Light so fast it goes backwards

[zane_stone]

Interesting work going on in Rochester...

http://www.scienceblog.com/cms/lights-m ... 10590.html

[CCC]

It appears that some material with an index of refraction lower than that of vaccuum has been found. In such a material, light would theoretically travel faster than c (light may be able to cross that boundary with less than infinite energy because it is massless at rest) - but anything travelling at >c would need to go backwards in time, so...

This might require some slight jiggling of relativity to fit the experimental data, although how much I'm not sure. But I have yet to see another theory which predicts that going at >c results in going backwards through time.

[equinoxe]

It appears that some material with an index of refraction lower than that of vaccuum has been found. In such a material, light would theoretically travel faster than c (light may be able to cross that boundary with less than infinite energy because it is massless at rest) - but anything travelling at >c would need to go backwards in time, so...

No, this is incorrect. The stuff that they talk about is group velocity. Read here:

http://gregegan.customer.netspace.net.a ... Notes.html

and here:

http://gregegan.customer.netspace.net.a ... 20/20.html

This might require some slight jiggling of relativity to fit the experimental data, although how much I'm not sure. But I have yet to see another theory which predicts that going at >c results in going backwards through time.

There is noneed for any "jiggling" of relativity, you need to understand simple wave theory correctly.

BTW, the sentence "the light is massless at rest " is incorrect. The correct statement is: photons have zero mass. Always.
Photons travel with the speed cin vacuum, c/n in a medium of refraction index n.

[CCC]

No, this is incorrect. The stuff that they talk about is group velocity. Read here:

http://gregegan.customer.netspace.net.a ... Notes.html

and here:

http://gregegan.customer.netspace.net.a ... 20/20.html

From those, it would seem that group velocity is merely an optical illusion, similar to the one seen when driving past a pair of fences with thin vertical slats. Which would mean that nothing would be going FTL, or backwards.

Which would move the whole thing from an interesting example of superliuminism and time reversal to a simple optical illusion. Pity.

There is noneed for any "jiggling" of relativity, you need to understand simple wave theory correctly.

Any theory requires the odd little alteration. Even if not for this experiment.

BTW, the sentence "the light is massless at rest " is incorrect. The correct statement is: photons have zero mass. Always.
Photons travel with the speed cin vacuum, c/n in a medium of refraction index n.

Then how, exactly, do you explain the phenomenon of radiation pressure?

That photons have zero rest mass I can agree with (that was what I meant), but they can certainly act like they have mass when they hit something while going at lightspeed. (Not much mass, true, but mass nonetheless).

[equinoxe]

From those, it would seem that group velocity is merely an optical illusion, similar to the one seen when driving past a pair of fences with thin vertical slats. Which would mean that nothing would be going FTL, or backwards.

Which would move the whole thing from an interesting example of superliuminism and time reversal to a simple optical illusion. Pity.

Why "pity"? Because it refutes a nonsense idea?

There is noneed for any "jiggling" of relativity, you need to understand simple wave theory correctly.

Any theory requires the odd little alteration. Even if not for this experiment.

Not relativity. And not in the context that you introduced above.

BTW, the sentence "the light is massless at rest " is incorrect. The correct statement is: photons have zero mass. Always.
Photons travel with the speed cin vacuum, c/n in a medium of refraction index n.

Then how, exactly, do you explain the phenomenon of radiation pressure?

The "pressure radiation" , explained by Einstein in his 1905 paper has nothing to do with photon mass. See paragraph 8 in the link below. No reference to any photon mass:

http://www.fourmilab.ch/etexts/einstein/specrel/www/

That photons have zero rest mass I can agree with (that was what I meant), but they can certainly act like they have mass when they hit something while going at lightspeed. (Not much mass, true, but mass nonetheless).

No, you are persisting in an erroneous idea. The photons cannot "exist" at any other speed than c, therefore "rest mass" makes no sense for them. They have zero mass at c! Please stop perpetrating wrong ideas, would you?

[CCC]

From those, it would seem that group velocity is merely an optical illusion, similar to the one seen when driving past a pair of fences with thin vertical slats. Which would mean that nothing would be going FTL, or backwards.

Which would move the whole thing from an interesting example of superliuminism and time reversal to a simple optical illusion. Pity.

Why "pity"? Because it refutes a nonsense idea?

No, because an optical illusion is a lot less exciting than time travelling light.

Any theory requires the odd little alteration. Even if not for this experiment.

Not relativity. And not in the context that you introduced above.

Perhaps not in the context of this experiment; but I have no doubt that it will still require alteration at some point in the future.

The "pressure radiation" , explained by Einstein in his 1905 paper has nothing to do with photon mass. See paragraph 8 in the link below. No reference to any photon mass:

http://www.fourmilab.ch/etexts/einstein/specrel/www/

I've done a bit of looking around on the internet myself; and it appears that I was using an obsolete definition for "mass" wherein the mass at speed is different from the rest mass due to the extra energy.

Thank you.

[equinoxe]

]

I've done a bit of looking around on the internet myself; and it appears that I was using an obsolete definition for "mass" wherein the mass at speed is different from the rest mass due to the extra energy.

Thank you.

The photon has only one type of mass since it can only exist at speed c. And it is zero. So the concept of rest mass/relativistic mass does not apply to the photon. I doubt that John Baez doesn't make this very clear, he knows his stuff.

[yobeht]

Relativity aside, these photons are traveling faster than c. Being so, the photons are invisible until they exit from the other side, slowing back down to C. At this point, the light beam is visible and it backtracks to the entrance of the experiment. The photons do not travel backwards, it is an optical illusion, due to >c speeds.