8 ms·

Another level of experimental blinding would be to avoid conferring with the other observatory (there are two components to LIGO, one in Hanford, WA and one in

by lutusp 11y ago

Another level of experimental blinding would be to avoid conferring with the other observatory (there are two components to LIGO, one in Hanford, WA and one in Livingston, LA) for confirmation/correlation until after one site's evaluation is complete. I don't know whether this is the current practice. In any case, if the observatories are actually able to detect gravitational waves and if there are no theoretical obstacles, eventually two black holes will merge, which will produce a completely fantastic observation consisting of a rapidly rising frequency (as the two BHs circle each other closer and closer, shedding their orbital energy as gravitational waves), followed by a very loud cosmological pop as the BHs finally collapse into each other.

Steuard 11y ago
My impression (from well off to the side in a tangentially related field, mind you) has been that false positives are common enough in the individual detectors that they really do need the comparison with the other location's data as a filter. (Not common enough for an actual coincidence to be at all likely, though.) I'm happy to be corrected if I'm wrong, of course! But I think the system has only been designed to be reliable as a whole.
- lutusp 11y ago
  > But I think the system has only been designed to be reliable as a whole. Yes, absolutely, I only meant that initial evaluations might be more objective if the two observatories cannot compare notes (at first) -- that might lead to a kind of confirmation bias. Only after one observatory satisfies itself that they have a sufficiently unambiguous signal should they be allowed to correlate with the other observatory's results. Considering the absence of plausible signals to date, this idea might be bending over backwards to avoid observational bias.
  - Steuard 11y ago
    I see your point, but I still have a sense that the false positive rate is just too high. I don't know the actual false positive rate, but I've somehow pictured maybe a dozen or so very brief (<1 sec?) signals per year. Under anything like those circumstances, an accidental coincidence would still be vanishingly unlikely (since there are 3x10^7 sec/year, it would be something like a one in a million chance), but the collaboration can't be expected to do full, detailed analysis of false signals a dozen times a year.
    - lutusp 11y ago
      > ... I still have a sense that the false positive rate is just too high. That might be true in principle, but considering what's at stake, I think the investigators will bend over backward to avoid making a claim that isn't ironclad. Remember that in real science, investigators look for explanations that will falsify their results, and only give credence to results that resist sincere efforts at falsification. (Contract this to the outlook of a pseudoscientist, who only looks for reasons to confirm his results.) Remember the incident in which some investigators claimed that neutrinos were exceeding the speed of light? In truth, they were looking for reasons to confirm their result, not falsify it. They were wrong, the real cause was a bad connection and ... they were pressured to resign their positions. https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly https://en.wikipedia.org/wiki/Faster-than-light_neutrino_ano... > ... the collaboration can't be expected to do full, detailed analysis of false signals a dozen times a year. That's true, but they could instead choose a handful of genuinely peculiar signals, signals unlikely to have natural causes, and try to falsify those. As above, there are a handful of cosmological sources that would produce -- not a brief signal -- but a pattern of signals, like a rising tone created by merging black holes. Those would be more interesting, as well as being easier to compare to false terrestrial sources.
ars 11y ago
> eventually two black holes will merge ... as the two BHs circle each other closer and closer I've heard this so many times, and I still don't see how it's possible! Because of time dilation this merger will take infinite time. No one has ever explained this satisfactorily to me. Why do people just seem to completely ignore relativity when dealing with black holes?
- deleted 11y ago
  [deleted]
- NolF 11y ago
  As I understand it, time only "slows" from the BH frame. From an observer POV it would happen as described.
  - ars 11y ago
    Other way around actually.
- daxfohl 11y ago
  It would take infinite time to an observer outside the horizon. It would probably look something like a cell dividing, in reverse, over infinite time, such that the end result is indistinguishable from a single hole.
- jessriedel 11y ago
  This is the exact same conceptual issue as the formation of a single black hole (BH). Consider a spherical shell of matter (i.e. GR dust) collapsing inwards to form a BH. As the dust particles get closer and closer together, one can roughly say that time slows down for them and that none of the particles ever pass through their mutual horizon. Rather, the argument goes, the particles just becomes exponentially close to where that horizon would form. From the outside, this is indistinguishable from a BH for all practical purposes, since the only way to isolate the still-free dust particles is to zoom exponentially close to the horizon and then drag yourself out (which requires an exponential amount of energy). The problem with this argument is that there is no preferred global time coordinate with respect to which you declare that "the BH has formed". Rather, the spacetime events corresponding to "each dust particle crosses their mutual horizon" (i.e., "the formation of the BH"), along with the rest of the interior of the BH, comprise an additional spacetime region you can choose to attach to your manifold or not. This is a free choice because causality flows one-way across the boundary; no experiments outside the BH can be sensitive to whether the region "really" exists. There are multiple ways to extend the normal time coordinates (at infinity) into the interior of the BH, and a natural choice is the proper time of in-falling particles/observers. In that case, the BH definitely forms at a finite values of that time coordinate. Merging two existing BHs doesn't introduce new fundamental issues. Quantum mechanics does introduce new fundamental issues, but formally the jury is still out on whether it resolves this question. The majority opinion is that the interior of the BH exists just as much as regions beyond your cosmological horizon (in an expanding cosmology) exist, from which you are likewise causally disconnected

by lutusp 11y ago

Steuard 11y ago

My impression (from well off to the side in a tangentially related field, mind you) has been that false positives are common enough in the individual detectors that they really do need the comparison with the other location's data as a filter. (Not common enough for an actual coincidence to be at all likely, though.) I'm happy to be corrected if I'm wrong, of course! But I think the system has only been designed to be reliable as a whole.

lutusp 11y ago
> But I think the system has only been designed to be reliable as a whole. Yes, absolutely, I only meant that initial evaluations might be more objective if the two observatories cannot compare notes (at first) -- that might lead to a kind of confirmation bias. Only after one observatory satisfies itself that they have a sufficiently unambiguous signal should they be allowed to correlate with the other observatory's results. Considering the absence of plausible signals to date, this idea might be bending over backwards to avoid observational bias.
- Steuard 11y ago
  I see your point, but I still have a sense that the false positive rate is just too high. I don't know the actual false positive rate, but I've somehow pictured maybe a dozen or so very brief (<1 sec?) signals per year. Under anything like those circumstances, an accidental coincidence would still be vanishingly unlikely (since there are 3x10^7 sec/year, it would be something like a one in a million chance), but the collaboration can't be expected to do full, detailed analysis of false signals a dozen times a year.
  - lutusp 11y ago
    > ... I still have a sense that the false positive rate is just too high. That might be true in principle, but considering what's at stake, I think the investigators will bend over backward to avoid making a claim that isn't ironclad. Remember that in real science, investigators look for explanations that will falsify their results, and only give credence to results that resist sincere efforts at falsification. (Contract this to the outlook of a pseudoscientist, who only looks for reasons to confirm his results.) Remember the incident in which some investigators claimed that neutrinos were exceeding the speed of light? In truth, they were looking for reasons to confirm their result, not falsify it. They were wrong, the real cause was a bad connection and ... they were pressured to resign their positions. https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly https://en.wikipedia.org/wiki/Faster-than-light_neutrino_ano... > ... the collaboration can't be expected to do full, detailed analysis of false signals a dozen times a year. That's true, but they could instead choose a handful of genuinely peculiar signals, signals unlikely to have natural causes, and try to falsify those. As above, there are a handful of cosmological sources that would produce -- not a brief signal -- but a pattern of signals, like a rising tone created by merging black holes. Those would be more interesting, as well as being easier to compare to false terrestrial sources.

ars 11y ago

> eventually two black holes will merge ... as the two BHs circle each other closer and closer I've heard this so many times, and I still don't see how it's possible! Because of time dilation this merger will take infinite time. No one has ever explained this satisfactorily to me. Why do people just seem to completely ignore relativity when dealing with black holes?

deleted 11y ago
[deleted]
NolF 11y ago
As I understand it, time only "slows" from the BH frame. From an observer POV it would happen as described.
- ars 11y ago
  Other way around actually.
daxfohl 11y ago
It would take infinite time to an observer outside the horizon. It would probably look something like a cell dividing, in reverse, over infinite time, such that the end result is indistinguishable from a single hole.
jessriedel 11y ago
This is the exact same conceptual issue as the formation of a single black hole (BH). Consider a spherical shell of matter (i.e. GR dust) collapsing inwards to form a BH. As the dust particles get closer and closer together, one can roughly say that time slows down for them and that none of the particles ever pass through their mutual horizon. Rather, the argument goes, the particles just becomes exponentially close to where that horizon would form. From the outside, this is indistinguishable from a BH for all practical purposes, since the only way to isolate the still-free dust particles is to zoom exponentially close to the horizon and then drag yourself out (which requires an exponential amount of energy). The problem with this argument is that there is no preferred global time coordinate with respect to which you declare that "the BH has formed". Rather, the spacetime events corresponding to "each dust particle crosses their mutual horizon" (i.e., "the formation of the BH"), along with the rest of the interior of the BH, comprise an additional spacetime region you can choose to attach to your manifold or not. This is a free choice because causality flows one-way across the boundary; no experiments outside the BH can be sensitive to whether the region "really" exists. There are multiple ways to extend the normal time coordinates (at infinity) into the interior of the BH, and a natural choice is the proper time of in-falling particles/observers. In that case, the BH definitely forms at a finite values of that time coordinate. Merging two existing BHs doesn't introduce new fundamental issues. Quantum mechanics does introduce new fundamental issues, but formally the jury is still out on whether it resolves this question. The majority opinion is that the interior of the BH exists just as much as regions beyond your cosmological horizon (in an expanding cosmology) exist, from which you are likewise causally disconnected