Phase velocity faster than light?

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Discussion Overview

The discussion revolves around the concept of phase velocity in the context of atomic physics, particularly whether it can exceed the speed of light. Participants explore the implications of phase velocity exceeding light speed, its mathematical nature, and historical context within quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the concept of phase velocity exceeding the speed of light and seeks clarification.
  • Another participant references a Wikipedia article explaining that phase velocity can exceed c in certain media but emphasizes that this does not allow for the transmission of information faster than light.
  • A different participant suggests that the phenomenon is merely a mathematical artifact and asserts that nothing is actually traveling faster than light.
  • This participant also notes that matter waves were an interim theory leading to quantum mechanics and encourages focusing on the correct theory rather than historical aspects.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of phase velocity exceeding light speed. There are competing views regarding its significance, with some emphasizing its mathematical nature and others discussing its historical context.

Contextual Notes

There are limitations in understanding the implications of phase velocity, particularly regarding its relationship to information transfer and the historical development of quantum mechanics. The discussion also reflects varying levels of familiarity with the underlying concepts.

shanepitts
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I am currently taking an undergrads course in atomic physics, and I have just grazed the concepts of matter wave functions, group waves, group velocity, and phase velocity. For the latter, it has been stated that phase velocity can exceed the speed of light. I don't fathom this. Could anyone please help me with this concept?
 
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Does this help?

Per wiki: http://en.wikipedia.org/wiki/Faster-than-light#Phase_velocities_above_c

The phase velocity of an electromagnetic wave, when traveling through a medium, can routinely exceed c, the vacuum velocity of light. For example, this occurs in most glasses at X-ray frequencies.[11] However, the phase velocity of a wave corresponds to the propagation speed of a theoretical single-frequency (purely monochromatic) component of the wave at that frequency. Such a wave component must be infinite in extent and of constant amplitude (otherwise it is not truly monochromatic), and so cannot convey any information.[12] Thus a phase velocity above c does not imply the propagation of signals with a velocity above c.[13]
 
Thanks
 
shanepitts said:
I am currently taking an undergrads course in atomic physics, and I have just grazed the concepts of matter wave functions, group waves, group velocity, and phase velocity. For the latter, it has been stated that phase velocity can exceed the speed of light. I don't fathom this. Could anyone please help me with this concept?

Its purely a mathematical artefact - nothing is really traveling FTL.

Also matter waves was simply an interim theory on the way to full blown Quantum Mechanics that appeared when Dirac published his transformation theory.

Don't spend too long on what is just really of historical interest. Try, as soon as possible, to come to grips with the correct theory. So you understand its REAL basis check out:
http://www.scottaaronson.com/democritus/lec9.html

Your course may not get to this for a while, or indeed at all, depending on its depth, but just keep in the back of your mind this is what QM is REALLY about - the rest is basically historical background.

Thanks
Bill
 
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