i’m in a physics rabbit hole, doing too many problems and now i’m wondering, how is light moving? why?
edit: thanks for all the replies! this stuff is fascinating to learn and think about
In: Physics
**This is *very much* an ELI5 explanation to lay out the basic concepts. As with a lot of things at this stage, the actual mechanics of it are… *complex*.**
Light is always moving because *everything* is always moving — or would be, if it had its way. The default speed of the universe is the speed of light. If there’s nothing to slow you down, there you are, bopping around at *c*.
So… why are *you* not moving at the speed of light? Well, it’s because of your interaction with *other stuff*. If you have a particle that interacts with something called the Higgs field, it has *mass*. This mass has a lot of cool properties — being able to touch it is a very popular one — but another is that it requires more energy to get you moving. That makes sense, right? You need to push a dump truck a lot harder to get it moving than you need to push a bike; heavier (or ‘more massive’, with a couple of ELI5 fiddly bits) things require more energy to move faster. If you have *any* mass, though, it’s impossible to get enough energy in you to get you to the speed of light. You’d need an infinite amount, and that’s just not going to happen. The only things that can move at that speed are things that are already going at that speed — and that don’t have any interaction with the Higgs field at all.
A photon is a massless particle. It doesn’t interact in a meaningful way with the Higgs field (in a vacuum, at least), which is what gives mass-having particles their mass, so there’s nothing to slow it down. It just runs at its own speed, which is the speed that everything in the universe would run if it didn’t have anything getting in the way.
#Extra Credit/Follow Up Questions/Pedantry Corner:
**Why do massless things move at the speed of light in the first place?**
You’ve kind of got the question backwards there. It’s not that massless things move at the speed of light; it’s that we call things that move at the speed of light ‘massless’. That’s how we define what ‘massless’ means.
If you think back to Bill Nye and ‘inertia is a property of matter’, that’s what we’re talking about. (Inertia is the property by which something will either stay at rest or stay in uniform motion in a straight line, unless that state is changed by an external force.) Inertia is something that only things with mass have.
**Why does gravitational lensing happen?/Why are massless particles unable to escape from a black hole?/How is a massless particle affected by gravity?**
If photons are massless, how are they affected by the gravity of large and dense objects like planets or black holes? Well… they’re not. Not exactly, anyway.
You’ve probably seen the physics demonstration of spacetime, where [a large elastic sheet is weighed down by a heavy object](https://www.youtube.com/watch?v=MTY1Kje0yLg), and items rolled across it in a straight line seem to curve their paths. It’s not that the items are being rolled differently, but the fabric of ‘spacetime’ is being warped, so what *looks* like it should be a straight line is now curved. Black holes warp the spacetime around them so much that nothing can get out, including light; the mass of the photon (or not) doesn’t make a difference.
**The Higgs field only accounts for a small percentage of mass in the universe…**
Yes, technically; only a very small percentage of mass comes directly from interaction with the Higgs field, including the mass of subatomic particles called *quarks*. The rest of it comes from what happens when you cram together a HUGE amount of energy into the tiny space of a proton or neutron, held together by the Strong Nuclear Force. (If you remember Einstein’s equation of mass-energy equivalence, E=mc², what that basically means is that you can convert a tiny amount of matter into an enormous amount of energy, and vice versa. This is the principle by which nuclear fission occurs; when you ‘split’ an atom, that energy is no longer held in place as matter and just goes everywhere.) The three quarks inside every proton and neutron are held together by a buttload of energy, which ‘becomes’ matter due to mass-energy equivalence. Even though the energy *itself* doesn’t interact with the Higgs field — as I understand it, anyway — the quarks do.
If not for the Higgs field, you wouldn’t have quarks; if you didn’t have quarks, you wouldn’t have the building blocks of matter as we know it today. If it helps, you can think of it as just an extra step. (And remember, this is ELI5; there’s only so much detail you can go into before it becomes impenetrable.)
**It’s the speed of causality, not the speed of light…**
Sure, technically — they’re the same thing, and things like gravitational waves also travel at that speed — but ‘the speed of light’ is much more likely to be something your average Joe has heard of. (Again, this is ELI5; it’s about explaining things to people who don’t have a background in this sort of thing.) ‘The speed of causality’ is probably a more accurate description, and if it helps your understanding, go for it, but [if ‘speed of light’ is good enough for Wikipedia](https://en.wikipedia.org/wiki/Speed_of_light), it’s good enough for me.
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