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Mudassir Ali
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Virtually every physicist in the world believes that the true theory of gravity, when we find it, will be a quantum field. That’s, in part, because all the other forces of nature are quantum, and that has been shown experimentally. The includes electromagnetism, the strong nuclear field, and the weak field. (Of course, those “three” theories have been successfully unified into a single quantum theory known as “the standard model.”
Most of the effort in the theoretical study of gravity has been directed towards (1) quantizing the gravitational field in a meaningful way, and (2) unifying that theory with the standard model. Some people think they are close: string theory does unify them, but string theory is still too vague and ill-defined to make any successful predictions, so it may be wrong. (My own opinion is that it will be shown to be wrong, but I am a minority among physicists in this.)
So is it a quantum phenomena? My answer, which draws ire from many of my colleagues, is that it may not be quantized. It is amusing that many people actually get angry at this suggestion. I have the sense that many theorists take the quantization hypothesis as canonical, as given by God, as something that should not be questioned. Yes, question the assumption of unitarity, of TCP, of the Big Bang —- but don’t question the assumption that gravity will be quantized!
Why might gravity be different? Einstein showed that gravity can be understood through geometry. In fact, it is most easily understood that way. And the geometric approach led Einstein to outstanding predictions, predictions that were verified experimentally.
Now it turns out that in the weak field approximation, Einstein’s geometric theory can be shown to be equivalent to a weak field theory. They predict all the same things. So does that mean that gravity is really a field? In a sense it is, since the term field could refer to the space-time metric. But does that equivalence indicate that the metric will be a quantifiable field? Could it be an exception? Could it be the one and only field that is not a quantum field?
It would help the theorists convince me if their progress in quantum gravity led to predictions that could be verified experimentally. Perhaps the closest is the work of Steven Hawking, who showed radiation from a black hole would be a necessary consequence of the quantization of gravity. But that has not been verified experimentally?
Suppose we actually get to observe a small black hole and determine experimentally that it does not radiate. Then I guarantee the following (in the same sense that Joe Namath “guaranteed” victor in the 3rd Super Bowl): that within a few days there will be numerous papers submitted by renowned theoretical physicists postulating that gravity is not quantized!
In physics, and in all of science, we’ve learned to beware of consensus. There was a consensus in geology and geophysics that “plate tectonics” was nonsense. There was a consensus that the gravitational constant of Einstein was zero. There was a consensus that no process in physics could violate parity symmetry. Now there is a consensus that the ultimate theory of gravity will be a quantum theory. Beware of especially aware of the dangers of consensus when there is no experimental evidence to support it.