Is spacetime an absolute reference frame?

[chuckset]

STUPID QUESTION ALERT!

Is spacetime an absolute reference frame?

 

[JesseM]

No, because the notions of being "at rest relative to spacetime" or "in motion relative to spacetime" have no physical meaning.

 

[chuckset]

So, is spacetime only used as a benchmark for accelerated motion?

 

[JesseM]

So, is spacetime only used as a benchmark for accelerated motion?

What do you mean by "used as a benchmark"? I don't think it'd be meaningful to say you're accelerating "relative to spacetime" either, though acceleration is absolute (you know you're accelerating because you feel G-forces).

 

[chuckset]

For Newton's thought experiment about two rocks in deep space that will pull taut, with respect to what is it rotating? I'm trying to read Brian Greene's book "The Fabric of the Cosmos" but I'm getting very confused. Forgive me if this is a stupid question.

 

[JesseM]

For Newton's thought experiment about two rocks in deep space that will pull taut, with respect to what is it rotating? I'm trying to read Brian Greene's book "The Fabric of the Cosmos" but I'm getting very confused. Forgive me if this is a stupid question.

I think it would be misleading to say it's rotating relative to spacetime since spacetime doesn't have any landmarks that you can measure your motion relative to. I would probably say it's rotating relative to inertial frames of reference (coordinate systems), frames that can be identified because the equations for the laws of physics take a particular form when stated in terms of their coordinates. But maybe others would have different ways of describing what's going on here.

 

[chuckset]

Ok thanks for your responses and thanks for helping me!

 

[atyy]

I think it would be misleading to say it's rotating relative to spacetime since spacetime doesn't have any landmarks that you can measure your motion relative to. I would probably say it's rotating relative to inertial frames of reference (coordinate systems), frames that can be identified because the equations for the laws of physics take a particular form when stated in terms of their coordinates. But maybe others would have different ways of describing what's going on here.

I agree that it's relative to an inertial frame. I'm not sure exactly what Greene says, but maybe there's an idea that the global inertial frames of special relativity reflect the symmetries of the flat metric - which can be colloquially identified with spacetime, and which in general relativity becomes curved and further identified with the gravitational field?

 

[ManyNames]

Isn't the aether a concept used as an absolute reference frame? In fact, I'm sure Einstein said relativity was unthinkable without one.

Today, we've replaced the word aether for the zero-point energy field, which acts like a quantum aether.

 

[JesseM]

Isn't the aether a concept used as an absolute reference frame? In fact, I'm sure Einstein said relativity was unthinkable without one.

Today, we've replaced the word aether for the zero-point energy field, which acts like a quantum aether.

Einstein only said that spacetime was similar to an aether in the sense that it has physical properties of its own, but in relativity it is unlike the traditional notion of aether in that it doesn't have any rest frame of its own, so it is meaningless to talk about your velocity relative to spacetime (whereas it would make sense to talk about velocity relative to the aether). For example, see the last paragraph of http://www.tu-harburg.de/rzt/rzt/it/Ether.html where he writes:

Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.

The same is true of quantum fields, quantum field theory is Lorentz-symmetric so quantum fields look the same way in every frame, they don't have a rest frame.

 

[PhilDSP]

Einstein only said that spacetime was similar to an aether in the sense that it has physical properties of its own, but in relativity it is unlike the traditional notion of aether in that it doesn't have any rest frame of its own

That's quite interesting. Langevin's concept of aether was that it remained always at rest with regard to a particle or object in any frame. And maybe that was the later conclusion of Poincare and Lorentz. Therefore in following the motion of a particle translating between a laboratory frame and a frame moving in tandem with the particle, length and time contraction arises.

http://en.wikisource.org/wiki/The_R...trons_to_Other_Branches_of_Science#cite_ref-1