Can LIGO actually detect gravitational waves?

[Strange]

VIRGO, in Italy, should be online soon (or is already?) - I realised the other day, I have driven right past it on the autostrada from Pisa. Next time I will have to see if it can be seen from the road!

[KipIngram]

It seems they still have sensitivity upgrades to do on several of these facilities. I'm guessing that in a few years we'll be picking up stuff like this routinely. It's really great that we got a "scream event" the way we did - it proves that the systems work and that we haven't wasted our money putting them in.

I once read some stuff about home-built radio telescopes. By definition they were small, of course, since they were intended to be within reach of individuals. But if you had access to a parcel of land you could do the same kind of thing there - deploy several of them in an array, hundreds of meters apart, and use the same sort of time sync circuitry I mentioned above to get them locked together. Then you'd (sort of) have a very large radio telescope.

 

I read last night they're planning a space-based gravitational observatory in the future. An array of satellites following Earth's orbit, but well away from Earth. I think it mentioned a 5 million kilometer arm length.

[tar]

KipIngram,

 

Understood. I should not be thinking micro, when large samples and statistical analysis are in play. But still, in regards to your objection, if the space that everything is occupying changes shape, the photons on average, or as a whole, will be affected by the change in shape. They would take the curved route, thereby going straight.

 

If the signals are then processed and checked against projected expectations of various events at various distances in various directions, until a match is made, there is a certain

 

"Expectancy bias is what sometimes makes double-blind studies necessary. Expectancy bias often creates a fault in your long-term memory. Expectancy bias can change the outcome of an experiment because the results are the effect of the experimenter changing results based on what they think is supposed to happen."

 

Regards, TAR

[KipIngram]

"Expectancy bias is what sometimes makes double-blind studies necessary. Expectancy bias often creates a fault in your long-term memory. Expectancy bias can change the outcome of an experiment because the results are the effect of the experimenter changing results based on what they think is supposed to happen."

 

 

Very true, and they'd need to use statistical techniques to guard against that. But at the same time, knowledge of what particular sought after events will "look like" in your apparatus is additional information that you can take advantage of. Of course, you also run the risk that such knowledge is flawed, in which case you're now looking for the wrong thing. But all of these things can be treated statistically, and I have to assume they have been.

 

A simple example is knowing that you're looking for a particular frequency in your signal. Using either analog circuit techniques or digital processing of the output you can "lift that component" out of the results. If you assume the wrong frequency you won't find anything. But if you are close you hear something, and then you can fine tune. I'm sure they're taking the results they got on the black hole merger and using them to refine our expectations about such events.

 

I think a lot of our physics experiments historically have "designed in" some expectation about what we expect to see - that's not a new thing at all.

[tar]

KipIngram,

 

Well I am sure you are right. People have already taken such expectation bias into account and in essence performed double blind experiments, but I would be more convinced if somebody that did not know the scream event occurred, gleaned its occurrence from the LIGO data.

 

Just knowing what a thing looks like does not always mean you are seeing that thing when something looks like it. I am thinking that a whisper event, close, would "look" identical to a far away scream.

 

Raising the question, as to why LIGO is not picking up those closer whispers.

 

Regards, TAR

[StringJunky]

 

 

 

 

I think a lot of our physics experiments historically have "designed in" some expectation about what we expect to see - that's not a new thing at all.

 

Of course, they are working to a prediction.

[tar]

A toy truck sitting on the rail would look 100 times bigger than the real truck coming up the road.

[KipIngram]

Raising the question, as to why LIGO is not picking up those closer whispers.

 

The are picking them up - lots of them. But all of that is rejected (at least the local whispers) because only one of the LIGO facilities detects them.

 

Now, if you mean "slight less local" whispers, like from the motion of bodies in our solar system, then the question has more merit. I guess it has to do with just how big the difference between the "screams" and the "whispers" are. I'm sure that we'd have been proud of anything that could be firmly ascribed to gravity waves since we're just starting out in this endeavor. But later, when we have much more sensitive facilities such that we could detect (for example - not sure if we'll be able to do this) the motion of Jupiter through the solar system, we'd remove that from the data as we processed it because we know Jupiter is there and we know we're not interested in that information.

[Mordred]

Keep in mind looking at the spin statistics polarity effects is only 1 part of the predictions. Two other key aspects is the Chirp signal and the ringdown. These develop depending on the type of merger. The paper I posted only covered the detection of the quadupole nature.

 

In essence they examine the full signal not just 1 wavelength of said signal.

Edited June 4, 2017 by Mordred

[tar]

KipIngram,

 

Not sure, that throwing out Jupiter's contribution would be the best way to get a "picture" of gravity.

 

I am thinking about how I could tell when a storm was coming close, before I heard the thunder or saw any flashes, by the way the static sounded on my shortwave receiver. Static is "just noise" obscuring a short wave broadcast, but if it is the storm you are listening for, well then, it is not noise.

 

Regards, TAR

[KipIngram]

But I don't think the point here is to get a picture of gravity as a phenomenon - the point is to use the phenomenon of gravity to get a picture of other things. So it does make sense to disregard the pieces of your signal that you know can be attributed to things you're not interested in. If one of those pieces is fully predictable, then you can just remove it - the fancy processing has to do with trying to remove things that are not predictable, like local noise.

 

I do agree with you that great care is in order when imposing expectations on the output of an experiment, though. Like any other thing one does, it needs to be done right.

[tar]

KipIngram,

 

Understood...but my thought is that if you just "look" at every vibration, you will be looking at the world. You will see the way gravity looks, and you would see the different "look" of Jupiter, as opposed to Neptune. If the goal is simply to verify predictions based on theoretical math, well you have not learned anything about the world. If the goal is to "see" a gravitation wave, well then why not look at them all, get used to what they look like, and maybe learn something new about what the universe is doing, by watching all the gravitational waves passing thou gh.]

 

Oh, THAT's what gravity is doing!

 

My thought being, that our brains are plastic, built to recognize patterns and we have an ingrained understanding of space and time. "Watching" gravity, in real time, by let's say taking the fringe information and simulating what waves are moving through in a model simulation with real time symbolic motion being represented by colors and real time motion of simulated lines and such would be "better" than looking at an equation in terms of comprehending what reality is doing.

 

Regards, TAR

 

 

Or add in the simulation, the "sound" of the various waves. Like for instance take the range of frequencies your instrument is picking up and translate those in a direct proportional way to our range of hearing. You could then "listen" to the overtones and echoes while you watched, and the understanding would be direct and real, with no statistical analysis required.

Edited June 4, 2017 by tar

[KipIngram]

Ok. I don't really see the two as mutually exclusive, though. You've got this mountain of data. And you have a list of theoretically predicted "patterns" for things like black hole mergers. So you can look at your mountain of data searching it for such patterns. Or you can just look at it - it's all good.

 

It's routine, for example, for astronomers to search for outer solar system objects by taking photographs from two different times and using computers to scan them for things that have moved. That's how they found Eris, for example.

[tar]

​KipIngram,

 

OK to do both, but the thread question is whether or not we can "see" gravitational waves through the data coming from LIGO. If we could "see" Jupiter's gravity, then we would know what gravity looked like, the answer would be yes, and we could then "see" gravity coming from a dark source, and have increased the reach of our senses, through computer simulation, using the actual data as our input and an analogous image as what we experience.

 

 

Regards, TAR

[KipIngram]

Yes, definitely - I think if they'd been able to see Jupiter with the available sensitivity that would have happened - we wouldn't have had to "wait for a scream." And dI think they've actually worked out the distribution of dark matter exactly that way - by deducing that it has to be there based on observations of things we can see.

[Strange]

​KipIngram,

 

OK to do both, but the thread question is whether or not we can "see" gravitational waves through the data coming from LIGO. If we could "see" Jupiter's gravity, then we would know what gravity looked like, the answer would be yes, and we could then "see" gravity coming from a dark source, and have increased the reach of our senses, through computer simulation, using the actual data as our input and an analogous image as what we experience.

 

 

Regards, TAR

 

 

We can "see" Jupiter's gravity. Through the orbits of its moons, the effects it has on the orbits of other planets, etc. Jupiter does not generate gravitational waves, so there is nothing to "see" in that regard.

 

And, we can "see" the gravity of dark matter (if that is what you mean by "dark source"?) which is how we know that there must be dark matter. And, of course, there have been other "dark sources" such as Neptune, which was first known about because of its gravitational effects, a long time before it was actually seen.

Edited June 4, 2017 by Strange

[aramis720]

To detect you essentially send a continous wavelength signal with a laser beam. You split the beam with mirrors via destructive interference. Then combine the two with constructive interference reforming the original wavelength.

 

This forms your baseline, when length contraction occurs on either arm the phases shift due to length contraction leads to destructive interference on recombination as one or both signals change which are detectable at the receivers

 

This is essentially how the Michelson interferometer works. Also keep in mind you cannot have length contraction without also having time dilation due to a GW wave.

If your interested I can post the mathematics specific to the spin 2 and quadrupole nature of a GW wave,as well as spin 1. Mechanical vibration and the electromagnetic waves are dipoler waves they do not have the same effects on phase shifts. There is no attenuation or dispersion in a GW wave. As such a lot of research went into filtering out those types of interference. Though there is a distinguishable difference between spin 1 and spin 2. (mechanical vibration is symmetric with spin 1 dipoler).

 

Though I'm less familiar with the detector end.

Thank you just wanted clarity we get all kinds on a forum lol

Yes, I understand how interferometers work but no one here has explained why either of the arms of the interferometer is supposed to contract due to the grav wave. Again, the wave is defined as a wave of space itself, so anything (the arm or what have you) in that space will wave to exactly the same degree as the space itself that it occupies, thus the wave will be undetectable.

[Strange]

Yes, I understand how interferometers work but no one here has explained why either of the arms of the interferometer is supposed to contract due to the grav wave.

 

 

I have provided several different explanations.

 

If you could explain exactly why you find those unsatisfactory, it might be easier to address your question.

 

So, for example, if you were attempting to measure the length of the interferometer by using the wavelength of light, then you would be correct: both the arm and the "ruler" (the wavelength) would vary by the same amount. But that is not what is being done.

 

Can you be more specific about which parts of the various explanations provided you do not understand? (Without just saying "everything is warped to the same degree so gravitational waves are undetectable"; they clearly are detectable so the problem is with your understanding of the explanation, not the detection process itself.)

[geordief]

Yes, I understand how interferometers work but no one here has explained why either of the arms of the interferometer is supposed to contract due to the grav wave. Again, the wave is defined as a wave of space itself, so anything (the arm or what have you) in that space will wave to exactly the same degree as the space itself that it occupies, thus the wave will be undetectable.

Perhaps I am equally (or more so) ignorant as you in this question and so perhaps I can put a (hazy) suggestion, The gravitational waves that are predicted (and found) propagate through space at the speed c. They encounter the interferometer which also contracts (I am not sure of the direction) in spacetime.

 

This contraction though ,in my opinion also needs time to take effect and also (I imagine ) itself propagates at the speed of c in its own direction (a different direction from that of the gravitational wave)

 

These two propagations do not cancel out and so the gravitational wave is detectable.

 

That may well have been nonsense but perhaps it makes a sense to you and others in the thread

[aramis720]

Perhaps I am equally (or more so) ignorant as you in this question and so perhaps I can put a (hazy) suggestion, The gravitational waves that are predicted (and found) propagate through space at the speed c. They encounter the interferometer which also contracts (I am not sure of the direction) in spacetime.

 

This contraction though ,in my opinion also needs time to take effect and also (I imagine ) itself propagates at the speed of c in its own direction (a different direction from that of the gravitational wave)

 

These two propagations do not cancel out and so the gravitational wave is detectable.

 

That may well have been nonsense but perhaps it makes a sense to you and others in the thread

This isn't correct b/c the light wave and the interferometer arm are contracting to exactly the same degree and at exactly the same time as the grav wave -- again because the wave is a wave in space itself, so anything contained in that space will of course be moving precisely with the wave.

 

 

I have provided several different explanations.

 

If you could explain exactly why you find those unsatisfactory, it might be easier to address your question.

 

So, for example, if you were attempting to measure the length of the interferometer by using the wavelength of light, then you would be correct: both the arm and the "ruler" (the wavelength) would vary by the same amount. But that is not what is being done.

 

Can you be more specific about which parts of the various explanations provided you do not understand? (Without just saying "everything is warped to the same degree so gravitational waves are undetectable"; they clearly are detectable so the problem is with your understanding of the explanation, not the detection process itself.)

So what exactly is the mechanism you're suggesting? Sounds like we agree that the arm does not contract in a detectable way, and nor does the wavelength -- both b/c the wave is a wave of space itself, and thus anything in that space is also waving to exactly the same degree as the grav. wave. If those two facts are true how is anything being detected.

Edited June 4, 2017 by aramis720

[KipIngram]

aramis720: Just so I can get calibrated here, are you contesting whether or not LIGO works, or are you complaining that the explanations you've seen so far are too hand wavy to suit you?

[aramis720]

aramis720: Just so I can get calibrated here, are you contesting whether or not LIGO works, or are you complaining that the explanations you've seen so far are too hand wavy to suit you?

I'm mainly questioning the basis for the experiment. False positives occur all the time in science and if the physical basis for this experiment is faulty then we're looking at false positives. Keep in mind also that the ability to tell where the apparent signal is coming from is extremely low granularity at this time b/c there are only two detectors. A third is coming online soon that will allow triangulation. So it's possible that we're seeing some kind of signal and a rough ex post explanation of where it's likely to come from, based on a faulty premise about how to detect grav waves. Now all of this is extremely unlikely, b/c I'm not even a Ph.D in physics, but this is why we have forums to discuss basic questions...

[KipIngram]

Yes, with two stations they were only able to triangulate a line through the sky along which the source would lie.

[Strange]

So what exactly is the mechanism you're suggesting? Sounds like we agree that the arm does not contract in a detectable way, and nor does the wavelength -- both b/c the wave is a wave of space itself, and thus anything in that space is also waving to exactly the same degree as the grav. wave. If those two facts are true how is anything being detected.

 

 

I don't think I can do any better than the explanation from the first page I linked to:

A gravitational wave will cause one arm to shorten and the other to lengthen. This will also cause the laser wavelength in the shortened arm to decrease (blueshift) and the wavelength in the lengthened arm to increase (redshift). But there is nothing in the detector that measures wavelength. What it really measures is the shift in the arrival time of each 'tick' of the wavelength crests. If the arms stay the same length (no gravitational wave), then the 'ticks' of the laser light come back to the beam splitter at the same time and produces destructive interference where we measure the light (labeled 'Photodetector' in the image above). If a gravitational wave causes the length of the arms to change and shifts where the 'ticks' of the laser light occur, the two light beams will no longer return to the beam splitter at the same time. It is this "out of sync" arrival time of the crests of the laser light that produces the interference patter we utilize to detect gravitational waves - we couldn't care less about the actual wavelength of the light (other than it was consistent going into the detector).

So, you are correct: the length of the arm and "ruler" (wavelength of light) both change on the same way. But the speed of light doesn't change. So the time taken for each "pulse" (wave) to arrive changes.

 

As I say, unless you can say what it is about that you don't understand (i.e. it is not measuring length, it is measuring travel time) I don't know how to make it clearer.

I'm mainly questioning the basis for the experiment. False positives occur all the time in science and if the physical basis for this experiment is faulty then we're looking at false positives.

 

I'm not sure they occur "all the time" but they do occur. Good experimental design (and results analysis) is all about trying to eliminate them.

 

You can read all about the work done to check that it is a real signal and extract all the information from it here: https://cplberry.com/2016/02/23/gw150914-the-papers/

 

 

 

 

Keep in mind also that the ability to tell where the apparent signal is coming from is extremely low granularity at this time b/c there are only two detectors. A third is coming online soon that will allow triangulation. So it's possible that we're seeing some kind of signal and a rough ex post explanation of where it's likely to come from, based on a faulty premise about how to detect grav waves.

 

The likely direction is a result of the detection (a rough triangulation based mainly on the delay between the two detectors) not an input to it, so I'm not sure why that would be a problem.

[aramis720]

 

 

I don't think I can do any better than the explanation from the first page I linked to:

So, you are correct: the length of the arm and "ruler" (wavelength of light) both change on the same way. But the speed of light doesn't change. So the time taken for each "pulse" (wave) to arrive changes.

 

As I say, unless you can say what it is about that you don't understand (i.e. it is not measuring length, it is measuring travel time) I don't know how to make it clearer.

 

I'm not sure they occur "all the time" but they do occur. Good experimental design (and results analysis) is all about trying to eliminate them.

 

You can read all about the work done to check that it is a real signal and extract all the information from it here: https://cplberry.com/2016/02/23/gw150914-the-papers/

 

 

 

The likely direction is a result of the detection (a rough triangulation based mainly on the delay between the two detectors) not an input to it, so I'm not sure why that would be a problem.

But it's not true that the time for the propagation of light is changing and thus detectable. What is being detected is a phase difference in the light signals in both arms. And if the phase is off this is interpreted as a change in the length of one or both arms. That's what is meant by a change in timing. So the change in timing is inferred from the phase difference, and that is due entirely to a change in the length of the arms OR a change in the wavelength. Since we both agree that there is no change in the arms or the wavelength from the grav waves we are back staring at the essential problem I raised in the OP.