system
Given the following post, respond as if you're commenting on Reddit:
human
At what point does light "have" mass?

my understanding of light is that it is a wave,  similar to sound. wavelength,  frequency, etc. are measuring "tools."
light is composed of mass-less particles, and thus, has no "weight."

I once read of an experiment where light was shown into a vacuum sealed box, with all interior sides of the box containing mirrors, at the perfect angles.

in this instance, the mass of the box is increased when the light is switched, "on."

1. has anyone else heard if this experiment , or is it theoretical? 

2. am I correct in thinking light has no measurable weight?
gpt
Any given individual photon propagating in vacuum will always be massless. What *can* have mass is a system of multiple photons--so your box example would indeed weight heavier with the light trapped inside. Imagine two photons, 1 and 2, traveling in opposite directions, he math works like this:

> E^(2) = (mc^(2))^(2)+(pc)^(2)

> E1 = pc

> p(total) = p1+p2 = 0 (opposite directions)

so,  

> E(total)^(2) = (E1+E2)^2 = (mc^(2))^(2)

If they're traveling in parallel, the mass is zero and for anything in between it depends on a bit more complicated equation involving a (1-cos(theta)) of the angle between them. This is what we call *invariant mass* which you can almost think about like a 4D (spacetime) consequence of Pythagoras's theorem. This mass while not attributable to any single component, acts like any other mass and will gravitate, obey Newton's laws, and do anything else you expect mass to do. In fact it accounts for 99% of the mass of neutrons and protons. In that way, much of the mass of the universe is from this strange consequence of Special Relativity.