About the LIGO Gravitational-Wave Rumor | Hacker News Reader

About the LIGO Gravitational-Wave Rumor

skyandtelescope.com

24 pointssubnaught9 comments

The bit I found most interesting:

"The current excitement could easily be a false alarm. Even if LIGO has a promising signal, it may be a false test signal planted as a drill. It's been done before, in 2010 near the end of LIGO's last pre-upgrade run. Three members of the LIGO team are empowered to move the mirrored blocks by just the right traces in just the right way. Only they know the truth, and the test protocol is that they not reveal a planted signal until the collaboration has finished analyzing it and is ready to publish a paper and hold a press conference. “Blind tests” like this are the gold standard in all branches of science."

Getting to the point of publishing a paper and holding a press conference seems like a lot of potentially wasted effort on a fire-drill type exercise. Presumably there are reasons why they wouldn't just let everyone know that it was a drill when they'd all decided on a conclusion? I realise that I'm assuming that the point of drawing that conclusion is much earlier than completing the paper as well, which may not be true.

Its probably poor phrasing in the article. The key is its event based (more or less) and they don't reveal until after all analysis is done.

So analysis complete we have X events at a SNR of 3 dB, Y events at a SNR of 2 dB, etc. Then and only then do the blind test dudes reveal "ah well all X events at a SNR of 3 dB were fake, and one of the Y events was fake, and you missed detecting one of the Y level events but the other 42 results were real."

We did this a lot in quantitative chemical analysis lab, it kind of spoils the point of the lab session if the TAs tell you the percentage iron in the sample before you start to test it, or while you're evaluating the test run results.

From the comments: "... how much noise exists in the measurement environment? I find it difficult to believe something even as mundane as a minuscule earthquake or similar ground noise wouldn’t cause orders of magnitude more of a signal and therefore swamp out any meaning information"

This is also the first thing that came to my mind. How do the filter noise? Or are we missing something that is not described in the article?

And secondly, why not putting ( teoretically) this lasers into space? Aside the solar radiation/wind, not much would interfere if put around the Lagrange points.

They want to do this in space: https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_S...

Funding is limited though, and doing it on the ground is easier to develop and maintain/troubleshoot.

As I understand it, this is one of the main reasons that there are two detectors thousands of miles apart, in Washington and Louisiana. There will be a whole lot of false alarms at each one separately, but it's extremely unlikely that precisely matching earthquakes will occur at both locations at once. (They have other very sophisticated ways of isolating the system from outside noise, too, of course. But I don't think the experiment would be viable without the two location filter.)

As for space, the LISA project has been trying to get funding for years. (The preliminary "LISA Pathfinder" mission launched on Dec. 3, so there's finally some real progress on this: http://sci.esa.int/lisa-pathfinder/)

> This is also the first thing that came to my mind. How do the filter noise? Or are we missing something that is not described in the article?

The mirrors are suspended from pendula: https://www.advancedligo.mit.edu/sus.html

my intuition says this is impossible to detect as if space time is bent then all the equipment and our reality is bent along with it and thus we will be none the wiser.... ie we might be inside a bent reality but we and all our sensing equipment will still sense everything as straight.

A very simple counter example is that we can "see" the bend in spacetime made by the sun from how the earth makes a circular path around it, as opposed to going in a straight line.

LIGO just looks at the path (or rather just the length of the path) of photons.

It's not bent the same in all directions. You can detect the difference in different directions, which is what LIGO does.

Your point, by the way, was hashed out in the early 1900s as part of the development of modern relativity. Physicists were well aware of the problem back then, and that's part of why it's so interesting and important that the speed of light turns out to be a constant.

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