Pushing Microwaves Faster Than Light

ContinuousPark writes: "According to this NY Times piece, Lijun Wang of the NEC Research Institute in Princeton has reported an experiment where "a pulse of light that enters a transparent chamber filled with specially prepared cesium gas is pushed to speeds of 300 times the normal speed of light". A second experiment by three scientists for the Italian National Research Council is reporting also superluminal speeds. And yet, this seems to be consistent with Einstein's theories. "

Comments

No login mirror

[Janthkin]

As usual, you replace the "www" with "partners" to get the no login required version of the article, found h ere.

More information

[spiralx]

Can be found here at Nature.

Whilst the difficulty in this experiment is in interpreting the results, one thing to remember is that the speed limit c for any information is a postulate of relativity, not something that has been proved. It appears to be true so far, but there is nothing to say that it always applies.

Huh?

[FascDot Killed My Pr]

Can someone with a degree in physics answer these questions:

"...under these peculiar circumstances, the main part of the pulse exits the far side of the chamber even before it enters at the near side."

At first I was going to flame NYT for such a stupid claim, but upon reading the rest of the article that appears to be what the scientists themselves are claiming. So my first question is: What is speed if not distance travelled divided by time elapsed? If the time elapsed is negative, how is the speed "300 times c"?

The second question is about the obvious "time travel" aspects. They say several times "you can't send info faster than c", but they don't indicate a reason. The closest they come to justifying this statement is (paraphrase) "there is a leading edge to the main pulse that arrives sooner".

So which is it? Did the pulse exit before it entered OR was there a "leading edge". You can't have it both ways. Either the signal travelled faster than light (in which case signaling superluminally is possible, by definite) OR it did NOT.
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Re:Huh?

[Sir Tristam]

So which is it? Did the pulse exit before it entered OR was there a "leading edge". You can't have it both ways.

Actually, you can have it both ways. When the pulse enters the chamber, the leading edge enters before the trailing edge. The time difference between when the leading edge exited and when it entered is the same as the time difference between when the trailing edge exited and when it entered. If you would have a fixed observer at the entrance, and another fixed observer at the exit, the fixed observer at the exit would see the pulse before the fixed observer at the entrance.

If you looked at the whole system, this is probably what the experiment would look like: A pulse is emitted from the exit of the chamber, and a mirror copy of that pulse appears to be emitted back up the chamber from the exit. While this pulse is travelling up the chamber, the microwave transmitter emits a pulse towards the entrance of the chamber. The transmitted pulse and the pulse travelling up the chamber meet at the entrance to the chamber, where they appear to destroy each other. Notice that the pulse travelling backwards up the chamber is a mirror image of the other two pulses (which are actually the same pulse.) If the leading edge of the pulse outside the chamber is to the left of the trailing edge, then the leading edge of the pulse is to the right of the trailing edge inside the chamber.

For those who are interested by this subject, Nick Herbert wrote Faster Than Light: Superluminal Loopholes in Physics in 1989; it still remains quite interesting and speculative.

Re:Less confusing, but little more info

[kinkie]

Short answer: evanescent waves can't carry information because thery convey no power.

Long answer (I'll try to keep it simple): when a beam of EMW (electro-magnetic waves) hits the interface between two materials, it gets partially reflected (and thus returns to the side where it came from), and partially refracted (that is, "goes forward"). Think of the effect you have when you look at a stick through the water's surface. How much of the incoming power is reflected and how much is refracted depends on the two materials involved.
We're mainly interested in the refraction part at this point, and particularly at the angle the EMW beam has with the interface surface. Under certain conditions, the propagation angle of the refracted ("forward") beam with the perpendicular to the interface's surface is bigger than the one of the incoming beam. If we increase the incoming beam's angle, so does the "forward" beam's, until it is completely parallel to the interface. At this angle ("critical angle") the physics of the whole setup change abruptly, and all the incoming power gets reflected. But some kind of EMF is still present in the "forward" part of the interface, generating some field patterns named "evanescent waves".
Those waves don't carry power "forward" (because it's all being reflected), but _can_ carry power along the interface.
This effect has also been studied for long-distance communications using the earth-to-air interface as a carrier.

Old News

[fred_the_slow]

The item is actually old news. For those of you who missed it originally, I will re-post here the text oif a previous, and more exciting. development:

Overclocker Creates Rift in Space-Time Continuum

Santa Cruz, CA - A rift in the space-time continuum was created today when overclocker Jamie Aperman ran a 750 MHz Coppermine Pentium III at 1.6 GHz. Overclocking has long been blamed for causing global warming, but this is the first occasion that the fabric of space-time has been damaged.

MIT Professor George Greznowski said, "It appears that the CPU was operating so fast that it began to execute instructions before they arrived. This execution of future instructions created a small tear in the fabric of space-time itself through which part of the motherboard passed into a parallel universe."

No one was injured in the accident, but a computer motherboard was partially damaged. Mr. Aperman better known as SpeedPhreeek said, "I'm pissed. I lost a brand new Alpha Cooler and Coppermine to a parallel universe. I called my insurance company and they don't cover losses to rifts in the space-time continuum."

Intel researchers have long warned of such damage to the space-time continuum, and added clock multiplier locks to their CPUs before they were required by Congress. A bill is now in the US Senate which would require a three day waiting period for purchasers of Alpha Cooling Fans and Peltier cooling devices. The bill would also require clock multiplier locks on all new processors.

Overclocking advocate Horace Spencer said, "This bill before Congress won't prevent overclocking. They'll just create a black market for non-locked processors. Most of the top overclockers already get their goods from Taiwan." (link no registration req. here: http://bbspot.com/News/2000/5/clock_rift.html)

ASCII art description

[Remus Shepherd]

I am a physicist, but not an optical physicist, so take the following with a grain of salt.

Wish I had a whiteboard. Let's try doing this with ASCII art.

Just before the pulse hits the chamber, things look like this:

/#####\-> |~~~~~~~~~~~~~~~|
Pulse Cesium chamber

Note that the pulse has a 'leading edge' -- a rise time before its maximum intensity. Once that leading edge hits the cesium, the cesium recreates the entire pulse on the other side:

/#####|\->~~~~~~~~~~~~~/|#####\->

So the pulse appears to have gone faster than light through the cesium. Another way to look at it is that the cesium, using nothing more than the leading edge of the pulse, spontaneously created a new pulse. Actually, it created two new pulses, as you can see after a little more time:

|~/#####\->~~~~~~~<-/#####\~| /#####\->

The two pulses within the chamber are moving towards each other, and they'll deconstructively interfere, cancelling each other out. (Actually, they cancel out as soon as the original pulse is completely in the chamber, but it's easier to draw this way.) Meanwhile the pulse outside the chamber is moving away from it and towards your measuring equipment.

So the pulse is not travelling backwards in time. The pulse isn't travelling far at all; it's being annihilated, really, but a copy of it is generated . It just happens to be generated some distance away.

Note that my drawings are flawed; the light pulse was probably longer than the cesium chamber. So the original pulse was already half-destroyed by the time the new pulse emerged from the other end. That would have been difficult to draw.

Why can't we use this to send information faster than light? Read the article again -- they're not really sure that you can't. One person is arguing that the information is packed into the leading edge of the pulse (sort of an optical gzip) and so you're compressing information but not sending it superluminally. Other people (Dr. Nimtz, third paragraph from the bottom) say that they really are sending information faster than light.

Personal opinion: This looks like some kind of wave phase propagation trick to me. We've always known that you can cause a phase shift in a beam of light to propagate superluminally, but the problem is that you can't encapsulate information in phase shifts adequately, due to (IIRC) the uncertainty principle. Not to say that this isn't an exciting experiment, but it doesn't appear to have a practical use. Now, the microwave experiment that travelled at 1.05 c excites me...I'd like to see if they can extend it to interstellar distances and through vacuum. :)

hmmm

[the_other_one]

If a processor worked faster than light it could generate output before the input was entered

This would be very embarasing when it spits out the answer let's say 42 and you forget what question you wanted to ask.

What's so special about c?

[lingsb]

OK, here's the reason why things can't go faster than the speed of light.

Relativity (Special Relativity at least) is based on 2 postulates:

1. The laws of physics are the same in any inertial (ie. non-accelerating) frame of reference. This means that the laws of physics are the same on earth (technicall only without gravity) and if your on a (hypothetical) spaceship traveling at half the speed of light a million light-years away from earth.
2. There is a speed, which when you 'transform' from one frame of reference to annother, remains the same. 'Tranforming' your frame of reference is simply looking at what the other person would see.

For example: you're on a train, and your friend is next to the track as the train goes past. This is quite a fast train - it's going at half the speed of light. You shine a torch ahead of the train: The light coming out of the torch is going at the speed of light (from your point of view - your 'frame of reference') Your friend standing by the side of the track also sees the light coming out of the torch. From her point of view, it's also going at the speed of light. It looks like we have a problem here: She sees the train moving at half the speed of light, and the light moving at the speed of light - the light is going at half the speed of light relative to the train. You on the train however see the light moving away from you at the speed of light.

Paradox? No. Einstein showed that it is actually our concept of space and time that is wrong. From your perspective on the train, everything else (including your friend) is actually squashed up in the direction of the motion of the train - parallel to the tracks. So the light has gone 'further'. Your friend sees the train squashed up (parallel to the tracks) and that time has slowed down on the train.

All this is effectively saying is that where the light is at a particular motion is not disputed by either you or your friend, so there is paradox.

So that's relativity. All you need is a speed which is the same in all reference frames. It doesn't have to be anything to do with light at all. There isn't anything which forbids 'faster than light' travel.

There is a consequence though: If something is travelling faster than light in one frame of reference, there will we another frame of reference where it appears to be travelling backwards - it comes out of the end of the chamber before it's entered the other side.

This causes problems with causality. Things happen before they are caused to happen.

BB.

PS. I'm studying physics at Oxford, England.