I What happens to gravitational waves?

[StandardsGuy]

Two black holes that are orbiting and collide give off mass in the form of gravitational waves before the collision. Do these waves get absorbed by something, or is this mass lost to the universe?

 

[phinds]

Two black holes that are orbiting and collide give off mass in the form of gravitational waves before the collision. Do these waves get absorbed by something, or is this mass lost to the universe?

The mass is given off, as you say, exactly as it is the case when accretion disks give off some of their energy as photons. In both cases, the thing "given off" just keeps on traveling through the universe (unless and until it encounters an object that impedes it (such as the LIGO detector, in the case of gravity wave), but the amount of the total wave given off (from the merger) that LIGO impedes is WAY below negligible.

 

[mathman]

Inverse square law reduces effects.

 

[StandardsGuy]

But I am trying to understand if mass (however small it is) is lost. If you knew if the universe is finite or if you knew if it is infinite, would it change your answer? Does the wave going through objects add mass to the object?

 

[phinds]

But I am trying to understand if mass (however small it is) is lost. If you knew if the universe is finite or if you knew if it is infinite, would it change your answer? Does the wave going through objects add mass to the object?

But they DON'T give off mass, they give off energy (if you're talking about gravitational waves) and that is attenuated when it travels through an expanding universe such as the one we live in.

 

[StandardsGuy]

You are arguing semantics and adding little of value. My questions are still unanswered. The black holes have given up a lot of mass. OK it is changed to energy in the form of gravitational waves. OK it spreads out. If these waves were electromagnetic waves, they would be absorbed by objects and likely changed to heat. What do the gravitational waves do? If you don't know say so. I can accept that, but don't keep patronizing me. With the expanding universe they may just add to the energy of the vacuum. Perhaps this is the energy that is expanding the universe. Thoughts?

 

[jim mcnamara]

Your neutron star example gave off energy. Mass and energy are related by a simple equation. Every school kid knows it. Einstein made it famous. Can you recall it? E=______ (fill in the blank) I'll bet you know it ,too.

You also probably know already:
Our sun converts Hydrogen into Helium. The process is called fusion. It is the source of the heat and light the sun emits. And guess what? If the sun start with a mass Hydrogen, say a metric ton, then the mass of Helium is less than mass than the Hydrogen we started with. The missing mass was released as energy.

This IS the answer to your question. If you do not understand we'll try to help. Please do not assert that your question is unaswered.

 

[berkeman]

You are arguing semantics and adding little of value. My questions are still unanswered. The black holes have given up a lot of mass. OK it is changed to energy in the form of gravitational waves. OK it spreads out. If these waves were electromagnetic waves, they would be absorbed by objects and likely changed to heat. What do the gravitational waves do? If you don't know say so. I can accept that, but don't keep patronizing me. With the expanding universe they may just add to the energy of the vacuum. Perhaps this is the energy that is expanding the universe. Thoughts?

Maybe you are a visual learner. What happens to the photons given off by this conversion of mass to energy that make it to space? Does that help clarify?

https://sciencing.com/effects-hydrogen-bomb-5399698.html

 

[StandardsGuy]

You are even more patronizing than phinds. Your example has nothing to do with gravitational ________. (can you fill in the blank?) My question was what happens to the energy? Does it get absorbed? If so, in what form?

 

[StandardsGuy]

Maybe you are a visual learner. What happens to the photons given off by this conversion of mass to energy that make it to space? Does that help clarify?

https://sciencing.com/effects-hydrogen-bomb-5399698.html
View attachment 251050

No. Electromagnetic waves are photons. Gravitational waves are "geometry" according to other threads on this site. There MAY be something called a graviton involved. Do gravitational waves add mass if/when they are absorbed?

 

[berkeman]

No. Electromagnetic waves are photons. Gravitational waves are "geometry" according to other threads on this site. There MAY be something called a graviton involved. Do gravitational waves add mass if/when they are absorbed?

Actually, you beat me to my trying to edit my reply. It looks like gravitational waves are not absorbed by much of anything...

https://www.ligo.org/science/GW-Potential.php

Gravitational waves will change astronomy because the universe is nearly transparent to them: intervening matter and gravitational fields neither absorb nor reflect the gravitational waves to any significant degree. Humans will be able to observe astrophysical objects that would have otherwise been obscured, as well as the inner mechanisms of phenomena that do not produce light. For example, if stochastic gravitational waves are truly from the first moments after the Big Bang, then not only will we observe farther back into the history of the universe than we ever have before, but we will also be seeing these signals as they were when they were originally produced.

 

[Drakkith]

Two black holes that are orbiting and collide give off mass in the form of gravitational waves before the collision. Do these waves get absorbed by something, or is this mass lost to the universe?

Both. The mass is radiated away in the wave, which can then be absorbed by objects as they interact with the wave.

 

[256bits]

You are even more patronizing than phinds. Your example has nothing to do with gravitational ________. (can you fill in the blank?) My question was what happens to the energy? Does it get absorbed? If so, in what form?

Was it LIGO that detected a gravitational wave from merging black holes.
And how much was the change in lengths between the two arms - a fraction of the width of a proton.
So if the flexing of a material object by that amount ends up as heat, one can see the amount of energy removed from the gravitational wave, if that would be the cause of the flexing, is very, very small.
As was mentioned in post @2 by @phinds.
And post @3 is applicable by @mathman

 

[Dale]

What do the gravitational waves do?

Most of the universe seems to be pretty transparent to gravitational waves. However, there is some small interaction, which is what is detected by LIGO. The waves are waves of spacetime curvature and spacetime curvature is closely tied to tidal gravity. Tidal effects can accelerate, decelerate, and heat matter, so all of those can happen with gravitational waves. But it is normally a very small amount.

 

[PeterDonis]

Do these waves get absorbed by something, or is this mass lost to the universe?

These aren't the only two alternatives. The waves themselves carry energy; that's where the mass lost in the black hole merger goes. If the waves pass through an object, they might give up some of their energy to the object, yes; but that doesn't have to happen.

The black holes have given up a lot of mass. OK it is changed to energy in the form of gravitational waves.

Exactly. Nothing is lost. So what's the problem?

If these waves were electromagnetic waves, they would be absorbed by objects

Not necessarily. They might travel forever through the universe, never hitting anything. But they would still be carrying the energy they took away from their source.

What do the gravitational waves do?

The possibilities are the same as for EM waves.

With the expanding universe they may just add to the energy of the vacuum.

No, this is not a possibility, either for EM waves or gravitational waves.

Perhaps this is the energy that is expanding the universe.

There is no such thing as "the energy that is expanding the universe". And in any case the expansion of the universe is a separate topic from the topic of this thread.

 

[NC_Seattle]

I asked a similar question a few years back.
https://www.physicsforums.com/threads/transfer-of-energy-from-gravity-waves.857217/
My understanding, based on Feynman's 'sticky bead argumant' is that gravity waves do transfer energy to mass they pass through. This added energy is translated into heat, which does add (relativistic) mass to objects.

Furthermore, if the 'closed universe' model is correct then those gravity waves will continue bouncing (refracting?) around the universe, continually adding mass back to normal matter until they are completely dissipated.

If our universe has a hot ending (in one tiny hot point) then at some point in time those gravity waves will begin to overlap and start pouring serious amounts of energy back into the increasingly hot universe.

If our universe has a static ending (steady state) then the gravity waves will keeping bouncing around and shedding energy until they don't exist anymore.

If our universe has a cold ending (spread out to infinity) then the gravity waves will spread out to vast distances, shedding less and less energy as they become more spread out.

 

[PeterDonis]

My understanding, based on Feynman's 'sticky bead argumant' is that gravity waves do transfer energy to mass they pass through.

This is true in principle, but in practice the amount of energy transfer for most gravitational waves and most objects is extremely negligible.

 

[Keith_McClary]

If these waves were electromagnetic waves, they would be absorbed by objects and likely changed to heat.

Actually only about $10^{-16}$ of the sky is stars. Some more might be absorbed by dust or gas.

 

[davenn]

Furthermore, if the 'closed universe' model is correct then those gravity waves will continue bouncing (refracting?) around the universe,

bouncing off what ?

If our universe has a static ending (steady state) then the gravity waves will keeping bouncing around

again, bouncing off what ?

 

[StandardsGuy]

Most of the universe seems to be pretty transparent to gravitational waves. However, there is some small interaction, which is what is detected by LIGO. The waves are waves of spacetime curvature and spacetime curvature is closely tied to tidal gravity. Tidal effects can accelerate, decelerate, and heat matter, so all of those can happen with gravitational waves. But it is normally a very small amount.

I like your answer except perhaps the last sentence. Each incidence is a very small amount because it is spread out, but the number of incidences goes up exponentially for the same reason. But maybe you meant each incident absorbes a small amount of what's there?

 

[Keith_McClary]

Each incidence is a very small amount because it is spread out, but the number of incidences goes up exponentially for the same reason.

As I noted above, the chance of hitting anything is $10^{-16}$. Space is very empty.

 

[sophiecentaur]

I like your answer except perhaps the last sentence. Each incidence is a very small amount because it is spread out, but the number of incidences goes up exponentially for the same reason. But maybe you meant each incident absorbes a small amount of what's there?

This reminds me a bit of the Olber Paradox but the other way round.

 

[StandardsGuy]

Exactly. Nothing is lost. So what's the problem?

Thanks for your answers. The mass is changed to equivalent energy so there is conservation, and there seems to be a consensus that it won't be changed back. Mass itself is less in the universe, so the attraction of gravity for it is gone, I think. That is not the case for two stars colliding to create a black hole. The gravity still exists in that case (but the mass is also gone).

There is no such thing as "the energy that is expanding the universe". And in any case the expansion of the universe is a separate topic from the topic of this thread.

My understanding is by the current theory it is called "dark energy", but I won't pursue it further.

 

[PeterDonis]

The mass is changed to equivalent energy so there is conservation

Yes.

there seems to be a consensus that it won't be changed back.

More precisely, in practice only a small amount will be changed back, because many of the waves will never pass through any matter, and those that do will generally only deposit a very small amount of their energy in the matter they pass through.

Mass itself is less in the universe, so the attraction of gravity for it is gone, I think. That is not the case for two stars colliding to create a black hole. The gravity still exists in that case (but the mass is also gone).

You are making two errors here.

First, a black hole is a source free (i.e., zero stress-energy) vacuum solution, just like gravitational waves. Both are made of spacetime curvature--two different specific configurations of spacetime curvature, but still spacetime curvature. So they are not different in the way you are saying here.

Second, "attraction of gravity" does not disappear when a source emits gravitational waves. For example, suppose you are orbiting a pair of black holes that are about to merge. You are far enough away to observe the merger without being destroyed by its effects, but close enough so that you can measure the gravitational attraction of the two-black-hole system. Even after the merger, when a significant amount of the original combined mass of the holes has been converted into gravitational waves, you still feel the gravity of the system the same as before: nothing changes in the gravity you feel until the gravitational waves emitted by the merger have passed you on their way out into the rest of the universe. And when the gravity you feel does change, it's not because of anything being converted into gravitational waves; it's simply because the energy carried by those waves has passed you so it's no longer below you, no longer part of the system whose gravity you are measuring.

 

[Delta2]

I thought that gravitational waves are absorbed by masses, pretty much like EM waves interact and absorbed by electric charges... Why this is not the case?

 

[snorkack]

Waves without rest mass still possesses moving mass and cause attractive gravitational forces.
Look at the starlight that is bent on passing just by Sun´ s edge.
It is not absorbed by Sun (or we could not see it).
It possesses some momentum (otherwise it would not have energy and direction) which is changed when it changes direction.
Sun must exert some attractive force on starlight in order to change its direction...
and by the law of action and reaction, it follows that the starlight must exert some attractive force on Sun as reaction to being bent.
Not much, and most of it is canceled out by starlight passing Sun in other directions. But still, electromagnetic waves such as light and radio waves that are passed through space without absorption, reflection or refraction are still bent by pure gravity, and they in their turn exert gravity on rest masses and electromagnetic waves, feeble as it is.
And so do gravitational waves carry mass and exert pure gravitation.

 

[PeterDonis]

I thought that gravitational waves are absorbed by masses, pretty much like EM waves interact and absorbed by electric charges... Why this is not the case?

Gravitational waves are absorbed by masses, just much, much less strongly than EM waves are absorbed by charged objects. The reason for the difference in strength of absorption is basically a combination of gravity being a much weaker interaction than EM to begin with, and gravity being a spin-2 interaction whereas EM is spin-1 (which means the lowest order absorption, just as the lowest order emission, is quadrupole instead of dipole).

 

[stefan r]

The ligo detector normally has 2 halves of a beam of photons. When a gravity wave passes through the ligo detector it still has 2 halves of a beam of photons. The photons are the same frequency in both cases. There is no reason to believe any energy was absorbed from the gravity wave.

I can not say that I know enough about gravitational waves to be certain that energy is never transferred anywhere. The detection part of the ligo detector is not absorbing any energy.

 

[PeterDonis]

I can not say that I know enough about gravitational waves to be certain that energy is never transferred anywhere.

Feynman showed in 1958 that an object can absorb energy from a gravitational wave and heat up. He modeled the "object" as a set of masses connected by springs. The fluctuating tidal gravity of the gravitational waves makes the masses move and stretches the springs: that transfers energy from the wave to the masses-springs system.

The reason a detector like LIGO absorbs (to a very, very good approximation) no energy from a gravitational wave is that there is nothing analogous to the masses and springs in the laser beams in the two arms of the detector: the photons in the laser beam do not (to a very very good approximation) interact with each other, so there is no analogue to the force involved in stretching the springs.