Why Going Faster-Than-Light Leads to Time Paradoxes

I like to imagine that as soon as we discover FTL and break spacetime, the simulation crashes and the higher beings are annoyed they have another bug to fix

I landed at the same "strict rules" conclusion back in 2007 when designing a sci-fi setting with FTL. I borrowed a bit from Douglas Adams and ended up with a version similar to one you hinted at. That any attempts to perform an action which would result in a paradox would be met with increasingly unlikely events to prevent it from occurring in the first place. Thus, before sending any FTL comms, or engaging in certain kinds of travel, you have to draw the light cones and think through the implications as shown in your video. Thanks for putting this together. It was nice to see someone think through the same things and tie it into a neat package.

Congratulations, you had my complete attention to the entire 25 minutes. Very well done video and explanation. Super interesting

Cosmological and chronological paradoxes are so interesting, rather enjoyed listening to this more than I'd expected.

One small thing: if the message is traveling in a different time perspective than the ship, in what time perspective will this ship be able to decode it? (a reference to Orson Scott Card's books about Ender). I think this is not considered in your diagram.

Possibly the best, most informative and most intelligible videos on an impossible subject I have ever seen. There are those moments in life you can't un-see. Mostly we think of those in negative terms. But things we actually grok come in moments of elucidation and just can't be unlearned. This video put understanding just on the tip of my tongue ... I just get the flavor of the intertwined complexities. Thanks so much!

Does anyone realize how crazy it is that we can even theorize something like this

I would posit that it is possible it's just all about frames of reference. You would not be able to affect anything in your personal referential past. You could observe it, but not change it. I think the diagram is off in using the frame of reference for the STL ship as the "speed" of their FTL message would be significantly "faster" than the message from Earth. The correction for this means that their message would NOT arrive prior to that of the sent message. (On the graph their message would appear to them (could they see the graph) to curve upward past the message point. Using star trek terms if you fired off a message at warp 1 its not going to travel at (warp 1 + full impulse) simply because you were going at full impulse. Basically this graph "forgets" that for all of the pink FTL lines, the straight "space" line is not straight. For them it is a curve, so if you factor that in and the flatten it back out it shouldn't violate causality. I am probably wrong somewhere, it's 2 in the morning...

I'm just curious, if time is constantly moving forward as in the perspective of earth wouldn't the message be returned from the ship later in time? any clarification would be appreciated

your level of explanation, and competence is amazing. You show great quality passion, high enthusiastic and love for astrology.

My issue with this is the STL operates under the assumption that the crew or even a computer is capable of responding the exact same moment the message is received. Even if it was a computer setup to receive a 1 and return a 0 there's still time there for the response to be formulated and sent out

Hello Sir! Time Paradox you explained resides in a slice of space and time as concluded by Eienstine. How do we validate this Paradox in reference to continuous expansion of time and space without the measure of acceleration rate?

"It almost seems like a cruel cosmic joke that the universe be arranged such a way that we are able to look out and glimpse its wonders and yet are trapped here by the speed of light." So well said

Very interesting video! However, a question popped up for me about the breaking of the causality rule and I can't seem to answer it myself.
So, according to the rule, "the world line defines the time axis, as perceived by those following it", and thus the time slice of the STL's crew has to be parallel to the that of the observer's (that is, them; the red dash line). However, I noticed that, according to the diagram, the hypothetical STL ship takes off from Earth (I assume the point of origin within the vertical Earth's time axis is the point of departure of the ship) to send the warning of the Supernova X to Vega before the Earth(lings) actually "sees" Supernova X. Does that mean that, in this case, it is implied that "seeing" is a different event than "perceiving", and that the Earth(lings) are aware of the Supernova X radiation coming their way before they actually "see" or rather experiment its effects (if any)? Also, assuming an STL ship was ready to be launched the moment something like Supernova X was detected (not "seen" as in the diagram; something prior to that), does that mean that both an STL ship and an FTL ship are sent to Vega within the same space and relatively the same time, just in different points in time on Earth relative to the "seeing" and "perceiving" of Supernova X? Should that be the case, then both ships would have been sent to space with the same knowledge of Supernova X and the same goal, and thus when the STL ship gets to Vega, it already knows about Supernova X happening right from its departure from Earth because its purpose was to warn Vegans about that in the first place, right? My point here is, the time slice for the STL's crew is supposed to be parallel to the red dash line, and as shown in the video causality was broken when applied, but the red dash line doesn't seem to start at the right point of the Earth's time axis if anything I said makes sense, and so its origin should be the same as the one from the FTL ship if "seeing" is the same as "perceiving", which is what I understand is the case. Then the testing with the time slice would make sense since the red dash line, with its new point of origin being the star on Earth in the diagram, would roughly have the same angle as the original green line that connects Supernova X, Earth and Vega, and thus the causality would be the same as with the FTL ship, the only difference being that the STL ship would get there long after Vegans noticed the Supernova X, right?
Furthermore, the red dash line, from what I understand, does not represent the actual moment when the STL ship reaches Vega: it represents how its crew perceived the time that passed from their departure from Earth until their arrival on Vega, but the real moment when they reach Vega is the red line, not the dashed one. So, the only "paradox" here would be that the crew thought they reached Vega on time to warn Vegans about Supernova X when, in reality, it happened decades or centuries ago. Shouldn't we then apply the time slice in the the red line's angle and not in the dashed one?
Otherwise, if an STL ship was sent to Vega before Supernova X was detected from Earth, then the diagram would be correct as it is but the causality wouldn't be broken: the STL ship would've been sent to Vega for a different purpose other than warn Vegans about Supernova X given that the crew from that STL's ship would not know about it: it just hadn't happened yet when they departed from Earth. In that case, just like in the last scenario, the only problem would be the crew's perception of reality or knowledge about it, but not the actual events that happened around them.

First thing that causality/FTL paradox diagram makes me think is that our basic understanding of causality may be inaccurate. That's the first place my brain reaches for.
It's like running through an equation that you already know the answer to but coming up with the wrong answer. You know the answer you got is wrong because you know what the answer already is, you're just working through the scenario the equation presents.
In life, when it comes to problems involving perception of any kind, I've learned that the first place you always want to look is at the human error factor.
I'm no quantum physicist or math wizard, sure. But when you know the answer you're coming up with doesn't match the answer you know to expect, do you blame the problem or do you double check your work?
Or to simplify it, when you take a measurement and then cut something to length and it doesn't work, do you check the measurement or do you check the cut length?
Problem then becomes that we don't have a cheatsheet to refer back to and there's no way to run a real world version of it to see how we get to the answer. At least not yet.

I think there's definitely a huge risk of putting something in the space of being 2 sizes at one time.

"The cosmic speed limit, the speed of light is so diminutive compared to a galaxy. It almost seems like a cruel cosmic joke that the universe be arranged in such a way that we are able to look out and glimpse its wonders and yet are trapped here by the speed of light. Fated to only ever peer through the bars of our cosmic prison and dream." -- This is so beautifully spoken...

I can’t do 73+29 in my head without panicking but these grand math problems fascinate me

There once was a young lad named Mike.
Who could run faster than the speed of light.
He took off one day,
In a relative way,
And returned on the previous night!

I so want to work with you and get a PhD. Unfortunately, I am neither a physics major nor good at physics!! But I absolutely love astronomy and everything you do on this channel.

Thank you for breaking my brain. I have always had a problem with relativity, but I am not an academic in such things, I'm a layman with little real understanding. This both helped me understand and broke my understanding...

So if I had a house on mars with a FTL facetime phone and was calling my friend on earth, I could communicate instantly as if we were face to face. But what if while we were FTL facetiming, we also had STL camera feeds of each other’s respective rooms? Could we then see each other in a present past and an instant present?

My explanation of time paradoxes has always been that the timeline is rewritten each time. For example in Terminator, John Connor rose to be a lead through other means. After the events of the movie happen, he becomes a better leader in the future and changes appearance (genes) due to his father being different than the previous timeline.

Imagine being the first person to reach another star only to find humans have already colonized it because of time dilation

In the Halo extended lore (books and etc), their use of slipspace creates time paradoxes like, constantly. It's not uncommon for a ship to arrive at it's destination hours, days, or WEEKS before it left. And everyone's collective reaction to the existence of these paradoxes is just to kinda shrug, say "hey man, don't worry about it too much," and move on

Loved your video. Have your same feelings and hopes about it. But "Alcubierre" is a Spanish lastname. From one of our regions, the Basque Country. It's pronounced something like this "Al (like Al Copone), cu (like quality), bi (like bee), e (like elegance) and rre (like recognition). He is mexican with Spanish origins. Thank you and good luck ;)

This is the one video that finally makes me understand paradoxes

Just wanted to put this comment and anecdotal information into the universe.
Your thumbnail image for this video, I've seen a close example of it in real life. Gravitational lensing / distortions surrounding an object with blueshift at front with redshift on rear, if I would've blinked at that moment I would have missed it. That was the most mind blowing night I've ever had, while stargazing deep in the mountains (other people were with me to confirm it wasn't a dream, there was a few more physics defying events that happened that night). I wish I was better at math to dive deeper into the topic, but there's only so much time in a week (ba dum tiss).
Love the video, great detailed explanations!

The easiest way through this scenario would be communication protocols like radio silence.

absolutely mind blowing video. loved it

"The universe is not required to be in perfect harmony with human ambition." -Carl Sagan

Excellent video! One question open for me is "How would the space axis look like for any FTL voyager? Would you flip it also relatively to the light speed? In that case in our frame of reference we would see their axes as inverted (meaning that they are travelling back in time?). Would a FTL traveller still see light moving always at light speed?

very well explained . thanks for sharing knowledge

Fascinating stuff. My brain feels like it wants to expand beyond the confines of my skull. I think once we understand the true nature of the universe we will better understand the possibilities and consequences of FTL.

If you look at FTL as a linear speed then you probably have access to instant teleportation anyway, I feel as though acceleration and deceleration of anything traveling FTL would help alleviate a lot of confusion around space/time paradoxes. You would be looking at a lot more problems to solve with linear FTL such as matter being instantly obliterated as it instantaneously goes FTL. I like to imagine it as a function curve which starts out as STL then crosses the null line horizontally then decelerates to the same linear function of STL but below the null line. kind of like a tilda ~
I know all this is completely incorrect but hey might give someone something to think about or explain how wrong exactly i am :)

Thanks Dr Kipping
If space is expanding FTL, then we are also expanding at an FTL speed relative to everything else. Sort of like two jets traveling at 600 mph together that appear to be stationary to each other . If we were in a spaceship that somehow could halt our expansion speed, could we "travel" to another star that is in expanding space towards us?

I like how Terry Pratchett said it better than the way Hawking did: Everything that happens, stays happened.

Bascailly, what I got from this is that as long as you don't have an STL ship traveling a significant percentage of the speed of light and using an FTL transmitter, then the odds of a paradox are bascially zero. And if we had FTL technology we probably wouldn't be using STL ships that travel at a significant percentage of the speed of light anyway. So it's a problem, but not a huge problem.

This video perfectly answered all the question I had on the subject, thank you!

It's been too long since I learned how reflecting lines about different axis' works; how does the ftl comm reply at 18:10 result in that line, rather than its reflection about the time axis (since it's traveling the opposite direction in space, like described at 8:53), which would then not break causality since the comm will still be traveling higher up the time axis rather than back down?

I have traveled through a deep dark hole many many times and seem to find myself amazed everytime

Interstellar is a great movie and it actually got me wanting to go to space, im 15 so i have my whole life ahead of me and i can't wait to see if i am actually able to accomplish my dream

What is most beautiful about this video - is actually what it has done to the comments section. Look at all the wonder, the questions, the curiosity, the debate. All these wonderful human beings trying to discover - this is what I love most about humanity, we're all in this thing together :)

This video is an exceptional exploration of the possibility of faster-than-light (FTL) travel and messaging.
The research referenced in this video is outstanding, and the music and film/TV clips used make it more interesting. The video is well researched and presented in an engaging manner. If you're interested in space travel and its possibilities, this is a must-watch! Subscribe to their channel to stay updated with their research.

Correct me if I’m wrong but the paradoxes would only happen if your going FTL but if your going at 0.99 the speed of light you’d just experience what a person on plane sees to a greater extent in terms of time dilation

imagine you are somewhere else in the universe. now imagine you wanted to travel here. there you go. just saved a hell of a lot of time energy and effort.

Man great word choice in the introduction, you play beautifully with words and caught my attention with this awesome introduction so much that i just had to write it in a comment, you have a new subscriber:)

14:50 I really tried to understand and comprehend this, was fine all the way up to the dotted stl axis..and I struggled :( rewatched and still can't get it. 😔
Edit: I watched the rest and as Marty Mcfly once said, (almost) 'I guess I'm just not ready for this stuff yet' 😅
A fascinating and intriguing topic, however.

I'm having a lot of difficulty wrapping my head around some things here. For example, why is the space axis a mirror-flip across the null line of the world line? For the "stationary" earth I get that it would look like a mirror-flip, but why is it? Also, when the transmission is sent from the STL ship after receiving the earth transmission, why is the slope negative? I feel like having this side-by-side with a similar diagram for the STL ship's perspective (with its world line being vertical, and the others being angled) would help clear things up...
Like, I get that there's some consideration here for time dilatation, but wouldn't that be bypassed too, as using something like the Alcubierre drive, the object is not actually moving FTL, just the space it resides in is, so it wouldn't suffer time dilatation... I'm just getting confused.

Well.. sometimes there's circumstances that create two or more different speeds of sound. So I wonder if there's circumstances that could create different speeds of light? I'm postulating if you could cause variable light speed then you could also have variable time. But now wait a minute. Time is more related to gravity. I'm going to have to think about it some more.

In my opinion if FTL was possible, then the apparent causality violations would only be illusions that would be observed from the ship's perspective. Once you stop, everything will appear to "fall" back in order

you are great and i love your videos keep up your good work

It was all very nice explained and I understood almost all of it. there is only one thing I am trying to figure out.
in the Diagram, why is the Space axis for the STL ship at an acute angle? wouldn't that mean that, even though they are traveling STL for us, it seems for them as though they are traveling faster than light? because, for them they are closer to the space axis than the time axis.
I hope my question is understandable.

If we're "trapped" by the speed of light, I would imagine that it's for our own good.

Yet again you combine scientific explanation with a thoughtful and beautiful message.
Thanks for one day deciding to start doing these videos, they mean a lot to me.

Each second traveling the speed of light, you travel back in time 2,25 seconds. Made the calculation and formula sometime in 1997, being in highschool somewhere in Philadelphia. Could do the math again.

What if we somehow calculate how to send the message back at a specific send rate so that it’s not received back in time?

If our efforts at FTL create paradoxes, what happens since parts space/time are currently traveling several times faster than light relative to us?

Perhaps every instance of paradox creates its own tumeline and universe.

Thinking about this more. I'm pretty sure the result is due to the "instantaneous" message not being instantaneous in all reference frames. In this diagram it's instantaneous in the sender's reference frame, but not in the recipient's.
That might be the correct as far as special relativity goes. But then again, before special relativity everyone thought the speed of light was different in different reference frames and no one considered that it could be the same in all reference frames.
What does the result look like if we assume faster than light messages are the same speed in all reference frames, just like we assume with light? I don't think this would break causality. Though it might break other things. I need to think about it more.

I always love the very grounding and philosophical thoughts proposed in these videos along with the theory. Really helps to place the science and the importance of these studies in a broader sense and why or why not humanity should pursue them.

Or, change your calendar as we know it into a light year calendar. Like for instance, time how long it takes to travel a light year in the same minutes and seconds use everyday, then adjust our calendar reflect those light days. Just a thought

Does anyone question whether or not if light actually has speed?

If the idea is to work together for something beyond that is attainable by our own mortal timeline, for the benefit of the next generation or of the survival of humanity, then the ants and the bees have already beaten us to it. The movie Interstellar has given us this message in its final scenes.

I'm not able to understand why the STL ship's space line (dotted red line) is at a lower angle than the ship's time (solid red line). Not saying you're , I may have missed something and would love to understand this model.
Edit: I see now why the model works like this, as it's explained that the space line is just a flipped time line over the 45° angle.

Very interesting and very nice video. Many thanks.

The problem only applies because the ship's worldline isn't parallel to either Earth's or Vega's. Perhaps FTL is only possible when both the origin and destination have parallel worldlines, which could be enforced by a requirement that the reference frames are entangled in a way that forces them to be in sync. I don't know enough physics to determine if that makes any sense tho...

The key to this explanation of FTL travel and time paradoxes seems to be the idea that a person's space axis is always a reflection of it's time axis about the null line. That is why when the STL ship travels close to the null line that it's space axis is also moved close to the null line. It isn't clear to me from this video why that is. But perhaps that is a larger and more complicated discussion.

So if Vega can get a warning can get a warning THEN later will see it, that means theoretically as soon as you get the warning you can travel at FTL to prevent it from happening thus going back in time

Watching videos like this , with futuristic images only reminds me of my childhood. I used to think that all this was scienfiction,

How small of a an object or particle would it take to damage an object going the speed of light

Imagine all the temporal anomalies we don't know about.

I’m in favor of the conclusion that if we encounter “paradoxes” in a scientific thought process we should consider that the concept is flawed. However, I would point out that the chart has problems when using the STL craft and it’s time slice. Even if one buys that the STL time slice is the correct angle here, it still matters where the craft is on that slice. The crew doesn’t get to instantly “know” when Vega sees things. You’d have to allow time for Vega to send the signal to the craft and by then the light from the super nova might have already reached the ship. In addition, the ship’s movement through space over the course of the events it does perceive would affect the value of this visual.

Logically, you could replace "light" with "sound" and "see" with "hear", and FTL with FTS etc. and by the exact same logic and graph, nothing can go faster than the speed of sound.

The messages I feel fall victim of the observer effect. Just that fact that a human mind is aware of it, changes the outcomes.

So since most of this is theory and assumptions I will question how this STL ship is capable of picking up non direct signals and messages at the speed of light (I imagine that FTL messaging would be more of a P2P affair than standard signals). Because while yes *technically* effect happens after cause but only if they receive this information at or faster than the speed of light. Which means that from the perspective of the crew, yes its a paradox, but its the same as if I got an email telling me about an oil spill and then got a newspaper that told me about how an oil tanker lost control and may or may not miss an island. In theory I could perceive this as a paradox if I chose to believe the newspaper to be more up to date than the email.

Something people are struggling with is why the line is moving downward in the STL Ship example with the FTL transmission. It's understandable, given how unintuitive physics is with common sense. Im not 100% sure either whats exactly going on, but I think I can grasp enough to put it in laymans terms.
So when the STL ship is moving through space, time around it is dialating according to how fast you move through space. The closer to the speed of light you are, the faster time seems to move around you as space literally speeds by. The problems begin when you reach the speed of light. What happens?
Firstly, you are now past the speed of light. Think of a car. You know how when a car drives by you at the same speed, it almost looks like it's not moving at all, even if you know it is? That's what would happen to time and space around you as you perceive it. To light itself. Now consider what happens when you begin to outspeed light.
It would move. But not forwards ahead of you. But behind you. Space and time would be so slow by your perspective that it would begin to move backwards, literally. The closer you get to another object, the further back in time that space will go. Atleast, in theory.
There's definitely alot more to this, but don't feel bad if you can't figure it out. This is literally the science people spend their entire lives attempting to solve. Understanding the fundamental underworkings of our literal existence isnt something you just happen to have the ability to do. You are flesh and blood, and your instincts as well as common sense do not apply to the chaotic, esoteric universe that lets us live in it.
In short, FTL might be fundamentally impossible because of physics, the theory of relativity, and paradoxs. To break a hardset rule might just not be physically possible, and we wont know for certain UNTIL we can figure out why Quantum Mechanics and Relativity don't mesh perfectly. Hope this helps, any physicists feel free to come and tell me if Im wrong somewhere. My understanding is very limited, so my word isnt law here.

Its always amazing howcsurprised humans are in their confusion over things they understand so poorly.

Your written script and voice work is top class, man. Documentary level for me, especially for a subject matter that I find intriguing.

This all makes complete sense and I understand what you’re saying. 🥴

Every point in the universe has a unique energy signature. If you can create a device that could detect it and another device that could create it then you would be able to phase out of one location and phase into the location you want to be at regardless of distance.

I think the thing is that people assosiate time with the speed of light. Seeing something has nothing to do with when it actually happens. Time still exists even if you are blind

i'm wondering how photons are experiencing space and time, since they are travelling at the speed of light there fore there space and time axis are the same. the more impressive fact for me that i can see in this diagram is that if you moving away from the object you are literally travelling backwards in time for that object

Regarding time paradoxes, I’d like to point out the dialog from Avengers End Game which seems logical that every moment a new universe is created with its own timeline and we live in multiverse.

Damn - these shows are amazing. I love the philosophy components and the human impact of cosmic questions.

How fast is the speed of sight? Would that be nice if we can look at a star and be there.

Seems quicker and easier to explore other dimensions then our own universe. I’m not aware of any world united effort to build interstellar ships FTL or otherwise. CERN and other facilities like it however are used to explore.

I would like to see a video relating schrodinger's cat theory with FTL.

Hello everyone , if one could exceed the speed of light causing a distortion in time to reverse itself. Would " effect equal cause " ? Thank you.

i think the problem is reaching the speed of light itself, not going over, from my physics recollections, the problem is to reach the speed of light your ship reaches limitless mass, hence the need for limitless energy, but if by any chance we skip this threshold, maybe it becomes possible...

What a wonderful episode. I really loved the prespective you provided at the end. To view this inate limitation of sub-FTL speeds as a blessing, because it might compel us to live for future generations beyond ourselves. Beautiful stuff ^_^

Time travel to the future is easy. I’ve done it, February 2nd 2010 to February 2nd 2020.
I’m still in shock and amazement.

I have a question, I watched a UAclips vid the other day and it mentioned in that one that galaxies are travelling away from us faster than the speed of light while the universe expands outwards. My question is why are we not travelling out at the same rate as them. It was mentioned some galaxy will pass the edge of the observable universe. Surely if we are moving away from what we believe is the centre we should be going at the same speed.

The FTL technology in Star Trek was inspired by theoretical physics.

It seems to me like the "protagonist" of causality is the one with agency. It would seem more intuitive, if the realities that the STL ship intercepts FTL message and sends a reply, and that Earth receives the reply before sending the message exist simultaneously. Of course, this could have some kind of crisis "prophecy" applications, wherein you just have to assume that unsolicited messages have a context that exists from the STL ship's perspective, but are absent in your own. It might be a good way to undo or mitigate cosmic catastrophes, to surround your solar system with detection satellites equipped with FTL relays.
You would just be in your office and you would receive a message saying "Don't tell anyone about the supernova about to happen." They would later land and rendezvous and report that you had previously send a message stating that a supernova happened, do they sent one back in time to tell you not to bother. In a way the STL ship would be its own pocket of causality, unrelated to the wider universe. To suppose this somehow breaks the universe would suppose any form of clairvoyance also does so. I'm aware the latter is fantasy, but that's also what's said of sufficiently advanced technology.

Our understanding of the physics of our universe is rudimentary, at best. People once truly believed that it was impossible to break the sound barrier.

Outstanding! FTL was always a dream since watching Star Trek as a kid. The fact that we can glimpse these wonders of the universe as water filled meat sacks is truly amazing. Even if it isn’t possible, I appreciate that logic as to why is isn’t possible. Space is big. We’re living in an age of high definition discovery.

This was mind blowing.

Is it possible that FTL travel is possible but going backwards in time is just not an option? Essentially accelerating infinitely into the future but still having to wait for everything to catch up?

If FTL is impossible, the upside is that the possibility of aliens attacking would be quite low.