Can LIGO actually detect gravitational waves?

[Strange]

strange,

 

I would rather look at it the other way around. What do the signals look like? THAT is what a gravity wave looks like, after passing through 1 billion other GWs. The signature of 1 billion other gws has to be on each and therefore one should expect sort of a fractal situation.

 

 

As far as we can tell they look exactly like the waves predicted by theory so "passing through 1 billion other GWs" has an immeasurably small effect, currently.

[tar]

Strange,

 

What about the orphan waves?

 

Can I not explain them by the effect on the one wave of the shape of space through which it traveled to get to LIGO?

 

Regards, TAR

Although my idea does not yield a testable claim, at the moment, it is a direction to look in.

[Strange]

Strange,

 

What about the orphan waves?

 

 

They are purely hypothetical.

[Mordred]

Our detectors wouldn't be sensitive enough. The arm lengths determine what frequency range we can detect.

 

For example our solar system produces GW waves but the frequency is far too low to detect.

 

LIGO has a decent practical range of detectable frequencies. However does have its limits.

 

Assuming a schotastic GW background exists the frequency range would be extremely low. The one paper I linked mentions an upper range.

Edited June 13, 2017 by Mordred

[tar]

Strange,

 

Well, I mean, what about the data that fits the model of hypothetical orphan waves? Could not that data be explained by the fact that passing through 1 billion GWs is going to leave a mark?

 

Regards, TAR

[Strange]

Strange,

 

Well, I mean, what about the data that fits the model of hypothetical orphan waves? Could not that data be explained by the fact that passing through 1 billion GWs is going to leave a mark?

 

 

Without a model, no one can answer that. If the effect is the same as the orphan waves hypothesis, then you need some other evidence to distinguish the two hypotheses.

[tar]

Modred,

 

Just wonder though whether the ripples might be of the same type frequency of a GW, its precursors and its after waves. That is, the billion near planes that the near plane of the shell of gw150914 passed through where oriented at a half a billion different angles, with the intersecting wave either coming from behind or in front, from the left or the right or from above or below,

 

That is, perhaps after a billion taps from each direction, the pebble is, on average, worn smooth. And the original signal is preserved.

 

Regards, TAR

that means the most recent taps would be the ones whose marks would still be clear on the wave

so it would be reasonable to assume that evidence of gw150914 might still be visible on gw151226

GW170104 would have evidence of GW151226 (imprinted with GW150914) strong on GW170104 with an obscure imprint of GW150914's direct impact on GW170104.

 

In fact there would be something to be learned about the distance and direction and characteristics of gws just prior GW150914, before we detected any, because their imprint would be strongest on the first detected and noticeable again on the next in both their original tap and on the wave that tapped the next and so on.

and if space is indeed like a rubber sheet, and a bit elastic, with a memory that returns to original shape, there is probably a little rebound, aftershock, settling that takes place​

 

which might account for some of the lesser events, unproved but obviously recorded (something must of been noticed between 150914 and 151226)

[Silvestru]

I'm only a lay person as far as science/Cosmology is concerned but I see it as rather easy to understand that gravitational waves, are simply a distortion of spacetime..... A wave passing through the aLIGO , will lengthen space-time ever so slightly along one arm of the detector, and compress it along the other arm...

Waves by definition have peaks and troughs and travel transversely progressively away from their source.

 

Why don't you express it in your own words if it is so easy to understand?

[tar]

maybe a better analogy than a pebble worn smooth (where abrasion wears material off) would be a lump of clay whacked with a board against a table and then set on the table at a different angle and whacked again...enough times at all the different directions and you get a smooth sphere, with evidence of the last whack, the most prevalent

now make your table and board out of clay and keep turning your clay between and whacking, but the imprint of the last whack will be on the whacker and the table and will be evident on the lump come the next turn and whack

the lump still has the same mass(original wave configuration), just deformed and reformed as other waves pass through

here the gradient would be the mass

 

it can not change

change shape and design but will always be the same gradient, the same wave or impulse moving through space, just lessening in energy density as the spherical shell grows larger

Edited June 13, 2017 by tar

[swansont]

except the ripples are caused by one gw passing through another

 

 

I've read nothing other than Mordred's comment that suggests that this happens. All other discussion of GWs say they only interact weakly with matter

 

(such as https://en.wikipedia.org/wiki/Gravitational_wave#Astrophysics_implications

Gravitational waves have two important and unique properties. First, there is no need for any type of matter to be present nearby in order for the waves to be generated by a binary system of uncharged black holes, which would emit no electromagnetic radiation. Second, gravitational waves can pass through any intervening matter without being scattered significantly. Whereas light from distant stars may be blocked out by interstellar dust, for example, gravitational waves will pass through essentially unimpeded. These two features allow gravitational waves to carry information about astronomical phenomena never before observed by humans.)

 

https://www.quora.com/Can-gravitational-waves-be-absorbed-scattered-Can-gravitational-waves-undergo-interference-Can-they-be-refracted-by-a-mass-distribution

 

​And that weak GWs do not affect each other

https://physics.stackexchange.com/questions/110162/what-happens-when-a-gravitational-wave-interacts-with-another-one

 

Secondly, even if there were such an effect, at what scale would you see an effect for waves that have a million-km or more) wavelength?

[tar]

"I've read nothing other than Mordred's comment that suggests that this happens. All other discussion of GWs say they only interact we wakly with matter​"

 

SwansonT,

 

Given that matter will not affect a gw does not preclude one gw from affecting another.

 

If space is temporarily stretched and squished as a gw passes then a gw passing through that deformed space will not know the space is deformed and will propagate happily at C. The wave front will be delayed where space it stretched and advanced where space is compressed. Once the waves completely pass through each other, perhaps you can say that one was unaffected by the other, but the pass through is never actually completed, as we have expanding sphere shaped shells overlapping in what would be a circle or an ellipse, experienced more locally as two very slightly curved planes intersecting in a slightly curved line.

 

I would think the scale upon which you would experience a million km wavelength event is in a periodic repetition of some pattern every 3 and a 1/3 seconds. (about the length of a breath )

 

Regards, TAR

Edited June 13, 2017 by tar

[swansont]

"I've read nothing other than Mordred's comment that suggests that this happens. All other discussion of GWs say they only interact we wakly with matter​"

 

SwansonT,

 

Given that matter will not affect a gw does not preclude one gw from affecting another.

At least one of my links says the opposite.

 

If space is temporarily stretched and squished as a gw passes then a gw passing through that deformed space will not know the space is deformed and will propagate happily at C. The wave front will be delayed where space it stretched and advanced where space is compressed.

How much deviation do you expect? The strain from the first event was what, 10^-18? We know the amplitude drops linearly with distance, and that was a little over a billion LY away. So unless you are within a few LYs of the event when it happened, the strain is smaller than 10^-9. That's how little you would affect another wave, if there is any effect at all.

 

I don't see you quantifying any of your claims.

 

Once the waves completely pass through each other, perhaps you can say that one was unaffected by the other, but the pass through is never actually completed, as we have expanding sphere shaped shells overlapping in what would be a circle or an ellipse, experienced more locally as two very slightly curved planes intersecting in a slightly curved line.

What are the odds of detecting two GW events essentially simultaneously (i.e. within a second of each other)?

[tar]

SwansonT,

 

The strain difference exists at the line of intersection between the two merge events. That is, the GW150914 is pulling and pushing point x in space for a second or so, at the same time GW151226 is pulling point x this way and that for a second or so. If, scalewise this size wave in space can delay and advance the light from a pulsar, enough to notice in the redshift and blueshift of a known steady repeating signal, then a second gw passing through a known gw that already passed through Earth would be affected, frequencywise to the same scale as an EM wave passing through a GW. Or so I would surmise.

 

Regards, TAR

the strain difference need not be measured, the frequency difference implies the strain on space where the wave actually is or was at the intersection

like a sounding device of sorts

you send out a GW and see what its affects are on an incoming wave

 

You send out GW150914 and read its echo on GW151226.

Edited June 13, 2017 by tar

[swansont]

SwansonT,

 

The strain difference exists at the line of intersection between the two merge events. That is, the GW150914 is pulling and pushing point x in space for a second or so, at the same time GW151226 is pulling point x this way and that for a second or so. If, scalewise this size wave in space can delay and advance the light from a pulsar, enough to notice in the redshift and blueshift of a known steady repeating signal, then a second gw passing through a known gw that already passed through Earth would be affected, frequencywise to the same scale as an EM wave passing through a GW. Or so I would surmise.

 

Regards, TAR

the strain difference need not be measured, the frequency difference implies the strain on space where the wave actually is or was at the intersection

like a sounding device of sorts

you send out a GW and see what its affects are on an incoming wave

 

You send out GW150914 and read its echo on GW151226.

 

 

 

The same scale being the operative term here. Unless you are detecting them simultaneously, one will have (at best) a minuscule fractional effect on the other. e.g. the 10^-18 strain will be increased or reduced by a factor of 10^-18

[Mordred]

Which is difficult enough to detect.

[tar]

OK I don't have a very good plan here, but the idea, I think is worth keeping in the back of the mind, in terms of things to look for, in the signals. Whether we can see it or not will be answered if we look for it and don't find it.

[tar]

https://www.bing.com/images/search?view=detailV2&ccid=klg3FiII&id=8605C8042591BB5A8BA014F91596FC4CAB4BF957&thid=OIP.klg3FiIIOMbvYsV1kJvUmQEsDf&q=ligo&simid=608024743366887719&selectedIndex=13&ajaxhist=0

 

Thread,

 

Is someone here able to tell us what we are looking at, when we see these waves? That is, the waves in the charts do not look like fringes. What do the fringes, the actual undoctored signals look like?

 

I have been, as you are aware attempting to visualize what a GW does to space as it comes through, to try and figure what one gw would do to another gw as it passes through. But I am not finding anywhere to see raw data, as in what did the GW150914 actually "do to" the equipment in the two locations.

 

It might be easier to visualize once a third LIGO is on line and direction and size can be better visualized, but in the link above are the high points of the curve where compression of space is increased and the low points where expansion of space is at a local max? That is if I count 20 high points in .2 of a sec, the distance of about 20,000 miles, does that mean as this series of waves comes through a spot 500 miles from a spot undergoing stretching is undergoing squishing? Does this tiny gravitational gradient actually exist and does it "flip", that is go from stretching one way to stretching 90 degrees the other way, 20 times in .2 secs?

 

Regards, TAR

that is if you are standing on a spot, looking toward the black hole merger, do you flatten out and bulge out the width of a partial proton 20 times in .2 secs?

important to me because the amplifying mirrors are bouncing laser beam into a gw headed right down an arm on the way out and going with a portion of the gravity wave on the way back, and this happens again and again, and each leg out is taken while a different half mile section of the gravity wave is in the experiment, and the next half mile of gravity wave that the laser goes through, or that comes through the space where the mirrors are, is of a different strain than the previous half mile worth of wave as well as different than the next

[beecee]

I don't believe there is any reasonable doubt that gravitational waves were detected...three times!

On another forum I once participated in, two or three anti GR people we had were also "questioning" the aLIGO results and many questions were raised.

I E-Mailed one of the Professors involved with aLIGO itself and here was his reply......

 

Dear Barry,

My name is Maximiliano Isi and I am a member of the LIGO Laboratory at the California Institute of Technology. I also happen to be the author of one of the papers cited at the end of your message. Thank you for your interesting email and apologies for the belated reply.

It is absolutely true that our observations do not allow us to fully rule out the existence of non-GR physics. This is just a consequence of the fact that experimental observations have the power to disprove theories, but not to prove them: scientific theories are falsifiable, but not demonstrable. The best we can do is to say that our measurements agree with GR up to some (high) confidence level. It is in that precise sense that we mean "Einstein was right" (a misleading phrase: we don't know whether GR is fully accurate, we just cannot prove it wrong with what we have seen).

We have several methods to make quantitative statements about the agreement between the GR prediction and the signals we measure, but I won't describe them here in detail. Note that in order to find signals in the detector noise we use templates that tell us what the waves look like, and those templates are the output of highly–advanced super-computer simulations of GR dynamics; this means that the signal cannot be too different from the GR prediction or we wouldn't have seen it at all!

Now, agreement with GR is not exclusive: an alternative theory might explain our observation just as well as GR or even better. Given any two competing theories (with different predictions), we can always ask which one is favored by our data, and we have well-established statistical methods to make quantitative statements to answer. Unfortunately, however, the mathematics of GW emission and propagation has only been worked out for very few of the viable alternatives to GR and in most of those cases the theories are similar enough to GR that the signals we'd expect to see are practically indistinguishable. The reason for this is that computing GW waveforms for interesting sources is an extremely complicated mathematical problem and (as mentioned above) it takes super-computers to do it even in GR, the theory we know best (and most alternatives are intrinsically more complicated).

So far this has all been about the relation between theory and experiment. However, most of the text in your message alluded to potential logical inconsistencies within GR itself. Since the main point relies on a thought experiment, let me begin to address this by clarifying that, although thought experiments can be a very useful tool, they are not proper logical arguments in themselves and do not formally tell us anything about the the validity of a theory. This is because natural language is too ambiguous to express formal statements: GR (as every other physical theory) is a mathematical framework and we need mathematics to discuss it properly. This is evident when you consider how both quantum mechanics and special relativity are full of paradoxes that seem to point to contradictions that go away when expressed mathematically. Paradoxes point to the inadequacies of our intuitions, not to those of the theory.

That said, I'd like to point out a few potential flaws in the argument presented in your email, without actually going into mathematical detail:

First, Feynman's sticky bead argument played an important role in re-igniting interest in GWs at a point in history when it wasn't clear whether they were real at all; however, the argument is not a core part of the GR framework and is usually not even referred to in modern treatments of the topic—our understanding of GR has come a long way since the 50's!

Second, our intuitions about space and time do not jive well with GR. Because spacetime can be curved, the fact that circumference of the loop in the example decreases does not say much about the radius. For example, imagine you went in a circle around a massive object (say, Sun) and measured the distance travelled (call it c, for circumference), and then travelled radially inwards towards the center and measured that distance too (call it r, for radius), then you would find that c < 2*pi*r because the massive body curve the spacetime around it. This is all to say that a shrinking circumference does not imply a shrinking radius (at least not in all frames).

Third and last, it seems to be implied in the text you quote that the existence of longitudinal gravitational waves would be in conflict with GR; however, this is only true in a narrow sense that needs to be explained. According to GR, at any point in space-time one should be able to find a particular form of the GW equations (the technical term for this freedom in the eqs. is gauge, think of it as a frame of reference though it's not the same) in which the wave can be expressed as the combination of two independent polarizations transverse to the direction of propagation. Notice some key aspects of this statement: there is a specific choice of gauge (or frame of reference, if you wish) in which the equations take this particularly nice form, and that choice can only be defined locally (i.e. a choice that works nicely in one point in space, will be bad somewhere else). This means that if you choose an arbitrary frame of reference, chances are the wave will not look transverse, though you could always switch to the specific frame and gauge which will make the waves look nice at that point (this is the so called transverse-traceless gauge). The bottom line is that you might think that you have longitudinal waves, but you can always explain that as a combination of independent, transverse waves.

Finally, I would like to add a few words about Carver Mead's G4v theory. Unlike most alternatives to GR, Carver's theory makes markedly different predictions than GR with respect to the polarization content of GWs. However, the relative orientation of our detectors makes LIGO not really good at distinguishing different polarizations in transient (short-lived) signals like the ones we have observed so far. Furthermore, we don't have a full prediction of what the GW trace of the merger of two compact objects would look like in G4v (Carver is working on it), so we cannot make a statement about which theory, if any, is favored by the data. So we will have to wait for more detections and more theoretical work until we are able to make a statement about G4v.

Once again, thank you very much for a very thought-provoking email and for your interest in LIGO and gravitational waves in general. By all means, do let me know if you would like me to clarify any of the points above or if you have any questions.

Best,

Maximiliano Isi
--------------------
California Institute of Technology
LIGO Laboratory, MC 100-36
Pasadena, CA 91125

[tar]

beecee,

 

"this means that the signal cannot be too different from the GR prediction or we wouldn't have seen it at all!"

 

Not sure I see Prof. Isi's logic here. If I had a theory about the magnetic effects created when a jet liner circles an airport, and theorized that this would effect the electromagnetic waves in LIGO in a certain pattern and I made such a template showing the pattern I expected, and searched the noise from LIGO with super computers and found the pattern that aligned with the template in the data from both experiments around the same time...I am not sure why that means, my theory is not falsified.

 

Regards, TAR

[beecee]

beecee,

 

"this means that the signal cannot be too different from the GR prediction or we wouldn't have seen it at all!"

 

Not sure I see Prof. Isi's logic here. If I had a theory about the magnetic effects created when a jet liner circles an airport, and theorized that this would effect the electromagnetic waves in LIGO in a certain pattern and I made such a template showing the pattern I expected, and searched the noise from LIGO with super computers and found the pattern that aligned with the template in the data from both experiments around the same time...I am not sure why that means, my theory is not falsified.

 

Regards, TAR

I see plenty of logic in what he says when taken in context with the full statement thus.....

"We have several methods to make quantitative statements about the agreement between the GR prediction and the signals we measure, but I won't describe them here in detail. Note that in order to find signals in the detector noise we use templates that tell us what the waves look like, and those templates are the output of highly–advanced super-computer simulations of GR dynamics; this means that the signal cannot be too different from the GR prediction or we wouldn't have seen it at all"!

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Putting it again in simple layman's terms of which I am an expert [ ] the detectors are designed to detect GW's aligning with highly advanced templates like BH/BH collisions as distinct from NS/NS collisions or NS/BH collisions as well as probably others.

Worth remembering I think the BICEP2 experiment and the pre announcement from that. Later experiments by a different group of scientists showed their conclusions to be less then probably accurate and the large possibility of dust being the cause.

The Professionals at aLIGO and elsewhere I'm sure would not be making the same mistake, considering the chances other ambitious scientists could pick up,if they were really mistaken.

Talk and debate on science forums such as this are just that. Those at the coal face and with their heads down and collective arses up, are doing/done the hard yards.

The over riding, irrefutable point is, the measured waveform agrees beautifully with the predictions of GR for a BH hole binary on now three occasions.

Edited June 16, 2017 by beecee

[StringJunky]

The over riding, irrefutable point is, the measured waveform agrees beautifully with the predictions of GR for a BH hole binary on now three occasions.

They actually detected them six times with two detectors; Three each.

[Mordred]

The Prof's logic is absolutely correct. The detector was designed to detect a specific type of wave. That being the quadrupolar nature. Hence the L shape, the detail many miss is that the detection also confirms spin 2. Prior to detection models merely favored, however spin 0 was a remote possibility.

 

Any spin statistics other than 2 and the detector would not have worked.

Edited June 16, 2017 by Mordred

[tar]

Mordred,

 

Understood, the LIGO was built to only pick up gws and It picked something up that looked exactly like what it was predicted a gw would look like, so the experiment worked.

 

I am not arguing that that is where we are.

 

What I am trying to picture, is what else had to be the case if a GW passed through. What effect would a GW have on another GW. What should it look like if we looked at the same wave half way between the two LIGOs.

 

Based on the actual noise, the actual reading, with no matching of computer simulations, could a trained eye look at the fringe pattern and say, there, there is another one? Or look. there is a NS/NS event?

 

Regards, TAR

For instance, if we had four LIGOs up could we use the three to triangulate on a GW and use the fourth as a blind. That is record the data on all four, but seal the results from LIGO 4. Search the data from the three and match it against the templates and supercomputer models and GR simulations and discover an event thought to be a GW. Then, knowing the direction from which the GW came, its strength and frequency, reconstruct the wave, in detail across the space that included LIGO 4, and determine exactly what the data on 4 should look like, across a specific 2 second time frame, before we unsealed the data?

Edited June 16, 2017 by tar

[beecee]

Mordred,

 

Understood, the LIGO was built to only pick up gws and It picked something up that looked exactly like what it was predicted a gw would look like, so the experiment worked.

 

I am not arguing that that is where we are.

 

What I am trying to picture, is what else had to be the case if a GW passed through. What effect would a GW have on another GW. What should it look like if we looked at the same wave half way between the two LIGOs.

 

Based on the actual noise, the actual reading, with no matching of computer simulations, could a trained eye look at the fringe pattern and say, there, there is another one? Or look. there is a NS/NS event?

 

Regards, TAR

For instance, if we had four LIGOs up could we use the three to triangulate on a GW and use the fourth as a blind. That is record the data on all four, but seal the results from LIGO 4. Search the data from the three and match it against the templates and supercomputer models and GR simulations and discover an event thought to be a GW. Then, knowing the direction from which the GW came, its strength and frequency, reconstruct the wave, in detail across the space that included LIGO 4, and determine exactly what the data on 4 should look like, across a specific 2 second time frame, before we unsealed the data?

 

 

I'm reasonably sure that the expert and professionals involved in the detection of gravitational radiation, have allowed for all reasonable contingencies.

If you have any reasonable assertion, supported by at least some evidence to show they have not allowed for all reasonable contingencies, then I would write up a scientific paper stating your case and showing by your "evidence" why their GR gravitational wave result maybe false, and submit it for appropriate professional peer review.

Let us know how you go.

[swansont]

beecee,

 

"this means that the signal cannot be too different from the GR prediction or we wouldn't have seen it at all!"

 

Not sure I see Prof. Isi's logic here. If I had a theory about the magnetic effects created when a jet liner circles an airport, and theorized that this would effect the electromagnetic waves in LIGO in a certain pattern and I made such a template showing the pattern I expected, and searched the noise from LIGO with super computers and found the pattern that aligned with the template in the data from both experiments around the same time...I am not sure why that means, my theory is not falsified.

 

Regards, TAR

 

 

There's a pretty large hurdle there, with "if I had a theory". You have no such theory, and no template, and this only serves to trivialize the difficulty involved in doing the actual science.

 

Based on the actual noise, the actual reading, with no matching of computer simulations, could a trained eye look at the fringe pattern and say, there, there is another one? Or look. there is a NS/NS event?

 

What would be the point of intentionally impeding the detection efforts in this way?

 

One thing that would result from using the templates is that some event that did not conform closely to GR would not be detected. IOW, you probably won't find new physics with it (should that exist), because you are only looking for patterns predicted by GR.