Can Light Travel Faster Than c in Diamagnetic Materials?

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SUMMARY

The speed of light in vacuum is defined as 1 / sqrt(mu_0 e_0), while in matter, it is modified by relative permittivity and permeability, expressed as c / sqrt(k_m k_e). In diamagnetic materials, k_m is indeed less than 1, but this does not imply that light travels faster than 3 * 10^8 m/s. The key distinction lies in the phase velocity versus group velocity; although phase velocity can exceed the speed of light in vacuum, the group velocity, which conveys energy and information, remains below this limit.

PREREQUISITES
  • Understanding of electromagnetic theory, specifically Maxwell's equations.
  • Familiarity with the concepts of relative permittivity and permeability.
  • Knowledge of phase velocity versus group velocity in wave propagation.
  • Basic principles of diamagnetism and its effects on light propagation.
NEXT STEPS
  • Study the implications of Maxwell's equations on light speed in various media.
  • Research the differences between phase velocity and group velocity in electromagnetic waves.
  • Explore the properties of diamagnetic materials and their impact on light behavior.
  • Investigate materials that exhibit negative phase velocity and their applications.
USEFUL FOR

Physicists, electrical engineers, and students of electromagnetism seeking to deepen their understanding of light propagation in different materials, particularly in the context of diamagnetism and wave mechanics.

conquerer7
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The speed of light in vacuum is 1 / sqrt(mu_0 e_0). In matter, this is modified using relative permittivity and permeability, so the speed of light in matter is c / sqrt(k_m k_e).

In diamagnetic materials, isn't k_m < 1? Then we'd have light going faster than 3 * 10^8 m/s in, say, nitrogen gas, which is diamagnetic. What's wrong?
 
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You're mostly correct, except for the assumption that C would be the variable that changes. C will stay constant, and relative permittivity will go up accordingly.
 
conquerer7 said:
so the speed of light in matter is c / sqrt(k_m k_e).
This is the phase velocity, not the group velocity. And don't forget the k_e-part, which is usually dominant in matter.

There are materials with a phase velocity larger than the speed of light in vacuum and even materials with negative phase velocity. However, the group velocity (the thing which can carry energy and information, neglecting losses here) is always smaller than that.
 

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