Re: Want to prove E=mc²? University labs should try this!

Its remarkable  that you (and others) don't understand how hopeless
this is.
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You send a laser beam with power P₀ = 5W into a cavity with inner
reflectivity R = 0.99.
It is irrelevant if the beam is diffused, when the light hits the inner
   walls of the cavity, it is reflected and the reflected power is P₀*R.
The reflected light will then hit the opposite wall and be reflected
again. The reflected power is now P₀*R*R.
When the light has been reflected n times, the reflected power is
P = P₀⋅Rⁿ.
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If n = 24170 the reflected power is P = 1.590e-105 W ≈ 0 W
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If the cavity is spherical with volume 1000 cm³, the diameter
is D = 12.4 cm. Which means that the light will be reflected
24170 times during 10 μs.
If you integrate the energy of the laser light in the cavity
it will be constant 2.05e-7 J
During the 10 μs 5⋅1e-6 J is put into the cavity, so 4.9795e-05 J
will be heat in cavity walls.
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The heat energy in the cavity walls will generally be:
  E = (5⋅t - 2.05e-7) J where t is the time in seconds.
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The temperature of the walls will increase, and steady
state will be when the outside of the cavity has a temperature
so that the radiated power (and convection loss) is 5 W.
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At that time, the cavity will be filled by black body radiation
with temperature equal to the inner walls.
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Have I missed something?