eli5 how do we know nothing is faster than light?
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The key is to learn special relativity, if you understand that, you understand why nothing goes faster than speed of light, or slower. Everything moves exactly at speed of light through spacetime, what changes is only direction.
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I thought I heard somewhere that the universe expansion is faster than light. Is that so and does special relativity not apply in this scenario?
The math for special relativity predicts that it would take infinite energy to accelerate any amount of matter to light speed and also that the speed of time will approach zero as you get to light speed. Beyond light speed it’s all impossible undefined behavior.
The reason we trust this mathematical model is because it makes a lot of other unlikely but easily testable predictions (about color shifts and about the passage of time at high speeds) which have so far all turned out to be true.
Another way to think of it is, we’re *all* moving at the speed of light (or causality). However, your true speed is the sum of your speed through space and that of your speed through time.
The faster you’re moving through time, the slower you’re moving through space.
See, you and I are moving REALLY, REALLY fast through time. We’re moving through time almost as fast as it is possible to do. Thus, our speed in space is actually really slow.
Light is moving as fast as possible through space, thus it doesn’t move AT ALL through time. Time doesn’t pass for the photon.
There’s a curved line on a graph that everything exists upon, as you start going really really fast through space your speed through time starts slowing down more and more, so your spot on the curve shifts but it’s always on that predictable curved line.
So it’s kind of like there is just THE “speed” and it’s all a question of what the ratio between your time speed and your space speed is.
Hypethetically, something could go faster than light. It just isn’t something we know about or are able to observe.
I’m going to break into philosophy for a bit but, if a tree falls in the forest and no one is there to hear it, does it make a sound? If a particle moves faster than light, but you can’t hear it, can’t see it, can’t feel it, can’t smell it, can’t taste it, and it doesn’t affect anything we can sense in turn, does the particle exist, and does it truly move faster than light?
It might, it might not, but until we have a way to observe whether it exists and what it’s doing from our perspective as a species, it does not exist for all practical purposes.
We can’t enter that forest. We don’t know what other trees are out there. The laws and theories presented in science are based on the forest we can observe, not the unknowable beyond.
The math says you can’t.
And we’ve tested the equations out. A lot. And they still work every time.
So the math says you can’t.
Everything moves at the same speed, and we call that speed “c”. It’s just that we don’t just move through space, we move through time. So, we’re all moving through “spacetime”. The faster we move through space, the less speed is available to move through time. Light travels through space at c so doesn’t move through time at all.
It’s not like there is slow and fast, because we all move at the same speed, just in different directions, and one of those directions is time.
It’s not that things can’t go faster than light – it’s that everything in the universe moves at exactly the same speed, all the time. That speed is the speed of light.
There is only the one speed. Nothing moves faster *or* slower. The proportion of that speed that is expressed as motion through space and motion through time is all that changes.
If something is moving with all of its speed through time, it doesn’t move through space. If something moves with all of its speed through space, it doesn’t move through time.
We noticed that light moves with all of its speed through space and none through time, so we named the universal speed after it. But don’t let the name fool you – you’re moving at the speed of light as well.
A long time ago, scientists discovered that massless things (incl. light) always move at the exact same speed, regardless of how fast the source or measurement system is moving.
A very smart guy named Einstein used this fact to do some math. He found that accelerating anything up to the speed of light, takes infinite energy. Which is impossible. Even more importantly, he found that getting mass to go faster takes an imaginary amount of mass. Which is even more impossible.
And things without mass go at exactly the speed of light – no slower, no faster.
Either way, Einstein’s math makes one thing very clear: nothing can go faster than light.
And this math that Einstein did, predicted a bunch of other things. Things scientists can test. Scientists like running tests, so they did (and still do) a lot of them.
And all those tests, match Einstein’s predictions exactly.
So scientists have only one option – accept that Einstein was right.
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