Slashdot Mirror
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.
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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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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"Convictions are more dangerous enemies of truth than lies."
There was a tiny mistake in dialect translation during the production of this story, they aren't working on any high-level theoretical physics down in Louisiana, they are working on something much more down to earth:
Backhoes.
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We cannot reason ourselves out of our basic irrationality. All we can do is learn the art of being irrational in a reasonable way.
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.
A few months ago, I can't remember where I read it (maybe at /.), a gravitational experiment was set up to measure the effect that an eclipse had on large pendulums like the one in the Smithsonian. Cameras were placed to detect small variations in the pendulum's drift. As I recall, the very early preliminary data suggested that pendulums within the eclipse zone had variations, whereas those outside the zone did not. Now that would be something if true ! Has anyone heard anything more about this ?
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.
Imagine if two probes (say Deep Space 8 and, er, 8.5) were equipped with lasers, so that they could shot at each other once they were outside major planetary influences. It's the only satisfactory solution (imho), because when you're on a planet there's no way you can say for sure that you've accounted for every microscopic movement. Granted, you've still got planet x out there somewhere, but what the heck. If the experiment was screwed up by the discovery of a new planet (or brown dwarf) I wouldn't complain too much.
I believe you can filter this kind of noise by having multiple detectors.
Several detectors 100s of miles apart will get the same signal from a distant black hole, but will get very different signals from local perturbations.
-Yarn - Rio Karma: Excellent
It is described in terms of warped space, but the waves this article are talking about are similar to a wave in the ocean (a transverse wave) only the ocean is space/time
some karma... and kinda lukewarm about it.
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. :-)
Sure does.
But a little bit of theoretical work with a binary system shows that energy can be lost from the system in the form of ripples in Space/Time - our good old fashioned gravity waves.
By analogy with electromagnetism it was then supposed that these waves take the form of discrete packets of energy - gravitons.
So if we presume that dear old gravity and quantum mech can be married at some level, its reasonable to assume that gravitons behave both like waves and particles (I recommend books by Richard Feynman if you want to get a good feel for this - and avoid Hawking like the plague, he can't write and he gets it wrong. Its amazing what a bit of media manipulation can do).
Richard Feynman - the greatest Physicist of the 20th Century.
http://news2.thls.bbc.co.uk/hi/english/sci/tech/sp ecials/total_eclipse/newsid_415000/41527 3.stm
Is what you are referring to I think.
V. interesting stuff !
Life is just a bowl of All Bran - Small Faces
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.
This theory states that no matter how hard it tries, the cat will never be able to catch a moving laser.
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]
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
See number 1, substituting 'stupid person' for 'cat'.
This theory states that, whoa, dude, when you shine the laser up your nose, it glows like Rudolph, man! *puff*[2]
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.
I once had a degree in Physics and some knowledge of the subject. Now all I've got is the degree. Any better informed physicists want to take a shot at this?
Einstein described gravity in terms of warped space/time, and the motion of particles through it as the shortest possible path through a curved space. His numbers worked (confirmed as early as the 1920's if I recall) so folks mostly bought it. Einstein hoped to express all forces as some form of curve in space, but that didn't work out for him - other forces are selective in their effects, while gravity happens to everybody.
Now, these days we tend to view things in terms of particles. The reasons are less experimental than logical. We're not talking about tiny billiard balls hurling through space, quantum mechanical particles are a little too weird for that.
Take the following well established notions:
1- Energy does not exist apart from a mass (or alternately that mass and energy are the same thing - the two statements are pretty much the same.)
2- Vacuum has no mass - it isn't a medium that can carry energy.
3- Gravity, like the other three forces, transports energy.
Given these three, we pretty much must conclude that gravity is transported by particles. It might be a particle which, like the photon, has zero rest mass, but if energy is moving through empty space, a particle, by definition, must be carrying it.
Of course, saying so doesn't answer any questions at all. Why should the action of gravity particles distort space/time? If you have a good answer and experimental data to back it up, the Nobel committee has a sizeable cash prize waiting for you.
The structure of space-time itself. It's not the ether, since electromagnetism propagation is different.
...phil
...phil
"For a list of the ways which technology has failed to improve our quality of life, press 3."
I did neglect to consider things like quantum interference, which in principle would make a difference. It would be hard to measure though unless the experiment was designed to look for it.
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!
Yes, I'm still a junky. Are you still a bitch?
A gravity wave refers to a "packet" of basically periodic warping of space. The warping of space affects the "distance" which the photons must travel before reaching the detectors. Thus, the phase of the photons will be different if there is a significant warping of space time along the path of the photons. The experiment will measure relative phase shifts of the photons to detect the gravity waves.
http://ligo.caltech.edu
Caltech's Press Release
I don't know why their writing articles on this now. LIGO has been in the works for quite a while now.
Just to give you an idea of how sensitive these things are: my roommate spent the summer working on calculating shifts in the earth's crust caused by the sun and moon, so that the correction factors can be applied. If I remember correctly, at one point Kip Thorne (the Feynman Professor of Physics here at Caltech) was working on correcting for the change in Earth's momentum caused by raindrops hitting the surface.
-ElJefe
But that only deal with Einstein's theories about time/space vs. speed.
The prediction of gravitywaves has, as stated in an earlier post, been proven by measuring the energyloss caused by gravitywave radiation in binary star systems.
But noone has yet detected *an actual gravitywave*!
Only by second hand observation.
And, of course, a new kind of instrument to look into space with is never wrong.
Look at what the telescope, then the radio-telescope and later x-ray/gamma ray/infra-wave detectors have meant for astronomy and our understanding of the univers!
The introduction of gravitywave observatories will most probably lead to so far unknown discoveries.
/.Mattsson - My native language is not English, so please don't whine over linguistic errors. (That's lame anyway...)
The whole reason why we talk about forces as being particles is that particles can move through a vacuum. Electromagnetism works across a vacuum because the force is carried by a particle: the photon. Radio waves do not need a medium to travel in because they are composed of photons.
"Gravitons" are what gravity particles are called. In principle, it works the same as radio, except with a different particle. Of course, there are important (and unexplained) differences between radio and gravity - that's what keeps physicists employed.
All the way to my quantum mechanics final exam.
:^)
I can't even remember where to begin to calculate the photon density in the field. If I did, I might be able to work that half out (although I'd probably get the wrong answer - seem to recall not doing that well on my quantum final either.)
The interference from a field that's parallel to the beam sure as hell won't be much. Of course at the edges it won't be parallel... I think I'll need some extra paper.
Thank God I quit physics.
Alcubierre's homepage (http://www.astro.cf.ac.u k/pub/Miguel.Alcubierre/index.html) has a broken link to the paper itself. I can't find another copy.
The New Scientist has an article about it here.
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)
Y'all should know, though, that the next generations of LIGO are already "in the slot", and they promise to increase the event rates quite a bit. The whole trick is to isolate the mirrors as much as possible from disturbances that aren't gravity waves. Fortunately, there are very clever people working on the problem as we speak.
BTW...perhaps the best isolation would be to shoot the whole Michelson interferometer mess up into orbit. I'm pretty sure that some guys at NASA are working on this too. It's worthwhile to check out the LIGO page. Set