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
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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