杏吧原创

PHYSICISTS are masters at describing the flickering subatomic world, at predicting how particles whizz about and bump into each other. But when they zoom out and consider the Universe as a whole, the laws governing atoms don鈥檛 quite fit.

They have been struggling with this problem for years, assuming that if they got the right theory everything would fall into place, but maybe they are deluding themselves. Perhaps we simply shouldn鈥檛 expect the laws of the microworld to explain the world on the largest scale.

Thomas Banks of Rutgers University and the University of California in Santa Cruz believes that we simply can鈥檛 build everything from the bottom up; some large-scale aspects of the cosmos may be just as fundamental as the laws that govern particles. Indeed, the action of the cosmos could even change the properties of individual particles: we could be living in a top-down Universe.

It鈥檚 verging on heresy. Throughout the history of physics, large-scale phenomena have always been explained by smaller scale ones. Gases, for example, can be described as swarms of hyperactive atoms; the atoms in their turn as tiny electrons orbiting dense nuclei, which can be broken down into protons, quarks and so on. The end of the process will come when we find the truly elementary particles and learn to describe the forces between them, preferably within a single theoretical framework. The behaviour of particles, matter, galaxies and the whole Universe should ultimately derive from this 鈥渢heory of everything,鈥� the source of all truth.

Already, physics has reconciled three of the four forces that push and tug matter. Electromagnetism and the so-called strong and weak forces that operate inside atomic nuclei can all be described in terms of their constituent particles: the quarks and electrons that make up matter bounce around under the influence of force particles, such as photons and gluons. But gravity is holding out: there is no theory that describes how gravity works on a subatomic scale.

Cue the world鈥檚 smallest string ensemble. According to string theory, you can think of elementary particles as loops wriggling in 10 dimensions, all differentiated by the kind of wriggle. These quantum rubber bands snap and fuse, giving rise to the four forces. One kind of string would show up as a hypothetical particle that carries the force of gravity.

Certainly there are still problems: despite progress over the past five years, we still don鈥檛 know what the basic principles of string theory are. And it鈥檚 extremely difficult to calculate anything with strings, so the theory is largely untested. But many physicists are confident that it will work out in the end, and string theory, or something very like it, will be installed as the theory of everything.

What throws a spanner into the works is the inconvenient lightness of empty space. Quantum mechanics, an essential part of any small-scale physical theory, says that everything fluctuates. And that includes the vacuum. This means that pairs of particles should be popping in and out of existence all the time, filling space with their energy. To distinguish them from the long-lived particles that make up matter, these ephemeral creatures are called virtual particles.

At first glance, this predicted vacuum energy isn鈥檛 at odds with reality. Over the past seven years or so, astronomers have gathered fairly firm evidence that empty space has an energy of its own. By analysing the brightness of supernova explosions, they have found that distant galaxies are receding from one another faster and faster as time goes by. Space itself seems to be accelerating outwards. To explain this behaviour, physicists鈥� first guess is that there must be some kind of invisible but ubiquitous energy permeating space 鈥� energy that generates a repulsive kind of gravity, known as the cosmological constant, that pushes the galaxies apart.

There鈥檚 one snag, though. Use the equations of quantum mechanics to tot up just how much energy there should be, adding in all the contributions from known subatomic particles such as electrons and quarks, and you get a huge figure. The strength of its repulsion should be so great that the Universe would have blown itself to smithereens long ago.

Superpartners

Can strings ride to the rescue? For a while it seemed that they might. That鈥檚 because adding extra kinds of particles to the Universe can actually reduce the energy of the vacuum. Whereas virtual force-carrying particles (bosons) add their mass to the vacuum, virtual matter particles (fermions) subtract some.

Change the balance between bosons and fermions and you could change the energy of the vacuum. Now, strings can vibrate in many different ways 鈥� more than enough to account for all the known subatomic particles. So in string theory, there are a lot of extra particles. Indeed, every kind of elementary particle could have a long-lost twin, called a superpartner: for every kind of matter particle there鈥檇 be a force-carrying twin; for every force particle there鈥檇 be a matter twin. Many of these particles, dubbed sparticles, have entertaining names 鈥� the selectron partners the electron, squarks partner quarks, and the Wino, the superpartner of the W particle which carries the weak force.

If the masses of the superpartners were the same as their siblings, though, the vacuum energies of the bosons and fermions would cancel each other out exactly. There would be zero vacuum energy, and zero acceleration. But we already knew that the masses aren鈥檛 equal: the superpartners must be very much heavier than their ordinary kin, or else we鈥檇 have found them in particle accelerator experiments.

Given that they have to be so heavy, you can go back and work out a rough figure for the total vacuum energy. And again, it doesn鈥檛 work out. It鈥檚 not quite so huge a discrepancy as before, but it鈥檚 still out by a factor of 10⁶⁰. An embarrassing gap to say the least.

This leads many physicists to think that a big conceptual shift is in order. The best ideas they have right now aren鈥檛 that great: that somehow new physics cancels out all but the barest sliver of vacuum energy; that there are many Universes or patches of this Universe with different random vacuum energies, and we just happen to live in this one; or even that gravity changes at large scales.

The particular shift Banks has in mind turns the whole problem on its head. He thinks that maybe string theory can鈥檛 explain the size of the Universe鈥檚 acceleration. Indeed, maybe there is no purely bottom-up explanation at all: the acceleration has to be taken as a fact of life. In a sense, it explains itself.

Here鈥檚 how Banks鈥檚 idea works. In calculating the vacuum energy, the usual assumption is that the number of possible virtual particle states is infinite, so getting the total energy means adding up an infinite series of terms. But according to physicists who study quantum theories of gravity, that may not be so. In quantum gravity, space-time is made of little quantum fluctuations, so the total number of quantum states available is limited. What鈥檚 more, it鈥檚 limited in a rather shocking way: instead of growing like the volume of a region, it is proportional to the regions鈥� surface area (New 杏吧原创, 27 April, p 22).

This 鈥渉olographic principle鈥� is profoundly counter-intuitive. It means that the bigger the region you are looking at, the more thinly the available quantum states are spread. That makes it rather hard to interpret; how big a region should you look at to calculate your vacuum energy, for example? The bigger the region you choose, the smaller the average vacuum energy you鈥檇 get.

Unless, that is, there is a meaningful boundary to the Universe. In an accelerating cosmos, the most distant galaxies are hidden from view because light cannot move fast enough to cover the space in between and tell us what鈥檚 going on out there. There is an absolute limit on all we will ever be able to see. In many ways it is like the event horizon that cloaks a black hole, as nothing from the other side can ever affect us, no matter how far in the future. The horizon in an accelerating space is called the de Sitter horizon, after the Dutch physicist who discovered this space-time as a solution to general relativity.

If the horizon defines the limit to what you can ever see, it should also limit the number of quantum states you can observe. And that could reduce the vacuum energy, because the number of virtual-particle states that can exist is also limited by this distant horizon.

Andrew Cohen of Boston University and David Kaplan and Ann Nelson of the University of Washington wrote in a 1999 Physical Review Letters paper that this should bring the vacuum energy calculation into line with the acceleration measured by astronomers. Scott Thomas of Stanford University also argues in a paper to be published in PRL that the fewer quantum states in a holographic theory would by themselves tame the cosmological constant problem. But there鈥檚 no holographic theory that works for de Sitter space yet, so these ideas can鈥檛 be put on a firm basis.

Banks has made a much bolder claim. He says we should just treat the vacuum energy as a measure of the number of quantum states in de Sitter space, and not as something to be calculated from first principles. He hopes that what he鈥檒l get is a situation that is self-consistent: the horizon trims the vacuum energy to just the right amount so that it produces the right acceleration to set that horizon. 鈥淭he cosmological constant determines and is determined by the number of states,鈥� says Banks.

To some people, this may seem suspiciously circular, or at least hopeful. Banks hasn鈥檛 won many converts yet. The idea is bold, concedes Leonard Susskind of Stanford University, a string theorist and co-originator of the holographic principle. 鈥淏ut it is very speculative and has not been backed up by any technical calculations.鈥�

Cosmological calculator

That鈥檚 not to say that no one is sympathetic. 鈥淭he idea that the cosmological constant is a measure of the number of states seems quite plausible,鈥� says Joseph Polchinski of the University of California in Santa Barbara鈥檚 Institute for Theoretical Physics. Willy Fischler of the University of Texas in Austin has made that claim too. And indeed, Polchinski鈥檚 co-worker Raphael Bousso has applied a holographic technique he derived to argue that any universe with a positive cosmological constant must have a finite number of states.

The most shocking aspect of Banks鈥檚 idea is that the shape and size of the cosmos can affect physics on a local scale. There have already been hints in modern physics that the small and large are intimately connected, but this pushes the idea to extremes. Could this radical shift of perspective show us how to fix our fundamental theories? Might string theory emerge stronger than before?

As it stands, any quantum field theory, including strings, assumes an infinite number of states. The weird thing, according to Banks, is that physicists haven鈥檛 found many versions of string theory with broken supersymmetry. In other words, string theory says that the superpartners ought to be the same mass as their ordinary partners; so a selectron would weigh as much as an electron, and so on.

So what breaks this perfect symmetry? Banks thinks it鈥檚 the de Sitter horizons. The acceleration of the Universe is changing their masses, generating the right vacuum energy.

How could the motion of distant galaxies change the properties of a tiny object on Earth? Well, quantum particles aren鈥檛 just isolated lumps of matter; they come attached to a cloud of virtual particles that trace out every path from the real particle throughout the Universe around it. These branching networks of virtual particles are attached to the real particle at their core, and when you measure the charge or mass of that particle that includes contributions from all the virtual particles too.

So the traditional way of calculating the mass of a superpartner means adding up an infinite number of states. But maybe that鈥檚 wrong. If there鈥檚 a de Sitter horizon limiting the number of states that any particle can have, then the correct calculation won鈥檛 have an infinite number of states. The superpartners would be feeling the edge of space and time.

Until just recently, Banks had little hard theory to support this idea. But he has now worked out a way that one particular supersymmetric particle, called the gravitino, might become weighed down. The details are highly technical, but the picture is simple enough: first, a virtual gravitino pops into existence near the real gravitino. It zips over to the horizon, buzzes around briefly, then zips back. The number of states it has a chance to interact with in that moment determines the real gravitino鈥檚 mass, which ends up at around 10^鈭�3 electronvolts.

According to the equations of supersymmetry, this in turn implies that the remaining sparticles are of the order of one trillion electronvolts, just above the capabilities of any existing particle accelerator. The Large Hadron Collider, now being built at the CERN laboratory in Geneva, will reach into this range, and could finally detect supersymmetric particles. But to test Banks鈥檚 theory, we will have to wait for more sophisticated accelerators still to pin down their masses and properties precisely enough to work out whether the theory holds true.

If Banks is eventually proved right, then we鈥檒l know that the tiniest speck of matter is curiously tied to the fate of the whole Universe. We鈥檒l also know that the ultimate quest in physics has hit a rather frustrating brick wall. A theory of (almost) everything, anyone?