Comments on: Barely Excited

By: Serge

Serge — Wed, 05 Aug 2009 03:57:02 +0000

@Gray Gaffer It’s not changing the length of tubes or pendulums inside the tubes. The noticeable change of length would require something unimaginable, like black holes collision nearby. The pendulum just swing tiniest amount, so the length which laser beam should travel before reflection change.

By: Gray Gaffer

Gray Gaffer — Tue, 04 Aug 2009 21:37:42 +0000

gpher65: so it is just a G force on the tubes changing their length, without affecting the space-time through which the laser beam is traveling and therefore not the beam? Or not as much? I thought gravity was equivalent to space-time distortion?

By: gopher65

gopher65 — Tue, 04 Aug 2009 16:48:02 +0000

I obviously meant that the thing that they were detecting wasn’t EM radiation, unlike almost every other type of telescope we have:P.

Don’t be unnecessarily obstinate.

By: Lab Lemming

Lab Lemming — Tue, 04 Aug 2009 10:40:26 +0000

“And this one won’t even be reliant on EM. Woot!”
So what sort of laser beam are they using?

By: gopher65

gopher65 — Tue, 04 Aug 2009 05:43:02 +0000

Gray Gaffer: LIGO’s sensitivity during its last run was so low that only a truly unbelievably ginormous event would have triggered a positive result. Each time they upgrade it the distance from which they can detect those ginormous events increases, and their ability to detect weak(er) nearby events increases, increasing the likelihood of a detectable event.

So far, because of the low sensitivity, the calculated probability of a detectable event happening within detection range has been very, very low. They haven’t actually expected to find anything yet. They’re currently “just” trying to upgrade their experimental setup to the point where they have a decent chance of occasionally catching something big happening.

I put “just” in quotes because their current task is no mean feat, but people seem to dismiss it as worthless because they aren’t yet to the point where they are producing usable observations.

It’ll be really cool when they “see first light” though:). Then we’ll have an entirely new type of telescope to play with . And this one won’t even be reliant on EM. Woot!

By: Gray Gaffer

Gray Gaffer — Tue, 04 Aug 2009 01:26:45 +0000

It occurred to me that a Gravity Wave is essentially a flexure of space-time. If this is the case, would not the laser beam be equally affected? The ruler would change in the same way as the ruled, hence no change would be measured? They have been running long enough for cataclysmic waves to have passed us by, surely, yet no positive signal measured to date. I thought the length change they are seeking was a space-time distortion effect, not a pure space gravitational force compression of matter with no space-time geometry distortions. But the experimental setup seems to be assuming the latter is what will happen.

By: Jeff

Jeff — Sun, 02 Aug 2009 11:07:27 +0000

@gopher65:

I seem to remember recently discussing the paper I think you’re referring to and concluding that the arguments of the author would put the “resolution” of the universe significantly larger than the scale we’ve already probed down to, presumably ruling out such reasoning.

By: Serge

Serge — Sun, 02 Aug 2009 08:24:14 +0000

I’m wondering how long before somebody freeze the cat to ground state and solve Schrödinger box mystery once and for all.

By: Lab Lemming

Lab Lemming — Sun, 02 Aug 2009 03:52:29 +0000

How do they correct for earthquakes?

By: gopher65

gopher65 — Sun, 02 Aug 2009 00:55:40 +0000

Not exactly on topic, but I’ve read that some recent results from… somewhere (my mind is like a sieve ) are starting to suggest that we may indeed live in a holographic universe. And if this is true, the “resolution” of our universe might be too low for us to directly observe (reasonably weak) gravitational waves on a setup like LIGO or LISA. For some reason that bums me out:(.

Do you have any thoughts on whether or not that is a giant load of crap?

By: Chris W.

Chris W. — Sat, 01 Aug 2009 22:23:58 +0000

PS: While I gnash my teeth over an editing oversight in my previous comment, I'll add the abstract of Wisdom's article (from Science, 2003):

Cyclic changes in the shape of a quasi-rigid body on a curved manifold can lead to net translation and/or rotation of the body. The amount of translation depends on the intrinsic curvature of the manifold. Presuming spacetime is a curved manifold as portrayed by general relativity, translation in space can be accomplished simply by cyclic changes in the shape of a body, without any external forces.

..and the concluding paragraph of the article:

The curvature of spacetime is very slight, so the ability to swim in spacetime is unlikely to lead to new propulsion devices. For a meter-sized object performing meter-sized deformations at the surface of the Earth, the displacement is of order 10^-23 m (17). Nevertheless, the effect is interesting as a matter of principle. You cannot lift yourself by pulling on your bootstraps, but you can lift yourself by kicking your heels.

By: Chris W.

Chris W. — Sat, 01 Aug 2009 21:12:47 +0000

Speaking of centers of mass, see this article that appears in the August issue of Scientific American. (Although mostly off-topic, but it describes some fascinating and little-known manifestations of spacetime curvature, recently investigated by Jack Wisdom of MIT.)

By: James

James — Sat, 01 Aug 2009 18:03:39 +0000

Of course, as you well know, the pendulum is not a simple harmonic oscillator. However there will be non-physicists reading your postings, so I think it’s a good idea to be accurate so as not to misinform.

For small displacements the restoring force – proportional to the sine of the angle – approximates the linear force of a true harmonic oscillator. This is true of many systems for small displacements.

Every undergraduate physicist learns how to quantise the harmonic oscillator – few, if any, quantise the pendulum.