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  Quantum gravity: How can the microscopic degrees of freedom of space-time be determined?

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Researchers in quantum gravity seem to agree that space-time is emergent: it is made of many microscopic degrees of freedom, most probably of Planck size. Google Scholar lists many proposals: loops, chain mail, strings, trinions, twistors, causal sets, to name just a few. It seems that all these proposals are interesting and most of them yield general relativity in some suitable limit. 

So far, there seems to be no consensus.  But many researchers work on their own model since 20 or 30 years. How do they know that they are working on the best model? Or on the correct model?  

How can or could an outsider determine which of these proposals is correct? What is the best way to compare the proposals against each other? In the end, the question is this: What is the best criterium to find out what space-time is made of?

asked Dec 18, 2022 in Chat by Michael [ no revision ]
recategorized Dec 18, 2022 by Arnold Neumaier

So far none is proved correct; what is promising depends on who judges. To form an opinion on what is promising one must study them all or wait till there is agreement. People work on one or two of the options hoping to illuminate their consequences so that one can possibly decide in the future what has a chance to succeed.

Well, in physics "waiting til there is agreement" is not the way to do research. We look for arguments, not for agreement. The question is about which arguments should be used - not about which authorities we should follow.

Different authorities suggest following different lines of argument. Thats how science proceeds when there is no agreement.

The question was precisely about the arguments / criteria! What are they?

Your question asks far too many questions simultaneously.

All researcher favor their own arguments and criteria. The criteria are vague and the argument convince only their supporters. otherwise there would be agreement. Don't expect to get a list of all the many possibilities!

1 Answer

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Your question was:

           What is the best criterium to find out what space-time is made of?

There is only one criterion in physics: correspondence with observations.

Every theory of quantum gravity needs to reproduce general relativity in the classical limit.

And every theory of quantum gravity has to reproduce quantum theory and particle physics.

At present, no theory fulfils the last criterium: no theory explains elementary particles and gauge interactions. So, at present, the answer is simple: no theory agrees with observation. 

answered Jan 14, 2023 by Butterfly [ no revision ]

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