Light so fast it goes backwards

[warpus]

Mathematically I understand it but conceptually, no.

The point is that information isn't being transmitted faster than the speed of light in this hypothetical case - but if you were on the moon it would appear that it was.

And that's exactly the same thing we have here! Information isn't travelling faster than light so nothing is violated - but from our frame of reference it appears as though the light is travelling faster than light.

 

[Tenochtitlan]

Well, he could be right.

The Diameter of the Moon is 3476 km according to the internet. Thus, his laser dot will be forced to travel this distance in the exact same amount of time it takes him to rotate the laser pen here on earth. Assuming he can do it than less than a hundredth of a second (and since it's only a 5 degree arc, this is possible, though very difficult), he would correct:

Distance travelled: 3476 km = 3,476,000 m
Time required: lets say 0.01 seconds, for the sake of argument

3,476,000 m / 0.01 s = 3.4678 x10^8
c = ~3.0 x10^8

So, if we assume he can move his pen 5 degrees in 0.01 seconds, he would be correct (unless of course simple math doesn't hold at this level, which I wouldn't be surprised to learn).

It's a trick. The dot is not moving, its a different dot each time that appears right next to the old one. I will try to explain it later

 

[History_Buff]

It's a trick. The dot is not moving, its a different dot each time that appears right next to the old one. I will try to explain it later

Yeah, I know what you mean, I spent a while sitting here working out the time frame between the first photon and the last, but the point is still almost valid

 

[Timko]

Hooray for people not understanding the differences between group velocity and phase velocity!

Yeah, it wasn't till I got to the end that it was obvious that nothing interesting was happening. Bleh.

 

[AL_DA_GREAT]

Quantum physics is to hard to understand. Just accept what you read. BTW it could be possible. time is screwy at the speed of light nothing realy is normal.

 

[FriendlyFire]

If that's true, then an object travelling above light speed would appear to a stationary observer to be moving in the other direction; a pulse of light would enter the fiber at X o'clock, move through the fiber, and exit the other end at, say, X - 3.

So, who knows? Maybe they finally broke the Big Barrier.

Technically yes. Light would appear to travel in reverse. It would appear as if eveything is going in "reverse" motion.

How a human can survive faster then light speed travel is a whole another thing

 

[Bozo Erectus]

I guess one way to go faster than the speed of light is to lower the speed pf light.

 

[betazed]

I guess one way to go faster than the speed of light is to lower the speed pf light.

This happens regularly. Check out Cherenkov Effect

 

[pboily]

Well, he could be right.

The Diameter of the Moon is 3476 km according to the internet. Thus, his laser dot will be forced to travel this distance in the exact same amount of time it takes him to rotate the laser pen here on earth. Assuming he can do it than less than a hundredth of a second (and since it's only a 5 degree arc, this is possible, though very difficult), he would correct:

Distance travelled: 3476 km = 3,476,000 m
Time required: lets say 0.01 seconds, for the sake of argument

3,476,000 m / 0.01 s = 3.4678 x10^8
c = ~3.0 x10^8

So, if we assume he can move his pen 5 degrees in 0.01 seconds, he would be correct (unless of course simple math doesn't hold at this level, which I wouldn't be surprised to learn).

Are you taking into consideration the fact that the light in question emanates from the Earth and that it takes a while for photons to reach the Moon, so that in fact, what travels on the surface of the Moon is not a light "beam" in the usual sense of the term?

EDIT: I see that it's been pointed out already...

 

[Bozo Erectus]

This happens regularly. Check out Cherenkov Effect

Ive actually heard of Cherenkov radiation, I just didnt know what it was, thanks. BTW, so what happpens to time while light is going backward or slowing down?

 

[Cheezy the Wiz]

Wow, now that's some crazy stuff. I got the TV analogy, its just freakin weird!

 

[Mise]

Isn't this just that thing where you increase the gain at one end of the pulse and decrease it at the peak, thereby creating a new peak at the leading end instead of at the middle? So it just appears as though the peak comes out before it goes in, whereas infact the new peak is just the leading end + some gain.

 

[Bozo Erectus]

Please, whats with all that technical jargon!

 

[betazed]

BTW, so what happpens to time while light is going backward or slowing down?

Nothing happens. In all of these effects (including the one posted in this thread) either the group velocity of light or the phase velocity of light is slowed down or speeded up. There is no law of physics which put any constraints on (or relates time dilation effects with) either the group velocity or the phase velocity of light (each of them can be made to exceed c, or can be made to be less than c).

 

[Bozo Erectus]

But then why is ones subjective time dependent on the percentage of the speed of light one is travelling?

 

[Truronian]

I don't see how this could work. The fundemental problem with something travelling faster than c is that it mucks up the Lorenz factor equation:

If v>c then you end up with an imaginary answer, which just doesn't translate. I can't see how negative speed is gonna help with this. Maybe this is what the article means wen it says it contradicts Einstein's work

Obviously I'm no expert but this is just my $0.02

Imaginary velocity may work though...

 

[Mise]

Actually the lorentz factor is the inverse of that, i.e. gamma = 1/sqrt(1-v^2/c^2)

So it diverges when v -> c. It's true that at v > c you get the sqrt of a negative number, but the point is that you can't get to c in the first place.

The other point is that c is just a number. It's not "the speed at which light is currently travelling", but "the speed of light in a vacuum".

 

[El_Machinae]

Imaginary velocity may work though...

Like when a baby has a fake steeringwheel on his carseat, and he makes the 'brrrrrp' sound while pretending to steer.

 

[hassan_lembeck]

That is my conclusion as well. This is usually, what people observe, when they claim to have seen "faster-than light" phenomena. Unfortunately, I don't have access to the full Science article from my home PC. But the link says that the researchers sent the laser pulse through an erbium-doped fiber, which is typically used to amplify laser pulses. The trick is, you "pump" the fiber with a certain laser of a certain color, energy is absorbed and stored in the erbium ions in the fiber. When you send a laser pulse of a certain different color through the fiber, that energy is released in the form of light at the color of the laser pulse currently in the fiber. That is, the laser pulse gets amplified.

But if the energy initially stored in the fiber is to small, it gets depleted before the entire pulse has been amplified. In that case, only the leading edge of the pulse gets amplified while the falling edge does not. That means that the center of the pulse moves forward at a speed faster than light although the individual photons constituting the pulse still travel at the speed of light. So the pulse that emerges from the fiber end is actually just the leading edge of the pulse, only amplified, and so it only carries the information that is contained in the leading edge.

I believe there are some semiconductor materials with which you can play kinda the same trick, "semiconductor saturable absorber mirrors" (SESAM)s. These guys will increase their absorbtion the more light you shine onto them. So if you shine a laser pulse onto them, the leading edge will go through, absorbtion increases and the rest of the pulse is absorbed. That will again give you a pulse whose center has travelled faster than light, although the pulse itself is just the leading edge of the original pulse...

Isn't this just that thing where you increase the gain at one end of the pulse and decrease it at the peak, thereby creating a new peak at the leading end instead of at the middle? So it just appears as though the peak comes out before it goes in, whereas infact the new peak is just the leading end + some gain.

 

[BirraImperial]

umm.. anybody else feeling stupid while reading this thread? (I know I am)