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Gravitational Waves!!! [and old Inflation Thread]
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| Watsisname | Date: Saturday, 23.01.2016, 08:21 | Message # 16 |
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| Gravitational wave detection could be a false alarm
I would definitely recommend to read the whole article and not just the headline. The fascinating part is why this could be a false alarm. Great insight as to how this group does "quality control", in a field where we currently cannot independently verify all potential signals, given the lack of required sensitivity across all detectors. It's actually pretty smart. 
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| midtskogen | Date: Saturday, 23.01.2016, 09:47 | Message # 17 |
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| I was a bit worried that injecting false signal means that they also have a pretty clear idea of what to look for in the data, and if you look for a specific pattern, then everything tends to look a bit like that pattern. It's simply how our mind works. I find it easier to trust discoveries that were unexpected.
Perhaps general relativity is one big illusion, which will be clear by a discovery not yet made.
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Saturday, 23.01.2016, 11:14 | Message # 18 |
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| Quote midtskogen (  ) I was a bit worried that injecting false signal means that they also have a pretty clear idea of what to look for in the data, and if you look for a specific pattern, then everything tends to look a bit like that pattern.
They do indeed have a clear idea of what to look for. It is dictated by the physics of gravitational waves. These waves are generated by variation in the quadrupole moment of the source, and they manifest as alternating orthogonal stretches and squeezes in the space-time geometry that propagate with the speed of light. So they look for changes in the pathlengths of the arms of the interferometer(s) that have this form.
However, there is no single pattern that has this form. It depends on the nature of the source! Is it an asymmetrically exploding star? Merging neutron stars? A binary black hole? Merging black holes? Merging black holes with spin axes tilted to the orbital plane? Echoes of the Big Bang? These will all look very different. There is a vast zoo of possible patterns we may find, not to mention patterns produced by phenomena we haven't even thought of.
In this sense, we don't have any idea of what particular pattern to be looking for, but we know how to recognize one if we find it, and how to characterize the source. We're in a very strong position in an exploratory sense, with gravitational waves potentially opening a whole new window on observational astronomy.
Quote midtskogen ( ) Perhaps general relativity is one big illusion
This statement makes no sense. General relativity is a model of physics. It makes predictions. Do you realize the number of very specific predictions that GR makes which have been experimentally verified, and the precision with which we do so?
Gravitational waves are literally the last remaining prediction of GR which has not been found directly, yet we already have powerful indirect evidence that they exist (the particular rate of decay of binary pulsars, as the waves carry away some orbital energy), the discovery of which earned a Nobel Prize.
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| midtskogen | Date: Saturday, 23.01.2016, 16:35 | Message # 19 |
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| Quote Watsisname ( ) This statement makes no sense. General relativity is a model of physics. It makes predictions. Do you realize the number of very specific predictions that GR makes which have been experimentally verified, and the precision with which we do so?
It's a scientific theory and model which requires a substantial amount of observations and none of them can contradict. If something contradicts a scientific theory, we could say that the theory is wrong, but I think that can be a bit unfair. We have observations that contradict Newton's theory and it's somewhat misleading to say that he was wrong. Rather, his theory holds up under certain circumstances and assumptions. What looks like something (under certain circumstances) that it isn't, is an illusion. So Newton's Euclidean universe with a time dimension separate from the spatial dimensions can be called an illusion rather than false.
In the 19th century it was known that there was something funny with Mercury's orbit. There were ways to interpret the data to get them agree with theory, though. Few wanted or expected Newton to be wrong. Still, general relativity turned out to be much more exciting than the ad hoc explanations for Mercury's orbit.
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Saturday, 23.01.2016, 22:07 | Message # 20 |
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| Quote midtskogen ( ) Rather, his theory holds up under certain circumstances and assumptions. What looks like something (under certain circumstances) that it isn't, is an illusion. So Newton's Euclidean universe with a time dimension separate from the spatial dimensions can be called an illusion rather than false.
Fair enough. I just never really think of it as an "illusion". I'd call it an approximation. I.e. it is a description of reality which gives very accurate predictions within a certain regime -- the regime within which the model was formulated. In the Newtonian regime, space and time are not obviously related, and the geometry is not obviously non-Euclidean. You could call that an "illusion". Or you could call it "insufficient precision to tell the difference." Because the difference is extremely small.
GR's model of space and time is a better model than Newton's. It works under a much broader regime. But it still reduces to Newton's Laws in the Newtonian regime. And GR is still just a model. There may be a future model which works even better. But that future model must also reduce to GR and Newton in those limits.
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| midtskogen | Date: Thursday, 11.02.2016, 16:05 | Message # 21 |
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| Gravitational waves from black holes detected (again)
NIL DIFFICILE VOLENTI
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| JackDole | Date: Thursday, 11.02.2016, 16:48 | Message # 22 |
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| Quote midtskogen ( ) (again)
Not again! As it stands, it is about the official confirmation that the discovery of 14 September last year was real and not a false alarm.
Don't forget to look here.
Edited by JackDole - Thursday, 11.02.2016, 16:51 |
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| midtskogen | Date: Thursday, 11.02.2016, 20:37 | Message # 23 |
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| Quote JackDole ( ) Not again! As it stands, it is about the official confirmation that the discovery of 14 September last year was real and not a false alarm.
I was referring to the announcement almost two years ago, when it was claimed that gravitational waves had been found, but later it had to be retracted. There is more reason to believe it this time, but for an outsider it's not easy to judge. My main concern is that if that if you look for patterns, random data tend to look like what you're looking for. There is more confidence in unexpected contradicting evidence than in expected confirming evidence.
NIL DIFFICILE VOLENTI
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| DoctorOfSpace | Date: Thursday, 11.02.2016, 20:46 | Message # 24 |
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| Considering the strength of this signal and the fact that it was picked up in two locations seems like fairly compelling evidence.
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| Watsisname | Date: Friday, 12.02.2016, 03:18 | Message # 25 |
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| Yeah. This is the whole point of having a network of detectors. It is conceivable that an unrelated local disturbance could cause a signal which mimics the behavior of gravitational wave passing through a single detector, even after the extreme care which is taken in isolating it from external vibrations. (Seriously, the array of oscillation-dampening pendulums they use to do this are incredible, not to mention the techniques for working around the random vibrations caused by quantum mechanics, which to me might as well be black magic.) However, you will not expect the same strong signal to be observed in two detectors separated by ~3000km, within a time interval of 10 milliseconds(!!), unless it really is a gravitational wave from an astrophysical source.
What we have here is actually a 5.1-sigma detection, which is pretty damn good! That's above the threshold for announcing the discovery of a new particle. A further cause for confidence that this is the real thing is that we have characterized the source, and the source makes sense. The waves are exactly what general relativity predicts for the inspiral and merger of two black holes. This includes the "ring-down", which is an important prediction of how black holes radiate away any irregularities according to the no-hair theorem. The impact of this detection in the context of experimental testing of GR really cannot be over-stated. This is super exciting! 
Here's the paper by the way. Check out the plots!
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| midtskogen | Date: Friday, 12.02.2016, 18:55 | Message # 26 |
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| I was wondering how they deal with seismic noise, so I checked the paper. They're using a laser with a ~1µm wavelength. Relatively small earthquakes can cause compression waves causing such displacements worldwide. If I say several a day, I don't think I'm off by orders of magnitude. According to the paper they have been able to isolate it pretty much perfectly from seismic noise above 10 Hz. Global seismic noise has noise has much lower frequencies. They don't cause noise all the time, though, and perhaps they can just filter out these frequencies altogether. Then there is seismic noise from the oceans (crashing waves), which will never fully away and form a constant background noise. They have a somewhat higher frequency (but still below 10 Hz). It's not clear in the paper how much of a problem this is and how they deal with it.
A second thought that I have is that the universe is big. Very big. Shouldn't gravitational waves from merging black holes be a pretty common event?
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Saturday, 13.02.2016, 02:10 | Message # 27 |
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| Their ability to isolate the detector from seismic noise over those frequencies is about 1010, which is pretty impressive, but they do still want to be very confident that what they are seeing is astrophysical in origin and not related to some terrestrial event. So each facility also has an array of seismometers and other environmental sensors, collectively recording over 200,000 channels of data with which to check for such correlations and characterize the background noise. This was a huge part of the data analysis -- they have a separate paper dedicated to it which you can find here (reference #69 in the main paper).
Quote midtskogen ( ) A second thought that I have is that the universe is big. Very big. Shouldn't gravitational waves from merging black holes be a pretty common event?
Of course, but the intensity of the waves diminishes with distance. So for a given sensitivity, you can detect a particular intensity of source out to a certain range. The volume of space that you can search grows as the cube of the sensitivity.
This is why having more sensitive detectors is so important -- they dramatically increase the chance that you will see something in a given interval of time. If I recall correctly, astrophysical models predicted that the first iteration of detectors could expect to see something once every 1 to 10,000 years. Which is not good. (This also speaks to how poorly constrained our models of black hole merger event rates and other gravitational wave producing phenomena in the universe are.) But with the latest upgrade to LIGO, we can expect many signals a year. It's almost certainly not a coincidence that the first verified gravitational wave event happened almost immediately after the upgrade.
Another note is that as you look out further, then the gravitational waves from binary black hole mergers start to become so frequent that the waves go into superposition, and essentially you are looking at a background noise instead of individual signals. This background should be detectable with LIGO in its final operating state.
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| midtskogen | Date: Saturday, 13.02.2016, 19:03 | Message # 28 |
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| Quote Watsisname ( ) Of course, but the intensity of the waves diminishes with distance.
Yes, but according to the paper the source was 230-570 Mpc away, and that kind of radius around Earth is not an insignificant portion of the universe. That volume includes millions of galaxies.
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Sunday, 14.02.2016, 00:19 | Message # 29 |
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| No kidding, it's a huge volume. Now what do you suppose is the rate of massive binary black hole mergers in this volume? Can you come up with a number?
Yeah, neither can I. Let's check what the experts have to say. From the paper on the gravitational wave background linked above:
And another paper dedicated to this topic, particularly in section 6.
Like I said -- prior to this detection, we had poor constraints on the rate of binary black hole mergers. It is a very hard thing to calculate because it depends on the star formation rate, the details of the progenitor explosions, and a lot of other factors. There were even a few authors predicting that there should be no binary mergers within the Hubble time (e.g. because the "kicks" during supernovae might be too strong and not allow a binary system to result, or at least not one which would inspiral within the age of the universe). Thankfully, those models were wrong. This observation now gives us much better constraints on the black hole merger rate, and it seems to be on the higher end of the predicted range.
Maybe this merger rate is just a lot less than you expect? It's only a few to a few dozen per Gpc3 per year. So we really do need to be able to "watch" a lot of a galaxies to have an good chance of spotting one in a reasonable amount of time.
Also, interestingly, it seems LIGO did not detect just one merger, but possibly several. It's just that this was the only one with sufficient confidence to announce. (The second strongest, another binary black hole merger, was too weak and had a false alarm rate of once per 2.3 years.)
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| midtskogen | Date: Sunday, 14.02.2016, 17:36 | Message # 30 |
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| The detection actually means two things: Both gravitational waves and black holes have been observed directly for the first time, or at least much more directly than before. The most certain thing in the paper is that the two signals detected were the same event (but we would be more certain if there were more station, and that would also mean that weaker signals could be confirmed). Exactly what was detected depends on more assumptions. If the black hole merger and masses could have been observed by other means independently, it would be so much easier to say that it must have been its gravitational wave that was detected. But instead we must say that if the theory we're confirming is right, a black hole merger, whose frequency is unknown, of this and this size, is the only thing that match the observation. This kind of fitting for the theory to be confirmed doesn't really do a very good job in excluding that the signal was from something that we haven't discovered yet.
An important result of this discovery, I think, is that the instruments are able to pick up something, and if the network is expanded and instruments improved, a door into a new world has been opened.
NIL DIFFICILE VOLENTI
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