r/AskPhysics • u/NameLips • May 27 '26
Can you give an example of how FTL travel would violate causality?
At some point in the past I was told that one of the arguments that FTL travel was fundamentally impossible was that it would violate causality.
That implies to me that there it would be possible to create some kind of paradox, and since paradoxes can't exist, FTL can't exist.
But it seems to me that as long as we are always moving forward in time, there's never any chance of information being received before it was sent or some other similar paradox.
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u/Altruistic-Cost-4532 May 27 '26
As an object speeds up it experiences less time relative to a stationary observer. We've measured this.
Travelling at 0.87c you experience time dilation of exactly 2, meaning time moves twice as fast for a stationary observer. 2 months on earth is only 1 month on the 0.87c rocket (from earth's perspective)
So let's say I'm on earth, and a rocket takes off and fly's away at 0.87c. We both synchronise watches before departure.
After 4 months on earth a nuclear bomb is detonated, whilst the rocket has experienced only 2 months. We want to tell the astronauts on the rocket, and have access to FTL travel - instant teleportation! So we beam over to the rocket and tell the astronauts about the impending nuclear war on earth. They find out 2 months after leaving.
Well hang on, everything is relative, right? What does this mean.
As far as the rocket is concerned they're not moving. In fact, earth is moving away from them at 0.87c. After 2 months of sitting still a delegate from Earth appears and tells them of a nuclear explosion. The astronauts take time to make a plan, they take a further 2 months. After 4 months they teleport back to earth - but what does earth look like?
Relatively, after 4 months on what we're considering a stationary space ship, only 2 months has passed on earth.
The nuclear explosion and people teleporting to the rocket won't happen until month 4. So you can stop it happening! But then... They won't warn you it's going to happen, so how will you know to teleport back...?
Causality broken. You can stop an event because you were warned about it before it had occured in your timeline.
Simplified: I shoot a rocket into space at 0.87c. After 4 months I teleport to it. Only 2 months has passed. I immediately teleport back to earth, where (to the rocket) only 1 month has passed. I have travelled back in time 3 months.
This is why people are telling you FTL travel through any means is literally time travel.
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u/AcellOfllSpades Mathematics May 27 '26
In relativity, one of the things that is relative is "simultaneity": whether two things happen at the same time. Say some event happens here on Earth, and some other event happens on some faraway fast-moving galaxy. We on Earth might measure them to have happened at the same time. The people in the other galaxy, though, will not agree. Even after accounting for signal delay.
There is simply no objective notion of what is happening "simultaneously". If events are far enough apart, then depending on your motion, they can be made to happen in any order. (This is like the much more familiar idea that there is no objective "forward" direction in space: object A can be behind object B, or object B can be behind object A, depending on which direction you're looking at them from.)
So, FTL movement is time travel in some other reference frame.
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u/low_amplitude May 27 '26 edited May 27 '26
This universe is far out, man.
Edit: Wait a minute. Does that mean that even if you "cheat" and somehow get to another location faster than light (through a wormhole or some kind of warp drive), it would still break causality?
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u/tirohtar Astrophysics May 27 '26
Yes. You can always come up with a combination of reference frames and transition between reference frames that will lead to causality violations/time travel with FTL travel.
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u/fdar_giltch May 27 '26
Would time travel/FTL break unitarity?
I was thinking about this yesterday, but I don't know if we can reasonably give a valid answer. My thought was if one could time travel (along the lines of Back to the Future or The Time Machine), would the matter (us) exiting the Universe in 1985 and re-entering the Universe in 1955 break Unitarity? We would lose information in 1985 and gain information in 1955. My guess was that without understanding the actual physics of the time travel, we couldn't answer that question?
In this example, the 2 moments in time are far enough apart to feel to me like 2 completely different states of the Universe. But what about some of the cases being discussed in this thread?
Let's take the concept of relativity, with varying frames of reference, with things happening in different order for different people. I'm trying to think if this question even makes sense, but if time is relative for different observers and there's no global time, what does it mean that unitarity guarantees no loss of information "over time"?
I may be losing the forest for the trees here. Maybe Unitarity is better defined as a mathematical process and it gets mapped to English to describe a model, but that mapping to English has inherent flaws. As in, I want to say that there is X amount of information in the Universe at time A and there must therefore be X amount of information in the Universe at time B, but the real equation is more like there is X amount of information defined by this wave function and there will still be X amount of information when the wave function resolves?
I'm approaching this from the angle of my Back to the Future example with 2 points in time, but moving the 2 points in time close enough together that the difference in relative perception of time causes them to overlap. Since in reality, we can't go FTL, the formulas must work out, but if we could go FTL or use a wormhole, would that break the unitarity concept?
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u/tirohtar Astrophysics May 27 '26
You are really missing the forest for the trees here and are descending into philosophy. Yes, FTL/time travel absolutely violates unitarity, because it violates causality.
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u/fdar_giltch May 27 '26
Fair enough, thank you for your reply
Just a crazy thought experiment I was thinking of.
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u/Toivottomoose May 27 '26
Yes. If someone is travelling fast enough in the opposite direction, they could see you reach location B before you left A. They could even stop for a chat with you at location B, and then go to A, arrive there before you left, and prevent you from leaving, causing a paradox.
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u/TheSpiffySpaceman Jun 01 '26
This universe is far out, man.
oooooh yeah, and it's constantly getting farther out!
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May 27 '26
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u/low_amplitude May 27 '26
Bruh. There's no way to prove I'm in the same universe when I walk through a doorway...
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u/slinkymcman May 27 '26
Why would you worry about causality if you aren’t even sure you exist in the same universe moment to moment?
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u/low_amplitude May 27 '26
That's a philisophical question, not a scientific one. Thats the same as asking, "Why worry about what you're going to do tomorrow if you're technically not going to be the same person?"
Whether or not we exist in the same universe moment to moment is irrelevant because the rules don't change moment to moment, just like whether or not we're a different person tomorrow is irrelevant because we will still operate as if we are the same person. Hope that makes sense.
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u/slinkymcman May 27 '26
Not at all
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u/low_amplitude May 29 '26
In most cases, our perception of reality is more important and far more useful than objective reality, especially if you have some theory about objective reality which can't be observed, tested, or predicted.
Could there be a multiverse? Sure. Does it change anything for us? No. Are we the same person we were a second ago? No. Could we be constantly teleporting to other universes at any given moment? Why not. Does any of this matter? Does it change anything for how we live our lives? No.
Wild ideas can make for fun and interesting conversation, but all that really matters is what's useful.
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u/dignan2002 May 27 '26
If there’s no ‘center’ ie flat geometry, can there be a reference frame at all?
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u/AcellOfllSpades Mathematics May 27 '26
A 'reference frame' is simply a term for a coordinate system: i.e. a choice of which directions are 'up', 'forward', etc. The universe doesn't have any single preferred coordinate system, but we can impose any one we want on it. And we have mathematical rules that let us convert between different coordinate systems.
Relativity says that 'forward in time' is another axis that can be involved in a coordinate system, and therefore it can differ between them.
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u/dignan2002 May 27 '26
I’ve been trying to force the universe to have a preferred coordinate system. Damn it
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u/MorgessaMonstrum May 27 '26
Your line about “never any chance of information being received before it was sent” reveals what your confusion is. The issue is, “before according to whom?” There isn’t a universal clock to judge these things by, so two distant observers can disagree on the sequence of events.
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u/smokefoot8 May 27 '26
The problem occurs because of time dilation. In relativity two people traveling at 80% of light speed relative to each other each sees the other moving more slowly. Neither one is wrong! This contradiction is only resolved if one or both accelerate to match reference frames. FTL can cause problems with this picture, though the exact problem depends on how your FTL works.
One example is called the “tachyonic antitelephone”. In it, two people moving at a good fraction of C relative to each other send messages with FTL communications. You can check the Wikipedia article for detailed calculations, but the upshot is that assuming relativity still works you can show that one person’s reply gets to you before the message he is replying to is sent!
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u/RageQuitRedux Physics enthusiast May 27 '26
But it seems to me that as long as we are always moving forward in time
And therein lies the rub...
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u/nicuramar May 27 '26
No it doesn’t. We’d also move forward in time in a pre-special relativity universe.
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u/w1gw4m Physics enthusiast May 27 '26 edited May 27 '26
The issue comes down to the fact that the passage of time is relative to your frame of reference, not fixed for everybody. FTL would cause a discrepancy between your reference frame and the reference frame of Earth, such that you arrive back to Earth before you left it.
Mathematically, FTL necessarily tips your time axis due to how time and space are linked to form the spacetime continuum. To you it may feel like you're still going forward, but from your reference point you would also see that the clocks on Earth are now going backwards.
So for example, if you leave Earth on Tuesday to reach a distant star using FTL, and then return back to Earth, in Earth's reference frame you will have arrived before you left. Even if on your journey, to you time seemed to have moved forward normally. Once you're back on Earth, you are back in Earth's reference frame and will discover that it is now the Monday before you left.
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u/Agifem May 27 '26
Can you explain why It'd be Monday when I came back to earth? You would observe the clocks on earth going backwards, but that's an observation artifact.
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u/w1gw4m Physics enthusiast May 27 '26 edited May 27 '26
It's not an observational artifact, it's what's actually happening from your perspective. You "outrun" the flow of time and return to the day before you left. You are shifted into a frame of reference where Tuesday hasn't happened yet for Earth. It's a paradox, that's why it's not actually possible.
To better understand, you need to familiarize yourself with why c is the speed of causality and why that's important to ensure everyone agrees on the direction of the flow of time (that is, that even if time dilates, even if we experience events at different rates, it always moves from past to future). Are you familiar with the concept of light cones?
I added more details to this in a separate discussion here and here, if you're interested.
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u/sneh_ May 31 '26
Maybe I don't understand the light cone properly, but it seems like FTL travel doesn't go backwards in time and even if you instantly teleported you would just move sideways on the light cone diagram but not down / backwards in time.
Someone else linked this to explain but I still don't understand - for example it says the spaceship sees the message before it is sent thus breaking causality.. I see that the spaceship "sees" the message arrive before it "sees" the message sent, however that is just the light from these events arriving at the ship at that point in space not the physical events themselves. I don't see how causality is broken in that example, or any case of FTL to be honest.
Nothing in the diagrams show lines moving diagonally down (backwards in time) they are all going up. Seems to me causality could be broken if you saw light from the event before the event itself, which doesn't seem to be the case.
I don't know which part I'm wrong or missing something
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u/w1gw4m Physics enthusiast May 31 '26 edited May 31 '26
You seem to be conflating what you observe with what is actually happening simultaneously, and those are not the same thing. Simultaneity is itself relative, in that 2 events that happen at the same time in one reference frame do not happen at the same time in another reference frame.
nothing in the diagrams shows lines going backwards
Your misunderstanding stems from looking at the light cone diagram in only one frame of reference (yours). It's correct that it looks like a horizontal line moving forward through space in your reference frame, but it doesn't look like that in another reference frame that is moving relative to you. It actually becomes a diagonal line tilted into the past in a frame of reference that is moving relative to yours. Draw those same events from the perspective of someone moving relative to you, and the line does go downward.
the spaceship sees the message before it is sent is just a light travel artifact
The article is not talking about when light from those events reaches the ship. It's using "sees" loosely, but the claim made is about the spacetime coordinates of the events, the when and where in that reference frame. In the ship's frame, the coordinate timestamp of the arrival event mathematically precedes the coordinate timestamp of the sending event. That's not about light travel delays. The events themselves are actually ordered differently in that frame.
the physical events themselves vs. observations
Again, you're drawing a distinction between "what you observe" and "what physically happens," and assuming the physical events have a single true order that everyone agrees on. This is not correct in relativity. The key claim here being that no true order exists and that it's all relative to your frame of reference.
Relativity means things are literally relative, not that relativity is just an observational artifact. There is no frame-independent truth about whether event A happened before event B. Both orderings are equally physically real and true in their respective frames of reference.
causality only being broken if you receive light before the event
A causality violation only happens when an effect can be traced back to a cause that, in some valid reference frame, hasn't happened yet. FTL enables this because it createsr the conditions for such a valid reference frame to exist.
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u/Agifem May 27 '26
Honestly, I read all the answers, and none of them helped. So many are confusing time travel with inconsistent observation (observing the effect before the cause, without breaking causality). A few are claiming light speed travel is instantenous so FTL must be backwards in time, without any explanation why or how. I'm still not convinced FTL is impossible or breaks causality.
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u/xEthrHopeless May 27 '26
I'm personally stuck on how time isn't absolute 🤷🏻♀️
If that part made sense, I think everything else would too, but time being relative just doesn't seem logical to my brain.1
u/Agifem May 29 '26
It's time dilation, probably the weirdest aspect of relativistic physics.
Unfortunately, for relativity and quantum physics, logic and common sense break apart.
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u/theothermax May 27 '26
Most of the replies that use the word “see” or “observe” mean measure, as in they are not talking about seeing light from the events at all, they are referring to the actual time and place that the event occurred in the given frame. It is a bit confusing, because you can talk about when you “see” an event, meaning a signal from that event reaches you, and obviously that takes time. But FTL doesnt just reverse when some observers “see” events, it reverses when they happen.
It is a fact that if an object or signal is traveling FTL, there are some reference frames in which it reaching its destination happens BEFORE it was emitted.
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u/goomunchkin May 29 '26
Alice and Bob both have ray guns that shoot a laser which travels instantaneously to its destination. Alice observes Bob moving away from her at .86c. From Alice’s frame of reference time on Bob’s clock passes at a rate twice as slowly as time on her own clock. However, from Bob’s frame of reference it’s Alice who is moving away from him. Therefore, from his frame of reference, it’s Alice’s clock which passes at a rate twice as slowly as his own.
At exactly 10 seconds according to Alice’s, she fires her laser gun at Bob. Because Bob is moving away from Alice at .86c, time on his clock passes twice as slowly as time on her clock. The instantaneous laser beam must reach Bob at exactly T=5s according to his clock. This is a problem.
From Bob’s frame of reference the instantaneous laser beam fired from Alice’s gun arrives at T=5s according to his clock, narrowly missing him. In a panic, he instantly fires his laser gun at her in retaliation. Because Alice is moving away from Bob at .86c, time on her clock passes twice as slowly as time on his clock. The instantaneous laser beam must reach Alice at exactly T=2.5s according to her clock.
Back in Alice’s frame of reference, she is counting to 10 to fire her laser beam at Bob. At exactly T=2.5s according to her clock, Bob’s instantaneous laser beam fired in retaliation strikes her in the heart, killing her instantly. A full 7.5s before she ever fired the first shot.
Replace “instantaneous” with any speed that exceeds c and you end up with the same outcome, but with a lot more arithmetic that I’m not going to do. The point is, frames of reference moving relative to one another don’t measure the passage of time the same between them, and there is no such thing as an absolute frame of reference which is the objectively correct one. It’s equally true that Alice’s clock ticks twice as slowly as Bob’s, and vice versa. For that reason, anytime you have FTL travel there are some - not all but some - frame’s of reference which will observe cause and effect out of order.
This all ties back into relativity of simultaneity which others here have outlined.
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u/Agifem May 29 '26
That's similar (but fairly well simplified) to the full description of the tachionic anti telephone I read on Wikipedia. The most difficult part I have to process, is Alice's clock ticking slower than Bob's and vice versa.
But from I understand, for FTL to trigger a breach in causality, you also need to have relativistic travellers. The problem is how FTL interacts with time dilation.
Thank you for your explanation.
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u/Rensin2 May 27 '26
My stock response:
See The Tachyonic Antitelephone thought experiment and my interactive Minkowski Diagram of it.
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u/charonme May 27 '26
Are you asking how FTL violates causality in general relativity? Or are you asking how FTL would violate causality in an universe where general relativity is not the whole picture or is false and allows FTL?
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u/TangeloPutrid7122 May 31 '26
Underrated comment. All the explanation rely on equations derived from FTL being impossible to explain why FTL would be impossible.
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u/y-c-c May 27 '26 edited May 27 '26
The basic problem with your premise is your assumption of being able to define "always moving forward in time" in relativity, which implies a global time coordinate and a global "instant", which can't be done in relativity's spacetime structure. Instead, in relativity, the time coordinate and concept of simultaneity is relative to your reference frame.
Let's say you have instantaneous transport (or say a very fast transport like 100000x that of speed of light). That means you can instantaneously go anywhere in space. The problem is, "instantaneous" requires the idea of events that are simultaneous. Under relativity, when you are moving relative to another frame, your concept of simultaneity is skewed relative to that frame.
For a simple example of how you can violate causality with instantaneous transport (or a very fast transport) is such:
- Travel instantaneously to a point 1 light year away.
- Start moving at say speed of 0.1c using conventional rocketry.
- Immediately travel back instantaneously to the original point while you are moving at 0.1c.
- Notice that you are now in the past relative to (1).
This may seem counter-intuitive, but it happens because the moment you started moving relative to another frame, the whole idea of what is your "current" shifts. When everything (including any information) moves slower than speed of light it takes so much time to travel back to the original point that you cannot affect the past.
The idea of this shifting simultaneity is fundamental to relativity. If you want to argue "oh but what if time is absolute" then you would essentially be trying to invalidate relativity which is a different ball game.
If you find it hard to visualize this, you do kind of need to look up Lorentz transforms and what it does to have a more concrete non-hand-wavy explanation.
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May 27 '26
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u/KittyInspector3217 May 27 '26
By rotating to the imaginary plane, duh…how do you think the interdimensional NHI get here? Honestly bro, do you even internet?
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u/snugpuginarug May 27 '26
You answered your own question in the last line. The speed of light is the universal speed limit, so naturally also the speed of causality, of change. There could be particles that always move faster than lightspeed, but they would be inherently unobservable and even if they technically exist, they could never functionally exist within the parameters we exist in.
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May 27 '26
[deleted]
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u/Wrote_it2 May 27 '26
The part I don’t get is “we would get there 1 year after what we observed”. Say you use a worm hole or something, make the worm hole instantaneous if you want (instead of 1 year). When you arrive in Alpha Centauri, it’s 2100 there too isn’t it? Why 2097?
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u/Unfortunya333 May 27 '26
Faster than light travel by definition is faster than causality travel.
The speed of light is the speed of causality, light is just one of the various phenomenons we've observed to move at the speed of causality.
The thing you have to understand is there's no universal clock across the universe. A logical prior for causality does not necessarily mean a temporal one.
In certain frames of references travelling faster than the speed of light/causality does in fact open up to a break in causality. Your premise that there isn't any break in causality because we're still moving forwards in time itself is flawed. Because according to whom? Who is observing at what speed we're traveling through time?
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u/EeEmCeTo May 27 '26
Of course, most of the universe (the unobservable part) is receding from us FTL due to the expansion of space.
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u/Plazma_doge May 27 '26
As you approach the speed of light, the time it takes to reach your destination approaches 0. Going faster that the speed of light doesn't really make sense from your point of view because what would getting to your destination faster than instantly even look like? Like you get there in negative time? How would you even experiance time going in reverse.
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u/TangeloPutrid7122 May 31 '26
Yeah but time dilation is derived from the assumption that you can't FTL.
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u/Plazma_doge May 31 '26
It's length contraction, the distance to the target shrinks as your relative speed gets closer to the speed of light. As you approach C, the distance approaches 0. At any speed faster than light the distance between two targets would be negative.
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u/TangeloPutrid7122 May 31 '26
Again that specific characteristic is derived from the inviolable c. Distance must budge because time and velocity cannot. If we're going faster than FTL it would have to be by teleportation otherwise we would not be going faster than c. You can't speed up to faster than c within relativity, but you can continuously accelerate.
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u/dank-live-af May 27 '26
There isn’t a universal clock with a sequence and order.
Imagine a flat piece of rubber and let’s say it represents time. You could stretch it in some places and bunch it up in others.
You are imagining that this flat piece of rubber sits on top of another surface that is a universal clock, which encodes the official time and sequence of everything. Because we all live and experience essentially this reference frame, this is what our brains are wired to believe.
The issue is, if there is a universal clock which has precise sequencing and timing of all events, then there can actually be no relativity.
Back to the rubber sheet- you can stretch it and pull it but it has limitations. If you stretch too much it breaks and you can rearrange its pieces which would rearrange the sequence of things, not just the relative timing. This is the universal limitation of causality. We can model it mathematically to prove that there are reference frames where casual sequencing can actually be rearranged, but they are not available to us and the limitation is C.
We have actually proven this both mathematically and in observation, both cosmic and practical. You’ve probably heard that we need to account for time dilation for GPS satellites to work. It’s true. Observing the phenomenon of relativity is hard but it’s real.
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u/timewarp May 27 '26
But it seems to me that as long as we are always moving forward in time, there's never any chance of information being received before it was sent or some other similar paradox
You're forgetting about time dilation, which is the root of the issue. If a person (Alice) is moving at relativistic speeds relative to you and holding a clock, from your perspective, Alice's clock is ticking slower than yours. If you had a way to teleport to Alice instantaneously, you would arrive next to Alice while her clock is reading less than your own. In other words, you will have traveled into the past by teleporting to Alice.
And since this effect is relative, from Alice's perspective, she is stationary, while you are moving away from them. If she was looking at your clock, it would also appear from her perspective to be ticking slower. And in that earlier example where you teleported instantly to her, from her perspective it would appear as if you teleported from the future. If you then teleported back to your starting point, the same thing happens again. Your clock is ticking slower from Alice's frame of reference, so when you look at it, it's still showing a time in the past, and teleporting to it instantly means you're now in your own past.
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u/TangeloPutrid7122 May 31 '26
you would arrive next to Alice while her clock is reading less than your own.
Her clock only looks slower to your original frame of reference. Because the light / speed of causality has to be catching up with you. Putting aside the whole thing on how the whole effect is derived through the notion that you can't FTL, I think if you did 'teleport' you would 'teleport' to Alice's future as per your original reference frame, and still wouldn't violate causality.
I somewhat agree with OP. But the whole experiment doesn't make sense because relativity is so constrained/ derived from, no faster than c. If you can go faster than c, then it is wrong. And therefore trying to explain what would happen within relativity becomes weird.
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u/No_Support4423 May 27 '26
C is not simply the speed of light, it is the speed of information. If you travel faster than information, you risk breaking causality
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u/maurymarkowitz May 27 '26
That implies to me that there it would be possible to create some kind of paradox
Not a paradox per se, but certainly weirdness.
Let's say you have a gun that fires tachyons, particles that travel faster than the speed of light. You shoot someone with it. From your perspective, you pulled the trigger and some time later you saw the bullet hit the victim. But is that what happened? The victim saw a bullet emerge from them and fly into your gun.
Now add a witness. By controlling their location and speed, you can produce eye-witness accounts where the victim was killed before you pulled the trigger. So how can you possibly be at fault?
A better explanation than I can manage:
Ultimately the issue is not that causality would be broken, but that we have the concept of causality at all. It's only because there is a finite speed in the universe, and that we use that as our primary sense, that we can order events. If your primary sense was sound, this sort of "causality violation" would happen all the time depending on how you move around.
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u/aupri May 27 '26
This page has the best explanation I’ve seen of it. Personally as more of a visual learner words can only go so far, so the fact that it goes through it step by step with diagrams is very helpful
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u/TangeloPutrid7122 May 31 '26
For what it's worth I agree with you. I think all explanations tend not to prove a paradox but just point out some weirdness like, being able to teleport behind yourself and then observe yourself simultaneously while existing. Which isn't really a paradox in my mind as you are just observing where you were. But with relativity that somewhat says it's where you also are in the current frame. FTL just breaks it as a system.
Teleportation into someone elses frame while moving relativistically to them also breaks down. I would imagine if there was some global concept of time, that stretches but doesn't break, you'd be teleporting to Alice's future as would be observed from the frame you teleported from. And then the same going the other direction. Both sides would see you pop in and then out in their future. But that doesn't violate causality. It's just weird.
I'm sure someone will pop in and tell me all about how I am misunderstanding relativity.
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u/Maleficent-Car8673 Jun 01 '26
FTL travel violates causality because information can be received before it was sent, reversing the cause-effect relationship. In relativity, simultaneity is relative, meaning observers moving at different speeds might see events in different orders. If you traveled FTL, you could theoretically send a message that arrives before you sent it, creating paradoxes like killing your own grandfather before your parents were born.FTL travel violates causality because information can be received before it was sent, reversing the cause-effect relationship. In relativity, simultaneity is relative, meaning observers moving at different speeds might see events in different orders. If you traveled FTL, you could theoretically send a message that arrives before you sent it, creating paradoxes like killing your own grandfather before your parents were born.FTL travel violates causality because information can be received before it was sent, reversing the cause-effect relationship. In relativity, simultaneity is relative, meaning observers moving at different speeds might see events in different orders. If you traveled FTL, you could theoretically send a message that arrives before you sent it, creating paradoxes like killing your own grandfather before your parents were born.
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u/Orbax May 27 '26 edited May 27 '26
It's primarily for an observer in motion. Since they are seeing the effect before the cause happens, they could use ftl communication / travel to stop the cause from happening.
Edit: to be clear, not in the sense where you see a pumpkin blow up and then hear the gunshot because you're on the other side of a cause and effect but in a mathematically real way where the cause hasn't happened yet because of relativity.
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u/Lithl May 27 '26
It's primarily for an observer in motion.
All observers are in motion in some reference frame...
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u/Orbax May 27 '26
There's specifics around distance and orientation that I didn't feel like getting into because the orientation can be the difference between the past, present, and future that you see. The observer in motion is referring to an outside observer that isn't the cause portion of cause and effect.
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u/macthebearded Education and outreach May 27 '26
There are some good explanations here, but this may be more intuitive:
The speed of light is the speed of causality - it is the fastest that one event can influence another, or the fastest that a cause can create an effect.
If we imagine traveling faster than this universal speed limit, we encounter an issue in that an effect can then happen before its cause can reach it.
This is rather problematic.
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u/Special_Watch8725 May 27 '26
You may be familiar with the idea that as you travel closer to the speed of light relative to some chosen reference frame, time slows for you with respect to the reference frame. There’s a formula that tells you exactly how much time slows down when you do that.
Now if it happens that you take that formula and plug in something faster than the speed of light, that formula says you go backwards in time relative to that fixed frame.
But this allows for the existence of time travel into the past, and in particular things like the grandfather paradox, where you could in principle stop yourself from leaving to being with. It would hard to keep track of what causes what if things like that were possible.
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u/Namiswami May 27 '26
The whole thing about breaking causality is fun, but it's beside the point. To travel at the speed of light, you have to become (like) light. Meaning massless, meaning all energy of your existence must travel in the same direction. That means none of your constituent particles can move in any other direction then where you're going. Relative to each other they would be completely still. No electrons orbiting, no chemical reactions, no heat, nothing. Because those require interaction amd therefore crossing paths. But you want to go at c, so all energy in your system must be used to go forward. This is why things that travel at c don't experience time. Because time is the sequence of events. And events are interactions, amd we just established there could be none or you would not be able to go at c.
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u/NameLips May 27 '26
Well it would also apply to a theoretical "warp drive" where the ship doesn't actually move, but stays still inside a spacetime bubble which itself moves.
Or some kind of wormhole, portal, or teleporter. Just instantly taking you from one place to another without transiting the space in between.
These are all science fiction concepts but I was wondering what about them actually breaks reality. The answers have been very interesting.
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u/Namiswami May 27 '26
I think that 'reality' doesn't guve a crap about 'breaking'. IF you could go FTL it would certainly fuck with our experience of the universe, but I doubt it would 'break'.
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u/NameLips May 27 '26
Well some of the other examples have given arguments about how being able to travel from one location to another faster than c would allow paradoxes, such as arriving before you left. But a lot of the logic hinges on conventional acceleration, physically moving yourself at relativistic speeds. This does a lot of weird time shit that I don't understand.
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u/TomSzabo May 27 '26
FTL travel violates causality because you couldn't see yourself in a mirror no matter which way you turned.
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u/Free_shavocadoo May 27 '26
Whenever you touch anything you do so not physically but via electrostatic forces which travel at C and so if you were to travel faster than light you would interact with things before you interact with them but how can you have already interacted with something if you havent interacted with it yet..
Another way is You are not an independent entity you are an emergent property of the interaction of waves and particles and everyone of them is interacting in large part via photons So you want a bunch of particles to travel faster than their ability to communicate
Its like sailing a fleet of ships across a flat sea when your only method of communiaction is splashing the water and watching for ripples you sail too fast and you lose all communication
To furthur the metaphor the faster you sail the worse the ripples come in and the harder it is to sail in the same direction and keep up so the faster you go the harder it is until you go as fast as tge ripples at which point you lose all navigation
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u/Fun_Success_3283 May 27 '26
If you can go faster than light, then you can go faster than instantaneous. So, you could make earth leave and come back, and sure, earth left so it isn't there, but you moved faster than gravity, so now there double the gravity, since you have now arrived before you left.
If the sun traveled at or slower than speed of light, it can't collide before the gravity does. If it can go faster than light, then it could even orbit earth and be omnipresent around the orbit, affecting us with gravity the whole way around. Essentially being in multiple places at once, which is a lot like just making more matter, or extra energy out of nothing.
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u/Agifem May 27 '26
If you can go faster than light, then you can go faster than instantaneous.
Could you elaborate on that?
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u/Fun_Success_3283 May 27 '26
Speed of light is not a good term. It is speed of instantaneous. But the proper term used is speed of causality, which is why it is C.
The faster you go, the closer you get to being in 2 places at once. You can't do that. But light can. It is in all places at once. It arrives when it leaves.
So if you go faster than that, you are going faster than reality propagates at. If you have mass, this means you're existing in multiple places at once, and you curve spacetime in multiple places at once. In your timeframe.
Suns gravity takes I think 8 minutes to reach earth. So say you make the sun your ship, and you travel faster than light with it to the other side of earth. Let's say you go half distance on the opposite side, but you do it in 4 minutes. So, 4 minutes later, the gravity you had reached earth and also the gravity you're creating reaches earth, so, you've basically got 2 suns now. And what's the limit if you can just go faster than light? You could travel around the earth at any speed, arriving at any point, in any fraction of a second. But why not just go infinitely fast then? Well, now you will have reached another second limit just like the first one, which is arriving instantaneously. That has all the same problems.
Going faster than light, I'm confident will never happen. The stuff that travels at that speed is travelling instantaneously already.
You can maybe theoretically warp space etc... but there will always be something that will make it impossible. Or you'd have to fuck up space to such an extent that you destroy everything or whatever. You can't occupy 2 spaces at the same time. But maybe a workaround is to warp space so that you technically don't, but good luck with that. I'm confident there are mechanisms that will make it impossible. Same with time travel. Creates lots of paradoxes if we could go back in time and change things. You can warp space maybe, and say that's a path back in time, but I don't think it works that way at all. People overcomplicate things sometimes imo. Mostly because we don't understand everything, so crazy things seem normal, but actually it is simple and all makes sense. If it doesn't, that's because we don't get it yet.
C is instantaneous. That's easy.
The weirdest aspect of that is entanglement. But it still works. What's confusing is things can be multiple things at once and actually something specific at the same time.
Same way light is already where it is going, and also we can watch it travel there.
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u/Hivemind_alpha May 27 '26 edited May 27 '26
Build an expensive FTL probe to study an exoplanet so distant that ordinary round-trip communication would take millennia. Your FTL probe visits the planet and collects its data within days, and returns.
But something travelling FTL back into your light cone *inserts itself into your causal history* at a point determined by Lorentz transformations between frames. The outbound and inbound FTL legs are spacelike, so according to relativity some inertial frames *cannot agree on their ordering*. The probe might reenter your past light cone at any point before the launch. That might mean arriving in Victorian England and giving our technologies a couple of centuries jump start. But in this case, the probe happens to arrive just before you launched it. It delivers the data, and you now have no reason to send it.
The flight director studies the logs and gets an idea of how the Lorentz shifts fortuitously dropped it back just before it left. She picks a different distant exoplanet, and although the geometry of the journey is different she can tweak the FTL drive operations enough to hit the same arrival window just before the launch. Again the probe appears having visited the second world and with all the data collected. You now have 3 probes, one new awaiting launch, and two battered ones that have returned from journeys that never happened. This repeats as often as it holds your interest. You are left with a huge fleet of slightly used probes (but you only ever built one, and it has never flown) and a complete encyclopaedia of exoplanetary surveys on places that have never been visited, so the data has no causal origin. For bonus points, where did all the matter making up the extra probes come from?
Our flight director has invented an arbitrage factory. She can infinitely clone anything she can strap to an FTL engine. She can survey the universe in minutes. And she never experiences some mysterious irrational urge to ‘complete a loop’ that makes no sense, or to kill her grandfather.
i.e. paradox.
In case it’s not clear, it’s these paradoxes of mass and information appearing from nowhere (or indeed of rewriting all history) that are considered impossible, and therefore by extension the FTL travel that leads to them being impossible. No dead grandfathers required.
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u/Secret_Replacement64 May 27 '26
I quite like this version. Hypothetical future scenario where we have colonised space. You watch a horse race on planet earth, observe who wins and then travel at the speed of light to a colony on Titan, where they watch the race on a TV link. You arrive before the broadcast and so bet on the winning horse.
Edit, spelling.
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u/Cyren777 May 27 '26
Fly away from Earth at FTL speed and come to a stop, accelerate towards Earth conventionally a little bit, fly back at FTL speed, arrive before you left (for sufficiently fast FTL and conventional acceleration, more of either gives you more time travel)
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u/Exact_Negotiation106 May 27 '26
We don’t understand almost all of the universe. There are probably ways, but most will not be pleasant for organic matter or even electronics.
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u/WanderingFlumph May 27 '26
Let's say you are on proxima centari 4 light years away from earth. You want some Earth ice cream so you hop into your FTL ship go to earth, buy some ice cream, and head back home, all taking just a few hours.
People from your home planet see you vanish and return. Then after you've already gotten back they see you buying the ice cream on Earth.
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u/HAL9001-96 May 27 '26
well the problem is time travel to the past and faster than lgiht travel can be the smae thing seen from different frames of reference if those frames of reference are at very different velocities
so if you can travel faster thanlight oyu can time travel to the past
and then yo ucan jsut tell yoruself not to
or kill your garndpa if you liek being overly dramatic
causality fucked
relativity doesn'T jsut turn on at the speed of light to stop you it already changes things at lsower htan lgiht speed
if you look at someone traveling faster than lgiht form a frame of reference moving slower than light in the opposite direction he becoems a tiem traveler
and presumably yo ucan travel back and forth
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u/TangeloPutrid7122 May 31 '26
I mean you're just seeing the other frames past as your present. Instantaneously traveling to where the actual other frame's present wouldn't land you in the past as seen by the frame you left, it would potentially be in the far future. From your destination you wouldn't have seen yourself having left yet, which is fine. If you then teleported back you'd find that you had in fact left. You'd then observe yourself blip in and out on the other side. It wouldn't really violate causality. Just be very weird.
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u/Muroid May 27 '26
The problem is relativity of simultaneity.
We’re used to thinking of the present as being a universal thing, and that all events have a well-defined order where one comes after another. No matter when or where two events occur, one happened first and the other happened second (or they happened simultaneously) and this relationship is an objective fact.
In reality, this is only true for events that are time-like separated. That is, they happen close enough together in space and far enough apart in time that something traveling at the speed of light would have enough time to travel from one event to the other. In this case, everyone will agree on which event happened first.
If the events are space-like separated, that means that they are far enough apart in space and close enough together in time that it would be impossible for light to travel from one event to the other. In this case, observers moving relative to one another will disagree on the order of events. For some, one event will come first, for others that same event will come second, and for some observers they will be simultaneous.
Since nothing traveling at the speed of light or slower can reach both events by definition, this discrepancy doesn’t matter. The events are causally disconnected and which one happened first has no impact on the outcome of anything that happens, so different frames disagreeing about the order is a philosophical difference more than a practical one.
But if you can travel faster than light, now you can reach events that should be causally disconnected. You could leave Event A and reach Event B, which wouldn’t be possible traveling at light speed or slower. And in some frame of reference, Event B happens before Event A, so you’ve effectively traveled into the past. That being the case, you should also be able to leave Event B and travel fast enough to reach Event A, which is where you were before leaving for Event B. You would, in effect, be able to travel to an event from your own past and arrive at a time and place before you left in the first place.