5 ms·

LIGO collaboration member here: this is a tunable knob, independent of the number of detectors, that we've set very intentionally based on feedback from astrono

by pinko 5y ago

LIGO collaboration member here: this is a tunable knob, independent of the number of detectors, that we've set very intentionally based on feedback from astronomers who want to follow up with optical/gamma/neutrino instruments following our BNS detections. We could reduce our false positive rate to a negligible number today, at the cost of _not_ reporting many likely discoveries. Also, a minor point but there are only two LIGO detectors online at the moment, with a third sister detector in Italy (named Virgo), and a fourth coming online soon in Japan (named KAGRA). There does in fact exist a third LIGO instrument, but it's currently mothballed, awaiting construction in India.

gus_massa 5y ago
Do you publish some number about how sure you are?
- pinko 5y ago
  Yes, we carefully explain the statistical confidence of our detections in each discovery paper, and there are dozens of methods papers that go into excruciating detail on those techniques. Prior to our first detection, the overwhelming prime directive of our collaboration was _not to make a false detection_ and we went to insane lengths to avoid one; e.g., we had a small team of people secretly injecting false signals -- "blind injections" -- into our data, so that we all expected to be regularly seeing them and wouldn't be tempted to gin the analysis to find a detection where there wasn't one. (Amusingly, because of this, it took weeks for many of us to believe the first detection was real, even after the blind injection team swore it wasn't one of theirs. In the end, we charged an independent team to do a forensic analysis of everything from the security cameras and seismometers in the observatories, to every computer and disk the data passed through, to convince ourselves this couldn't possibly have been maliciously injected by hand by a rogue scientist. It was a wild few weeks!) Now that it's clear gravitational waves exist, however, we focus on optimizing the false alarm rate for astronomers (who want it well above zero so they don't miss anything) rather than optimizing for ~zero false detections.
  - ars 5y ago
    When I see LIGO graphs they always appear linear - the change in frequency is linear with time. But with time dilation as black holes near each other, shouldn't the frequency change be exponential? Or does it cancel out - the frequency goes up as they get closer, and then time dilation lowers it back as it dilates into infinity?
  - im3w1l 5y ago
    That sounds hilarious, but you must be quite unique in having this approach right? Or are the particule physics people playing similar games?
    - pinko 5y ago
      Unique as far as I know, at least in a scientific collaboration like ours. But we were unusually sensitive to the risk of false detections given (a) skepticism of LIGO's prospects from other physicists, and (b) the lingering stain of a false detection claim, from a bar detector, a few decades earlier. We believed that if we had announced a discovery that turned out to be wrong, it would probably have meant the end of our experiment, and in practice the end of the field, at least for a long time. It was fortuitous that the first detection turned out to be gold plated and unambiguous, otherwise we would have probably published a bunch of "we saw something interesting but can't claim it as a GW discovery" papers on the next few weaker events before we would have felt comfortable making a confident claim.
      - airstrike 5y ago
        Thank you for all these replies and for the fantastic work you and the team do. This is now my favorite AMA
    - jasonwatkinspdx 5y ago
      Not quite the same lengths, but AIRC LHC uses two detectors with substantial design differences, and partitions the teams from sharing information for some of what they target, as a way to reduce systemic errors.
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by pinko 5y ago

gus_massa 5y ago

Do you publish some number about how sure you are?

pinko 5y ago
Yes, we carefully explain the statistical confidence of our detections in each discovery paper, and there are dozens of methods papers that go into excruciating detail on those techniques. Prior to our first detection, the overwhelming prime directive of our collaboration was _not to make a false detection_ and we went to insane lengths to avoid one; e.g., we had a small team of people secretly injecting false signals -- "blind injections" -- into our data, so that we all expected to be regularly seeing them and wouldn't be tempted to gin the analysis to find a detection where there wasn't one. (Amusingly, because of this, it took weeks for many of us to believe the first detection was real, even after the blind injection team swore it wasn't one of theirs. In the end, we charged an independent team to do a forensic analysis of everything from the security cameras and seismometers in the observatories, to every computer and disk the data passed through, to convince ourselves this couldn't possibly have been maliciously injected by hand by a rogue scientist. It was a wild few weeks!) Now that it's clear gravitational waves exist, however, we focus on optimizing the false alarm rate for astronomers (who want it well above zero so they don't miss anything) rather than optimizing for ~zero false detections.
- ars 5y ago
  When I see LIGO graphs they always appear linear - the change in frequency is linear with time. But with time dilation as black holes near each other, shouldn't the frequency change be exponential? Or does it cancel out - the frequency goes up as they get closer, and then time dilation lowers it back as it dilates into infinity?
- im3w1l 5y ago
  That sounds hilarious, but you must be quite unique in having this approach right? Or are the particule physics people playing similar games?
  - pinko 5y ago
    Unique as far as I know, at least in a scientific collaboration like ours. But we were unusually sensitive to the risk of false detections given (a) skepticism of LIGO's prospects from other physicists, and (b) the lingering stain of a false detection claim, from a bar detector, a few decades earlier. We believed that if we had announced a discovery that turned out to be wrong, it would probably have meant the end of our experiment, and in practice the end of the field, at least for a long time. It was fortuitous that the first detection turned out to be gold plated and unambiguous, otherwise we would have probably published a bunch of "we saw something interesting but can't claim it as a GW discovery" papers on the next few weaker events before we would have felt comfortable making a confident claim.
    - airstrike 5y ago
      Thank you for all these replies and for the fantastic work you and the team do. This is now my favorite AMA
  - jasonwatkinspdx 5y ago
    Not quite the same lengths, but AIRC LHC uses two detectors with substantial design differences, and partitions the teams from sharing information for some of what they target, as a way to reduce systemic errors.
- 5y ago

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