I know what the speed of light is basically the speed limit of everything in the universe, and no information can travel faster than light itself. Light also exhibits many interesting properties and also serves as a constant for many formulae and equations in physics. The space-time continuum changes depending on what fraction of the speed of light at which you are travelling.
However, why light? Why is light and only light the one “phenomenon” that dictates most of physics? What makes light so special? My apologies if the question is phrased kind of strange.
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Light isn’t special, at least not in the way you’re asking it to be.
The speed of light is, in fact, the speed of *causality.* It’s the speed that *any* massless particle, or field wave, will travel. Gravitational waves also propagate at *c* (in other words, if the sun suddenly disappeared, Earth would continue orbiting it for eight minutes). If something is one light-year away, there is absolutely nothing you can do that will have any effect on it in less than one year.
We call it “the speed of light” because light was the first thing we realized was going that speed.
We call it the speed of light because we only later realised that the speed is much more fundemental.
Any massless particle will be traveling at the speed of light, and massive ones will always be slower. It is more appropriate to call it the cosmic speed limit or maximum possible speed. As often in science what stuff is called is affected by our history of discovery.
The second part is where light is special is in particle physics. There forces need particles to transmit them. Here the electromagnetic force just so happen to be transmitted by photons.
In general light just interact with alot of things, making it very important for exploring the world, whether you are going for a walk or determining the elemental composition of a distant galaxy.
Light(=photon) is massless (as of today), and it’s possibly the only fundamental particle that have this characteristic. I’d it’s what it makes light so special. That said, any fundamental particles are special.
Because light is a very good reference point when talking about something like relativity. No matter what speed you’re going, the speed of light stays *constant.*
A good example I always heard was the baseball metaphor. Pretend you’re standing at the back of a moving bus going 20mph. You throw the ball towards the front at 20mph. Well, relative to you it’s going 20mph, but to somebody on the outside, the ball is going 40mph.
Light does not do this. Lets say the bus is still going 20mph and you are standing in the back and shine a flashlight towards the front. Well, according to the baseball method of adding it, the light should be going 20mph+C, right? No, it doesn’t since nothing can necessarily exceed the speed of light, and you cannot really ‘accelerate’ light, so the beam cannot go faster than the speed of light already is. The difference between this and the baseball metaphor, relative to you ***and*** the person outside the bus, the light is moving at the exact same speed.
This is why it’s super important, it’s essentially the only thing in the universe that stays constant no matter the reference point. Just like you said, it stays constant across every formula and equation you can think of.
As far as know there are 4 fundamental forces in the universe. Every other “force” is based on these 4. Two of those fundamental forces act on VERY small ranges (ie less than the size of atoms). The other ones are gravity and electromagnetism.
Gravity requires a LOT of mass to work but it extends to very long distances. It keeps planets orbiting stars, gives us the weight we feel on earth. Because of this, at human levels, gravity is generally felt as a “constant”. For us, it pulls us and all the stuff on earth towards the ground. We don’t “feel” the gravity of the sun etc because earth’s gravity is large enough to make all other sources of gravity generally irrelevant day to day (except for tides – caused by the moon in ELI5 terms)
The last remaining force electromagnetism is what causes all the other interactions that we study. Electromagnetic forces can operate at “medium distances” and “medium strength”. This is very useful because nearly everything humans do are at this scale. Things like chemical reactions, why we don’t fall through our chairs, what makes electricity and magnets work, radio waves, light, human biology and physics – all of these are basically various forms of electromagnetism in operation. The object that “carries” electromagnetic forces is the photon. Some photons interact with our eyes and we call it light. Many other varieties of photons are not visible to humans. Since pretty much everything that we experience except gravity operates through photons, knowing how it works is pretty much the bulk of physics.
Now this doesn’t explain WHY it is this way. It is this way because the universe appears to work this way.
Light is easy to access and incredibly powerful measuring tool when used right. As others said, light itself is not that special (I’d say it is regardless, but for different reasons), the constant c denotes the speed at which causality travels. It’s like a lag between cause and effect, where c says “when you’d observe the effects if cause is a certain distance away” (astronomical distances; not on Earth).
The universe is made up of several different fields that are all around us, all the time. When particles travel through them they are affected. One of these fields is called the Higgs field. This is the one that gives particles mass when they travel through it. Light, unlike most particles, isn’t affected by the Higgs field, therefore it has no mass and can travel at the fastest speed possible. Because we figured out that Light is unaffected by mass, we used it as the marker for the constant maximum speed of the universe.
Another interesting way to think of it is to consider not just space, but “spacetime” – the four dimensions we all exist in (3 dimensions of space and one of time.)
Then we can say that everything and everyone – you, me, your cat, and the light around us, are all moving, all together, all at once, at the speed of light through “spacetime”. This “speed limit” is not just a “limit” but is the fixed speed at which everything moves through “spacetime”
Picture “spacetime” as “north and east” and you and I are standing together in an open field. we’re equally matched runners and start running as fast as we can – me due north and you due east. we are going the same speed but I am making progress moving only in the north direction, I don’t move at all to the east. You, the reverse, you make all your progress moving east but none to the north.
If instead I run, say, “northeast” then while we are going the same speed, this time some of my speed is to the north and some to the east. I’m moving to the east slower than you because some of my motion is also to the north.
We are running at a fixed speed “as fast as we can go” but making more or less progress in one direction vs the other depending on our direction.
So movement through “spacetime” is similar. There is only one “speed” anything and everything can move through spacetime. It’s not a range or a limit, it is one fixed speed. The “speed of light”
Some things, like light, see all their motion through space (“north”) and none through time (“east”). This is because photons have no mass. Heavier things that do have mass (like you and me), also moving through spacetime at the speed of light, have slower motion through space (“north”) because some of our motion (in fact most) is also through time (“east”).. the faster we can move through space, the slower we’ll move through time, and vice versa (just like the more we run north the less progress we make to the east)
Everything, everywhere, all at once is moving at the speed of light through spacetime. Light has no mass and so it only moves through space and doesn’t experience time. Anything with mass will have to split their motion at the speed of light between “space” and “time” and depending how fast you move in one of those directions, you necessarily end up moving slower through the other.
So what makes light so special? Nothing other than its being massless.
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