Gravitational Waves from Mergers

[RJ Emery]

Why would gravitational waves be generated from the merging of two black holes or other types of mergers?

What is different about the physical characteristics of the merged entity compared to the sum total of the pre-merged entities?

 

[marcus]

Why would gravitational waves be generated from the merging of two black holes or other types of mergers?

What is different about the physical characteristics of the merged entity compared to the sum total of the pre-merged entities?

energy, for one thing.
a lot of the energy in a binary system is dissipated as grav waves, as the two things spiral in towards each other faster and faster

so the combined BH object would not "weigh" as much as the sum of the two separate objects

circular motion involves fairly constant acceleration, and the acceleration of any sort of "charge" whether electrical or gravitational mass, produces waves in the corresponding field.

think about a circular radio-antenna

two compact objects spiraling in towards each other can actually dump quite a bit of their mass as waves

a head-on collision also involves a form of acceleration (namely DEceleration) so there are waves there too, just more of an abrupt pulse.

here is something fun to do:
google "moving charge" and it will send you here:
http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html

officially, this is about a moving ELECTRIC charge but don't worry, it is really fun and gives the right idea.
at the upper right menu where it says "linear" change that to "circular"
and then press "go"
you will see the charge move in a circle and the waves will spread out, as if from a ciruclar antenna.

I think this Caltech thing is one of the simplest and greatest physics animations on the web.

 

[RJ Emery]

energy, for one thing.
a lot of the energy in a binary system is dissipated as grav waves, as the two things spiral in towards each other faster and faster

So, by the time of the merger itself with a corresponding burst of gamma rays, the gravitational waves will have largely been emitted?

 

[Chris Hillman]

Links to excellent on-line expository papers

So, by the time of the merger itself with a corresponding burst of gamma rays, the gravitational waves will have largely been emitted?

The gravitational radiation expected from a merger event (e.g. merger of two black holes) according to gtr follows a very specific qualitative sequence; the quantitative features are also distinctive. Thus the interest in LIGO/VIRGO since these will provide tests of unprecedented strength, tests of gtr in the highly nonlinear strong field regime. (Existing tests probe the weak-field regime.) This includes periodic waves with frequency increasing as the binaries near each other, with a final frequency "chirp" followed by a burst of radiation during merger itself, followed by a characteristic "ringdown", followed by exponentially decaying radiation. This scenario has been carefully explained by leading experts like Kip Thorne in various expository papers which are comprehensible (if you skip the math) by laypersons. See http://math.ucr.edu/home/baez/RelWWW/HTML/grad.html#gw for suggested reading.