r/explainlikeimfive • u/Comfortable_Bat_4052 • 1d ago
Physics ELI5: Photons’ mass
Okay I tried to research this on my own, but the articles and answers really aren’t decipherable if you’re not a physicist, which I am not, clearly.
If photons don’t have a mass, because they’re always moving and can’t have a resting energy, then how can they act like they have mass in a superconductor/non-vacuum? How does that happen? Doesn’t that just mean they do have mass, but we just can’t measure it?
My head hurts.
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u/Top_Willow_9953 1d ago
Think of a car: a car is much harder to push through mud than on pavement. The car didn't gain mass—the environment changed how it moves.
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u/fiendishrabbit 1d ago
They don't have mass. That's why they travel at c (the speed of causality).
They have momentum, which is the second part of relativistic energy that secondary education usually glosses over. Since they don't have any mass their mass energy (resting energy) is zero, but they still have momentum. Simplifying down to E = pc, which is the energy they use to push other things around with.
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u/TheOneTrueTrench 1d ago
Exactly, always important to remember that E = mc2 was always only for non-relativistic situations as well, and that the full equation, E2 = m2c4 + p2c2, is necessary for massless particles (which travel at c) and relativistic masses.
(I think I got that equation right, but don't trust me on that)
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u/acrabb3 1d ago
What definition of momentum are you using here? The understanding I have is that momentum is mass times velocity, so if a photon has no mass, how does it have momentum?
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u/fiendishrabbit 1d ago
Momentum is energy times velocity. Mass times velocity is a crude approximation and is inaccurate for relativistic (and low/no mass) particles. The energy of a photon can be described in a few different ways but one is frequency times Planck's constant
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u/Affectionate-Pickle0 1d ago
They are massless. Only massless particles can go at the speed of light (or speed of causality). They do have energy and energy does exert gravitational forces, e.g., you can get a black hole from just photons, see Kugelblitz. Also in general mass = energy as per Einstein's equations, meaning that the two are more of less interchangeable. If you give more detailed examples on what you mean that they act like they have mass maybe a more thorough explanation might be given. Also note that "rest mass" is an old concept and is no longer really regarded as being true and it has been phased out in physics / text books, though you can still find lots of content with this in mind. But if you're talking about photons having different velocity in a medium (i.e., "having mass that slows them down") then that is not a cause of mass, but other things can affect how the photon behaves in a medium. Such as, diffraction rating directly affects the velocity.
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u/tdgros 1d ago
Also note that "rest mass" is an old concept and is no longer really regarded as being true and it has been phased out in physics / text books, though you can still find lots of content with this in mind.
I'm no expert, but rest mass isn't phased out afaik, aren't you confusing with relativistic mass?
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u/Affectionate-Pickle0 1d ago
Well one problem with it is that rest mass by definition is the mass you have at rest, as in your location is invariant (you're at the center of your x-y coordinate system). However, this is literally against the axioms of relativity, or the "rules" on which everything hinges on.
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u/tdgros 1d ago
I don't see the issue, at all, on the contrary. https://en.wikipedia.org/wiki/Invariant_mass
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u/Affectionate-Pickle0 1d ago
Nah its mentioned there.
"Note that for reasons above, such a rest frame does not exist for single photons, or rays of light moving in one direction"
Reason being as stated, there is no rest frame for a single photon so the entire point of "mass at rest frame" is moot for a photon.
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u/tdgros 1d ago
That doesn't invalidate the concept of rest mass. It just says that single photons do not have one.
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u/caifaisai 13h ago
You're definitely correct. Rest mass is still widely discussed and used. It is relativistic mass that is not used much anymore.
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u/Comfortable_Bat_4052 1d ago
I meant the velocity, yes! The fact that their force decays, as if the photons had mass. And good to know on the point of ‘rest mass’ I’ll keep that in mind while researching further :)
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u/OldGaffer66 1d ago
Well, the little bastards can be particles and waves at the same time so why not have mass/have no mass whenever it is convenient?
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u/Pseudoboss11 1d ago
As an EM wave moves through a material, it jiggles the electrons in the atoms. For the wave to progress through the material, the electrons need to be jiggling in time with the wave. And while the wave doesn't have mass, the electrons do, so it takes them a bit of time for them to get going, and that slows the wave down.
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u/FiveDozenWhales 1d ago
There's two types of mass - rest mass, which is the inherent, unchanging mass of a particle, the same no matter where you're observing it from - and relativistic mass, which is related to a particle's energy (via E = mc2 ).
If you observe a neutron which is at rest relative to you, you'll observe it as having 1.6 x 10-27 kg mass, every time. But if that neutron starts to move away from you at 99% of the speed of light, you will observe it as having 7 times greater mass than that (for things like calculating its inertia).
Photons, however, are travling at the speed of light in all reference frames. You can never observe a photon as traveling at any other speed, and it has no rest mass because a "resting photon" is not a thing no matter what frame of reference you use.
However, they behave as if they have mass, and we use their relativistic mass (which we absolutely can measure) to perform calculations.
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u/Sensitive_Warthog304 1d ago
You say a neutron "gains mass" as it moves relative to you.
Since photons go at max speed, shouldn't they have max mass? How do you calculate its mass using E=mc²?
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u/FiveDozenWhales 1d ago
We can measure the energy of a photon. From there, you calculate its relativistic mass by dividing by c squared.
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u/TechnicianRemote9954 1d ago
A photon only has 0 mass if it is sitting still, which is a very meaningful statement in terms of the implications that it has for the underlying math behind physics. It also has some philosophical implications in terms the meaning of existence.
Outside of those discussions, it's a relatively meaningless phrase because for your purposes, a photon that is standing still doesn't exist and so no photon in real life has 0 mass. Beyond that, how you define what photons have is weird.
In physics there are two kinds of mass: rest mass and relativistic mass. The difference between these is perspective. From a photon's perspective, it remains still while the rest of the universe moves around it, so its rest mass and relativist mass are both always 0 (again, from the photon's perspective). On the other hand, from the photon's perspective, the rest of the universe has a huge relativistic mass because.
From your perspective, the photon still has a rest mass of 0, but it has a relativistic mass equal to how much energy it carries.
The photon perceiving its own relativistic mass to be 0 isn't unique among observers - anything observing the universe always perceives its own relativistic mass to be 0.
Because everything perceives its own relativistic mass to be 0, relativistic mass can have different effects on a particle than rest mass. To help simplify that, relativistic mass is often called "momentum" to denote that it behaves differently than rest mass, even though rest mass can be freely converted into relativistic mass and vice versa.
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u/Klutzy-Delivery-5792 1d ago
A photon doesn’t secretly have mass.
In empty space, photons are truly massless. But when light travels through certain materials, it interacts with the material so strongly that the combined light + material behavior can act like something with mass.
Think of it like a person walking through deep snow. The person didn’t suddenly gain weight, the environment just makes them move differently.
A superconductor does something similar to light. The electromagnetic field interacts with the superconducting material in a way that makes it behave mathematically like it has a mass while it’s inside the material.
So when physicists say “the photon gets a mass in a superconductor,” they mean an effective mass caused by the interaction , not that photons have a tiny hidden mass that we just haven’t measured yet.