Scrutable

We’re regularly detecting black hole mergers now with LIGO and associated detectors. The ripple in space that is detected is on the order of angstroms, but these events are billions of light years away. If I were to place a detector at the event horizon of a black hole at the moment it merged with another how big would the ripple in space be?

Grumble wrote: ↑

Thu Sep 03, 2020 6:39 am

We’re regularly detecting black hole mergers now with LIGO and associated detectors. The ripple in space that is detected is on the order of angstroms, but these events are billions of light years away. If I were to place a detector at the event horizon of a black hole at the moment it merged with another how big would the ripple in space be?

I can't answer that, but I think I saw a comment that; for the first detection, if we were at the distance of the Earth from the Sun, then the ripple would have been about a nanometre. Might have been a micron, but in that sort of ballpark.

OK I don't know enough relativity to be able to argue, but we exist embedded in spacetime, surely if there are ripples then, anything we use to measure them will be distorted along with the spacetime? It seems equivalent to Michelson and Morley trying to detect the ether. Obviously there is something I don't understand.

Boustrophedon wrote: ↑

Thu Sep 03, 2020 10:15 am

OK I don't know enough relativity to be able to argue, but we exist embedded in spacetime, surely if there are ripples then, anything we use to measure them will be distorted along with the spacetime? It seems equivalent to Michelson and Morley trying to detect the ether. Obviously there is something I don't understand.

That’s why the detectors are at 90° to each other. Space time is distorted in one direction relative to the other resulting in a different path length for the lasers.

Grumble wrote: ↑

Thu Sep 03, 2020 10:23 am

Boustrophedon wrote: ↑

Thu Sep 03, 2020 10:15 am

OK I don't know enough relativity to be able to argue, but we exist embedded in spacetime, surely if there are ripples then, anything we use to measure them will be distorted along with the spacetime? It seems equivalent to Michelson and Morley trying to detect the ether. Obviously there is something I don't understand.

That’s why the detectors are at 90° to each other. Space time is distorted in one direction relative to the other resulting in a different path length for the lasers.

Yebbut the question is why does the thing you use to measure (the laser beam) not also stretch. If you used a long ruler to measure something and there was a bit of stretched space in the middle, the middle of the ruler would also stretch so you couldn't detect the stretch.

I’m probably going to cock this up but AFAICS the laser pulse travels through space time, it’s not a part of it.

Martin Y wrote: ↑

Thu Sep 03, 2020 11:07 am

Grumble wrote: ↑

Thu Sep 03, 2020 10:23 am

Boustrophedon wrote: ↑

Thu Sep 03, 2020 10:15 am

OK I don't know enough relativity to be able to argue, but we exist embedded in spacetime, surely if there are ripples then, anything we use to measure them will be distorted along with the spacetime? It seems equivalent to Michelson and Morley trying to detect the ether. Obviously there is something I don't understand.

That’s why the detectors are at 90° to each other. Space time is distorted in one direction relative to the other resulting in a different path length for the lasers.

Yebbut the question is why does the thing you use to measure (the laser beam) not also stretch. If you used a long ruler to measure something and there was a bit of stretched space in the middle, the middle of the ruler would also stretch so you couldn't detect the stretch.

What you're measuring is the difference in length between the two parts of the L.
If the ripple comes in along the east/west axis, that one momentarily shortens as the ripple passes along it. That means it's shorter than the north/south axis for the 13 milliseconds for it to pass

Yes, I get that bit. You're measuring the difference between two axes. But that's still not addressing the point that you can only measure a change if the thing you're measuring stretches but your ruler does not (or vice versa).

Martin Y wrote: ↑

Thu Sep 03, 2020 5:47 pm

Yes, I get that bit. You're measuring the difference between two axes. But that's still not addressing the point that you can only measure a change if the thing you're measuring stretches but your ruler does not (or vice versa).

The light doesn’t stretch

Grumble wrote: ↑

Thu Sep 03, 2020 6:34 pm

Martin Y wrote: ↑

Thu Sep 03, 2020 5:47 pm

Yes, I get that bit. You're measuring the difference between two axes. But that's still not addressing the point that you can only measure a change if the thing you're measuring stretches but your ruler does not (or vice versa).

The light doesn’t stretch

AIUI, as everybody knows light doesn't speed up or slow down in any reference frame. Instead it's frequency shifted towards blue or red. The transient difference in frequency between the two axes creates a brief wobble in the interference pattern.

basementer wrote: ↑

Thu Sep 03, 2020 6:59 pm

Grumble wrote: ↑

Thu Sep 03, 2020 6:34 pm

Martin Y wrote: ↑

Thu Sep 03, 2020 5:47 pm

Yes, I get that bit. You're measuring the difference between two axes. But that's still not addressing the point that you can only measure a change if the thing you're measuring stretches but your ruler does not (or vice versa).

The light doesn’t stretch

AIUI, as everybody knows light doesn't speed up or slow down in any reference frame. Instead it's frequency shifted towards blue or red. The transient difference in frequency between the two axes creates a brief wobble in the interference pattern.

The speed of light is constant in all reference frames, but the Interferometer measures the phase difference between two beams with the same wavelength/frequency. When the light from the two arms hits the detector, it's going in the same direction, so there'll be no frequency shift between the two beams. But the frequency shift in one arm causes that beam to have a different phase when it gets to the detector, causing a fringe shift in the interference pattern if there's a change, which is what's measured. Think of the length of the arms in units of wavelength, when one beam is frequency shifted, the arm has changed length due to the different wavelength so reaches the detector with a different phase to the other.

So from an outside observer's frame of reference the arm is momentarily no longer exactly n wavelengths long, but in the arm's frame the light is momentarily red or blue shifted?

Yes, the length of the arm changes and the light gets red/blue shifted. The shift is different in the two arms and the frequency/phase difference is what causes the interference pattern to change.

Article, and links to further articles, here: https://stuver.blogspot.com/2012/09/q-i ... by-gw.html

Thanks.

Martin Y wrote: ↑

Fri Sep 04, 2020 6:10 pm

So from an outside observer's frame of reference the arm is momentarily no longer exactly n wavelengths long, but in the arm's frame the light is momentarily red or blue shifted?

Frames of reference don't come into it, the two arms are moving in the same frame (if they are not, something has gone VERY wrong) and it is the difference in path length (or time of flight, if you like) between the two arms that is the measurement.

I think Phillip's link above explains things well, better than I could.

It didn't really help me because if the arm gets longer and the wavelength gets longer then the arm is still the same number of wavelengths long so there's no phase shift on the return signal.

Also, it linked to what was said to be a really good explanation but the link seemed to be broken.

This paper has a slightly different explanation:

http://www.pef.uni-lj.si/bojang/Gradiva/AMJ1997.pdf

Light in the arms does get red/blue shifted as the arms change length, but light entering later "sees" the altered lengths.

Gfamily wrote: ↑

Thu Sep 03, 2020 5:26 pm

What you're measuring is the difference in length between the two parts of the L.
If the ripple comes in along the east/west axis, that one momentarily shortens as the ripple passes along it. That means it's shorter than the north/south axis for the 13 milliseconds for it to pass

I think the ripple is actually a tidal type distortion, where a circle is distorted into an ellipse. You see a difference when the wave comes in perpendicular to both arms (or equivalently, you see the component of it that's perpendicular to both arms).

dyqik wrote: ↑

Sat Sep 05, 2020 1:47 pm

Gfamily wrote: ↑

Thu Sep 03, 2020 5:26 pm

What you're measuring is the difference in length between the two parts of the L.
If the ripple comes in along the east/west axis, that one momentarily shortens as the ripple passes along it. That means it's shorter than the north/south axis for the 13 milliseconds for it to pass

I think the ripple is actually a tidal type distortion, where a circle is distorted into an ellipse. You see a difference when the wave comes in perpendicular to both arms (or equivalently, you see the component of it that's perpendicular to both arms).

Yes, I meant to go back to add IABMCTT, but my phone decided to reboot instead..

philip wrote: ↑

Sat Sep 05, 2020 12:44 pm

This paper has a slightly different explanation:

http://www.pef.uni-lj.si/bojang/Gradiva/AMJ1997.pdf

Light in the arms does get red/blue shifted as the arms change length, but light entering later "sees" the altered lengths.

Thanks. That paper dives straight in to addressing exactly what puzzled me: Although the light in transit in the arm when the gravity wave arrives stretches with the arm, the light which follows it into the already stretched arm is not stretched. So it will share the same extended time of flight as the stretched light but not share the stretched wavelength to cancel out the phase shift.

A much more mathematical paper: https://arxiv.org/abs/gr-qc/0511083 which discusses how it can be difficult to express quantum results in terms of physical explanations. It's beyond my maths, and I did some of this at University years ago.

philip wrote: ↑

Sat Sep 05, 2020 5:42 pm

A much more mathematical paper: https://arxiv.org/abs/gr-qc/0511083 which discusses how it can be difficult to express quantum results in terms of physical explanations. It's beyond my maths, and I did some of this at University years ago.

That's useful for someone like me because I understand the quantum mechanical part but don't know any general relativity. The quantum mechanical part isn't particularly heavy.

What the paper really says is that you need general relativity to understand gravitational wave detectors. It doesn't make sense to try to understand gravitational waves with an intuition based on Newtonian gravity because in that theory there aren't even gravitational waves.

shpalman wrote: ↑

Sat Sep 05, 2020 6:38 pm

philip wrote: ↑

Sat Sep 05, 2020 5:42 pm

A much more mathematical paper: https://arxiv.org/abs/gr-qc/0511083 which discusses how it can be difficult to express quantum results in terms of physical explanations. It's beyond my maths, and I did some of this at University years ago.

That's useful for someone like me because I understand the quantum mechanical part but don't know any general relativity. The quantum mechanical part isn't particularly heavy.

What the paper really says is that you need general relativity to understand gravitational wave detectors. It doesn't make sense to try to understand gravitational waves with an intuition based on Newtonian gravity because in that theory there aren't even gravitational waves.

Ha, I also managed to avoid any relativity in my degree.

I did special relativity, that's fairly basic and impossible to avoid if you want electromagnetism to make any sense. General relativity is a whole other thing.

I just went ahead and played with microelectronics instead