Testing the Theory of Relativity

HD 456 wrote to us with an MSNBC story about the new observatories that are starting to come online for the purpose of gravity wave research. One is located outside New Orleans, the other in Washington State, the facilities will shoot lasers down 2.5 mile tunnels in an attempt to detect changes in gravity caused by black holes. Redundancy in facilities is being used to avoid having false background noise skew results. In addition to garnering more information on black holes, the scientists also hope to collect data concering the origin of matter.

Comments

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[Signal 11]

Contary to the belief of Fred Moody (as pointed out by another reader) - merely observing a black hole will not create one. Only stupid people generate black holes. Fortunately those are kept between their two ears, and they are buried with them. Relax, Fred. Just don't go near stupid people - they have a tendancy to suck.

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Universal observations from inside a tunnel

[osu-neko]

I just love how we're observing things like black holes light-years away by looking at things inside dark tunnels. Kind of reminds of observing solar neutrinoes by looking at things in a dark cave.

Of course, since everything in the universe interacts with everything else gravitationally, I ought to be able to deduce the superstructure of the universe and the motion of anything and everything in it by observing how my tea leaves settle on the bottom of my cup. With sensitive enough instruments, of course. My grandmother, for instance... :-)

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Gravitational Waves Exist!

[Thorsett]

Despite the suggestion to the contrary in the MSNBC story, gravitational waves have already been shown to exist. Joe Taylor and Joel Weisberg and their collaborators have demonstrated, using a binary star system, that Einstein's prediction of the rate at which energy is radiated away by accelerating masses is correct to better than one half of one percent. (This was the subject of the 1993 Nobel Prize in Physics, awarded to Taylor and Hulse.) These results also prove, for example, that gravitational waves travel at the same velocity as electromagnetic waves (ie, the speed of light, or 300,000 km/s).

LIGO is an exciting project that may open a whole new field of gravitational wave astronomy and directly probe the properties of such exotic objects as black holes and neutron stars. But it will do it using well-established physical principles.

Relativity FAQ

[Duke of URL]

Here's a usenet relativity FAQ if your interested in some info.
Here's some info on black holes too, while I'm at it.

How it works

[cybercuzco]

This is basically a Michalson interferometer on a monster scale. The Michalson interferometer splits up a light source, in this case, a laser, into two beams, sends them down tow tunnels at right angles to each other and then recombines them and projects the result against a screen or detector. This thing is EXTREMELY sensitive to changes in the path lengths. A change in the path length of half a wavelength of the light used is enough to cause an interference pattern at the dtector. The longer the paths used, the more sensitive it is to disturbances. This thing can detect the gravitational pull of a human walking by the end of it. Needless to say they get lots of interference, like the moon, cars, trucks, squirrels etc. Which is why they need a whole bunch of them spread out. Presumably a gravity wave will hit all the detectors at the same time, since gravity moves as fast as it wants to and doesnt have to be held back by the speed of light rule. Anything that doesnt occur at both detectors at the same time is thrown out. Pretty neat really, Einstein was right about everything else, i feel confident he'll be found right once again. Damn Sylvan fissure ::pounds skull::

Re:How it works

[kwijebo]

You're right that LIGO is basically an enormous Michalson interferometer, but in order to increase its apparent length it measures the superposition of a laser beam that has bounced back and forth across its length many times. As you say, the longer the path used, the more sensitivity to displacements in length it has.

But it is NOT true that LIGO can detect the gravitational pull of a human walking by the end of it. A litte background: gravitational waves are created by changes in the gravitational quadrupole moment of a system. Electromagnetic waves, in contrast, are created by changes in the electrical dipole moment of a system.

Getting changes in the quadrupole moment of a system requires doing something funky with the angular momentum of the system. That can happen when black holes collide (they radiate a bunch of their angular momentum away), and I don't know when else it might happen. But it definitely DOESN'T happen (on any reasonable scale) in any terrestrial processes, or any that we know of in our solar system.

It is true that a passing truck, or even a passing person, can cause mechanical vibrations that will affect LIGO's measurements. LIGO is an incredibly accurate system for measuring differences in distances between its perpendicular lengths. Gravitational waves should, according to GR, cause a change of length in one axis and not the other. Mechanical vibrations will also cause a difference in position for the mirrors, so LIGO has a VERY complicated system of mechanical dampers to minimize this effect.

Finally, the last time I checked, most GR experts were of the opinion that gravity waves propagate at the speed of light, not at infinite speed.

annoying-ass article

[mattorb]

I think this article is largely missing the point. GR has been amply proven, at least to the extent that LIGO isn't going to add much to the proofs -- gravity waves exist (see earlier post about a Nobel given out for this work).

A large part of what makes LIGO interesting -- at least to those who believe it will work (see below) -- is the prospect of eventually being able to do some real astronomy with the thing; that is, the idea that particular astrophysical phenomena would send out unique and detectable gravitational wave signatures.

Kip Thorne, one of the world's bad-asses on this subject, I think talks about LIGO in his book from a few years back, Black Holes and Time Warps. Highly recommended if you're interested in this sort of thing.

Finally, it's worth pointing out (as the article did not) that there are real questions about the odds of getting useful data out of the thing. Admittedly, I trust Thorne's opinion on this a lot more than most, but there's definitely a pretty narrow zone where a) we'll detect lots of gravitational waves with LIGO and b) we wouldn't have detected them already. (If I'm not mistaken, there have been small-scale versions of LIGO done already.) I wish I could point you to a link on this, but I can't think of anything useful.

Just my two cents. :-)

The distinction isn't relevant

[vlax]

Your magnet will have no effect. Photons have no charge, and are uninfluenced by the other photons in the magnetic field. Energy does not exist apart from mass and vacuum has no mass, so it's difficult to say that photons aren't matter. Of course, it's also semantically unimportant to distinguish between matter and energy at all.

Laser Principles

[Hrunting]

Other principles they will be testing:

1. The Cat Theory
   This theory states that no matter how hard it tries, the cat will never be able to catch a moving laser.
   
2. The Eyeball Theory
   This theory states that no matter how large the warning on the side of the laser, someone will inevitably see what happens when they shine it in their eye.[1]
   
3. The Policeman Theory
   Shine a laser through a donut and one can theoretically throw a policeman into a brain lock as they try to defend themselves from the obvious sniper while also try to obtain the donut
   
4. The Stupid People Theory
   See number 1, substituting 'stupid person' for 'cat'.
   
5. The Nasal Theory
   This theory states that, whoa, dude, when you shine the laser up your nose, it glows like Rudolph, man! *puff*[2]
   
6. The Austin Powers Theory
   This theory states that sharks with frickin' laser beams on their heads are more deadly than mutated sea-bass.[3]
   

And yes, I want one of these in pen form.

[1] They go blind
[2] I in no way condone the use of illegal drugs
[3] Being conducted in conjunction with the Darwin Society.

Re:looking for what??

[phil reed]

What, prey tell, is the medium which allows these waves to propergate?

The structure of space-time itself. It's not the ether, since electromagnetism propagation is different.


...phil

Gravity: a force or a wave or a particle

[laertes]

As far as I recall, general relativity defines gravity as the curvature of space-time. Gravity waves are the result of changes in the curvature of space time. Gravity waves are theorised have these properties:

Gravity waves will be accompanied by gravitons, a hypothetical particle that has zero rest mass and twice the spin of a photon.

Gravity waves and gravitons propagate outward at the speed of light.

Gravity waves compress mass in one direction perpendicular to the direction they travel, and expand it in a direction perpendicular to both the direction of compression and direction of travel.

Gravity waves are moving ripples in space-time.

Black holes coliding make big gravity waves.

Gravity waves pass through matter.

This experiment it is trying to get empirical evidence on all of the above claims. This has been a goal of some physicists since the theory was proposed in 1916. However, this goal has previously been beyond experimenters technological reach. It takes today's most sophisticated lasers and detectors to isolate a gravity wave from far away. Any local vibrations reaching either the lasers (like noise, or earthquakes) or the detectors will be easily confused with gravity waves. However, the mass of nearby objects does not interfere, just the vibrations they produce.

As for the design of the installation: it is in the shape of an L, because (as I mentioned before) gravity waves both compress and expand matter as they pass through it. On laser moves faster, and the other slower. This is different from a Michalson inferometer, which checks if normal gravity (that is: curvature of space-time) bends light. A Michalson inferometer isn't used to determine the nature of gravity waves.

Gravity affects all of the universe simultaneously (although it doesn't affect it much, it does affect it). Gravity waves are held back by the speed of light limit though. So, the two installations would get waves at different times, depending on the orientation of the earth to the event.

Of course, this is all conjecture, and that's why we US taxpayers get this installation. If this had already been proven, we wouldn't need these two new observatories.

PS: Check out the observatories homepage for more info!

Gravity Probe B

[QuantumET]

For people interested in tests of general relativity...

Gravity Probe B is a satellite that will be launched in a few years' time. It plans to check for one untested prediction of general relativity (the frame-dragging effect of massive spinning objects like the earth) by placing several hyperaccurate gyroscopes in orbit and measuring the change in the rotation axis of those gyroscopes from this effect.

It's been under works for 30 years now... here's the website for the project.

The whole system has to be incredibly accurate... I worked with this over the summer, and the technical details are scary (for example, the gyros used are the smoothest spheres ever made by man... if they were expanded to the size of the earth, the greatest height difference between valleys and peaks would be about 16 ft)