Gravitational Waves: Mass Interaction Questions Answered

[NotASmurf]

Hey all. Since gravitational waves travel at the speed of light, when they are "escaping" from mass, doe's their intensity lower? If so what mass would be needed to completely absorb a gravitational wave given the wave's properties? Any help appreciated.

 

[PeterDonis]

Since gravitational waves travel at the speed of light, when they are "escaping" from mass, doe's their intensity lower?

In other words, do they behave like light waves and "redshift" as they climb out of a gravity well? I believe the answer is yes, but I have not seen any detailed treatments of this.

what mass would be needed to completely absorb a gravitational wave

A wave can't really be "completely absorbed". Consider the analogy with electromagnetism: an object absorbs EM waves if it can take their energy and turn it into heat--i.e., into vibrations of its atoms (or molecules). But that heat will gradually get transferred, and at least part of it will go into EM radiation again--for example, objects at room temperature emit (mainly) infrared radiation, i.e., EM waves. So the original EM waves didn't get "completely absorbed"; they just got converted into other EM waves.

Similarly, a mass might convert the energy of incoming gravitational waves into heat, but that heat will get radiated away again, and some of it (at least in principle) will be radiated as gravitational waves again. So the gravitational waves won't get "completely absorbed".

 

[NotASmurf]

So even if a gravitational wave from an external source hits a black hole, it will be partially absorbed, but will still be re-emitted? or would it all be converted to hawking radiation?

 

[PeterDonis]

even if a gravitational wave from an external source hits a black hole, it will be partially absorbed, but will still be re-emitted?

Black holes are not ordinary objects, so no, not the way you mean.

or would it all be converted to hawking radiation?

Yes. Technically that counts as "re-emitted", but you distinguished it from other possible ways of being re-emitted, none of which are possible for a black hole.