AT THE most powerful particle accelerator in the world, the twin colliding beams of protons have been switched off for a few hours. All seems quiet, but both the giant machine and the foundations of physics are about to be shaken by a tiny time bomb. Hiding within a copper plate deep inside one of the accelerator’s massive detectors is a peculiar interloper: a particle that is waiting to explode, and with its incandescent fragments write a message from beyond our universe.
If this particle does appear at the (LHC) near Geneva, Switzerland, it could change the nature of physics. Physicists might have to abandon their goal of explaining the fundamental basis of our reality and just accept that the properties of matter and energy in our universe arose at random. It could mean not only that we live on a small planet in an insignificant solar system in one of a trillion galaxies in the universe, but our own universe is just one insignificant slice of an unimaginably vast and diverse multiverse.
To many physicists, that is anathema; but not to Savas Dimopoulos of Stanford University in California or his colleague Nima Arkani-Hamed at Harvard University. In 2002, they first began to wonder what a multiverse might mean for particle physics.
Advertisement
This was at a time when the multiverse was being discussed, albeit reluctantly, as a solution to a cosmic problem. Astronomers had discovered a repulsive force pushing the galaxies apart, caused by an inherent energy present in space. Often called the cosmological constant, no one knows what is generating this force.
On the face of it, physics has a ready-made explanation. According to quantum theory, the vacuum, or the space between particles, is not totally empty. It is home to short-lived “virtual” particles that flicker in and out, created by the fundamental quantum fuzziness of the world. Although that might be a hard concept to swallow, it is an enormously successful idea. The calculations of quantum field theory show that these virtual particles cluster around the ordinary, solid, long-lived particles of matter, changing their properties in ways that accurately match many experimental observations.
So the cosmological constant could be the net energy of all those randomly appearing and disappearing virtual particles. The problem is, it takes some embarrassing fiddling of the figures to get the number measured by astronomers, an energy density of about 1 joule per cubic kilometre. The quantum calculation needed to arrive at the number is horribly sensitive to the precise values of particle masses and charges. So far, the best answer physicists have is that the cosmological constant could have any value up to 10120 times what astronomers observe.
It is relatively easy to devise a model of particle physics in which virtual particles with positive and negative energies cancel out exactly to zero, but why they should almost cancel each other out, leaving us with a tiny residual energy, is much harder to see.
One physicist had already predicted this, however. In the 1980s, Steven Weinberg at the University of Texas in Austin adopted a controversial line of argument called the anthropic principle, which roughly states that the universe has to possess properties that make it hospitable to life, otherwise we wouldn’t be here to see it.
He started by pointing out that if our cosmological constant were only 100 times as big as observed, we would be in trouble. Its repulsive force would have stretched out the thin gas of the early universe, preventing it from ever collapsing into stars and planets. But if you have a lot of universes, each with a random value of the cosmological constant, there’s going to be at least one with an energy density of roughly a few joules per cubic kilometre. That would enable the existence of planet-dwelling life forms who would then be in a position to observe this value of cosmological constant.
Such a range of universes might sound like wild speculation, but some respected cosmological models imply that there could indeed be many universes, perhaps even an infinite number. In the theory of eternal inflation, for example, our own universe is just one offshoot of an endlessly growing “tree” of universes.
And according to recent developments in – one attempt to merge all the forces of nature in a single theoretical framework – the basic physics of the universe is almost unconstrained. In this “string landscape”, nearly anything can happen. You can have any number of particles with different masses and different forces.
Put eternal inflation and the string landscape together, and you have a vast array of universes with almost any imaginable properties. Universes of antimatter. Universes ruled by magnetism, or filled with nothing but black holes. Universes with all possible values of the cosmological constant.
That is why early in the 21st century, when the string landscape appeared, the anthropic principle was being given serious consideration. Along with Weinberg, many physicists thought it might explain the rough value of the cosmological constant.
Dimopoulos and Arkani-Hamed thought it might do more. If the anthropic principle and the multiverse could explain the cosmological constant, perhaps it could also shed some light on the nature of the Higgs boson, the theoretical particle that is thought to give all others mass, and one of the particles the LHC was built to detect. “We were asking, what would happen if you apply the same ideas to the hierarchy problem?” says Dimopoulos.
The hierarchy problem has to do with the mass-energy of the Higgs boson, believed to be in the ballpark of 100 gigaelectronvolts, or 100 times the mass of the proton. When particle physicists try to calculate it, their numbers often come out far higher than the mass of the particles the Higgs is supposed to contribute to, indicating something has gone wrong somewhere.
The most popular solution to the hierarchy problem is a theory called supersymmetry (SUSY), which says that every one of the known fundamental particles has a heavier twin. When all the extra particles of supersymmetry join in the virtual dance around the Higgs, they can trim its waistline. The energy of a virtual supersymmetric particle is equal and opposite to the energy of its partner, so the contributions made by each pair of particles cancels out. Squark balances quark; photino balances photon, and so on. The Higgs remains healthily chunky, not morbidly obese.
So far so good. Along with the Higgs, one of the great hopes for the LHC is that it will discover a bucketful of these supersymmetric particles. But in its plainest form, SUSY makes some seriously wonky predictions. We should already have discovered some of these “superpartners” at existing particle accelerators, for example. The cloud of virtual superpartners should also affect ordinary particles in unpleasant ways. Most disturbingly, they would destabilise protons, making all ordinary matter fall apart. And the presence of superpartners would allow all sorts of other unknown happenings in the particle world, such as muons spontaneously turning into their lighter cousins, electrons.
“Plain supersymmetry makes some seriously wonky predictions”
“After a lot of experiments there has not been any hint of SUSY,” says Dimopoulos. For each individual one of those predictions, he says, you can find plausible explanations for why they are not seen. “But by the time you look at the whole package of ‘things that should have happened but didn’t’, you start getting a somewhat baroque structure.”
So perhaps supersymmetry is not the answer to the Higgs hierarchy problem at all. Instead, thought Dimopoulos, if you accept that the value of the cosmological constant is selected by our existence in a hospitable universe, then why shouldn’t the Higgs mass be chosen in the same way?
However, he and Arkani-Hamed were reluctant to abandon SUSY altogether. Its particles have other uses. They can form dark matter, an otherwise puzzling presence in the cosmos. And physicists think that they may also allow three of the four forces of nature to be unified into one superforce, which would have ruled the hot, early universe.
Best of both worlds
Instead, Dimopoulos and Arkani-Hamed set out to construct a version of supersymmetry that incorporates anthropic ideas. Their solution was to split SUSY in two, so that while some of the superpartners weigh in at around 100 gigaelectronvolts, others become immensely heavy. Specifically the squarks (partners of quarks) and sleptons (partners of leptons, such as electrons and muons), all have energies of a billion GeV or more.
These squarks and sleptons are now so massive that they play no role in low-energy phenomena. They no longer make protons decay, or cause all those other unwelcome and unseen particle reactions predicted by the simplest versions of SUSY. Meanwhile, at very high energies they do come into play, so they can still help to unify the forces of nature.
The partners of force-carrying particles – such as gravitinos and photinos – remain middleweight, at only a few hundred GeV. Just as in simple supersymmetry, the lightest of these “-inos” makes a good dark-matter particle. It will be stable, it will neither emit nor absorb light and it should be created in the cauldron of the big bang in about the right quantities to account for the dark matter that astronomers infer. “We still wanted to have a dark-matter particle and unification. Within that we think we have found the simplest possible theory,” says Dimopoulos.
When Dimopoulos and Arkani-Hamed their idea in 2004, there was a flurry of papers by other physicists exploring the idea. However, anthropic arguments remain controversial, and despite the authors’ heavyweight reputations, split supersymmetry is no exception. “I’m not a big fan of it,” says John Ellis, a theoretical particle physicist at the CERN laboratory, where the LHC is being prepared for its first run later this year. His criticism is that the anthropic approach gives you too much freedom to answer any troublesome question in physics. “At some point you might just as well say ‘let’s fine-tune everything’ and go home,” Ellis says.
Theorist Frank Wilczek at the Massachusetts Institute of Technology is also unhappy with the idea of split SUSY. “I think it’s a logical possibility, but if we really have to appeal to anthropic considerations I think that’s a big retreat. It would mean the explanatory power of theoretical physics would be limited.”
Such a bold theory requires clear evidence. The LHC just might provide it, in the form of a metastable version of the gluino, the superpartner to the gluon which carries the strong nuclear force and binds quarks together into protons and neutrons.
In ordinary SUSY, the gluino decays very rapidly. But in split SUSY it tends to last much longer, because in the process of decaying it has to briefly create a superheavy squark. Making this squark takes a lot of energy, which the gluino can borrow briefly from a random quantum fluctuation, but such huge fluctuations are rare, so you generally have to wait a long time for the decay to happen.
If the colliding beams of the LHC do create such metastable gluinos, they would leave distinctive traces in the machine’s particle detectors. A gluino might come to a halt in a solid piece of machinery or in the surrounding soil. It could sit there ticking away for minutes or months until it finally decays, sending a spray of particles out through the detector (see Diagram).
Chris Hill at the University of Bristol in the UK works on one of the LHC’s particle detectors, called the (CMS). His interest was piqued when he read a about stopped gluinos, and decided to work out how to spot them. “It occurred to me that the absolute best time to look for these things was when nothing else was going on,” he says.
From time to time, the LHC will be shut off before being refreshed with new proton beams. There will be gaps of up to a few hours when there are no beams in the machine and no high-energy particle collisions lighting up the detectors. During these quiet times it should be much easier to see an occasional stopped gluino going bang.
“The best time to see a gluino go bang is when the LHC is switched off”
Together with Andris Skuja and Fedor Ratnikov at the University of Maryland in College Park, Hill has worked out how to program the CMS to pick up these events, which will look very unlike the usual particle reactions it was designed for. Their strategy should be in place for the first LHC collisions later this year. Based on their calculations using split SUSY, Hill says CMS could find tens of stopped gluinos per day.
That powerful piece of evidence would have dizzying implications. “It would be a strong indication that there is a string landscape or a multiverse,” says Dimopoulos. “I think the majority of opinion would come around to that point of view.”
Wilczek admits that finding a delayed-action gluino would be highly suggestive. “We wouldn’t literally see the multiverse, but it would encourage that kind of thinking. It would also be a huge boost for the whole idea of unification and supersymmetry. No question it would be a major discovery.”
The universe might be a little more coy, however. It may turn out that the gluino’s lifetime is a few nanoseconds. That’s long compared with those of other massive particles, but not long enough to leave such a clear signal in the detectors of the LHC.
And even if the LHC does find Dimopoulos’s stopped gluinos, not everyone will be persuaded that arguments based on the multiverse are good science. “My opinion of anthropic reasoning is likely to remain unprintable,” says Ellis.
Nevertheless, Ellis does think that the idea of split SUSY has been useful in one way. “It is getting the community to think about unusual things that might show up at the LHC. There will be platoons of graduate students ready to go off and analyse these possibilities.”
Perhaps they will find a messenger from the multiverse, or perhaps they’ll find something even more unsettling – some subatomic sign that reality is unlike any theory yet dreamed of.
Cosmology – Keep up with the latest ideas in our .
Quantum World – Learn more about a weird world in our comprehensive special report.
