LIGO Announcement - Live Now!!!

[Darnok]

2 minutes ago, kurja said:

Heh

Peeps with tin foil hats mustbe thrilled ,)

So Tesla's math and experiments are in tin foil section now?

[Ravenchant]

2 hours ago, Hcube said:

Sounds like there won't be very much suspense for the Nobel prize for physics

There's the matter of deciding which individuals exactly get to share the prize, should be a thankless job. Anyway, now that a whole field of astronomy has been finally validated (not to say it wasn't "valid" before, but making direct measurements is a great leap) we should see plenty of research grants I wonder if the plan for follow-up missions to LISA pathfinder will be adjusted. A strain resolution of 10^-21 in two decades suddenly sounds a lot less groundbreaking...

Edited February 12, 2016 by Ravenchant

[micr0wave]

I'm not sure if i understood it right, but what puzzles me: How can you measure distortion of spacetime when you do the measurement from 'inside' that distorted spacetime ? How do you compensate for the distortion ?

[sevenperforce]

14 minutes ago, micr0wave said:

I'm not sure if i understood it right, but what puzzles me: How can you measure distortion of spacetime when you do the measurement from 'inside' that distorted spacetime ? How do you compensate for the distortion ?

It's a measurement of the change in the distortion of spacetime.

Does anyone know if they immediately pointed Hubble toward the signal to see what they could see?

[p1t1o]

2 minutes ago, sevenperforce said:

It's a measurement of the change in the distortion of spacetime.

Does anyone know if they immediately pointed Hubble toward the signal to see what they could see?

I wondered the same thing as microwave, but I'm still not sure how you measure a change if you, yourself and your oberving equipment is also embedded in the changing medium, and is therefore changing itself?

Is it the case that though *space* is being warped, physical object within that space are not? In that way, a measurement of change can be made. Otherwise, it feels like trying to measure how much a ruler changes length, using another ruler, that also changes in length by the same amount - the measurement would be the same in "stretched", "squashed" or "normal" configurations. Clearly a measurement was made, but its still boggling my mind a bit

As for Hubble, as far as I understood, they could only extrapolate the direction that the signal came from to within quite a large "banana shaped" portion of the sky.

Also, would a merger of 2 black holes have a significant signature in the visible spectrum? Its hard to imagine it having much of one if they were non-active/accreting.

[sevenperforce]

Just now, p1t1o said:

I wondered the same thing as microwave, but I'm still not sure how you measure a change if you, yourself and your oberving equipment is also embedded in the changing medium, and is therefore changing itself?

Is it the case that though *space* is being warped, physical object within that space are not? In that way, a measurement of change can be made. Otherwise, it feels like trying to measure how much a ruler changes length, using another ruler, that also changes in length by the same amount - the measurement would be the same in "stretched", "squashed" or "normal" configurations. Clearly a measurement was made, but its still boggling my mind a bit

Each of the LIGO detectors use a pair of laser beams that extend several miles and meet at the center. An interferometer is installed to measure the precise alignment of these two beams; any change in beam shape will produce very noticeable misalignment.

Because the two beams are perpendicular to each other, a gravitational ripple will always alter one of them more than it will alter the other. This is what produces the misalignment which trips the interferometer.

Imagine that you're standing on a pier, holding cables that run to a pair of buoys each several hundred yards away. If a wave causes the buoys to shift their location, then one of the cables will pull more taut than the other based on where the wave came from, and you'll be able to feel this change. It's just that instead of cables, we have laser beams and a very precise interference measurement device.

Just now, p1t1o said:

Also, would a merger of 2 black holes have a significant signature in the visible spectrum? Its hard to imagine it having much of one if they were non-active/accreting.

60 solar masses is a pretty big black hole; it's hard to imagine there wouldn't be something swirling around or being lensed or something.

[p1t1o]

8 minutes ago, sevenperforce said:

Each of the LIGO detectors...

I understand, on some level, how the detectors are supposed to work. But on another level, I still don't.

In terms of your buoy analogy - to me it seems like when a wave make one buoy further away, how come it doesn't affect the length of the rope attached to that buoy by the same amount?

Is EM radiation unaffected by the spacetime distortion?

[sevenperforce]

Just now, p1t1o said:

I understand, on some level, how the detectors are supposed to work. But on another level, I still don't.

In terms of your buoy analogy - to me it seems like when a wave make one buoy further away, how come it doesn't affect the length of the rope attached to that buoy by the same amount?

Oh, it does. But since the wave's direction of propagation is going to be better aligned with one cable than with the other cable (since the cables are perpendicular), the two cables won't have exactly the same change in length, and that's how you'll know.

It's not a perfect analogy, because with the cable example you could simply feel the tug, and that's not quite right. It's more like the two cables don't quite touch, but cross at the pier, and you've balanced a pencil very carefully between the two of them. Now, if one of them becomes slightly longer than the other, it will change the alignment of the two cables and the pencil will fall off.

 

[micr0wave]

4 minutes ago, sevenperforce said:

Oh, it does. But since the wave's direction of propagation is going to be better aligned with one cable than with the other cable (since the cables are perpendicular), the two cables won't have exactly the same change in length, and that's how you'll know.

It's not a perfect analogy, because with the cable example you could simply feel the tug, and that's not quite right. It's more like the two cables don't quite touch, but cross at the pier, and you've balanced a pencil very carefully between the two of them. Now, if one of them becomes slightly longer than the other, it will change the alignment of the two cables and the pencil will fall off.

 

yes, when you look from outside it makes perfect sense, but 1mm will be 1mm in distorted spacetime too which sounds to me as there will be no forces applied, so it will only make sense to me when, as p1t1o said, the em radiation isn't affected by the distortion.

[p1t1o]

Oh, it does. But since the wave's direction of propagation is going to be better aligned with one cable than with the other cable (since the cables are perpendicular), the two cables won't have exactly the same change in length, and that's how you'll know.

It's not a perfect analogy, because with the cable example you could simply feel the tug, and that's not quite right. It's more like the two cables don't quite touch, but cross at the pier, and you've balanced a pencil very carefully between the two of them. Now, if one of them becomes slightly longer than the other, it will change the alignment of the two cables and the pencil will fall off.

 

Sorry if I'm being [science pun] dense but...

I familiarised myself with how the interferometer works, and how it is [supposedly, in my eyes] able to detect very small changes - including the differential between the two perpendicular arms.

But I still dont see how, if [back to the buoys] I have the 2 ropes, and the wave passes over one rope - if the distance to the buoy changes, but the length of the rope changes the same amount, I won't see anything at my hand, where I hold the ropes - no pencil will be compelled to fall. Even if the wave affects both buoys/rope pairs differently, I would never see any change at the ropes in my hand - assuming that the rope [laser] and buoy position [mirror] are affected in the same way.

As far as I know, you could squash the distance of one arm to 1% of its original, but because space - and my ruler - are also squashed, I will still measure it to be the same length.

Obviously I'm not grasping some part of this

 

PS: basically what micro said

[sevenperforce]

Just now, micr0wave said:

yes, when you look from outside it makes perfect sense, but 1mm will be 1mm in distorted spacetime too which sounds to me as there will be no forces applied, so it will only make sense to me when, as p1t1o said, the em radiation isn't affected by the distortion.

 

Just now, p1t1o said:

But I still dont see how, if [back to the buoys] I have the 2 ropes, and the wave passes over one rope - if the distance to the buoy changes, but the length of the rope changes the same amount, I won't see anything at my hand, where I hold the ropes - no pencil will be compelled to fall. Even if the wave affects both buoys/rope pairs differently, I would never see any change at the ropes in my hand - assuming that the rope [laser] and buoy position [mirror] are affected in the same way.

As far as I know, you could squash the distance of one arm to 1% of its original, but because space - and my ruler - are also squashed, I will still measure it to be the same length.

Indeed. With a single laser, you'd never be able to tell; you have no way of measuring the distortion.

However, you can measure the difference between the two lasers. And since one laser is aligned with the direction of the ripple while the other is not, one laser will be distorted and the other will not be. And so they will register a misalignment even though you can't directly measure the distortion.

[PB666]

8 hours ago, micr0wave said:

I'm not sure if i understood it right, but what puzzles me: How can you measure distortion of spacetime when you do the measurement from 'inside' that distorted spacetime ? How do you compensate for the distortion ?

Manley video much better explanation than mine, so edited out reply.

Edited February 12, 2016 by PB666

[micr0wave]

so there's a difference how waves and particles are affected by distortion ?

[sevenperforce]

7 minutes ago, micr0wave said:

so there's a difference how waves and particles are affected by distortion ?

Not so much, no.

But a laser beam running parallel to the motion of the gravitational ripple will be squeezed/stretched more than a laser beam running perpendicular to the motion of the gravitational ripple. The ripple squeezed the entire Earth, but it squeezed it along only one axis.

[p1t1o]

No offence PB666 but that was about as clear as mud! Axis theta? Defy Newtonian Physics?

 

**edit**

Is it the *frequency* that is changing rather than the *path-length*?

Because that would tally with what I know about gravitational red/blueshift and does not require a measured *distance*.

Edited February 12, 2016 by p1t1o
avoid double post

[lajoswinkler]

2 hours ago, Darnok said:

Wow Tesla was right Aether exists

http://imgur.com/gallery/0VhrXPV

https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

 

This is zero connection to aether. And please don't mock that engineer, he has enough troubles with the tinfoil hat team taking a dump on his work.

[SargeRho]

2 hours ago, Darnok said:

Wow Tesla was right Aether exists

http://imgur.com/gallery/0VhrXPV

https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

 

While the LIGO is effectively a giant michelson-morley detector, the aether isn't the only thing it can detect. The Aether has been thoroughly disproven.

[micr0wave]

But what i can't understand (my brain lacks a dimension or two probably ) all that stretchin and squeezing still leaves 1mm being 1mm, no matter how hard i squeeze it.

So, as far i understand, the time to travel for a laserbeam at 1c through 1ly will take 1year, no matter in which angle the distortion crosses the beam, assuming this, it doesn't matter how many beams are crossing at the mirror, they all needed 1 year to travel 1 lightyear.

I'm not questioning the results, i just try to get it into my brain how the distortion doesn't affect the measuring of it or how it's compensated.

[Darnok]

1 minute ago, SargeRho said:

While the LIGO is effectively a giant michelson-morley detector, the aether isn't the only thing it can detect. The Aether has been thoroughly disproven.

How?

https://en.wikipedia.org/wiki/Aether_theories

Quote

We may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an aether.

IMO you are confusing "Luminiferous Aether" with Aether - particle that possibly creates space and those gravitational waves are nothing more, but waves of particles. And those particles affect in very minor way everything around us, also those particles are affected by energy and matter in very weak way.

[Ravenchant]

3 minutes ago, p1t1o said:

No offence PB666 but that was about as clear as mud! Axis theta? Defy Newtonian Physics?

This clip does a good visualization. Unless the "wavefront" propagates exactly at the centreline between the two beams, they get squeezed and expanded by different amounts.

http://i.imgur.com/0VhrXPV.webm

[sevenperforce]

12 minutes ago, micr0wave said:

But what i can't understand (my brain lacks a dimension or two probably ) all that stretchin and squeezing still leaves 1mm being 1mm, no matter how hard i squeeze it.

So, as far i understand, the time to travel for a laserbeam at 1c through 1ly will take 1year, no matter in which angle the distortion crosses the beam, assuming this, it doesn't matter how many beams are crossing at the mirror, they all needed 1 year to travel 1 lightyear.

Well, 1 mm is being stretched to 1.000001 mm for a split second, then it rebounds back. You'd never have any way of measuring that.

But 1 mm on the perpendicular axis is being stretched to something like 1.000005 mm. So for that split second, the two rulers are 0.000004 mm out of alignment.

[p1t1o]

1 minute ago, Ravenchant said:

This clip does a good visualization. Unless the "wavefront" propagates exactly at the centreline between the two beams, they get squeezed and expanded by different amounts.

http://i.imgur.com/0VhrXPV.webm

I am 100% familiar with this, my problem is that, as far as I understand, one arm could be dancing a jig and the other a waltz, but if the laser is dancing just as much as the arm it is travelling down, the detector at the apex should see two completely static and unchanged beams - because the beam itself is being distorted just as much as the space it is travelling through.

 

Unless the beam *isn't distorted*, or isn't distorted *as much*.

 

If the phase-change, however, is caused by a change in frequency, and not by one beam literally arriving at a different time to the other, this I could get. In that way, to use the "ruler" analogy, even though my ruler changes length in the same way as the thing I am trying to measure, thus always giving me the same distance measurement no matter how much space is distorted, I can still tell something has changed as the ruler will change colour. Does that carry even a shred of sense, I'm struggling here!

 

 

[micr0wave]

yes, i get that, but 1.000001 mm is only 1.000001 mm if you look from 'outside', being within the stretched space it will be 1mm.

Maybe my misconception is this 'inside' 'outside' thing.

[Rakaydos]

4 minutes ago, micr0wave said:

yes, i get that, but 1.000001 mm is only 1.000001 mm if you look from 'outside', being within the stretched space it will be 1mm.

Maybe my misconception is this 'inside' 'outside' thing.

Except you have photons coming if "from outside" continuously. Really, the detector is measuing hw much TIME it takes light to travel through distorted space, since light always travels the same speed. So an arm that is distored longer, the next photon has to travel longer, and when it bounces back it's slightly out of phase.

[sevenperforce]

Well, the interferometer does function based on the interference between the wavelengths of the two beams, IIRC, but it's the phase shift (due to the change in the total length of the beam) that is being measured; the wavelength change is not really detectable.

Here, I'll try to do a visual depiction.

Suppose you have two lasers that cross perpendicularly. Where they meet, they form an interference pattern:

Now, suppose that a gravitational wave comes along from the left of your screen and changes the length of the horizontal laser:

The interference pattern is now "bent" and points in a slightly different direction because the horizontal laser's phase meets the vertical laser's phase at a different point than it did before.

Make sense?