Comments on: Catching the waves

By: Barely Excited | Cosmic Variance | Discover Magazine

Barely Excited | Cosmic Variance | Discover Magazine — Fri, 31 Jul 2009 16:10:23 +0000

[...] purpose of the LIGO experiment is to search for gravitational waves in the universe. They haven’t found any yet, [...]

By: Matt S.

Matt S. — Tue, 14 Jul 2009 13:05:35 +0000

There’s also a gravitational wave detector located in Germany, called GEO 600. The arm-length is shorter than LIGO or VIRGO, but they used some nifty tricks to get sensitivity up to almost the level of LIGO (well, a magnitude lower, but still not bad).

By: Gravitational Wave Astronomy Summer School « Brute Force Physics

Gravitational Wave Astronomy Summer School « Brute Force Physics — Fri, 10 Jul 2009 10:13:31 +0000

[...] gravitational waves will be made in the next few years. If professional physics conferences like the one Daniel at CV posted on the other day are anything like this, I see why there are so many of them. If you are an upper [...]

By: coolstar

coolstar — Wed, 08 Jul 2009 23:51:06 +0000

A sobering footnote to the discovery of the first binary pulsar is that Hulse was unable to find an academic post in physics and astronomy. He did apparently work for many years at the Princeton Plasma Physics lab as did Congresscritter Dr. Rush Holt. I’d bet hard currency that Swarthmore now boasts of turning down Dr. Holt (a good congressperson and a good person, period) for tenure.

By: Brian137

Brian137 — Wed, 08 Jul 2009 00:42:34 +0000

Apparently, Advanced LIGO will be so sensitive that quantum effects will become significant.
http://physicsworld.com/cws/article/news/33755

By: hanfordgradstudent

hanfordgradstudent — Tue, 07 Jul 2009 21:44:13 +0000

nota bene: The spike at ~340Hz is a collection of lines from the vibration of the wires which are used to suspend the optics like pendula. The reason that it looks like a single peak is because the frequencies of the vibrations from each optic’s wires are very similar and the resolution of this noise curve is too poor to differentiate them. What frequency those vibrations appear as noise is dependent on what material the wires are made of and how thick they are. The thinner the wire, the lower the frequency of that noise peak.

Human hearing is an incredibly useful tool that is regularly used in LIGO. Staff at the observatories often plug the output of the interferometer into an audio-mixer and listen on a pair of headphones. One does not need to pitch up the output since human hearing ranges from 20Hz to 20kHz, which covers the entire spectrum in Dan’s plot up at the top of the post. Audio is used to diagnose problems in detector subsystems and investigate short (non-gravitational wave) blips in the data called glitches. The gravitational wave signals we are looking for, such as coalescence of a binary star system will have frequencies in the audio band. There are few websites where you can listen to simulated signals. Just check out http://www.ligo.org.

By: Optickal

Optickal — Tue, 07 Jul 2009 13:19:37 +0000

The comments have made the essential point that not only do you need high sensitivity but also high isolation from noise to see these incredibly tiny distortions in spacetime. LIGO’s environmental channels, which monitor ambient noise, show that wave motion and oceanic storms a thousand miles distant can be picked up by LIGO’s sensors.

LIGO will indeed start a new observational run this month, with a sensitivity about twice that of the previous run (which was, by the way, at better than design sensitivity!). This run will last about 1-1/2 years, so an indisputable “event” just might be seen, but that observation would still be fortuitous. We’re all banking on Advanced LIGO, which will go online in 2014-2015 and will probe a volume of space 1000 times greater than LIGO could; but even then, it will take some time for the observatories to reach optimum sensitivity.

By: Arrow

Arrow — Tue, 07 Jul 2009 09:10:06 +0000

Daniel says “If the ball has always been moving along its current path, there is no new information to transmit. We already know where it will be at the next instant.”

Information is still transmitted – that the ball keeps moving in the same direction, information that there is no change is still information.

“If the ball suddenly accelerates (e.g., it stops moving, or changes direction, or speeds up), then a wave has to propagate to let us know (modulo near field effects).”

I think the point is that near field effects can also transmit information which is not in the form of waves.

By: nota bene

nota bene — Tue, 07 Jul 2009 08:01:43 +0000

So LIGO is essentially the most sensitive microphone ever invented. I never thought of it that way. Pretty cool. I wonder if some of the machine’s raw output could be pitched up into audible frequencies.

What’s the source of the spike in the 5th run between 300 and 400 Hz?

By: daniel

daniel — Tue, 07 Jul 2009 04:00:17 +0000

@TimG, hopefully the comments from Chris W. help. Indeed, the electromagnetism example can be quite instructive. An electron in constant velocity motion doesn't radiate, but an accelerated electron does. Purcell's undergraduate E&M textbook has a nice discussion of this exact point (with some very helpful pictures). In the same way, the ball rolling along the floor at constant velocity, although it does move the needle on the gravitometer, is not radiating gravitational waves. If the ball has always been moving along its current path, there is no new information to transmit. We already know where it will be at the next instant. If the ball suddenly accelerates (e.g., it stops moving, or changes direction, or speeds up), then a wave has to propagate to let us know (modulo near field effects). In the same manner as the electron. To see this explicitly requires GR; Eanna and Scott have a nice review which goes through all the gory details.