You always hear this phrase if you watch something about astrophysics ‘Nothing can move faster than light’. But speed is only relative. How can this be true if speed can only be experienced/measured relative to something else?
In: Physics
The explanation that worked best for me: the speed of light is not so much the speed of *light*, but of *causality*: nothing can cause an effect on something else faster than that speed. Since light is not limited by anything (at least in a vacuum), light travels at max speed of causality, ergo, the speed of light is the fastest that anything can go.
Wow, OP. You’ve asked the very same question that Einstein asked himself to come up with one of the most revolutionary ideas in physics!
You are correct that speed is relative. If I’m walking up an escalator at 2 m/s and the escalator is moving at 5m/s then my speed relative to a person standing still at the bottom of the escalator is 7 m/s, but to someone else on the escalator who is standing still and waiting patiently for the escalator to transport them to the next floor my speed is 2 m/s.
But light travels at the same speed from all perspectives. Say a spaceship is traveling at 90% the speed of light. If I shine a torch from the back of the spaceship to the front and someone on the ground can see through the spaceship’s window, then the light from the torch will appear to move at the speed of light to both of us. But the escalator example would suggest that to the person on the ground, it should be traveling at 90% of the speed of light + the speed of light i.e. at 190% of the speed of light. So how can it appear to move at the speed of light to both of us? Well, if the person on the ground is looking through the window and everything in the ship (including not only the beam of light from the torch, but the people inside the ship) is moving in slow motion, then the beam of light can appear to move at the speed of light.
Mind blowing, eh? To solve the paradox, time must be relative! Time inside the ship appears to be slowed down to the person on the ground, and conversely everyone outside the ship looks like they’re running around like ants to the people inside. Actually, there’s a bit more to it than that, since distances are affected too. But thinking about it like this is a good starting point.
It is hard to ELI5, but here goes:
You are standing beside a train, you shine your flashlight at someone else standing beside their train, they shine their flashlight when they see yours. You measure the time for the round trip, you get the speed of light.
You both get on your trains, heading toward each other. You shine your flashies, you measure the round trip.. wow, same speed of light.
You both go real fast, say leaving the station at half light speed each toward each other. You shine your flashies, you measure the round trip.. same speed of light.
Now, the reason for this complicated, but essentially your point of view, your perspective, is where all the distances and times are measured from. And those numbers don’t work by adding up, when they get closer to the speed of light, the figuring starts to distort.
For example if you hop on a spaceship and head toward a distant planet and start accelerating. The rest of the universe, including the distance to that planet, will seem to get shorter (not just because you are moving that way). Essentially you turn everything into pancakes. But, people on that other planet see that happening to you.
The takeaway, is that is actually how movement works, our ability to add and subtract distances and speeds is actually the weird little simplified version of reality that we get to live in.
This is long, but it breaks down the problem/answer in details.
The rule concerning the speed of light, more specifically, would be that nothing can travel faster than the speed of light *relative to you.*
Now, you as an observer to something else, would be what is referred to as a ‘reference frame’. Think of yourself as the big clock in town square that everyone else uses to synchronise their own wristwatches. You’re the baseline, the standard, the metric against which other things are measured, because as far as you’re concerned, you’re the centre of the universe. You and your outwards perception of the world is simply **zero.**
All speed is relative. If I’m standing still and a car whizzes by at 100mph, then I see it moving at 100mph. But if I’m in a car next to it that’s doing 95mph, then I only see that car inch forward at 5mph. Any speed you experience is the relative difference between you and all other moving things. Are you moving forward at 95mph, or is the Earth is turning backwards at 95mph? There is literally no actual empirical way to answer that question, because physically there is no difference.
Meaning, if I travel forward at 10mph and something else travels towards me at 10mph, it feels the same as if I’m staying still and it moves towards me at 20mph, right? Correct. Two cars moving towards each other at the same speed would takes the same amount of time to collide as one car staying still and the other car moving at twice the speed. The relative motion between the two is identical.
So, if I’m moving towards a photon at 1mph, and that photon is moving towards me at the speed of light, then the relative speed between us is the speed of light +1mph, right?
Nope. It’s just the speed of light. If one of the two objects is moving at the speed of light, the speed between you and it is only *ever* the speed of light. It doesn’t matter how fast you’re moving. 0mph, a million mph, it simply makes no difference.
This phenomenon sounds like a space-breaking paradox. If it works at 100mph, why not an arbitrarily high speed? What happens, where’s the shift? Despite the oddness of it, this behaviour is an absolute proven fact, as sure as gravity pulls you down and the sky is blue. Even if you move towards something travelling at the speed of light and that something moves towards *you* at the speed of light, the relative speed between you is… still just 1x the speed of light.
This trippy phenomenon is called *frame invariance,* and is the founding principle on which Einstein based his theory of relatively, which describes how time is relative and is not static between two different frames of reference. Frame invariance says everything I’ve described in a single sentence: *”The speed of light is invariant* [does not change] *between inertial* [no acceleration; constant speed between the two] *frames of reference”.*
So, the two assertions you’ve made – that speed can only ever be relative and that nothing can move faster than light – are both true, both at the same time. If you’re moving towards something at the SoL and it moves towards you at the SoL, the speed between you is still just the SoL. It would take the same amount of time for that photon to hit you as if you just stood still and waited for it to arrive. If you were travelling alongside a photon at the SoL and you were just ever so below the SoL, it would still move away from *you* at the SoL. It would move away from a stationary observer at the SoL, even though you’re moving relative to them, and so on. It makes no sense, but our monkey brains simply are not equipped to conceptualise how seemingly broken physics starts to become when you approach speeds this high.
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