Can anyone explain or correct this?

[leolee5633]

http://www.youtube.com/watch?v=hW7DW9NIO9M&feature=player_detailpage

On the above one minute physics video, the guy explained E=mc² using the idea of Doppler effect, but that doesn't seem right to me, can anyone help?

Thanks very much!

 

[mfb]

I think there would be other ways to fix energy conservation and you have to be more careful with the Doppler effect (needs an integration over the solid angle), but the basic idea is fine: The kinetic energy of the cat somehow depends on the energy content of the cat. Otherwise, "radiating+getting speed afterwards" would need the same energy as "getting speed + radiating" - but the light energy would be different.

 

[leolee5633]

I think there would be other ways to fix energy conservation and you have to be more careful with the Doppler effect (needs an integration over the solid angle), but the basic idea is fine: The kinetic energy of the cat somehow depends on the energy content of the cat. Otherwise, "radiating+getting speed afterwards" would need the same energy as "getting speed + radiating" - but the light energy would be different.

Thank you, but is that guy correct in calculating the Doppler energy gain to be (1+v^2/2c^2) not (1-v^2/2c^2)?
Also he is using classical kinetic energy to derive the formula, is that appropriate?

 

[mfb]

but is that guy correct in calculating the Doppler energy gain to be (1+v^2/2c^2) not (1-v^2/2c^2)?

That is a bit sloppy, he should integrate over all emitted photons / space directions.

Also he is using classical kinetic energy to derive the formula, is that appropriate?

That is another problem - you can derive that the classical formula does not work, but it is problematic to derive a new formula, based on the (wrong) old one. The result happens to fit, but that could be just by chance.

 

[leolee5633]

That is a bit sloppy, he should integrate over all emitted photons / space directions.


That is another problem - you can derive that the classical formula does not work, but it is problematic to derive a new formula, based on the (wrong) old one. The result happens to fit, but that could be just by chance.

I see, thank you very much for you help.