# Photons inside a box

One of my friends told me that the definition of mass is the amount of matter. I told him that mass is not the amount of matter, because when we heat an object, the mass of the object increases.

I gave an example: Photons moving around inside a closed massless box having walls of perfectly reflecting mirrors gives mass to the box, because the definition of mass is $\sqrt{E^2-p^2}$ .

Inside the box, when we consider the photons as a whole they don't have momentum, thus mass becomes $m=E$ when $c=1$.

But I'm really confused what happens when the box starts moving, because in that case the momentum as a whole of the photons is not zero? What will be the mass of the box then?

• Think about it in terms of momentum of the photons. What happens to their momentum when the box starts moving? Sep 26, 2014 at 4:59
• If there are many photons inside the box then momentum on the average of all the photons is zero , just like the average velocity of the molecules in a container is zero in kinetic theory of gases? but what happen when there is only one photon inside the box?
– Paul
Sep 26, 2014 at 9:36
• What would happen if it wasn't a single photon but a (perfect) tennis ball? What makes you think that a single photon would behave qualitatively differently from the scenario of a perfect ball? Sep 27, 2014 at 1:05

Your reasoning is quite correct and you are definitely on the right track. The rest mass of the sealed, reflecting box most definitely increases, by dint of the equation $E^2 = p^2 c^2 + m_0^2 c^4$ you cite. You simply need to think in a bit more detail about what happens when you shove the box: for simplicity, simply assume a one-dimensional cavity with motion only along the cavity's optical axis. The mirror at one end will begin to blue shift the light whose motion is in the opposite direction to the motion and at the other end there will be a red shift back. Whether you analyse this situation classically (messy and hard) or as photons (easy) as done in my answer here, you get the same answer: there is a difference between the impulses imparted to the mirrors at either end by dint of the different light wavelengths, and when you do the calculation it shows that you need to impart an impulse $E \Delta\,v/c^2$ (owing to the presence of the light with energy $E$ alone) to the box to change its speed by $\Delta\,v$. A slightly different method of getting to this conclusion is method 2 in my answer here. Either way, one can see that a great deal of the "rest mass" in the World indeed arises from the confinement of massless objects, as discussed further in my answer here and here.