Why there must be a way of including gravity into a quantum theory?

[Tio Barnabe]

Why there must be a way of including gravity into a quantum theory? What leads scientists to believe that ultimately all the interactions must be subjected to quantisation?

Would that be because we describe all other interactions in quantum theory and so we must include gravity on this framework if we want to have a theory describing interactions between the different type of "forces"?

 

[Nugatory]

Why there must be a way of including gravity into a quantum theory? What leads scientists to believe that ultimately all the interactions must be subjected to quantisation?

We cannot ignore quantum effects in very small regions of space. We cannot ignore gravitational effects when these are strong. Thus, any theory that properly handles intense gravitational effects in small regions of space must include both gravity and quantum mechanics.

As for how this theory would relate to the current theories of quantum mechanics and general relativity? To be consistent with the many experiments that have already been done and support these theories, it must reduce to GR as quantum effects become less significant, and to QM as gravitational effects become less significant. This is analogous to the way that special relativity reduces to Newtonian physics at low speeds where relativistic effects are negligible.

 

[Tio Barnabe]

I see I think.

Any chance of one day we come up with a totally different theory for the interactions in our universe? I mean, a theory that has nothing to do with quantum theory, but that still predicts the same things as quantum theory predicts? Maybe a new family of theories where it would be easier to include interactions with gravity? Or would such new theories be not worth of pursuing?

 

[Nugatory]

Any chance of one day we come up with a totally different theory for the interactions in our universe? I mean, a theory that has nothing to do with quantum theory, but that still predicts the same things as quantum theory predicts? Maybe a new family of theories where it would be easier to include interactions with gravity? Or would such new theories be not worth of pursuing?

It's possible. Of course this hypothetical new theory must have the property that it agrees with GR and QM everywhere that those theories are known to work (that is, under all conditions except very small scales and very strong gravitational effects); if it didn't then it wouldn't agree with the experiments that have already been done and support those theories.

However, in the absence of a candidate theory this discussion is just idle speculation.

 

[jerromyjon]

What leads scientists to believe that ultimately all the interactions must be subjected to quantisation?

Wishful thinking? It simply follows from logical deduction that if you break everything down into its constituent pieces and you add all the values of the pieces together you get a total that agrees with reality. It works in many areas of physics in many ranges of applicability but not always.

Would that be because we describe all other interactions in quantum theory and so we must include gravity on this framework if we want to have a theory describing interactions between the different type of "forces"?

Gravity is the "odd one out", the other three forces (EM, weak and strong nuclear) all fit together rather well in the standard model.

Maybe a new family of theories where it would be easier to include interactions with gravity? Or would such new theories be not worth of pursuing?

I think it could be fantastic and yield many new insights about our amazing universe! I doubt it would be easier, though. Just as an example you can take the center of mass of an object and describe it's motion quite simply using Newtonian mechanics or you can try to model billions or trillions of atoms and all their motions to get a slightly more accurate description... If all you care about is where and when something will land Newton is your man, if you want to know why you have to dig deeper.

 

[Tio Barnabe]

if you break everything down into its constituent pieces and you add all the values of the pieces together you get a total that agrees with reality.

Carefully. That's not the definition of quantisation. It rather has a precise mathematical meaning.

I think it could be fantastic and yield many new insights about our amazing universe! I doubt it would be easier, though. Just as an example you can take the center of mass of an object and describe it's motion quite simply using Newtonian mechanics or you can try to model billions or trillions of atoms and all their motions to get a slightly more accurate description... If all you care about is where and when something will land Newton is your man, if you want to know why you have to dig deeper.

It would amazing and I agree with you.

 

[jerromyjon]

Carefully. That's not the definition of quantisation. It rather has a precise mathematical meaning.

I wasn't attempting to alter the definition. I was simply stating my simplified view of the problem. When you look at the BH information paradox as a prime example, gravity and quantisation don't play well together. Relativity makes quantum physics appear incomplete, or is it the other way around?

 

[Tio Barnabe]

I wasn't attempting to alter the definition. I was simply stating my simplified view of the problem.

Ok. I was not trying to defy your statement, just alerting you that that's not the most official way quantisation is defined.

Relativity makes quantum physics appear incomplete, or is it the other way around?

That's true, but I think we have to use one theory at each time. I think it doesn't make sense to use a theory to analyse a given situation, discovering that a certain phenomenum is predicted by the theory, and then trying to incorporate another theory to analyse that same phenomenum, which is predicted only by the original theory. It simply doesn't make sense to me.

 

[jerromyjon]

discovering that a certain phenomenum is predicted by the theory, and then trying to incorporate another theory to analyse that same phenomenum, which is predicted only by the original theory. It simply doesn't make sense to me.

It hasn't worked out very well, so far, so it makes sense that it doesn't make sense.
I'm thoroughly convinced that one if not both theories have to be scrapped and a more direct, unified approach needs to be envisioned. That could be my naive, non-mathematical intuition talking but simulations never seem to go the same path as nature...

 

[atyy]

There is already a way of quantizing gravity, just like there is a way of quantizing classical electrodynamics. In both quantum general relativity and quantum electrodynamics, the theories are effective quantum field theories, meaning that they are valid at lower energies (which is all that is relevant for current experiments) but not at much higher energies.

 

[star apple]

I want to ask something related to this. Our QFT at present is not background independent. We used the technique where you need to take backreaction into account. Is this a separate thing to quantum gravity?.. the main purpose of quantum gravity being to compute for strong gravity at the Planck scale or singularities.. but if you just want to create background independent QFT.. is this also called quantum gravity?

 

[jerromyjon]

There is already a way of quantizing gravity

I've been following loop quantum gravity for quite some time, still like you say there are limits. I'm not sure of what or why it breaks but at some point it still is not an all-inclusive theory.

 

[jerromyjon]

Our QFT at present is not background independent.

Of course, we can't fathom a universal wave function...

 

[Tio Barnabe]

they are valid at lower energies [...] but not at much higher energies

Would it has to do with renormalization?

 

[jerromyjon]

Would it has to do with renormalization?

If it were, with supercomputers they could "realign" the parameters to make sense of it...