Why photons have zero mass

Home Physics The Theory of Relativity. If photons have zero mass, why do they feel the effects of gravity? This is a great question! This page was last reviewed on February 1, About the Author Manolis Papastergis Manolis is a sixth year graduate student who studies galaxies with Arecibo. Similar Questions that might Interest You If light has no mass, then what draws it into a black hole? Intermediate How do gravitons escape black holes to tell the universe about their gravity?

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Editor's Pick. What is the universe expanding into? Intermediate What do I need to do to become an astronomer? Beginner What's the difference between astronomy and astrology? The water wave therefore carries momentum even though it has no mass. The water itself has mass, but the wave has no mass. A water wave is not a packet of water traveling along. In fact, the water that the wave is traveling through stays more or less in one place.

Rather, the wave is a rippling domino-effect of motion. As another example, consider a long jump rope held taught at both ends by two girls. If one girl shakes her end of the rope violently enough to send a wave down the rope to the other girl, the wave can jerk the other girl.

The rope has not transported any mass, but it still carries momentum through its waving motion. In this way, waves can have no mass but still carry momentum.

In addition to being a particle, light is also a wave. This allows it to carry momentum, and therefore energy, without having mass. Now, in the case of quantum electrodynamics, it is a gauge theory with the symmetry group U 1. This U 1 -group which also gives rise to conservation of electric charge also has a gauge field associated with it, which is the electromagnetic field well, technically it is the vector potential of the EM-field.

Well, great, we now know what a photon is, but what does any of this have to do with the photon not having any mass? The defining property here is that a photon is classified as a gauge boson. It actually turns out that mathematically, there is no way to add a mass term for the photon without breaking gauge symmetry, which is what the whole Standard Model is practically based on.

Other gauge bosons such as the W and Z bosons can, however, obtain a mass regardless through something called spontaneous symmetry breaking and the Higgs field, but this does not occur for the photon. So, in fact, the quantum field theoretical explanation for why a photon does not have mass is that gauge symmetry forbids it. Therefore, if we want quantum electrodynamics to work like it should and also agree with experiments, the photon cannot have a mass this is quite an important point as all experimental evidence also suggests the photon to be massless.

Even though these lead to some very interesting consequences in, for example electromagnetism for massive photons, electromagnetic waves should have three polarization states , they simply do not correspond to reality or anything that has been experimentally verified apart from the superconducting phenomenon, which is a whole other story.

This of course indicates that the mass of a photon is also extremely small it could likely be proven to be exactly zero, if it was possible to measure it accurately enough.

The simplest way to measure the mass of a photon is to measure electric fields produced by charged particles. There are even methods for measuring the mass of a photon that involve measurements of planetary magnetic fields. An even better result was obtained from experiments involving measurements of solar winds a solar wind is basically plasma coming from the sun, which carries a magnetic field with itself.

It turns out that if the photon had a mass, this plasma would behave differently, namely, there would be current produced and the plasma would get accelerated. I'm the founder of Profound Physics, a website I created to help especially those trying to self-study physics as that is what I'm passionate about doing myself. I like to explain what I've learned in an understandable and laid-back way and I'll keep doing so as I learn more about the wonders of physics.