Many questions about the LIGO Gravity Wave Experiment...

  • #1

Main Question or Discussion Point

I'd like to understand the details behind the LIGO experiment a bit better:
  1. How do scientists know that the gravitational waves detected by LIGO originated from two specific black holes (located a billion light years away)?

  2. Does the LIGO result confirm the existence of black holes?

  3. Was it only after gravitational waves were detected by LIGO, that scientists tried to figure out from which location the gravitational waves originated? i.e. post-hoc deduction

  4. There was a "dud" gravitational wave detection in 2014 by the BICEP2 telescope located at the South Pole. What makes scientists so confident this time that the LIGO results aren't a dud?
 

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  • #2
Orodruin
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How do scientists know that the gravitational waves detected by LIGO originated from two specific black holes (located a billion light years away)?
They do not. There are large error bars on exactly where the merger occurred. You cannot pinpoint a specific position.

Does the LIGO result confirm the existence of black holes?
Yes.

Was it only after gravitational waves were detected by LIGO, that scientists tried to figure out from which location the gravitational waves originated? i.e. post-hoc deduction
Yes. Before the gravitational wave arrived, there was no way of knowing a merger was going to occur there.

There was a "dud" gravitational wave detection in 2014 by the BICEP2 telescope located at the South Pole. What makes scientists so confident this time that the LIGO results aren't a dud?
The BICEP2 result was not an actual gravitational wave measurement, but an indirect measurement of the CMB polarisation which seemed to have properties which could be explained by gravitational waves in the early Universe. Scrutiny showed that the effect was likely due to foreground dust and not actual gravitational waves in the early Universe.

The signal at LIGO is very different. It is a direct measurement of a gravitational wave so the background is known. It was detected independently at the two different observatories. It matches exactly the prediction for a black hole merger. The expected rate of random fluctuations giving a similar signal is one in 200000 years.
 
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  • #3
How do scientists know that the gravitational waves detected by LIGO originated from two specific black holes (located a billion light years away)?
Because with two gravitational wave detectors, they have a partial reconstruction of the source of the waves. This means that if you measure a pulse of light (EM waves) twice, you can get some information about the direction of the source, because you know how far apart the two detectors are and what the timing difference between the signals.

Does the LIGO result confirm the existence of black holes?
No, it provides indirect evidence for their existence, but it is pretty compelling. There is other indirect evidence, mind you, by observing objects in the vicinity of what are clearly black holes.

Was it only after gravitational waves were detected by LIGO, that scientists tried to figure out from which location the gravitational waves originated? i.e. post-hoc deduction
Yes. The reconstruction required measuring the gravitational multiple times. The analysis for where the gravitational wave originated is extremely broad. It is a huge swath of the sky.

There was a "dud" gravitational wave detection in 2014 by the BICEP2 telescope located at the South Pole. What makes scientists so confident this time that the LIGO results aren't a dud?
Totally different physics was measured. In 2014, it was an indirect detection, so there was always the possibility that what they were observing weren't gravitational waves (They were measuring it through CMB polarization, which has two real sources, magnetic field and gravitational waves; it turns out, against early expectations, that it was the former and not the latter). Contrarily, the LIGO detection is a direct detection of gravitational waves, so they literally measured a gravitational wave itself. You can't really take that back.

As time goes on, they will detect many more gravitational waves, which will increase our confidence, but this detection alone sits just over 5 sigma (1 in 3 million chance it's random noise).
 
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  • #4
Orodruin
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No, it provides indirect evidence for their existence, but it is pretty compelling. There is other indirect evidence, mind you, by observing objects in the vicinity of what are clearly black holes.
If it walks like a duck and quacks like a duck ...
 
  • #5
If it walks like a duck and quacks like a duck ...
Like I said, it's extremely convincing indirect evidence. It's essentially impossible to envision this as failing to be black holes without overturning GR. And while these are extremely important theoretical points, it's still irrefutable that this isn't a direct detection of black holes.


(EDIT: English parlance is somewhat bad (particularly in the advent of blogs like Woit's), so let me be clear that when I say "important theoretical points" I don't mean important points that are themselves "just theoretical." I mean points that pertain to well-tested scientific theories and/or scientific principles.)
 
  • #6
Thanks FieldTheorist and Orodruin - very comprehensive replies.

Supposing that these gravitational waves indeed originated from these two black holes - do we expect LIGO to detect more gravitational waves from this same event?
 
  • #7
Orodruin
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Supposing that these gravitational waves indeed originated from these two black holes - do we expect LIGO to detect more gravitational waves from this same event?
No, the detectable gravitational wave from the event has passed us - just like the light coming from the supernova SN1987A passed us in 1987 and is not coming back. Just like the light from SN1987A, the gravitational wave spread out from the event at the speed of light and since it is an isolated event, it will no longer be detectable once the wave passes by.

Of course, just as there are many supernovae occurring in the Universe, there will likely be other black hole mergers to observe in gravitational waves.
 
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  • #8
pervect
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I'd like to understand the details behind the LIGO experiment a bit better:
  1. How do scientists know that the gravitational waves detected by LIGO originated from two specific black holes (located a billion light years away)?

  2. Does the LIGO result confirm the existence of black holes?

  3. Was it only after gravitational waves were detected by LIGO, that scientists tried to figure out from which location the gravitational waves originated? i.e. post-hoc deduction

  4. There was a "dud" gravitational wave detection in 2014 by the BICEP2 telescope located at the South Pole. What makes scientists so confident this time that the LIGO results aren't a dud?
The calculations are outlined in their paper, available online at https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_Detection_of_GW150914.pdf

A very brief summary. A plot of frequency vs time for the inspiral was made. The paper describes the details of the signal filtering. A brief quote from the paper:

The basic features of GW150914 point to it being produced by the coalescence of two black holes i.e., their orbital inspiral and merger, and subsequent final black hole ringdown. Over 0.2 s, the signal increases in frequency and amplitude in about 8 cycles from 35 to 150 Hz, where the amplitude reaches a maximum. The most plausible explanation for this evolution is the inspiral of two orbiting masses, m1 and m2, due to gravitational-wave emission. At the lower frequencies, such evolution is characterized by the chirp mass [11] L. Blanchet, T. Damour, B. R. Iyer, C. M. Will, and A. G.Wiseman, Phys. Rev. Lett. 74, 3515 (1995)]
The paper goes through an analysis of how compact the mass must be to emit a 150 hz signal, and concludes from the chirp mass and the size constraints that a black hole is the only plausible candidate.

As far as confidence goes - only time will tell. Certainly the intangible factors like the sheer size of the team and the amount of internal review seems very promising.
 
  • #9
vanhees71
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If it walks like a duck and quacks like a duck ...
Well, it doesn't always mean that it really is a duck! Particularly black holes are not very attractive guys. They are singularities of the general relativistic gravitational field (pretty similar to point charges in classical electrodynamics which can be characterized as singularities of the electromagnetic field), i.e., manifestations of our ignorance. Unfortunately we have no clue, what else such massive objects of around 30 solar masses might be than "black holes", but who knows which kind of unknown matter might exist that could explain it. As I said, so far we have no idea!
 
  • #10
Orodruin
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Well, it doesn't always mean that it really is a duck!
Well, I think this is the point of the proverb. If an object has the observable properties of a given type of object, there is really no way to distinguish it and you might as well refer to it as being of that type.
 
  • #11
vanhees71
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Sure, but as I stressed, I find it a bit unsatisfactory to explain something as a singularity of our description. I'd rather think that the description of the "duck" is incomplete in such a case. Of course, I'm well aware, that in the present stage of empirical knowledge it's impossible to come up with a better idea.
 
  • #12
Orodruin
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In that case we would probably still continue calling it a duck even after the more precise description was in place, we would simply say that "this is a better description of a duck".
 
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  • #13
PAllen
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An example is the fuzz ball model. It is generally called a candidate model of BH in a QG theory. It agrees with classical GR on macroscopic horizon properties, but differs microscopically (e.g. a Planck length beyond the classical horizon will show differences), and it has no singularity. IMO, this is a model of a quantum BH, not something that is not a BH.

I guess it depends on defining properties. To me, the singularity represents theory incompleteness, and is an inessential feature - if it exists, it can't be detected and it presumably doesn't exist. I would define BH by the macroscopic horizon properties and by the totality of GR predictions about externally detectable classical phenomenology. Whatever has these attributes is a BH.
 
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  • #14
Orodruin
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Whatever has these attributes is a BH.
In other words, whatever walks like a duck and quacks like a duck is a duck.
 
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