I know that nothing with mass can go as fast as light. I think that “there is a universal speed limit” and that :light achieves that limit” are two different statements. So, I am curious about the first one. Is it just an axiom?
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As you speed up, you experience time differently. The speed of causality, or the the speed light travels is when that object is no longer experiencing time. If you were moving at the speed of light, you would arrive at your destination instantly. You can’t go any faster because you arrived instantly. The so called limit is how a stationary observer sees it traveling.
There is a concept called spacetime. We all move through spacetime at the “speed of light”. But the faster you move through space, the slower you move through time. So at the limit of speed, your time is 0. You would need to be going backwards through the time period to offset the space portion. This is not likely possible.
The universal speed limit is a consequence of Mass and Energy being the same thing (from the famous E=MC2 equation).
So to get something to move faster you have to give it energy, the more mass it has the more energy you need to give it to achieve the same speed. That’s pretty intuative, it takes more energy to speed up a car than a baseball.
But giving something energy means it has more mass equivalence so speeding up something that is already going fast takes more energy then speeding up something that is going slow.
Because of this speeding something up to the speed of light takes infinite energy because of calculus.
Imagine you’re standing on a conveyer belt like at the grocery store, only it’s huge – so big that it seems to go on forever in both directions. Now imagine this belt is moving left to right at 1,000 metres per second.
The belt, though, is a smart belt and it adjusts its speed based on how fast you are moving forwards so that your speed is always 1,000 m/s.
So if you’re walking at 3.6km/h (or 1m/s) by Pythagoras we can determine its new speed as sqrt(1000^2 – 1^2) = 999.9995 m/s, so not a huge difference. If Usain Bolt ran 100m in 10s (so 10m/s) the speed would change to sqrt(1000^2 – 10^2) = 999.95 m/s. If you were driving a car at 120km/h (33.3m/s) the belt would move at 999.444 m/s and if it could pull an aeroplane flying above it at 200m/s (720km/h) along, its speed would drop to 979.8 m/s.
If you move at mach 2 (twice the speed of sound, 660m/s) the belt is still moving at 751 m/s and if you reach 866 m/s it moves at 500 m/s, starting to significantly slow down.
We actually move through four dimensional space-time and time is like the conveyer belt. The faster we move through space, the slower we move through time. The reason nothing can travel faster than light is because we already ARE moving at the speed of light; we’re just moving through space-time, not just space, and most of our motion is in the time direction, unless we start moving at speeds that approach light speed.
It is less that there is a speed limit and more that we normally think of speed wrong.
It is a bit like asking why is the north-pool the farthest north you can go or why is -460°F as cold as you can go?
We normally think of speed as open ended an open ended scale where you can always add an extra mile per hour to get a mile per hour more.
It doesn’t really work like this at all.
The faster you go the less the whole adding a mile per hour to increase you speed by a mile per hour works.
There are no velocities that you can add together that come out to more than the speed of light.
In many ways our way of thinking about speed and distance and time in everyday life is wrong or at least just a simplified approximation of how the world really works. This approximation works at small normal human scales, but breaks down if you go to too big a scale.
You may compare that like we normally approximate the world as flat when drawing a map of a small place like a city, but how that works less and less the bigger you go.
The universal speed limit is a limit in the same way that you are limited to arriving at a destination after you started.
Light is just one of the things that go that fast. Gravity might be another and causality itself is thought to happen at that rate.
If there was no universal speed limit, there would be no logical consistency/causality.
That does not mean a universe without a speed limit is impossible, but life as we know it would not be able exist in such a universe.
Yes, it is an axiom for relativity, and thus for all modern physics. We have no _absolute_ proof for it, but everything we know suggests so, which is as good as things can get.
As to “why”… this kind of metaphysical questions will most likely never find a true and proper answer. Even if it some day follows from an even more basic and yet more general theory of nature, this just moves the goalpost. In the end, we can at best make guesses.
For example and not entirely serious now, if one believes that the universe is a just an elaborate simulation, then it might just be there to limit interactions and thus computational complexity; or some programmer used a data type of fixed size (which also somewhat explains Planck lengths).
Because the universe would be inconsistent. If light speed was arbitrary and unlimited, there would be weird frames of reference where cause preceded effect, or grand children would be able to meet and influence their infant grand parents… all kinds of nonsense.
This is going to be an unsatisfying anwer, but the fact is that we don’t know and we don’t really bother to ask. Science is concerned with asking “how” questions, not “why” questions. Asking why a fundamental property of the universe is what is simply isn’t a meaningful question for science. It doesn’t tell us anything useful.
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