Can Phase Velocity Exceed the Speed of Light and Enable Time Travel?

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Discussion Overview

The discussion revolves around the concept of phase velocity and its relationship to the speed of light, particularly whether a phase velocity exceeding the speed of light could imply the possibility of time travel or traveling backwards in time. The scope includes theoretical implications of relativity and the nature of wave properties.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that if something has a phase velocity greater than the speed of light, it might travel backwards in time in one frame.
  • Others argue that relativity states normal matter cannot travel faster than light in any frame, suggesting that phase velocity does not imply backward time travel.
  • A participant mentions that phase velocity is a property of a wave and does not involve mass moving at that velocity.
  • There is a discussion about the observation of phase velocity in multiple frames of reference and its distinction from the transmission of information or matter.
  • One participant introduces a thought experiment involving the speed of a point at the intersection of two moving rulers, suggesting that certain points can travel at arbitrarily high speeds.

Areas of Agreement / Disagreement

Participants generally disagree on the implications of phase velocity exceeding the speed of light, with no consensus reached regarding whether this could allow for time travel or backward movement in time.

Contextual Notes

There are limitations in understanding the definitions and implications of phase velocity and its relationship to relativity, as well as the conditions under which certain velocities are observed.

byron178
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When something has a phase velocity greater than the speed of light,will it travel backwards in time in one frame?
 
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Nope.
 
ghwellsjr said:
Nope.

but relativity says that if something were to travel faster than light than in one frame it will travel backwards in time.
 
ghwellsjr said:
Nope.
Hmm, how do you conclude that?
 
byron178 said:
but relativity says that if something were to travel faster than light than in one frame it will travel backwards in time.
No, it doesn't. It says the normal matter cannot travel faster than light (in any frame).
 
HallsofIvy said:
No, it doesn't. It says the normal matter cannot travel faster than light (in any frame).
I would agree with that but do you think that that would imply that the phase does not go backwards in time in another frame?
 
Passionflower said:
I would agree with that but do you think that that would imply that the phase does not go backwards in time in another frame?

ive read that its like an illusion the phase velocity traveling faster than light.
 
Passionflower said:
I would agree with that but do you think that that would imply that the phase does not go backwards in time in another frame?
Apparently then the problem is that you do not understand what "phase" velocity means. "Phase" velocity is a property of a wave. There is nothing with mass moving at the "phase" velocity of a wave.
 
HallsofIvy said:
"Phase" velocity is a property of a wave. There is nothing with mass moving at the "phase" velocity of a wave.
Yes, and I implied that that was not the case?

Phase velocity can be observed, and thus it can also be observed in many frames of reference.
 
  • #10
The propagation of emf in a waveguide has a wave velocity greater than c.

What does "backwards in time" mean?
 
  • #11
Passionflower said:
Yes, and I implied that that was not the case?

Phase velocity can be observed, and thus it can also be observed in many frames of reference.

Plenty of things can be observed to be faster than speed of light. But they don't transmit information or matter.

Consider two very very long straight rulers, both moving at uniform speed (<c of course) perpendicular to their elongated direction. The point at which they cross has speed proportional to 1/tan(theta), where theta is the angle between them. So this point can travel at any arbitrarily high speed. Just like any point of the wave front of a dispersive wave.
 
  • #12
mathfeel said:
Plenty of things can be observed to be faster than speed of light. But they don't transmit information or matter.
Yes I am aware of that.

Did anyone claim the contrary?

Not every single posting with a question about relativity is designed to question its validity as some apparently seem to think.
 

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