"Jacques" wrote in message
...
Hi everybody,

I have a question which bothers me since long time and maybe with your
help
I can find the answer at last. Since I heard the formule E=mc^2 for the
first time it struck me that there seems to be no logical relation between
the energy contained in a mass and the speed of light.

One derivation of the formula considers a set-up similar to two plates fixed
at each end of a length of drainpipe
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(Should be viewed as a fixed font. )

Consider a photon of wavelength lambda emitted from the left hand plate
towards the right hand plate.
The photon has a momentum p = h/lambda to the right, and the plate/drainpipe
setup has the opposite momentum (i.e. moves to the left).

After time t (which depends on the speed of light and the separation between
the plates), the photon hits the right hand plate and the momentum goes into
the plate/drainpipe setup, stopping its movement to the left.

In effect a photon has travelled the length of the pipe from left to right,
and the pipe has moved a much smaller distance from right to left.

As there has been no external force on the pipe, its centre of mass will not
have moved; so we have to consider the photon movement as equivalent to
transfering a mass from the left to the right end.

The amount of movement of the pipe depends on the momentum of the photon
(h/lambda). We can also use the wavelength of the light to give us its
energy E = hc / lambda.

Solve the problem to give the movement of the centre of mass of the system
when a photon of energy E travels from left to right and you'll find that
the mass equivalence of the photon, m = E/c^2