Is it possible that speed of a gravitational waves are greater than c?

1. Apr 5, 2012

Ravi Mandavi

Since at event horizon, the escape velocity of black hole must be greater than speed of light but even light can't escape from black hole so is it possible that speed of gravitational wave > c?

2. Apr 5, 2012

alexg

When the Black Hole formed, the gravitational field was already there, permeating space. It doesn't go away because an event horizon formed.

3. Apr 13, 2012

skeptic2

It would be hard to explain a 21 billion solar mass black hole by saying that the gravitational field was already there when the star collapsed. I think most cosmologists accept that the field grows as the BH accretes mass. But from where does the gravitational field originate?

While a static field is not affected by an event horizon, the process of accreting mass is hardly static. If the gravitational field were an indication of the mass at the singularity then there is a problem of how the increases in the mass of the singularity, resulting in the changes in the gravitational field, propagate from the singularity backwards in time and faster than light in order to escape the event horizon.

On the other hand, there is an explanation that as matter falls through the EH, it leaves its gravitational field frozen at the horizon.

4. Apr 13, 2012

Chronos

Under quantum electrodynamics, gravity should be mediated by virtual particles. Virtual particles are not bound by event horizons.

5. Apr 14, 2012

Aidyan

I wonder instead why physicists/astronomers are so sure that gravitational waves travel with the speed of light? Why can it not be slower? Is it just an assumption that waits to be tested, or is there a deeper reason for that?

6. Apr 14, 2012

phyzguy

The reason we believe that gravitational waves travel at the speed of light is that gravity is very accurately described by Einstein's equations of General Relativity. General Relativity has passed every experimental test to which it has been subjected, and General Relativity predicts gravitational waves, and predicts that they travel at the speed of light. While gravitational waves have never (yet) been detected directly, their consequence has been seen in the Hulse-Taylor binary pulsar, the discovery and explanation of which earned them the Nobel prize. When gravitational waves are directly seen, probably within the next decade, I would bet a large sum of money that they are seen to travel at the speed of light.

7. Apr 14, 2012

Chronos

Lorentz invariance is the part of GR that limits the propogation speed of fields to 'c' and it is assumed Lorentz invariace also applies to gravity. Hulse and Taylor received the 1993 Nobel for their work on binary pulsars, which showed timing increases that precisely matched those predicted by GR. If gravity were not Lorentz invariant, the results would not have matched the predictions of GR.

8. Apr 14, 2012

jambaugh

Remember that we have not quite yet observed gravitational waves. The theory that predicts them predicts their speed as c. Until or unless they are observed it is not appropriate to simply speculate whether this one point may vary. You must rather bite off a bigger chunk.

One would be speculating whether there is an alternative theory, also predicting gravity waves but traveling at different speeds, which is self consistent and consistent with observed phenomena.

9. Apr 14, 2012

DaveC426913

Good, you answered you own question.

All mass (thus all sources of gravity) started off either before or outside the BH, thus their gravitational fields were indeed already in effect when they fell into the BH.

10. Apr 14, 2012

Orion1

$$P = \frac{dE}{dt} = - \frac{32}{5}\, \frac{G^4}{c^5}\, \frac{(m_1m_2)^2 (m_1+m_2)}{r^5}$$

$$\frac{dr}{dt} = - \frac{64}{5}\, \frac{G^3}{c^5}\, \frac{(m_1m_2)(m_1+m_2)}{r^3}$$

If gravitational wave velocity was greater than luminous velocity, then the amount of power radiated and the orbital decay of binary pulsars would be much less.

If gravitational wave velocity was less than luminous velocity, then the amount of power radiated and the orbital decay of binary pulsars would be much more.

However, the measurements of these functions in nature indicate agreement with these equations in which the intrinsic velocity of gravitational waves is exactly equal to luminous velocity.

Could the gravitational wave velocity be measured directly from nature with is equation?

Gravitational wave velocity:
$$c = \left[ \left( \frac{64 G^3}{5} \, \frac{(m_1m_2)(m_1+m_2)}{r^3} \right) \left( - \frac{dr}{dt} \right)^{-1} \right]^{\frac{1}{5}}$$

Reference:
Gravitational wave - Wikipedia

Last edited: Apr 14, 2012
11. Apr 14, 2012

Chronos

Jimbaugh, do you think Hulse was not compelling evidence of gravitational waves and their Lorentz invariance? phyzguy, apologies I was still pecking away when you posted essentially the same thing.

12. Apr 14, 2012

Nabeshin

This is an incredibly naive analysis... If you assume gravitational radiation does NOT travel at c, a lot has to change within relativity. If it's slower, well you have a massive graviton which among having a speed <c also admits longitudinal polarizations. The whole situation becomes significantly more complicated. If it's faster, all kind of crazy things go out the window and I don't even know where to start there.

13. Apr 14, 2012

jambaugh

Yes I do. But I do not consider it direct observation. The pulsar observations make it that much harder to speculate outside of GR. One must explain them.

If however we can build a grav. wave detector we needn't speculate, we could measure the speed. Thus it is meaningful to ask what value we might measure.

Until then, this question is of a different type, (i.e. what does theory say) and so supposing a different answer than theory supposes a distinct theory.

Now personally I'll be shocked if we observe other than speed c grav waves...beyond shocked, flabbergasted.

14. Apr 14, 2012

Chronos

Thanks for the clarification, jambaugh. As you probably recall, Kopeikin and Formalont attempted a direct measurement of the speed of gravity about a decade ago, but, it is considered controversial. I believe it was Steve Carlip who basically accused them of making a backdoor measurement of the speed of light.

15. Apr 15, 2012

Orion1

16. Apr 17, 2012

poioip

what basically happens is that gravity bents space & time fabric. the light travels parallel to space & time fabric so at a point the fabric is stretched and light have to travel twice as long so that's why they say light cannot escape a black hole. As Eisenstein said that c is the optimum speed limit so lets say a particles with mass zero (energy) can travel at the velocity of c

17. Apr 18, 2012

Nabeshin

18. Apr 18, 2012

strat1227

Pardon if this is a naive understanding of it, but I was under the impression that describing gravity in waves is just a way for us to visualize it, but in actuality that's not what's happening.

My understanding is just that mass alters space-time, so for example while the moon appears to be going in a circle to us, it's actually not going in a circle through space-time, because that's an entirely different plane than our spatial 3 dimensions.

So if that's the case (if it's not then please just correct my misunderstanding and ignore this question), why does it take any time at all for gravity to permeate? If you drop a billiard ball onto a outstretched blanket, all points of the blanket are affected at the same time. Similarly, if you just spontaneously created a large amount of mass in the middle of empty space, would not the space-time field be instantly altered?

19. Apr 18, 2012

Nabeshin

No, even within your analogy your intuition is failing here. If you imagine a really large sheet and dropping a bowling ball on it, say the sheet goes from New York to Paris, do you really think the guy in Paris will feel the ball that was dropped in NY instantly? No, he will not. The disturbance propagates at something like the sound speed of the material (I have no idea what this is for a taut sheet, but certainly finite!). The analogy with spacetime is decent, in that the disturbance propagates our at the speed of light.

Note that within GR, if I just imagine the moon orbiting the earth, nothing is changing about the spacetime (assume the moon mass is so much smaller than the Earth that it doesn't alter the geometry). The Earth creates some well, and the moon is just moving in a straight line through that well. 'Gravity' is encoded in the curvature of that well, which never changes, so it doesn't make any sense to talk about anything propagating here. Only when I change the situation (remove some mass somehow, or something like that), does the geometry need to respond, and these ripples travel outwards like waves on a pond at precisely the speed of light.

20. Apr 18, 2012

strat1227

Ok, that explanation makes sense :) I was thinking about it from a mathematical standpoint and not a physical one ... Mathematically speaking, all points would feel it at the same time, but that obviously can't be the case physically

That was the question that inspired my post, if I just created a bunch of mass spontaneously in the middle of empty space, how long would it take surrounding planets to feel the effects of it.