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
It’s a fallout of how electromagnitism works. A changing electric field causes a magnetic field, a changing magnetic field causes an electric field. Given that light is wiggling electric and magnetic fields, it can’t not move.
It’s like asking why a water wave is always moving…you can’t stop a wave, it’s not stable in one position because it’s a dynamic phenomenon that’s driven by it’s own change. It’s only stable if it moves.
This is just me as a layperson trying to figure it out a bit myself, but it seems that a photon begins traveling in the first place because it is emitted from some source, and does so with nothing to stop it until it is absorbed somewhere, by its interaction with other particles.
Traveling with its full value of C in the spatial rather than temporal it experiences no time, and also has no mass, so perhaps even when reflected off of an object it never loses its energy until absorbed.
Again, I’m not a physicist, but I love to learn about it try to visualize it.
**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.
You might as well ask ‘what is light’ at the same time. It doesn’t really have a ‘propulsive’ force. Once emitted, it moves at the speed it does unless slowed down by a medium. But once the medium’s gone, it goes back to its normal speed. Kinda like a wave in that sense. But it also has momentum and other things that make it seem like a particle; but you can’t have a particle just passing through solid objects indefinitely. Light is weird stuff that we don’t really understand why; we just understand that it is the way it is.
More importantly, when a photon flies into your mouth, are you eating it?
Think of light like Goku. Goku wears weighted clothes. Those clothes give him mass. When he takes off the clothes, he goes faster.
Light is the same way, but it took off every bit of mass, which makes it move at the fastest possible speed.
So the “speed of light” isn’t just the speed of light, but the fastest anything can possibly move in the universe without breaking fundamental physical laws.
The reason we will never reach the speed of light is because we have mass.
From a photon’s perspective – there is no sense of motion, because there is no time – as your experience of time relative to the rest of the Universe slows as you approach the speed of light. So traveling at the speed of light – the photon exists at all points simultaneously – with no sense of past, present, or future.
Watch v-sauce’s video “what is the speed of dark”. That will help you go further down the rabbit hole.
To tack onto this, if light doesn’t experience time (unless absorbed or slowed by a medium) does that mean there are still photons out there emitted from the Big Bang?
I took up to physics 3 in college touching on relatively a bit and found it difficult to get conceptualize, but thinking about it this way blows my mind.
If we were on earth and could somehow watch a live video feed of a person on a spaceship traveling at 0.5c, would their actions appear to us to be moving at half speed?
Also, what happens when a photon moving at the speed of light impacts a surface? I know that it will either be absorbed or reflected but what is physically happening when the photon is absorbed and converted to heat
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