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Physicists Detect Gravitational Waves, Proving Einstein Right

nytimes.com

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Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#51

Post by Steuard »

elorant wrote: post #15

Are gravitational waves supposed to be that weak or is it because of the distance between us and those black holes? Do they lose power as they travel through space?

Both!

As others have said, intensity (power per unit area) decreases according to the same inverse square law that governs most effects due to localized sources in three dimensions of space. In this case, you're looking at a distance of over a billion light years, and then squaring it: that's a pretty enormous "per unit area"!

But gravity itself is also a tremendously weak force compared to the others. That may seem surprising at first, but it becomes pretty clear when I point out that a cheap little refrigerator magnet exerts enough force to overcome the gravitational pull of an entire planet right beneath it. Gravitational waves are pretty much just ripples on the top of that already tiny force.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#52

Post by strange_quark »

omgitstom wrote: post #27
pinkrooftop wrote: post #18

Would it be possible to listen for information transmitted via gravitational waves? Would there be any benefit over radio?

I think if you really wanted to think outside of box for this, quantum entangled particles is your best bet for instantaneous low energy communication

I'm sorry but no, you cannot transfer information with quantum entanglement. What entanglement says is that if you have a photon and I have a photon and they are entangled and you make a measurement on some attribute of your photon, my photon will assume the complimentary state. However, the state your photon assumes when you measure it is random and once you measure it, you lose the entanglement. So, there's no way for you to encode any information in your entangled photon. Yes, I can infer what state your photon was in as soon as you measure it, this is useful for encryption as we can then compare notes after making a measurement and make sure nobody tampered with our entangled photons.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#53

Post by ufmace »

elorant wrote: post #15

Are gravitational waves supposed to be that weak or is it because of the distance between us and those black holes? Do they lose power as they travel through space?

Near as I can tell, they're intrinsically extremely weak, so much so that the only thing we've been able to see it all is extreme events like these black hole mergers. In theory, pretty much any time anything moves, some level of gravitational waves should be generated, though no telling if we'd ever be able to detect them.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#54

Post by drx »

mikeash wrote: post #39
antognini wrote: post #10

Earlier quoted context omitted.

The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sens…

Is there any idea how far away these black holes were? It would be interesting to know the volume of space it can potentially detect evens.

TFA says "they had heard and recorded the sound of two black holes colliding a billion light-years away" and "1.2 billion years ago".

And from the paper: "The source lies at a luminosity distance of 410+160-180 Mpcc corresponding to a redshift z=0.09+0.03-0.04.". (https://dcc.ligo.org/LIGO-P150914/public) Which corresponds to 1.337+0.522-0.587 billion ly (or between 750.2 million and 1.859 billion ly).

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#55

Post by Terretta »

nickhalfasleep wrote: post #34
pinkrooftop wrote: post #18

Would it be possible to listen for information transmitted via gravitational waves? Would there be any benefit over radio?

This event was the equivalent of three of our suns turned into pure energy. Pretty expensive to send a message.

> This event was the equivalent of three of our suns turned into pure energy.

"The collision unleashed the energy of a billion trillion Suns in a fraction of a second."

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#56

Post by Steuard »

Roodgorf wrote: post #49

Are there any potential competing theories this detection could also support? I'm wondering how much room there is here for confirmation bias, but I suppose that's a pretty hard thing to measure without the benefit of hindsight.

Even before this discovery, it's been pretty solidly established that any alternative theory to General Relativity would need to behave essentially identically to GR in the limits where we've been able to test it. So, for example, the "low energy limit" of string theory is general relativity (plus other content, in most cases). I'm not sure whether the loop quantum gravity folks have a working low-curvature limit yet (I'm out of touch), but that would be a requirement for them, too.

At first glance, I'd guess that this discovery only strengthens that conclusion: even a small deviation from GR might well change the detailed behavior of an immensely high curvature situation like a black hole merger, and what we saw seems to have been a spot on match for the GR-based models.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#57

Post by davrosthedalek »

misnome wrote: post #48
losvedir wrote: post #4

> And then the ringing stopped as the two holes coalesced into a single black hole, a trapdoor in space with the equivalent mass of 62 suns. All in a fifth of a second, Earth time. Am I reading this correctly, that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided? Was that extremely coincidental? Or do these events happen all the tim…

In the press conference Kip Thorne mentioned that the estimates were for several events per year, before sensitivity upgrades.

But he neglected to mention the error bars on this, which AFAIK are huge at least for BH-BH mergers. Every time we built a new instrument, we saw something new, whether in astrophysics, or nuclear physics, or particle physics. Maybe the BH-BH rate is much higher than expected.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#58

Post by bzbarsky »

elorant wrote: post #15

Are gravitational waves supposed to be that weak or is it because of the distance between us and those black holes? Do they lose power as they travel through space?

A bit of both. https://en.wikipedia.org/wiki/Gravitational_wave#Wave_amplit... runs through the numbers for the gravitational waves the Earth produces by orbiting the sun; it claims that at 1 light-year the amplitude would be 1 part in 1e26.

For comparison, the wave that was detected is claimed to be "four one-thousandths of the diameter of a proton". That's about 7e-18 meters, on a baseline arm length of 4 km, so about one part in 6e20 -- about 175,000 times stronger than the waves Earth's orbit produces. And that was about 40x as strong as minimal sensitivity on LIGO, according to the article ("can detect changes in the length of one of those arms as small as one ten-thousandth the diameter of a proton").

Obviously if we were closer to the black hole collision we'd see much stronger waves. But you really do need very massive bodies accelerating very much (or equivalently orbiting very fast) to produce something that's detectable by LIGO over interstellar distances at all. The key part from this article is that the orbital period was about 1/250 of a second at the end; compare to Earth's orbital period. Going back to the formula given in the above Wikipedia entry, the frequency dependence is hiding in the "1/r" factor for the amplitude. 1/r is proportional to w^{2/3} (though it's not clear to me whether that's still true in a general-relativistic treatment; it's true enough for the Earth's orbit), which tells you how the wave amplitude scales with frequency...

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#59

Post by dragonwriter »

tomp wrote: post #23
antognini wrote: post #10

Earlier quoted context omitted.

The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sens…

If this events are so rare (that we don't even know how rare they are), how is it possible that they achieved the required certainty (5 sigma)? I guess you could count one looong wave as a series of one-time events/measurements, but it could as well be a loooong interference.

Others have answered other aspects of this, but as I understand it, it is not the case that we don't know how rare they (BH-BH events) are because they are so rare, we don't know how rare they are, because we don't have a really good model for them. So, we don't know how often we'd expect to detect them, once we had a detector.
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