Shape Dynamics: How Can It Offer Same Predictions as GR & SR?

[lindberg]

TL;DR Summary

How does shape dynamics reproduce the predictions of special relativity?

Developers of Shape Dynamics (as Julian Barbour and Tim Koslowski) claim that Shape Dynamics will offer in most cases the same predictions as GR, and implicitly as SR. How can this possibly be true, since they get rid of time altogether and thus of any notion of simultaneity?
How would they get the reciprocity we encounter in SR to appear in their theory?
Thank you.

 

[PeroK]

Summary: How does shape dynamics reproduce the predictions of special relativity?

Developers of Shape Dynamics (as Julian Barbour and Tim Koslowski) claim that Shape Dynamics will offer in most cases the same predictions as GR, and implicitly as SR. How can this possibly be true, since they get rid of time altogether and thus of any notion of simultaneity?

There is no physical thing called simultaneity. That's the irony. People spend all that time digesting the relativity of simultaneity,when in fact it's not a thing.

There was a thread on here a little while ago debating whether it might be better to teach SR without mentioning simultaneity at all!

 

[Orodruin]

There was a thread on here a little while ago debating whether it might be better to teach SR without mentioning simultaneity at all!

https://www.physicsforums.com/threads/teaching-sr-without-simultaneity.1011051/

any notion of simultaneity?

Simultaneity in special relativity is purely conventional and a matter of definition. The entire point of the relativity of simultaneity is that the typical convention results in different notions of simultaneity for different inertial frames. It is not really saying anything physical.

 

[lindberg]

@Orodruin thanks for the link! I was not aware of it. I honestly always (well, at least since I've discovered the basics of SR) took it as a rule that the notion of simultaneity has to be thrown away by the window to get SR.

 

[Orodruin]

@Orodruin thanks for the link! I was not aware of it. I honestly always (well, at least since I've discovered the basics of SR) took it as a rule that the notion of simultaneity has to be thrown away by the window to get SR.

This was the point I was making in that thread. Simultaneity does lose much of its meaning in SR, yet the relativity of simultaneity as typically defined in SR does persist in how we tend to treat it in our courses.

 

[Dale]

That said, while I haven't read Barbour's shape dynamics stuff, the stuff that I have read from Barbour was a big waste of time. If it is not a dramatic departure from his earlier work then I wouldn't spend much effort trying to learn it or reconcile it to anything else.

 

[lindberg]

That said, while I haven't read Barbour's shape dynamics stuff, the stuff that I have read from Barbour was a big waste of time. If it is not a dramatic departure from his earlier work then I wouldn't spend much effort trying to learn it or reconcile it to anything else.

Why was it a waste of time? What did you find wrong about his ideas?
I have read that he gets accused of doing mathematical philosophy, not physics, since what he suggests does not give any new predictions (except for black holes being replaced by "breathers" that would behave initially like black holes, but spit out light and matter at some point; this point might take longer to achieve than the existence of the universe... which on the testability scale is... meh).

 

[lindberg]

This was the point I was making in that thread. Simultaneity does lose much of its meaning in SR, yet the relativity of simultaneity as typically defined in SR does persist in how we tend to treat it in our courses.

To be honest, if there is anything I have understood about SR it is the essential role of relativity of simultaneity, which is responsible for both length contraction and time dilation. Now to quote the description of Mercati's book on the topic:
"The most important feature of this theory is the replacement of relativity of simultaneity with a more tractable gauge symmetry, namely invariance under spatial conformal transformations. "

Now I have to admit I have not read the book yet, I should purchase it. But how the hell could someone possibly do such a thing? I will need to look at the recommended thread, maybe I will find an answer there. But I need to admit I cannot see how the empirical success of SR would be reproduced by Shape Dynamics, without falling back into the long-exorcised Lorentzian relativity with clocks running slow and rulers getting shorter...

https://oxford.universitypressschol.../oso/9780198789475.001.0001/oso-9780198789475

 

[Orodruin]

To be honest, if there is anything I have understood about SR it is the essential role of relativity of simultaneity, which is responsible for both length contraction and time dilation.

Except that time dilation and length contraction are not actually central elements of the theory (as they are the direct results of an arbitrary convention). They are also generally misunderstood concepts whose misapplication is responsible for most SR ”paradoxes”.

The central point is spacetime geometry.

 

[Dale]

Why was it a waste of time? What did you find wrong about his ideas?

It wasn’t that they were wrong, they were useless. He kept talking about getting rid of time but all he did was replace all of the $t$ variables with $\theta$ variables that did the exact same thing as $t$ in all physics formulas and experiments but have a different philosophical connotation. Waste… of… time…

I have read that he gets accused of doing mathematical philosophy, not physics, since what he suggests does not give any new predictions

Yes. That is my view, at least of his older stuff. Not only did he not give any new predictions, he didn’t even do any interesting math. Substituting variables is unimpressive.

 

[lindberg]

@Dale I think I get what you mean. His work belongs more to metaphysics than physics, since he merely gives a new interpretation to whatever SR and GR have already brought.
Well, apparently he has some other physicists working with him (Sean Gryb, Tim Koslowski, Henrique Gomes, etc.), so maybe over time shape dynamics will evolve differently. I guess the stalemate quantum gravity is encountering does push people into exploring different paths. Which is... why not?

 

[Orodruin]

The point is that if it makes no different predictions then it is not a different path. Just a reparametrisation of the same path. This may be worthwhile if it leads to new insights, which I am not certain this does.

 

[robphy]

I am unfamiliar with Shape Dynamics.
However, it appears to be another possible route toward a quantum theory of gravitation.
( see https://en.wikipedia.org/wiki/Quantum_gravity#Candidate_theories )

soapbox:

Following the Correspondence Principle, one would expect each candidate theory to​

reproduce much of the known experimental results​

while making a new prediction that makes some headway​

toward a successful formulation of quantum gravity.​

​

Since quantum gravity has been stuck for a while (due to limited experimental guidance),​

theorists try to make some progress.​

​

Each candidate theory explores the implications of​

holding sacred some declared set of fundamental concepts and principles,​

while weakening or generalizing others.​

​

Some are more popular, more well-funded, more respected, etc... than others.​

​

Because of this, some ideas are more developed than others,​

where "development" could mean progress toward success,​

as well as progress in realizing that an approach may be wrong or too difficult​

and that one must back up a few steps and try something else.​

​

(My dissertation involved aspects of one of the Other Theories.)​

Following your link to "Shape Dynamics: Relativity and Relationalism" by Flavio Mercati,
the chapter entitled "Relativity Without Relativity" (a title in the spirit of JA Wheeler)
https://oxford.universitypressschol...98789475.001.0001/oso-9780198789475-chapter-7
suggested to me to google it.
It led to
Relativity without relativity (2002)
Julian Barbour, Brendan Foster, Niall Ó Murchadha
https://arxiv.org/abs/gr-qc/0012089
The answer to your question may be implicit in there... but it's a rather technical presentation.

Another type of resource to seek out are dissertations.
Here's one I found by googling "Shape Dynamics"
https://www.imperial.ac.uk/media/im...tations/2020/Joshua-Hoffmann-Dissertation.pdf
The introduction of dissertations might be helpful.
The chapter title "Deriving Local Lorentz Invariance" caught my eye... but the chapter is rather involved.

One can also try to find seminars (say, on YouTube or on physics-department websites).
Here's one that seemed promising
Tim A. Koslowski (New Brunswick) gives a talk at the Mini-Workshop on the Foundations of Shape Dynamics (23 June, 2014) titled "Shape Dynamics". ( &t=19m46s looked promising... but it wasn't clear to me)

I have nothing more to offer
other than these strategies for finding answers to your questions.

Good luck.

 

[lindberg]

@robphy thanks a lot for such a detailed answer and for having taken the time to look into the topic!

 

[lindberg]

@Dale and @Orodruin thanks a lot, guys.