Why Don't Earth and the Moon Emit Gravity Waves Like Neutron Stars?

[enceladus_]

If two spinning, neutron stars in a tight orbit lose energy by giving off gravity waves, why don't other objects in orbit do the same? I don't understand what makes the two neutron stars unique. Why wouldn't the moon and the Earth give off gravity waves as well? I also know that the moon is slowly moving away from us, so this is very confusing to me.

 

[PeterDonis]

If two spinning, neutron stars in a tight orbit lose energy by giving off gravity waves, why don't other objects in orbit do the same?

According to GR, they do; but the rate of energy loss is a lot smaller so we can't directly measure it. The systems with two neutron stars are the only ones we know of that have a rate of energy loss due to gravitational waves that is within our capability to measure with our current technology.

I also know that the moon is slowly moving away from us, so this is very confusing to me.

This is a different effect; it's due to the tidal interaction between the Earth and the Moon, and doesn't involve emission of gravitational waves.

 

[Nugatory]

If two spinning, neutron stars in a tight orbit lose energy by giving off gravity waves, why don't other objects in orbit do the same? I don't understand what makes the two neutron stars unique. Why wouldn't the moon and the Earth give off gravity waves as well?

They Earth does give off gravity waves. They're just very weak, so weak they produce no noticeable effects even over millenia.

The Earth goes around the sun once a year, so only sends one crest-to-trough-to-crest wave in a year. The two orbiting neutron stars do an orbit in minutes and have maybe a million times the mass of the earth, so they're radiating away a million times as much energy in each pulse, and doing the pulses 100,000 times as quickly.

 

[enceladus_]

Ahh, I see. That makes a lot of sense. Can I also conclude that out bodies are experiencing gravitational waves from a wide variety of sources, and thus have our shape contorted on an unimaginably small scale?

Thank you both very much

 

[PeterDonis]

Can I also conclude that out bodies are experiencing gravitational waves from a wide variety of sources

Yes.

and thus have our shape contorted on an unimaginably small scale?

In principle, yes; but there are so many other things contorting our bodies on much larger scales that the gravitational waves would not have a chance to contort anything.

 

[enceladus_]

Thanks again!

 

[phyzguy]

According to GR, they do; but the rate of energy loss is a lot smaller so we can't directly measure it. The systems with two neutron stars are the only ones we know of that have a rate of energy loss due to gravitational waves that is within our capability to measure with our current technology.

Orbital decay due to gravitational wave emission has also been detected in some closely orbiting white dwarf binaries. See this paper for example. Imagine two white dwarfs orbiting with a 12 minute period!

 

[enceladus_]

Thanks for that, very interesting.