Gravity Wave Detector Ready For Business

Arthur Embleton writes "The BBC has an article about a Gravity Wave Detector. There are two L shaped set-ups. One in Washington, the other in Louisiana. They've got a Laser pointing at a mirror 4km away, watching for the reflection and if it is distorted then it shows that there has been a gravitational pulse, possibly by two Black Holes colliding. The detectors are apparently so accurate they can measure to one-thousandth of the width of a proton! How did they test that it works?"

Comments

Really..

[NegativeK]

How did they test that it works?

I think that's the problem. These detectors should work in theory, but gravitation waves are so minute when they get to us that it's _really_ hard to be able to get a reading on them. My bet is the first to provide fairly solid evidence of gravitational waves gets a Nobel.

Re:Really..

[Anonymous Coward]

I betting that's why they build two. You get the same event at the same time on both then say, "What else could it be." I think the hardest problem they will have is that they also made the worlds most sensitve seismic detector. Luckily seismic waves travel much slower than gravity waves.

so many things about it

[lingqi]

1) here is an excellent presentation about how it works, etc. something about the sensitivity is on the order of measuring saturn moving toward the sun by the distance of the diameter of a proton.

2)here toward the bottom of the page you can LOG IN to their system and view all the logs. the password and login is blatantly displayed on the site. we should all email the site admin to have this changed.

3) I hope they figured it out for 300 million dollars, but wouldn't changes in gravity wave stretch / compress the tubes AND CAUSE REDSHIFT / BLUESHIFT in the lasers therefore cancelling out the effect?

Re:so many things about it

[lingqi]

no that's not right. the point of the two axis is so that since the gravity stretch / compress effects are vertical / horizontal, you will get different differences between the tubes, therefor costing the light a different amount of time to return. If the light is at constant frequency, this would cause phase shifts between the beams, something you can measure. (this is from their "how it works" website, by the way)

However, I am proposing that since there will be corresponding red/blue shifts in the two shafts, the lights will oscillate the same amount of cycles before merging again, therefore nullifying any potential phase difference, and hence eliminating the possibility of obtaining any result.

I'd think that building these tubes parallel (with a lot of distance between) would be better, because then you would really get phase-shifts from the finite speed of the gravity waves, a positive shift and then a negative shift between the two beams, as the wave affects them sequentially.

Of course, I am just armchair researching - like I said, sure hope they got this right for 300 million dollars.

Re:so many things about it

[Smidge204]

Placing the tubes at a right angle makes perfect sense... because you don't know which way the gravity wave will come from.

What you are saying makes sense only if there were some way a gravity wave could make one tube shorter and one tube longer the same amount at the same time, AND the same photon had to travel through both tubes.

However, it seems that blue/red shift issues are moot anyway, since the device works by basically measuring a relative shift in arrival time between two photons, not my measuring spectrum shift.
=Smidge=

I must correct myself

[lingqi]

for number one above, from here:

Observing this fantastically tiny effect is equivalent to detecting the motion of Saturn if it were to move closer to the sun by the diameter of a single hydrogen atom

so it's not the diameter of a proton but the diameter of a hydrogen atom. A lot better, but well, still pretty small.

"This week's finds" on LIGO

[mike_stay]

John Baez has some really good info about LIGO in several of his "This Week's Finds in Mathematical Physics" columns. week198 is the most recent to mention it. Baez is a great place to start if you like understanding connections between all kinds of different areas of math & physics (which, of course, includes everything else :)

Re:"This week's finds" on LIGO

[Anonymous Coward]

Baez is a great place to start if you like understanding connections between all kinds of different areas of math & physics (which, of course, includes everything else :)

And what a versatile person; folk music AND advanced maths..

Oh, wait.. you said JOHN Baez?
Oops.

Re:Doesn't matter

[Mt._Honkey]

Millions of dollars in federal grant money later, do you think anyone gives a rat's ass?
Don't bother us with details like "how do you verify that it's calibrated?". There's a board of directors who have pensions! Nice little retirement nesteggs!

I know that this is a troll, but I am going to respond anyway. I know/work with physicists at Fermilab, and I can vouch for how hard these people work to make sure that everything works perfectly and that data generated is valid. They have teams of people checking to make sure that every assumption made is correct (I'm involved in one such group). Physicists like those at Fermilab, or LIGO, or other such facilities are among the most dedicated, competent people you will ever meet. They aren't there for the money, they are there for the science. Even after they "retire", many continue to work for decades for free. Physicists are there to get shit done, and I'm sure that their retirement benefit packages are not their primary motives.

If that were the case, I doubt that they would have gone through 4 hard years of painful undergraduate courses, followed by even harder grad school, then working through a post-doc position... all to secure a good pension. People like that just go into business.

They're in it for the hunt, the dream, the achievement... the advancement.

Interference

[spin2cool]

But the weak nature of gravity means these disturbances are unimaginably small. . . One of the major tasks for engineers has been to insulate the installations from vibrations - from passing lorries and earthquakes - that might swamp the real data

As a physics student, I know of many who question the reliablility of such instruments, especially when they're on the surface of the Earth. The earth's crust is composed of constantly moving, shifting layers of rock that create almost constant imperceptible geologic disturbances. It's nearly impossible to completely negate these.

The scientists responsible for the experiments claim that the non-proximity of the two locations will negate any interference, but there is plenty of seismic data that shows that even the smallest tremors can be picked up by delicate equipment on the other side of the globe!

Re:Interference

[MindStalker]

Yes, but gravity waves travel at light speed, seismic waves travel, at a much smaller known speed (about twice the speed of sound is it??) As long as you keep perfect time sync between the stations its easy to measuse how fast the wave traveled and tell which it was.

More info

[crapulent]

Here are the slides [pdf] from the Oct 2002 NSF review. Lots of pictures, graphs, technical details, etc. for anyone interested. In a nutshell they are aiming to measure strain on the order of 10^-21 over the frequency range of 100Hz - 1kHz. Using two facilities separated by 3000km allows them to search for correlated events and weed out localized noise. IANAP.

More slides here.

LIGO home page.

HTH.

Re:Insulation from vibrations noise...

[Frans Faase]

The idea is to find a place where there are not many fibrations and to make the system rigid. Movements parallel to the detector axis are (theoretically) not noticed. The remaining vibrations will simply make the instrument temporarily blind. That is no problem as long as they do not occur continiously. It is possible to distinguis between vibrations and changes in lenght. Using multiple instruments all over the world also helps distinquishing between local vibration events and globally caused changes in length due to gravational waves.

Re:Insulation from vibrations noise...

[stevelinton]

I wonder how exactly they are doing this - what kind of technology can be used to hold two things 4km away at precisely (give or take a few thousandths of the width of a proton) the same relative position all the time.

They don't. They damp out a certain amount of vibration via clever mountings, etc.
Then they make sure that all the rest happens at very specific frequencies. You can think of a guitar string. When you jolt a guitar, the string will "sing" at its tuned note. I think the LIGO mirror supports are incredibly precisely tuned.

Now they only look for gravity waves at other frequencies, mainly ones away from where seismic noise mostly is.

Finally, they compare respoonses from two remote detectors and look for "matching" events separated by the speed of light, instead of the speed of seismic waves.

Showing that it works

[Twylite]

How did they test that it works?

As everyone is well aware, a gravaton pulse has a 78.2% probability of overloading the power conduits leading to microfractures in the dilithium chamber and a chain reaction that causes a rift in the space-time continuum.

Basically, Seven of Nine appeared briefly, bad mouthed someone about something they may do one day in an alternate future, recalibrated the sensors not to detect her, and never appeared in the first place.

Scientists analysing the situation need only to look for a slight seemingly-random deviation in the operational parameters and one operator who feels insulted for no particular reason, in order to prove this theory.

hmm...

[C21]

wouldn't this work better in deep space. Failing that, wouldn't tossing it up in low earth orbit be better? I can't imagine how theyre going to get past the incredible amount of vibrations, tweaks, tilts that the earth provides.