Is General Relativity Really About Physics on Curved Spacetimes?

[atyy]

So Hobba was right in the other thread we were discussing when he said ""Up to about the plank scale the assumption it is flat is fine, with gravitons making it behave like it had curvature or actually giving it curvature (we can't determine which) works quite well."

Yes, I think Hobba was right.

 

[waterfall]

Yes, I think Hobba was right.

Ok. But there was something you said later in the thread that perplexed me. You said:

"BTW, although massless spin 2 can be equivalent to Einstein gravity in spacetimes that can be covered by harmonic coordinates (or similar), I don't think the reverse is true that the existence of a spin 2 field is sufficient to produce Einstein gravity.

Zhang and Hu, A Four Dimensional Generalization of the Quantum Hall Effect
Elvang and Polchinski, The Quantum Hall Effect on R^4

Bekaert et al, How higher-spin gravity surpasses the spin two barrier"

How could that be. You said massless spin 2 in harmonic coordintes can produce Einstein gravity, then you followed it immediately with the conflicting passage " I don't think the reverse is true that the existence of a spin 2 field is sufficient to produce Einstein gravity." But you just mentioned in the first sentence that it can! This has been perflexing me for a week so hope you can explain the context of what mean in your conflicting paragraph. Thanks.

 

[atyy]

Ok. But there was something you said later in the thread that perplexed me. You said:

"BTW, although massless spin 2 can be equivalent to Einstein gravity in spacetimes that can be covered by harmonic coordinates (or similar), I don't think the reverse is true that the existence of a spin 2 field is sufficient to produce Einstein gravity.

Zhang and Hu, A Four Dimensional Generalization of the Quantum Hall Effect
Elvang and Polchinski, The Quantum Hall Effect on R^4

Bekaert et al, How higher-spin gravity surpasses the spin two barrier"

How could that be. You said massless spin 2 in harmonic coordintes can produce Einstein gravity, then you followed it immediately with the conflicting passage " I don't think the reverse is true that the existence of a spin 2 field is sufficient to produce Einstein gravity." But you just mentioned in the first sentence that it can! This has been perflexing me for a week so hope you can explain the context of what mean in your conflicting paragraph. Thanks.

A chair can be made of wood, but not everything made of wood is a chair.

 

[waterfall]

A chair can be made of wood, but not everything made of wood is a chair.

Ok. So you mean full GR includes black holes dynamics *near* singularity which spin-2 field over flat spacetime doesn't cover. Good. Thanks for the clarification.

 

[atyy]

Ok. So you mean full GR includes black holes dynamics *near* singularity which spin-2 field over flat spacetime doesn't cover. Good. Thanks for the clarification.

Yes, that's true, but not what I meant. I meant that there may be spin 2 fields that produce "gravity" that is different from that of GR, even below the Planck scale.

http://arxiv.org/abs/1007.0435

 

[waterfall]

Yes, that's true, but not what I meant. I meant that there may be spin 2 fields that produce "gravity" that is different from that of GR, even below the Planck scale.

http://arxiv.org/abs/1007.0435

I actually read the paper above. It's talking about higher spin (more than 2) that produce "gravity" that is different from that of GR. It's not talking about spin 2.. so maybe you are mistaken above?

Also your analogy "A chair can be made of wood, but not everything made of wood is a chair." is not related to the above paper but as an answer to my other question, isn't it?

 

[waterfall]

Yes, that's true, but not what I meant. I meant that there may be spin 2 fields that produce "gravity" that is different from that of GR, even below the Planck scale.

http://arxiv.org/abs/1007.0435

I think what you meant was that since spin-3 or spin-4 describe GR. Then spin-2 describe "gravity" that is not 100% GR. This is very important to distinguish because it means spin-2 over flat spacetime is not equivalent to GR even those describe by harmonic coodinates.

The meaning of equivalent is "=". So when something is not matched 100%. They are not equal. So when you said before it is equivalent and later said spin-2 is not sufficient to produce GR. Then your statements conflict. Try to be consistent in descriptions especially when dealing with such complicated subject. Thanks.