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Because the event is extremely violent.

Just as a nuclear bomb converts a small amount of mass to radiant energy (grams?), a merger can convert mass into other forms of energy, too.

Gravitational waves, of which there are probably many passing through us all the time from many sources, carry a lot of power. The only reason we don't see them every day is that spacetime itself is extremely stiff.

Isn't it also that power decreases over the cube of the distance to the event? A truly gargantuan amount of power (1 solar mass equivalent) distributed across a 3-dimensional shell dozens or hundreds of light years across should be minuscule at any Earth-sized point along that shell.

They put the distance at 440−190+180  Mpc. 440 megaparsecs is is around 1.2 billion light years away(!).

Wouldn't it be inverse square, not inverse cubed? Or do gravity waves behave different than other things we're used to, like light?

Actually the nice thing about gravitational waves is that we are sensitive to their amplitude, which scales as 1/r, and not to their energy, which scales as 1/r^2.

When you say that spacetime itself is extremely stiff, I can't help myself but to visualize a lattice of discrete points through which the wave propagates through. I understand this would be the wrong way about thinking about it, but is there another way?

This isn't really a wrong picture to imagine. In a way, this is what the experiments are trying to measure. You have a mirror and a laser at two points of your lattice (the space itself), and the "region" between them compresses and expands, which makes the laser take a slightly different time to bounce back.

Far away in linear approximation (which is what we saw here), the lattice picture is quite appropriate as a first order analogy.

That's an open problem right now. Lots of proposals, insufficient evidence to resolve the question.

Stoset is right. The reason why we do not notice them is due to the inverse square law. This event was 1.3 billion light years away. 1 Solar mass of energy being turned into a gravitational wave nearly instantly creates a very strong gravitational wave. But by the time it comes here, it deforms things by distances that we normally reserve for subatomic particles. What this finding shows is that this happens somewhat regularly in our Universe, it was just that we were previously unable to achieve the precision to measure such small deformations.

Not inverse square law, but simply inverse law.

As energy radiates outwards, it becomes diluted on a sphere that increases its surface area as r^2. Thus the local energy density goes with inverse square law in the distance to the observer. But we don't measure the local energy density of gravitational waves, but the local amplitude. And in wave mechanics energy is the square of the amplitude. So the amplitude goes down with 1/r.

Not really. You could have been within a few thousand miles of this collision and survived. In fact, it would be well below your ability to notice if it were 1 AU away.


Needless to say, you would not survive if 1 solar mass of energy were released in any other form (even neutrinos). The key feature is that that tremendous amount of energy results in only a minuscule deformation of spacetime, i.e., spacetime is extremely stiff.

By the way, the distances are way smaller than merely subatomic. The strain sensitivity is 10^-21, which for the ~ 1 km arms of LIGO is a shift of just 10^-8 the width of an atom.

> you would not survive if 1 solar mass of energy were released in any other form (even neutrinos)

But it is close!

A supernova releases "few times 10^45 J of neutrino energy" [1], so let's say 5. 5e45 J is about 6e28 kg, while solar mass is 2e30 kg. And neutron radiation from a supernova would get fatal when closer than about 2.3 AU [2]. So we have a factor of 30 from the masses and a factor of 5 (1^2 AU vs. 2.3^2 AU) from the distance.

So about 1/150 of solar mass released in neutron radiation would be survivable at the distance of 1 AU.

[1] http://wayback.archive.org/web/20120313045458/http://www.and...

[2] https://what-if.xkcd.com/73/

Yea, I mentioned this because I just read it a couple of days ago :)

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