# B Professional debates about Spacetime

#### PeterDonis

Mentor
in LET, the physical mechanism of time dilation is due to the ether
No, it's not, because "being at rest relative to the ether" only maximizes your proper time in a very special set of cases (and which cases they are is in principle unobservable). The ether cannot explain why proper time is ever maximized for an observer who is not at rest relative to the ether--or, since we don't know which inertial frame is actually the ether rest frame, it is better to say that the ether cannot explain why proper time is ever maximized for two different observers who are in relative motion (it's maximized for each observer between a different pair of spacetime events, but that's how it works--you can't just decree that everyone has to only travel between some fixed pair of events).

Geometry is how spacetime produces time dilation. There is no other physical mechanism.
Yes, and as I said, this is true for any interpretation of relativity.

Wave function is how matter produces observable. There is no other physical mechanism.
But this is only true in some interpretations of QM, not all of them.

#### vanhees71

Gold Member
What different interpretations are there of spacetime, corresponding to the different interpretations of QM?
Well, if you read the philosophical literature of that there's an entire plethora of different interpretations. Of course that's all pretty irrelevant for the physics case. Fortunately spacetime is not so much under public debate as quantum theory, where people much more than in the case of space think that some philocophical issues are of relevance in physics. ;-)).

#### lucas_

No, it's not, because "being at rest relative to the ether" only maximizes your proper time in a very special set of cases (and which cases they are is in principle unobservable). The ether cannot explain why proper time is ever maximized for an observer who is not at rest relative to the ether--or, since we don't know which inertial frame is actually the ether rest frame, it is better to say that the ether cannot explain why proper time is ever maximized for two different observers who are in relative motion (it's maximized for each observer between a different pair of spacetime events, but that's how it works--you can't just decree that everyone has to only travel between some fixed pair of events).

Yes, and as I said, this is true for any interpretation of relativity.

But this is only true in some interpretations of QM, not all of them.
Since we focus on the minimal interpretations. This explains why we have a hard time coming up with quantum gravity.

For example. Imagine the waves in the water in the beach. Let's say some spots of the waves were the observables in QM. And curvature of the beach was like the manifolds of spacetime. And scientists trying to figure out how to relate the curvature of the beach to some spots of the waves in the ocean. I guess this is our attempt at quantum gravity now?

This is what I meant trying to come up with ways how to view QM and Relativity from different perspective. Just to feel the problem with coming up with quantum gravity. Do you think minimal interpretations can solve quantum gravity? What are the main problems of QG in your opinion?

About LET. We can't distinguish LET by experiments and figure out the ether frame. But who knows if quantum particles can use the ether frame for Bell-like correlations. If Bell's Theorem didn't prove either nonreality or locality wrong. We could junk LET. But with Bell's Theorem and Aspect experiments and all those tests producing positive results. We may need the ether. Hope someone can collect all the important threads about it into some kind of FAQ so readers can just read the FAQ. I understand why it's not allowed for discussions and I won't be discussing if further.

#### PeterDonis

Mentor
I guess this is our attempt at quantum gravity now?
What you describe doesn't seem to bear much resemblance to either string theory or loop quantum gravity, which are the two main contenders right now.

Do you think minimal interpretations can solve quantum gravity?
I don't think the question of how to solve quantum gravity has much, if anything, to do with the question of interpretations of QM.

What are the main problems of QG in your opinion?
I see two:

(1) Quantum mechanics and General Relativity seem almost incompatible, so it's really hard to imagine a theory that somehow contains both of them as approximations in appropriate limits.

(2) Gravity is really, really weak to begin with, and any quantum aspects of gravity are much weaker still, so it's really hard to imagine how any putative theory of quantum gravity could make any predictions, over and above the predictions we already have from relativity and quantum mechanics in their respective domains, that could be tested now or in the foreseeable future. It's really hard to develop a theory in the total absence of experimental clues.

We can't distinguish LET by experiments and figure out the ether frame. But who knows if quantum particles can use the ether frame for Bell-like correlations.
This is the sort of speculation that PF's policy on LET prohibits.

I won't be discussing if further.
Ok.

#### lucas_

What you describe doesn't seem to bear much resemblance to either string theory or loop quantum gravity, which are the two main contenders right now.

I don't think the question of how to solve quantum gravity has much, if anything, to do with the question of interpretations of QM.

I see two:

(1) Quantum mechanics and General Relativity seem almost incompatible, so it's really hard to imagine a theory that somehow contains both of them as approximations in appropriate limits.

(2) Gravity is really, really weak to begin with, and any quantum aspects of gravity are much weaker still, so it's really hard to imagine how any putative theory of quantum gravity could make any predictions, over and above the predictions we already have from relativity and quantum mechanics in their respective domains, that could be tested now or in the foreseeable future. It's really hard to develop a theory in the total absence of experimental clues.
Can you really assume SR, GR as approximately described by Newtonian mechanics, and then include higher-order non-Newtonian correction terms in a power series in $\frac{1}{c^2}$?

"For empirical purposes, there is no need for General Relativity, for example. Or Special Relativity, for that matter. You can just (as is done in the post-Newtonian expansion) assume that physics is approximately described by Newtonian mechanics, and then include higher-order non-Newtonian correction terms in a power series in $\frac{1}{c^2}$. Let the terms in that expansion be determined experimentally. There is no need for a theory such as General Relativity that attempts to understand the differences in terms of a concept of curved spacetime."

#### PeterDonis

Mentor
Can you really assume SR, GR as approximately described by Newtonian mechanics, and then include higher-order non-Newtonian correction terms in a power series in $\frac{1}{c^2}$?
For many purposes, yes. This is called the post-Newtonian approximation:

A good discussion of this approach, and its "unreasonable effectiveness", is in this paper by Will:

The "unreasonable effectiveness" part includes the fact that this method works pretty well for predicting the gravitational wave signatures from black hole mergers, even though a black hole is a case which, intuitively, should not be treatable by this method since the region at and inside the horizon cannot be viewed as simply being a perturbation of flat spacetime (which is the underlying rationale of the approach in the first place).

However, this method does not work for cosmology, which suggests that the real limitation of the approach is not being able to view gravity as a perturbation of flat spacetime, but being able to view it as associated with isolated massive objects separated by large regions of empty space. The universe, globally, cannot be described that way, because the universe as a whole is not an "isolated object" in this sense. So there is at least one domain in which viewing gravity as just Newtonian plus correction terms does not appear to work and a different approach is needed.

#### Dale

Mentor
But with Bell's Theorem and Aspect experiments and all those tests producing positive results. We may need the ether.
Absolutely, unequivocally, 100% no. We may use the ether if we wish, but we do not in any way need it.

#### lucas_

For many purposes, yes. This is called the post-Newtonian approximation:

A good discussion of this approach, and its "unreasonable effectiveness", is in this paper by Will:

The "unreasonable effectiveness" part includes the fact that this method works pretty well for predicting the gravitational wave signatures from black hole mergers, even though a black hole is a case which, intuitively, should not be treatable by this method since the region at and inside the horizon cannot be viewed as simply being a perturbation of flat spacetime (which is the underlying rationale of the approach in the first place).

However, this method does not work for cosmology, which suggests that the real limitation of the approach is not being able to view gravity as a perturbation of flat spacetime, but being able to view it as associated with isolated massive objects separated by large regions of empty space. The universe, globally, cannot be described that way, because the universe as a whole is not an "isolated object" in this sense. So there is at least one domain in which viewing gravity as just Newtonian plus correction terms does not appear to work and a different approach is needed.
In Hossenfelder peer reviewed blog site.. there was this interview with the father of loop quantum gravity... https://backreaction.blogspot.com/2019/06/a-conversation-with-lee-smolin-about.html

Or you could have heard from it in the general field about Barbour treating space as fundamental while Smolin treating Time as fundamental and space emergent.

I just want to know what interpretation does this emergent or fundamental time or space thing fall under, and why:

1. Minimalist
2. LET
3. Block Universe
4. New interpretation?

#### PeterDonis

Mentor
I just want to know what interpretation does this emergent or fundamental time or space thing fall under
And I already responded to that:

I don't think the question of how to solve quantum gravity has much, if anything, to do with the question of interpretations of QM.

#### lucas_

And I already responded to that:
I wasn't asking what interpretation emergent (or fundamental) time or emergent (or fundamental) space fall under in quantum mechanics but in relativistic interpretation like Minimalist, LET or Block Universe.

You mentioned: "
I don't think the question of how to solve quantum gravity has much, if anything, to do with the question of interpretations of QM."

#### PeterDonis

Mentor
I wasn't asking what interpretation emergent (or fundamental) time or emergent (or fundamental) space fall under in quantum mechanics but in relativistic interpretation like Minimalist, LET or Block Universe.
Quantum gravity has nothing to do with interpretations of relativity either.

#### lucas_

Quantum gravity has nothing to do with interpretations of relativity either.
Smolin was NOT talking about quantum gravity. Just that time may be fundamental and space emergent. So must it be applied to minimalist where spacetime geometry cause observations, or LET or Block Universe. I can't fit it to any.

#### PeterDonis

Mentor
Smolin was NOT talking about quantum gravity. Just that time may be fundamental and space emergent.
Which is purely speculation on his part, for the purpose of helping him decide where to focus his theoretical efforts. It is not a theory.

So must it be applied to minimalist where spacetime geometry cause observations, or LET or Block Universe. I can't fit it to any.
You'll have to ask Smolin. It's his speculation.

#### lucas_

Which is purely speculation on his part, for the purpose of helping him decide where to focus his theoretical efforts. It is not a theory.

You'll have to ask Smolin. It's his speculation.
This is his context (I don't think he would reply to my email and I don't know his email):

"
The combination of a fundamental time and an emergent space implies that there may be a fundamental simultaneity. At a deeper level, in which space disappears but time persists, a universal meaning can be given to the concept of now. If time is more fundamental than space, then during the primordial stage, in which space is dissolved into a network of relations, time is global and universal. Relationalism, in the form in which time is real and space is emergent, is the resolution of the conflict between realism and relativity."

I just want to know whether to use minimalist, LET or block universe as frame of reference to understand it. In minimalist where geometry creates observations. It is agnostic whether time or space is fundamental or emergent or does minimalist interpretation use both emergent time and space? in LET and block universe, both time and space are fundamental or emergent. Or better yet.

In each of of the following. Is time emergent or fundamental, how about space?

1. Minimalist time is ( )emergent or ( )fundamental
space is ( )emergent or ( ) fundamental

1. LET time is ( )emergent or ( )fundamental
space is ( )emergent or ( ) fundamental

3. Block Univ. time is ( )emergent or ( )fundamental
space is ( )emergent or ( ) fundamental

Please check the blanks whichever applies.

Thank you.

#### PeterDonis

Mentor
In each of of the following. Is time emergent or fundamental, how about space?
I guess I didn't make myself clear. This is Smolin's speculation, so nobody else except Smolin could possibly answer your questions.

And therefore there is no point in continuing this thread, and it is closed.

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