# The weight of a cavity of radiation

A sealed cavity full of light weighs more than an empty one. So consider a cavity consisting of parallel mirrors, with a pair of beams bouncing between the mirrors. The question is whether that weight will change if we arrange for the beams to completely cancel one another at every point along their length?

• How do you achieve that? – gatsu May 2 '13 at 8:28
• By making the mirror separation an odd multiple of the half-wavelength. But it doesn't have to be precise; we don't need to get into spectral purity or Heisenberg, etc. Partial cancellation is enough for us to still be able to ask whether the weight changed or not. – Andrew Palfreyman May 2 '13 at 9:24
• There is no such thing as pure cancellation of two existing waves everywehere in a cavity. What happens is that you form a standing wave[en.wikipedia.org/wiki/Standing_wave] where the energy is zero at the nodes and double at the anti-nodes. Overall, the energy and therefor ethe mass is unchanged – gatsu May 2 '13 at 10:38
• Yes, good; I knew that and temporarily forgot it. So how can this be done? How about 2 collinear beams 180 degrees out of phase? – Andrew Palfreyman May 2 '13 at 16:30
• That's equivalent to a beam with zero intensity. – Colin K May 2 '13 at 20:50