Umm...can anything travel faster than the speed of light?

Arjuna34 wonders;
What actually happened, though, is that the light emitted “early” wasn’t the same light that went in. The strange part, though, is how the emitted light knew to be emitted so quickly.

I wonder about that myself. Any answers?
Peace,
mangeorge

Well, I don’t think the train analogy is correct at all. Trains aren’t waves. Now, look at an ocean wave coming up against two close-together, side-by-side rocks. The ocean wave rises to a peak as it hits these rocks and continues on between them. The original wave dumps its contents into a narrower wave that, if one could neglect friction, would propagate much faster, on between the two rocks. If the space between the rocks be narrow enough, and maybe kelp covering the rocks could make them slick enough, the ongoing, much narrower but faster wave could peak at the far side of the rocks before the original wave peaks at the leading surface of the rocks (but has already dumped enough of its contents into the fast wave to get it to its peak). (If that all sounds fishy, well, hell, that’s the way oceans are. . .and I’m usually all wet after entering them.)

The Wash. Post article was better than the LA Times one, but the SF Chron, in abbreviating the Times article, messed it up even more:

Nature (full text):

 HTML:

http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v406/n6793/full/406277a0_fs.html

 PDF:

http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v406/n6793/full/406277a0_fs.html&content_filetype=pdf

Washington Post:

http://www.washingtonpost.com/wp-dyn/articles/A9431-2000Jul19.html

L. A. Times:

http://www.latimes.com/news/state/20000720/t000068166.html

S. F. Chronicle:

I read the experimenters’ letter to Nature, but I’m not really capable of digesting it properly.

Ray (Surf’s up.)

Let me recommend a fantastic book called “Hyperspace” written by a physicist named Michio Kaku. He traces the development of theoretical physics starting with the contributions of Georg Bernhard Riemann in 1854. It then follows an orderly progression through Einstein, Kaluza-Klein, Quantum Theory, and the various models created in an effort to achieve Grand Unification.

It really is great stuff for those of us who love this sort of thing :)!

I must echo the “Hmmm,” of Arjuna34 and mangeorge. The articles seem to be saying that the emitted waveform isn’t the same waveform per se that entered the chamber, but it is identical. So something had to be travelling faster than c to get the exiting wave to arrange itself like that, right? Also, if what was being transmitted so quickly was not the light wave itself but the shape of the lightwave, wouldn’t that be transmission of information? Be gentle with me; I’d like to understand, but lack the training to make complete sense of what I’ve read.

Oh, and in the last paragraph of the letter to Nature, it seems as though the authors say that this experiment may not show that light can travel faster than c at all, but that it might just expose flaws in the current method for measuring the speed of a light wave. If that’s the case, why are they quoted in the lay articles as saying that the light went faster than c?

Because, as Chronos mentioned, the mainstream media mangled the story, as they often do with science news. I haven’t read the Nature letter- I think I’ll have to read it before commenting any further about it :slight_smile:

[BEGIN RANT]
From my cynical point of view, the mainstream media would rather have an attention-getting headline like “Scientist beats light barrier” than “Scientist performs esoteric laser experiement, meaning unclear”, regardless of the truth, just to grab your attention. They’re primary mission isn’t to inform, but to sell advertising.
[END RANT]

Arjuna34

I thought that one of the alternative explanations for the apparent results of this experiment was that there seem to be some correlations between things in the universe–that something happening at point A can cause something at point B to happen simultaenously. We’re talking at the exact same time. AND the experiment might have been illustrating this. Not that the light traveled, but that it was in fact two lights, which happened at the same time.

I probably SLAUGHTERED that explanation, I hope one of you science types can fix it. This, with better clarity, is how Brian Greene answered an audience question about this.

here we go again…from CNN this time

http://www.cnn.com/2000/TECH/space/07/20/speed.of.light.ap/index.html

The problem with that explanation, Cranky, is that you’ve got to define “at the same time”… which means that you’ve got to either have the two events in the same place, as well, (not the case here), or you’ve got to set a reference frame. If you can find a reference frame where they’re the same time, then you can also find a reference frame in which either one is first.

(Forgive me, but I too am an ignorant but interested layman. No rough stuff please.)

Before I get to my question I want to throw another faster-than-c scenario (which was treated with an equal amount of eyeball-rolling in another thread) into the pot: the guy who was able to “filter out” slow-moving light and “transmit” the Star Spangled Banner (or some other tune) using only speedy (faster than c) light.

So, just from this one thread we’ve got cesium-boosted light pulses (the OP), a laser-point spot (Chronos), miniscule particles in the upper atmosphere (silent rob), electrons in nuclear reactor water (Arjuna), and my musical, patriotic speedylight.

Some or all of these examples may be able to go faster than c – but NONE of them can “transmit information,” according to physics mavens. So, it seems that what’s really important is not the speed of anything (clever catchall phrasing, eh?), but whether it can “transmit information.”

So here is what we laymen want to know: What EXACTLY is meant when physicists say “transmit information”? It’s obviously a very precisely defined term, so let us have it.

It’s not really that complicated. If you want to transmit information you must have a source of information and a target. Let’s take a laser pointer at some point P. Let’s have the spot move from point A to point B faster than light could travel from A to B. No information goes from A to B. You might think that the presence of a spot at B is information and you would be correct. But the source of the information is not point A, the source is point P. Someone at point A cannot use the spot to tell someone at point B anything, since A and B are both targets not sources.

Transmitting information is more general than traveling. That is, if a body can move from point A to B, the presence of the body can transmit information - the body could be a piece of paper with writing on it. But information can be transmitted without the movement of a physical body - say I click on SubmitReply. This transmits information without any physical object moving from me to you.

If we show that information cannot be transmitted faster than light, we have shown that nothing can travel faster than light. But if we only show nothing can travel faster than light, we leave open the question can I transmit a message.

[QUOTE]
*Originally posted by DrMatrix *
**

All of these tenuous explanations aside, the experiment cited was developed and conducted to determine how, when you swipe your Debit Card through a quantum sensor, the money can leave your bank account faster than the speed of light.

Here’s another way to explain “transmit information”. Say I flip a coin, and want to send the result (heads or tails) to my colleague. That piece of information is one “bit”, since there are two possibilities. Now, the question is, how long does it take to transmit that information?

All the faster-than-c setups so far may let you transmit “something” faster than c, but it’s not information (or mass).

A case-by-case analysis:

  1. Your reference to filtering out the “slow light” and keeping the “fast light”. I’m not familiar with this, but if the analogy to standard frequency-domain filtering holds, you’d have to wait for the slow light to arrive before you could separate it from the “fast” light.

  2. The cesium-boosted pulses can’t transmit info faster than light, even though the front of pulse come out before the back of the pulse entered, because to transmit a stream of bits, you’d have to wait for the ENTIRE pulse to travel, not just the front edge. This may not seem entirely obvious, but a rough analog is a rifle- sure, the bullet travels 1000 ft/s, but if you can only pull the trigger 10 times per second, you can only send 10 bits/second. In this case, the speed of light limits how often you can “pull the trigger”. This analogy is pretty flawed, but gives you an idea of the limitations with out going into wave packets and Fourier analysis :slight_smile:

  3. The laser spot can move on a wall faster than c with a suitable setup of angles, mirrors, etc., but in every case the thing moving the spot (a mirror moving back and forth at lightspeed, etc.) becomes the limitation, and you end up with slower-than-c information rate. For example, you’ve got a laser bouncing off a mirror onto a wall, and for every 1 inch of mirror movement back or forth, the laser dot moves a mile. If you move the mirror back and forth at lightspeed, the dot is moving faster. However, if you try to send information with the dot by patterns of movement, you’re still stuck moving the mirror, which is bound by c, so the dot doesn’t help things.

  4. My electrons in a nuclear reactor pool are traveling faster than light travels in the water, but not faster than light in a vacuum (c), so you couldn’t transmit anything faster than c with them.

  5. I’m not sure what silent rob was talking about.

Hope that helps! Let me know if I’ve bungled something up- I’m not a professional :slight_smile:

Arjuna34

[QUOTE]
*Originally posted by tcburnett *
**

Of course the credit-card industry has access to alien technology which lets them take your money at faster-than-light speed, but deposit money to you at the 34mph light speed discovered last year :slight_smile:

Arjuna34

First of all thank you Doc and Arj for your gracious, patient posts. (And, of course, thanks tc for your humorous asides.)

Please indulge me a little more, as I have a follow-up Q. (This Q was actually suggested by an earlier poster, but went unanswered.)

Why can’t the mere presence of the cesium-boosted light pulse (albeit just the front “end”) be information enough?

For example, say I arrange with someone on the moon (Moon Maid) that as soon as Madonna’s baby is born I will send a laser pulse their way; naturally, I will have pumped the atmosphere – all way to the moon’s orbit – with cesium vapor. By watching for my pulse, will Moon Maid not know that Madonna’s baby was born before she hears it, say, on the radio?

If your response is that despite our prearranged signal, Moon Maid can’t REALLY REALLY KNOW the kid was born (…because maybe I flipped the laser on accidentally, or someone sabatoged the experiment…) therefore no REAL information was transmitted, I would say:

Okay. But she certainly must REALLY REALLY KNOW that someone turned on the laser beam. Isn’t that fact alone, information???

Thank you for your continued patient replies.

I’ve found a quote from the Nature article that may help to explain, minus detailed information, how this experiment worked:

The last phrase, “the superluminal propagation of a pulse has been viewed as the result of the amplification of the pulse front edge and absorption of its tail,” shows how an apparent increase in the speed of light is observed.

In essence, no pulse of light can “turn on” instantaneously Instead, the intensity must gradually increase, plateau, and then gradually decrease with time, in a shape that when plotted looks much like a normal “bell” curve. The “location” of the wave is defined, as I understand it, as the location of the point of maximum intensity.

The researchers created a complex optical setup which, in essence, amplified the front of the wave and partially absorbed the rear. The point of maximum intensity, and thus the location of the wave, are shifted forward, and thus the wave appears to go faster.

No information is transmitted faster than light because of the simple fact that the “front” of the wave, which was generated at the absolute beginning of the light pulse, is not moved forward; only the “peak” of intensity is.

I can’t understand much of the mathematics, so I don’t know if this is totally right. Nonetheless, it seems to make sense to me, and explains how a pulse can seemingly travel faster than c without violating casualty.

JasonFin is correct (I think :slight_smile: ). Saying “the pulse exited the tunnel before it entered” is only true when you consider the pulse as entering when the peak enters. What happened was that the peak exited before the peak of the pulse entered, but NOT before the very beginning of the wave entered. In fact, the beginning of the wave packet entered long before the peak exited. The wave packet lasted 3.2 microseconds in time, and the peak came out 62 nanoseconds before it entered (1 microsecond = 1000 nanoseconds = 1 millionth of a second). But the peak started out in the center of the packet, which is 1600 nanoseconds after the beginning of the pulse.

Arjuna34

In QM you are not sure exactly where a particle like a photon is. The only thing you can say for sure is, that it is somewhere within the wave form. The peak is just where it would most likely be found if you went and looked for it. I would imagine that the peak could exit one side before the peak entered on the other side as long as the wave doesn’t exit before the it enters.

Speed Is Relative

The speed of light is not relative. Every other speed is relative. The speed of light is absolute and the same for all observers.

While we’re on the subject of c and various violations thereof, does anyone have any comments on the following article?
http://www.newscientist.com/news/news_224832.html

Seems a few theoretical physicists, wacky and fun-loving individuals that they are, are proposing theories in which the speed of light decreased during the early expansion of the universe.