# Can LIGO actually detect gravitational waves?

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Strange    2431
Posted (edited)

I've argued that it wouldn't b/c the meter stick (the interferometer arms and their effect on the light fringe phase) itself is changing to exactly the same degree that space and spacetime are being distorted.

If the arms were being used as a meter stick, you might be right. But they aren't.

The change in the arms is what is being measured. By using light. Which does not change in the same way.

Edited by Strange

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PS. I have no religious dog in this fight. I'm a philosopher of science and I'm writing a couple of books in this area so seeking some dialogue with intelligent observers about key issues.

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Strange    2431

I'm a philosopher of science

Really.

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beecee    77
Posted (edited)

Or do you just refuse to accept it, despite informal and mathematical explanations? What (religious?) beliefs do you have that make it impossible for you to accept a mathematical result?

I have certainly seen closeted god botherers of various denominations, rage on in denial about GR and GWs and much of science in general elsewhere: The fact that science and GR have virtually eliminated any need for a deity, annoys them no end.

Strange, I've addressed in detail in this thread (last week) why the argument that apparatus and light are affected differently doesn't add up.

Only to your own satisfaction.

Do you agree that any expansion or contraction of the balloon will not be detectable using the lines themselves or any coordinate system based on the lines? And if so why wouldn't this apply equally to the 3D version? I'm not asking you to repeat prior points or give me equations. I'm asking you to think about it from a basic logical perspective.

aLIGO is not about expansion and/or contraction, it's about the warping/waving and undulations in spacetime we call GW.

But you continue to ignore that.

I'm asking you to think about it from a basic logical perspective.

Yet it is you who keeps repeating invalid conclusions despite being given answers and many links, and of course the other point you are not apparently game enough to address, that is to E-Mail aLIGO for some real hands on data on why you are wrong.

Until you do that, it appears you are not interested in answers that divert away from your own invalid interpretation.

Edited by beecee

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Of course they're being used as a meter stick. That's the entire function of the interferometer: to measure differences in the speed of light from different directions, in this case by measuring wavelength fringe differences, which relies on differences in the mirror positions to work. And in my 2D thought experiment the perpendicular lines function as a meter stick in a similar way. Why is it so difficult to get anyone here to state their agreement that the 2D lines can't be used to measure expansion or contraction of the balloon? I'm not trying to trick you. I'm simply trying to establish some points of agreement to then get to the real world example.

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beecee    77
Posted (edited)

And of course the third point you have failed to address, is why the three GW detections so far have all been BH mergers, but each detection is/was unique in distances from us, the sizes of the BH's and I would hazard a guess, the size of the chirp or noise....

https://www.ligo.caltech.edu/page/faq

Q:

If a gravitational wave stretches the distance between the LIGO mirrors, doesn't it also stretch the wavelength of the laser light?

A:

A gravitational wave does stretch and squeeze the wavelength of the light in the arms. But the interference pattern doesn't come about because of the difference between the length of the arm and the wavelength of the light. Instead it's caused by the different arrival time of the light wave's "crests and troughs" from one arm with the arrival time of the light that traveled in the other arm. To get how this works, it is also important to know that gravitational waves do NOT change the speed of light.

With that in mind, imagine now that you and a friend want to compare how long it takes you to drive to the end of the interferometer arms and back. Just like LIGO's laser light waves, you leave the corner station at exactly the same time, take different paths, and travel at precisely the same speed. You expect to meet up again at the same time. But if a gravitational wave passes while you are on your journey, one of you will end up traveling down the longer arm, and one of you will travel down the shorter arm. Since you're still going the same speed, one of you will take longer to return than the other!

The arrival times change because when the arms of the interferometer change lengths, so too do the distances the light waves travel before exiting the interferometer. What gravitational waves do not change, however, is the speed of light. This means that a wave of light that happens to be in a longer arm during a gravitational wave has to travel farther before exiting, so it takes longer to leave than the beam that was in the shorter arm. The light waves no longer match up when they exit, so they interfere with each other. The laser light acts not as a ruler, but as a stopwatch.

But what if suddenly the length of one path got longer while the other route got shorter? One of you would have to travel a little farther and take longer to reach your destination than the other; you would no longer arrive at the same time! Furthermore, by precisely measuring the difference in arrival times, and knowing your rates of speed, you could actually calculate how much farther or less far you each had to drive in order to arrive when you did

The change in the arms is what is being measured. By using light. Which does not change in the same way.

Correct as detailed above in aLIGO questionare.

Of course they're being used as a meter stick. That's the entire function of the interferometer: to measure differences in the speed of light from different directions

WRONG, the speed of light does not change ever....as detailed on the aLIGO questionare and a multitide of previous answers,

Edited by beecee

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Beecee, you've misunderstood my point. Interferometers were developed to measure potential changes in the speed of light. Remember Michelson Morley? The LIGO interferometers are actually called Michelson interferometers. In the case of LIGO, interferometers are being used to detect minute distortion of the arm lengths from potential GWs, and my point this whole time has been that under the definition of GWs they can't measure any changes to the arm lengths because the arms distort by exactly the same amount as the GWs.

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beecee    77

Beecee, you've misunderstood my point. Interferometers were developed to measure potential changes in the speed of light. Remember Michelson Morley? The LIGO interferometers are actually called Michelson interferometers. In the case of LIGO, interferometers are being used to detect minute distortion of the arm lengths from potential GWs, and my point this whole time has been that under the definition of GWs they can't measure any changes to the arm lengths because the arms distort by exactly the same amount as the GWs.

It appears the whole world has misunderstood your point

All I need do is refer you back to all the answers you have been given and particularly the aLIGO answer above.

You are wrong, whether you accept that or not makes no difference to what most reputable professionals and lay people alike now accept, based on the observational evidence.

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Ah, I'm wrong because I'm wrong? You can't or won't address my basic point. I appreciate your efforts to point me to various resources but I've read the peer reviewed papers that directly address these issues and I've posted here my objections to the arguments presented in these papers and my requests for clarifications. Still no one has addressed the basic logical points that must be the foundation for any scientific paradigm to work. Consider my 2D ballon metaphor and tell me how the lines and their coordinate system could detect any changes to the balloon's surface? You previously suggested that my mentioning contraction or expansion of the balloon made this thought experiment irrelevant to LIGO and GWs. You misunderstood again bc any wave is simply a series of expansions and contractions of space. The point of course is, in both 2D and 3D versions: how can you detect any changes of the medium with a device that exists in and is distorted by the medium that is trying to be measured?

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beecee    77
Posted (edited)

Ah, I'm wrong because I'm wrong? You can't or won't address my basic point.

No your wrong because you have interpreted it wrong as many have informed you, and after the recent links and answer from the horses mouth so to speak, you still refuse to accept the answer and your total misrepresentation and misinterpretation.

Again despite your repeated denial.

The aLIGO does not measure expansion and or contraction as you interpret and envisage, rather it measures undulations/waves. It does not measure any change in the speed of light as that is a constant and does not change.

The three detection events were all each unique...one near massive BHs merging, one with stellar stellar sized BHs and the other Intermediate size, all at different distances and each with their own distinctive chirp.

And finally the answer/s as given by my link here....

https://www.ligo.caltech.edu/page/faq

As others have now suggested, [it was answered as far back as post 3] it appears your refusal to accept that you were and are wrong, maybe due to an agenda of sorts or possibly just delusions of grandeur.

Whatever the reasons, you still have the choice to confront aLIGO with your imaginary find, something which so far you have ignored.

You previously suggested that my mentioning contraction or expansion of the balloon made this thought experiment irrelevant to LIGO and GWs. You misunderstood again bc any wave is simply a series of expansions and contractions of space. The point of course is, in both 2D and 3D versions: how can you detect any changes of the medium with a device that exists in and is distorted by the medium that is trying to be measured?

No I did not misunderstand...you did not think it through. Your invalid balloon analogy and expansion, is only concerned with even/constant expansion over all of the balloon...GWs as I said are undulations that vary over time and distance.

Edited by beecee

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Strange    2431

Beecee, you've misunderstood my point. Interferometers were developed to measure potential changes in the speed of light. Remember Michelson Morley? The LIGO interferometers are actually called Michelson interferometers. In the case of LIGO, interferometers are being used to detect minute distortion of the arm lengths from potential GWs, and my point this whole time has been that under the definition of GWs they can't measure any changes to the arm lengths because the arms distort by exactly the same amount as the GWs.

And that is exactly why it works. The arm lengths change. The speed of light doesn't. Therefore (unlike the M-M experiment) there is a phase shift.

I'm not sure what the reason for your denial of these simple facts is, but it has gone beyond rationality.

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StringJunky    1485
Posted (edited)

I'm not sure what the reason for your denial of these simple facts is, but it has gone beyond rationality.

When one gets to this stage of persistent conviction that one is right, in the face of contrary expert opinion and sources, one needs to let go and start again without that conviction and see how it pans out. Conviction blinds people.

Edited by StringJunky

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Mordred    833

When one gets to this stage of persistent conviction that one is right, in the face of contrary expert opinion and sources, one needs to let go and start again without that conviction and see how it pans out. Conviction blinds people.

Yes the characteristic hurdle I see he needs to change is how strain works. Yet we have tons of experience both in physics and engineering measuring strain.

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tar    235

Thread, the original MM experiments I thought proved there was no ether, that is there is no medium through which light travels.

The LIGO proves however that there is a medium through

GWs travel. A medium that the GWs energy can stress and strain this way and that.

aramis720, and me as well a little, are trying to square these two findings in terms of what is the medium through which light travels, and what is the medium through which GWs travel.

Is it the same medium or do each item, light and gravity ride upon their own medium?

Regards, TAR

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Mordred    833
Posted (edited)

Tar field interactions can exhibit medium like properties particularly with f=ma relations.

A medium is a specific type of field. (a matter field).

The LIGO detector is a matter field, The GW wave can be characterized as a force field.

When force fields interact with matter fields you will get medium like properties and dynamics. However when 1 force field interacts with another force field. The (electromagnetic) the dynamics are not identical. F=ma becomes less useful as there is no rest mass term.

Forget ether the only way to understand how the different fields interact is to study the equations of state. ( Or if you want a more challenging route. The coupling constants and how they apply to field interactions.)

Matter fields resist changes in inertia more than force fields as matter fields are more strongly coupled.

The lasers and arms represent two distinct field interactions with a GW wave interaction.

1) Arms = matter+force field interaction

2) Lasers=force +force field interaction.

the invariant (rest) mass of each field is distinctive so when a force vector from another field acts upon it.

The Displacement must not be equal as the mass is not identical.

The M\$M was looking for interaction described by 1 but instead showed interaction 2.

(highly oversimplified heuristic view)

Edited by Mordred

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tar    235
Posted (edited)

Thank you Mordred, I think I almost understood that.

I am still however left with the question of why GWs are thought to travel at or below the speed of light. That is, why mathematically should they be thought to propagate through a matter field at the same rate.

Regards, TAR

that is, if the electromagnetic waves coming from a BH merger are traveling through space warped by gravity for 1.3billion years, and a GW is traveling through space warped by gravity for 1.3 billion years, and the dynamics of interaction between the light and the space and the GW and the space are different, how is it assumed they will reach the Earth at the same moment? (or come in from the same direction)

Edited by tar

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Mordred    833
Posted (edited)

I don't know maybe this will help.

Not many understand what is meant by monopole, dipolar or quadrupole. So lets show the difference as simple as possible.

A monopole is a scalar quantity. (no vector component) First order tensor 1d.

q or -q using charge.

A dipole is a vector component represented by a line between two coordinates.

{q}-------{-q} the two charges are seperated distance if displacement depends on mass and acceleration relations.

This is a Second order tensor 2D (Pauli matrixes are examples.

A quadrupole is a 3d 3rd order tensor

{q}------{-q}

|,,,,,,,,,,,,,,,,|

|,,,,,,,,,,,,,,,,|

{-q}----{q}

The tensor form is

$\mathbf{q}=\begin{pmatrix}q_{xx}&q_{xy}&q_{xz}\\q_{xy}&q_{yy}&q_{yz}\\q_{xz}&q_{yz}&q_{zz}\end{pmatrix}$

Edited by Mordred

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tar    235

well if I understand, then the design of the experiment is giving me a problem that I hope is worked out in that the distance between the mirrors is changing while the gw is coming through, and the laser is bounced back and forth between the mirrors hundreds of times for leverage, before the beam exits to the detector That is, on each bounce of each laser the distance of the mirrors is different with a different amplitude of stress on each mirror and the distance between the center and end of each arm, while the wave is coming through, and the wave has multiple peaks and troughs, in that 20 wavelets came through in a little over a second

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Mordred    833
Posted (edited)

well if I understand, then the design of the experiment is giving me a problem that I hope is worked out in that the distance between the mirrors is changing while the gw is coming through, and the laser is bounced back and forth between the mirrors hundreds of times for leverage, before the beam exits to the detector That is, on each bounce of each laser the distance of the mirrors is different with a different amplitude of stress on each mirror and the distance between the center and end of each arm, while the wave is coming through, and the wave has multiple peaks and troughs, in that 20 wavelets came through in a little over a second

essentially correct thus far

The only way to understand the strain changes between the two arms is under 3 dimensional analysis.

Replace the charges I used above with position. (though keep in mind I didn't include the ct coordinate for relativity in the above) keeping it as simple as possible.

The 4th order is the transverse traceless gauge. (The traceless is a particular type where $h_{\mu\nu}=0$ in the transverse direction. One where the effective degrees of freedom reduce to 2. (see spin above) hence plane waves.

The above is needed to understand that only the (+2,-2) spins provide the degrees of freedom but that is a full lesson unto itself. Spin 2 being (+2,+1,0,-1,-2) mathematically (+1,0,-1) factor out.

The +2 and -2 is your two independant polarization states.

some literature will describe a GW wave with the following added relation.

"a GW wave has no dipolar moment as it propogates."

Dipole moment has spin statistics (+1,-1) Pauli matrixes (2d Hilbert space)

Euclidean space is 3d, which modelled under 4d is the Minkowskii matrix

$\eta=\begin{pmatrix}-c^2&0&0&0\\0&1&0&0\\0&0&1&0\\0&0&0&1\end{pmatrix}$

Scalar is spin 0.

That what is meant by that statement. The effective degrees of freedom due to spin for a GW wave is (2,-2).

Now my advise to everyone not previously familiar with the above. Reread any references you like on the Quadrupole section. Including references already posted on this thread.

In particular the Ferrouri vs Carroll links.

PS the use of "q" is represents any charged field. A charged field is anistropic. An uncharged field is homogenous and isotropic.

So any field can represented in the above by field charge values at each coordinate.

Using Pythagous we only require 3 charge points to describe the square relation above as under GR the transforms preserve pythagous.

Anyways under reductions transverse traceless describes the two polarization as the required two independant polarizations. (The proofs require considerable understanding of GR to make sense of)

Edited by Mordred

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beecee    77

Thread, the original MM experiments I thought proved there was no ether, that is there is no medium through which light travels.

The LIGO proves however that there is a medium through

GWs travel. A medium that the GWs energy can stress and strain this way and that.

aramis720, and me as well a little, are trying to square these two findings in terms of what is the medium through which light travels, and what is the medium through which GWs travel.

Is it the same medium or do each item, light and gravity ride upon their own medium?

Regards, TAR

Tar, with all due respects to Mordred who obviously has answered your query in complete fashion, imo things have now got over complicated....From post 1, the question was asked, thus......

"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"?

And that most certainly has been answered with many links including from aLIGO, illustrating how and why the questioner was simply wrong with his inference from the outset.

And as others have noted, it seems to now have become a religious like conviction with him, in the face of overwhelming data that says otherwise.

I have seen this type of rock hard contrariness many times elsewhere.

Facts...the experiment was successful....GWs were discovered three times from three varying size BH mergers, each resulting in a unique signature....Einstein's GR was once again, further shown to be correct within its field of applicability.

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