Quantum tunneling and information (NOVA episode)

[Ring]

Actually that should read

This link will answer your questions about (not) faster than light information transfer via tunneling .

http://www.socorro.demon.co.uk/gunter.htm

[g8rguy]

FYI, this was briefly discussed on the board here.

[Revtim]

*Originally posted by Ring *
**This link will answer your questions about faster than light information transfer via tunneling (not). I was going to excerpt from it, but I couldn’t figure out how to do it and keep it clear. So click on it or continue to wallow in hopeless ignorance.
http://www.socorro.demon.co.uk/gunter.htm **

Well, thanks for the link (although tought to read because of no paragraph breaks!), but to be honest, I still don’t understand the reasoning why the experiment didn’t send a signal FTL.

An exerpt:

To his now bemused audience, he played a tape in which among the background hiss strains of Mozart could be heard. This was the “signal” that had travelled faster than light. But that very word “signal” triggered heated discussion, with some participants claiming that the symphony could not be regarded as a signal. Among them was Chiao. “It’s not a signal in Einstein’s sense because of the timescales involved. I agree that when the music crosses the barrier it is shifted forward in time, compared with music that travels by a conventional path, but only by a very small amount. It is so small that you can predict what will happen to the music simply by looking at how the original audio waveform is changing. There is no threat to causality.”

What is it about a short timescale that prevents the music from being a “signal in the Einstein’s sense?”

Also, thanks for the link gr8rguy, I’ll check out that thread a little later (maybe it explains my above question), when I have more time.

[Revtim]

Ring, from the other thread:

Well the link is about anomalous dispersion which is different from quantum tunneling, but the effect is basically the same.

The difference is that a pulse traveling through an anomalous dispersive medium is rephased while a particle that tunnels changes its wave shape. But neither phenomenon involves faster than light signal transmission.

Are you saying that because the wave changes shape, it destroys the information? If so, then how do you explain the clearly audible music?

[Ring]

1.A pure monochromatic (unmodulated) wave cannot carry information.

2. The simplest form of a modulated wave is a wave packet.

3. A wave packet consists of wave components that constructively interfere (build up) inside the packet and destructively interfere (cancel) outside the packet.

4. A wave packet has width.

5. Information, historically, has been considered to be transmitted when the center of the packet (largest amplitude) reaches a detector.

6. However, the wave front, (which arrives before the center) actually has the all the same info as the center of the packet. It just has a smaller amplitude.

7. Tunneling causes the shape of the packet to shift forward.

8. If an experimenter considers information to have been sent at the moment of highest amplitude, and the position of the highest amplitude shifts forward then it will appear to have been transmitted faster than light. But nothing has actually moved ftl.

[Philster]

*Originally posted by Ring *
**…
8. If an experimenter considers information to have been sent at the moment of highest amplitude, and the position of the highest amplitude shifts forward then it will appear to have been transmitted faster than light. But nothing has actually moved ftl. **

The most intelligent thing to do is to approach the results knowing that nothing travels faster than the speed of light. While it might appear this way, nothing moved ftl.

Why do people tackle the most established theories?

>>>If you are going to get into quantum mechanics, and you “observe” something going faster than light, you probably should brush up on QM basics before you conclude that “x” travelled ftl.

Does anyone agree? I mean, it seems obvious to me.

[Philster]

Pardon…I was supporting SDMB poster Ring. Hope that was clear.

[Revtim]

Just to be clear here, I’m asking a question, not trying to debate that the info was sent FTL. I’m just having trouble understanding why this experiment looks like it sent info FTL, but actually didn’t. I’m not attacking any theories, Philster, I’m just trying to understand what happened here.

And even if I was attacking an established theory, that would be a good thing. If established theories were never questioned, we’d still be living in caves banging rocks together.

Anyway, thanks again for the info Ring. If I understand correctly, in essence the wave packet was still travelling at light speed, but the informational contents shifted forward. But because the information is technically also at the beginning of the wave packet, the shift forward wasn’t really a gain, since it was at the forward position already. And, when the comparison was made between the two streams (the unobstructed vs the tunneled), it was really comparing the unobstructed packet’s MIDDLE to the tunneled packets FRONT. Is this roughly correct?

[Philster]

*Originally posted by Revtim *
**
And even if I was attacking an established theory, that would be a good thing. If established theories were never questioned, we’d still be living in caves banging rocks together.

**

Agreed. I didn’t mean to imply that YOU were questioning a holy grail of theories. I’ve read about too many people concluding that something exceed ‘c’ when they should be worrying about their observations and test quality.

My bad.

[Revtim]

That’s OK Phil. What do you think of my last paragraph in my previous post, does that sound accurate to you?

[Ring]

And, when the comparison was made between the two streams (the unobstructed vs the tunneled), it was really comparing the unobstructed packet’s MIDDLE to the tunneled packets FRONT. Is this roughly correct?

Yes, that’s about it. But please understand I’ve been pretty roughshod in explaining this and I’ve just glossed over a lot of stuff.

If the wavefronts triggered the measurement then nothing would have even seemed to have exceeded lightspeed.

[SmackFu]

This episode is being re-run tonight, if anyone’s interested. Depending on your time zone, it might be starting in a few minutes.