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h6294443
2004-Jan-15, 12:45 AM
I am still fairly new around here, but enjoy lurking.

I am in a discussion with a fella on another board about FTL issues.
He keeps telling me that Feynman allows for FTL travel of particles through QED.



As for the FTL discussion, c is a limit on the collapsed Schroedinger waveform, not on any individual Feynman diagram path within a quantum superposition.

Can someone help me out? Is it possible or not? I thought that any particle that has mass cannot equal or exceed C in velocity.

Also, he maintains that photons can be slowed down through something other than photon-electron interaction as described by Feynman in QED.
Any help here?


I have read the book. Controlling photon velocity has been an area of great advances in the past couple of years -- long after QED was written.

milli360
2004-Jan-15, 02:37 AM
There's that travel back in time--that would be much faster than light, right?

h6294443
2004-Jan-15, 03:04 AM
I am not so much worried about the time travel implications, as this probably is not applicable to macro objects.

I want to know what's to this....

Here's a link I found regarding this:

http://www.cerncourier.com/main/article/42/3/13

milli360
2004-Jan-15, 03:20 AM
I am not so much worried about the time travel implications, as this probably is not applicable to macro objects.
Time travel is faster than light.

Zamboni
2004-Jan-15, 04:08 AM
Time travel is faster than light.

I thought time travel is only 88 miles per hour?

:)

Hamlet
2004-Jan-15, 02:26 PM
Time travel is faster than light.

I thought time travel is only 88 miles per hour?

:)

Yes, but you need to apply 1.21 Gigawatts (pronounced Jigawatts) to the flux capacitor. :lol:

Cougar
2004-Jan-15, 03:09 PM
Is it possible or not? I thought that any particle that has mass cannot equal or exceed C in velocity.
Well.... I'm no expert in this area, but if I recall correctly, quantum tunneling is essentially finding particles where they're not supposed to be, and I believe this includes superluminal travel. If you intersect "superluminal" and "tunneling", you'll find hundreds of webpages to choose from. HERE (http://www.aei-potsdam.mpg.de/~mpoessel/Physik/FTL/tunnelingftl.html) is an interesting discussion on the matter by someone who is certainly more expert in the area than me....

wedgebert
2004-Jan-15, 03:31 PM
Well.... I'm no expert in this area, but if I recall correctly, quantum tunneling is essentially finding particles where they're not supposed to be, and I believe this includes superluminal travel. If you intersect "superluminal" and "tunneling", you'll find hundreds of webpages to choose from. HERE (http://www.aei-potsdam.mpg.de/~mpoessel/Physik/FTL/tunnelingftl.html) is an interesting discussion on the matter by someone who is certainly more expert in the area than me....

Well, I believe the actual limitation is that a particle with mass cannot be accelerated to or past c. If you could find a way to just suddenly be travelling faster than c, there wouldn't be a violation.

Anyways, tunneling isn't really superluminal travel. A particle is represented by a wave. The higher the section of the wave, the more likely the particle is to be at that point in any given moment.

However, the particle can go from anywhere in the wave to any other point in the wave whenever it wants. I guess a simple way to look at tunneling would be that the object the particle tunnels though occupies a portion of the particles wave, thus removing or limiting the chance of the particle being at that point. Thus there is a higher likelyhood of the particle being on the otherside.

The biggest limiation however, is that you cannot carry information at superluminal velocties. [/i]

swansont
2004-Jan-15, 03:37 PM
Is it possible or not? I thought that any particle that has mass cannot equal or exceed C in velocity.
Well.... I'm no expert in this area, but if I recall correctly, quantum tunneling is essentially finding particles where they're not supposed to be, and I believe this includes superluminal travel. If you intersect "superluminal" and "tunneling", you'll find hundreds of webpages to choose from. HERE (http://www.aei-potsdam.mpg.de/~mpoessel/Physik/FTL/tunnelingftl.html) is an interesting discussion on the matter by someone who is certainly more expert in the area than me....

It's been a while, but... I think the issue is that when you do a Feynman path integral, you integrate over all possible paths, which has to include some (many) that would require superluminal speeds. But these aren't observable - you can't actually measure the particle doing this.

As Patsy might say, "It's only a model." (Shhh!)

The model works, but one has to be careful how far you go in interpreting all aspects of the model as reality.

Sierra
2004-Jan-21, 12:10 AM
Just imagine the particle (e-) as a white ball inside a cloud of black smoke (you never know where it is). Now is viewing from a certain distance the cloud seems to be a round small black ball and you do not need to know the exact location of the white ball to calculate the speed of the whole object. If you get closer then it becomes dizzy and burred. Now you place this cloud just beside a little wall so small that that the cloud reaches the other side.
Suddenly the cloud shifts to the other side of the wall. If you measure the speed from center point of the old position to the center point of the new position you see that the cloud must have moved faster than light... but the thing is: The cloud is not the position of the particle. You never actually saw the particle moving. It could have been almost inside the wall and then moved to the other side and making a new cloud there. Then it would not have needed a speed faster than light... what you observed was just the cloud. -> Heisenberg

I hope that was correct ;)

TwelfthVarmint
2006-Mar-30, 07:26 PM
Note from antoniseb: This is an old thread restarted by a new member. Welcome TwelfthVarmint

wedgebert said, "Well, I believe the actual limitation is that a particle with mass cannot be accelerated to or past c. If you could find a way to just suddenly be travelling faster than c, there wouldn't be a violation."

Which is fine if you're a particle, but in a macro scenario, where you're going to want to stop eventually (i.e. space travel) you've got a problem. Assuming you discover a way to be "suddenly" traveling at C, and further asuming that this doesn't destroy you, you now have infinite mass. Since your mass is now infinite, any Tsiolkovsky maneuver to accelerate negatively is going to require an infinite exhaust velocity for your reaction mass. Since you can't accelerate it past C up to infinity (and don't remind me that it is already traveling faster than C along with you, since you still have to accelerate it in the opposite direction if you want it to slow your spaceship down) this form of travel becomes completely impractical unless, of course, you conveniently find yourself "just suddenly" traveling slower than C again in just the right place to do so.

TravisM
2006-Mar-30, 09:40 PM
If you can "suddenly" travel C+ you'd probably be able to "suddenly" stop. Hm... I wonder about C#...? :D

Nereid
2006-Mar-30, 11:47 PM
Moved, from ATM to General Science ...

Ken G
2006-Apr-01, 02:03 AM
What is impossible classically is for two particles to pass by each other with a relative speed larger than c. It's not very precise to talk about how fast something is "going", and the distances between particles separated by large distances can certainly increase faster than c in some observers' frames. And yes, as it has been pointed out, quantum mechanics relaxes many of the classical rules, but serious violations (i.e., greater than the uncertainty principle) have a very low probability. As for classical music being transmitted at 4.7 c, nope.

John Dlugosz
2006-Apr-07, 08:35 PM
I am in a discussion with a fella on another board about FTL issues.
He keeps telling me that Feynman allows for FTL travel of particles through QED.


Pick up a copy of Feynman's "QED" and see what he really meant, in context. It's an easy read, transcribed from 4 one-hour lectures for non-physics students.

A real particle is never observed to travel faster than light. By postulating that "all possible" paths don't worry about such things, when you sum over all of them, you get the correct "real" answer.