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Once the two event horizons overlap the two black holes will necessarily merge. The "peanut shaped" event horizon of the new black hole will become ellipsoidal

Once the two event horizons overlap the two black holes will necessarily merge. The "peanut shaped" event horizon of the new black hole will become ellipsoidal as the black hole emits copious gravitational waves. This happens in approximately the time it takes light to travel a Schwarzschild radius. For a supermassive black hole, this could be on the order of a few seconds. This is a consequence of the no-hair theorem. The no-hair theorem states that the only possible stable black holes are Schwarzschild, Kerr, and Kerr-Newman black holes, which are all ellipsoidal in shape (or spherical in the case of a Schwarzschild black hole). When the event horizons overlap the new black hole has a decidedly non-ellipsoidal shape, so it has to become ellipsoidal and it does this through the emission of gravitational waves. Fittingly, this process is known as "balding."

HCIdivision17 12y ago
Do you have a reference to what the shape of these gravity waves would be like? Intuitively, I would guess they'd be star-cracking in magnitude, if the shape snaps that hard. Like, black holes smashing into each other is bad, but the snap from going from a peanut to a pill shape must be utterly catastrophic (I honestly haven't any appropriate adjectives for this - black holes sort of exist as stable catastrophies...)
- antognini 12y ago
  Yes, you don't want to be anywhere near the black holes as this happens. The strength of gravitational waves is measured by its "strain." The strain is the fractional change in length due to the gravitational wave. Gravitational wave detectors like LIGO are trying to measure strains of ~10^-20, so the distance between two points a light-year apart would change by ~0.1 mm. Obviously this is a very subtle effect. But near the merging black holes the strain is ~1. So if you start out 2 meters tall you will end up 4 meters tall in about 1 second.
ChuckMcM 12y ago
ok, mind blown AGAIN. So if their event horizons ever "touch" it sets of a massive series of gravity waves, which presumably LIGO and others will detect. But isn't there a time component to the gravity wave that would change the local time as it passed through? For some reason I was thinking that LIGO was measuring phase compression on light as an indication of a gravity wave wavefront.
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HCIdivision17 12y ago

Do you have a reference to what the shape of these gravity waves would be like? Intuitively, I would guess they'd be star-cracking in magnitude, if the shape snaps that hard. Like, black holes smashing into each other is bad, but the snap from going from a peanut to a pill shape must be utterly catastrophic (I honestly haven't any appropriate adjectives for this - black holes sort of exist as stable catastrophies...)

antognini 12y ago
Yes, you don't want to be anywhere near the black holes as this happens. The strength of gravitational waves is measured by its "strain." The strain is the fractional change in length due to the gravitational wave. Gravitational wave detectors like LIGO are trying to measure strains of ~10^-20, so the distance between two points a light-year apart would change by ~0.1 mm. Obviously this is a very subtle effect. But near the merging black holes the strain is ~1. So if you start out 2 meters tall you will end up 4 meters tall in about 1 second.

ChuckMcM 12y ago

ok, mind blown AGAIN. So if their event horizons ever "touch" it sets of a massive series of gravity waves, which presumably LIGO and others will detect. But isn't there a time component to the gravity wave that would change the local time as it passed through? For some reason I was thinking that LIGO was measuring phase compression on light as an indication of a gravity wave wavefront.

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