MadSci Network: Physics

Re: Which is the difference between front, group, signal and energy velocities?

Date: Sat Apr 10 17:03:07 1999
Posted By: Frank Berauer, Technology Transfer Engineer Microelectronics
Area of science: Physics
ID: 919309403.Ph

Hi Jose,

I will try to explain the complex theory behind wave dynamics in
simple terms:
A photon can be imagined as a packet of waves. Imagine it like
people travelling together in a bus. The bus speed is the group
velocity and determines when the packet reaches its target. For
photons, this velocity is always the speed of light.
Just like people in a bus, waves can move within the packet and
thus temporarily be faster or slower than the packet speed. Does
the bus reach the target faster because of this? No.
Unlike a bus, the length of a photon is not fixed. During its
travel, it generally expands. Thus the front is faster than the
back. Does an expanding bus reach its target faster? Again the
answer is No.
Although the front and wave velocities might exceed the speed of
light, no signal can be carried by these, because we can detect
only entire photons and thus have to wait until "the bus arrives"
to now that we got a signal.

Greetings from Indonesia,
        Frank Berauer

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