IT’S A comforting thought in these times of change: fashions come and go,
reputations rise and fall, but diamonds—or, at least, the atoms from which
they are built—are forever.
Or are they? Theoretical physicists have been questioning the supposed
permanence of matter for years. They already know that neutrons aren’t stable:
once plucked from the hearts of atoms they decay into protons, electrons and
antineutrinos in a little over 10 minutes. Protons are certainly more stable
than that. But for years the suspicion has been growing that protons might also
be doomed.
Although neutrons are unstable, when they decay they do at least have the
decency to keep things in the family and produce some protons to compensate. But
many theorists now believe that the proton can pull off an altogether more
radical trick, and decay into particles bearing no obvious relationship to their
progenitor, marking the end of matter as we know it. Their conviction is based
on attempts to forge so-called grand unified theories (GUTs) of the forces that
bind together the subatomic world: the weak force, which causes many subatomic
particles to decay; the strong force, which binds nuclei together; and the
electromagnetic force, which holds atoms together. Even in their most primitive
form, GUTs point to an astonishing new feature of our Universe. They predict
that every proton—whether bound into atoms or not—should be slowly
but ineluctably disintegrating. Or, to put it more poetically, diamonds may not
be forever after all.
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Yet theorists are now admitting that there’s something pretty bizarre about
the prediction. For despite their very best efforts to catch protons in the act
of falling apart, matter looks as rock-solid as ever. For years, researchers
have been looking for even the merest hint of proton decay. And so far, they
have found diddly-squat.
What has gone wrong? Have physicists not been patient enough? Or have they
made some fundamental goof in their attempts to describe the whole subatomic
world in a few lines of algebra? Among theorists there’s a growing feeling that
the day of reckoning is approaching: they must either show that the proton
really does decay, or come up with some pretty good reason why it doesn’t. At
stake is decades of worldwide effort to bring the whole of nature under one
theoretical roof.
These efforts had their first triumph in the late 1960s, when theorists
Sheldon Glashow and Steven Weinberg in the US and Abdus Salam in Britain
succeeded in unifying two of the subatomic forces: electromagnetism and the weak
nuclear force. Both were shown to be different aspects of another, overarching
“electroweak” force.
Until then, sceptics had always been able to argue that a Theory of
Everything, which encompasses all the known forces of nature, including gravity,
was nothing but a mirage—albeit one that had fooled Einstein himself. But
the emergence of electroweak theory changed that. It emboldened some theorists
to ponder how to unite the electroweak force with the strong nuclear
force—the next step towards a Theory of Everything.
Almost immediately, theorists spotted that this unification led to an amazing
new phenomenon: proton decay and the disintegration of atoms. The first people
to cotton on were Salam himself and his colleague Jogesh Pati. In the summer of
1972, kicking around mathematical ways of unifying the strong and electroweak
forces, they spotted one simple, yet profound implication of such a theory.
Quarks, the particles that make up all matter influenced by the strong nuclear
force, would somehow have to be linked to the particles involved in the
electroweak force—leptons such as electrons and neutrinos. Indeed, the
mathematics of the situation went further, implying that these two fundamentally
different particles must blur their identities and turn into one another. But if
quarks can turn into leptons, then so can all the particles made up of
quarks—including protons. And that, Salam and Pati realised, meant that
protons must be unstable.
For all the sophisticated mathematics behind their argument, they found it
hard to convince others: their first paper was rejected as nonsensical by a
leading physics journal, and it took a personal approach by Salam to the
journal’s editor to get the paper into print. Even then, most of the physics
community thought that proton decay was nonsense. But by 1974, Harvard theorists
Glashow and Howard Georgi—another architect of the original electroweak
unification—had also figured out that unification meant proton decay.
They reached this conclusion while trying to construct a GUT using a powerful
mathematical concept known as symmetry groups. Quantum theorists had already
found that nature has a penchant for theories that collect together particles
according to the diktats of symmetry groups with weird monikers like U(1) and
SU(2). In one of the most impressive displays of chutzpah in theoretical
physics, Georgi and Glashow made this penchant the basis of their first stab at
a GUT. Instead of trying to persuade electroweak theory to encompass the strong
force by tacking on extra bits to the symmetry groups of that theory, Georgi
decided things might be easier if he searched for a new group that did all the
unification in one go. Rummaging through the various possible symmetry groups,
Georgi discovered that SU(5) was the smallest, simplest group capable of doing
the trick.
Despite being far more elegant than Salam and Pati’s approach, the theory of
proton decay put forward by Georgi and Glashow ran into the same barrage of
outraged scepticism. Not until the late 1970s did anyone put the idea of proton
decay to the test. The slightly greater enthusiasm stemmed from the fact that
Georgi and others had dug some solid predictions out of the esoteric algebra of
SU(5). According to the theory, a proton should decay into a positron—the
antimatter equivalent of an electron—and a neutral pion
(see Diagram). But
the prospects of witnessing the dramatic decay of matter hardly seemed bright.
The equations of SU(5) showed that the decay process is dependent on the
“unification energy” of the electroweak and strong forces—the energy you
would have to re-create if you were to witness the reuniting of these
fundamental forces into one GUT force. And that energy is incredibly high:
equivalent to a temperature of 1029 Kelvin, which has only ever been reached in
the very first moments of the big bang.
According to the equations of SU(5), the rate of proton decay is intimately
related to this energy. Plugging in the figures, Georgi and his colleagues found
that SU(5) led to a proton lifetime so long that it made the big bang seem like
something that happened a moment ago. Calculations suggested a typical proton
life span of around 1031 years—a thousand billion billion times longer
than the age of the Universe.
How on Earth could you test such a mind-boggling prediction? Researchers
decided to exploit the fact that the proton decay process is random, like
radioactive decay. This means that any particular proton has a 1 in 1031 chance
of decaying in any year. So bring together 1031 protons—say in a tank of
water the size of a swimming pool—and there’s a good chance you’ll see one
go off in a year.
And that is just what teams of researchers around the world have been
attempting to do since the early 1980s: watching vast collections of protons for
years on end in the hope of catching one turning into a positron and a pion. The
most intensive effort yet has been made in the so-called Super Kamiokande
experiment, where physicists have spent years patiently watching 50 000 tonnes
of ultra-pure water in a mine thousands of metres below Mount Ikenoyama in
Japan.
Yet even a decade ago, theorists knew that the game was up for SU(5).
Experiments had failed to uncover any evidence for the decay of the proton, thus
showing that its life span must exceed 1032 years—10 times longer than
suggested by the original SU(5) prediction.
Theorists tweaked the basic ideas of SU(5) to come up with more refined
estimates, but these just made it worse—increasing the gap between theory
and experiment a thousandfold. By the early 1990s, the lesson seemed clear. A
grand unification of the electroweak and strong nuclear forces might be
possible, but SU(5) wasn’t the way to do it—at least, not unaided.
Not that theorists lost much sleep over this. By then most of them had become
convinced that, in its original form, SU(5) never stood a chance as a GUT. This
is because it didn’t incorporate what many theorists now believe to be a key
ingredient of any theory with pretensions to grand unification:
“słÜ±č±đ°ů˛ő˛âłľłľ±đłŮ°ů˛â”.
Put simply, supersymmetry provides a way of unifying particles separated by
the single most basic division in the subatomic world, their “spin”. Like most
ideas in quantum theory, spin doesn’t mean quite the same as it does in everyday
usage. Although in some situations subatomic particles behave as if they are
spinning like tiny tops, in reality they are far more ineffable objects. Even
so, each can be put into one of two fundamental families, according to the
amount of spin it possesses. Fermions all have spins that are multiples of a
half (1/2, 3/2, 5/2 units), while bosons have whole-number spins (0, 1, 2
units).
Building bridges
Theorists are eager to bridge the Great Spin Divide because unification
demands the bringing together of all the particles of matter and the forces
between them. It turns out that all the particles of matter—electrons,
protons, neutrons—belong to the fermion family, while all the particles
linked to fundamental forces—such as the photon, carrier of the
electromagnetic force—are bosons.
Supersymmetry provides the mathematical means to bridge that divide by
revealing the essential similarity between fermions and bosons. But this
unification comes at the price of more theoretical complexity—and,
crucially, more particles. For according to supersymmetry, every one of the
familiar denizens of the subatomic world has a supersymmetric counterpart:
quarks come with “squarks”, and leptons come with “sleptons” (the supersymmetric
counterpart of an electron, for example, is a selectron).
The good news for researchers puzzling over proton decay is that all these
extra particles increase by a factor of about 30 the unification energy at which
the electroweak and strong forces merge to become the single GUT force. Better
still, the lifetime of the proton depends very sensitively on the value of that
energy, increasing as the fourth power. This means that the supersymmetric
version of GUT predicts a proton lifetime of around 1034
years—comfortably above the limits set in the late 1980s.
But there’s a nasty sting in supersymmetry’s elegant tail. For while it
explains why no one has yet seen a proton decay into the positron and pion
expected from SU(5) theory, it reveals that this is a pretty unlikely fate for a
decaying proton in any case. According to the supersymmetric version of GUTs,
it’s actually far more likely that the proton will decay into a combination of a
so-called positive kaon and an antineutrino.
So the introduction of supersymmetry has solved one big problem with proton
decay—the total lack of any sign of its decay into a positron and
pion—only to raise another: why hasn’t anyone seen a proton decay into a
kaon and an antineutrino? To add to their frustration, theorists aren’t even
sure if current experiments should have detected this type of proton decay by
now. The formula for the rate of decay of the proton by this route demands a
value for something called the Higgs colour triplet mass—and no one knows
what this value should be.
Actually, the situation is even worse than that. Theorists do have reasons
for expecting that the Higgs colour triplet mass should be no more than around a
thousand times that of the proton itself. The trouble is that the formula for
proton decay given by supersymmetric GUTs shows that the lower this triplet
mass, the shorter the proton lifetime. And plugging this best-guess value into
the formula gives a life span of only 500 000 years or so.
This, to put it mildly, is hard to square with the current observational
limit announced last year by a team from a group of researchers working on the
Super Kamiokande experiment of at least 5 Ă— 1032 years. So the task before
theoreticians is now clear: if they are to explain the current observations on
proton decay—or the lack of it—they must find a way to make this
triplet mass trillions of times heavier.
Some, like Frank Wilczek of the Institute for Advanced Study in Princeton,
are sanguine about the chances of solving this problem. Indeed, Wilczek suspects
the answer may already exist, wrapped up in the theory of how particles acquire
their mass. Over 30 years ago, Peter Higgs of Edinburgh University showed that
particles can be imbued with mass by interacting with what are now called Higgs particles
(“Masses and molasses,” żěè¶ĚĘÓƵ, 10 April, p 32).
Wilczek suspects that a related effect he investigated in the early 1980s with
colleague Stefano Dimopoulos may boost the Higgs colour triplet mass high enough
to get theorists off the hook. “It’s a pretty good trick for doing this, and if
you make the most straightforward assumptions, the rate is close to existing
limits,” says Wilczek. Even so, he admits it could be tight: “Many of us would
be relieved if decays through the kaon and antineutrino path were observed
˛ő´Ç´Ç˛Ô.”
But other theorists think that there’s a much deeper explanation of proton
decay. Like Georgi 25 years ago, they too believe that the problem calls for
grand visions rather than tweaks. Indeed, they see Georgi’s approach as not bold
enough. They reckon that proton decay can only be fully understood by unifying
not just three, but all four of the fundamental forces—including gravity.
“Without the inclusion of gravity, grand unified theory is too unconstrained,”
says Henry Tye, a theorist at Cornell University. “And the only way we know how
to incorporate gravity is via superstring theory.”
Among physicists, superstring theory is the Great White Hope for a theory of
everything, and its crucial feature is that it sees elementary particles not as
mere points, but tiny vibrating strings whose behaviour follows the dictates of
supersymmetry. Thus, the properties of every particle depend intimately on how
its constituent string behaves—and that can have dramatic consequences for
the proton. In particular, says Tye, superstring theory suggests that the
lifetime of the proton depends on the typical energy scale at which superstring
effects become important.
But, crucially, the resulting formula shows that proton lifetime increases
not as some itsy-bitsy power of this energy scale, but as some colossal power,
such as 24. “So even if the superstring energy scale is many orders of magnitude
below the GUT scale, we still end up with a very long lifetime, say 1072
years”, says Tye. If this line of argument is correct, then the failure of
researchers to find a shred of evidence for proton decay turns from a threat to
a triumph. Indeed, they had better not find evidence for proton decay any time
soon.
With superstring theory and the search for a Theory of Everything still in a
state of flux, not all theorists are happy about using superstrings to put to
rest issues like proton decay. “Given the present state of knowledge, string
theory may do any number of things,” says Wilczek. “The trouble is we don’t
know which—if any—it actually will do.” Even so, if evidence for
proton decay does not turn up soon, theorists will be forced to face a dilemma.
Should they take this to mean their attempts to unify the forces of physics are
flawed and must be abandoned, or that they are on the right track, but just
haven’t pushed their ideas far enough yet?
What is clear is that nearly 30 years after the mere suggestion was dismissed
as ludicrous, the decay of the proton is emerging as one of the biggest
challenges facing those trying to forge a Theory of Everything. Who
knows—perhaps the world’s most brilliant theorists are wrong, and the
admen were right all along. Maybe diamonds really are forever.