4 ms·

We predict that the speed of light and the speed of GWs is the same. One benefit of EM followup is that you can test this hypothesis (you should see roughly sim

We predict that the speed of light and the speed of GWs is the same. One benefit of EM followup is that you can test this hypothesis (you should see roughly simultaneous GW and EM radiation); if it isn't true, then our understanding of general relativity is flawed. Yes, if you have a long event, that smears out the time during which EM radiation might have been emitted. But for just observing the event, you expect a long-lived afterglow. It starts out hot and radiates gamma and x-rays, then cools through UV, visible, IR, etc. all the way to radio waves for weeks after the event. You can learn a lot about the event and the material that was ejected from it from a detailed analysis of this cooldown.

SomeStupidPoint 9y ago
On long flights, don't we expect the light to arrive after the gravitational waves because it interacts more strongly with the particles that both sets of waves traverse? Both kinds of waves are slowed when passing through media, but because EM is much stronger than gravity, the effect from interstellar dust should be much stronger on EM than on gravity. Even a 1-in-a-million slowdown of the EM versus gravity would mean that everything outside of ~100ly would arrive at least an hour later. Every intergalactic bit of light is passing through at least a few thousand lightyears of galactic dust, and potentially much more.
- stefco_ 9y ago
  That is a different effect. Interstellar dust is extremely diffuse; it is just vacuum with the occasional chunk of matter absorbing some fraction of light. It can then reemit blackbody radiation, but that is different than what happens in continuous media, like glass or water. In continuous media, Maxwell's equations still describe linear wave propagation but with reduced velocity.
  - tzs 9y ago
    What about pulse dispersion in the pulses from pulsars? (And even more so in fast radio bursts). Doesn't that require more than just absorption plus blackbody reemission? Not that it would make much difference. The delay due to dispersion goes down quadratically with increasing frequency. At the lowest frequency that Google tells me is used in radio astronomy, 13 MHz, the delay would be (if I've understood the dispersion equations correctly and done the math correctly) about a minute compared to visible light or gravity waves, and so not very useful. If you went down to 1 MHz, the delay would be a much more respectable 3 hours, but I'd expect that there would be way too much noise from terrestrial sources to do any useful radio astronomy anywhere down around there.
    - stefco_ 9y ago
      Yeah, but I believe that those effects are usually from near the source (which is part of why we can use details of the spectrum over time to infer source structure and composition). There might be cases I'm not aware of, though.
mirimir 9y ago
For BH-BH mergers, it seems that the result is a larger BH, less mass converted to GW. And I can't imagine that any of that BH mass gets ejected. Maybe my imagination is limited, however. Even so, there's likely mass in accretion disks, and it will likely get very hot in the bh-BH merger process. That's what you referred to, I guess.
redcalx 9y ago
Any idea what the duration of these GW events is, are we single blips talking microseconds or milliseconds, or a series of blips with distinctive patterns over several seconds/minutes? or something else?
- stefco_ 9y ago
  BH/BH mergers are in our sensitive frequency band for less than a second, and they get louder and higher-frequency until their peak volume, at which point they ring-down to silence. Neutron stars should spend longer in our sensitive band. There are also probably pulsars and (maybe) other signals out there that are nearly periodic and that we are hearing all the time! But those would be much much quieter (think of a burning flame vs. an explosion; the faster you release your energy, the more noticeable it is). Furthermore, those sources should be pretty close to a pure sine wave. That's hard to detect, because so much of our noise looks similar. We look at compact binary coalescences so closely in part because they are so distinct that we can be fully confident that our detector didn't produce them (we have a very talented and suitably paranoid approach to studying our background noise without prior assumptions of its form). You can't be so sure with these faint, continuous sinusoidal signals UNLESS you do a fully coherent search over long periods of time. Our detectors are not equally sensitive in all directions, and they can pick up direction as a phase-delay between the detectors in our network. As the earth rotates, both detected direction and strain amplitude from a a continuous source should vary. You can imagine doing what is basically a very, very long timescale fourier transform of the signal; the longer you integrate over, the more you filter out sine waves with frequencies nearby your chosen frequency. But this is a computationally expensive approach because longer integration times mean you need to test more frequencies as well as doing the longer integration. There are approaches to finding good compromises re: fully-coherent search windows, and I'm sure there's plenty of other clever voodoo that goes into the continuous waveform searches, but I don't know much more than that.
  - redcalx 9y ago
    Thanks very much. From what you're saying, I wonder then in a hypothetical scenario in which we were 100% confident that all local noise (all non-GW based noise) was eliminated, I imagine we'd be left with lots of actual GW based noise, from which a greater number of specific events could be picked out with the type of analysis you're talking about. I wonder then if there's work to understand how much GW noise there is, and work on lowering the local non-GW noise floor further, etc. I guess that just means ever larger detectors - and of course correlating results from detectors around the world. All very cool and interesting work anyways.

SomeStupidPoint 9y ago

On long flights, don't we expect the light to arrive after the gravitational waves because it interacts more strongly with the particles that both sets of waves traverse? Both kinds of waves are slowed when passing through media, but because EM is much stronger than gravity, the effect from interstellar dust should be much stronger on EM than on gravity. Even a 1-in-a-million slowdown of the EM versus gravity would mean that everything outside of ~100ly would arrive at least an hour later. Every intergalactic bit of light is passing through at least a few thousand lightyears of galactic dust, and potentially much more.

stefco_ 9y ago
That is a different effect. Interstellar dust is extremely diffuse; it is just vacuum with the occasional chunk of matter absorbing some fraction of light. It can then reemit blackbody radiation, but that is different than what happens in continuous media, like glass or water. In continuous media, Maxwell's equations still describe linear wave propagation but with reduced velocity.
- tzs 9y ago
  What about pulse dispersion in the pulses from pulsars? (And even more so in fast radio bursts). Doesn't that require more than just absorption plus blackbody reemission? Not that it would make much difference. The delay due to dispersion goes down quadratically with increasing frequency. At the lowest frequency that Google tells me is used in radio astronomy, 13 MHz, the delay would be (if I've understood the dispersion equations correctly and done the math correctly) about a minute compared to visible light or gravity waves, and so not very useful. If you went down to 1 MHz, the delay would be a much more respectable 3 hours, but I'd expect that there would be way too much noise from terrestrial sources to do any useful radio astronomy anywhere down around there.
  - stefco_ 9y ago
    Yeah, but I believe that those effects are usually from near the source (which is part of why we can use details of the spectrum over time to infer source structure and composition). There might be cases I'm not aware of, though.

mirimir 9y ago

For BH-BH mergers, it seems that the result is a larger BH, less mass converted to GW. And I can't imagine that any of that BH mass gets ejected. Maybe my imagination is limited, however. Even so, there's likely mass in accretion disks, and it will likely get very hot in the bh-BH merger process. That's what you referred to, I guess.

redcalx 9y ago

Any idea what the duration of these GW events is, are we single blips talking microseconds or milliseconds, or a series of blips with distinctive patterns over several seconds/minutes? or something else?

stefco_ 9y ago
BH/BH mergers are in our sensitive frequency band for less than a second, and they get louder and higher-frequency until their peak volume, at which point they ring-down to silence. Neutron stars should spend longer in our sensitive band. There are also probably pulsars and (maybe) other signals out there that are nearly periodic and that we are hearing all the time! But those would be much much quieter (think of a burning flame vs. an explosion; the faster you release your energy, the more noticeable it is). Furthermore, those sources should be pretty close to a pure sine wave. That's hard to detect, because so much of our noise looks similar. We look at compact binary coalescences so closely in part because they are so distinct that we can be fully confident that our detector didn't produce them (we have a very talented and suitably paranoid approach to studying our background noise without prior assumptions of its form). You can't be so sure with these faint, continuous sinusoidal signals UNLESS you do a fully coherent search over long periods of time. Our detectors are not equally sensitive in all directions, and they can pick up direction as a phase-delay between the detectors in our network. As the earth rotates, both detected direction and strain amplitude from a a continuous source should vary. You can imagine doing what is basically a very, very long timescale fourier transform of the signal; the longer you integrate over, the more you filter out sine waves with frequencies nearby your chosen frequency. But this is a computationally expensive approach because longer integration times mean you need to test more frequencies as well as doing the longer integration. There are approaches to finding good compromises re: fully-coherent search windows, and I'm sure there's plenty of other clever voodoo that goes into the continuous waveform searches, but I don't know much more than that.
- redcalx 9y ago
  Thanks very much. From what you're saying, I wonder then in a hypothetical scenario in which we were 100% confident that all local noise (all non-GW based noise) was eliminated, I imagine we'd be left with lots of actual GW based noise, from which a greater number of specific events could be picked out with the type of analysis you're talking about. I wonder then if there's work to understand how much GW noise there is, and work on lowering the local non-GW noise floor further, etc. I guess that just means ever larger detectors - and of course correlating results from detectors around the world. All very cool and interesting work anyways.