5 ms·

I hate LIGO because it shouldn't work on paper, and few seem to actually understand it. 1. If you have a light wave in the universe, and you "hang" a mass by e

I hate LIGO because it shouldn't work on paper, and few seem to actually understand it. 1. If you have a light wave in the universe, and you "hang" a mass by each valley and trough of the light wave, and then you squash and stretch the universe, the masses stay connected to the valleys and troughs of the light wave. Said another way, when you squash and stretch space, lightwaves within that spacetime are also squashed and stretched. 2. LIGO claims to work by measuring the squashing and stretching space with light. However, as we just learned, squashing space also squashes light waves. So the LIGO arms and light waves in the arms are all compressed together. Said another way, LIGO can't work because the ruler itself is squashing as space squashes, so you can't measure if space is compressing. LIGO does work, but if you can't explain how it works despite this description, then you don't understand LIGO.

karlshea 3y ago
> LIGO does work, but if you can't explain how it works despite this description, then you don't understand LIGO. So uhh... how does it work in spite of that description?
- nyssos 3y ago
  It has two arms at right angles to each other, and measures the difference between the two length changes via laser interferometry. This gets you the component of the wave polarized along the corresponding axes. No clue why GP thinks it "shouldn't work": it's an extremely difficult engineering problem, but the physics of it is relatively straightforward.
  - karlshea 3y ago
    Thanks!
  - onetimeuse92304 3y ago
    Yeah, the physics is straightforward. Even "laser interferometry" is super simple to explain. It is getting enough resolution and noise immunity from laser interferometry where all the interesting stuff is.
  - munchler 3y ago
    I think GP's point is that you can't measure changes in the length of an arm if the ruler you're using is changing in the same way. I think this misrepresents the situation, but I can't say how, so I can't dismiss it quite so quickly.
    - nyssos 3y ago
      The "ruler" you're using is the other arm, which doesn't change in the same way: one axis stretches while the other compresses.
      - NoMoreNicksLeft 3y ago
        True, but with only two arms, I think LIGO is somewhat directional, is it not? Stronger signals from some black holes depending on which way they're oriented relative to the arms. And if there were many different simultaneous signals, some could mask the others. Clearly, we need a 3-armed, space-based LIGO. Or better, a dozen of them.
        _Microft 3y ago
        There are two LIGO interferometers in the US [0], Virgo in Italy [1], GEO600 in Germany [2], something in Japan [3] and something being planned out being built in India. There is LISA as space-based project in the works too [4]. [0] https://en.m.wikipedia.org/wiki/LIGO https://en.m.wikipedia.org/wiki/LIGO [1] https://en.m.wikipedia.org/wiki/Virgo_interferometer https://en.m.wikipedia.org/wiki/Virgo_interferometer [2] https://en.m.wikipedia.org/wiki/GEO600 https://en.m.wikipedia.org/wiki/GEO600 [3] https://en.m.wikipedia.org/wiki/KAGRA https://en.m.wikipedia.org/wiki/KAGRA [4] https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...
  - da-bacon 3y ago
    "but the physics of it is relatively straightforward." Amusingly Einstein both identified gravitational waves in his then new general relativity and then changed his mind about whether they existed. Indeed he had found three different types of these waves, and two of them were simply coordinate artifacts (they could be made to travel at any speed, Eddington famously quiped that they could be made to move at the "speed of thought"). The third type however didn't have this problem (though again here Einstein would write a paper where he claimed these gravitation waves required singularities, but these turned out these were all coordinates singularities .... sort of like how late and long coordinates misbehave at the poles) In some ways the physics of it is straightforward, in other ways....not so much.
  - stevebmark 3y ago
    I think you have an intuitive mental model of how it works, but don't actually understand it, or aren't able to explain it well. This question is important enough that there have been papers written on it. I suggest you read https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/530040/If-light-waves-are-stretched-by-gravitational?redirectedFrom=fulltext https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300... which may strengthen your understanding.
    - nyssos 3y ago
      > This question is important enough that there have been papers written on it. I suggest you read https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300 https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300... This is a physics education journal: its papers are supposed to be interesting and accessible to undergrads, not on the frontier of the field. What the linked paper amounts to is noticing that the sensitivity of a detector depends on the relative length scales of the arms and the waves you're trying to detect. This is true, and also one of the first things you would consider when figuring out what size your detector should be. It's a good homework problem, but not news to anyone actually working on LIGO.
      - stevebmark 3y ago
        I legitimately find your cognitive dissonance interesting. I think you're on the other end - you may understand the fundamentals so well that your brain isn't processing the question / conundrum in the way that others see it. Honing in on the the scales of the arms as your reaction the paper is why I think this, as that's not related to the problem. And, well, you said it yourself, you don't understand the question: > No clue why GP thinks it "shouldn't work" It's also possible you think that this is all about laser interferometry, and aren't properly considering how it could work in the context of compressing space, since a laser interferometry system in compressed space wouldn't produce interference.
onetimeuse92304 3y ago
What you really mean is that you don't understand how LIGO works and neither do any of the reporters who try to explain it. And so the explanations that are available in public do not really give an insight into what is happening. That's unfortunately true for a lot of physics. For example, there are very few good explanations available for what is electric current and it is pretty funny observing even people with lots of experience offer the same lame, false explanation.
unblough 3y ago
> Said another way, LIGO can't work because the ruler itself is squashing as space squashes, so you can't measure if space is compressing. I think the chief confusion here is that you may be thinking the light arrives at the detector in the same amount of time regardless of spacetime curvature. That is, the the ruler is itself squishing. But what needs to be considered is the constant speed of light. This implies that what happens is, in the presence of additional curvature, and constant speed of light, the additional distance traveled will have appeared to slow the light. In the laser interferometer this registers as interference. https://m.youtube.com/watch?v=ajZojAwfEbs https://m.youtube.com/watch?v=ajZojAwfEbs It is also worth noting that any claims of detection are thoroughly investigated and confirmed with other detectors. > It is difficult for a single LIGO detector to confirm a gravitational wave signal on its own. The initial discovery of gravitational waves required that the signal be seen in both detectors (Hanford and Livingston). https://www.ligo.caltech.edu/page/what-is-ligo https://www.ligo.caltech.edu/page/what-is-ligo
- Akronymus 3y ago
  > the additional distance traveled will have appeared to slow the light. I thought it made the light reduce in frequency. Or did I misunderstand?
  - mnw21cam 3y ago
    You didn't misunderstand. A sudden stretching of space cannot change the number of peaks and troughs as they go past, but since those peaks and troughs are now slightly further apart, the frequency of the light is slightly lower, and the light takes slightly longer to travel. This applies to light that is already in transit. Of course, as the gravitational wave passes by, the length/frequency returns to normal again. New light that is emitted at one end of the journey while there is a stretching status will have the same frequency as normal, and just see the longer journey. In fact, the gravitational waves that LIGO is able to detect are slower than the time it takes for light to make the journey, so the stretching is effectively gradual, and the detector is basically an extremely accurate length measurement. The gravity waves aren't fast enough to make the light changing frequency a thing that needs to be worried about.
Zamicol 3y ago
LIGO has three rulers. The difference among the rulers results in measurement. (Two rulers are on site, the "third" is the other locations.)
shric 3y ago
People have down voted the parent comment but I'm not sure why. Perhaps it was the tone, but it caused a series of informative explanations that may not have arisen without this post.

karlshea 3y ago

> LIGO does work, but if you can't explain how it works despite this description, then you don't understand LIGO. So uhh... how does it work in spite of that description?

nyssos 3y ago
It has two arms at right angles to each other, and measures the difference between the two length changes via laser interferometry. This gets you the component of the wave polarized along the corresponding axes. No clue why GP thinks it "shouldn't work": it's an extremely difficult engineering problem, but the physics of it is relatively straightforward.
- karlshea 3y ago
  Thanks!
- onetimeuse92304 3y ago
  Yeah, the physics is straightforward. Even "laser interferometry" is super simple to explain. It is getting enough resolution and noise immunity from laser interferometry where all the interesting stuff is.
- munchler 3y ago
  I think GP's point is that you can't measure changes in the length of an arm if the ruler you're using is changing in the same way. I think this misrepresents the situation, but I can't say how, so I can't dismiss it quite so quickly.
  - nyssos 3y ago
    The "ruler" you're using is the other arm, which doesn't change in the same way: one axis stretches while the other compresses.
    - NoMoreNicksLeft 3y ago
      True, but with only two arms, I think LIGO is somewhat directional, is it not? Stronger signals from some black holes depending on which way they're oriented relative to the arms. And if there were many different simultaneous signals, some could mask the others. Clearly, we need a 3-armed, space-based LIGO. Or better, a dozen of them.
      - _Microft 3y ago
        There are two LIGO interferometers in the US [0], Virgo in Italy [1], GEO600 in Germany [2], something in Japan [3] and something being planned out being built in India. There is LISA as space-based project in the works too [4]. [0] https://en.m.wikipedia.org/wiki/LIGO https://en.m.wikipedia.org/wiki/LIGO [1] https://en.m.wikipedia.org/wiki/Virgo_interferometer https://en.m.wikipedia.org/wiki/Virgo_interferometer [2] https://en.m.wikipedia.org/wiki/GEO600 https://en.m.wikipedia.org/wiki/GEO600 [3] https://en.m.wikipedia.org/wiki/KAGRA https://en.m.wikipedia.org/wiki/KAGRA [4] https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...
- da-bacon 3y ago
  "but the physics of it is relatively straightforward." Amusingly Einstein both identified gravitational waves in his then new general relativity and then changed his mind about whether they existed. Indeed he had found three different types of these waves, and two of them were simply coordinate artifacts (they could be made to travel at any speed, Eddington famously quiped that they could be made to move at the "speed of thought"). The third type however didn't have this problem (though again here Einstein would write a paper where he claimed these gravitation waves required singularities, but these turned out these were all coordinates singularities .... sort of like how late and long coordinates misbehave at the poles) In some ways the physics of it is straightforward, in other ways....not so much.
- stevebmark 3y ago
  I think you have an intuitive mental model of how it works, but don't actually understand it, or aren't able to explain it well. This question is important enough that there have been papers written on it. I suggest you read https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/530040/If-light-waves-are-stretched-by-gravitational?redirectedFrom=fulltext https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300... which may strengthen your understanding.
  - nyssos 3y ago
    > This question is important enough that there have been papers written on it. I suggest you read https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300 https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300... This is a physics education journal: its papers are supposed to be interesting and accessible to undergrads, not on the frontier of the field. What the linked paper amounts to is noticing that the sensitivity of a detector depends on the relative length scales of the arms and the waves you're trying to detect. This is true, and also one of the first things you would consider when figuring out what size your detector should be. It's a good homework problem, but not news to anyone actually working on LIGO.
    - stevebmark 3y ago
      I legitimately find your cognitive dissonance interesting. I think you're on the other end - you may understand the fundamentals so well that your brain isn't processing the question / conundrum in the way that others see it. Honing in on the the scales of the arms as your reaction the paper is why I think this, as that's not related to the problem. And, well, you said it yourself, you don't understand the question: > No clue why GP thinks it "shouldn't work" It's also possible you think that this is all about laser interferometry, and aren't properly considering how it could work in the context of compressing space, since a laser interferometry system in compressed space wouldn't produce interference.

onetimeuse92304 3y ago

What you really mean is that you don't understand how LIGO works and neither do any of the reporters who try to explain it. And so the explanations that are available in public do not really give an insight into what is happening. That's unfortunately true for a lot of physics. For example, there are very few good explanations available for what is electric current and it is pretty funny observing even people with lots of experience offer the same lame, false explanation.

unblough 3y ago

> Said another way, LIGO can't work because the ruler itself is squashing as space squashes, so you can't measure if space is compressing. I think the chief confusion here is that you may be thinking the light arrives at the detector in the same amount of time regardless of spacetime curvature. That is, the the ruler is itself squishing. But what needs to be considered is the constant speed of light. This implies that what happens is, in the presence of additional curvature, and constant speed of light, the additional distance traveled will have appeared to slow the light. In the laser interferometer this registers as interference. https://m.youtube.com/watch?v=ajZojAwfEbs https://m.youtube.com/watch?v=ajZojAwfEbs It is also worth noting that any claims of detection are thoroughly investigated and confirmed with other detectors. > It is difficult for a single LIGO detector to confirm a gravitational wave signal on its own. The initial discovery of gravitational waves required that the signal be seen in both detectors (Hanford and Livingston). https://www.ligo.caltech.edu/page/what-is-ligo https://www.ligo.caltech.edu/page/what-is-ligo

Akronymus 3y ago
> the additional distance traveled will have appeared to slow the light. I thought it made the light reduce in frequency. Or did I misunderstand?
- mnw21cam 3y ago
  You didn't misunderstand. A sudden stretching of space cannot change the number of peaks and troughs as they go past, but since those peaks and troughs are now slightly further apart, the frequency of the light is slightly lower, and the light takes slightly longer to travel. This applies to light that is already in transit. Of course, as the gravitational wave passes by, the length/frequency returns to normal again. New light that is emitted at one end of the journey while there is a stretching status will have the same frequency as normal, and just see the longer journey. In fact, the gravitational waves that LIGO is able to detect are slower than the time it takes for light to make the journey, so the stretching is effectively gradual, and the detector is basically an extremely accurate length measurement. The gravity waves aren't fast enough to make the light changing frequency a thing that needs to be worried about.

Zamicol 3y ago

LIGO has three rulers. The difference among the rulers results in measurement. (Two rulers are on site, the "third" is the other locations.)

shric 3y ago

People have down voted the parent comment but I'm not sure why. Perhaps it was the tone, but it caused a series of informative explanations that may not have arisen without this post.