B Professional debates about Spacetime

[PeterDonis]

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:

https://en.wikipedia.org/wiki/Post-Newtonian_expansion
https://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism
A good discussion of this approach, and its "unreasonable effectiveness", is in this paper by Will:

https://arxiv.org/pdf/1102.5192.pdf
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]

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:

https://en.wikipedia.org/wiki/Post-Newtonian_expansion
https://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism
A good discussion of this approach, and its "unreasonable effectiveness", is in this paper by Will:

https://arxiv.org/pdf/1102.5192.pdf
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]

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."

I wasn't asking about interpretations of QM but relativity.

 

[PeterDonis]

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]

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]

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.