• Question: How close are physicists to finding a theory that makes general relativity and quantum mechanics work together? Does the fact that they can't both be true mean one of them is wrong?

    Asked by anon-251974 on 29 Apr 2020.
    • Photo: David Sobral

      David Sobral answered on 29 Apr 2020:

      I would say likely closer but not that close. Currently one of the problems is the fact that we have too many ways to try to do this and there are very little experiments or data to guide us and to reject most theories once and for all.

    • Photo: Adam Baskerville

      Adam Baskerville answered on 29 Apr 2020: last edited 29 Apr 2020 8:55 am

      This is a profound question and one with a complicated answer. What we are really concerned with is how general relativity unifies with quantum field theory and it is known that there are no contradictions between special relativity and quantum field theory. General relativity also works well as a low-energy quantum field theory but breaks down when we look at situations of the order of the Planck scale. We refer to this as the problem of “nonrenormalizability.” Nonrenormalizability does not mean there is a contradiction in the theory; it means the theory is incomplete.

      The main problem arises when we study gravity as is shows high-energy behaviour inconsistent with local quantum field theory. If we want to study small distance behaviour In quantum mechanics, we can scatter particles at very high energies. This does not apply to gravity where the short distance/high energy relationship breaks down. This tells us that quantum gravity, at very high energies, is not a quantum field theory as we know it and will turn out to be something more bizarre than what we already understand.

      None of this shows a contradiction between general relativity and quantum mechanics, but means that quantum field theory, the framework we use to describe all non-gravitational forces, is not sufficient for understanding gravity. Rather than quantum field theory being incorrect, think of it as being incomplete. In terms of when this will occur, I think we are far away from a complete theory that unifies the two.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 29 Apr 2020:

      I’m an experimental particle physicist, and my answer would be “not very close” – there is a theoretical framework, string theory, that can in principle do this, but theorists have been working on it for years and it is still not a “useful” theory, in the sense of making experimentally testable predictions.

      The question of whether one of the theories is “wrong” because the two do not seem to be consistent raises a very interesting point – it’s an excellent question, thank you! The answer is really “yes, and no”: if they are genuinely inconsistent, one (or both) of them must be wrong at some fundamental level, but this does not mean that it is wrong as currently applied. To explain what I mean by this, consider Newton’s law of gravity. We now know that this is “wrong”: gravity is not described by Newton’s laws, but by the general theory of relativity. However, for most applications Newton’s law is “right”: if you want to send a spacecraft to Mars, you use Newton’s law, not general relativity, because Newton’s law is ever so much easier to work with, and it will give the right answer. Newtonian gravity is an APPROXIMATION to general relativity which holds in most everyday situations, unless you are dealing with extremely high precision (the atomic clocks on GPS satellites), very strong gravitational fields (such as around black holes) or speeds close to the speed of light. In the same way, we know that GR and quantum mechanics work well in their respective applications: GR fails if we try to extend it to the quantum size scale, but when you are dealing with astrophysical bodies it is highly accurate. If we do succeed in finding a working theory of quantum gravity, GR will turn out to be an approximation to that theory that works well as long as you do not try to apply it to quantum size scales, just as Newton’s laws of gravity work well if you do not try to apply them to regions with very strong gravitational fields.

    • Photo: Anne Green

      Anne Green answered on 29 Apr 2020:

      I’m a theoretical astroparticle physicist, so this isn’t my area of expertise. However I agree with Susan that we’re ‘not very close’, for the reasons she explained.

    • Photo: Paul Saffin

      Paul Saffin answered on 30 Apr 2020:

      The difficulty with this is that Quantum Mechanics (QM) and General Relativity (GR) both work so well in their own domains (big things for GR, and small things for QM) so we don’t really want to give up either.
      Ideally one would like a single theory that works for big and for small, so people have tried to make GR into a quantum theory, without much success. The problem is that we have no reason to expect the final theory to be some sort of glueing together of GR and QM, and may look completely different, like String Theory. This is a bit like trying to make the ideal entertainer by putting the head of Dua Lipa on the body of Will Smith – that’s never going to work!

    • Photo: Liza Sazonova

      Liza Sazonova answered on 7 May 2020: last edited 7 May 2020 2:28 pm

      This might be a more naive answer (and I don’t work on this), but I like to think that we really don’t know – it could be tomorrow, or it could be in many years.
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      We have a huge amount of evidence that both GR and QM are correct on the scales that we see. We see GR effects in daily life (for example, we need to correct for space-time curvature in GPS), and we see quantum effects in everything small enough, like semiconductors in computer chips.
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      Maybe there are places where GR and QM don’t work, or break down. This is why people do lots of research into black holes, because those things are so extreme that we don’t really know how GR and QM work there yet! Looking at these exotic things can give us a clue of what should the unified theory do.
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      In the end, I think it will come down to someone being creative and imaginative enough to do come up with such a theory. We have all the tools – centuries of mathematics – to do this in theory, but no one has figured out how yet. Someone needs to take that breakthrough step.
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      Einstein did this when he described the world with relativity: a completely different approach to classical mechanics that was used before, and no one else thought of this. I think we need something similarly novel and unexpected to resolve the QM / GR problem.
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      Maybe it will be you! 🙂
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      That said, it will probably be a while until experimental evidence will be able to back it up… this is one of the problems with string theory: maybe it could resolve the problem, but we can’t prove it with experiments!

Comments

  • Photo: Kim

    Kim commented on :

    Attempts to find the Theory of Everything is the one of the things I find most exciting about physics. I wanted to just drop a comment on the notion these theories possibly being ‘wrong’, which I find quite interesting from a philosophy of science point of view. Both quantum mechanics and general relativity make spectacularly precise and reliable predictions for their appropriate lengthscales. I suspect (though someone please correct me if I’m wrong!) NASA mostly still uses Newtonian gravity to plan satellite orbits/spaceflight. Newtonian mechanics is probably ‘wrong’ insofar as we realise it isn’t the full ‘truth’, but hopefully you can see that ideas of ‘right/wrong’ and ‘truth’ are quite complicated things when it comes to science ~