Theoretical Physics [OT thread]

[capnq]

The SF novel dmm is thinking of is Brain Wave, by Poul Anderson.


[This message has been edited by capnq (edited 01 March 2001).]

[jimbob55]

The information contained in a particle of any type includes its spin and mass. A particle can't travel faster than light. It can make light speed in the case of a photon, but travelling faster than light is against the rules.
Any subatomic particle is subject to uncertainty (it don't know where it is or it don't its vector). A wavefront of a particle would be smeared out in a probabilistic sense, so that the point at which the detector sees it is a critical probability threshhold. The uncertainty may allow the particle to appear to move faster than c, but it isn't. If you measured enough wavefronts and eliminated all that were slower than light speed you could come up with some evidence that your chosen wave was breaking the rules but on average everything stays below light speed.
This concludes my dogmatic and incomprehensible rant about stuff + things.

[dmm]

quote:

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Originally posted by Kimball:
...I don't know that I buy the "gravity waves" thing. I tend to believe the warped space theory of gravity that Einstein proposed.

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The gravity waves come straight from Einstein's theory. Think about it this way: suppose one member of a binary star system suddenly supernovas, taking out its partner. Locally, the warping of space changes "instantly", but far away no one knows yet that anything has happened. The light from the nova hasn't reached them. Everything looks normal. According to relativity, the warping of space far away will also remain unchanged at first. The change in the warping will propagate outward at the speed of light. That is what is meant by a gravity wave.

You might be thinking of gravitons, which at this point is more of a name than a serious theory. They are supposedly the particles that mediate the gravitational force, the quanta of gravity, analogous to photons. But no one has come up with a working theory of quantum gravity, so the jury is still out on whether or not gravitons actually exist.

[dmm]

quote:

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Originally posted by Dracus:
The Japanese discovered 2 years ago a particle that in it's natural state travels faster then the speed of light

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Maybe it travels faster than light in air? That's quite possible, and quite common. Neutrinos do that, and they constantly bombard us, and the Japanese have been working on some cool neutrino experiments for the Last few years. Maybe you're thinking of that? Particles are allowed to exceed the "local speed limit" which is always less than c. When they do, they give off forward-directed light known as Cerenkov radiation, which slows them down until they're below the local speed limit.

[a philistine]

quote:

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Originally posted by Dracus:
The Japanese discovered 2 years ago a particle that in it's natural state travels faster then the speed of light. Everything on the Earth is being bombard by this particle as it moves through space. I can't remember what they called it. I will try to locate the article. It has been a year since I read it.

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"Tachyon" is a name which has been given to hypothetical particles which always travel faster than light. They can never travel slower than the speed of light.

I believe that there has been no "proof" of the existence of tachyons, although there has been a reported sighting in the 70's or 80's by someone studying cosmic rays that has never been reproduced, and there are some theories that tachyons are necessary to explain certain observable (or theoretically observable) conditions, which indirectly support their existence, but, AFAIK, nothing conclusive has yet been shown.

If you can find the article, please post a link.

--A Philistine

[Derek]

Ok, couple of things:

Gravity waves: physicists are spending large amounts of money on gravity infereometers to detect the presence of gravity waves. No-one has done so yet, but there is hope. Gravity wave detection is extremely difficult (if they even exist), because it is very hard to account for all disturbances possible. However, it is known that gravity, like other forces, travels at c (speed of light in a vacuum)

Tachyons: a postulated particle, never detected, whose important property is that it never goes slower than c.

FTL beams: Recently (Last summer), a group of physicists posted in Nature that they were able to fire a laser beam (laser beams are made up of photons, which are both wavelike and particlelike depending on what you are looking for) through a caesium filled chamber with a distance on the order of centimeters. The front wave of the beam contained all the information of the entire wave, and somehow caused an identical beam (although with much less amplitude) to be emitted from the far side of the chamber before the beam had fully entered the near side of the chamber. Measuring everything, these physicists were able to show the group velocity was much, much greater than c. Group veloccity is a hard thing to understand; it is very related to actual velocity, but is not the same. Take a couple of courses in Quantum Mechanics (an interesting subject in itself) and you might have a handle on group velocity. Anyways, these physicists believe that they did not violate relativity for various reasons. My best advice is to read up on this experiment yourself, for it is quite a well done experiment, and the results are still being interpreted.

The Japanese did the same thing, but with a much different wavelength of EM wave, and only exceeded c by 17% They are also not sure of what happened.

Kimball: I love it when engineers talk physics. It adds humor to my day...


Derek

[a philistine]

quote:

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Originally posted by Derek:
Ok, couple of things:

Gravity waves: physicists are spending large amounts of money on gravity infereometers to detect the presence of gravity waves. No-one has done so yet, but there is hope. Gravity wave detection is extremely difficult (if they even exist), because it is very hard to account for all disturbances possible. However, it is known that gravity, like other forces, travels at c (speed of light in a vacuum)
{snip}Derek

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Wasn't there observational (non-experimental) proof of gravity waves from rotating pulsars? I think it resulted in 1993 Physics Nobel?

--A Philistine