Waves in Vacuum: Can They Travel Faster than c?

[gonchenshi]

According to relativity, any wave that can travel through vacuum can do so only at c, because since vacuum looks the same for every observer, so should the mechanism allowing the waves to travel (since that takes place in the vacum) and therefore the speed at which the wave travels (I think), and since all other speeds will be different for observers in other frames. So how can matter waves travel faster than c? I konw wiki says it can because it doesn't carry information, but there has to be a mechanism allowing it to travel...er, right?

 

[Bob_for_short]

No, matter does not normally travel faster than c. What travels faster than c is not matter.

There are events independent of each other. In some reference frames they may happen simultaneously but at distant places. Of course they have independent reasons. For example, we cannot and do not say that the event A made happen the event B because of super-luminary transfer of trigging signal.

 

[gonchenshi]

There are events independent of each other. In some reference frames they may happen simultaneously but at distant places. Of course they have independent reasons. For example, we cannot and do not say that the event A made happen the event B because of super-luminary transfer of trigging signal.

But the reason a wave travels is because the previous state of the wave results in the next state of the wave (eg. light travels because the changing electromagnetic field changes the electromagnetic field around it). So for a wave, state A did make happen state B, er... right?

 

[ryuunoseika]

According to relativity, any wave that can travel through vacuum can do so only at c, because since vacuum looks the same for every observer, so should the mechanism allowing the waves to travel (since that takes place in the vacum) and therefore the speed at which the wave travels (I think), and since all other speeds will be different for observers in other frames. So how can matter waves travel faster than c? I konw wiki says it can because it doesn't carry information, but there has to be a mechanism allowing it to travel...er, right?

i still don't completely understand your logic, but i think i should point out now that waves of matter can't travel through a vacuum because of what they are, waves of matter. Matter is not a vacuum. Vacuums are the absence of matter. Waves traveling through matter are not traveling through a vacuum because matter is not a vacuum.

 

[ryuunoseika]

According to relativity, any wave that can travel through vacuum can do so only at c, because since vacuum looks the same for every observer, so should the mechanism allowing the waves to travel (since that takes place in the vacum) and therefore the speed at which the wave travels (I think), and since all other speeds will be different for observers in other frames. So how can matter waves travel faster than c? I konw wiki says it can because it doesn't carry information, but there has to be a mechanism allowing it to travel...er, right?

i think I've figured out your problem! you're making the fundamental error! the error that sparked relativity in the first place:

if you travel east at 1/2 the speed of light and your shine a flashlight pointing east then the light coming out of the flashlight is traveling at 3/2 the speed of light.

this is false, because in relativity velocities do not add classically. 1mph + 1 mph ≠ 2 mph. it actually equals 1.999999999999999 and so on mph, because, as you approach the speed of light, it becomes more and more difficult to increase one's velocity.

 

[gonchenshi]

i still don't completely understand your logic, but i think i should point out now that waves of matter can't travel through a vacuum because of what they are, waves of matter. Matter is not a vacuum. Vacuums are the absence of matter. Waves traveling through matter are not traveling through a vacuum because matter is not a vacuum.

But that's like saying that since electromagnetic waves are waves of light (photons), they don't travel through vacum, either.

 

[gonchenshi]

if you travel east at 1/2 the speed of light and your shine a flashlight pointing east then the light coming out of the flashlight is traveling at 3/2 the speed of light.

this is false, because in relativity velocities do not add classically. 1mph + 1 mph ≠ 2 mph. it actually equals 1.999999999999999 and so on mph, because, as you approach the speed of light, it becomes more and more difficult to increase one's velocity.

This has nothing to do with my problem. I have no difficulty with velocity addition, I do have difficulty understanding how relativity can allow waves to travel through vacuum at speeds faster (or just different from) c.

 

[A.T.]

I do have difficulty understanding how relativity can allow waves to travel through vacuum at speeds faster (or just different from) c.

Maybe your problem is not the type of the wave (em-waves vs. matter waves), but the definition of wave velocity (phase velocity, group velocity):
http://en.wikipedia.org/wiki/Wave_propagation#Wave_velocity
http://en.wikipedia.org/wiki/Phase_velocity#Matter_wave_phase
http://en.wikipedia.org/wiki/Group_velocity#Matter-wave_group_velocity

 

[Al68]

According to relativity, any wave that can travel through vacuum can do so only at c, because since vacuum looks the same for every observer, so should the mechanism allowing the waves to travel (since that takes place in the vacum) and therefore the speed at which the wave travels (I think), and since all other speeds will be different for observers in other frames. So how can matter waves travel faster than c? I konw wiki says it can because it doesn't carry information, but there has to be a mechanism allowing it to travel...er, right?

Can you reference the wiki article and explain what "matter waves" you're referring to?

 

[gonchenshi]

Can you reference the wiki article and explain what "matter waves" you're referring to?

The de Broglie relations, mostly.

 

[Al68]

Can you reference the wiki article and explain what "matter waves" you're referring to?

The de Broglie relations, mostly.

In that case, A.T. made the right point. The real issue is whether you are referring to group velocity or phase velocity.

The phase velocity of electromagnetic radiation can also exceed c, and just like the phase velocity of "matter waves" exceeding c, no information or energy is carried faster than c.

The speed limit of c applies to signal velocity, or wave front velocity, (which is normally equal to group velocity), and is basically the speed that information and energy is carried by the wave.

 

[Austin0]

In that case, A.T. made the right point. The real issue is whether you are referring to group velocity or phase velocity.

The phase velocity of electromagnetic radiation can also exceed c, and just like the phase velocity of "matter waves" exceeding c, no information or energy is carried faster than c.

The speed limit of c applies to signal velocity, or wave front velocity, (which is normally equal to group velocity), and is basically the speed that information and energy is carried by the wave.

Hi Al68

When considering phase velocity of a single photon is there any difference between wave front velocity and phase velocity? As I understand phase velocity it is simply the velocity of a wave peak. Is this incorrect??
Does the idea of phase velocity greater than c pertain to groups of photons and the possible phase translation due to interference ?
Or other??
Thanks

 

[HallsofIvy]

But that's like saying that since electromagnetic waves are waves of light (photons), they don't travel through vacum, either.

No, because photons are not matter.

 

[Al68]

Hi Al68

When considering phase velocity of a single photon is there any difference between wave front velocity and phase velocity? As I understand phase velocity it is simply the velocity of a wave peak. Is this incorrect??

That's right. But a single photon is not normally modeled as a wave at all.

Does the idea of phase velocity greater than c pertain to groups of photons and the possible phase translation due to interference ?
Or other??
Thanks

It's about wave dispersion. Here's a decent link: http://en.wikipedia.org/wiki/Phase_velocity.