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Propagation of Light, Group velocity and Phase velocity

Prasanna asks:

“how can light travel in the form of waves in space, which has no medium? if light doesn’t travel in the form of wave in the space then in equation. ‘phase velocity=c^2/group velocity ‘ phase velocity becomes zero and also c=0, which indicates velocity of light =0, but light actually travel in space . How can this be possible?”

Answer:

I feel that there is some sort of misunderstanding in your concept.

Why should phase velocity become zero?

Please remember that light is an electromagnetic wave, where the propagation is due to the mutually reinforcing time varying electric and magnetic field vectors varying in mutually perpendicular directions.

The phase velocity is a the speed at which a point of fixed phase (say the crust of the wave)  propagates, which is not always the speed at which the electromagnetic information travels.

Group velocity is the speed at which electromagnetic information travels.

Hope that the following links will help you sort out the issue.

  1. http://www.microwaves101.com/encyclopedia/phasevelocity.cfm
  2. http://en.wikipedia.org/wiki/Group_velocity
  3. http://en.wikipedia.org/wiki/Phase_velocity
  4. http://en.wikipedia.org/wiki/Speed_of_light
  5. http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SpeedOfLight/FTL.html

Wormhole

Deepa asked:

“What is a worm hole?”

Ans:

A wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime.

A wormhole is, in theory, much like a tunnel with two ends each in separate points in [wiki]spacetime[/wiki]. image

There is no observational evidence for [wiki]wormholes[/wiki], but on a theoretical level there are valid solutions to the equations of the theory of general relativity which contain wormholes. The first type of wormhole solution discovered was the Schwarzschild wormhole which would be present in the Schwarzschild metric  describing an eternal black hole, but it was found that this type of wormhole would collapse too quickly for anything to cross from one end to the other. Wormholes which could actually be crossed, known as traversable wormholes, would only be possible if exotic matter with negative energy density could be used to stabilize them

Faster than light Travel
[wiki]Wormholes [/wiki]allow superluminal (faster-than-light) travel by ensuring that the speed of light is not exceeded locally at any time. While traveling through a wormhole, subluminal (slower-than-light) speeds are used. If two points are connected by a wormhole, the time taken to traverse it would be less than the time it would take a light beam to make the journey if it took a path through the space outside the [wiki]wormhole[/wiki]. However, a light beam traveling through the wormhole would always beat the traveler. As an analogy, running around to the opposite side of a mountain at maximum speed may take longer than walking through a tunnel crossing it.

Ref: Wikipedia

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