Basic physics of GW150914

Benjamin Knispel
Benjamin Knispel
Joined: 1 Jun 06
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Topic 204925

Dear all,

the LIGO Scientific and Virgo Collaborations have just published a nice paper about “The basic physics of the binary black hole merger GW150914”. As they write on (highlighting by me),

The paper is a tiered introduction for a wide audience: the most important points can be accessible to anyone with a high school or undergraduate background in physics, and more advanced topics are explained assuming only a basic but wide science background. It is offered as a primer to anyone who wants to be prepared for the era of gravitational-wave astronomy, or wants to learn more about general relativity, black holes and gravitational waves – an open invitation to the physics of the exceptional event GW150914.

You will find the paper (open access) in The “Annalen der Physik”.


Cheers, Benjamin


Einstein@Home Project

Joined: 28 May 05
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Hallo Benjamin! Thank´s, it

Hallo Benjamin!

Thank´s, it is verry interresting. I downloaded it and will careful study it soon. 

It is the most detailed text for an intermediate level, I know.


Kind regards and

happy crunching


Mike Hewson
Mike Hewson
Joined: 1 Dec 05
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FWIW I have looked up the

FWIW I have looked up the Hoop Conjecture mentioned. Kip Thorne came up with it over 40 years ago. The Key Idea is that you can't actually pass a tape measure through a black hole, from one side to the other ( going in via the horizon on one side and coming out of the horizon on the opposite side ) in order to measure the radius. The classical 'radius' concept fails in relativity. However clearly the black hole on the outside appears to subtend an angle when viewed at some distance from it, so it is natural to want a radius/diameter to speak of.

This makes me think of the ball gauge used by cricket umpires :


.... and so, ignoring detail of the stitched seam for this discussion, this will give you a standard to compare a given ball's size w.r.t. cricket regulations.

Moreover one can place the ball in different orientations in this gauge and thus also see how round it is ie. after a few decent knocks it may not be the sphere as manufactured ( to some tolerance ). I see on TV occasions where the umpires, players etc are wondering whether the ball is indeed round enough to be played with. So they fiddle and rotate the ball in the gauge acting as a shape template. For example one might use a chicken egg and easily demonstrate how the distance from one point to some other point, both on the shell's surface, would vary.

Roughly : The Neat Idea is to find/make/imagine some circular hoop with a minimum circumference such that you can rotate it around some given enclosed mass which either is, or perhaps will shortly be forming, a black hole. Minimum here meaning : you cant' find/construct some other circular hoop with a shorter circumference that does that also. One can thus derive a 'radius' on the assumption that classical geometry holds - divide the circumference by 2 PI - and then you get a number about 3 km per solar mass enclosed for black holes.

'Circular' here is the usual Euclidean/flat space idea. A circle is a curve/locus of points each of which is the same distance from a single specific point, called the centre of the circle. Importantly a circle is a planar geometric object as is that ball gauge above, so in effect slices the thing held within it. A black hole's radius as derived above is a 'proxy' number for a slice we can't actually make.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

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