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#11
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Advanced LIGO has detected gravitational waves from a binary
David Staup wrote:
On 2/13/2016 10:40 AM, Jos Bergervoet wrote: On 2/13/2016 11:08 AM, Gregor Scholten wrote: What I'm really asking myself is: That black hole collision took place in 1.3 billion light years distance, and is still detectable. How strong would the gravitational waves be if the collision had taken place in a much nearer location, e.g. in 1 million light years or 1000 light years distance? Strong enough to yield effects visible to bare eyes? Strong enough to destroy Earth? Slightly raising the stakes, I think their gravitional waves would not destroy the earth, even if the two black holes were replacing the sun! (We would of course have to give the earth an 8 times higher orbital speed in the first place, to maintain its distance). The distance to the sun is about 10^14 times smaller, so the waves would be some 10^14 times stronger and the suspended mirrors in the LIGO detector would not move 4 atto-meter, as they did on Sep 14, but a whole 0.4mm! This doesn't look like more than a micro-earthquake so the earth would not be destroyed and even the delicate LIGO detector would easily survive this (but the presence of two black holes instead of the sun might cause other problems..) Comparing it to EM: we can detect the Pioneer spacecraft radio transmitter now that it is at 3 10^12 meter distance, What if we were 10^14 times closer? That would be comparable to holding a transmitting cell-phone at 1 cm from your ear (in fact its transmitter is just slightly stronger than the average cellphone, both are a few Watts). This surprises me, the equivalent of 3 solar masses radiated away in less than a second from 96 million miles away and we wouldn't notice? We would certainly notice a change in the orbit of the earth, some time later, from the missing mass. I haven't done the sums, but I feel confident that the signal should also be visible in the instantaneous earth-moon distance, if the lunar laser reflectors were being operated. The effects would also be immediately visible in GPS, Jan |
#12
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Advanced LIGO has detected gravitational waves from a binary
Phillip Helbig (undress to reply) wrote:
David Staup writes:=20 This surprises me, the equivalent of 3 solar masses radiated away in less than a second from 96 million miles away and we wouldn't notice? It's not just the energy, but rather the effect it produces on whatever it interacts with (or not). Right. I just ran some numbers, and I came to the remarkable conclusion that the total solar power output (4E+26 watts) could harmlessly pass through you if it was in the form of gravitational waves rather than heat and light. I used the formula c^3 h^2 f^2 pi / (8 G) to convert strain to flux. G is the gravitational constant, h is the strain, c is the speed of light, and f is the frequency in Hz (250 in this case). The peak strain of the recent event was 1E-21, so I get a peak flux of 10 milliwatts per square meter. No wonder they always list the sensitivity of LIGO in terms of strain rather than in terms of watts per square meter. The latter doesn't sound nearly as impressive! Indeed, if the event had given off light rather than gravitation waves, it would have been not only bright enough to see from here, but bright enough to read by! As a sanity check, I divided the reported peak power output of the event, 3.6E49 watts, i.e. 200 solar masses per second annihilated, by the area of a sphere 1.3 billion light years in radius. I get about 20 milliwatts per square meter. What accounts for the factor of two discrepancy? Probably polarization. LIGO, if I understand correctly, is sensitive to only one of the two polarizations. ("Only" 3 solar masses were annihilated, because the event lasted less than a second.) Lets get closer to the event and see what happens. I hope you're reading this with a fixed font. Distance flux (W/m^2) strain N 1.3E25 m (1.3E9 ly) 1E-2 1E-21 4E33 1.3E22 m (1.3E6 ly) 1E+4 1E-18 4E39 1.3E19 m (1.3E3 ly) 1E+10 1E-15 4E45 1.3E16 m (1.3 ly) 1E+16 1E-12 4E51 1.3E13 m (66 AU) 1E+22 1E-9 4E57 1.3E10 m (8 M miles) 1E+28 1E-6 4E63 The last column is the number of gravitons per square meter per second. I get that by multiplying the flux by the frequency and dividing by Plank's constant. In each case, I assume you're floating in space, in a good spacesuit, facing toward the event. I assume that a strain of one part in a million isn't going to hurt you, especially if it's front-to-back rather than head-to-toes. Note that that last distance is much less than 1 AU. 1E+28 watts per square meter -- your cross-sectional area is probably roughly one square meter -- means 25 times the sun's total power output is going through you. I wonder what it would feel like. Of course I'm also assuming it was a "clean" event, i.e. nothing but gravitational waves was given off. If it consisted of nothing but two black holes, that's pretty much certain. But if there was other stuff in the area, all bets are off. Indeed, there was a weak gamma ray burst half a second after the event, which may or may not be a coincidence. We don't know the direction of either the event or the gamma ray burst, except very roughly. Supernovae radiate a huge amount of energy in neutrinos, but these hardly affect anything else. Neutrinos aren't nearly as stealthy as gravitons. According to Randall Munroe, a typical supernova will emit 1E57 neutrinos, and they will be lethal at about 2 AU. During the peak tenth of a second of the event at the closest distance I list, 100,000 times as many gravitons will harmlessly pass through you as the *total* number of neutrinos given off by a supernova! -- Keith F. Lynch - http://keithlynch.net/ Please see http://keithlynch.net/email.html before emailing me. |
#13
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Advanced LIGO has detected gravitational waves from a binary
Keith F. Lynch wrote:
As a sanity check, I divided the reported peak power output of the event, 3.6E49 watts, i.e. 200 solar masses per second annihilated, by the area of a sphere 1.3 billion light years in radius. I get about 20 milliwatts per square meter. What accounts for the factor of two discrepancy? Probably polarization. LIGO, if I understand correctly, is sensitive to only one of the two polarizations. Also, red shift. 200 solar masses per second at the event equals about 180 solar masses per second on Earth, as the event is receding from us at about a tenth of the speed of light. Also, the received frequency at the time of peak power, about 250 Hz, was originally about 270 Hz. That's an impressive orbital period (1/270 second) for two 30-solar-mass objects. The last column is the number of gravitons per square meter per second. I get that by multiplying the flux by the frequency and dividing by Plank's constant. Sigh. Of course I should have *divided* by the frequency, not multiplied. So my estimated numbers of of gravitons were about five orders of magnitude too high. The total number of gravitons emitted by the event was about 3E+78. I hope someone checks my math. -- Keith F. Lynch - http://keithlynch.net/ Please see http://keithlynch.net/email.html before emailing me. |
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Advanced LIGO has detected gravitational waves from a binary black hole collision/merger
In article ,
Jos Bergervoet writes: Why are the 2 predicted curve in this picture slightly different? https://www.ligo.caltech.edu/image/ligo20160211a As the caption notes, one of the curves has to be inverted and shifted by 7 ms because of the different detector locations and orientations. The rest of the differences are presumably random noise. The signal to noise is only 5 sigma, after all. Figure 1 of the published paper is clearer. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
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Advanced LIGO has detected gravitational waves from a binary
Nicolaas Vroom wrote:
What I'm really asking myself is: That black hole collision took place in 1.3 billion light years distance, and is still detectable. What I'm asking myself is: Consider a sphere of 1.3 billion light years (as a matter of thought) around our Sun. Than nowhere at this sphere 1.3 billion years ago a similar event i.e. inspiral and merger BH, took place. Because if it did we would see both. Also nowhere at the path, during its travel time of the wave, from the origin of the event to the moment (point) of detection, a similar event took place. Because if it did etc. Assume that this path is a straight line. It seems to me that you do not clearly distinguish between the path in space and the path in spacetime. If we consider the path in space, numerous similar events could have taken place without being detectable for us today, because the emitted gravitational waves would have already passed us a long time ago. Than also starting from any point on the sphere mentioned above, in a straight line to the point of detection a similar event took place. Because if it did etc. What I want to point out is to detect solely only one inspiral BH is quite remarkable. What I would expect is that there would be more binary BH's mergers in progress. Apparently this is not the case. Your conclusion is wrong. There could be many mergergs being in progress. But the gravitational waves did not reach us yet, but may do so in e.g. 1 year, or 10 years, or 1000 years. Consider supernovae for comparison. Assume we observe a single supernova now. That does not mean that there were no other supernovae ongoing. The matter of fact is simply that the light of other supernovae does not reach us now, but may reach us in 1 year, or 10 years, or 1000 years. |
#16
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Advanced LIGO has detected gravitational waves from a binaryblack hole collision/merger
On 2/16/2016 8:48 PM, Steve Willner wrote:
In article , Jos Bergervoet writes: Why are the 2 predicted curve in this picture slightly different? https://www.ligo.caltech.edu/image/ligo20160211a As the caption notes, one of the curves has to be inverted and shifted by 7 ms because of the different detector locations and orientations. The data are shifted, not the predicted curves, those are inverted but not shifted as can clearly be seen in the ring-down region at the end. The rest of the differences are presumably random noise. There will be noise in the data, but why in the predictions? The signal to noise is only 5 sigma, after all. Figure 1 of the published paper is clearer. OK, we look at: https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_Detection_of_GW150914.pdf There in Fig. 1, the predicted curves are called "Numerical relativity" and they *still* have differences in shape, that are not inversion or shift transformations! It is becoming more and more intriguing. Possible explanations: 1) For some reason (to make the curve look more "natural"?) someone decided to add random noise to the computed results. And they added *different* noise for Hanford and Livingston. To me this seems a silly eplanation. 2) The results are different polarization componentsa (after all you only need a 45 degree tilt to see the independent other polarization for a spin-2 field.) 3) The numerical routines generate some numerical errors visible as small random looking "ripples" in the computed result. This seems likely since complex curved space-time will enforce a complicated non-uniform grid in the 4 coordinates. A combination of 2) and 3) seems most likely to me (I didn't find any proof for it in the text, but I may have overlooked it in the extensive list of papers that have accumulated.) -- Jos |
#17
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Advanced LIGO has detected gravitational waves from a binary black hole collision/merger
On 18/02/16 09:20, Jos Bergervoet wrote:
OK, we look at: https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_Detection_of_GW150914.pdf There in Fig. 1, the predicted curves are called "Numerical relativity" and they *still* have differences in shape, that are not inversion or shift transformations! It is becoming more and more intriguing. Possible explanations: 1) For some reason (to make the curve look more "natural"?) someone decided to add random noise to the computed results. And they added *different* noise for Hanford and Livingston. To me this seems a silly eplanation. 2) The results are different polarization componentsa (after all you only need a 45 degree tilt to see the independent other polarization for a spin-2 field.) 3) The numerical routines generate some numerical errors visible as small random looking "ripples" in the computed result. This seems likely since complex curved space-time will enforce a complicated non-uniform grid in the 4 coordinates. A combination of 2) and 3) seems most likely to me (I didn't find any proof for it in the text, but I may have overlooked it in the extensive list of papers that have accumulated.) As far as I understand as a non-expert in the field of GWs, in the PRL they write that the numerical-relativity and the sine-Gauss wavelet fits are ploted within the "detector filter". That's the usual way to compare calculations with data. Also in our field (relativistic heavy-ion collisions) you have to run your theoretical results for cross sections and related observables through the detector-acceptance filter. Sometimes that's a simple cut but often it's also a numerical routine developed by the experimentalists taking into account details of the detector. I guess, that's the same here. The analysis of the GW signals out of the detector noise is far from trivial, and I cannot understand this in all details, of course. Note that there are a lot of papers by the LIGO+VIRGO collaboration on the arXiv, where you can find many more details. Among them are some papers about the analysis of the detector noise and GW signal reconstruction: http://arxiv.org/abs/1602.03843 http://arxiv.org/abs/1602.03845 http://arxiv.org/abs/1602.03844 -- Hendrik van Hees Goethe University (Institute for Theoretical Physics) D-60438 Frankfurt am Main http://fias.uni-frankfurt.de/~hees/ [Mod. note: quoted text trimmed --mjh] |
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Advanced LIGO has detected gravitational waves from a binary
Op dinsdag 16 februari 2016 22:17:56 UTC+1 schreef Gregor Scholten:
It seems to me that you do not clearly distinguish between the path in space and the path in spacetime. If we consider the path in space, numerous similar events could have taken place without being detectable for us today, because the emitted gravitational waves would have already passed us a long time ago. Consider an event (BH merger) at a certain distance r0 from an observer at a moment t0. The gravitional waves propagates towards observer. At t1 this wave is at a distance r1 at t2 at a distance r2 and at t10 at a distance r10 = 0 i.e. the wave reaches the Observer. In this case I divide the path in 10 steps but you can also consider 100 steps i.e. any number. The point is if at any of these positions (tn,rn) also a second independent BH merger takes place than at (t10,r10=0) the observer will receive a super position of two gravitational waves. And that is not what we want (?) The next thing you can do is draw a sphere with radius r0 around Observer. If at t0 at this whole sphere there is also a BH merger than the gravitational waves from that event will also reach the observer at (t10,r10=0). Again a superposition and that is not what we want. It should be mentioned that the concept of a sphere with radius r0 is an approximation. The same thing you can do for all the rn at tn. At each of the spheres rn at tn there should not be any second BH merger in progress, because if it does you will receive sets at t10,r10=0. To get a different idea about of what I see as a problem: suppose gravity waves act instantaneous. This means that at this instant if you observe 1 BH merger, there should not be any second BH merger in progress at a radius of roughly 2 billion light years, Consider supernovae for comparison. Assume we observe a single supernova now. That does not mean that there were no other supernovae ongoing. The matter of fact is simply that the light of other supernovae does not reach us now, but may reach us in 1 year, or 10 years, or 1000 years. To detect a single supernova the story is different. At each instant you can detect many supernovae simultaneous assuming their directions come from different positions on the sphere surrounding us. Nicolaas Vroom |
#19
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Advanced LIGO has detected gravitational waves from a binary
In article , Nicolaas Vroom
writes: Consider an event (BH merger) at a certain distance r0 from an observer at a moment t0. The gravitional waves propagates towards observer. At t1 this wave is at a distance r1 at t2 at a distance r2 and at t10 at a distance r10 = 0 i.e. the wave reaches the Observer. In this case I divide the path in 10 steps but you can also consider 100 steps i.e. any number. The point is if at any of these positions (tn,rn) also a second independent BH merger takes place than at (t10,r10=0) the observer will receive a super position of two gravitational waves. And that is not what we want (?) The next thing you can do is draw a sphere with radius r0 around Observer. If at t0 at this whole sphere there is also a BH merger than the gravitational waves from that event will also reach the observer at (t10,r10=0). Again a superposition and that is not what we want. It should be mentioned that the concept of a sphere with radius r0 is an approximation. OK. Consider that such an event lasts a few seconds or whatever. One can get some idea about the chances of two or more overlapping (the one at the farther distance taking place farther in the past, of course). People have done the calculations and estimates of the numbers of such events. I don't think "confusion", as this is probably called, that is, more than one event observable at the same time, is an issue here. To get a different idea about of what I see as a problem: suppose gravity waves act instantaneous. This means that at this instant if you observe 1 BH merger, there should not be any second BH merger in progress at a radius of roughly 2 billion light years, But they travel at the speed of light. If not, GR is incorrect, then you can't trust the rest of your calculations either. IIRC one could detect the time-of-arrival distance between the two detectors. Consider supernovae for comparison. Assume we observe a single supernova now. That does not mean that there were no other supernovae ongoing. The matter of fact is simply that the light of other supernovae does not reach us now, but may reach us in 1 year, or 10 years, or 1000 years. Same with gravitational-wave events. To detect a single supernova the story is different. At each instant you can detect many supernovae simultaneous assuming their directions come from different positions on the sphere surrounding us. In principle the same with gravitational waves. However, since they last much shorter than a supernova, probably only one is visible at any given time. |
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Advanced LIGO has detected gravitational waves from a binary black hole collision/merger
Jos Bergervoet writes:
OK, we look at: https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_Detection_of_GW150914.pdf There in Fig. 1, the predicted curves are called "Numerical relativity" and they *still* have differences in shape, that are not inversion or shift transformations! It is becoming more and more intriguing. Possible explanations: 1) For some reason (to make the curve look more "natural"?) someone decided to add random noise to the computed results. And they added *different* noise for Hanford and Livingston. To me this seems a silly eplanation. The numerical relativity curves are "projected onto each detector", which I'd assume means they started with a theoretical prediction and then projected it onto the detector response--that's what one does for comparison with an observed signal. So look at figure 3(b) of that paper, and you'll see that the two sites have some significant differences in instrument noise. They may have simulated the noise and the band-reject filters, or just the filters, it probably says in one of the papers I haven't looked at yet. But either way is going to result in small differences in the theoretical expectations for the two instruments. -dan |
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