# What actually is the event that we think we have detected with gravitational waves? [duplicate]

This answer shows the "event" that is creating excitement. It looks to the untrained eye like a single "blip" on a detector. It appears to last less than a second.

It is, later in the answer, referred to as a "black hole merger".

Are we seriously saying that two black holes merged in under a second?

## marked as duplicate by Danu, ACuriousMind♦, John Rennie black-holes StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Feb 17 '16 at 6:56

• All this is explained in simple terms and with short videos at the press announcement of the discovery. Here is the webcast youtube.com/watch?v=_582rU6neLc – anna v Feb 14 '16 at 7:35
• Good to know. That webcast is over an hour. It will also be good to have a succinct answer here on physics.stackexchange.... – GreenAsJade Feb 14 '16 at 7:41
• too broad to be answered here. Why do you think the presentations lasted 45 minutes? – anna v Feb 14 '16 at 7:46
• 'cmon. It's a yes/no answer. And it has been answered in one paragraph, below. – GreenAsJade Feb 14 '16 at 7:52
• It answers one of your many questions in the text. – anna v Feb 14 '16 at 8:10

Yes - the black holes actually merged in less than a second. If my memory serves me from the press announcement, at the time of final collision, the two black holes were moving relativitistically, at approximately 50% the speed of light. When you have compact massive bodies orbiting one another with very small orbits, you get very high speeds.

• It may be worth adding that they feel very sure that it is a black hole collision because of the close match between various characteristics of simulation of that event and the actual measured waveform. – GreenAsJade Feb 14 '16 at 7:58
• What I've also come to appreciate is that the gravitational wave pulse that was observed is the very last moment of the merger. – GreenAsJade Feb 14 '16 at 11:18
• Right - obviously, this system has existed for a while. – Sam Blitz Feb 14 '16 at 16:01
• It's worth noting that these guys have almost certainly been orbiting each other for ages, radiating undetectably small amounts of energy and slowly spiraling in. The strength of the waves and radiated power increasing as they grew closer together. The event that lasted less than a second was just the cataclysmic finale to a drawn out ballet. – dmckee Feb 14 '16 at 17:13
• Yes - that's what I realised: not something clearly explained. – GreenAsJade Feb 15 '16 at 11:26

The signal was fit to that produced by two black holes merging, with each having a mass of roughly 30 times that of our sun. For a solar mass the Schwarzschild radius is about 3 km. So that means the black holes, if off by themselves, would have an event horizon radius of about 90 km.

Only right near the end of their fate during the merger was the gravitational wave strong enough to make it above the detector noise threshold. Yes gravitational waves were emitted during the entire process, but we can only detect the violent end which releases the strongest waves.

In the process of merging, the black holes were moving relativistically. So moving on the order of 100 km many times in that short chirp recorded is quite reasonable.