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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

#151

Post by tantalor »

idanoeman wrote: post #124
sandworm101 wrote: post #105

Earlier quoted context omitted.

>> that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided? Counterintuitive, but yes. Because it happened billions of years ago, it happened a long long way away. The sphere of objects billions of years away/ago is far larger than those closer to us. So such a detector should be detecting exponentially more very old objects than new o…

*Polynomially more objects. The volume of a constant-thickness spherical shell is O(r^2).

*Quadratically more objects.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#152

Post by Florin_Andrei »

bzbarsky wrote: post #126
swombat wrote: post #103

Earlier quoted context omitted.

On the other hand, we may well detect the 3 solar masses radiated away as energy. That decreases as an inverse square law, so as one solar mass is about 10^30kg, and 1kg gives off about 10^17 J, we're talking about an explosion releasing something around 10^47 J. For comparison, a 1 kiloton nuclear bomb gives off about 10^15 J. So, inverse square that explosion... 1 light year is about 10^16m, so we square that and g…

The big question is how much of the energy would get transferred in practice. I agree that 3 solar masses worth of electromagnetic radiation at 1 light year distance would feel like a nuke going off. What I don't know is to what extent the energy of the equivalent gravitational waves (which _would_ have a lot of energy I agree) would actually get transferred to things we care about, like the atmosphere and us. If it'…

If the gradient is small (as it should be 1 ly away) then the coupling ought to be very weak.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#153

Post by anchpop »

boardwaalk wrote: post #138

If they build a third observatory, can they triangulate where in the universe the events are occurring?

They'd have to build 4, actually. Imagine 3 intersecting spheres, there would be 2 possible positions remaining (just as with 2d triangulation, having 2 circles would leave 2 points remaining and we'd need a 3rd to find out which was correct

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#154

Post by astrosi »

bzbarsky wrote: post #133
ISL wrote: post #97

Earlier quoted context omitted.

The longer the interferometer arms, the better you can do in sensitivity. The reason LIGO has 4000 m long arms is that it makes the experiment 4000x more sensitive than something you can do on a bench. (and their laser stabilization is excellent, improving things further)

Sure, but LIGO is sensitive at something like 1 part in 1e20, which is a lot more than 4000x better than 1 part in 1e6. I agree that their laser stabilization is likely much better, their vacuum is likely a lot better, etc. I was just surprised by how much better, I guess; 10 orders of magnitude is a lot.

Part of the reason for that is that LIGO isn't exactly a Michaelson interferometer in that it has an extra pair of mirrors in each arm. If you look at this schematic [1] then in a traditional Michaelson interferometer you would only have the mirrors that are at the end of both arms.

With LIGO there is an extra set of mirrors within the arms this allows the light from the laser to bounce between them ~100 times or so increasing the effective path length greatly.

[1] https://www.nsf.gov/news/speeches/colwell/rc03_ligo/img009.j...

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#155

Post by lutorm »

stouset wrote: post #145
boardwaalk wrote: post #138

If they build a third observatory, can they triangulate where in the universe the events are occurring?

My intuition is that this is unlikely, but I'd love to see someone do the math. Given the scale of interstellar distances, any locations on our planet (and even in our solar system) are going to effectively function as a single point. Given arbitrarily-accurate measurement, it could work, but I'd bet physical limitations will prevent that from being a possibility. To my mind, it'd be roughly like trying to triangulat…

It definitely would work. The distance to the event is irrelevant, it's the light travel time between the detectors compared to the accuracy with which you can pin the event down in time that matters. The light travel time across the Earth is of order a hundredth of a second, which is a significant fraction of an event that takes ~ a tenth of a second.

However, the error ellipse will probably be quite larg, and given that they come from cosmological distances it's unlikely that they would be anything but isotropically distributed (like gamma ray bursts are).

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#157

Post by pdonis »

davrosthedalek wrote: post #61
Steuard wrote: post #56

Earlier quoted context omitted.

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…

Well, they extracted a lot from the waveform: Distance, the two masses, the resulting mass. I could imagine that a competing theory gives the same waveform maybe with different values for these parameters.

A "competing theory" would first have to match the GR predictions in all the other regimes where it's already been tested. But doing that is an extremely strong constraint on a theory, to the point where the only theory that can meet it is GR itself. Physicists know this because alternative theories to GR have been constructed and tested, and they have all failed. See, for example, here:

https://en.wikipedia.org/wiki/Alternatives_to_general_relati...

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#158

Post by xenophonf »

drx wrote: post #54
mikeash wrote: post #39

Earlier quoted context omitted.

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).

That's around 1/60th the diameter of the observable universe!
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