# Why can photons travel faster than c?

1. Dec 10, 2004

### Gamish

why can photons travel faster than c, through the conditions presented by quantum tunneling. This makes no sense. If photons travel faster than c, then c is not really constant. If we have actually measured the tunneling of photons that travel faster than c, then that means that from the frame of reference of the observer, it saw the photon travel faster than c, which according to SR, the observer traveled faster than c.

"In the beginging, something has always, there was something, in which there was no beginning. If something did come from nothing, than that something was actually something in the first place. Now was this "something" energy, or a being. If it was a being, it can explain the lack of entropy in the universe. "

2. Dec 10, 2004

### NateTG

First off this probably belongs in the QM section rather than SR/GR.

That said, I am not a QM expert, but I think that there are several issues with your question. I believe that several of them are due to your question being about QM interpretation - it's about the state of a 'particle' while it is not being measured:
The notion of photon speed is somewhat ambiguous in quantum mechanics because you're talking about a 'wavefunction' which doesn't have a well-defined position.
c is the spoeed of light in vacuum. AFAIK tunneling does not occur in vacuum.
One of the well-known conflicts between GR/SR and Quantum Mechanics is that quantum mechanics assumes that the speed of light is infinite, and SR/GR does not.
It's not at all clear that it's the same 'photon' at the beginning and end of the tunneling even if the wave functions are identical.

3. Dec 10, 2004

### Gamish

erm...

ok, if I'm correct, scientist have made photons travel faster than c, how is this posible tho? Maybe I should carry this over to QM board....

I understand it :surprised

4. Dec 10, 2004

### chroot

Staff Emeritus
Photons never travel faster than c. In fact, photons never travel at any speed other than c.

The "faster-than-light" experiments you're talking about refer to the phase velocity of a wave going through a medium with anomalous dispersion. The group velocity of a wave, at which the energy and information of the wave travels, is always less than c. The phase velocity is unrelated to the actual propagation of energy or information, and is arbitrary. It can even be much greater than that of light. There is no violation of relativity.

Also, you might have heard of "virtual photons" going faster than light, and this is permissible by quantum mechanics. These virtual photons are not photons. They cannot be detected (because, if they were, they'd no longer be virtual). They do not really "exist;" they only represent contributions to the total amplitude of a real event occuring, like tunneling.

- Warren

5. Dec 11, 2004

### pervect

Staff Emeritus
I think Gamish is probably referring to the following effect as far as "faster than light tunelling"

Because the amplitude of the signal goes down exponentially when tunelling through a barrier, the phenomenon isn't very useful for transmitting information over long distances.

As I recall, the authors used a method that produced two correlated photons, and sent one photon through a path that tunelled, while the other photon travelled through a path without any tunelling They authors found that the correlated photons interferred with each other only when extra distance was added to the path with the tunnel, indicating it was faster than the path without the tunnel.

I'm sure there is more detail in the Physical Review article.

The results are consistent with a model that indicates the photon simply does not travel through the barrier. A "quantum foam" model would be one explanation of this effect that's consistent with the results. But this isn't necessarily the only explanation. Another explanation is that the wave nature of the photon makes localizing it impossible, so that the velocity measurement becomes uncertain by the width of the photon's wave packet.

One interesting point is that energy was not transferred any faster via the path with the tunnel on the average, due to the severe attenuation in the tunneling path. This is to be expected because of some theorems in QFT.

6. Dec 11, 2004

### jcsd

These kind of things are physicists parlour tricks i.e. often cleverly designed (though in the case of tunnelling the result and the theory is very basic in quantum mechanics) that seem to violate some physical principle when in actuality they do nothing of the sort.

In this case it cleraly does not violate SR as no information can be transmitted usingf this method even in principle, also at no time in the experiment can you actually say that an indivdual photon is travelling faster than c.

7. Dec 11, 2004

### pmb_phy

One photon 'disappears' and another one 'appears', the time between of which is covered at a speed that would take a particle speed greater than c. Nobody has ever observed a photon moving faster than c. In fact in QM it is meaningless to speak of the trajectory of a particle as one does in classical mechanics.

Pete

8. Dec 11, 2004

### pervect

Staff Emeritus
Yes, there are a lot of tricky things going on with defintions in these sorts of experiments. One of the many issues is one of wave-particle duality - a wave does not have a precise location.

$$\begin{verbatim} / / \ / \ ------/ \---------- \end{verbatim}$$

Unfortunately, as the meaningless lines above show, I can't get my ascii art to work, so I can't talk draw the diagrams I need to illustrate the point I wanted to make :-(

That's supoosed to be a triangle shaped "pulse" above. Next, below it, I intended to show another pulse below it that had two characteristics

1) it was "ahead" of the first pulse
2) it was also completely inside the first pulse

but I can't draw the illustration.

9. Aug 24, 2009

### Nemesis212

Ok well quantum tunneling is in the same area in physics as E=Nhf or, plank energy. In plank energy to higher the electromagnetic oscillations the lighter and faster mass can become. A famous formula for this is the "galaxy clock" showing as oscillations increase light eventually breaks its own speed limit. now if this is true then quantum tunneling is based off of quantum oscillations. Because, if you observe what a quantum tunneling effect is you will notice a spiral effect. the same as electromagnetic curvature which would allow higher oscillations. meaning. that a photonic particle under high enough electrostatic and or electromagnetic conditions can and will travel faster then light. I personally think that quantum tunneling is a fact because there are experiments that have proven its presents. and observed tachyons have seem to move "backwards" giving the impression that there moving faster then light. Then you always have the "zero-point" energy and subspace theory. that states that if there are parallel universes then there must be spaces between them that states the rules of physics must not exist in a space that is not part of a physical universe. Also this goes on the line of how could we harness these electro magnetic amplitudes that cause oscillations that cause matter to travel faster then light? would it be as simple as energizing an electrostatic or electromagnetic field around some space craft?? the main issue is how in depth is this going to have to go to harness it.