Does light carry any mass?
light has no rest mass. That is to say, you cannot bring a photon to a halt and measure its mass. It does not exist, just as photons standing still (in the vacuum) do not exist.
But an actual photon has mass-energy. If you were to create a box lined with perfect mirrors and let a ray of light in, bouncing back-and-forth forever, you could measure, in principle, a small increase in the box’s weight. (And while such a box may not be realizable, something similar does exist: for stars, especially supergiant stars, a significant portion of their mass is in the form of trapped radiation, i.e., a “photon gas”.)
And “something” is not a synonym for “mass”. A photon has other properties, such as its angular momentum. When it comes to gravity, it’s all those properties together that determine how an object produces, or reacts to, a gravitational field. Rest mass is just one of several parameters (albeit at low speeds, in weak fields, as in our everyday experience, it is the most significant of them.)
Finally, as to why the photon is like this: They don’t usually teach this in undergraduate courses, but Maxwell’s equations are mathematical identities. The only thing that is needed to realize them is a (mathematical) definition of the electric charge. It is possible to define the charge differently, and end up with a modified set of equations: the so-called Proca equations, which describe a hypothetical form of electromagnetism in which the photon does have a non-zero rest mass. Why Nature chose Maxwell over Proca… it is not a question physicists may ever be able to answer. Although I note that in actuality, Nature chose both: The Z boson of the weak interaction is, for all practical intents and purposes, just like a very heavy, massive version of the photon.
light has no rest mass. That is to say, you cannot bring a photon to a halt and measure its mass. It does not exist, just as photons standing still (in the vacuum) do not exist.
But an actual photon has mass-energy. If you were to create a box lined with perfect mirrors and let a ray of light in, bouncing back-and-forth forever, you could measure, in principle, a small increase in the box’s weight. (And while such a box may not be realizable, something similar does exist: for stars, especially supergiant stars, a significant portion of their mass is in the form of trapped radiation, i.e., a “photon gas”.)
And “something” is not a synonym for “mass”. A photon has other properties, such as its angular momentum. When it comes to gravity, it’s all those properties together that determine how an object produces, or reacts to, a gravitational field. Rest mass is just one of several parameters (albeit at low speeds, in weak fields, as in our everyday experience, it is the most significant of them.)
Finally, as to why the photon is like this: They don’t usually teach this in undergraduate courses, but Maxwell’s equations are mathematical identities. The only thing that is needed to realize them is a (mathematical) definition of the electric charge. It is possible to define the charge differently, and end up with a modified set of equations: the so-called Proca equations, which describe a hypothetical form of electromagnetism in which the photon does have a non-zero rest mass. Why Nature chose Maxwell over Proca… it is not a question physicists may ever be able to answer. Although I note that in actuality, Nature chose both: The Z boson of the weak interaction is, for all practical intents and purposes, just like a very heavy, massive version of the photon.
