Gravitation and Space Time warps

[Lazernugget]

Okay, so here's my questions:

1. In the theory of the expanding universe, the universe, well, expands. But space it's self would have to expand faster than light could travel or at the same speed for that to work. Umm, Hello? The space would travel faster than light. Would it not? If so, explain that please.

2. In general relativity, Gravity is explained by warps in space time. What exactly causes those warps? The warps depend on mass, so does the Higgs Boson have affect on the size and effect of those warps in space? Or is it the mysterious force carrier Graviton that exists within all matter with mass, meaning if we could somehow contain free Gravitons we could warp space by first neutralizing the gravitons and then dispersing them to fold and bend space?

If you have questions about my question, feel free to ask, and please try to help answer.

-Lazer

 

[WannabeNewton]

Okay, so here's my questions:

1. In the theory of the expanding universe, the universe, well, expands. But space it's self would have to expand faster than light could travel or at the same speed for that to work. Umm, Hello? The space would travel faster than light. Would it not? If so, explain that please.

https://www.physicsforums.com/showthread.php?t=508610

2. In general relativity, Gravity is explained by warps in space time. What exactly causes those warps? The warps depend on mass, so does the Higgs Boson have affect on the size and effect of those warps in space? Or is it the mysterious force carrier Graviton that exists within all matter with mass, meaning if we could somehow contain free Gravitons we could warp space by first neutralizing the gravitons and then dispersing them to fold and bend space?

General Relativity does not deal with the Higgs Boson nor with gravitons (even though it implies their existence). To put it simply, the source of curvature in GR is the Energy - Momentum tensor and this affects the geometry/curvature. You have to hold on until a QFT framework incorporating gravity is fully developed.

 

[jfy4]

General Relativity does not deal with the Higgs Boson nor with gravitons (even though it implies their existence). To put it simply, the source of curvature in GR is the Energy - Momentum tensor and this affects the geometry/curvature. You have to hold on until a QFT framework incorporating gravity is fully developed.

Don't you think rather that GR assumes their existence, as opposed to implies their existence? While the development of a QFT of gravity would hopefully have a graviton (or an analogue) I don't think it's classical limit, GR, implies that one exists, I think it assumes it. As far as the Higgs, I don't see that falling out of GR, or a quantization of GR either, although, I couldn't know since it hasn't been done in full...

What do you think?

 

[WannabeNewton]

Don't you think rather that GR assumes their existence, as opposed to implies their existence?

Maybe I worded it wrong. I meant more along the lines of implied it from the symmetry of $h_{\mu \nu }$ and the group velocity being equal to unity from $\square \bar{h_{\mu \nu }} = 0$. As far as the higgs goes I agree with you that it doesn't have much going for it right now.

 

[bcrowell]

Don't you think rather that GR assumes their existence, as opposed to implies their existence?

Huh? GR was developed before quantum mechanics, so it can't have assumed the existence of a graviton.

 

[Lazernugget]

Thanks for the replies, they're all great. So, I have Q #1 explained, but I still think I could use a bit more clarification on my Q #2

 

[jfy4]

Huh? GR was developed before quantum mechanics, so it can't have assumed the existence of a graviton.

Looks like I worded it wrong! How I meant the above was in the sense that, given a correct quantum theory of gravity (and the graviton which we think will be part of it), it implies GR. On the other hand, given a classical limit, it does not imply the quantum theory.

Are you okay with that?

 

[jfy4]

Thanks for the replies, they're all great. So, I have Q #1 explained, but I still think I could use a bit more clarification on my Q #2

The stress-energy-momentum tensor, $T_{\alpha\beta}$, is the source for the Einstein tensor, $G_{\alpha\beta}=R_{\alpha\beta}-\frac{1}{2}g_{\alpha\beta}R$, which gives a description of the curvature of space-time. For Einstein's field equations, the curvature described is due to matter (anything that isn't the gravitational field).

 

[Naty1]

... given a correct quantum theory of gravity (and the graviton which we think will be part of it), it implies GR. On the other hand, given a classical limit, it does not imply the quantum theory.

That's the way it usually works. As h approaches zero, QM approaches classical.

 

[bcrowell]

Looks like I worded it wrong! How I meant the above was in the sense that, given a correct quantum theory of gravity (and the graviton which we think will be part of it), it implies GR. On the other hand, given a classical limit, it does not imply the quantum theory.

Are you okay with that?

Sure, makes sense to me.

 

[K^2]

1. In the theory of the expanding universe, the universe, well, expands. But space it's self would have to expand faster than light could travel or at the same speed for that to work. Umm, Hello? The space would travel faster than light. Would it not? If so, explain that please.

In GR, speed of light is a local limit, because it only makes sense in flat space-time. If your space-time sufficiently curved, two remote objects can be moving relative to each other much faster than speed of light. Consider Alcubierre Warp Drive for an extreme example.