aramis720

It's only undetectable if you use a ruler of some sort. So we don't use a ruler.

 

 

No.

 

 

The length changes, as measured by something that depends on time rather than length. If the length changes but the speed of the probe is constant, what happens to the time?

 

 

The speed of light is not changing. This has been mentioned by several people in the discussion. I don't know why you think it is; the local value is always c.

We're going in circles here. The speed of light is invariant with respect to what? I understand, of course, that c is invariant, but various parties have offered here that it is the invariant speed of light that gives rise to detectable effects from the interferometer. I'm asking how this is the case. If you assert that the interferometer can detect grav waves b/c of a difference in phases of the two light signals sent up and back in each arm, there must be a change in wavelength or length of the arms or both. Most here have agreed that there is no change in either, so what will happen? No difference in phase overlaps of the two light signals. "Time" in this context is inferred from change in phase fringes, but there can't be any change in phase fringes based on the definitions offered in this context.

 

 

That's not what you were saying in your initial posts. I read the arguments to be that whatever "ruler" you might use also stretches and contracts, so you can't use a ruler to make the measurement.

 

e.g.

"I guess I'm still not seeing how the interferometer arm is detecting anything at all b/c if a gravitational wave is defined (as it is) as a ripple in spacetime itself, then anything in that ripple of spacetime will be distorted exactly the same degree to which spacetime is distorted."

 

and

 

"But again there is no detectable change in any dimension because the waves are literally waves of space, so anything occupying that space (whether it's falling masses or interferometer arms or a space gerbil) will be distorted by EXACTLY the same degree as space itself."

"Change" implies detectable change. So if the arms are waving to exactly the same degree as the space they occupy (as they must under the definitions of these terms in this context), then it's entirely undetectable. So the lengths of the arms are not changing. Which brings us back to my initial question: what is being measured?

 

 

The bending of light did not change the local speed

So how in this case is the speed of light being changed by grav waves, and in such a way that any change in speed would be detectable by an interferometer?

 

Since C can't change something has to, why not length?

Why would the invariance of c in this case lead to any length change? Again, the grav wave is waving space itself. The speed of light would be as affected by this wave as anything else in that space, as we know from various experiments finding the bending of light from gravitational masses like in 1919.

 

 

I had this same question some time ago. Strange's link in post #3 is IMO a good explanation. You have to think of the light as a clock rather than a ruler. Yes, the wavelength changes, but you are not detecting a change in wavelength, or counting the number of waves. You are detecting a phase difference from a difference in travel time between the two arms. Since c is an invariant, if the length changes (regardless of the wavelength of the light) the travel time will vary.

But the length of the arms isn't changing. That's been my point all along.

 

But what did they detect then? How do you think they detected it with the precision that they did if it was undetectable? How did they get the mathematics of the prediction to match the experiment?

 

If if was undetectable because the instruments contracted by the same degree as the space around it, as you said, how could they have gotten a false positive? They would have gotten a null result. Don't you see a contradiction there?

There's no contradiction. The wave forms reported in the experiment are modeled and then data is matched to the modeled wave forms. Since both detectors got the same signals something was detected, yes, but if my objections here are correct that detection event wasn't from grav waves. We certainly shouldn't put the cart before the horse and argue that the premises must be correct b/c they found what they were looking for. If the premises aren't correct the experiment can't work, period. We then should look to what was detected in the apparent false positive. Again, I'm almost certainly wrong about my objections here, since I'm not even a physicist. But I do follow this stuff pretty closely and I still haven't seen any responses to my concerns that satisfactorily explain how the alleged detection event is taking place.

 

Yes but no, you do realise it has been detected?

The point of my OP was that the premises underlying this experiment seem questionable. False positives happen pretty regularly in science.

 

 

Yes.

 

Maybe what you are missing is that the two arms are affected differently (because of the nature of the waves). Therefore, the time taken for light to travel each arm will be different (because the speed of light doesn't change and time = distance / velocity), therefore there will be a phase difference between each arm.

 

You seem to have misunderstood. As you can see, I didn't say they are not being contracted (alternately stretched and squeezed, in fact). I said they are both affected by the same amount. So if you tried to use the wavelength of the light to measure the length of the arm, you would measure a difference.

 

But if you use the speed of light, then you will see a difference. The speed of light is not affected. It travels different distances in each arm.

 

 

Again, I said the speed of light does NOT change.

 

I guess this is why you don't understand the explanations. You seem to be interpreting things to mean the opposite of what is said...

 

 

Because if light takes a different amount of time to travel one arm than the other, it will have a different phase when it gets back.

I could have stated more clearly my summary of your previous statements, I'll agree on that, but my point remains. You had agreed that length and wavelength change to the same degree exactly as the wave, and thus are undetectable, that was my point. You now clarify that because the speed of light remains constant that the interferometer will register a difference. But think again about what is going on with the measurement apparatus. Light does not exist in some other realm than the space we exist in. So if space itself is being distorted so is any light occupying that space. You write:"Because if light takes a different amount of time to travel one arm than the other, it will have a different phase when it gets back." But why would it take a different amount of time to travel one arm? We know, of course, that light is affected by gravity in the same way as mass -- this was the basis for the famous 1919 Eddington experiment looking at the curving of light around the sun during a full eclipse.

How can you have phase variance without time being involved? Seriously we have made every effort to explain the detection methods to you.

 

The part you continuously mistakenly misunderstand is the polarity nature of a GW wave. Calculate the wavelength and then apply a quarter of that to get your required detector length.

 

Then recognize that within that Precise same wavelength you have in essence 4 simultaneous movements.

 

Both -x and +x contract while +y and -y expand. You have an L shape inerferometer it is impossible for both arms to have the same vector components.

 

For the last time if you have length contradiction you MUST have an influence on the time components. It is impossible not to.

 

From the very first link posted by Strange.

 

This is two simultaneous movements with different vector components in each arm. Why is that so hard to understand? It is no different than gravitational redshift which relies on SR and GR being correct. Those blue/redshifts alter the frequency of the beams and causes phase shifts as a result. These frequency changes are detectable.

 

Lets put it in electrical terms. You have altered a frequency of light by a frequency of GW. This will induce a propogation delay which results in a phase shift.

 

We don't care about the detector walls.

 

We only care about the phase shifts.

 

Step 1) split a signal with amplitude of x. When you recombine the two signals as they are identical you have complementary waves = constructive interference. The amplitude will be the sum of the two waves.

 

Hit it now with a momentary GW wave. Arm on left contracts and its signal is delayed, signal on the perpinficular DOES NOT undergo the identical affect. It will blueshift not redshift. Its signal also goes out of phase.

 

HOWEVER both signals remain out of phase with each other so you get destructive interference on recombination.

 

The recombined signal will have an amplitude less than the sum of the amplitudes of the two signals.

 

The walls of the detector has nothing to do with the experiment. It is strictly its influence on the beam itself that is involved in the detection. When you have differences in gravitational potentials you gravitational redshift which involves both length and time dilation.

My point has been that there can't be any detectable length contraction b/c space itself is contracting, so anything occupying that space is contracted precisely the same amount -- and is thus undetectable. There is no contraction of either arm. Isn't that a pretty clear point?

 

 

The phase difference is due to the difference in light travel time.

 

 

We don't agree on that.

 

You seem to have convinced yourself to such an extent that you are not willing to consider that you might be mistaken.

 

You have just explained why there is a change in length: "so anything occuping that space will wave in exactly the same way as the grav wave".

 

The gravitational wave causes a change in space, i.e. length.

You just stated in the previous post that you did agree that neither the arm nor the light wave are being contracted. Here's your quote: "you are correct: the length of the arm and "ruler" (wavelength of light) both change on the same way." Then you stated that even though the arm and the wavelength of light weren't changing, the speed of light must nevertheless change with respect to the interferometer b/c of the special relativity postulate that the speed of light is always constant. I then asked you why you made this conclusion b/c that's not how interferometers work. They work based on comparing phase overlaps. If the arm lengths or wavelengths change the phases won't overlap exactly. So there's no literal time measurement. Could you explain your reasoning further here? I'm always willing to consider that I may be mistaken (in fact, it's almost certain that I am mistaken here b/c of the vast weight of authority against me), yet no one here has answered my questions satisfactorily and there seems to be an ongoing confusion about my question. For example, your last two lines show that you don't understand my point about grav waves distorting space and anything in it. The point of anything in that space that is being waved also waving is that it makes such waves in principle undetectable b/c there is no physical difference that can be detected -- if indeed grav waves are defined as waves of space itself. So what I'm suggesting is the conceptual structure by which physicists are suggesting grav wave detection may in fact be erroneous.

The speed of light is constant for all inertial observers and because of that fact the differences can be noted for the travel times in each arm.

How? The only measurement is phase overlaps or lack thereof. There's no time measurement at issue here.

Because that's what nature does when two blackholes merge. The same thing is not happening in two different directions and that is what's measured; as one is contracting the other one is expanding.. What's so hard about that? Things are not 'waving' in the same direction everywhere.

It seems that I'm not communicating well my basic point. Here it is: if a grav wave is defined, as it is, as a wave of space itself, anything occupying that space will wave to EXACTLY the same degree as the grav wave. So what can be used to detect such a wave when anything used to detect the wave must by necessity occupy the same space that is waving? Is this clear now? It doesn't matter what direction the wave is coming from. Any wave direction will wave space itself and anything occupying that space in exactly the same way.

There is a change in the length of the arms.

 

Why is there a change in length? Again, a grav wave is defined as being a wave of space, so anything occuping that space will wave in exactly the same way as the grav wave. So why is there a change in length of the arm?

 

 

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.

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...

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.

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.

So your stating GR length contraction does not occur ?

 

It is detectable within a range of wavelengths when you change the x axis lengths you change space. Thats what Ligo is designed to detect.

 

It's not just math.

 

Because not all particles in the same space is changing in the same direction.

 

As we have stated numerous times. Why do you think the arms are 4 km in length if memory serves.? wave length polarities Ripples.

Let's zero in on the interferometer arm that is allegedly contracting. The arm is in the x direction of space, let's say. The grav wave comes in at the same x direction, waving up and down. So any physical object, whether it's an interferometer arm or a light wave, that occupies that space will wave to exactly the same degree as space itself. It won't matter how long or short the arm is because any distortion in that arm will be undetectable b/c it's distorting in exactly the same amount as space itself. Is that clear?

what is the simplist possible explanation. hrrm k you get length contraction on the x axis at the same time length expansion on the y axis. Then next cycle reverse the two.

 

Measure the contraction/expansion with a modified Michelson interferometer.

But what I'm suggesting is that as these terms are defined in the theory itself there is no contraction or expansion of these arms that is detectable b/c it's space itself that is contracting and expanding, so anything in that space will contract and expand in exactly the same degree as space.

Waves in spacetime is changes in length as per length contradiction. So the x dimension of length does contract. What you have above does apply. That's precisely is what were detecting Ripples in spacetime geometry dimensions (ct,x,y,z)

Changes in length of what? Space itself, right? So, if we agree that space itself is contacting, how can anything occupying that same space measure that contraction?

The ripples are in different directions. Hence the required length to frequency ratio and shape of the arms.

 

Picture a ball squeeze it, the sides not being squeezed expand. Then next cycle this reverses. This action induces strain.

 

Did you read the wiki section on "effects of passing?" In essence you are measuring changes in the detector arms lengths.

 

[latex]\delta L(t)=\delta L_x-\delta L_y=h L(t)[/latex] where h is the measured strain.

But again there is no detectable change in any dimension because the waves are literally waves of space, so anything occupying that space (whether it's falling masses or interferometer arms or a space gerbil) will be distorted by EXACTLY the same degree as space itself. So how is the oscillating squeeze of the ball you describe and as depicted on the wikipedia page taking place when there is no detectable change?

 

 

It is the change in distance that is being measured. Because the distance changes, the time taken by the light changes. From the article I linked:

 

There is a link to another (very simple) explanation at the end of the article:

http://www.americanscientist.org/issues/pub/2004/11/wavy-gravy

Thanks for this additional link but I don't find it helpful b/c Shawhan is simply re-stating the conclusion as though it's an explanation. So the idea is this: the interferometer arm is stretched by the gravitational waves and therefore light takes a bit longer to travel that arm and therefore the phases of the light in the two arms doesn't match anymore, by a tiny amount (this is how all interferometers work). BUT, again, if the space in which the arm is located is stretching, the arm itself doesn't stretch in any detectable way. And therefore there is no additional distance for light to travel. It's not as though light continues to travel in a separate dimension at the "unstretched" speed. Light is occupying the same spacetime all physical things occupy. So we're back to the basic question of how LIGO is supposed to work.

The length of the detector arms must be of sufficient length to encompass a quarter gravitational wave. In order to understand this you must first be aware of the nature of a quadrupole wave.

 

Ligo can only detect a small range of gravitational waves just as an antennea can be tuned to pick up certain frequencies by increasing or decreasing the antenna length.

 

In the GW wave you have simultaneous x and y changes. As the x axis contracts the y axis expands and vise versa. This is specifically why the L shape is required. The deviations of the x and y axis of the GW wave induces strain which is what is being measured.

 

You have to look at the type of strain spin 2 quadrupole waves induces. The animation on this page shows the simultaneous movements.

 

https://en.m.wikipedia.org/wiki/Gravitational_wave

 

Using a L shape detector allows you to measure both the x and y axis changes.

I guess I'm still not seeing how the interferometer arm is detecting anything at all b/c if a gravitational wave is defined (as it is) as a ripple in spacetime itself, then anything in that ripple of spacetime will be distorted exactly the same degree to which spacetime is distorted. So how can an interferometer arm(s) occupying that spacetime detect the wave in any way?

We've already detected gravitational,waves with LIGO three separate times now. Here's a primer on how: https://www.ligo.caltech.edu/page/ligos-ifo

Thanks, but it doesn't address the point I'm asking about: if space itself is waving/warping, how can any interferometer, no matter how long, detect it? It seems to me pretty clear that it can't, in principle, because any attempt to measure it will use tools that are distorted by exactly the same amount as space itself is warped. No?

 

 

This is actually a really good question. What is being measured is the travel time of light along the two arms. The speed of light is not affected by the gravitational waves, so the minute difference in the time to travel along each arm can be measured (as a phase difference).

 

More here: http://stuver.blogspot.it/2012/09/q-if-light-is-stretchedcompressed-by-gw.html

Thanks, but I don't see how measuring the speed of light gets around the problem I'm referring to. Any speed of light refers to distance traveled divided by time, but if that distance is distorted by gravitational waves any measuring instrument will also be distorted by exactly the same amount, making such detection in principle impossible, or so it seems to me. Or am I missing something?

I have a basic question that I've never actually seen answered in discussions of LIGO and gravitational waves: if these waves are warping space itself (actually spacetime), then all matter occuping that space will be warped to exactly the same degree that space is warped, making such warps in principle undetectable. So if an interferometer like LIGO, with two perpendicular arms, is set up to measure such waves, what is it actually measuring? Any distortion of the arms in the direction of the waves will not be detected because that arm(s) will be distorted to exactly the same degree that space itself is distorted. Help?