system
Given the following post, respond as if you're commenting on Reddit:
human
When two beams of laserlight are put in perfect, destructive interference, where does their energy go?

Where does the energy go when two beams of laserlight are put in perfect, destructive interference so that all the waves cancel out each other and there is no beam anymore?

Why is the law of conservation of energy not hurt by this phenomenon?
gpt
If the two lasers are perfectly aligned and at the same frequency so that there is destructive interference *everywhere*, the lasers will simply not emit radiation in the first place because their emission mode has become forbidden. Instead, the electrons will transition by some other route than lasing such as electron-electron scattering or phonon emission. In the end, the energy that would have been in the laser beam if its emission mode had not been suppressed ends up as heat in the laser.

You have to realize that lasers don't automatically emit light when you turn them on. They only lase when the stimulated radiative transition has a higher probability rate (roughly speaking) then other possible transitions modes for the electron. In a full analysis, calculating the stimulated radiative transition rate would require taking into account the resonant cavity, and where the light may go after.

This is fundamental to laser design. A laser is basically an amplifying medium at a natural frequency and a resonant cavity. The peak frequency of the amplifying medium is often not exactly lined up with any of the resonant frequencies of the cavity. The amplifying medium does not emit at its peak natural frequency because this frequency is forbidden by destructive interference effects of the cavity resonance. Instead, the amplifying medium emits at the frequency of one of the cavity resonances.

When talking about interference, the energy usually goes to the regions of constructive interference. But when there is destructive interference everywhere, the light is simply not created in the first place because it is forbidden from doing so.