# B Why does "gravity" need to be unified?

1. Aug 29, 2017

### thetexan

If I understand space time correctly Gravity is not a force acting on a body but rather the effect of a mass on space time causing a distortion which effects the movement of another mass through that space time.

If Gravity is not an actual force then there are only three forces left to be unified, electro, strong and weak. And hasn't that essentially been accomplished already?

tex

2. Aug 30, 2017

### ISamson

3. Aug 30, 2017

### Staff: Mentor

We have no idea what happens if gravity and quantum mechanics are relevant at the same time. Our existing theories just fail to make predictions. There has to be something new, and it is expected that the right answer leads to a unification of the interactions for high-energetic processes.
We don't know the right way to combine the strong interaction with the electroweak interaction either, but that is an easier problem.

4. Aug 30, 2017

### kodama

is it possible that nature does not combine the strong interaction with the electroweak interaction or does this lead to a theoretical difficulty?

5. Aug 30, 2017

### phyzguy

I would look at it by saying that we know that GR can't be the whole story. GR predicts that the matter inside a black hole falls into a region with zero radius and infinite density, which is non-physical. We suspect that at the very high densities which result inside of a black hole, quantum mechanics must modify GR in such a way that the densities stay finite. When GR and QM are combined, we suspect that the other known forces will be part of the same theory. But today, nobody knows how to combine these things.

6. Aug 30, 2017

### kodama

QFT also predicts elementary particles are zero radius

7. Aug 30, 2017

### Staff: Mentor

They don't lead to singularities, however, as they cannot be localized with infinite precision.
That is the point. GR doesn't have that feature, quantum mechanics has it.

8. Aug 31, 2017

### dextercioby

I presume your comment comes from the fact that there was failed work in the 1970s over the so-called GUTs in which a compact gauge group was proposed to replace the gauge group of the Standard Model, so I am thinking this GUT idea has to fail somehow even without taking gravity into consideration. People think: oh, gravity must be unified with the other three interactions, but the real question should be: why do we need this when the Standard Model works 90-95% fine and does not unify chromodynamics to electroweak?

9. Aug 31, 2017

### Staff: Mentor

It works fine in the regions we can access experimentally in the lab. We would like to have a theory that also works everywhere else, like inside black holes or in the very early universe.

10. Aug 31, 2017

### nikkkom

(1) SM _did_ unify EM and weak forces. So unification sometimes does happen.
(2) "Unifying" gravity with QFT is a _different type_ of "unification" that expanding gauge group G1×G2 to a larger group. It's about having a quantum theory of gravity. Because quantum theories have fewer problems than classical ones when UV limit is taken. (Classical theories, in general, give predictions which were demonstrated to outright not match experiments, although finding such experiment specifically for gravity would be tough).

11. Aug 31, 2017

### romsofia

Well, one reason we need it is because physicsts nowadays love trying to picture the universe as quantum fields. We have 3 options: Quantum fields emerge from spacetime, which would make the argument that spacetime is more fundamental than the quantum fields, and thus wouldn't fit into the standard model (no need for force carrier). Spacetime emerges from quantum fields, which would imply that gravitons "create" spacetime (analogous to how photons "create" the electric and magnetic fields), so it would fit into the standard model. Last option would be that spacetime IS a quantum field, and once again we need some sort of force carrier (graviton) to communicate interactions (similar to other quantum fields), so we would see it pop up in the standard model. The way I see it, 2 out of 3 options have gravity going into the standard model.

You can find a lot of papers showing that spacetime emerges from quantum fields, but I don't really follow the logic behind this. I believe that spacetime is more fundamental than quantum fields purely based off the reasoning that it affects everything and has infinite range. As opposed to the weak, E+M, or strong force which are limited in their interactions, gravity is not. So it only make sense that gravity would be more fundamental.

But, what I think doesn't really matter if I, or someone else, can't show it. But to show it, really isn't good enough anymore. We have papers that show all 3 (effective field theory deserves it's own recognition here), but some more than others. I think spacetime being an effective field theory is "hot" right now, followed by spacetime emerging from quantum field, followed by spacetime being a quantum field, and the least popular would be spacetime is more fundamental than quantum fields.

12. Aug 31, 2017

### ohwilleke

Going from the other direction, the Standard Model is formulated in a way that respects special relativity, but not general relativity. But, general relativity makes true predictions that differ from a world without general relativity. Therefore, the Standard Model is not fully accurate unless it can be formulated in a manner that incorporates general relativistic phenomena.

Fortunately, that doesn't have a lot of practical implications for calculations because gravity is weak relative to SM forces at the scales where the SM is applied to experiments. But, there is a fundamental theoretical inconsistency between and incompatibility of the two theories so SM + GR can't be a complete description of nature.

13. Sep 5, 2017

### trainman2001

If gravity is not a force, how does it create gravity waves that seem to move at C. Sort of seems like a force to me.

14. Sep 5, 2017

### nikkkom

What is that fundamental inconsistency?
I only know that SM was proven to be renormalizable, but "simple" quantum gravity (adding spin-2 graviton to SM) is not renormalizable - it is only an effective QFT.
However, for practical purpose of calculating quantum corrections, effective QFTs are perfectly usable. (Which in case of gravity, gives such an incredibly small number that it's not experimentally testable. Pity).

15. Sep 5, 2017

### ohwilleke

The fundamental inconsistency is that Standard Model is formulated in Minkowski space. But, general relativity must be formulated using Riemannian_geometry in a manner that the SM does not. Space-time has different properties in the SM and in GR.

16. Sep 5, 2017

### ohwilleke

Gravity is a force. It can do work. The fact that it can be formulated geometrically (as in GR) as well as or instead of as a QFT in which a non-contact force is transmitted by a carrier boson (the graviton) is not a contradiction. It is two ways of explaining the same thing, just as many thing in physics can be explained by algebraically and geometrically.

17. Sep 5, 2017

### nikkkom

Hot is this a "fundamental" inconsistency? Add terms to Lagrangian where metric tensor couples to other fields, and you have an effective QFT for gravity. For example, this old paper:

https://arxiv.org/pdf/gr-qc/9405057.pdf

"how we can do this" was never a problem. The problem is that the result is non-renormalizable. It is "only" an effective theory.

18. Sep 5, 2017

### Canis Lupus

Are gravity waves gravity though?

19. Sep 6, 2017

### romsofia

If renormalizable was the only issue, the problem would've been solved, and example from the 70s: https://journals.aps.org/prd/pdf/10.1103/PhysRevD.16.953

I'm not sure what you mean by "how can we do this" not being a problem though, so I won't comment on that.

An article that i'm currently going through that would be relevant to this thread is: https://arxiv.org/pdf/0907.4238.pdf

Last edited: Sep 6, 2017
20. Sep 6, 2017

### atyy

Gravity is a force, just like the the others. To get a complete theory of quantum gravity, we do not know whether gravity has to be unifed with the other forces, or not.

String theory tries to get quantum gravity by unifying gravity with the other forces.

Asymptotic safety tries to get quantum gravity without unifying gravity with the other forces.

21. Sep 6, 2017

### martinbn

Strictly speaking it doesn't predict that. For example what do you mean by zero radius? I am guessing that you have in mind $r=0$, where $r$ is the usual coordinate. But this coordinate cannot, even if you stretch the meaning, be interpreted as a radius. It is not even spacelike, it is timelike. Any statement about zero radius is at the very least inaccurate and misleading.

22. Sep 6, 2017

### Staff: Mentor

23. Sep 6, 2017

### Canis Lupus

Can you briefly, perhaps with a simple reference, provide your understanding of the difference between the two terms "gravitational wave" and "gravity wave". It is seems the two are clearly defined differently. I'd appreciate an understanding of that difference if you have the time.

24. Sep 6, 2017