Speed Of Light Broken With Off Shelf Components

jukal writes "An interesting article at NewScientist.com: " Now physicists at Middle Tennessee State University have broken that speed limit over distances of nearly 120 metres, using off-the-shelf equipment costing just $500.", " it may be possible to use this reflection technique to boost electrical signal speeds in computers and telecommunications grids by more than 50 per cent. Electrons usually travel at about two-thirds of light speed in wires, slowed down as they bump into atoms. Hache says it may be possible to send usable electrical signals to near light speed. ""

Comments

Links & a question

[alienmole]

Of course, we're going to have the usual back and forth about how this isn't really breaking the speed of light, it's just the group velocity, etc. For those unfamiliar with the issue, the following links might help:

http://scienceworld.wolfram.com/physics/Superlumin al.html
http://www.weburbia.com/physics/FTL.html
http://physicsweb.org/article/world/13/9/3

The thing that really seems interesting about this is that they're doing this with cheap equipment, which will make experimenting with this a lot easier.

Can anyone explain how this would be used to increase subluminal transmission of electrical signals, as mentioned in the article? This whole group velocity thing has always seemed like a bit of an illusion to me, and none of the explanations I've seen has really clarified how it's anything more than that.

Phase vs. Group velocity

[Mendenhall]

Here comes this problem again. The article explains it, but buries it at the bottom.

What the group has attained is a transmission line with a phase velocity greater than the speed of light. This is actually not too hard to do with a resonant line (which they have), but they have constructed a cute, cheap way to demonstrate it. The group velocity, which is the speed at which information moves, is still less than c, and they explicitly say so.

The best use for a setup like this is to bring a good demonstration of the difference between the two to an undergraduate laboratory setting, to hammer into students forever the importance of the difference.

Re:Phase vs. Group velocity

[m.dillon]

Right, but it's slightly more complicated then that. What is really going on is that the nearly resonent wave is interfering with the signal wave, canceling out the stretching effect you get. That is, different frequency components of any EM wave will travel at different velocities with the fastest component going near the speed of light. So the information packet stretches as it goes down the wire. Conventional electronics cannot predict the entire wave from just the fastest component but the universe can as a quantum mechanical effect. You can't pass information without multiple frequency components (even just changing the phase will temporarily create additional frequency components, which stretch). In anycase, since conservation of energy is required by the universe (at least so far), the canceling out of the slower components of the wave causes the energy associated with those components to accumulate in the faster components of the wave. These faster components happen to be moving at near the speed of light so, overall, you wind up with a non-attenuated (or less attenuated) signal at the far end whos entire contents reaches the far end at near the speed of light.

Now the complication: you cannot simply create a resonent wave to cancel out the slower components at point X because you do not know what those slower components are at point X (they haven't arrived yet). But since the signal itself knows (quantum mechanically speaking), you can use reflections of the signal itself, at near resonence, to cancel out portions of itself which have not yet arrived. Confused yet? The result is that the cancelation gives the whole signal 'a push'. This cancelation effect appears to move faster then the speed of light because it is canceling a wave that has not yet arrived. This is the phase velocity they are talking about I think. but it is only using information that has traveled at the speed of light (quantum mechanically speaking the universe only needs the leading edge of the attenuated signal to know the whole signal), so there is no way this technology could be used to actually achieve FTL data transfer.

This is for real, a number of universities have been working on it for years. How useful it winds up being in the end is a matter of opinion, though.

-Matt

Selling a bridge?

[andika]

Today I found this 'selling a bridge' twice, and I can't understand what it means. Is it an idiom?

I use dictionary.com as my main online dictionary, but up to now, I haven't found a good idiom reference online. Any suggestions?

Re:Selling a bridge?

[badboy_tw2002]

Common saying in US: You could con someone that gullible by selling them the Brooklyn Bridge. He's saying if you believe the story, you'd believe anything.

Re:Selling a bridge?

[MedManDC]

Here's a good idiom resource: Wayne Magnuson: English Idioms. Unfortunately, the bridge selling idiom is not there.

Basically, it means that if you believe that story, you'll believe anything, as in "come to me because I have a bridge (sometimes the Brooklyn Bridge) I want to sell you."

A guy in Arizona bought the London bridge....

[Desmoden]

many years ago even though it was falling apart (which is why the brits were selling it).

Re:A guy in Arizona bought the London bridge....

[perlyking]

Not quite.. the thing is not that it was falling down but that the guy thought he was buying Tower Bridge.

Who are you scolding?

[Catskul]

I hope you arent scolding the /. editors for this, because if you look at the article it has an almost identical headline.

Speed of light broken with basic lab kit

group velocities can exceed c

[alienmole]

Can't argue about New Scientist - it seems to have lost all credibility, perhaps since it began publishing on the web, I'm not sure. Luckily, we have Slashdot to correct it! ;o))

Regarding phase velocity vs. group velocity, both phase velocity and group velocity can exceed c - see Superluminal, second paragraph. Group velocities exceeding c have been done for decades - for a bit of a history, see No thing goes faster than light.

The innovation in this case seems to be that it's doable with cheap equipment, and over fairly long distances.

Re:This article is so bad it's not funny.

[teece]

The article is interesting, but really only to physcis students with a no budget for interesting experiments.

As for that "electrons usually travel at two thirds the speed of light" nonsense, who is the editor?

I have calculated the drift speed of electrons myself (you could too, it isn't hard). It depends on a couple factors, but the normal US 120V circuit humming along at maximum capacity (15 A) has an electron drift speed along the wire *orders of magnitude* lower that 2/3*c. I don't remember the exact number, but it was something likt 6 CM per hour! Eg, a snail moves faster.

The e/m field propation is at the speed of light, not the electron motion. Perhaps he didn't meant drift speed. Individual electrons can and do move much faster, but their paths are quite random, in all directions. The aggregate speed comes out very low.

Tim

Re:Could it be used for AM communications?

[bmwm3nut]

no, you still get into problems with the frequencies traveling at different speeds (dispersion). think of an AM wave, you have a set carrier frequency and then you modulate it's amplitude to convey the information. you can take a fourier transform of the wave to see the component frequencies. if you do this, you'll see a large peak at the carrier frequency, but there will be other smaller side peaks (side bands) in there too. if you only had one frequency present, all you'd get would be a sine wave which carries no information. you need to constructivley and destructively add waves of different frequency to carry information. once you have more than one frequency, you get into problems with phase velocity and group velocity, and no matter how hard you try, the information will not travel faster than the speed of light.

Re:186,000 miles per second

[darthpenguin]

One second is the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.

Re:186,000 miles per second

[orac2]

One second is defined as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."

The meter is then defined in terms of this. There really are very few basic, basic units, and the kilogram is currently the only one which still relies on an actual physical prototype, and NIST are currently working on a 'electric' kilogram.

Even if it was possible.

[mmol_6453]

Even if it is/were possible (has anyone actually gone to the trouble to email the scientist who supposedly did the experiments?), there would be some severe expected problems.

They're talking about interfering waves. That means pulsating DC, if not straight AC. Get this up to a frequency to even be useful (ala GHz to compete with CPU or networking technology), and suddenly you're broadcasting your signal. (Though coax's construction does cause some muting of this, IIRC) And putting it on silicon is a thing for Intel to do.

And just for proof that it's not possible: "superposition."

It says that waves will pass through each other and come out the same on the other side. Easiest to see in a ripple tank, or maybe in a physlet.

Doh!!!

[HardCase]

Whoops, let me correct this...where I said group velocity, insert phase velocity.

The group velocity is the speed at which the information travels. Obviously that's the thing that we'd dearly love to increase.

-h-

Re:a better analogy

[Fortuna Wolf]

Except that this analogy is wrong.
In some cases electrical signals work like that, but don't travel instantaneously.
No object is totally rigid, its forbidden somewhere in the laws of physics. The balls will compress slightly and then a wave either in the movement of the balls or their getting compressesed and then expanding. Its akin to taking a stiff object and swinging it, if you swing it fast enough and its long enough, the end won't break the speed of light because its not completely rigid.

Re:a better analogy

[kalidasa]

wait, say I have a string 1AU long, and I swing it with a peroid of 6 seconds, why would the end not be going faster than light?

Figure out the mass of it . . . it will take a hell of a lot of energy to whip a string 1 AU long. Eventually you'll start running into relativistic effects at both ends of the string; dilation of both time and length, massive increases of the string's mass (remember, when an object gets up to relativistic speeds its mass dilates upward, and more force is required to accelerate it at the same G; the mass of the tip of the string will approach infinity as its velocity approaches c).

Cherenkov Radiation anyone?

[Orgasmatron]

The speed of light is broken all the time. It causes Cherenkov Radiation...

http://rd11.web.cern.ch/RD11/rkb/PH14pp/node26.htm l

And yes, I know people usually mean the speed of light in a vacuum