Commentary

Invoking New Physics

As I read reviews of current work on cosmology, I ran across the following sentence concerning why type 1a supernovae are dimmer than expected in the matter-dominated theory of cosmic expansion: “The most likely explanations are an acceleration of the cosmic expansion, dimming by dust, and evolution of the SN luminosity. A priori, all these interpretations are equally likely.”¹ This statement is stunning: how can each of these possibilities be equally likely when the universe has only one state? Or perhaps this is a statement of experience; the writer is saying that whenever he comes up with plausible explanations, each one has as much probability of being true as any of the others. But more broadly, this implies that any theory the community comes up with is as plausible as any other, so that the probability of any one theory being correct is a function of the amount of imagination within that community, with the probability of a given theory being correct falling as more theories are developed. Clearly these interpretations are nonsensical. The only type of probability that can be assigned to the truth of a theory is one that expresses the experience of the researcher, and in the experience of this researcher, a theory that invokes new physics to explain an observation is a long shot.

This is not to say that new physics never appears in astronomy. There are several very good examples where new physics solved long-standing problems.

One of the earliest problems of modern astronomy was the resolution of the age of the Earth with the age of the Sun. Early work in geology suggested that the age of the Earth is several billions of years old. This, however, presented a problem for astronomers studying the Sun. Before the twentieth century, the only known sources of energy were chemical and gravitational, and for powering the Sun, the gravitational energy provided the greater energy reserve. But the lifetime of a Sun powered only by gravitational energy is about 30 million years. The resolution of this problem, of course, is that the nuclear fusion of hydrogen into helium is the source of the Sun's energy; our understanding of these processes developed at the beginning of the 20th century.

Solar spectroscopists of the 19th century encounter new physics in 1868 when they found lines in the solar spectrum that corresponded to no known chemical. To explain the lines, they assumed that an unknown chemical was present in the Sun; they named this chemical helium after helios, the Greek name for the sun. Helium was later found on Earth in 1895, almost thirty years after its astronomical discovery.

In more recent times the observed solar neutrino flux could not be explained without new physics. The nuclear fusion processes in the Sun produce a type of neutrino called the electron neutrino.² Because neutrinos interact weakly with electrons, protons, and neutrons, the neutrinos escape the Sun with a minimal amount of interaction. Therefore, if one observes solar neutrinos, one is directly observing the nuclear reactor at the core of the Sun. The problem is that the solar neutrino experiments of four decades ago observed a neutrino flux that was about one third the value expected from the standard theory for the Sun. Work on improving the theory of the solar interior did not resolve the problem. The problem was finally resolved in 2001 when new neutrino experiments demonstrated that two-thirds of the electron neutrinos are converted into two other types of neutrino—muon and tau neutrinos—before they arrive at Earth.

So we have several examples where astronomical observations required the development of new physics. But how often is new physics needlessly proposed? This is a difficult question to answer, because who keeps track of how many theories are incorrect? Certainly in the case of gamma-ray bursts dozens of theories employing new physics were proposed, and all were proven wrong; gamma-ray bursts are not from exotic objects, but are from core-collapse supernovae.

We can go down a list of hypothetical physics that has not appeared necessary; magnetic monopoles, evaporating primordial black holes, cosmic string, quark nuggets, and antimatter stars are among the colorful beasts of this fanciful menagerie. But none of these have produces observable signatures, and when a theory has invoke any one of these as a solution for an unexpected observation, it has been disproven.

I have found that there is a definite order in judging an explanation for an observational result. The highest probability explanation is that the result is a systematic error or a statistical fluke (an amazing number of three-sigma results are published and later contradicted by further observations). Second, explanations based on a better understanding of the standard physics responsible for the phenomenon have the next highest probability of being correct; much of this is simply a reflection of the complexity of astronomical phenomena and the long time it take to develop theories that are sophisticated enough to reproduce the observations. The least likely explanation is that we are seeing the effects of new physics. From the examples of problems that required for their solution new physics, we see that new physics has been required perhaps once every half-century. With this type of track record, we should always regard a theory that employs new physics as a longshot.