# LIGO physicists eyeball a new gravitational wave

Physicists working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) have spotted gravitational waves rippling through the universe for the third time, according to results published in Physical Review Letters on Thursday. The discovery of gravitational waves was announced for the first time in February 2016. …

1. #### Not quite

"As mass travels through spacetime, a trail of gravitational waves is emitted"

As a mass *accelerates* through spacetime gravitational waves are emitted.

Being in an orbit counts as being accelerated. As the orbit speeds up when the black holes approach each other, the frequency of the waves increases until they merge and you get the massive backwash followed by silence.

1. #### Re: Not quite

An orbit has acceleration in space. An orbit in space is a projection of a geodesic ("straight line") in spacetime.

Ken

2. #### Two solar masses (in energy) escaped

So much for "nothing can get out of a black hole"

1. #### Re: Two solar masses (in energy) escaped

Interesting and mind-boggling question. I would propose that the energy did in fact not come out of the holes, but from the space-time itself, which got whipped around by the orbiting holes. Aren't black holes nothing but holes punched into space-time?

I really would love to hear an explanation from someone who really understands this stuff.

1. #### Re: Two solar masses (in energy) escaped

Same principle as a space probe getting a gravity assist by swinging around one planet on the way to another. Just on space steroids.

IIRC up to 30% of the mass of a black hole can be stolen by throwing crap at it and missing by just the right amount.

1. #### Re: Two solar masses (in energy) escaped

@Black Betty

"Same principle as a space probe getting a gravity assist by swinging around one planet on the way to another"

When a probe gets a gravity assist from a larger body it 'steals' energy from the velocity component of the larger body's momentum, not from its mass component, which remains unchanged.

2. #### Re: Two solar masses (in energy) escaped

"IIRC up to 30% of the mass of a black hole can be stolen by throwing crap at it and missing by just the right amount."

So a bit like the Earth and flying?

2. #### Re: Two solar masses (in energy) escaped

"I really would love to hear an explanation from someone who really understands this stuff."

Can't claim to be any kind of expert but my understanding is that the energy is radiated away in increasing amounts as the masses approach - this fits with the waveform detected where the amplitude increases with time until the point of merger. Maybe it will need much improved sensitivity to explore the very interesting region of time during which the merger happens.

3. #### Re: Two solar masses (in energy) escaped

The gravitational field of the black holes obviously extends beyond the event horizon. The gravity of two multi-stellar masses will indeed "whip the space time around" as they try to accelerate each other, liberating energy as they produce gravity waves. This obviously reduces the energy of the local black hole system. Because the energy of the system is proportional to its mass (E=MC^2), that liberation of energy will effectively reduce the mass of the black hole pair ... by a mechanism known as "magic stuff". :-)

2. #### Re: Two solar masses (in energy) escaped

Einstein's field equations state that nothing can get into or out of a black hole. Time stops at the event horizon.

1. #### Re: Two solar masses (in energy) escaped

"Einstein's field equations state that nothing can get into or out of a black hole. Time stops at the event horizon."

But they can apparently slowly "evaporate" due to Hawking radiation:

1. #### Re: Two solar masses (in energy) escaped

For large black holes "slowly" is no exaggeration - if my arithmetic is right, for the final black hole in this event, that's more than a hundred million, billion, billion, billion, billion, billion, billion times the current age of the Universe (roughly, or over 10^72 years).

Ken

3. #### Re: Two solar masses (in energy) escaped

So much for "nothing can get out of a black hole"

Gravity has been doing it forever!

1. #### Re: Two solar masses (in energy) escaped

One theory to explain how gravity (gravitons if those exist) seemingly escapes the black hole is that it's space itself that holds and propagates (sort of remembers) the gravity field. It does not need to be connected to the mass inside. Within the space around the black hole there is then information about how much mass is inside the event horizon. But that is just one theory. It don't fully explains gravitons etc. there is a lot of phenomenas where quantum theory and general relativity don't play with each other.

4. #### Re: Two solar masses (in energy) escaped

The point is that so far all the results have been in-line with general relativity. Using less precise descriptions can lead to odd-sounding conclusions, but "GR rules".

Ken

3. #### Nice story ... pity about the lousy pictures

The guardian seems to have got hold of a decipherable version for the 'Ligo's gravitationa-wave detection' image.

4. #### Wot I want to know ...

... is why has that pretty blue 'n yellow critter got one eye bigger that the other?

Nurse!

5. #### disappointed ...

misread headline, was expecting a fascinating story about physics labs assembled from basic (plastic) building bricks ....

6. Why is this story in 'Security' ???

1. Why is this story in 'Security' ???

That wave is headed our way! R-r-r-r-u-n!

2. Because some data agrees with predictions of GR, so lots of people are still secure in their understanding of reality?

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