eli5 how do we know nothing is faster than light?


eli5 how do we know nothing is faster than light?

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The branch of physics that deals with things going really fast (special relativity) breaks if you go faster than light. Masses & energies head for infinity, time stops working, total weirdness ensues.

It’s *possible* that this is a math artifact of an incomplete model but the special relativity equations have been proven to an astonishing degree of accuracy so, right now, all evidence says they’re correct and no evidence says they’re wrong (at least about the speed of light part).

Basically, *if* you can go faster than light, there’s a whole branch of physics that’s totally incompatible with everything we currently understand. It’s possible but there’s currently no evidence for that (and a huge amount of evidence against it).

We know we can’t push a regular object up to light speed because that takes infinite energy and we don’t have that. That’s not quite the same as not being able to have something that’s already going faster than light and just stays there, but it’s not even clear how we’d detect something like that (and we have no idea how to create it).

We know that mass increases with relative velocity. At light speed, the mass of any particle would be infinite and we do not observe any infinitely-massive particles.

Photons have no mass. Their velocity therefore does not require them to be infinitely massive.

We looked at lots of things moving, and saw some strange things happening. Specifically, we did an experiment called the “Michaelson Morley Experiment” that showed, it doesn’t matter what direction you move, or even if you’re moving at all, light you see is always moving at the same speed.

Then, Einstein decided to use that observation to make his theory of relativity. This theory makes a bunch of really weird predictions, like that space and time warp and move in just such a way as to always keep things slower than light.

We are able to test those predictions, and thus far, every single test we have made of Relativity has come back confirming it.

If we find a test that deviates from Einstein’s predictions, then we’ll have to change our theory. But it still has to make the same correct predictions Einstein made, which means it still has to keep objects below light speed.

We have no law that accounts for faster than light travel at the moment so we can’t theorise such a thing using given laws. We can’t see the either as they are so fast (if they exist?)
These laws change with time, we didn’t know about sub particles before someone theorised them (higs bosin?)

There’s a few ways to approach this topic, all related but coming from different perspectives. And each is grounded in the best understanding of physics we have, tested to amazingly precise accuracy.

First, we know that the faster an object (with mass) moves, the less time it experiences relative to slower moving things (this is really tricky to explain properly, but it’s well-established experimentally). As something approaches the speed of light, an observer who is not moving at that speed would see the clock on that something tick slower and slower (and a clock doesn’t just mean an actual clock – it’s anything that changes at regular intervals). The closer it gets to the speed of light, the more the clock appears to slow down. And if it ever reached the speed of light (which is not possible for any object with mass – more on this below), its clock would stop and it would experience no passage of time at all.

Second, it takes energy to move an object faster and faster, and the faster you want it to go, the more energy you need. As an object approaches the speed of light, the energy needed to increase its speed even a tiny tiny bit is huge. And if it could reach the speed of light (which it can’t), infinite energy would be required. This is simply not possible.

Third (and this is the flip side of the second point), the faster an object moves, the more “massive” it becomes – in a sense, it weighs more and more as it moves faster and faster. As it approaches the speed of light, its mass approaches infinity. Infinite mass is impossible.

All of this stems from Einstein’s special relativity. It’s always possible that this model of how the universe works is wrong, but all evidence we have to date indicates otherwise.

With respect to things without mass (technically “rest mass”) like photons or gravitational waves, they can move only at the speed of light. They do not experience the passage of time. A photon that was created in the big bang and travels through the universe for billions of years would not experience any time.