Is it possible that there is no such thing as quantum gravity?

Mudassir Ali
Feb 27, 2020 01:40 PM 0 Answers
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Mudassir Ali
- Feb 27, 2020 01:40 PM

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.

Mudassir Ali
- Feb 27, 2020 01:40 PM

Yes of course. People making the counter argument are making the same obvious error.

“All other forces (3 of them) have been quantized therefore gravity must be as well.”

In other words, if something happens three times it will always happen. Great argument!

But it’s even worse. Humans have no real deep understanding of fundamental physics. We have done an impressive job discovering many symmetries in the Universe and putting together a handy mnemonic that allows us to calculate and make use of them to accomplish some very useful things. That’s great, but we have no clue why things are the way they appear and can make no solid argument for why they have to be that way.

It’s reasonable to suggest that there will be some theory which allows us to calculate what will happen in conditions where both quantum mechanics and general relativity play crucial roles. But we have no single theory and none of them have any experimental support. It’s perfectly possible that theorists today are completely on the wrong track and that we are no closer than the ancient egyptians were towards understanding nuclear physics.

It’s even possible that humans (even all intelligent forms of life) never discover such a theory. Perhaps there is a paradigm shift ahead that completely changes how we think about physics and science just as there have been in the past and that our current difficulties and understandings will be seen as fundamentally misguided.

Consider the following pattern of human thought and realize how many times it has described how humans have made progress.

Our success at A convinced nearly everyone of B which was responsible for all the wasted effort of our best minds trying to understand C. With the more modern view of D, we now view E as the more fundamental question and C as a meaningless pursuit.

It wasn’t until I stepped away from the field of physics that I realized how poorly physicists understand this pattern and how so much of what they believe is really going on is more a matter of fashion and faith. And like all progress in human thought, it is more important that we all believe the same metaphors and less important whether those metaphors are actually true. That’s because these shared metaphors are what enable useful communication including ultimately the communication to abandon that metaphor and adopt a more useful one.

Mudassir Ali
- Feb 27, 2020 01:40 PM

Not really. You can make a pretty simple argument that such a thing is necessary.

Consider a two slit experiment. Send one photon at a time through the slits. You still get an interference pattern so, as Bohr put it, every particle only interferes with itself.

That photon has to be seen as being in a superposition of states, one passing through each slit. But a photon has energy so it makes a gravitational field, however tiny. And those two states have two different positions. And the gravitational field strength depends on distance from the particle so the gravitational field of the photon must be in a superposition of states.

General relativity doesn’t allow superpositions for gravity. So general relativity cannot be the last word on the subject.

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