Undergrad How did the theory of quantum gravity begin?

[south]

TL;DR

When General Relativity was already successful, for a number of reasons and with a number of goals, physicists sought some connection with quantum theory. I am interested in those reasons and those goals.

Hello everyone. I have the following questions:
1. How and why did the theory of quantum gravity begin?
2. What is the objective of those who are dedicated to this research?

 

[Ibix]

You can't superpose spacetimes, so you cannot use GR to write down the gravitational effect of a quantum particle in a superposition of states. There is some disconnect there. GR also produces singularities in an incredible range of circumstances, suggesting it is making incorrect predictions in some circumstances.

I don't think there is any particular point to research in quantum gravity beyond understanding the circumstances where GR fails. It would give us more insight into the more extreme situations we find in the universe, including the Big Bang and black holes, but neither is a pressing concern to us. There may be practical benefits to the such a theory, but not knowing what the result is it's hard to say.

 

[timmdeeg]

Are we sure at all that a theory of quantum gravity exists?

 

[south]

Hi Ibix, hi timmdeg, thanks for answering my question. I'm beginning to understand that it's not easy for those who research quantum gravity. I'd also like to read what you can expand on and add, in terms suitable for a layman like me.

 

[Ibix]

Are we sure at all that a theory of quantum gravity exists?

There has to be a better theory because GR can't handle quantum sources and all sources are, at some level, quantum objects. Whether it will look like other quantum theories is more open to question. I believe a quantum field theory of massless spin two particles has been worked out and does most of the job, but is non-renormalisable. There are various alternative approaches, but no clear winner. And GR is very, very accurate, and until we find an experimental situation that GR doesn't predict accurately it's hard to know how you'd pick one.

 

[south]

I believe a quantum field theory of massless spin two particles has been worked out (...)

I've read that spin-2 is what is used for solid reasons. Does the phrase sound reasons imply the impossibility of spin-1?

 

[Ibix]

The massless spin-1 theory is electromagnetism.

 

[south]

The massless spin-1 theory is electromagnetism.

Okay. And is there any possibility, even remote, of conceiving of gravity as an electromagnetic phenomenon? Has anyone ever tried this?

 

[javisot20]

Okay. And is there any possibility, even remote, of conceiving of gravity as an electromagnetic phenomenon? Has anyone ever tried this?

https://es.m.wikipedia.org/wiki/Gravifotón

 

[timmdeeg]

There has to be a better theory because GR can't handle quantum sources and all sources are, at some level, quantum objects.

As I understand it a new theory should be proven by observation. Supposed someone comes up with a QG theory which is consistent mathematically how could we trust its the correct theory?

Or a bit in more detail. Lets assume matter collapses and forms a black hole. Do we expect in this case that QG corrects GR regarding its wrong prediction? But how about the state of the matter?

 

[south]

https://es.m.wikipedia.org/wiki/Gravifotón

Thanks javisot20 for answering my question. The Kaluza-Klein theory gave physicists permission to speculate by expanding the number of dimensions, until that number became a difficult problem for physicists. Imagine for a layman like me. The link you left is very useful.

 

[Ibix]

Okay. And is there any possibility, even remote, of conceiving of gravity as an electromagnetic phenomenon?

No. There have been attempts to combine electromagnetic and gravitational phenomena in one unified theory (e.g. Kaluza-Klein), but if successful that would make them both part of something larger, not make one part of the other.

 

[Ibix]

As I understand it a new theory should be proven by observation. Supposed someone comes up with a QG theory which is consistent mathematically how could we trust its the correct theory?

Experiment, as you say. As with GR vs Newton, we would expect that a new theory would give different predictions everywhere, but experiment tells us the difference must be really, really small in everyday cases (or we'd have noticed already). We expect the differences to be large in black holes, because there we've reason to believe that GR is going badly wrong.

A candidate future gravity theory, though, might also say something like "the Cavendish experiment result will differ from GR predictions on the nth decimal place". And maybe there's some weird corner where that n is small enough that the results are detectably different but we haven't yet tried that particular combination of prameters. Then we could test the theory. A Cavendish experiment on the atomic scale might be one such - incredibly hard to do and beyond us today, but at least you don't need a black hole.

So in summary, theoreticians knocking inconsistent theories on the head is useful work. Getting predictions out of consistent ones is also useful because then the experimentalists have a defined problem to work on.

 

[south]

Experiment, as you say. As with GR vs Newton, we would expect that a new theory eould give different predictions (...)

Thank you Ibix. I found your last two answers very useful and informative. Reading them I felt I was in good hands.

 

[timmdeeg]

A candidate future gravity theory, though, might also say something like "the Cavendish experiment result will differ from GR predictions on the nth decimal place". And maybe there's some weird corner where that n is small enough that the results are detectably different but we haven't yet tried that particular combination of prameters. Then we could test the theory. A Cavendish experiment on the atomic scale might be one such - incredibly hard to do and beyond us today, but at least you don't need a black hole.

Thanks, sounds very interesting. I wasn't aware of the Cavendish experiment.

 

[haushofer]

A recent paper which could be a nice addition to this topic:

Does Quantum Gravity Happen at the Planck Scale?​

https://arxiv.org/abs/2501.07614

 

[south]

A recent paper which could be a nice addition to this topic:

Does Quantum Gravity Happen at the Planck Scale?​

https://arxiv.org/abs/2501.07614

I downloaded the PDF. The first thing I did was to skim through it and immediately liked the emphasis given to the conceptual aspect. Thank you very much.