How can photon have energy, but no mass if “m=E/c^2” (E= mc^2)
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Because mass is mass not energy.
Energy is energy not mass
Although mass can carry energy (kinetic) it is not itself that energy, it can impart energy to other mass by collision, but no mass is transferred.
And energy can impart energy (light heating up a surface) but does not add mass in the process.
If you had a machine that could create mass from energy, you can use E=mc^2 (rearranged) to figure out how much mass you could create from a given amount of energy.
Or if you have a machine for converting mass to energy (nuclear bomb for instance) you can calculate the conversion using E=mc^2
We still don’t really know know what light is, we can describe what light does, but not exactly what it is. So if you were looking for “what is it, that it carry’s energy” well that’s hard to explain and starts getting into “quantum” physics and the untraviolet cascade, plank length and so forth.
Holy cow! I’m reading the explanations, and there must be some genius five year olds in here.
*mind blown upon seeing plank constant mentioned*
I didn’t understand those equations until I was seven!
*joke*
E= mc^2 asks how much energy is ‘locked’ within mass of an object. If this energy is released in pure form it will be released as photons. So this simplified version of the equation does not explain energy of photon itself in relation to its mass as photon is the solution here not the question.
Who knew photons were the masters of being both light and heavyweights? Impressive little things!
I always understood it as not that photons don’t have any mass, but that they don’t have any rest mass. Is this wrong?
E = mc^2 is the simple version of the equation that relates energy to relativistic mass. And yes, light does have relativistic mass (which is equivalent to gravitational mass). In fact Einstein came up with this equation to show that light had mass, which it does, if by mass you mean specifically relativistic or gravitational mass, or mass-energy content.
What we are all used to is that light is massless, by which we mean light has no rest mass (which is more like the amount of “stuff” in something). You get that from the more useful equation that combines rest mass with kinetic energy via:
E^2 = m^2 c^4 + p^2 c^2.
Here, the m is rest mass and the p is momentum. For light, 100% of its energy (or mass) comes from it’s momentum, and none of it comes from rest mass – it has no rest mass. This is true for anything travelling at the speed of light.
You can use these equations to show that for objects with mass, travelling at low speeds, we observe kinetic energy to be roughly 1/2 mv^2 = 1/2 p^2 / m (when kinetic energy makes up a small fraction of the total mass-energy). But for light, this approximation is completely irrelevant, and momentum doesn’t mean the same thing we are used to (in how it’s related to velocity).
How can photons have energy, but no mass?
Because photon (particles) are abstractions, not physical objects.
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