark was discovered and indeed was heavy, this added to the
attractiveness of the possibility that supersymmetry breaking might
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 32
