A common misconception is that the major problems of physics have mostly been solved, and all that’s left is some minor sweeping-up. I doubt that the readers here would agree, for huge surprises and mysteries continue to surface in physics. In a new piece in the New York Review of Books, “Physics: What we do and don’t know” (free download), Nobel Laureate Steve Weinberg discusses these puzzles, which occur on scales both large and small.
I found a bit of the piece tough going, as if Weinberg were writing for professionals instead of science-friendly and educated people. I had trouble with this, for example:
Speculations of this sort ran into an obvious difficulty: photons have no mass, while any new particles such as W+, W-, and Z0 would have to be very heavy, or they would have been discovered decades earlier—the heavier the particle, the higher the energy needed to create it in a particle accelerator, and the more expensive the accelerator. There was also the stubborn problem of infinities. The solution lay in an idea known as broken symmetry, which had been developed and successfully applied in other areas of particle physics since 1960. The equations of a theory may possess certain simplicities, such as relations among the photon, W+, W-, and Z0, which are not present in the solutions of the equations that describe what we actually observe. In the electroweak theory there is an exact symmetry between weak and electromagnetic forces, which would make the W+, W-, and Z0 massless, if it were not that the symmetry is broken by four proposed “scalar” fields that permeate the universe, from which the W+, W-, and Z0as well as the electron get masses. A new particle discovered last year appears to be the predicted quantum of one of these scalar fields.
But the bulk of the piece is a very nice summary of what we do and don’t know, and I recommend it warmly. Here are some of the mysteries, with Weinberg’s take indented:
1. What is dark matter?
It turns out that particles already known to us are not enough to account for the mass of the hot matter in which the sound waves must have propagated. Fully five sixths of the matter of the universe would have to be some kind of “dark matter,” which does not emit or absorb light. The existence of this much dark matter in the present universe had already been inferred from the fact that clusters of galaxies hold together gravitationally, despite the high random speeds of the galaxies in the clusters. So this is a great puzzle: What is the dark matter? Theories abound, and attempts are underway to catch ambient dark matter particles or remnants of their annihilation in detectors on Earth or to create dark matter in accelerators. But so far dark matter has not been found, and no one knows what it is.
2. What is dark energy?
In 1998, using the apparent brightness of exploding stars to measure the distance of far galaxies, two groups of astronomers found that the expansion of the universe is not slowing down at all, but rather speeding up. Within the rules of the general theory of relativity, this could only be explained by an energy that is not contained in the masses of any sort of particles, dark or otherwise, but in a “dark energy” inherent in space itself, which produces a sort of antigravity pushing the galaxies apart.
From these measurements, and also from studies of the effect of the expansion of the universe on the cosmic radiation background, it has been found that the dark energy now makes up about three quarters of the total energy of the universe.
3. How does gravity fit into the “theory of everything”? Weinberg got his Nobel (along with Abdus Salam and Sheldon Glashow) for unifying the electromagnetic and weak forces. But one “force” has so far defied unification.
Even so, the standard model is clearly not the final theory. Its equations involve a score of numbers, like the masses of quarks, that have to be taken from experiment without our understanding why they are what they are. If the standard model were the whole story, it would require neutrinos to have zero mass, while in fact their masses are merely very small, less than a millionth the mass of an electron. Further, the standard model does not include the longest-known and most familiar force, the force of gravitation. We commonly describe gravitation using a field theory, the general theory of relativity, but this is not a quantum field theory in which infinities cancel as they do in the standard model.
4. Is string theory right?
Since the 1980s a tremendous amount of mathematically sophisticated work has been devoted to the development of a quantum theory whose fundamental ingredients are not particles or fields but tiny strings, whose various modes of vibration we observe as the various kinds of elementary particle. One of these modes corresponds to the graviton, the quantum of the gravitational field. String theory if true would not invalidate field theories like the standard model or general relativity; they would just be demoted to “effective field theories,” approximations valid at the scales of distance and energy that we have been able to explore.
String theory is attractive because it incorporates gravitation, it contains no infinities, and its structure is tightly constrained by conditions of mathematical consistency, so apparently there is just one string theory. Unfortunately, although we do not yet know the exact underlying equations of string theory, there are reasons to believe that whatever these equations are, they have a vast number of solutions. I have been a fan of string theory, but it is disappointing that no one so far has succeeded in finding a solution that corresponds to the world we observe.
String theory, combined with the idea of cosmic inflation, leads naturally to the concept of multiverses, “pocket universes” in which the laws of physics would differ from those of our own universe. This possibility, if confirmed, would finally dispel the persistent theological argument for the “strong anthropic principle”: the idea that the laws of physics were designed by God to make possible sentient life that could apprehend and worship Him (i.e. humans). To my mind, that argument is the last redoubt of a natural theology that’s been eroded to virtually nothing by science.
Unfortunately, multiverses may well be impossible to observe, even though they fall naturally out of existing theories of physics. Some physicists, like Paul Davies, claim that multiverses were concocted by physicists solely to dispel the idea of God, but they’re dead wrong. It’s a serious idea that’s been around for a while.
5. Are there multiverses?
Inflation is naturally chaotic. Bubbles form in the expanding universe, each developing into a big or small bang, perhaps each with different values for what we usually call the constants of nature. The inhabitants (if any) of one bubble cannot observe other bubbles, so to them their bubble appears as the whole universe. The whole assembly of all these universes has come to be called the “multiverse.”
These bubbles may realize all the different solutions of the equations of string theory. If this is true, then the hope of finding a rational explanation for the precise values of quark masses and other constants of the standard model that we observe in our big bang is doomed, for their values would be an accident of the particular part of the multiverse in which we live. We would have to content ourselves with a crude anthropic explanation for some aspects of the universe we see: any beings like ourselves that are capable of studying the universe must be in a part of the universe in which the constants of nature allow the evolution of life and intelligence. Man may indeed be the measure of all things, though not quite in the sense intended by Protagoras.
What better way to spend a lazy Sunday than contemplating the mysteries of the cosmos?
Here’s a picture I took of Steve and Alex Rosenberg at the “Moving Naturalism Forward” meeting about a year ago: