Does an Atomic Bomb's Explosion in a Contained Box Affect Its Weight?

[bwilhelm]

Homework Statement

I am having some trouble with this one,
Suppose an atomic bomb was exploded in a box that was strong enough to contain all the energy released by the bomb. After the explosion the box would weigh:
a) more than before the explosion
b) less than before
c) no change

Homework Equations

The Attempt at a Solution

My first thought was that the weight of the box would decrease due to the loss of mass when it was converted to energy.
But is the mass converted to photons? Photons in motion would contain mass right? And if the box was able to contain everything wouldn't the weight remain the same?

 

[Dick]

Yes. Think about it. Suppose we a had a process that could completely convert mass to photons and back again. Now we put it in your box. It has weight w1. Now convert the mass to photons. If it now has weight w2<w1, then we could push it up some distance, convert the photons back into mass and move the heavier box back down. Since we did less work pushing the box up then we got by letting it back down we would have a perpetual motion machine.