# Are we wrong to try and unify quantum mechanics and relativity?

## Main Question or Discussion Point

Hi,

We're still seeking a satisfactory way to unify quantum mechanics and general relativity together, correct? Why do physicists make the assumption that there is one set of rules governing everything? Is it because that's what we tend to see in nature? Or because it's just a nice idea? Is it a desire, an expectation or a prediction? What if both sets of rules are correct and cannot be combined? In day to day life for example it's quite possible to have two sets of rules that don't overlap. The rules of football and poker for example. They both work and they both explain to an observer what's going on in a game of football and a game of poker, but unification of the two sets of rules is just the wrong way of thinking about it.

Thoughts? Thanks.

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dextercioby
Homework Helper
For the record, special relativity and quantum mechanics have been 'unified' since the end of the 1920s. The problem is with general relativity.

Thanks, yes. Post edited.

mfb
Mentor
We know there are situations where both are relevant at the same time - notably black holes, (theoretical) ultra-high-energetic particle collisions and probably the big bang. They have to work together in some way, we just don't know how.

If you try to start a poker session in a football match, something will happen, and you need rules that go beyond the two separate cases to describe it.

stevendaryl
Staff Emeritus
Hi,

We're still seeking a satisfactory way to unify quantum mechanics and general relativity together, correct? Why do physicists make the assumption that there is one set of rules governing everything? Is it because that's what we tend to see in nature? Or because it's just a nice idea? Is it a desire, an expectation or a prediction? What if both sets of rules are correct and cannot be combined? In day to day life for example it's quite possible to have two sets of rules that don't overlap. The rules of football and poker for example. They both work and they both explain to an observer what's going on in a game of football and a game of poker, but unification of the two sets of rules is just the wrong way of thinking about it.

Thoughts? Thanks.
Well, GR says that gravity is spacetime curvature due to the presence of matter. QM says that matter is described by a wave equation. So figuring the effect of matter on gravity in detail would certainly involve both GR and QM. So at some level, there has to be a unification. In practice, unification doesn't matter very much because at the microscopic level (particle, atoms and molecules), the effects of GR are negligible, and at the macroscopic level (stars, galaxies), the effects of QM are negligible. But in theory, both apply at all times.

bhobba
Mentor
Hi,

We're still seeking a satisfactory way to unify quantum mechanics and general relativity together, correct? Why do physicists make the assumption that there is one set of rules governing everything?.
Not all physicists think there is one set of rules governing everything. I believe there is because I suspect there is some startling symmetry waiting for us to discover. But that is just opinion. Nature may be such as you peel away each layer there is another layer.

But regards to General Relativity and Quantum Mechanics the situation is more subtle thatn popularisations would have you believe:
http://arxiv.org/abs/1209.3511

Thanks
Bill

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WannabeNewton
Hi,
Why do physicists make the assumption that there is one set of rules governing everything?
Because we can easily quantize general relativity in the low energy regime, wherein the metric tensor is just a spin 2 tensor field propagating on flat space-time, and compute tree-level Feynman diagrams and loop corrections for graviton interactions just like with photons in electromagnetism with no issues whatsoever. There must exist some high energy theory of quantum gravity which reduces to this low energy 'effective field theory' of gravitons.

That being said, there do exist arguments for why a quantization of GR is not necessarily the right way to go. Most of these arguments rely on showing in one sense or another that GR is an emergent theory much like statistical thermodynamics. C.f. http://arxiv.org/abs/gr-qc/9504004

DaveC426913
Gold Member
...it's quite possible to have two sets of rules that don't overlap.
But they do overlap.

Where would you draw the line between them? Atoms? Molecules? Cells? Volvos?

The rules of football and poker for example. They both work and they both explain to an observer what's going on in a game of football and a game of poker, but unification of the two sets of rules is just the wrong way of thinking about it.
But we can play a game of poker on a football field. Now what rules will you use?

But we can play a game of poker on a football field. Now what rules will you use?
I think I understand what you mean, and yes, ultimately the playing field (atoms, laws of nature etc) is the same (being equivalent to the laws of QM and general relativity applying in the same universe) but the rules differentiating and describing the two games are incompatible. Being offside means nothing in the game of poker. We can make new rules, to integrate the two games, but as they stand the rules themselves clash—even though the ball and the pack of cards are made of atoms. The rules sit above the 'stuff' governed by those rules. Does that make sense?

mfb
Mentor
The rules sit above the 'stuff' governed by those rules.
And that tells you they are not the fundamental rules governing the games. There has to be something more fundamental, some set of rules that allows to describe what a poker table on a football field would lead to.

DaveC426913
Gold Member
Yes, mfb has hit it on the head.

Our current rules are incompatible. Since the universe obviously does work with both QM and GR, we know that the rules we currently have are incomplete.

It is important that you recognize that there are places in our universe where both QM and GR do come into play at the same time. Our current rules for QM and GR do not cover those scenarios. We call it a singularity - meaning, literally, that our rules stop working.

And that tells you they are not the fundamental rules governing the games. There has to be something more fundamental, some set of rules that allows to describe what a poker table on a football field would lead to.
But even if we did find one set of rules governing the universe, it is still possible to have other sets of rules (society laws, poker, football, etc) that work within that universe, and only overlap in the sense that the stuff governed by the second set of laws is ultimately governed by the fundamental set of laws. What if QM is the most fundamental set of rules for the universe, and GR is another set of rules that that sits on top of those rules.

It is important that you recognize that there are places in our universe where both QM and GR do come into play at the same time.
Just as there are in poker and football ... the cards physically move around the table according to the same fundamental laws governing the physical movement of the ball. It seems to me that two sets of laws can exist which overlap in some areas, but not others. Both are right, even though they cannot be entirely unified.

DaveC426913
Gold Member
What if QM is the most fundamental set of rules for the universe, and GR is another set of rules that that sits on top of those rules.
That would be just fine. That's still one set of rules that explains both the quantum and cosmological universes.

That would be just fine. That's still one set of rules that explains both the quantum and cosmological universes.
My point exactly. It works and it's fine ... so why assume we can unify them any further?

DaveC426913
Gold Member
My point exactly. It works and it's fine ... so why assume we can unify them any further?
Because they are currently inadequate to describe our universe! We don't have a model that fully describes what we see.

We don't have a model that fully describes what we see.
Correct me if I'm wrong—we don't have one, but we do have two models (QM AND GR) which describe things very precisely at different scales, each of which is incomplete, but both of which may turn out to be correct and complete—aside from the fact that they are not unified. What I'm trying (perhaps not very well) to say is that aren't we imposing our own expectations on nature by trying to find one complete set of all encompassing laws to describe the universe, when, actually, two sets of laws may be the answer—as bizarre and counter intuitive as it may seem (remembering that bizarre and counter intuitive arguably describes QM, which is well established and verified).

To put it another way, what if all these problems were solved individually:

http://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics

... and the resulting outcome was two sets of laws. We could either conclude that we seriously messed up somewhere along the line and go back to the drawing board, or we could accept that nature is just plain weird.

stevendaryl
Staff Emeritus
Correct me if I'm wrong—we don't have one, but we do have two models (QM AND GR) which describe things very precisely at different scales, each of which is incomplete, but both of which may turn out to be correct and complete—aside from the fact that they are not unified.
If they aren't unified, then they can't be complete. Consider two identical particles of mass $M$ and charge $Q$. If we ignored quantum mechanics, we could describe their interaction using GR and electromagnetism. If we ignored gravity, we could describe their interaction using QM. So we have two descriptions of their interactions, and those two descriptions can't possibly both be right. Presumably, their actual interaction involves both quantum mechanics and gravity, so neither theory by itself is correct.

So we have two descriptions of their interactions, and those two descriptions can't possibly both be right.
Why not? Simply because that's counterintuitive?

stevendaryl
Staff Emeritus
Why not? Simply because that's counterintuitive?
If two theories make contradictory predictions, then one or the other prediction is wrong. It's not just not counter-intuitive.

If two theories make contradictory predictions
Is this the case? Are there any examples you could quote? That may answer my question in some ways, although I still feel my reasoning stands.

stevendaryl
Staff Emeritus
Is this the case? Are there any examples you could quote? That may answer my question in some ways, although I still feel my reasoning stands.
I gave you a specific example. What happens if you have two identical particles of mass M and charge Q and release them near each other? QM predicts that they repel each other (reflected in the fact that the probability of the particles being found close together gets smaller and smaller as time increases). GR predicts that if the masses are great enough, then they will be attracted to each other. Those are contradictory predictions.