# Am I on the right path here...

Gweedz
Joined: 16 Nov 05
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### RE: Guido, while you're at

Message 19913 in response to message 19912

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Guido, while you're at the library, you might be interested also in learning about LISA

I read about that online a while ago with the free-floating masses trailing the earth's orbit. Very interesting.

I would assume it would be easier to detect waves from nearby objects such as the mooon or jupiter than very distant black holes and binaries. I guess not.

Gweedz
Joined: 16 Nov 05
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### RE: The best non technical

Message 19914 in response to message 19901

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The best non technical book on GR I"ve come across is "The riddle of gravitation" by Peter G.Bergmann, a coworker of Einstein, 1968, but I don't know if is still available and not out of print.I edited its Italian edition in 1969 for Edizioni Scientifiche Mondadori.
Tullio

I didn't find that book, but I did find others on the subject.
Thanks anyway.

Ben Owen
Joined: 21 Dec 04
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### RE: Since gravity is

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Since gravity is energy, and energy is related to mass, and all objects release gravity "waves", then shouldn't the mass of these objects decrease over time?

There are several different things going on here, and they seem to have gotten mixed together. I'll focus on the main question, and come back later to some of the others.

Guido, it sounds like you're thinking something like: "E=mc^2 says the total mass and energy of an object are the same, up to multiplication by a constant. If something is radiating energy away in the form of gravitational waves, shouldn't it be losing the same amount of mass? (up to the factor of c^2)"

That's a good question, and the answer is yes. What happens to the source of the waves depends on the details of its configuration.

For a binary, the orbit shrinks. Eventually the stars (or black holes) hit each other. If they're neutron stars, you get a bang (gamma-ray burst etc) and ... it's complicated. If they're black holes, you get no visible signature - but you get a burst of gravitational waves and are left with a rapidly rotating black hole.

For a rotating neutron star with mountains, like the ones your computer is looking for, the energy loss shows up as a slowing down of the rotation frequency. The gravitational wave frequency is proportional to that, so it goes down too. This effect is very small - an amazing source would take 300 years to slow down a Hz - but these days Einstein@Home is looking at long enough chunks of data to need to start accounting for it in the analysis.

Back to binaries: After the burst of gravitational waves has gone, you're left with a remnant that has a slightly smaller mass. This is a permanent change in the gravitational field, which shows up as a DC (non-waving) part of the gravitational wave signal, and is called the "memory" effect. Surprisingly, this was only discovered about 15 years ago. Everyone slapped themselves for having missed it, because in hindsight it's obvious it should be there.

Unfortunately, while frequency changes are pretty easy to see, the gravitational wave memory itself is going to be very hard to measure, even with advanced LIGO, unless we get lucky and a binary pops off nearby. (I think that's what Bruce was referring to.) Too bad, because it would be direct observational evidence of one of the unusual features of Einstein's theory ("gravity gravitates"). Not that we really doubt that, thanks to all the indirect evidence, but it's always nice to observe it directly.

Hope this helps,
Ben