Gravitational waves - what determines their speed?

[Sorcerer]

I was reading up on gravitational waves at http://www.einstein-online.info/and it states that they propagate at the speed of light. If they are only distortions of space, which therefore have no mass/energy they shouldn't be bound by the rules preventing them exceeding the speed of light. Does this constraint indirectly say that they are composed of particles with mass/energy? If so, why don't they travel slower than the speed of light?

 

So why is the speed of light chosen for the speed of gravitational waves propagation, why not above or below?

[Mordred]

Gravity propogates at the speed of light. Even information propogates at the speed of light.

 

If our Sun blew up it would take 8 minutes before we would notice any effect either visually or via gravity.

[pavelcherepan]

As far as I know there exists a hypothetical boson that mediates gravity - graviton. Like a photon it's thought to have a 0 rest mass and thus can only travel at c. It hasn't been observed yet, just like gravitational waves.

[Sorcerer]

Gravity propogates at the speed of light. Even information propogates at the speed of light.

 

If our Sun blew up it would take 8 minutes before we would notice any effect either visually or via gravity.

 

This didn't really answer the question, I was particularly asking why. Light/photons as far as I'm aware actually tend to travel slightly less than the speed of light unless travelling through a perfect vacuum, which because of the virtual particle, vacuum energy, doesn't exist. What are gravitational waves "made" from, and why don't they interact to slow down slightly too, like light.

 

Since gravity waves carry information, how is it encoded, if it is simply encoded in the shape of space-time, is it then only able to be decoded by observing the interaction with space-time of particles. Shouldn't this then be viewed as the transmission of information between particles?

[MigL]

They can be encoded in any way that can be decoded at the other end as that would convey information.

And if that information travelled faster than c it would 'break' causality.

 

A boson that mediates an infinitely long range force like gravity or EM has to, by necessity, be massless.

And massless particles always move at c .

[Mordred]

c is also the speed limit for any interactions. Regardless of type. You can't measure gravity without something to interact with gravity.

 

When you read spacetime curvature what your really reading is the interaction strength upon particles that reside in that spacetime. Gravity doesn't affect a volume. It affects the mass/energy density of the particles occupying that volume.

 

Just like energy doesn't exist on its own being a property of particles. Mass also doesn't exist on its own. It is also a property of particles.

 

No particle can exceed c, so gravity cannot be transmitted in particle to particle interactions faster than c. So if you have a gravitational field of say test particles changes through that field will propogate at c or less.

That's essentially what a gravitational waves is. The rate of interaction due to gravity upon the mass of a system

https://en.m.wikipedia.org/wiki/Test_particle

[Sorcerer]

Thanks, I'm wondering if current experiments looking for gravitational waves assume they will propagate at exactly c? And if they assume so, but the waves propagate slower, how would that effect their measurements.

 

Wouldn't this mean lower energy waves, perhaps even more difficult to detect than currently thought? Since they'd be dissipating some energy interacting more strongly with the vacuum.

 

If gravity waves interact with the vacuum, could they somehow be altered by it, what if interaction with vacuum energy and virtual particles lessened gravity's effect at middle distances, but lent back energy and strengthened it over larger distances? Would this work as a suitable alternative for dark matter?

Edited January 9, 2016 by Sorcerer

[Mordred]

They look for it at c and below. You must remember we can only measure how a force or influence affects objects or particles. So for example if we try to measure gravity waves. We must measure the changes in the interstellar or intergalactic medium.

 

For example how a hydrogen cloud or plasma cloud reacts after some change in a gravitational body. We can't measure gravity directly. Only indirectly by its influence.

 

So the medium that the influence is moving through will slow down the propagation rate.

[Sorcerer]

Sounds about right. I know it's probably a slip, but gravity isn't a force.

 

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My brain got speculating a bit: As gravity waves propagate they disperse and become smaller, what if over short to medium distances they followed an inverse square law as Newtonian gravity indicates. At these distances they predominantly interact with matter (because they happen to form near dense clusters of matter) and their interaction with Zero Point Energy (ZPE) is of minor consequence.

 

As the wave propagates out into areas more dominated by voids, the interaction with ZPE plays a larger part. Normally in the void it would be expected the wave wouldn't slow, but its interaction with ZPE by lending and regaining energy quickly as it excites the vacuum and creates VP pairs which annihilate, slows the wave slightly, but maintains its total energy.

 

This gradual decrease in the speed of propagation, without a decrease in strength has the property of allowing the wave to interact for longer with medium range areas of the vacuum. Some of the VP's it excites into existence are split away before annihilation. During the mechanism that aids this one half of the VP appears on the front of the wave crest and another is left behind in the trough.

 

This reduction in energy modifies the inverse square law very slightly at medium distances, so gravity is slightly weaker here than normal, it also lessens the waves chance of continuing to excite VPs into existence from ZPE.

 

Finally the wave reaches a third stage, where it is very weak and distant, just as expected by the inverse square law, only with a slight reduction. But, it is at this point that the VP pairs previously created have their highest probability of decaying back into the vacuum. This borrowed energy is returned to the gravity wave and it is strengthened to just the right amount so that gravity's range of effect has an energy spike at its farthest reaches.

 

This extension of gravity at long range may be enough to hold galaxies together and be a plausible candidate for Dark Matter.

Edited January 9, 2016 by Sorcerer

[Mordred]

Whether you call it a force or a result of spacetime. It's still an interaction.

 

The inverse square decrease in strength is a calculatable and tested consequence involving how a force or interaction decreases as a result of the area of influence.

 

I could post the calculation from Elements of Astrophysics. However that would have to wait. It's simply due to volume change as the force or interaction spreads out to cover a greater volume

[ajb]

So why is the speed of light chosen for the speed of gravitational waves propagation, why not above or below?

It is not chosen, it comes from examining small perturbations of the local geometry in general relativity. Much like the speed of light c, was not chosen for electromagnetic radiation, it came from examining the equations that describe these waves.

 

Heuristically, as already mention the quanta of gravitational waves are gravitons and these are massless and so travel at the speed of light.

[geordief]

This is way above my level of understanding but is it possible ,in theory that gravitation waves (caused I understand by changes in the gravitational field ) might actually have their own interference pattern if two gravitation "wavefronts" from 2 distinct areas combined?

 

I understand that laser interferometry is being deployed in the aim of detecting gravitational waves but that is not what I am thinking of (well I don't think it is).

 

I hope I am not too far off topic.

[ajb]

This is way above my level of understanding but is it possible ,in theory that gravitation waves (caused I understand by changes in the gravitational field ) might actually have their own interference pattern if two gravitation "wavefronts" from 2 distinct areas combined?

 

I understand that laser interferometry is being deployed in the aim of detecting gravitational waves but that is not what I am thinking of (well I don't think it is).

I am not sure what you are asking here...

 

However, due to recent advances in computing power people have identified what expected signals interferometry experiments will detect based on probable sources of gravitational radiation.

[geordief]

I am not sure what you are asking here...

 

However, due to recent advances in computing power people have identified what expected signals interferometry experiments will detect based on probable sources of gravitational radiation.

A bit unclear myself too.

 

Suppose we have two distinct areas in the universe that cause gravitation disturbances ,such as two separate binary pulsars in separate parts of our galaxy perhaps.

 

As I understand it both of these systems would produce disturbances in space-time that propagate at the speed of light .(as a wave , would that be right?)

 

So the waves from each of these separate systems will come into contact with one another at some point and interfere with each other .

 

Would that be a reasonable assumption ? Could that interference be detected ? (not the laser interference hopefully produced in a measuring apparatus)

Edited January 9, 2016 by geordief

[ajb]

So the waves from each of these separate systems will come into contact with one another at some point and interfere with each other .

 

Would that be a reasonable assumption ?

I think that is a reasonable assumption. I am not sure exactly what sources have been modelled and what possible interference can be expected. That is a good question.

 

If the gravitational waves are weak then they will only locally interfere and pass through each other. If they are strong then much more interesting effects can happen such as black hole formation. I imagine that most of the sources of gravitational waves are weak and so interference is not really much of an issue. But we should really ask an expert in that field.

Edited January 9, 2016 by ajb