Can LIGO actually detect gravitational waves?

[swansont]

 

But back to the gravity gradient. The link gave me a 404 error

 

 

Fixed, but it's the same link I provided earlier.

swansont,

 

The simulation is way misleading. There are big stars in the foreground and tiny stars further back giving an impression that you are looking at massive in diameter holes with clouds of stars around their perimeter. I would say it is meant to give the impression of a view from 10s of thousands of light ​​years...but hey its a simulation, not worth arguing over, it does not mean anything, there is no real components to it.

f

But back to the gravity gradient. The link gave me a 404 error so I am just going by your two test mass description, and I am not sure whether the test masses are far from the source of the gradient or the two masses creating the gradient. The strain on the space within the interferometer is what I thought we were measuring, and the distance between the mirrors is not exactly the distance of the path of the beam, because the beam is sent back and forth quite a few times before recombined. So the strain on the mirror at the end of the path and at the leveraging mirrors near the half silvered one, is different at any t only by the gradient between, not the full beam path distance implied gradient. All this is probably figured, but I am concerned as to what we figure the distance between the mirrors means in terms of what happened with the black holes, 1.3 billion years ago. That is, if the gradient between the mirrors squishes the space between the width of a partial proton, and that is because some huge mass was accelerating very close to another huge mass 1.3 billion years ago, wouldn't a comet making its close approach to the Sun cause a little chirp itself?

 

 

The gradient is at the source; it's required to give off the gravitational wave. And, as I noted, it requires an acceleration of a mass.

 

Yes, a comet would cause a gravitational wave (as any orbit would) albeit an exceedingly small one. The chirp of the LIGO signal is caused by the infall — "chirp" is referring to the increasing frequency of the signal as the masses get closer to each other, so a comet would not chirp in any noticeable way.

well forget the comet question. Comets are just a little dust and gas, not massive at all. But when a planet orbits, I understand that it is in essence, falling around the Sun, or a moon falling around its mother planet. They are accelerating and therefore would have a non zero integral.

 

 

Yes. And not particularly massive compared to the sun, and the frequency of their orbit is smaller. The frequency of the earth's orbit is 31.7 nanoHertz, and I pointed out an f^4 dependence before. So for a black hole orbit at 0.1 Hz, that's already 28 orders of magnitude smaller based on just the orbit frequency. There's another 6 for the mass. You gain back 14 orders of magnitude from the distance.

[tar]

SwansonT,

 

The original link in #94 did work and was a nice layout. Thanks. I think I have it now.

 

In terms of frequency of black hole mergers though, I am not sure of the thinking.

 

Let's say for instance we have one going on now at the center of our galaxy, like let's say it happens tomorrow at 6. We would not get the wave for 30,000 years, or however far we are at light speed from the event. However if in 10 nearby galaxies there were 10 incidents that happened and each was exactly as far away as each incident was long ago, we could get all 10 waves next Thursday.

 

So it seems the range of a wave would dictate what volume of space we could expect a wave from, but I do not understand on what basis we are predicting the frequency, or so to speak, if we should expect one or two or three or so every two years. Past performance is no guarantee of future performance.

 

But that is neither here nor there, the thread question is whether LIGO can detect gravity waves, and it appears like it not only can, but did. Three times as a system.

 

What do you think, aramis720?

 

Regards, TAR

On the frequency, I am thinking that any black hole mergers in the local area, say within 100million lys, that happened 101million years ago or before...we have already missed. They came through and we didn't have our ears on. And most of those that did happen within the last 100million years are on average still scores of millions of years from getting here and bending our space. So if we think of shells of space at further and further distances, each of a certain thickness in terms of radial distance from here, the volumes of the further shells would be greater and hold more stars and galaxies and if black hole mergers are equally distributed per volume, larger volume shells, would have more mergers. So each shell as you go out would have more candidates, and we should expect more distant than local gws .

Edited June 7, 2017 by tar

[Mordred]

There is several formulas involved none are easily understood. So I won't bother latexing them. 1 is for the strain, 1 for chirp. The strain being h+ and h×. This formula requires extensive knowledge of GR tensors to be properly understood.

Edited June 7, 2017 by Mordred

[swansont]

 

In terms of frequency of black hole mergers though, I am not sure of the thinking.

 

Whose thinking?

[tar]

The people at Caltech that said "which might occur on a yearly basis within a volume of radius 6×1020km (20 megaparsecs)." That is 6x10(to the 20th)km. The superscript 20 failed to copy over from the link.

Edited June 8, 2017 by tar

[Strange]

In terms of frequency of black hole mergers though, I am not sure of the thinking.

 

 

You can find the original analysis (and all the other details of the detections) online.

 

https://cplberry.com/2016/02/23/gw150914-the-papers/#rates

 

I imagine this will have been updated since, based on the other detections made. I'm sure it wouldn't be hard to find the information.

[tar]

Strange,

 

The Rate p​aper dealt with the sensitivity of the equipment as well as what signals are to be called GWs to what level of confidence, and ramped up expectations into the future observation cycles based on known or projected sensitivity.

 

That is not how I was looking at it. Given the sensitivity of the system when we saw GW150914, and a guesstimate of the rate of black hole mergers or neutron star collisions, events that could produce a detectable GW across a steady state universe taken all at once, once, all of the events we will ever witness (in the next 100 years) have already occurred, unless an event happens within 100 lys which is not likely unless two neutron stars dash into the area and collide, which would also be the last gw we see.

 

So I was not looking at our ability to detect, but looking at how many are on their way here, and how many have passed, as in if you were anywhere in the universe, how often would a GW, like the three we have seen, come by.

 

Regards, TAR

I suppose I don't like the way people think about the universe, because it is not always clear what people are calling now, as I am always arguing for the two now position, and not everybody complies in their thinking, or their descriptions in terms of what they are calling GW150914. Is it the merger, which happened 1.3 billion years ago 1.3billion lys away, or is it the event when the wave passed and we sensed it with LIGO, or is it the wave itself, which still exists and is known presently to the universe as an ever expanding concentric sphere surface a light second thick?

in my thinking every black hole merger that ever happened is now a wave in space, that 1 second thick expanding shell

 

On Earth, we are either inside a black whole merger's wave, in which case it already passed, or we are outside the shell in which case the wave is on its way at the speed of light toward us.

Edited June 8, 2017 by tar

[Strange]

So I was not looking at our ability to detect, but looking at how many are on their way here, and how many have passed, as in if you were anywhere in the universe, how often would a GW, like the three we have seen, come by.

 

 

The rate at which they occur, and the distance at which they occur, and hence the answer to this question is a factor in working out how many we could detect. (Combined with the range of sizes of these events and the sensitivity of the detectors.)

 

 

 

I suppose I don't like the way people think about the universe, because it is not always clear what people are calling now, as I am always arguing for the two now position, and not everybody complies in their thinking, or their descriptions in terms of what they are calling GW150914.

 

That is because no one else is confused by this.

 

 

 

Is it the merger, which happened 1.3 billion years ago 1.3billion lys away, or is it the event when the wave passed and we sensed it with LIGO, or is it the wave itself, which still exists and is known presently to the universe as an ever expanding concentric sphere surface a light second thick?

 

What is "it" in this question?

 

Do you what is "GW150914"? In which case, I would say that it is the detection event. By the usual process of metonymy, it is also sometimes used to refer to the collision that happened all those years and light-years away. (Or, perhaps, the waves themselves, which are now receding from us.)

 

 

in my thinking every black hole merger that ever happened is now a wave in space, that 1 second thick expanding shell

On Earth, we are either inside a black whole merger's wave, in which case it already passed, or we are outside the shell in which case the wave is on its way at the speed of light toward us.

 

That sounds reasonable.

 

Although, the "1 second thick" bit will be highly variable depending on the exact nature of the event, and where you draw the cutoff - for example, a pair of orbiting black holes, or stars, will have been losing energy by gravitational radiation for years before they actually collided. This loss of energy was the first indirect evidence for gravitational waves and is also the reason why they end up colliding in the end.

[tar]

so it is important to note that the simulations do not "show" anything that is currently extant in the universe and the picture of two giant holes close to each other, is a picture of what things might have looked like WAY before the merger and not what things looked like during the second before merger

 

in other words, there are not black holes out there merging that we will EVER sense, as if they are out there and we have not found them yet, and once we find them we can take their picture

[Strange]

in other words, there are not black holes out there merging that we will EVER sense, as if they are out there and we have not found them yet, and once we find them we can take their picture

 

 

Not quite sure what that means. There seems to be one too few or one to many negatives!

[tar]

Strange,

 

Sorry it is a vague construction. I guess I mean basically that the universe is extremely large and extremely long lived, and sensing a black hole merger 1.3 billion lys away is quite a shakey proposition. Not that the wave was not caused by the holes merging, but that we cannot tell, from here, what is currently in that spot where the holes merged. We have no way to send out a reporter and take a picture of the event. And if we look at that spot now I suppose we will see a freshly formed massive black hole...but we really don't know a darn thing about what happened in that spot over the last 1.3 billion years.

 

Regards, TAR

Edited June 8, 2017 by tar

[Strange]

Strange,

 

Sorry it is a vague construction. I guess I mean basically that the universe is extremely large and extremely long lived, and sensing a black hole merger 1.3 billion lys away is quite a shakey proposition. Not that the wave was not caused by the holes merging, but that we cannot tell, from here, what is currently in that spot where the holes merged. We have no way to send out a reporter and take a picture of the event. And if we look at that spot now I suppose we will see a freshly formed massive black hole...but we really don't know a darn thing about what happened in that spot over the last 1.3 billion years.

 

Regards, TAR

 

 

That is true. And, of course true of all observations. When we see the Sun rise, we don't know what might have happened to it in the last 8 minutes!

[zapatos]

Strange,

 

Sorry it is a vague construction. I guess I mean basically that the universe is extremely large and extremely long lived, and sensing a black hole merger 1.3 billion lys away is quite a shakey proposition. Not that the wave was not caused by the holes merging, but that we cannot tell, from here, what is currently in that spot where the holes merged. We have no way to send out a reporter and take a picture of the event. And if we look at that spot now I suppose we will see a freshly formed massive black hole...but we really don't know a darn thing about what happened in that spot over the last 1.3 billion years.

 

Regards, TAR

 

That is also true of what happened in the spot your television resides during the eight hours you were at work.

[tar]

zapatos,

 

Yes, but that is my sort of point about noticing the wave. It is really not tremendously important for you to know which way my television was facing or what channel I was on a particular day, and not only would your speculation be wrong when you pieced together the scarce facts you had into a theory of what happened in that spot, but you would be wrong in two general ways that had little to do with the specific speculations. One, I am accidently retired and was probably not away from the spot the eight hours you are speculating about monitoring, and two, due to the rotation of the Earth and the revolution of the Earth around the Sun and the Sun's revolution around the Milky Way's core, that spot has been vacated by not only my TV but a whole lot of Earth and Solar System. That spot, may or may not be within the Heliosphere by now.

 

Which brings up another point about reconstructing an event that happened so far away, based on one little distortion of the LIGO experiment. We do not know what effect the Heliopause has on a gravity wave. How it attenuates the signal. How the wave is stretched and compressed as it passes through the zillions of other waves it would have to have navigated in the last 1.3 billion years. Does the galaxy have a galaxypause, where a bunch of particles are piled up against the radiation pressure coming in from surrounding galaxies? Do the equations we use subtract the effect that this mass has on the wave and how it attenuates it? What if we are attributing a characteristic of the LIGO signal t o the spin or mass of one of the holes, when the characteristic is an artifact of some other masses, that we are not figuring for?

 

Regards, TAR

which brings up another issue

 

a gravity wave is an unstudied creature

 

we theorize it travels through space at the same rate as photons/light but we also theorize it attenuates space itself

 

photons ride on a complex wave through magnetic field orthogonal to electric field where it is hard to imagine what is media and what is message

 

light is bent around massive galaxies as space is curved by the gravity

 

If light is lensed in this way, are gravity waves as well? That is if we figure a gravity wave is coming from the same spot as a galaxy we know is there, or was there by the light, is it certain that the gravity wave would have taken the same route through space to where we can overlay the image of the gw with our image of the galaxy and be sure they actually represent the same event?

for instance the time of day the spot on Earth the time of year the angle of the Solar system's equator compared to the center of the galaxy would all be contributory to deciding if the wave was coming in from away from the Milkyway core or through it. And if there is a black hole at the center of our own galaxy, this would have to make a difference in the attitude of the GW. Is all this figured for?

[Strange]

Which brings up another point about reconstructing an event that happened so far away, based on one little distortion of the LIGO experiment. We do not know what effect the Heliopause has on a gravity wave. How it attenuates the signal. How the wave is stretched and compressed as it passes through the zillions of other waves it would have to have navigated in the last 1.3 billion years.

 

 

Unless our theories are seriously wrong, we know exactly how much effect they have. (Zero, if you are curious.)

 

 

 

If light is lensed in this way, are gravity waves as well? That is if we figure a gravity wave is coming from the same spot as a galaxy we know is there, or was there by the light, is it certain that the gravity wave would have taken the same route through space to where we can overlay the image of the gw with our image of the galaxy and be sure they actually represent the same event?

 

Gravitational waves are not lensed in this way. So there could be a misalignment between the apparent position of the optical source. But as few would know it was affected by lensing, we could take this into account.

Edited June 9, 2017 by Strange

[tar]

Strange,

 

We could take it into account, but do we?

 

If we are associating a GW with an optically verifiable mass, and the gravity took a different course though warped space then did the light by which we build the image of the optically verifiable mass, then there is a delay equal to the difference in path length between the image gained optically or by radio wave or x-ray detector, and the image gained by calculating the waves within the chirp.

 

Given the distances involved a different path could mean days or weeks or months or years or decades or maybe thousands of years. So to associate a GW event with a electromagnetic field event, you would have to account for a timing difference and a direction difference.

 

Perhaps analogous to tying a particular clap of thunder echoing off the side of the garage, with a particular flash of lightning lighting up the side of the garage.

 

Regards, TAR

Edited June 9, 2017 by tar

[Strange]

Strange,

 

We could take it into account, but do we?

 

 

We have not yet seen an optical counterpart to a black hole merger. In the case of two isolated black holes, there would be no visible result. If nearby stars (and other matter) got dragged into the merger then that might make alls sorts of interesting fireworks.

 

In the unlikely event that this happened to be aligned with, say, another galaxy such that it was lensed than yes, that lensing would be taken into account in calculating the actual position. It could also be a benefit as lensing can give you an enlarged (if distorted) view of the distant object.

 

 

 

Given the distances involved a different path could mean days or weeks or months or years or decades or maybe thousands of years.

 

I think you are overestimating the scale of the lensing effect.

[imatfaal]

 

...

I think you are overestimating the scale of the lensing effect.

 

The addition in path of light being lensed by a galaxy between us and source is of the same order as the s-child radius of the intervening object. So for a Galaxy of 10e12 solar masses about 3e12km or, dividing by c, about 4 months in time

[Strange]

 

The addition in path of light being lensed by a galaxy between us and source is of the same order as the s-child radius of the intervening object. So for a Galaxy of 10e12 solar masses about 3e12km or, dividing by c, about 4 months in time

 

 

Thanks for that. I was trying to think how to calculate an approximate value.

[tar]

Strange,

 

"How the wave is stretched and compressed as it passes through the zillions of other waves it would have to have navigated in the last 1.3 billion years. "

 

You said there would be no effect, but I think you mean passing through an electromagnetic field type wave would have no effect. I was not thinking of EM waves, I was thinking of GM waves and failed to specify in the sentence.

 

I was envisioning space shaped like the rubber sheet with bowling balls on it with a million gw event firemen holding the sheet. Each tug on the edge of the sheet would ripple through the whole sheet with time. So my zillion waves are ripples of space, the sheet. So the question I was having was, doesn't one gw have to navigate the compressions and rarefactions of space caused by another GW, as well as transverse the gravity wells caused by all masses?

 

That is, if a photon has to stay within the rubber, does not a gravity wave have to as well?

 

Regards, TAR

So specifically I am thinking that although we only experience an event a year, that means that there are currently on the order of lets say 100,000 gw shell like wave fronts currently in the Milkyway. Each ripple has to pass through all the other ripples to get to us. Like refraction at the boundary of two substances the angle of incidence of the wave, like the rank of soldiers entering the swamp, would turn the wave. So other masses and other gravity waves, if they indeed are warping space itself, are to be taken into account when considering the path and origin of a GW. At least I would guess.

[swansont]

Strange,

 

Sorry it is a vague construction. I guess I mean basically that the universe is extremely large and extremely long lived, and sensing a black hole merger 1.3 billion lys away is quite a shakey proposition. Not that the wave was not caused by the holes merging, but that we cannot tell, from here, what is currently in that spot where the holes merged. We have no way to send out a reporter and take a picture of the event. And if we look at that spot now I suppose we will see a freshly formed massive black hole...but we really don't know a darn thing about what happened in that spot over the last 1.3 billion years.

 

Regards, TAR

Like much of modern-era physics, we have to go by what our instruments tell us.

[Strange]

Strange,

 

"How the wave is stretched and compressed as it passes through the zillions of other waves it would have to have navigated in the last 1.3 billion years. "

 

You said there would be no effect, but I think you mean passing through an electromagnetic field type wave would have no effect. I was not thinking of EM waves, I was thinking of GM waves and failed to specify in the sentence.

 

 

I was referring to gravitational waves passing through matter ("the heliopause") which has no effect.

 

I don't believe that waves passing one another would have any effect - there might be local interference effects where they cross, but this should all even out when they leave each other. Rather like shining one light beam though another; it has no effect.

[tar]

"In the case of two isolated black holes, there would be no visible result."

 

 

But there could be EM results, like some sort of x-ray or gamma ray burst. And after all, a hole is a hole. A mass as dense as a black hole would do something to the light backlighting it. Either lens it, or absorb it. And are not black holes suppose to have fountains of energy spewing out "the poles"? ...but then again if the two holes are so close as to merge, it would be difficult to say what was caused by the one or the other or the both, of the EM variety of evidence.

"Rather like shining one light beam though another; it has no effect."

 

Rather odd statement on a thread based on the interference, no signal effect of two halves of a beam coming back together 1/4 wavelength out of phase.

that is, one needs to consider what happens to space if two gravity waves are there at the same time, coming in from orthogonal directions

Edited June 9, 2017 by tar

[Strange]

"In the case of two isolated black holes, there would be no visible result."

 

 

But there could be EM results, like some sort of x-ray or gamma ray burst. And after all, a hole is a hole. A mass as dense as a black hole would do something to the light backlighting it. Either lens it, or absorb it. And are not black holes suppose to have fountains of energy spewing out "the poles"? ...but then again if the two holes are so close as to merge, it would be difficult to say what was caused by the one or the other or the both, of the EM variety of evidence.

 

 

The interaction of black holes themselves (as I understand it) cannot generate EM radiation. Obviously if there were matter surrounding them (accretion disks, nearby stars, etc) then the picture would be very different. That is what I meant by "isolated" versus other cases.

 

If they were two black holes with no accretion disks there would be no jets (these are formed from the inflaming matter).

 

 

 

"Rather like shining one light beam though another; it has no effect."

 

Rather odd statement on a thread based on the interference, no signal effect of two halves of a beam coming back together 1/4 wavelength out of phase.

that is, one needs to consider what happens to space if two gravity waves are there at the same time, coming in from orthogonal directions

 

If you shine shine two arbitrary beams of light through each other they will not affect each other. An interferometer is a very special case. And I did say there could be interference effects where they cross. But they would emerge unchanged.

 

If you had two coherent sets of gravitational waves arriving at the same time, then perhaps you could detect interference between them. But that does not sound like a very realistic scenario. If they had passed through other, then there would be no effect.

[imatfaal]

 

 

...If you had two coherent sets of gravitational waves arriving at the same time, then perhaps you could detect interference between them. But that does not sound like a very realistic scenario. If they had passed through other, then there would be no effect.

 

This is an interesting scenario - not sure what happens. Gravitational waves are the product of a quadrapole moment - would that allow interference in the same way. I would guess - for starters - that for any interference you would need frequency / wavelength coincidence but also h₊ and h_x similarity