# 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?

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#### phinds

Gold Member
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.

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#### 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

Gold Member
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.

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#### 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

Mentor
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

Mentor
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?

#### 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?

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

Mentor
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...

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

Staff Emeritus
2018 Award
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

Gold Member
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 gravitional 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

Mentor
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

Mentor
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.
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

Mentor
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

Gold Member
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

Gold Member
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

Mentor
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

Homework Helper
Gold Member
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?