If light doesn’t experience time, how does it have a limited speed?


If light doesn’t experience time, how does it have a limited speed?

In: 1056

We don’t know. We have no idea why the speed of light (the speed of causality really) is what it is. We are pretty certain our physical law would work just as well no matter what the speed of light is, but things might seem different if we were in that world, especially if lightspeed was “everyday speed” slow.

What we do know is that this speed limit is the only speed massless particles (without rest mass) can travel, and that at that speed, time doesn’t pass. It’s as if the speed of causality/speed of light is a combination of movement in space and time – move faster through space, you move slower through time, and when you’ve reached the speed limit, there’s no more time left to move through.

It is impossible to say anything meaningful about what light experiences. We’re the ones observing the speed of light, not the light itself.

Even without special relativity speed can only be measured by someone else, from your own point of view you’re always stationary.

Because everything moves through spacetime at the speed of light and photons are a bit of a special case. Right now sitting there reading this, you’re moving at the speed of light. It’s just all in the time direction, so instead of perceiving it as motion in meters per second, you perceive it as moving through time at one second per second. For a photon experiencing no time, it must have all of it’s motion in space and travels at the speed of light.

The speed that light travels is basically the “refresh rate” of the universe. It’s not that light itself is limited by some kind of speed limit. It’s simply the speed at which ANY information is passed along inside the universe. It’s like cosmic download speed that can’t be exceeded. Anything that happens in one part of the universe requires X amount of time to transmit that information to another part of the universe.

Your question doesn’t really make sense. When we talk about experiencing time, or speed, we need to clarify relative to what. Light, or anything moving at c, doesn’t have a valid reference frame making this comparison impossible.

Light moves at the speed of causality. This is the fastest speed something can move and affect something else. So light, like everything else, is limited by the speed of causality but since it has 0 mass it moves at the speed of causality which we generally just call the speed of light.

Edit: Typo

There is no such statement of “limited speed” ever made.

The correct and more baffling statement is that it has a constant speed in reference to the observer. Or in other words, the speed seems to be the same in all directions, no matter if you are already moving or not.

It is as if reality becomes non linear with movement.

Because we’re not measuring the speed of light from the viewpoint of the light itself, but from an outside observer.

Life, the Universe, and Everything happens at the speed of Reality. We used to call it the speed of light, but now we realize it’s really the “Speed of Reality.” Nothing happens faster than that. We’re not sure why, it might just be the fabric of the universe, for which we have some nifty equations you can learn about when you’re older than 5.

I think it’s easier to think of it this way: for some reason the universe has a speed limit, nothing can move faster than it. So light is just moving at the maximum speed the rules of our universe allow.

Think of it this way. We should not be using the phrase “speed of light” because it’s making you think about that speed wrong. Light travels at a speed we call “C” and C is simply the maximum speed for ANYTHING in the universe. All massless particles travel at C. The speed at which you feel changes in space-time is C. And since speed, time and mass are all intertwined somehow, C is when there is no time and mass becomes infinite. None of this makes any sense of course. The “why” behind it is almost certainly far behind our mind’s ability to grasp.

The trick is that everything is moving at exactly c, but in spacetime.
We, entities with mass, are “timelike observers”. This means that, when we don’t do anything special, we’re moving at c-speed in the direction of time (in human language, this means that time passes and we’re not moving through space). We can try “tilting” our movement towards the direction of space and we can, to a certain amount, but we can’t arrive at 45°: that is what we commonly call “light speed” because a “lightlike observer” moves at c-speed in space and time together. This is what we mean by “don’t experience time”: if light moves in time while it moves the same amount in space, our definition of “experiencing time” fails.

It’s a complicated but fascinating subject, if you have more questions try asking on physics.stackexchange.com or drop me a DM!

(source: I’m a theoretical physicist)

It doesnt have a limited speed, thats just the limit of speed itself. The less massive something is, the faster it can go. If you take all of the something out of it, whatever it is, it goes as fast as fast can go.

We dont know why that speed is the max speed. We do know that the faster you move through space, the slower you move through time. So when you go at 100% speed you go at 0% time. If the photon could experience, it would be experiencing the entirity of the universe in a moment.

This https://youtu.be/dGbN0e_urqw series of 3-4videos explain it in an eli5 way. I would highly recommend

As far as we understand it, light does not experience time. To light, things happen instantly. This obviously cannot be the case in the real world. What’s really happening is that c is the speed of causality, i.e. the speed at which things happen. It more or less exists because if things happened instantly it would all be over. So if you replaced the sun with a pool ball it would take 8 minutes for the effects to be felt.

I suggest you to check this YouTube [video](https://youtu.be/au0QJYISe4c)

I’ve never seen such cool and easy to grasp explanation. It made me realize that we’re all moving a the speed of light. Photon just traded time for speed.

So light, or massless particles that travel at the speed of light experience no time AND no distance. From a light particles “experience” it blinks into existence and out of existence when absorbed. A photon that from our vantage point that traveled 10 billion light years traveled that distance and took 10 billion years to get here. That is OUR perspective, from the light particle perspective it experience zero time and zero distance. This is called time dilation and length contraction. In relativity you have to look at the frame of reference. Our frame sees the time and distance, from the frame of reference of the photon it experience no time and no distance.

Because the speed at which you don’t experience time is the speed limit.

You’re always moving in two kinds of directions: time and space. The reason you can’t go faster when time is not moving is because you put all your motion in the “space” directions so you have none left over for the “time” direction.

The opposite is standing still: this is when time is moving fastest for you, because you out your motion in the time direction so you have none left over for the space directions.

It’s just something that’s baked into the universe itself. Like the current top comment says, it would be better if we actually referred to the speed of light as the speed of causality. Light is just the top racer who is stuck at that speed limit.

As for it being baked into the universe, it’s a bit like asking why if you walk in a straight line on a sphere, you end up where you started. That’s a feature of living on a sphere. The speed of causality is a feature of living in our universe.

As to why this speed is what it is, that has to do with the complex shape of the universe, similar to how the diameter of the sphere will determine how long it’ll take you to get back where you started on your trip across the surface of the sphere.

The time part of OP’s question is stuck in some confusion about the unitary nature of space-time and the mathematical singularity of infinity. Easier to say that to ‘experience’ time requires a conscious observer for the question to make any sense. That in turn will take you to Einstein’s wonky thought experiments with time dilated observers. Time will pass normally within every observer’s frame of reference of itself (even near light speed) but will look very weird when they observe something else. Like a very slow observer (us) observing something very fast (light).

I always figured that traveling at light speed, if you faced the direction that you were traveling, then you would see nothing, blackness. Turn 180° and you’d see the rest of your “ship”. Face either side and you’d see a split of black and normal vision, separated by your own personal visual event horizon. That’s what makes sense to my brain