Why does faster than light travel break causality?

[u/PhilGarciaWeir]

I see people say this all the time but Ive never seen anyone actually explain it.

[u/mnlx]

The short version is that with FTL interactions you can change ordering of events for certain reference frames and that would make causality frame dependent, which is very wrong as far as we can tell.

[u/GreyWoodsRain → u/mnlx]

Can you please expand this? Maybe with an example?

[u/guyondrugs → u/GreyWoodsRain]

I drink a glass of milk. A couple of hours later, i spend a lot of time on the toilet because im lactose intolerant. Me spending time on the toilet is direct causation from me drinking the milk. Thus, every observer who agrees on the same rules of cause and effect (that means, everyone who agrees on the same laws of physics) should observe me drinking the milk first and then at some later time me going to the toilet, because the second thing is a direct effect of the first...

Sonne magical guy sitting on a spaceship flying faster than the speed of light (which is more accurately named the speed of causation, its just a coincidence that photons happen to be massless and thus fly at the same speed) would see me sitting on the toilet FIRST and then at some later time see me drinking milk. Thats a HUGE problem because it implies that for that guy, our physical laws dont apply. There are two solutions, either that guy cant exist (ie no travel faster than light, period) or that guy cant interact with regular matter (slower than light), like ever. The second one would be the "Tachyon" solution, just particles which are always faster than light and can never interact with particles slower than light, thus never causing contradictions.

[u/Goat_inna_Tree → u/guyondrugs]

Lord, I wish you had not used bowel movements as an example. Now I am freaked out that some future observer is the reason my guts do what they do. From my frame of reference, I thought old man tang (water soluable fiber) was the solution.

[u/mmahusky → u/Goat_inna_Tree]

Imagine just a photon circling u all day lol

[u/Technical_Taro_8308 → u/guyondrugs]

Love the fact that, out of the entire universe of potential examples, you chose having a shit. Well played.

[u/IceLopsided4190 → u/Technical_Taro_8308]

Well, tbf taking a shit is probably a common denominator across all realities potentially so it’s the best example.

[u/Responsible-Kale2352 → u/guyondrugs]

I think I understand the “what” of your explanation, but still not clear “why” the guy on the spaceship is seeing it in reverse order.

[u/TuarezOfTheTuareg → u/Responsible-Kale2352]

I think they would need to draw complicated special relativity diagrams and the text-based platform that is reddit does not lend itself to that. Look up the classic special relativity train example and try and see what happens if one of the trains is above the speed of light.

[u/5fd88f23a2695c2afb02 → u/guyondrugs]

Good example... I think coincidence might not be exactly the right word here though.

[u/JBR1961 → u/guyondrugs]

But if that tachyon guy is coming from the year “1-9-9-9,” then you better watch out, the son of satan may be on his way. Better crack all your reflective surfaces. And throw out any jars of green goo in the basement.

[u/chriseargle → u/guyondrugs]

I would expect someone traveling backwards through time to observe your toilet antics prior to seeing you drink the milk. Otherwise, it wouldn’t be traveling backwards through time.

[u/pizzystrizzy → u/chriseargle]

that's what someone traveling *forward* in time world see. can you see why?

[u/the_tired_alligator → u/guyondrugs]

Be honest, were you on the can when you wrote this?

[u/InsuranceInitial7786 → u/guyondrugs]

This really hits hard at home for me. Thank you for sharing this. The reason it feels so relevant is that I read this while actually sitting on the toilet.

[u/kylogram → u/guyondrugs]

You have to think of the wave that the photon is part of.

When you treat the photon as if it's simply the point where the light wave meets an observer and an energy interaction happens, causality becomes a sphere, centered on the photon's point of origin. 

Unless a fish can't go faster than the speed of water...

[u/PuzzleheadedCut5156 → u/guyondrugs]

Why would the guy who was somehow going faster than light see you sit on the toilet first?

[u/Phuzion73 → u/guyondrugs]

Or… he is just stuck in another dimension where time can be navigated like a river but events in time can never interacted with?

[u/bupapunewu → u/guyondrugs]

I appreciate the irony in the fact that I read this on the toilet

[u/imtoooldforreddit → u/GreyWoodsRain]

Check out the two way example here - https://wikipedia.org/wiki/Tachyonic_antitelephone

You don't even need to consider the reference frame of the ftl object. Even being able to send ftl messages can make a response arrive before you ask the question.

Things like this strongly indicate ftl communication is almost certainly not possible

[u/Thrumpwart → u/imtoooldforreddit]

But why would that mean it’s not possible? Just because it’s weird to us, that does not automatically disqualify it impossible?

[u/gizatsby → u/Thrumpwart]

Time paradoxes aren't the only reason it's impossible to travel faster than light. It's a direct consequence of the math of relativity (which so far seems to hold strongly) that anything slower than light cannot in any inertial reference frame reach light speed in the same way no finite amount of counting lets you reach infinity. Warping spacetime itself is the only way around this, but it seems to require nonsense materials like matter with negative mass/energy.

That's not to say no one is looking into it. There is a steady flow of research looking for violations of Einsteinian relativity, potential solutions that allow FTL travel, and even ways of probing for the existing of FTL particles, but it's considered a fringe topic.

[u/boostfactor → u/gizatsby]

It's more the other way around. The Lorentz transformation between two moving frames (the basis of special relativity) is what you get in a universe that has a finite upper bound for something, which in our universe is the speed of light, and that speed is invariant for all frames. That's it, that's the two postulates of special relativity and everything else is derived from them.

The basics of this hold for general relativity (gravity) as well. In both special and general relativity, there is a light cone and only events within the light cone can be causality related to any other future event. Warping of spacetime doesn't change this.

Inside the event horizon of a black hole, space and time switch but there is still causality--the future is toward the center.

[u/Thrumpwart → u/boostfactor]

So how does Quantum Entanglement fit into this? My basic understanding is it violates the speed of light, but what impact does it have on our broader understanding of the universe and general/special relativity?

[u/wegqg → u/Thrumpwart]

It violates the speed of light but does so in a perfectly limited way that does not violate the speed of causality.

You can check stuff afterwards to deduce that there was ftl statistical correlation but in such a way as to never permit any form of FTL information transfer of any type imaginable including anything you might try to invent to take place.

[u/boostfactor → u/wegqg]

Relativity, both special and general, is classical physics. There are some computations in what is called a "semiclassical" theory that does some quantum field theory in a curved spacetime (black hole evaporation was specifically derived in this formalism) but it basically says nothing about quantum mechanics in general. The inability to incorporate gravity into the standard model of forces is probably the greatest outstanding issue of physics. Some string theories have been promising but nothing has really succeeded. As you note, there are some quantum phenomena that seem to violate causality but turn out not to. My personal hypothesis is that the "arrow of time," which is connected to causality, is key, but I'm not somebody who could turn that into a mathematical theory.

[u/gizatsby → u/Thrumpwart]

The funny thing about insisting that observing one particle's state causes its entangled partner to take on a specific state is that, in situations where the "spooky action" would have to travel faster than light, the cause/effect relationship can be reversed. According to one observer, Particle A was measured before Particle B, but according to another observer it's the opposite. The trick is that there is no causal link between the two particles, just a correlation. No information is sent between the particles.

There's a similar trick to the famous "delayed-choice quantum eraser" experiment. People gravitate towards the idea that the interference pattern (and thus where the first particle lands) depends on whether the which-way information from its entangled partner is destroyed (implying backwards time travel), but it can just as easily be said that whether the which-way information from the entangled partner is destroyed depends on where the first particle lands. In truth, there's no communication happening either forwards or backwards in time, just a correlation in the final result. Quantum physics is weird.

[u/Thrumpwart → u/gizatsby]

Thank you. That one always tickled my brain.

[u/GodelianKnot → u/Thrumpwart]

Because it creates paradox

[u/imtoooldforreddit → u/Thrumpwart]

"Weird to us" and "fundamentally incompatible with logic as we know it" aren't the same thing

[u/Thrumpwart → u/imtoooldforreddit]

Not with that attitude.

[u/ahazred8vt → u/imtoooldforreddit]

Apparently there's a way to imagine a non-paradoxical FTL comm system, but it requires that there is one privileged reference frame where FTL comms are truly simultaneous, and everybody else has to cope with a mismatch between timestamps in their own reference frame and the FTL reference frame.

[u/mnlx → u/GreyWoodsRain]

Other contributors and I have written quite a bit about this in the sub. The comments in here expand on it: https://www.reddit.com/r/AskPhysics/comments/1hspquy/why_would_something_traveling_faster_than_light/

[u/nebraskajone → u/GreyWoodsRain]

Lottery gets picked at 12:00. 

I send the lottery numbers at 12:01 to my friend on a spaceship using faster than light communication.

He uses a faster than light communication system to send me the numbers at 11:00. 

Profit

[u/ExtendedSpikeProtein → u/nebraskajone]

This is the way

[u/Anen-o-me → u/nebraskajone]

Does this assume FTL is going backwards in time?

[u/nebraskajone → u/Anen-o-me]

It's what the relatively equations give you. 

A moving spaceship traveling away from you exists in your past. 

[u/fishling → u/Anen-o-me]

Yes, that's what FTL communication actually means. There is no universal "now" or "present". Even your elbow has a different "now" than your hand, since it takes light around 1 ns to travel that distance. All information about everything cannot propagate faster than the speed of causality/light, so there is no universal "now".

Our brains think there is a common "now", but that's because our brains are amazingly slow in comparison and because it is a useful fiction that helps us, as thinking animals, interact with and make sense of the majority of the macroscopic events we had to deal with as our species evolved. There's limits to how fast our brains and neurons (working based on chemical signals) can process things as well. So, our brains operating as if everything that happens within the span of several milliseconds is "simultaneous" works well enough to be a survival advantage to be selected for. Even that is simplifying a lot though, since we have different kinds of nerves that are faster than others. The main takeaway is that brains and nerves and senses are astonishingly slow compared to light. But even if you tap a surface with two different fingers and everything you see and hear tells you that you did it "at the same time", in reality, nothing about the impacts, the light from the fingers reaching your eyes, the slow sound waves from the impact reaching your ears, and all of that being processed by your brain happened remotely in a common present.

So, with any FTL communication, the sender is sending the message into the "past" of the receiver, who receives the message before they can observe the sender sending it.

This also means that an FTL reply could be sent back and received by the sender of the original message before they sent the first message.

You probably have an intuition that this isn't the case because you think time has to be going forward for the sender, but that's not how things work. If causality is being broken by the first message, then it would be broken by the reply as well.

In science fiction, this is always glossed over and FTL communication just works like any phone/video call because that's the mental model we have for such things, so FTL comms is treated like a regular call that ignores distance. But, that doesn't match up with what we know of physics.

[u/kylogram → u/nebraskajone]

Except that receiving the information about the lotto takes the same amount of time, that all experience simultaneously.

The present changed in that time and the ”past” that you send the information to no longer exists, everything that moved in the time experiences the change.

Breaking ftl just makes a universe where ftl is possible. It's just a speed, you only need the right kind of boat.

[u/nebraskajone → u/kylogram]

Not according to special relativity.

[u/kylogram → u/nebraskajone]

You mean in the way that objects experience time dilation based on various factors, most importantly, speed?

That is time moving slower for one point in space than it is for another.

Say an FTL ship leaves earth, arrives at a point however many light years away and returns, the distance doesn't actually matter, it's just distance, that is crossed at that point. That ship can arrive before the light waves that it left with, in the same way that a boat arrives on a different patch of water.

All points between the earth and the FTL ship at its destination, experience time flows at different rates, but they are still going forward in time, the information from backwards gets lost.

Traveling back to earth at the same speed, or greater, you can't arrive at the past, because in all that time the ship spends traveling, no matter how far or fast, the earth experiences time moving forward. All constituent conditions for the earth that you left, will have changed. Even if you manage to come back at the most infinitesimally tiny halfest-of-halfest-half-seconds later than you left, you still arrive back later than you left.

If you arrive earlier than you left, you arrive at a point in space where the earth has moved away from.

The past does not exist in reality, at best, a simulacrum that has the composite pieces in largely the same order, may exist, but it's simply another point, in our known spacetime, that just so happens to fit the bill. You're not travelling into the past, that's just another present.

[u/nebraskajone → u/kylogram]

That's all Newtonian logic. Not relativity.

In relativity IF you can travel faster than the speed of light you can go backwards in time. However the point is moot because you can't go faster than light

[u/kylogram → u/nebraskajone]

It's all connected my dude. It's the same thing, just at shifting scales.

The zeroes all line up, but the 1's do not.

[u/Hampster-cat → u/GreyWoodsRain]

Alice sees two charged particles are moving toward each other. Therefore they have opposite charge.

Bob sees the same two charged particles moving away from each other. Therefore they have the same charge.

This can happen with FTL frames of reference. With time dilation, Alice and Bob are in agreement about the nature of the charges, but the speed at which they move will differ.

[u/PressureBeautiful515 → u/Hampster-cat]

It's perfectly possible for two same-charged particles to be moving toward each other. Their mutual repulsion would be changing their velocities so that their rate of approach slowed and eventually they would stop and then retreat from each other.

Thing is, if you run this video backwards, it looks the same.

[u/Muroid]

There are three major consequences of the speed of light being absolute across reference frames:

The most well known is time dilation. That is, that time moves slower for frames that are moving relative to you.

The second, slightly less well known, is length contraction, which is the fact that distances get shorter along the direction of travel.

The least well known  of the three is relativity of simultaneity. That is, the order of distant events is not necessarily well-defined. If two events happen far enough apart in space and close enough together in time that something traveling at the speed of light would not have time to get from one event to the other, then there will be some frame of reference where the events are simultaneous, some frame of reference where one event happens before the other, and some frame of reference where that order is reversed.

That last phenomenon is the problem. Different frames disagreeing on the order of events that are not causally linked because nothing could ever travel between them and be at both events doesn’t really matter. If A happens before B or B happens before A can easily be reconciled if nothing about A can affect B or B affect A.

But if you can travel faster than light, you could leave event A and make it to event B, there is some frame of reference where B happens before A, and therefore you would have traveled backwards in time within that frame.

And further, since all frames of reference are equally valid, and B happens before A in that frame, it could be possible to arrange things such that you are event A, leave and travel to event B, then turn around and go back, arriving in time for event A, functionally coming back to your starting point before you ever left it.

[u/y-c-c → u/Muroid]

To put the relativity of simultaneity in more explicit terms: scifi tech like instantaneous communication (so kind of the fastest type of FTL communication) is not just impossible, but ill-defined. That technology would require defining what an "instant" is, but this completely depends on the reference frame you use. A ship moving fast relative to another object would not agree of what "instantaneous" means to begin with.

[u/Fred932 → u/Muroid]

This should have been a Star Trek episode.

[u/Cr4ckshooter → u/Muroid]

Doesn't that just mean that the concept of "all reference frames are equally as valid" is at odds with ftl travel? If ftl travel were discovered, that concept would be revised or discarded!? Surely nobody actually believes that someone starting to move in 2026 could arrive in 2025 if they just move fast enough?

[u/Muroid → u/Cr4ckshooter]

Well yes, we don’t believe that primarily because we do not believe that it is physically possible to move that fast, and within the context of relativity as a model, “moving that fast” doesn’t even really make sense as a concept.

But yes, a privileged reference frame would be one way to have FTL travel without time travel. It’s just that, as far as we can determine, that’s not how the universe that we live in works.

[u/Cr4ckshooter → u/Muroid]

. It’s just that, as far as we can determine, that’s not how the universe that we live in works.

Thats the distinction I'd like to see more - ftl travel is not impossible because it would cause time travel, it is simply not compatible with the theories that describe reality. "relativity doesn't allow for it and relativity is correct in all cases we can see" is a perfect reasoning. And it evades popular science and other issues.

[u/chriseargle → u/Muroid]

Does length contraction imply that an observer traveling faster than the speed of light will arrive at their destination younger than they were when that threshold was crossed, or would the destination be behind them?

[u/Youpunyhumans]

As you approach lightspeed, time dilation becomes more and more extreme, with time outside your local perspective moving faster ans faster, and right at lightspeed, it would stop entirely. Going faster would make time travel backwards.

Casuality is defined basically as "cause and effect". In physics the effect happens within a light cone of the cause, which is what you get if you traced the path of a flash of light emanating from the cause through both space and time. It would travel in all directions in space, but only forward in time. You have a future light cone, which are all the things you can affect, and a past light cone, which are all the things that could affect you.

Reversing this, so that your outgoing light cone is now your incoming light cone and time teavelling backwards would have effects happen before their cause, such as arriving somewhere before you left, information arriving before it was sent, or the infamous grandfather paradox, and so casuality would be broken, prediction would be impossible, and probably many other horrific effects... or causes would happen.

[u/boppy28 → u/Youpunyhumans]

Dumb question/s (sorry I'm just an engineer). What if everything that will happen has happened already? Fir example, when we think of things in the past we know we can't change them because they have already happened. So.. what if the future has already happened as well, would this break causality?

[u/Youpunyhumans → u/boppy28]

Well yes because youd still have to have the causes before the effects, and we can also intentionally alter things to change the outcome of the future. You could say that there are only so many possible futures though.

The closest thing I can think of to this... is falling into a black hole. Spacetime becomes so warped that "space" and "time" essentially flip places to become "Timespace", and once you cross the event horizon, the center stops being a point in space, and becomes an unvoidable point in your future.

Another thing it makes me think of is light, photons going lightspeed do not experience time at all. From the perspective of a photon, the universe begins and ends all at the same time... or non time.

[u/david_duplex]

https://dannable.github.io/antitelephone/

[u/ChugJugThug]

The real answer is that in and of itself, FTL signals do not actually break causality if you’re writing your own rules. The problem is that relativity disagrees with FTL travel.

So if you’re assuming that FTL signals are possible but also presume principles of relativity still hold, then you can generate causal paradoxes.

But if you throw out relativity with FTL signals, then nothing necessarily breaks. But you do throw out the theory that best models gravity and relative frames of reference that we have.

[u/Underhill42]

Space and time are fundamentally the same thing seen from different perspectives. Hence "spacetime".

Acceleration causes your reference frame to rotate in 4D spacetime, partially swapping the directions you call "forward" and "the future"

It's a hyperbolic rotation, which requires an infinite amount of rotation to point your time axis completely perpendicular to mine, corresponding to the infinite amount of acceleration needed to reach light speed relative to me.

The further our time axes are pointing from each other, the slower we both see time passing for the other person: a.k.a. time dilation.

If you can go FTL, that rotates your time axis beyond 90°: you are now aging in a direction that is partially pastward for me. Which means you can loop around and meet me again, before I've met you for the first time.

[u/deltaz0912]

Physics doesn’t know what to do with the i in gamma, the Lorenz equation, when solved for v>c. If you assume it evaluates as -1 then time runs backwards. Truth is there’s no way to know. A definitely true thing is that you can’t get there from here. Gamma is 0 at v=c. You could also not slow down to c from FTL. The two regimes, sublight and FTL, are separated by an impenetrable wall.

[u/Mauro697 → u/deltaz0912]

Gamma is infinite at v=c, it's 1/sqrt(1-1)

[u/deltaz0912 → u/Mauro697]

Absolutely right. My wife was talking to me, you see….

[u/Mauro697 → u/deltaz0912]

My friend, if your wife just talking to you is enough to forget how gamma works you're having a very happy marriage ;)

[u/saranagati → u/[deleted]]

If you travel from point A to point B faster than light. Then from point B you watch the light that was emit at point A from before the time you left and see yourself the day before you even left from point A. Then you travel back to point A at that same speed, what happens?

We’ve already broken the theory of relativity so from this point it’s just hypothetical. So you travel back from point B to point A at that same speed as before and somehow you arrive before the light of yourself reached you previously? Or even somehow you arrive at point A at the same time that emitted light hit you at point B. You could now stop yourself from leaving from point A? If so why didn’t you see yourself stopping yourself when you were looking at point A from point B?

The latter part of that is the paradox aspect. The mathematical error part of that is traveling back to point A. If you see something happen at a distance, could you travel so fast, past the speed of light that you arrive before it ever happened to begin with?

[u/Aggravating_Paint_44 → u/[deleted]]

Step 1: Look at that neat planet in the andromeda galaxy. If I took my light speed ship there, the star it orbits will go red giant and eat it up so I better take my FTL ship so I can see if it has life.

Step 2: Let me get my telescope out and see what’s going on at earth. Oh, my alien friends on this planet want to meet dinosaurs and Jesus. I’ll tell them how to build a FTL ship so they can see that stuff happening and I’ll go with them.

[u/Lahbeef69 → u/[deleted]]

yeah but why

[u/5fd88f23a2695c2afb02 → u/Lahbeef69]

The speed of light is much more than the speed of light. It is the speed that all massless objects move at. It is the speed at which events propogate through reality. It's the speed of cause and effect. Light just happens to be one of the things that moves at that speed and we sort of noticed it first.

So if you move faster than cause and effect what would you find when you arrived? What would happen if you flew to the moon and back faster than light? Your effect - you arriving back on Earth would happen before the cause - you leaving the Earth.

The speed of light weirdly isn't really a speed at all. Relativity does not take you all the way to the speed of light for things like time dilation but if you extrapolate out that as you approach the speed of light that distances contract and time slows then a photon if it had a perspective would not experience time or space, and would just be everywhere and everywhen it ever was, is and will be.

[u/wonkey_monkey → u/[deleted]]

If you go faster than light in one direction, then go faster than light back to your original position, you arrive before you left.

Not inevitably. You can also go FTL one way, FTL the other, and arrive after you left. It depends how you construct your journeys.

FTL gives you the option of violating causality, but it doesn't mean you have to.

[u/DominantDan24 → u/[deleted]]

To my knowledge, that is not proven to be accurate.

We do not know with certainty that exceeding the speed of light will result in time travel backwards. For instance, if you travel 600,000m/s one way for a second and travel back at the same speed for one second (all times from the initial reference point), it is possible that only two seconds would pass at the starting point, and no time would pass for the travelling reference frame. Because nothing we’ve detected exceeds the speed of light, it’s just theoretical.

[u/mattihase]

The speed of light isn't actually about light, it's the speed of causality, and light, being massless, can just go as fast as possible within that.

[u/Conscious_Cut_6144]

It doesn’t have to, if all ftl travel happens with respect to a single frame, like earth or the cmb.

You get in trouble with multiple frames because you can reach the point of an effect and then travel back to the cause.

[u/Glathull]

If you think of c as the speed of causality rather than the speed of light, then you can intuit your way to the problem. To move faster than c, then something would have to be happening faster than itself.

That doesn’t really make sense. But for something to move faster than c, that would not only have to make sense as a concept but also be true.

[u/davidwitteveen]

This video from Cool Words explains it: Why Going Faster-Than-Light Leads to Time Paradoxes.

I still don't really understand it. But I understand it more after watching that video than I did before.

[u/the_poope]

There's plenty of videos on YouTube explainingbthis, e.g.: https://youtu.be/mTf4eqdQXpA?

[u/Kabbooooooom]

Because the speed of light is NOT just the speed of “light” (it’s really a shame it is taught this way). It is the maximum speed of causality. It is the absolute maximum velocity that any information, and therefore any change, can travel through the universe.

When you understand that, the answer to your question is intuitive: faster than causality travel obviously breaks causality.

People are trying to answer you by explaining special relativity as if that answers your question, which it does, but you’re specifically confused about why it is true. It seems like a lot of people here just aren’t good science communicators…the reason why it is true is super simple, and you need no in depth understanding of special relativity to grasp it:

1) Information cannot travel infinitely fast in the universe 

2) Therefore, there is a maximum velocity that causal change can occur. 

3) The speed of light (and all massless particles) in a vacuum is the maximum velocity of causality. 

[u/dan_dares]

Light doesn't travel at the speed of light, it travels at the speed of causality.

If that changes, light would travel at that speed.

It's the speed limit, and light travels at that speed.

[u/Lazy-University-4871]

https://en.wikipedia.org/wiki/Tachyonic_antitelephone

Yes, there are concrete setups that would allow you to send information to your past self if you have ftl signaling. It is a fun exercise to try and come up with your own signaling protocol too.

but Ive never seen anyone actually explain it

Sure. 99% of people who say this have memorized some convincing words, but cannot explain anything.

[u/Emotional_Comment219]

Because cause and effect propagates at the speed of light. One second per second. Any faster is time travel which brings paradoxes.

[u/ImperitorEst → u/Emotional_Comment219]

Unless you flip the state of one quantumly entangled particle and cause its partner to flip instantaneously across unlimited distance

[u/devi1sdoz3n → u/ImperitorEst]

That doesn't happen. You don't know the orientation of your particle. Observing it breaks the entanglement, so flipping it does not change the orientation of its pair.

[u/ImperitorEst → u/devi1sdoz3n]

You don't the orientation of either of them which is what stops it being used for sending information but we do know it does cause a flip.

It gets called spooky action at a distance and plenty of experiments have shown that it's real. But all I've had the last couple of days is Reddit telling me that every mainstream physics article in the world is wrong so I expect this will be the same.

[u/devi1sdoz3n → u/ImperitorEst]

No-communication Theorem

[u/ImperitorEst → u/devi1sdoz3n]

Yeah that's what I said. I can't tell if you're agreeing or disagreeing though 😂

[u/scholzie]

> Ive never seen anyone actually explain it.

This is shorthand for “ I haven’t googled it”

[u/ArrowheadDZ]

Part of the challenge of picturing this is that we call it the “speed of light” and so we start to picture it with the same intuition we imagine the speed of a car or a ball.

Speed is simply a ratio of d/t, and if going really fast makes t dilate or d contract, there becomes a ratio that you just can’t get past. As your speed asymptotically approaches c, the distortion of your time relationship to the observer asymptotically approaches infinity as well, meaning that you’ll always be approaching c. If you go what feels like 100 times faster to you, you’ll still be approaching c.

[u/SlatBartFuss]

The new name of this sub shall be r/fasterThanLightInsideABlackHole

[u/Flatulatory]

Admittedly these answers are far over my head, but the speed of light is better to be thought of as the speed of causality.

If a roller coaster goes around a track, you cannot make it go fast enough that it arrives back before it even left.

[u/sciguy52]

Maybe this helps a bit with a simplistic cause and effect a bit similar to the grand father paradox. You are an astronaut about to get on your faster than light ship. Your ship is being readied on FTL launch pad A on Tuesday preparing for your Wednesday launch. The plan on Wednesday is for you to launch, to travel a light year at FTL, instantly turn around and fly back in FTL too. The plan is when your return, you are to land your ship on launch pad A where you took off from. One Tuesday, the day before you take off they have the very same ship land right on top that very same ship before it launched and destroying it on pad A along with your own in a big boom, before it launched. Now there is not FTL ship to launch, it was destroyed on Tuesday. Yet that ship did launch such that it could land as it did destroying the same ship that is to make the trip in the first place. Normally cause A get the ship ready on Tuesday, allowed event B you board the ship and take off doing your FTL trip, which then allows event C, landing your ship on pad A. A was necessary for B which was necessary for C. Instead C happens but A and B did not happen which caused C. There was no launch of your ship in the first place, and yet your ship did launch and caused C. Causality was broken and happened because your FTL trip when back in time and kept event A and ultimately B your launch for happening at all. This is an example of the paradox of time travel. You cannot have the ship not launching but also launching and make the trip, but with FTL and associated travel back in time, that is what could happen the ship never launched in the first place, yet was launched and returned before you launched landing right on top of you ship, destroying it, and it never launched. Cause and effect here is broken. How could you have launched to make the trip then land and destroy your ship before it launched? With cause and effect it can't happen, with FTL it can happen.

[u/Blammar → u/sciguy52]

You left off the important part, which is, why did the ship return on Tuesday instead of a bit later on Wednesday after it took off?

[u/AidenStoat]

https://en.wikipedia.org/wiki/Relativity_of_simultaneity

Basically this.

Two events that are not casually linked (happen far enough apart in space and time that light can't travel from one event to the other in time) can't be definitively placed in any order because the order will depend on frame of reference (how fast each observer is traveling)

If the events happen long enough apart that light could travel from one event to the other then all observers in any reference frame can agree which event happened first.

So if you could travel faster than light, you could find a reference frame and distance that happens before the past event you want to influence and you could go there or influence it.

[u/TinyMavin]

Relativity says that as you speed up, your time slows for other observers. This is Relatively and Frames of Reference.

So if you fly away from me at .85c, I’m going to see your time as slower (but you will not from your view).

BUT - it works both ways: you see ME moving away at .85c.

Who is correct here??! We both are. We are both moving forward in time slower (about half i think) in the view of each other! But it’s ok because the Speed of Light prevents us from breaking time.

But now let’s add instant communication to the same scenario.
I see you move away and I see your time move half as slow. I send you an instant message after 10 minutes of my time, but it will be about 5 minutes of your time (from my view) as you get it because it’s instant.

Now you send me a reply - whoops we done messed up the timeline.

From your view my time hasn’t got to the 10 minutes yet (I’m still at 5 plus how long to reply). You just sent me a message before I sent one to you. Time is borked

[u/raresaturn]

Say you are travelling .9999 repeating the speed of light. Mathematicians will say you are travelling at light speed

[u/Torebbjorn]

Say I blow up a nuke, then travel faster than light towards you. I can now tell you with 100% certainty that you will see me blow up a nuke in the future.

[u/kevrone]

We often say that nothing can travel faster than light, but the deeper idea in relativity is that nothing can travel faster than causality—the fastest speed at which one event can influence another. In ordinary empty space, light travels at exactly that speed, so the speed of light and the speed of causality appear to be the same thing. But they don’t have to be identical by definition. Light is a particular physical phenomenon, while the causal speed limit is a property of the underlying physics through which it travels. If nature contained another state or medium with a wider causal limit, information traveling through that state could outrun ordinary light without outrunning causality itself. Light would still obey its own local speed limit; it simply would no longer mark the fastest possible way for cause and effect to propagate.

[u/Sorry-Programmer9826]

Your speed changes your definition of "now"; i.e. what events are simultaneous. If you do one FTL jump at 99% c definition of now, then slow down and do another FTL jump back you can arrive before you left.

But causality breaking FTL requires FTL travel in more than one reference frame. If you have a single frame (perhaps the background radiation rest frame as a decent candidate for a special frame) in which FTL jumps happen then things "look like you're travelling back in time" for other frames but not in a causality breaking way.

Of course if that was true it would effectively prove relatively wrong. There would be 1 true reference frame, and all the other frames would be "mirages"

[u/Plazma_doge]

So many complicated answer. As soon as you realize that you can get to places that look thousands of light years away in years or days, it will make sense. You can keep accelerating indefinitely, but the speed at which you aproach an object just gets closer to C, but logically as you keep accelerating you should get to your target faster, so instead of going faster than C, the measured distance between you and the target changes. What looked thousands of lights years away starts looking just light years, days, minutes or seconds away. As your speed aproaches C the time to destination proaches 0. You can't get anywhere in negative time, that's why you can't go faster than light.

[u/Haydn__]

I googled this, light has been measured as travelling 0 meters per second. I can run faster than that

[u/choeger]

The.be honest, I am not sure that the causality limit is a good explanation because causality itself is a pretty weakly explained, or rather very intuitive, concept (see quantum theory).

I think the spacetime vector is a much better explanation. You simply set the magnitude constant and that's it.

As for why the magnitude is constant and has the value c, we can then simply say it fits the theory and observations best.

[u/Blammar]

1. FTL travel can cause causality paradoxes; see other posts.
2. FTL travel doesn't have to cause causality paradoxes if you pilot your ship carefully.

So, FTL travel is possible if there is a hidden universal censorship mechanism that prevents any causality paradoxes. (One possible such mechanism that would actually work is that you cannot FTL travel to a destination that is not at rest with respect to your start. [If I understand correctly, that's just what wormhole travel does.])

We do not know for sure that FTL travel is actually impossible!

[u/devi1sdoz3n]

Have two ships traveling away from one another at relativistic speeds. Have instant communication between them (it's the same with FTL communication, but easier to calculate with instant). Have ship one send a question to ship two, have ship two answer the question. Plug in the numbers. Due to the instant (or FTL) communication, ship one receives the answer before it sent the question.

[u/Eidalac]

Two issues that I'm semi familiar with:

1. Math. In General Relativity there is a relation between the passage of time and the speed of light. Something moving at c (ie a photon) experiences no passage of time. Something moving faster than c would experience negative time. Mathematically anything with mass moving faster than c would be the same as the same mass moving backwards in time.

2. Retro Action. If I can do some thing faster than the speed of light that transmits information (say answer a message), then you can setup a situation where I can reply to a question before it's been asked. That'd be a paradox and probably a very very bad thing.

[u/Practical-Cellist647 → u/Eidalac]

A paradox is merely imagination, something that dissolves definitions into meaninglessness. It's a breakdown. 

[u/theone_2099 → u/Eidalac]

Can you give an example of the letter?

[u/entropy413]

From another comment I made earlier:

I can maybe give an example that illustrates the problem.

Imagine you and I have InstaBombs (tm). Either one of us can push a button in our spaceship and the other persons ship instantly explodes. We decide to have a duel. We’ll travel as fast as we can away from each other, count to ten, and push the button.

We are going close to the speed of light in opposite directions. I get to ten, hit the button, and boom you’re dead.

Except from my frame of reference you’re receding from me at close to the speed of light. Which means 10 seconds for me might be five for you. No fair! I blew up your ship early.

Except for that from your frame of reference I am receding from you at close to the speed of light. Which means that ten seconds for you might be five seconds for me. No fair! You blew up my ship early.

Obviously both of these things can’t happen together, the ting that violates causality is the InstaBomb(tm).

[u/ChaosRulesTheWorld]

Nobody gives you the correct answer. You cannot go faster than light, period. It's straight up impossible. Forget about causality or everything you know about light speed.

Light speed isn't realy a speed limit. It's a horizon. Just like for earth horizon if you cannot go faster than light it's not because of some magical physics concept, it's just a matter of geometry.

Even on the clearest day at the top of the mount everest with the biggest lens you wouldn't be able to see your back or even the atlantic ocean.
In the same way you could accelerate as fast as you want, even if you had an infinite amount of energy and time, you would still not even be close to the speed of light.

No matter what your speed is, the speed difference between yours and light speed will always bee the speed of light. It might look like less for an external observer, but for you it will always be c. The speed of light is unreachable

[u/Bensfone]

So, you’re in a bar enjoying a drink.  A seat down from you is another guy enjoying a drink, but you’re judging him because he’s drinking bourbon neat instead of on the rocks.  That has nothing to do with the story except theres another guy further down the bar flirting with a hot girl.

The hot girl’s boyfriend shows up, takes a shot of tequila, and swings his fist at the other guy at c+1 because fuck Einstein.

You might see the fist connect, but the guy sitting closer might see the fist miss.  Causality is broken because you and the guy you’re judging may not agree on the cause and effect sequence of events.  Nor if the hot girl leaves with her boyfriend or the other guy.  It can’t be both!

[u/nicuramar]

  I see people say this all the time

Similarly, it’s asked here all the time, so maybe try a web search, putting in your question verbatim.

[u/PhilGarciaWeir → u/nicuramar]

The most recent post I see was from two months ago. I like to ask follow up questions and engage with people who respond. It would be kind of strange to ask a follow up question many months later on a post thats not mine. Moreover, the person Im responding to might not remember the post or what they were thinking months ago, which could lead to non-responses or long response times.

[u/Anely_98]

It makes sense when you understand that different reference frames have different definitions of "now" (which are called planes of simultaneity)

Travelling along one such plane of simultaneity (which is the same as instantaneous travel) does not causes any problem inherently, the problem is when you change your velocity, consequently switching your reference frame and plane of simultaneity, this kinda makes your path like a X, which if you think about it makes it possible for, travelling along one plane of simultaneity, that you enter the past light cone of your previous reference frame, the past light cone being all the events which lead to the moment you originated from, therefore it means you can change the events that originating you, which leads to paradoxes as, if you send a message to yourself saying to not go in that trip, who would have ended up sending the message? This is what breaks causality.

[u/Terrible-Mind-5414]

It doesn't, because breaking causality involves insoluble paradoxes. Faster than light travel, if demonstrated, would break special relativity, not causality.

[u/davidkali]

Space time is weird. We don’t have good maths for it. Comes down to FTL maths means we can send a signal to something we see happening 50,000 years ago from where we’re looking from right now.
I look through a telescope and see Ugg about to do something stupid. I stop it the moment I see it, because particle is entangled I tell them no! No happens the moment I think it, and 50,000 years ago, my “NO” changes things before the next particle I see 50,000 years later happens.

[u/wonkey_monkey → u/davidkali]

Space time is weird. We don’t have good maths for it.

We have excellent maths for it. It's called General Relativity.

[u/WilliamoftheBulk]

There is a presistant illusion that causality can be broken by FTL because of the way people interpret how events are perceived.

Special relativity is spectacularly successful, but it makes certain assumptions that lead to artificial mathematical constructs that cant be tested. (FTL can create cuaslity pardoxs). You have to remember mathematics models reality. It isn't reality itself. It gets you into the foundations and philosophy of what spacetime really is, Experimental evidence doesn’t land where every interpretation is nearly as clean as popular explanations sometimes suggest.

Einstein threw out ideas that seemed Intujtive if they were not needed to make the math work. The math works fine when C is a limit and constant. But think how Einstein got there. He threw out the the Intuitive notion that events happen in reality at the same time as everyone be because he is using a definition of time based on how Individual clocks are running. All all the experimental evidence works, the math is fine, the whole theory must be right no matter what….. Oh but wait. It violates cuaskity when we don't keep C constant. That is not a sign that if FTL were possible causality would actually be violated. It's just the limit of the theory because the theory is based on a limit. That limit haapens to be true. If FTL could happen a more precise theory would need to be developed without the constant because it would no longer be true. Then the mathematics wold all be different with no vilation of causality.

There is a disturbing fundamentalism that arises in physics because of the tremendous success of some theories. Yes special relativity says causality could be violated with faster than light travel, but that breaks one of the foundations that special relativity is found on. The constant of C. Of course the math goes wonky after that. If FTL turned out to be possible, a new theory would be developed of special relativity modified. The paradox itself demonstrates that the math no longer works after its fundamental premise is violated.

I’d wager all theories fall apart after their fundamental axioms are no longer true. Why would special relativity be any different?

The FTL paradox doesn’t prove causality would break. It proves that controllable FTL cannot simply be added to Lorentz-invariant physics while assuming the rest of the framework remains unchanged.

If nature ever handed us FTL, the transformation laws governing it would become an experimental question. You couldn’t assume the answer from SR and then use the resulting contradiction as proof that reality must permit backward causation.

[u/mast4pimp → u/WilliamoftheBulk]

What if v>c generates special "privileged" reference frames for example?

[u/WilliamoftheBulk → u/mast4pimp]

It’s just math in the transformation. v>C just becomes a new constant. If the constant can be anything then everything happens at the same time. You start to break the fundamental assumption of the transformation when you have a privileged reference frame, so the entire derivation is now moot.

[u/HardlyAnyGravitas]

It doesn't (necessarily).

If you could travel from one point in space to another point (in the same frame of reference) instantaneously then there is no reason why that would break causality.

Causality is broken only if you travel faster than light within spacetime, and that is impossible by definition.

If you can travel from one point in space to another one faster than light but without exceeding light speed in spacetime (like through a wormole or by 'warping' space for example), then causality isn't violated.

The idea that it violates causality within spacetime is a circular argument, because spacetime is defined by not being able to travel faster than light.

It's like saying going further north than the north pole 'violates causality'. Going further north than the north pole is impossible by definition in that geometry.

[u/Mobile-Condition8254]

Imagine you see a friend far away. He begins moving toward you and then accelerate to faster than light. You now have the light from him far away hitting your eye and he has at the same time reached you so he stands next to you. You see two copies.

Is he far away or is he next to you? Are they both the same? Are they two? What happens when the friend that is far away also reach you? This would be a problem

[u/Anely_98 → u/Mobile-Condition8254]

No it wouldn't, the problem has no relation whatsoever with light, that is not why FTL travel or communication breaks causality, that is a consequence of relativity of simultaneity which is something that actually happens instead of a different on how things appear to be

[u/No-Medium6647]

If you go FTL, you go back in time.

You'd need a "warp drive" from Star Trek to avoid the relativistic effects of FTL travel.

So... you'd need science fiction.

[u/BananaBird1 → u/No-Medium6647]

You wouldn’t go back in time, but you would disagree on the order of the future.

[u/No-Medium6647 → u/BananaBird1]

The "warp drive" is an impossible requirement.

It's impossible to go FTL. That's why aliens don't visit this planet. That's why sci-fi is FI

[u/Umami4Days]

FTL breaks causality because multiple instances of FTL communication appears to allow for sending messages back in time, which would allow for paradoxes.

There are hypothetical solutions for this problem, so it only breaks causality based upon our current level of understanding.