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

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

If FTL does exist, the universe might have some way of blocking any information/matter from interacting with the ship/crew travelling FTL. Similar to how the event horizon of a black hole stops any information from escaping it where time and space flip beyond it ( which would also break causality if information could escape ).

It's very rare and special when someone can explain such a complicated topic in an easy to understand and follow way. Thank you.

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

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 watched the entire video, and I get the sense you broke the rules of your own diagram. You drew two diagonal lines at the start, indicating the speed of light. You set the premise that any line between the time axis and this diagonal line is something moving slower than light (STL). Any line between the diagonal line and the space axis are faster than light (FTL). You then start speculating about FTL transmitters able to send a message instantaneously, meaning horizontally to the right. I'm with you so far. This is where you break every rule that has been set. You draw a line that goes downward. We've not established any reason why a line would do that. Your made up transmitter can only go up to a limit of a horizontal line (instantaneous messaging). These paradoxes start popping up because you've suddenly decided this message is being sent to the past. The FTL transmitter is now a time machine for no reason. Like I said there might be some physics I don't understand. However in the diagram you've been using, there has been zero explanation of why any line would ever go down. The established rules were that lines could only be in the STL or FTL directions. Once you start going downward you break your own rule set.

The greatest explanation so far. I've been trying for months to understand this by reading forums and watching youtube videos. None were able to explain this. I still need to understand it more intuitively instead of through looking at lines, but this definitely opened my eyes. This is by far the best and most simply explained video so far on this. Thank you

Well, I don't know about time travel, but a fascinating thing to think about is that if you could travel FTL and have a way to look at earth from any distance, you could go far enough away from earth to see the past.

"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

What if the solution is a combination of two rules? 1. Anything that is going FTL cannot physically interact with anything going STL. 2. When exiting FTL, your world line is also precisely what it was before you engaged FTL. For example, an FTL ship cannot instantly travel to an STL ship, and exit out of FTL with zero relative velocity to the STL ship. This would make the STL ship intercepting an FTL message impossible. The only way to send messages FTL is by way of literally sending a physical ship that exits out of FTL at the destination. A message that simply travels through space and can be arbitrarily intercepted by any observer is just not possible. The FTL ship could also fly out to the STL ship at FTL, but in order for it to ever get back to Earth earlier than it left, it would have to accelerate to have a zero relative velocity to the STL ship from this example. But then that acceleration would take time, preventing it from getting to Earth before it left. I dunno, I’m no physicist, so I feel like I have to be missing something here, because that type of restriction doesn’t feel too arbitrary to me. This could just be me not yet picturing some other diagram following those two rules that still break causality. Seems like the isolation within FTL, and your relative velocity from your departure being conserved in the jump, both feel like they might be more of like natural consequences of how an FTL drive might actually work.

This video is amazing. Thank you for this simple explanation of such complex issues. I have discussed your diagrams with a couple of colleagues last night and we had an amazing time 😊

Omg this diagram finally explained why and how space and time are connected. Thank you so much. I always thought I understood this concept but I was just repeating info not fully grasping it.

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

Revisting this. Professor, I just saw an article featuring your terrascope idea, featuring you, of course 😊. I was so excited for you

There's a great scifi series on a subreddit where humans are using C+ canons that send projectiles using something akin to "slipspace" to travel faster than light to target, and in universe from the target's perspective they get hit before the canons are even fired.

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.

"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...

Phenomenal job of explanation & presenting in a minimalistic way 👏🏻👏🏻👏🏻👏🏻👏🏻

I enjoy that you are actually including the possibility of breaking the universe in this video. That is why we stopped increasing speed, in my warp engine space ship.

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

Hiya, aerospace engineering grad student here! This seems really well-researched, and yes, this _is_ an effect that happens exactly as you say - and it's also not time travel. Not in the way we conventionally think of it, anyway. This model is actually referring to _information_ travel, which is surprisingly pretty different! And no, even if you switched the FTL signal to, say, a person traveling FTL to tell the Vegans (lol vegans XD), everything still holds up to how we understand relativity. This is because this thought experiment doesn't factor in the *quantum effects of information.* Take a pair of electrons, for example - let's make these two electrons quantum-entangled (something that can actually happen!), which is a fancy way of saying that their properties are connected and shared - when one electron does something, so does the other. We have experimentally proven that, when one electron's spin flips, so does the other - _instantaneously._ Which shouldn't be possible, so long as we assume everything is capped at the speed of light. AND this happens regardless of the electron pair's distance from one another. So in theory, if we separated these two entangled electrons across the entire Milky Way, then flipped one, the other would also flip at the exact same moment as the first. This is because with _information_ travel, there's no light-speed limit to be found - no speed limit at all, really. So when one electron flips, the information of that flip _instantly_ travels to the other electron, and so it flips at the exact same time. Crazy, right? All this just to say that, according to quantum physics, information is not bound to the rules of relativity, in regards to both space AND time. This is exactly what happens in this thought experiment - due to the nature of the different axes of space, the information onboard the STL ship comes in all jumbled and in the wrong order, but because information doesn't actually have a given speed limit (the entangled electrons are proof of this), nothing in relativity is broken! The rest of the universe follows the horizontal axis (mostly), and causality remains unbroken - since the space axis is relative to the observer, the *perspective of the order of events can change* because _information has no given speed limit,_ it's only limited by how it's perceived - so the events themselves still happen exactly in the order they happen. The line parallel with the space axis, traveling up and down, is really an _information perception line._ Think about it this way - forget the signal. What if you had a ship that traveled at exactly the speed of light? Both its time and space axis would be at 45 degrees, and thus its perception line would also be at 45 degrees. In that frame of reference, all three events - supernova X, Earth observation of X, and Vega observation of X - would happen at the exact same time. And what if we add back in the signal, and the ship was an FTL ship? Flip the STL ship line and the STL space axis line at 14:47, and we're FTL! The perceived (note: _perceived_ ) order gets _really_ janky with this example - first, the Vegans receive the warning from Earth; then the Vegans see X; then Earth sees X; then X happens. It's almost backwards! But that's okay, because *information - and thus our perception of that information - can arrive in whatever order it wants, or rather, in the order that the frame of reference allows.* No time travel, just information travel. Just thought I should mention that, to alleviate the confusion! Physics, sadly, just _really_ doesn't like actual, real time travel. (My inner Doctor Who fan is crying) Edit: Whoopsie, so I brought this post by my thesis advisor, aaand turns out I may be just a _wee_ bit absolutely incorrect about how transmission of information via quantum entanglement works. Say you have a pair of entangled electrons that you've measured to have a net spin of 0. If you measure one electron's spin to be some value x, due to quantum entanglement, the other electron's spin must be -x. Simple enough, right? All entanglement says is that mutually shared info is conserved. _Technically_ it means you also instantly know that the other electron's spin is some value -x once you measure the first electron to be spin x, but the pitfall for many people (including myself, sadly) is extending this logic to the _transmission_ of information. Just because an electron is entangled with another doesn't mean flipping that electron will have an instant effect on the other. Transmission of information is, much like everything else in the universe, bound to the speed of light. Physically flip an electron, and the other electron will flip after an appropriate amount of time, NOT instantly. (Note that this also is in an ideal setting, where spin only happens along a single axis - normal electrons can have spin along any axis they want, which has all sorts of brain-breaking issues that I will NOT try and summarize for both your and my sanity.) Having said that, though, the logic of the information perception line still holds true to a degree. Proper FTL travel doesn't yet exist, so we can't properly test the effects that FTL might have on causality. The thought experiment proposed in this video calls into question some of the issues FTL might have, but this post is more calling into question the claim that what's happening is time travel. An error I made was assuming the information in this setting was quantum information, which makes no sense since this isn't really a quantum system! You find quantum info in a quantum system, which this isn't, so the information perception line is really just the perception of normal, tangible information. (And again, quantum info like everything else is bound to the speed of light. Just pointing out another error I made.) And yes, it still looks like causality is being broken, but still only from an information standpoint. Events still carry on as they do, it's just the order of _detection_ of events that's getting weird. Which is totally fine! Harking back to the entangled electrons, if we separate the pair far enough and make enough observations in a second, communication via the speed of light would take longer than the interval between observations, meaning you can find out the other entangled electron's spin faster than the other side could tell you what that spin is. So in a bit of a backwards way to what I said earlier, information _still_ finds cheeky ways of "beating" the speed of light without actually beating the speed of light. If information can do that, then there's no reason why this doesn't extend further ie. to this thought experiment. Information gets wonky, but we already know that's okay. No time paradoxes here! To be fully honest here, I ain't an expert in this field - I'm a hands-on build stuff kinda guy, the stuff discussed in both what I've said as well as in this video are only tangentially related to what I study. Thanks so much to all the people who've (politely) criticized my errors - y'all were the reason why I brought this to my thesis advisor in the first place. Science is all about correcting incorrect assumptions, so thanks to all of y'all for setting me straight! And feel free to point out any other errors I've made in this edit - there's only so far proofreading will get you lol

My mind is blown. When you switch the communications to "instantaneous" and re-explained the diagram, everything started making sense. I have had so much trouble trying to conceptually understand these potential issues with FTL travel/communication and this has helped so much. Still hope we figure out a way around it, though! Like you said, it's criminal to be able to see all this cool stuff out there and not actually be able to go see it ourselves.

at 17:00, the video illustrates how the "STL ship's time axis" should be vertical (plus, then FTL works again, according to this model)

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

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.

Thank you Professor, very interesting. I myself think that the only reason we even discuss FTL travel is because SF writers had to 'invent' it in order to write space travel stories. One thing: I believe you pronounced the word *albeit* as 'al-bite'. Actually albeit is simply a contraction of 'although be it' or 'although it be', and so is correctly pronounced *all-be-it*.

What if for the reply signal that gets sent back (for the Supernova FTL Warning example), could we potentially just calculate the point in time/space where Earth would be, and send a signal with a specified frequency to travel the exact speed needed to meet that calculated point?

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

Always a GREAT day when UKposts dings you because there is a new Cool Worlds!! 🛎 👍

Great video. Speed of “thought” as form of FTL travel? Perhaps currently the only way. Also, love Hawkings ideas that time paradox’s are impossible.

By far best explanation compared to all others. Great job. Could be perfect if at 14:30 (most frequently played viewer likely replying because it was a bit confusing) if you just explained more why the 45 degree flip dotted line for STL rather than mention to refer to a previous video for that understanding. Still good job.

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

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 :)

It's amazing how slow you plan to go faster than light. I have other things to do. If I have time, when I'm finished goofing off, I might check in to see if you made it to light speed.

Great vid bud, very much enjoyed it! 👍

respect to all the cameramen who time travelled into the past to bring us footage of all their FTL ships for use in this video

15:18 If we are looking at things from the ship's crews' perspective, how can you say that the first thing the crew perceives is Vega receiving the message from Earth? How did that information get from Vega to the ship? Was it instantaneous? Doesn't this information take time to travel to the ship from Vega? Yes, the time line has reached Vega, but it hasn't reached the ship yet. In fact, it appears that the ship learns about the message from Earth to Vega when the ship arrives at Vega (this might be an artifact of the speed of the ship, and might not be true for a different speed). More importantly, according to your timeline, the STL ship thinks Vega has received the message from Earth BEFORE THE SHIP HAS LAUNCHED. 18:11 Justify the world line for the return message from the ship. No matter how I look at it, the message from the ship back to Earth arrives later, not earlier. I don't see any way to get your reply world line from any of the other lines you've drawn.

What a fantastic misunderstanding of the subject. Glorious video, classic UKposts.

I wish every video was so capturing and beautifully made - just epic

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

The speed of light should really be taught as the speed of causality. The speed of light is an incidental stenographer of the speed of causality.

The universe doesn't care if it makes sense to us.

This is an amazing video and I love the breakdown and diagram! I do have one part though that is preventing me from fully grokking this concept, possibly the most significant element required for me to understand the time paradox. Around 18:10 it’s not explained why the FTL message from the STL ship goes backwards in time. The space component makes sense. But I assume the response transmission to turn off the FTL transmitter would go UP and to the left, not down and to the left. I assume there’s a reason but I don’t feel like that fundamental piece was thoroughly explained. Love the work done, keep doing great work, just want to understand that piece more. Thanks :)

The direction of the return ftl message from the stl ship is unexplained here. I would love to know the reason for that, seems like the most important part. Wonderful vid, very informative and easy to follow!

I think as soon as something like FTL is achieved, we'll soon learn that there is some other, infinitely stranger arbitrary limitation way beyond our grasp that we could never have even conceived of before. I have dreams about it all the time, since I was a child in fact.

Sorry to double-post; but this is one of the best videos on this subject. I often discuss why FTL communication alone can create paradoxes; but it is a tough concept to internalize so I have to go back and re-think it through. (Either by watching a video like this or drawing my own space-time diagram.) Speaking or spacetime diagrams, maybe you can siimplify it. Perhaps you can consider just "earth" and the sub-light ship communicating instantaneously (FTL.) Instead of a message and response becoming flipped, consider a continuous "open channel" (like a real-time phone call with both the home planet and the ship without ever hanging up.) Since time goes slower for the ship right from the beginning anything said from one will be heard from the other in opposite order, with the difference between the two getting greater and greater. (As an aside, I suppose on Earth the voice from the ship should sound in slow motion, while the vocals from earth should sound speeded up on the ship.. this is incidental, but it demonstrates since the vocals coming from the earth will be super fast containing much more information why it will appear to be from the *future* from the ship's perspective. Perhaps have the earth to ship channel constantly reading the news....the crew of the ship will hear the news more and more from the "future." I put that in quotes since it's cool from the ship's perspective. It's only the "future" if the ship then instantaneously relays that information back to Earth. ) Thus the time difference between call and response will continually increase as the distance/time between the two increases.

Maybe I'm vastly misunderstanding things, but I feel like no matter what you do you'd be unable to break causality. Time is moving forward for everyone, just at different speeds, right? So no matter what you do(even sending a signal FTL) you'd be just unable to break causality. Maybe this answers that "does light travel faster in one direction" question(there was a Veritasium episode on this idea), so in this case trying to break causality would actually "slow" the speed of light to preserve causality. So your message, to you(the STL ship in the diagram), is still going FTL, but to Earth your message would be STL and they'd get your "turn off FTL communications message" *after* sending their first message. I guarantee my logic here would make Stephen Hawking froth in rage, but my college dropout brain refuses to accept causality could ever be broken.

I like how he predicted every point where I got confused or skeptical and delivered a good answer without me ever asking anything lol About the talk in the end about exploration without FTL, it was quite poetic, but individuals can actually experience it one day and travel to many stars, considering there is no barrier in the laws of physics that say living things can't last for billions of years. More than that, the way in which time is perceived can be drastically modified, where a million years travel can feel like a mere hour. Just imagining human-like consciousness in a digital substrate is enough to see how this things are possible

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.

Your videos are amazing. Truuely stunning

14:54 Dont you have to start at the space line of the STL space axis like you did in your world time example? Vega wouldnt receive the warning from the ships perspective at this moment. It would know about the event, once the ship lands on vega. Wich makes sense since more time has passed in both systems. And why would the message travel backwards in time but forward in space once the STL ship responds?

Let's start perhaps with a more modest goal: Alcubierre-style drive that targets STL flight instead of FTL. Even that would be a tremendous step forward for our civilization.

Well this is the best explanation I've seen that explains why FTL will break causality. Every other show or channel just mentions that a violation will happen; you actually break it down and explain it clearly step-by-step. Amazing. You deserve a much wider audience. I will start referring others to your channel.

Even if FTL is impossible things like Alcubierre drive and wormholes could , hypothetically, still be used for slower-than-light or light-speed travel. STL or light-speed wormholes might also be useful as makeshift pocket dimensions, since the throat would have to be longer or the same length as the distance between the two mouths in normal space. They could also be used for uninterceptable communication

so my question here is at around 18:10 you're demonstrating what happens if the STL ship intercepts the FTL transmission, then sends a transmission back saying to "turn off FTL Transmission", what rule applying says that the returned transmission must move backwards in time? Why would it not move to upper position of the diagram of which case Earth would receive that transmission before Vega see's the Supernova keeping the order of causalities in line? Why must the returned transmission from the STL ship back to Earth be moving backwards in time?

It would've been useful to see that diagram from the STL ship's perspective, since it's not very clear why their reply travels at that angle

Attempting the impossible almost always brings great discoveries even if it's not the one that is being sought

Isn't the STL line off? The slower it goes the more vertical it will be, right? But then, the mirroring at the speed of light would be more extreme, to a point where the angle becomes zero, if the STL ship does not move at all, hence the time slice would be horizontal too. The message back, seems to travel backward in time in any reference system. Why? Just because it is drawn this way? Shouldn't it be angled up, and everything is restored? Looks to me, that in addition like the space/time coordinates of the STL ship are confused with that of Earth and Vega. Or vice versa and this causes the paradox. Confused.

Excellent lecture on the subject without the math! Thanks 😀

That was awesome,, answered a question I've had a difficulty conceiving for so long.. it now makes sense and as always opens the door to more questions. Great video and thankyou very much..

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.

Don’t take this wrong but the style of this video’s cover is really like 3b1b which is one of my favorite channel, and also thank you to make contribution to spread the knowledge and explaining everything to us😂

Great work explaining this causality breaking phenomenon, the visual and the diagram definitely shed some light on this mystery for me. Thanks. Here is my take on this thought experiment: the crew was traveling STL above the null line and the time travel paradox hinged on the assumption that Earth and Vega remained stationary in space while the crew was traveling through space-time. I think we get the paradox because of the assumption that Earth and Vega are stationary in space. Do we need to further assume the crew ship travels faster before Earth and Vega are set in motion through space? Is that even possible if the crew is traveling STL? Will we get the same time paradox if all three are in motion through space? Do the laws of physic allow freezing Earth and Vega through space while traveling STL? I'm just curious and trying to stay thoughtful. Thanks for the video, I truly enjoyed it.

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.

Best explanation of time travel paradoxes I've seen in a long time, thank you for this! I feel so amazed that on one hand we can theorize and explain stuff like this yet a lot of people will still doubt that we ever went to the moon.

If you’ve ever listened to people’s near death experiences they’ll often times describe going to the light and then time no longer being a concept, as if everything is happening all at once.

You have to understand Special Relativity to understand why we seek FTL travel. From your perspective, travelling near the speed of light allows you ro get almost anywhere almost instantly. The problem is, everyone back on earth would never know what became of you. Warp drives allow travel without disturbing spacetime, thus allowing you to return home in time for supper.

I legitimately wish you could see my face the moment i notice there is a new Cool Worlds video in my subscriptions sitting there waiting for me, i love this channel so much, the work and effort that goes into every video does NOT go unnoticed, thank you for giving us such high quality brain food. oh, btw, I NEED MORE lol.

18:11 "Bizarrely, it would follow a world line like this." - Would be great if you can elaborate more on what makes you draw the line that way.

I’m struggling to understand why you placed the space axis for the STL ship, if we created a new chart with just them line it would be well below the null line no? It was at a very acute angle

18:09 why would the STL ship's message answer go back that way on the diagram? I don't get it at all. It's very confusing because there were no example of someone answering back without FTL shenanigans.

Have you done the math with the actual Lorentz Transform equations? In the case of instantaneous travel, there are 3 events that occur at different locations and different times according to the two different reference frames. A message is sent from Earth the the STL ship and then back to earth both via FTL. It is absolutely vital in this situation to define the first two controls: The Earth time + location the message was sent, and the STL time + location the message was received. Once these controls are put in place, the diagram as shown no longer works (FTL breaks the rules of this diagram). However, the underlying Lorentz Transform equations are still valid. You will find an observer at a distance can witness events that are out of place, but NO message can be sent by any means before the origin, even with instantaneous travel! I have done the math several times with the correct controls, and Sabine Hossenfelder released a video that agrees with this finding.

If a system like an FTL communication system did exist, you could probably avoid the paradoxes by having unique communication "channels" where only certain references frames could interact with it, or just not have FTL communication between STL ships and main communication systems. You could also probably analyse what ships are travelling based on missions you're running and you could analyse possible paradox creating courses of events and avoid them. What would happen if you ran the same thought experiment but with an FTL ship?

What if the acceleration after light speed rather than adds more speed causes a bigger time dilation that preserves causality by from your prespective making everyone elses time pass faster?

Amazing video. To me everything about it was perfect. The structure, the atmosphere, the visual depiction, the music and most importantly the topic and conclusion of the video. I rarely discover such gems on here, so thank you a lot!

Sir, I only found out about you less than a month ago but have almost instantly became obsessed with your content. Keep it up man, I love everything you put out.

Say ftl communication was possible, maybe an ftl message can only maintain that speed in it's original transmission and is somehow obstructed during the response transmission (assuming an instantaneous relay) causing it to arrive back at a later point in time than the original transmission. Maybe ftl travel in general is only possible in one direction, free of paradoxes, due to some sort of deterioration encountered only on return trips that follow a paradoxical route.

I don't understand a word of it, but I am spellbound.😊

Edit: Cleaned it up. As others have mentioned, the only constraint necessary for a vector to have a >= T (Time) value. The problem is that T and S axis are different for different frames of reference. The one frame can have a vector with positive T (maintaining a dot product == C), but be a negative T vector (or dot product C) when translated to a different frame of reference. I think then the problem, all vectors having a positive T (and vector dot product == C), would be solved if there was an absolute frame of reference. A possibility frame would be the expansion of the universe. If all vectors are then constrained to having a positive T (and dot product of C) on the universal frame, regardless if a vector from one frame translates into a negative T vector in another. The universal constraints would then not be violated. That also means that "time travel" would really only be within a local frame, not the universal frame. Local paradoxes resolve on the Universal Frame. I think that makes sense. And hopefully I used the correct terms; my math is old and shaky. IANAP (I am not a physicist)

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...

Despite David's simplified explanation, the concept of space-time paradoxes continues to confound and overwhelm my understanding. Nevertheless, It's so cool..

I liked Contact, nice movie. Thanks for putting it in the video.

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

I have an affinity for how this gentleman explain science jargon in elucidating terms to the lay person and makes it also both profoundly educational and entertaining. He's never ever boring.

Excellent, thank you!

Thx for the great video!

Amazing video! I really enjoyed it and it had addressed alot of questions that were unanswered and persisted on my mind regarding systems FTL

This is the first video I watch of you and I am amazed by the quality of the edit and the way you explain these concepts. Thank you so much :)

How would the stl ship receive/perceive info about Vega receiving a ftl message? Are they just receiving earths ftl message to Vega? because on the diagram the ftl message axis intersects with the stl ships axis after Vega receiving the message.

the other option or direction is that we assume "time" travels at the same speed in every corner of the universe which indeed can cause paradoxes. if we don't assume time is equal everywhere, it means sending FTL messages to the fictional Vega planet about X can either arrive before or after the event happened depending on how fast time travels in that corner of the universe or system. However, my knowledge only goes so far.. if you disagree with this i fully understand but explain to me why this is not possible when FTL is?

I'm no expert, but, it seems that the ultimate paradox of sending the messages back in time before the event happened only appears if you switch between the two frames of reference. Which you do (we are talking about material around 19 min of video). To explain, you mark the signal of supernova travelling from earth to the stl ship instantaneously as observed from the earth and then mark how the signal to switch the FTL transmitter travels from the stl ship to earth as observed by the stl ship. If you were to draw the signal lines corresponding to single observer only, they will almost overlap and be parallel. Given a slight time delay in the machine that receives and interprets the signal and sends the signal back (a computer constrained by the speed of light for example) the signal to switch off will be always slightly later. Causality violation will never occur from a point of view of a single observer and no-one would ever be able to experience it, including the machine that sends out the supernova signal and receives the switch off signal, unless they can be in two places at the same time and be able to switch between the two frames of reference freely.

From the messages point of view, even if it was travelling instantly it still has to travel "through" all the light emitted from the destination. If its destination is 1 light year away, while travelling towards it it will see the destination experiencing a full year. If it then turns around and looks at the source it will be a year in the past from when it left but if it immediately heads back it will not arrive a year in the past it will see the source experiencing a year in an instant and arrive at or after the time it originally left.

you didn't explain why the STL ships signal would go back in time.

Thats funy to see this since I wrote an article about this almost 30 years ago, explaining why its impossible to have ETs here. Even if its was possible for they to travel faster then light, they would come here, since they could go anywhere at any time, why they would come here at this time ... Good video.

I love watching this channel. I have such a longing for knowledge of the universe, space and all the theoretical studies and possibilities they potentially propound. I feel like I'm learning with every video, yet after each one ends, I feel more stupid than before I started.

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.

11 months later, still epic

you can't turn ftl message wordline to stl ship frame refference(because ftl message is still absolute to the universe time-space frame) so when stl ship receive message its reply is inverse to receiving line so reply is receive at earth after. And that dashed line of stl is only time reference for in-ship time so it is moveable(alongside time axis) and that way, where it cross ship's full line is when something happens in crew perspective time. So when they arrive on vega and get the information from ftl ship it appears on dashed line on crossing with vega's full line. So no paradox occure!!

Thank you for taking the time to educate the rest of us! This was clearly a lot of work on your part and I found it so helpful and thought provoking #subscribed ❤️