Comments on: Einstein’s cosmic messengers http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/ Random samplings from a universe of ideas. Mon, 23 Nov 2009 14:59:24 -0600 http://wordpress.org/?v=2.8.4 hourly 1 By: some guy http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/comment-page-1/#comment-45182 some guy Fri, 31 Oct 2008 08:21:46 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/#comment-45182 This would be better set to the music of Stars of the Lid. This would be better set to the music of Stars of the Lid.

]]> By: Lawrence B. Crowell http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/comment-page-1/#comment-45183 Lawrence B. Crowell Thu, 30 Oct 2008 00:40:09 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/#comment-45183 That is a fair account. The Einstein field equations predict gravity waves with quadrupole and higher moments in a way analogous to how Maxwell's equations predict EM waves. Of course a theoretical "should" can never take the place of an experimental "does." The Hulse-Taylor measurement of pulsar orbits does illustrate that orbital energy is going away at a rate predicted by general relativity. This is a good signpost, but it does not clinch the case for gravity waves. If over time we can't detect gravity waves with ever more sensitive detectors then a whole lot of physics work done over decades is in danger of collapse. Lawrence B. Crowell That is a fair account. The Einstein field equations predict gravity waves with quadrupole and higher moments in a way analogous to how Maxwell’s equations predict EM waves. Of course a theoretical “should” can never take the place of an experimental “does.” The Hulse-Taylor measurement of pulsar orbits does illustrate that orbital energy is going away at a rate predicted by general relativity. This is a good signpost, but it does not clinch the case for gravity waves.

If over time we can’t detect gravity waves with ever more sensitive detectors then a whole lot of physics work done over decades is in danger of collapse.

Lawrence B. Crowell

]]> By: Matt http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/comment-page-1/#comment-45180 Matt Wed, 29 Oct 2008 23:23:05 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/#comment-45180 Horse, you're quite right in that so far no convincing evidence for a directly observed gravitational wave has yet been seen, but it's almost certainly just a sensitivity issue at the moment. I'd say (and I have to admit some bias in that my job is gravitational wave detection) that the existence of gravitational waves is well over a 99% certainty - their inferred presence has already won a Nobel Prize for Hulse and Taylor and GR has matched predictions with subsequent observations at every occasion. Data from the last LIGO science run was at it's design sensitivity, which gave it a reach of about 15 Mpc for binary neutron star mergers (one of the strongest expected type of signal), but the (rather uncertain) theoretical rate of these events suggest that at this range we'd expect only about 1 per 100 years. The detectors are currently being upgraded to about twice there previous sensitivity for another data taking run next year, which will increase their range by a factor of two, but the volume of space that they cover by about 8 times and give a corresponding increase in event rate (in fact the event rate gets larger due to the more homogeneous distribution of galaxies once you get past the rather sparse local 10 Mpc). By about 2014 we should have the next generation advanced detectors with 10 times better sensitivity than present, corresponding to a 1000 increase in observed volume at which time we should be seeing at least several events per year from binary neutron stars, plus hopefully many other local sources - if we don't see anything in the advanced detector era (or with the space-based detector LISA due to be launched a few years later) then some rethinks in our understanding might be in order, but not until then. Horse, you’re quite right in that so far no convincing evidence for a directly observed gravitational wave has yet been seen, but it’s almost certainly just a sensitivity issue at the moment. I’d say (and I have to admit some bias in that my job is gravitational wave detection) that the existence of gravitational waves is well over a 99% certainty – their inferred presence has already won a Nobel Prize for Hulse and Taylor and GR has matched predictions with subsequent observations at every occasion. Data from the last LIGO science run was at it’s design sensitivity, which gave it a reach of about 15 Mpc for binary neutron star mergers (one of the strongest expected type of signal), but the (rather uncertain) theoretical rate of these events suggest that at this range we’d expect only about 1 per 100 years. The detectors are currently being upgraded to about twice there previous sensitivity for another data taking run next year, which will increase their range by a factor of two, but the volume of space that they cover by about 8 times and give a corresponding increase in event rate (in fact the event rate gets larger due to the more homogeneous distribution of galaxies once you get past the rather sparse local 10 Mpc). By about 2014 we should have the next generation advanced detectors with 10 times better sensitivity than present, corresponding to a 1000 increase in observed volume at which time we should be seeing at least several events per year from binary neutron stars, plus hopefully many other local sources – if we don’t see anything in the advanced detector era (or with the space-based detector LISA due to be launched a few years later) then some rethinks in our understanding might be in order, but not until then.

]]> By: Horse http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/comment-page-1/#comment-45181 Horse Wed, 29 Oct 2008 22:44:07 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/10/29/einsteins-cosmic-messengers/#comment-45181 I'm interested in your thoughts about the significance of LIGO results to date: from what I can understand from a quick skim of the papers listed, no GW have been seen at all. Does this suggest merely a sensitivity issue, or some bigger gap in our understanding of GWs. I’m interested in your thoughts about the significance of LIGO results to date: from what I can understand from a quick skim of the papers listed, no GW have been seen at all. Does this suggest merely a sensitivity issue, or some bigger gap in our understanding of GWs.

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