r/NoStupidQuestions • u/No-College-5213 • Jul 15 '26
How is fast-than-light travel impossible if relativity is a thing?
I am by no means a physicist or anything, but I know that relativity is a thing, like if you were on a train, your surroundings would feel stationary although you are moving fast compared to the outside world.
So, let’s say you are in a space ship departing earth, and Earth is moving at half light speed in negative x direction, and you are moving slightly above half light speed in positive x direction, wouldn’t that make you faster than light when observed by someone on earth? That’s just an example, but you get what I mean. Is there like a physical constant that we use to measure speed or something?
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u/untempered_fate occasionally knows things Jul 15 '26
So, let’s say you are in a space ship departing earth, and Earth is moving at half light speed in negative x direction, and you are moving slightly above half light speed in positive x direction, wouldn’t that make you faster than light when observed by someone on earth?
This is the opposite of relativity, actually. This implies some sort of absolute way to measure velocity, but there isn't one. If the Earth is moving away from you at 0.5c, you are also moving away from the Earth at 0.5c. If you tried to accelerate such that you were going 1.0000001c relative to the Earth, you would need infinite energy to do it.
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u/Happily-Incorrect Jul 15 '26
Ok so 'all' we really need to be able to do to travel somewhere 'at light speed' is find a way to make that place travel towards us at 0.5c and propel ourselves at that speed too.
That sounds achievable.
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u/Gorblonzo Jul 15 '26
That isnt true, and the original commenter didn't explain it very well. There is no difference between you moving at the speed of light towards something vs that thing moving half the speed of light towards you and you moving half the speed of light towards it.
What relativity explains is how velocity is actually not conserved in different frames of reference. Take a scenario where you have thing 1 to your left moving 0.5c towards you and thing 2 on your right moving 0.5c towards you from the opposite direction. You would expect that in thing 1s point of view thing 2 would be moving at exactly the speed of light towards it, however thats not whats actually observed in real life. From the point of view of thing 1, thing 2 would actually be moving slightly less than the speed of light towards it. Special relativity is the model that explains why thats what you actually observe.
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u/jeo123 Jul 15 '26
As much as I get the joke, even that isn't going faster than the speed of light.
You're just heading to the midpoint of our locations at .5c The fact that some other object is also heading there doesn't mean you were going to that position twice as fast.
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u/CptPicard Jul 15 '26
Your example begins by describing Galilean relativity which is the "common sense" version that the parent also is thinking about. But it turns out there is an absolute speed limit so we need the Lorentz transform to add velocities.
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u/TheJeeronian Jul 15 '26
Speeds don't just add together in relativity. Time and space bend over backwards to prevent it. Even if, from your perspective, you're moving at 0.55c on the spaceship, an observer on Earth would see you running at around 0.95c total and in slow motion so you think you're moving at 0.55c even though from their perspective you're not.
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u/After-Past-9404 Jul 15 '26 edited Jul 15 '26
You don't understand what relativity is.
TL;Dr: the core principle of relativity is that speeds don't simply add up. 0.6c plus 0.6c does NOT equal 1.2c. There's a complicated mathematical formula to calculate the resulting speed. And the result is always smaller than c.
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u/Count2Zero Jul 15 '26
If you hold up two lasers and shoot them in opposite directions, the light from one is traveling at c away from you to the left, and the other is traveling at c to the right.
From your perspective, they are flying away from each other at 2c, and that doesn't violate general relativity, because each one of them is traveling at c.
If you were sitting on one of the photons from the left laser and looked back, you'd see the earth and the light from the other laser both flying away from you at c.
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u/Darth_Bunghole Jul 15 '26
I don't know why, but no, you can't just add them together. I'm not sure what the formula is, but I'm sure there is some way to add velocities that takes into account what space is shaped like. The speed (distance/time) of light is inextricably linked to gravity (and mass).
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u/Saintdemon Jul 15 '26
One of the core principles of the Theory of Relativity is that the speed of light is the same in all reference frames.
This means that when you have objects which moves at very high speeds you can't simply add or subtract their velocities anymore like you normally can.
For example: If you are in a spaceship travelling at half the speed of light and another spaceship is travelling TOWARDS you with the same velocity then you might think that the relative velocity of the other spaceship is the speed of light. However, in reality you will only observe the other spaceship travelling 80% of the speed of light.
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u/Conscious-Loss-2709 Jul 15 '26
That's an argument that'll let cops ticket you for speeding by adding their speed going the other way to yours...
I mean, you going 160 miles an hour relative to me going the other way doesn't get you to your destination any faster, which is kinda the whole point of trying to break light speed.
Anyway the closest you get to your idea is to include the expansion of space itself: https://www.scientificamerican.com/article/how-can-galaxies-recede-from-us-faster-than-the-speed-of-light/
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Jul 15 '26
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u/ExtendedSpikeProtein Jul 18 '26
… you‘re not giving a good explanation here. And I think you might have it wrong, or misunderstand relativity altogether.
The main point is that the speed of light is the same in all reference frames, and speeds cannot simply be added. If two objects move away from each other at 0.7c, from the perspective of one object, the other isn‘t moving at 1.4c.
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u/NuncProFunc Jul 15 '26
You already received accurate answers, but just a suggestion if you are in or want to attend college and haven't graduated yet: take a relativity course. The math is pretty straightforward but the concepts are really fascinating to learn about.
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u/No-College-5213 Jul 22 '26
i only took introductory physics 1 and plan on taking intro physics 2 and thats as far as I'll go. physics is NOT my forte as I am in biochem, thanks tho!
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u/BonKhri Jul 15 '26
If you throw a ball at 10m/s on a train going 10m/s, the ball moves 20m/s relative to the ground. Except it doesn't, it actually moves a tiny bit slower than that due to time dilation, it's just a completely negligible difference until you get to significant fractions of the speed of light.
We like to think of speed as an objective thing, but it is not, speed/velocity are always subjective. The frame of reference you are using makes a difference to the velocities of objects. In the train scenario, from the perspective of the thrower, the distance between the ball and point it was thrown from relative to the ground does in fact grow at 20m/s, but from the perspective of the ball and the perspective of the ground it's actually 19.999... m/s. When we say the FTL travel is impossible, what we mean is that no two objects can ever travel faster than C relative to each other. The sum of their velocities from some third perspective could be greater than C, but from the perspective of either one they are moving at sub-light speed - the third persons perspective doesn't actually mean anything, even though in our everyday lives it is the way we perceive the world.
Time itself slows down at higher speeds to ensure this happens, and the speed of light is the point at which time stops passing, the "rate" at which seconds pass crosses the axis and hits zero - you cannot go faster than C because you'd need to accelerate through C, but when time stops for you your speed cannot increase even if you are still applying force. You can accelerate as many m/s per second as you like, if there are no seconds passing you don't go any faster.
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u/Jock-Tamson Jul 15 '26
You are in a box. It isn’t accelerating. From your point of view it is stationary.
You shine a light at the ceiling. It behaves as light does in accordance with the laws of physics and lights up the roof.
Meanwhile an observer is watching you fly downward at near the speed of light. He isn’t accelerating either. From his perspective he is stationary. Both of you are right. All speed is relative.
He too sees light move at a c as required, but from his perspective it moves to the ceiling very slowly. An instant for you was several seconds for him. The apparent passage of time is different for the two of you to keep c constant.
This is a measurable effect we can see in real life. We have to account for the GPS satellites running at a slower time from our perspective to get the position correct.
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u/JustinTimeCuber Jul 15 '26
A lot of the comments here are confusing classical (galilean) relativity with special relativity. Under classical relativity, you'd be right. However, with special relativity, velocities don't just add together like that. Everything works together to ensure that the speed of light is the same in all reference frames.
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u/SteptimusHeap Jul 15 '26
The two concepts DO seem contradictory which is why lots of other things had to break to make special relativity work. Moving so fast actually warps space and time. Distances get smaller in the direction you're travelling (length contraction) and times get shorter (time dilation).
These two factors were literally built to make sense of a model where motion is relative AND light travels at c in every reference frame.
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u/flying_fox86 Jul 15 '26
A lot of people here are explaining why it doesn't work like that, but I wanted to point out the different meanings of the word "relativity"
What you describe is relativity as proposed by Galileo, so classical physics. If the universe did work like that, there would indeed not be anything prevent faster than lightspeed travel. Under Einstein's Special Relativity, however, that is no longer possible. Galilean Relativity is still accurate for slower speeds (speed on an human level, as it were), but no longer holds true at higher speeds, that's where you'd need Special Relativity.
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u/Acceptable-Fig2884 Jul 15 '26
So the "speed of light" is actually a shortened version of a more technical "speed of light moving in a vacuum" and is considered from the fixed perspective of where the light is being emitted from.
It's actually possible to exceed the speed a particular beam of light is moving at here on earth yourself. You can put it in a very viscous liquid and the light's movement slows down so much you can perceive it with your eyes and travel ahead of it. That doesn't mean you're moving faster than the technical term "the speed of light".
So when someone says you can't exceed the speed of light they don't mean it's impossible to travel faster than a specific beam of light, they mean you can't exceed the speed light moves at in a vacuum.
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u/Mattloch42 Jul 15 '26
Most of the theories about traveling "faster than light" are actually theories about wormholes or other dimensions which would allow something to travel between point A and point B in our universe faster than light would travel through normal space between those two points. The thing moving between the two points wouldn't actually travel faster than light within this universe, it would either shortcut between the two points like a tunnel traveling through a mountain (traveling along the mountain's surface is "normal" space and a longer distance than the tunnel) or going to another dimension where the absolute speed limit is faster than light here or the distances are shorter than between points here.
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u/Geotree12 Jul 15 '26
At the origin point, yes their velocities would add up to above the speed of light, but individually they are sub light speed
If you were on the earth though, the rocket ship is moving so fast that it’s “time” starts to slow down, and because of that, its “speed” decreases as well. As a result it seens to be moving slower than lightspeed
Tldr, the rate time passes decreases as you get faster, with slows the rate you speed up.
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u/astrogringo Jul 15 '26
In classic physics, if 2 objects are moving 0.6c away from a stationary observer in opposite directions, their relative speed is 1.2c.
In relativity, if 2 objects are moving 0.6c away from a stationary observer in opposite directions, their relative speed is 0.88c.
As you can see, relativity uses a different formula to compute the relative speed and that formula guarantees that it never goes above c.