I’m wondering if interstellar travel is possible. So I guess the starting point is figuring out FTL travel.
In: 1302
FTL travel is going back in time. This can create causality paradoxes. Which the universe seems to rly hate.
The speed of light is the limit because its the speed of causality.
For anything with mass to go just as fast as light they need infinity energy… there is just no such thing.
the secret is time dilation. The speed of light is constant for all observers no matter what speed they are moving at. So if you speed up to a significant proportion of the speed of light, time ACTUALLY slows down for you (and length starts to contract). How much time slows down is relative to how fast you go, so if you were to get really really close to light speed and waved, everyone else in the universe would see you waving very very slowly.
Here is a good series and visual intuition for it. https://www.youtube.com/watch?v=1rLWVZVWfdY&list=PLoaVOjvkzQtyjhV55wZcdicAz5KexgKvm
To get something to go faster you have to apply force to it.
The heaver an object is the more force it takes to make it go faster.
Because of “weird physics stuff” as an object approaches the speed of light its mass increases. All of this added together means that it would take infinite force to accelerate something to the speed of light because it would approach infinite mass the closer to the speed of light it went.
Einstein’s equation says accelerating anything with mass to the speed of causality (also light) is impossible because it would require infinite amount of energy
it does not however say anything about somehow (teleporting, bending space and other ideas that may or may not be possible) traversing distance in x seconds that would normally take light to traverse y years
The best example I’ve heard is: If you could have waves transmitting your phone call travel faster than light, your “I’m good” would be said before you heard “How are you?”. Which is fascinating concept, and, impossible.
One of the reasons we think it’s impossible is that the speed of light is less about light itself and more about information. In order for *things* to happen information needs to be passed around to allow other things to respond and, in the case of life, perceive. If you could exceed the speed of light, you would be sharing information faster than information can be shared. Cause and effect would break down because other things should have reacted to your presence, but couldn’t because you arrived before you arrived.
EDIT: This blew up. A number of people have asked some good questions, so I’ll cover off two of the recurring and related ones: are black holes faster than light because light gets trapped, and how do we know that the maximum speed limit isn’t just higher?
Black holes are not FTL. They don’t trap light because they’re stronger. They trap light because light is running around the lip of a really steep bowl, and eventually light gets tired and slows down, and begins to spiral in to the centre, [like a penny in a wishing well collector](https://youtu.be/XTipCQxJ6Ak?si=kgE-mmxp-007qSEH). Light is too slow to climb back out, but not because the black hole is FTL.
Knowing this, we can answer the second question. We do have evidence that suggests the maximum speed isn’t higher: nothing else seems to exceed the speed of light either, but does move at it, such as gravity. If everything that can move at light speed seems to stick to that speed as the maximum, it seems odd to suggest that they could move faster but don’t. You then have to ask: if the maximum speed limit is higher than the speed of light, why are multiple phenomena that aren’t light moving at the **same** speed as light? If we find something that does you can guarantee that we’ll update the textbooks, but the more we understand, the more we seem to find phenomena that don’t go faster than light, which is suggestive.
For anyone asking “why isn’t the speed of light N+1?”, take it up with God. I can only really either refer to design or to random chance with regard to universal constants (I favour the latter personally, but that’s just me). Its presently not a question we can even hope to test, so any answer will be either pseudo or fully religious.
I strongly recommend to watch as many videos as possible from this fantastic YouTube channel called “ScienceClick English” and, in particular, their 1st one: it will open your mind a lot more than most other videos or papers. This channel has a unique approach to simplify things so much that anyone can grasp concepts that usually are accessible to highly specialized people
[https://www.youtube.com/watch?v=au0QJYISe4c](https://www.youtube.com/watch?v=au0QJYISe4c)
It seems to be a property of the universe, postulated by theory and confirmed by multiple experiments. The consequences of which are the basis for Einstein’s theories of General and Special relativity, both of which have been confirmed many times by observation and experiment. If faster than light travel is possible, it will mean that those theories and observations will be have to be explained in a new way.
To answer your follow on question, interstellar travel is possible.
The nearest star is 4 light years away, so if we could travel at half the speed of light it would take eight years to get there (as viewed from Earth).
With time dilation (see responses from other commenters) the people on the spaceship would experience it as taking even less than 8 years.
The difficulty is creating a spaceship which can go half the speed of light (relative to Earth), because that’s a really fast speed, so would take a lot of energy to get to that speed.
Currently if we sent out a spaceship it would take tens of thousands of years. In that time we could build a faster spaceship which would overtake it, so sending one out now wouldn’t be that beneficial.
The speed of light is better described as the speed of causality. The universe is built with a speed limit for cause and effect. Light just happens to travel at that speed. As we travel closer to that speed, time passes slower for us.
Let’s say we wanted to travel to a far away star system, a thousand light-years away. Let’s also say that (via some magical process) we had a spaceship that could accelerate us up to 99.99% of the speed of light almost instantaneously (and that we could survive that acceleration – because magic). From our perspective, the distance to that far-away system would drastically decrease. We would get there relatively quickly, no problem, and only experience a short period of time on the trip.
From the perspective of an observer on earth, however, it would still look like the trip took over a thousand years. If we were to go there, have a stroll around the solar system, then come back, Earth would be over two thousand years in the future but we would have only experienced a fraction of that.
Another way of looking at it is that we are always moving at the speed of light, if you also consider the axis of time. The faster you move through space, the slower you move through time, and vice versa. A video that explains this better than I could is here: https://youtu.be/au0QJYISe4c?si=gDl3Sie67lv_IQGg
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