The way I think I understand it, even if we had some “element 0” like in mass effect to keep a starship from reaching unmanageable mass while accelerating, faster than light travel still wouldn’t be possible because you’d be violating causality somehow, but every explanation I’ve read on why leaves me bamboozled.
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Part of why it’s confusing is because causality is, more simplified, cause & effect.
Think of it in terms of say, writing an email and sending it out. If we violate causality, the email would already be sent out before you wrote it.
Another example with sound; bouncing a basketball makes a sound. If we violate causality, then it’d be the other way around, you hear the bounce sound before the ball bounces.
This leads to all kinds of paradoxes – not necessarily as ‘deep’ as the grandfather paradox or other time-travel paradoxes, but still leads to situations that can’t or don’t make sense, *because* they violated causality at some point.
Imagine your friend Bob is in his bedroom playing with a ball. Bob throws the ball to you, and you catch it. There’s a cause (Bob throwing the ball) and an effect (you catching the ball). This makes sense because Bob is right there in the same room as you, and you can see each other.
Now imagine that Bob is on the moon and you’re on Earth. Bob throws the ball really fast towards you, faster than the speed of light. According to science, the ball can’t actually go that fast, but let’s pretend for a moment that it can. Even if the ball could travel that fast, you still wouldn’t be able to see it coming because the light from the ball’s movement wouldn’t have had time to reach you yet.
So when you suddenly catch the ball out of nowhere, it seems like it happened for no reason, because you didn’t see it coming. That’s what violating causality means – there’s no clear cause and effect. This creates a paradox where something can happen before, during, and after something else all at the same time, and science doesn’t know how to make sense of that.
So I can walk at a speed of 5mph. If I am on a train that is going 30mph, I can walk forward and be going 35mph, even though I am still only capable of walking 5mph. You would say that I am walking 5mph in the reference frame of the train, and I am walking 35mph in the reference frame of the ground
In physics, we call the math that lets us describe the same motion in two different reference frames a coordinate transform. At low speeds, the coordinate transform is fairly simple (5mph+30mph=35mph). At high speeds, it becomes more complicated, to the point that if something is traveling faster than light in one reference frame, that is equivalent to saying it is traveling backwards in time in another reference frame.
It would violate cause-and-effect. This explains it better than I can: https://en.wikibooks.org/wiki/Special_Relativity/Faster_than_light_signals,_causality_and_Special_Relativity
You have to think of causality as a reaction. Whatever the “causality” event is in any hypothetical is a reaction to some other form of stimuli. THAT stimuli can only travel as fast as the universal speed limit.
It’d be like accurately yelling that gondor calls for aid 10 minutes before the light of the Beacons ever hit your land.
Because you would arrive before the information surrounding your arrival would arrive.
i.e., If you moved a Star faster than c, it would arrive at its destination before its gravity well arrived at its destination.
What might be the only possible way for a FTL travel would be worm hole as this wouldn’t really be violating FTL. You enter below FTL speed, you exit below FTL speed, you just appear somewhere else, but all the information of your arrival would arrive as it should.
One thing:
“Accelerating to faster than light” wouldn’t do anything, because you can’t do that. It’s not because it violates causality, that would be a consequence IF you did it. But you can’t do it not because some higher force find causality sacred, it’s simply because nothing with mass can reach the speed of light. That’s the only reason.
“Speed of light” is the maximum speed any massless particle (not just light) can travel. It’s the maximum speed information can be exchanged. Because there is an upper limit to information, sequence of events can be determined.
As a consequence of that, despite all the wackiness with Relativity, where two observers might disagree on the order of two unrelated events, they will agree on causality.
In other words, if A causes B and C causes D, two observers can disagree whether A happened before C or B before D, but they will agree that A happened before B and C before D.
With no speed limit, if information from C got to you before A, you could say C caused A. But someone else somewhere else would say A still caused C. At that point the world just stops making the bit of sense it still had.
Here is an example of faster than light travel breaking cause and effect. Other situations with similar results exist, but this makes it pretty clear and easy.
Imagine you have two ships that are passing each other at near the speed of light. So long as they do not change velocity or pass through a gravity well, relativity theory tells us that each ship sees the other as having time be slower on it. This time dilation effect has been tested by experiment many times, so we know that this is real.
Imagine that you also have a faster than light portal that allows you to instantly travel between the two. The two captains of the ships look through the portal all the time, and when they do they always see the other captain as living at half the speed that they are.
The two captains of the ships, Alice and Bob, get into a violent quarrel and decide to have a duel with pistols. They face each other through the portal, count out ten seconds, and fire.
Alice counts out 10 seconds and fires at Bob. However, from her point of view, Bob has only reached 5 seconds. Since the portal is instantaneous, her shot goes through the portal and hits Bob when only 5 seconds has passed.
Bob is thus shot only 5 seconds into the duel, but is not killed. He is outraged that Alice shot early. He fires back after 2 seconds. (Edit: From his point of view.)
Edit: I made a mistake in the next paragraph. Please read the correction below instead.
Because Alice sees Bob as moving at half speed, Bob shoots her 4 seconds after Bob is shot. Since Bob was shot at 5 seconds, Alice is shot at 9 seconds. Alice has killed instantly and thus never got to shoot.
Correction: From Bob’s point of view the shot came out of the portal at 5 seconds, but Alice is still at only 2.5 seconds because she is half as fast as he is. He does not see her shoot because she will not shoot for another 5 seconds, but he responds in 2 seconds believing that she has done so. Since he sees her as having been at 2.5 seconds when the shot came out of the portal, and in the two seconds he has taken to return fire she has only experienced one second from his point of view, his shot will enter the portal and hit her when he has experienced seven seconds and she has experienced 3.5 seconds, not 9.
Faster than light travel can almost always be turned into time travel.
The faster you go, the slower time goes.
We have proven this with clocks on space ships.
If you go as fast as you possibly can, time stands still.
If you go faster than as fast as you possibly can, you go back in time.
Going back in time violates causality. You can go forward in time as much as you want, but you can’t go backwards
In 3D, when you’re standing still, time is flowing by and you’re not moving through space. When you’re moving, time is still flowing by at the same rate, and you are moving through space.
In 4D spacetime, this isn’t the right picture. In 4D, you’re always moving at the same rate in a 4D direction. Your 4D velocity is constant all the time. If you are standing still in space, that means all your motion is happening in the time dimension. If you start moving through space, then the direction of your 4D arrow isn’t fully pointed along the time dimension, now it also points in one or more space dimensions a tiny bit. This means that your overall 4D velocity has not changed, but now some component of it is in the space direction and the rest in the time direction.
This is the reason you can’t travel faster than light: If it were possible to point your 4D velocity vector completely in the space direction such that the time component were zero, you would be traveling AT the speed of light, not faster. (You can’t actually do this because it takes an infinite amount of energy to point the 4D velocity vector completely into the space dimensions for any object that has mass. But if you could, you’d be going c.)
So it turns out that the total 4D velocity is an invariant for all things at all times. In order to go faster than c in a space direction, you’d have to increase the 4D velocity, which means changing an invariant, a thing that cannot change.
Too hard? Let’s make it simpler.
Imagine a 2D person in a sheet of paper. Paper person has a 2D matchstick one inch long. When they point the stick in some direction in the paper, the stick (and paper person) moves in that direction at a rate of one inch per second. So they just zoom around the paper all day, pointing the stick in some direction they want to go, and it moves them along. (When they get to the edge, they just wrap around to the other side of the paper. They don’t really know when this happens because it all just looks flat to them.)
One day, paper person discovers they can push the matchstick up out of the surface of the paper a little bit. When this happens, the component of the matchstick along the paper still makes them zoom around (think of this as the shadow of the matchstick that falls on the paper, which paper person can’t distinguish from the matchstick itself). And the component of the matchstick perpendicular to the paper moves the entire sheet up or down in that direction at the speed of that component.
So now imagine the matchstick is pointing at a 45 degree angle to the paper. Paper person is moving at reduced speed in the paper, and the entire paper is moving up at the same speed. How fast is paper person moving? Well, if you do the math, still one inch per second. No matter what direction the matchstick points, they’re always moving through 3D at an inch per second.
To paper person, they can’t really tell when the entire sheet is moving up or down, they only perceive their motion relative to the paper. If they point the matchstick completely up, they stop moving at all in the paper, and now the entire paper is moving up at an inch per second.
The question you’re asking is: Why can’t the paper person ever move faster than an inch per second? Because the matchstick is only one inch long. No matter the direction of the matchstick, the combination of movement in all the directions is always and forever going to be one inch per second.
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