THERE ARE THEORIES, AND THEN THERE ARE THEORIES
It is 1:15 A.M. in my study. Several hundred yards away, the Fermilab machine is colliding protons on antiprotons. Two massive detectors are receiving data. The battle-hardened CDF group of 342 scientists and students are busy checking out the new pieces of their 5,000-ton detector. Not all of them, of course. On the average, at this time, a dozen people will be in the control room. Partway around the ring the new D-Zero detector with its 321 collaborators, is being tuned up. The run, a month old, had the usual shaky start, but data taking will go on for about sixteen months, with a break for phasing in a new piece of the accelerator designed to increase the collision rate. Although the main thrust is to find the top quark, testing and extending the standard model is an essential part of the drive.
About 5,000 miles away, our CERN colleagues are also working hard to test a variety of theoretical ideas about how to extend the standard model. But while this good, clean work is going on, theoretical physicists are working, too, and I propose to give here a very brief, plumber's version of three of the most intriguing theories: GUTs, supersymmetry, and superstrings. This will be a superficial treatment. Some of these speculations are truly profound and can be appreciated only by the creators, their mothers, and a few close friends.
But first a comment on the word "theory," which lends itself to popular misconceptions. "That's your theory" is a popular sneer. Or "That's only a theory." Our fault for sloppy use. The quantum theory and the Newtonian theory are well-established, well-verified components of our world view. They are not in doubt. It's a matter of derivation. Once upon a time it was Newton's (as yet unverified) "theory." Then it was verified, but the name stuck. "Newton's theory" it will always be. On the other hand, superstrings and GUTs are speculative efforts to extend current understanding, building on what we know. The better theories are verifiable. Once upon a time that was the sine qua non of any theory. Nowadays, addressing events at the Big Bang, we face, perhaps for the first time, a situation in which a theory may never be experimentally tested.
GUTs
I have described the unification of the weak and electromagnetic forces into the electroweak force, carried by a quartet of particles: W+, W−, Z0, and the photon. I have also described QCD—quantum chromodynamics—which deals with the behavior of quarks, in three colors, and gluons. These forces are now both described by quantum field theories obeying gauge symmetry.
Attempts to join QCD with the electroweak force are known collectively as grand unification theories (GUTs). The electroweak unification becomes evident in a world whose temperature exceeds 100 GeV (roughly the mass of the W, or 1015 degrees K). As chronicled in Chapter 8, we can achieve this temperature in the lab. GUTs unification, on the other hand, requires a temperature of 1015 GeV, which puts it out of the range of even the most megalomaniacal accelerator builder. The estimate is derived by looking at three parameters that measure the strengths of the weak, electromagnetic, and strong forces. There is evidence that these parameters in fact change with energy, the strong forces getting weaker and the electroweak forces stronger. The merger of all three numbers occurs at an energy of 1015 GeV. This is the grand unification regime, a place where the symmetry of the laws of nature is at a higher level. Again, this is a theory yet to be verified, but the trend of the measured strengths does indicate a convergence near this energy.
There are a number of grand unified theories, a large number and they all have their ups and downs. For example, an early entrant to the GUT contest predicted that the proton was unstable and would decay into a neutral pion and a positron. The lifetime of a proton in this theory is 1030 years. Since the age of the universe is considerably less—somewhat over 1010 years—not too many protons have decayed. The decay of a proton would be a spectacular event. Remember we considered the proton to be a stable hadron—and a good thing, too, because a reasonably stable proton is very important to the future of the universe and the economy. Yet in spite of the very low expected rate of decay, the experiment is doable. For example, if the lifetime is indeed 1030 years, and we watch a single proton for one year, we have only 1 divided by 1030 as our chance of seeing the decay—1030. Instead, we can watch lots of protons. In 10,000 tons of water there are about 1033 protons (trust me). This would mean that 1,000 protons should decay in a year.
So enterprising physicists went underground—into a salt mine under Lake Erie in Ohio, into a lead mine under Mount Toyama in Japan, and into the Mont Blanc tunnel that connects France and Italy—to be shielded from the background of cosmic radiation. In these tunnels and deep mines they placed huge, clear plastic containers of pure water, about 10,000 tons worth. That would be a cube roughly 70 feet on each side. The water was stared at by hundreds of large, sensitive photomultiplier tubes, which would detect the bursts of energy released by the decay of a proton. So far no proton decays have been observed. This doesn't mean that these ambitious experiments have not proved valuable, for they have established a new measure of the proton's lifetime. Allowing for inefficiencies, the proton lifetime, if indeed the particle is unstable, must be longer than 1032 years.
Interestingly, the long and unsuccessful wait for protons to decay was enlivened by unexpected excitement. I have already told about the supernova explosion of February 1987. Simultaneously a burst of neutrino events was seen by the Lake Erie and Mount Toyama underground detectors. The combination of light and neutrinos was in disgustingly good agreement with models of stellar explosion. You should have seen the astronomers preen! But the protons just don't decay.
GUTs have a hard time but, ever resilient, GUT theorists continue to be enthusiastic. One doesn't have to build a GUT accelerator to test the theory. GUT theories have testable consequences in addition to proton decay. For example, SU(5), one of the grand unified theories, makes the postdiction that the electric charge of particles is quantized, and must come in multiples of one third the charge of the electron. (Remember the quark charges?) Very satisfying. Another consequence is the consolidation of the quarks and leptons in one family. In this theory, quarks (inside the proton) can be converted to leptons and vice versa.
GUTs predict the existence of supermassive particles (X bosons) that are one thousand trillion times heavier than protons. The mere possibility that these exist and can appear as virtual particles does have tiny, tiny consequences, such as the rare decay of protons. Incidentally, the prediction of this decay has practical, if very far-out, implications. If the nucleus of hydrogen (a single proton), for example, could be converted to pure radiation, it would provide a source of energy one hundred times more efficient than fusion energy. A few tons of water could provide all the energy needed by the United States in a day. Of course, right now we'd have to heat the water to GUT temperatures, but perhaps some kid now being turned off to science by an insensitive kindergarten teacher might have the idea that would make this more practical. So, help the teacher!
At the temperatures of the GUT scale (1028 degrees Kelvin) symmetry and simplicity have reached the point where there is only one kind of matter (lepto-quark?) and one force with an array of force-carrying particles and, oh yes, gravity, dangling there.
SUSY
Supersymmetry, or Susy, is the favorite of the betting theorists. We were introduced to Susy earlier. This theory unifies the matter particles (quarks and leptons) and the force carriers (gluons, W's...). It makes a huge number of experimental predictions, not one of which has (yet) been observed. But what fun!
We have gravitinos and winos and gluinos and photinos—the matterlike partners of gravitons, W's, and the rest. We have super-symmetric partners of quarks and leptons: squarks and sleptons, respectively. The burden on this theory is to show why these partners, one for every known particle, have not been seen. Oh, say the theorists, remember antimatter. Until the 1930s no one dreamed that every particle would have its twin antiparticle. And remember that symmetries are created to be broken (like mirrors?). The partner particles haven't be
en seen because they are heavy. Build a big enough machine and they will all appear.
Mathematical theorists assure the rest of us that the theory has a splendid symmetry in spite of its obscene proliferation of particles. Susy also promises to lead us to a true quantum theory of gravity. Attempts to quantize the general theory of relativity—our theory of gravity—have been beset with infinities up the wazoo in a way that could not be renormalized. Susy promises to lead us to a beautiful quantum theory of gravity.
Susy also civilizes the Higgs particle, which, lacking this symmetry, could not do the job it was invented to do. The Higgs particle, being a scalar (zero-spin) boson, is particularly sensitive to the busy vacuum around it. Its mass is influenced by the virtual particles of all masses that fleetingly occupy its space, each one contributing energy and, therefore, mass until the poor Higgs would grow far far too obese to save the electroweak theory. What happens with supersymmetry is that the Susy partners influence the Higgs mass with their opposite signs. That is, the W particle makes the Higgs heavier while wino cancels the effect, so the theory allows the Higgs to have a useful mass. Still, all this doesn't prove that Susy is right. It's just beautiful.
The issue is far from settled. Buzz words appear: supergravity, the geometry of superspace—elegant mathematics, dauntingly complex. But one experimentally intriguing consequence is that Susy willin'gly and generously supplies candidates for dark matter stable neutral particles that could be massive enough to account for this pervasive material that haunts the observable universe. Susy particles were presumably made in the Big Bang era, and the lightest of the predicted particles—perhaps the photino, the higgsino, or the gravitino—could survive as stable remnants to constitute the dark matter and satisfy the astronomers. The next generation of machines must either confirm or deny Susy ... but, oh, oh, oh what a gal!
SUPERSTRINGS
I believe it was Time magazine that forever embellished the lexicon of particle physics by trumpeting this as the Theory of Everything, or TOE. A recent book put it even better: Superstrings, Theory of Everything? (This is read with a rising inflection.) String theory promises a unified description of all forces, even gravity, all particles, space and time, free of arbitrary parameters and infinities. In short: everything. The basic premise replaces point particles by short segments of string. String theory is characterized by a structure that pushes the frontiers of mathematics (as physics has very occasionally done in the past) and the conceptual limitations of the human imagination to the extremes. The creation of this theory has its own history and its own heroes: Gabrielle Veneziano, John Schwarz, André Neveu, Pierre Ramond, Jeff Harvey, Joel Sherk, Michael Green, David Gross, and a gifted pied piper by the name of Edward Witten. Four of the prominent theorists worked together at an obscure institution in New Jersey and have become known as the Princeton String Quartet.
String theory is a theory about a very distant place, almost as far away as Atlantis or Oz. We are talking about the Planck domain, and if it ever existed (like Oz), it would have been in the very earliest flicker of Big Bang cosmology. There is no way we can imagine experimental data from that epoch. That doesn't mean we shouldn't persevere. Suppose one finds a mathematically consistent (no infinities) theory that somehow describes Oz and has as its low, low energy consequence our standard model? If it is also unique—that is, has no competitors that do the same thing—then we will all rejoice and lay down our pencils and trowels. Uniqueness is what superstrings doesn't enjoy. Within the major assumptions of superstrings are an enormous number of possible paths to the world of data. Let's see what else characterizes this stuff without pretending to explain it. Oh yes, as mentioned in Chapter 8, it requires ten dimensions: nine space dimensions and one time.
Now we all know there are only three space dimensions, although we have warmed up to the issue by imagining living in a two-dimensional world. So why not nine? "Where are they?" you rightly ask. Curled up. Curled up? Well, the theory started with gravity, which is based upon geometry, so one can visualize that six of the dimensions got curled up into a tiny ball. The size of the ball is typical of the Planck regime, 10−33 centimeters, about the size of the string that replaces the point particle. The particles we know emerge as vibrations of these strings. A stretched string (or wire) has an infinite number of vibration modes. That is the basis of the violin—or the lute, if you remember way back when we met Galileo's old man. The vibrations of real strings are classified in terms of a fundamental note and its harmonies or frequency modes. The mathematics of micro-strings is similar. Our particles come from the lowest-frequency modes.
There is no way I can describe what has excited the leaders of this theory. Ed Witten gave a fantastic, gripping lecture about all this at Fermilab some years ago. For the first time in my experience, when he concluded there was almost ten seconds of silence (that's a lot!) before the applause. I rushed over to my lab to explain what I had learned to my colleagues on shift, but by the time I got there I had lost most of it. The artful lecturer makes you think you understand it.
As the theory met increasingly more difficult mathematics and a proliferation of possible directions, the progress and the intensity surrounding superstrings dropped to a more sensible level, and now we can only wait. There continues to be interest on the part of very capable theorists, but I suspect it will be a long time before TOE reaches the standard model.
FLATNESS AND DARK MATTER
Waiting for a theory rescue, Big Bang still has puzzles. Let me select one more problem that has confounded physicists even as it has led us—experimenters and theorists alike—to some tantalizing notions about the Very Beginning. It is known as the flatness problem, and it has a very human content—the morbid interest in whether the universe will continue to expand forever or whether it will slow down and reverse to a period of universal contraction. The issue is how much gravitational mass there is in the universe. If there is enough, the expansion will be reversed and we will have the Big Crunch. This is known as the closed universe. If there isn't enough, the universe will continue to expand forever, growing colder—an open universe. Between these two regimes is a "critical mass" universe, one that has just enough mass to slow the expansion but not enough to reverse it—a so-called flat universe.
Time for a metaphor. Think about sending a rocket up off the surface of the earth. If we give the rocket too small a velocity, it falls back to earth (closed universe). The earth's gravitation is too strong. If we give it a huge velocity, it can escape the earth's gravitation and soar into the solar system (open universe). Obviously there is a critical velocity such that ever so slightly less speed results in fallback, and ever so slightly more results in escape. Flatness occurs when the velocity is right on. The rocket escapes, but with ever-decreasing velocity. For rockets on our earth the critical velocity is 11.3 kilometers per second. Now, following the example, think of a fixed-velocity rocket (the Big Bang) and ask how heavy a planet (total mass density of the universe) results in escape or fallback.
One can estimate the gravitational mass of the universe by counting the stars. People have done this, and, taken alone, the number is too small to halt the expansion; it predicts an open universe, by a very wide margin. However, there is very strong evidence for the existence of a distribution of nonradiating matter "dark matter" pervading the universe. When observed matter and estimated dark matter are combined, measurements indicate that the mass in the universe is close to — not less than 10 percent of nor more than two times—the critical mass. Thus it is still an open question whether the universe will continue to expand or will contract eventually.
There are many speculative candidates for dark matter. Most of them are particles, of course, with fancy names—axions, photinos —given by loving theorist-inventors. One of the most fascinating possibilities for dark matter is one or more of the standard-model neutrinos. There should be an enormous density of these elusive objects left over from the Big Bang era. They would be ideal candidates if ...
if they had a finite rest mass. We already know that the electron neutrino is too light, leaving two candidates, of which the tau neutrino is the favorite. Two reasons: (1) it exists, and (2) we know almost nothing about its mass.
Not long ago at Fermilab we carried out an ingenious and subtle experiment designed to detect whether the tau neutrino has a finite mass that would serve to close the universe. (Here cosmological needs drove an accelerator experiment, an indication of the particle-cosmology union.)
Imagine a graduate student on shift on a bleak winter's night, imprisoned in a small electronics hut on the wind-swept prairie of Illinois. Data have been accumulating for eight months. He checks the progress of the experiment, and as part of his routine he examines the data on the neutrino mass effect. (You don't measure the mass directly, but an influence the mass would have on some reactions.) He runs the entire sample of data through the calculation.
"What's this?" He becomes instantly alert. He can't believe the screen. "Oh, my God!" He runs computer checks. All are positive. There it is—mass! Enough to close the universe. This twenty-two-year-old graduate student experiences the incredible, breath-stopping conviction that he alone on the planet, among 5.32 billion of his fellow sapiens, knows the future of the universe. Talk about a Eureka moment!
Well, it's a nice story to think about. The part about the graduate student was true, but the experiment failed to detect any mass. That particular experiment just wasn't good enough, but it could have been, and ... perhaps someday it will be. Colleague reader, please read this to your uncertain teenager con brio! Tell him or her that (1) experiments often fail, and (2) they don't always fail.
CHARLTON, GOLDA, AND GUTH
But even if we don't yet understand how the universe contains the critical mass needed for a flat universe, we're pretty sure that it does. We'll see why. Of all the masses nature could have chosen for Her universe (say 106 times critical mass or 10−16 times critical mass), She chose something nearly flat. In fact, it's worse than that. It appears to be a miracle that the universe has survived the two opposing fates—immediate runaway expansion or immediate crunch—for 15 billion years. It turns out that the flatness at age one second had to be close to perfect. If it deviated by ever so little, on one side we would have had the Big Crunch even before we made a single nucleus; if the deviation were on the other side, the expansion of the universe would have progressed by this time to a stone-cold dead thing. Again a miracle! Much as scientists may envision the Wise One, der Alte, a Charlton Heston type with fake long flowing beard and a strange laser-induced glow, or (as in my own view) a Margaret Mead or Golda Meir or Margaret Thatcher type of deity, the contract clearly says that the laws of nature are not to be amended, that they are what they are. Thus the flatness problem is too much of a miracle and one seeks causes to make the flatness "natural." That's why my graduate student was freezing his ass off trying to determine whether neutrinos are dark matter or not. Infinite expansion or Big Crunch. He wanted to know. So do we.
The God Particle Page 50