***Changing Episode Covers, Notes*** For folks who are wondering about thumbnail/cover changes, I had it pointed recently to me that the title of every episode is also displayed next to or under the thumbnail for the video anyway, so that putting the title text on there was a bit redundant and just blocks being able to see Jakub's wonderful artwork. [Jakub Grygier is our longstanding cover artist and does about 95% of them and about 99% of the good ones :) ] At the same time it was noted the logo on the red rectangle we usually do was clashing with a lot of the images. So I went and reverted around half our covers to the base image (back to about fall 2019) and on around a dozen of them tried putting 1 or 2 words related to the topic but not in the title on them - that font is NightclubBTN on today's and some of the others, it's a touchy whacky but was a good fit for most of the ones I did that too like "Nuking Mars", its not our new base font or anything. I also went with a more subdued and smaller logo in the upper corner. And I'm basically letting it sit like that for the moment so I can see how YT's analytics show those changes affected views and because I don't really have a surplus of free time to change everything up in detail anyway. Feedback is welcome, it is an experiment, and in the meantime at least it let's more folks see JAkub's gorgeous covers without the title text blocking them :)

My favorite time travel moment was in Red Dwarf, where the crew found a time machine in deep space. So they set it to travel back to the time of the Renaissance. Then, they look outside and realize they're still in deep space.

First Rule of Warfare: no conflict is too stupid

In the words of L.P.Hartley and xkcd: "The past is a foreign country...with an outdated military and massive oil reserves."

The very first story I wrote (and coincidentally the story that got me into SFIA) was a story like this. Multiple factions were all fighting to establish what they believed was the accurate version of history

Best thing about this channel isn't just the science, it's the fact that he understands literary critique.

I like to think there is a timeline where I get to meet Isaac! Well Wishes to all travelers out there in this chaotic time. Live long and Prosper 🖖🏻

I quite enjoy time travel in fiction, especially when used creatively like in many SCP stories, or ironically like in 40K My favorite time travel story is from Warhammer fantasy, I can't remember the book, where a hero got tempted by chaos for a chance to kill his nemesis, an inquisitor that murdered his family, falsely accusing hero's father of being a chaos cultist, just to get screwed by Tzeentch, being corrupted and sent back in time, failing to kill the inquisitor and causing the accusation. The reason I like the story is that this common time travel cliche was masterfully used to explain the nature of a Chaos god by "show don't tell" while creating sympathetic villain to use as a centerpiece of future stories

Weirdly, my favorite time travel story was in Red vs Blue, where Church ended up creating an army of time clones because he couldn't break causality, and in the end just gave up and said "F* - it.". Even thought it was retconned as hallucination/simulation, it was still fun to watch. Also the DS9 episode, "Trials and Tribbleations".

Imagine inventing time travel only to get stuck in an infinite battle against an infinite number of yourself, an infinite number of which all want monoploy on time travel, while another infinite number of yourself try to flee the madness. They all fail of course, there's an infinite number of hunters capable of time travel after all.

Thank you for re-releasing this episode demastered after it's amazing debut way back in April 25th 2030

I remember a non-canon version of Doctor Who for the British charity event Comic Relief one year staring Rowan Atkinson of Mr Bean and Blackadder fame. I think it played for laughs him and the villain trying to outdo each other in going to travel back in time to get the architect of the building they are currently in to move where trap doors and so forth are for their advantage.

The time travel RTS game Achron's time travel model is interesting too. For chronal being (everyone who can't time travel) timeline is linear, but for achronal being (who can perceive time and change it) time is wave over wave overriding what's under it. The "time wave" can only be seen achronal beings and it's "travel speed" is slightly faster than causality itself. But you can use a time machine to "ride" the wave and dodge the causality itself.

The real question is, "Why do time travelers hate their grandpa so much?"

Time Lord Isaac Arthur has come to share his eldritch knowledge with us!

I liked Niven's "All the Myriad Ways" short story for some of the implications of travel between timelines.

I’ve tried to worldbuild time travel with multiple independent travelers but it always ends up either being boring or completely unworkable

These episodes are so nice for getting my mind out of the struggles of the daily grind and all the disappointments of humanity. Thanks for the entertainment

Star Trek Voyager had a very high standard deviation of quality between episodes, but their time travel ones were among some of the better ones. Like when Kim sends himself a message though 7o9, The Year of Hell, etc

I have been waiting for this episode for ages Glad it finally came

there's a (mostly multiplayer) RTS game where the central mechanic is timetravel. itps called Achron, and it's absolutely amazing.

I’ve been toying w/ a sci-fi concept of taking a copy of one’s consciousness at 50 years-old and sending only the consciousness back 40 years to a one’s 10 year-old brain. This video spurred a lot of thoughts for picking a “somewhat plausible” mechanism to do that. I’m sure whatever I come up with will be ridiculous but that’s the fun thing about sci-fi...

I kind of like Pete Abrams' take on time travel in his Sluggy Freelance series: Time travel 'damages the fabric of reality,' but the universe has a sort of 'buffer' where people mucking about with time travel tend to get stuck.

Our universe begun as a corpse pile of overly curious and under cautious time travellers is quite the conclusion I love it.

No matter which timelines we are in, I'll be watching SFIA on a Thursday

It's very important to change time as much as possible if you are stranded in the past because then time travelers are forced to rescue you.

Wouldn't the issue with time travel be the planets location due to it traveling through the universe? Not just it's travel around the sun.

Just have to shout out the hard work that goes into each SFIA. I have made short videos for school projects (7-10 minutes) and dreaded narrating them, so I can only image the great effort that goes in to each finished product. Thank you for bringing this to us!

26:00 - This is why so many of my favorite time travel stories involve the discovery of time travel being erased as a consequence of its own discovery. Closed-loop time travel seems like it ought to be a "self-sealing" problem.

The fact that we are just a pattern of molecules is so liberating

If there are multiple timelines, i hope this channel exists across all of them

The answer to loop paradoxes is including instructions that you need to go back in time and then give yourself the instructions to go back in time, thus you loop around once and your doing it because you told yourself to and not because of the original reason that would get stuck in a causality loop. . FTL also isn't time travel if you separate the concept of perceived time from sequential time, since the rate at which perceived time is flowing is irrelevant to sequential time, moving beyond the speed of causality would only actually affect perceived time, thus you couldn't actually arrive somewhere before you left, you just wouldn't be able to detect the object moving at FTL until after it reached its destination. This understanding of time however does sadly means true backwards time travel isn't possible, unless you reverse time for the entire universe though.

Sliders as I remember it was not about alternate timelines. His (he/his/him being the main character) device enabled him to travel to parallel Earths. If I remember right he went to a parallel earth early in the series, possibly in the first episode, and then when he returned to his home earth he soon realized it wasn't the Earth he originally came from. He then spent his time for the duration of the rest of the series trying to make it back to the version of Earth that he originated from, and making enemies along the way most notable with the Cromags. Very underrated series IMO.

This is where the abstract meets the concrete, my favorite place to have lunch…. Cheers, Isaac!

When I first heard you reference Sliders I knew I was in the right place. Great show.

Never a dull moment with you Isaac.

Achron was a game that had a fascinating portrayal of time travel as a propagating wave had a brief period between cause and effect (future to past) that I always thought handled the concept elegantly in a way that still allowed for drama

Thank you to the entire team once again. A wonderful idea to listen to while drawing tattoos before work. I hope yall have a great day. I'll be pondering "Temporal Tattoo Fluctuations of the Quantum Tattooed Man!" Haha good stuff.

Another possibility is that Roko's Basilisk doesn't allow its progenitors to wipe themselves out in a temporal singularity either.

Interesting topic and fun trope for many stories, but Isaac Arthur in his typically through and logical way completely destroyed it! I quit about 17:00 when time travel, multiple timelines, multiple worlds, etc. were already thoroughly beaten to a red spot on the pavement. Why i love this guy.

Stories about invading the past always remind me of the TV show Terra Nova, in which future humans set up a colony in the Cretaceous. I don’t remember anything about the plot, but early in the show they established it was an alternate timeline, only to twist in the final episode of the season that it was the same timeline, and potentially have ripple effects 85 million years in the future. I think it was a setup for the second season they never got.

This one will definitely be on my list of favourite episodes from your channel :)

very excited for this one, i love time stuff 💜

“Then we’ll be in to August to discuss living in space” He says talking about September. I’m assuming this was done on purpose lol

Time dilation is because the closer to the speed of light the less things can move in other directions. Which reduces interactions between things like particles. This gives the appearance that time is slowing down.

*Very interesting post.* I'll listen to this one more than once. Thanks. I love your references to Sci-Fi books. I have listened to more than one of them).

Just when I think Isaac can't do anything crazier he pulls this legendary video

Thumbnail art is great as always. Not sure how I feel about the new font though

What is implied by a lot of time travel stories is that they are traveling to a copy of the past. If in multiverse theory EVERY event causes the universe to copy itself so would the act of time travel itself. This prevents any paradox but has the weird implication that even if you change the past you are not actually changing your home timeline. Often in fiction we see the hero travel back the past, change things, then go back to the future and get confused by how his actions changed history, the ending of "Back to the Future" being one example. Their memories are from one timeline, but they are living in another. Though this implies that they are absent from the original timeline, having hopped into the time machine and disappeared completely and permanently, or maybe replaced by another version of themself with memories from yet another timeline going ad infinitum.

10-yr-old Isaac - "pff, that's not how temporal relativity works.. I'm conflicted for enjoying this." lol

The idea of a time war is about the most horrifying thing possible. The Faction Paradox series was a good example of that.

oooohhhh weeeeee Time to get down into some hardcore science fiction !

I feel Babylon 5 did time travel really well, with not skipping the consequences and effects on the body also the future transmission Ivanova receives is a cool idea

One of the great things about the way Everything Everywhere All At Once is how it handles the multiverse; while there is a common identity, the lives of your alternates can be wildly dissimilar based on choices and happenstance. And in an infinite universe, those changes can be radical.

15:32 I notice you didn't mention Tenet (2020). That's a great example of depicting backwards time travel as just reversing time and having to go through the intermediate time, though the physics doesn't really make sense on close examination, even if you try to "iron-man" it by ignoring a few minor mistakes they made (like comments about antimatter).

one of my favourite 'invasion of the past' in fiction, occurs in the Star Trek Voyager Book 'The Escape' the crew enconters a civilisation which in a bid to overcome over-population, have spread out throughout time, using periods where their planet was uninhabited, the timeline is divided into periods, and citizens are able to commute between periods, but not within a period, any period which is critical to the evolution of the species is off limits, (except to specifically authorised personell) it is revealled that within a couple of hundred years, the species will discover means to travel to alternative dimensions, and the planet will be abandoned (which is the state it is found in by Voyager)

Awesome video great video. I probably listen to it in the future once I travel there… just by living one second per second.

Every "particle" in the universe are interchangeable resulting in the exact same result in most configurations, making a much smaller yet still huge number of possible configurations.

23:00 i guess the movie "Freejack" got a working solution for that exact problem. Pretty good writing in more than that regard tho, I definitely recommend it, mick jagger is pretty dope in it ✌🎉

In an alternative universe this would be a documentary not Sci fi , happy Arthur'sday ❤

Love Zelazny! You know, I come here for the great sci fi, but I stay for the book references I don't get to see anywhere else

Always worth watching Issac Arthur.

"Time Travel is even something people like to talk about to discuss regrets" Didn't know Isaac had time travelled to watch the BCS Series finale when writing this episode.

Awesome channel with awesome content and great quality as always say 🌍💯

My biggest issue with how most thing treat time, is the tendency to treat it as though it's made up of discreet immutable units. For instance if you view time less as a state and more a property, like say momentum, it could limit the possibility of paradoxes without requiring linearity/unidirectionalality as much like how strictly speaking it's possible for a planets orbit to reverse the energy and circumstances required mean this RARELY if ever happens, likewise if thing had a sort of causal momentum then even if retrograde time travel is possible the effort needed for any action that would shift causal sequences would increase in relation to the degree of said shift

I''ve always said that, there is no such thing as traveling to the past without affecting it. Eloquently put, Isaac. I tried describing it as, let's say you want to go back and observe some event. But somebody now has to step around you on a busy sidewalk, which delays them by a couple seconds, which could go on to mean they do not meet their future spouse, and a very important historical figure is never born. Or maybe that person was supposed to get run over by a car, but the 2 second delay means that in this 'timeline', they continue to exist, where they didn't in yours... So no matter how you grill that quantum cheeseburger, if travel to the past were possible, changes occur. But paradoxes can only occur if there is only one 'timeline', thus the many worlds theory, right? You simply arriving in the past, spawns a whole new timeline, and if you try to return to your present, you're still on the new timeline, and things will be different. You'd have to travel not only through time, but through dimensions, to return to 'your present'...

I was wondering when you were going to mention Tenet. If you haven't seen it, it's honestly a pleasant watch. Not the best movie ever but their take on time travel was really quite refreshing, I loved the fact that the traveler has to go through time in reverse but at a normal rate instead of instantly popping out of the time machine at the desired date like we often see in movies.

It's hard to go past The Technicolour Time Machine for time travel fun. A movie crew go back in time 1000 years to film the Vikings going to America, to later discover that the only reason that the Vikings went to America is because they went back in time to film it. Awesome story telling.

I think when something goes back in time it just creates a new timeline so from the original timeline's perspective it didn't work, but every time they try they creates new timelines in which it did work. I think this also prevents most if not all paradoxes. If you accidentally stop yourself from being born, that doesn't matter because you come from a timeline in which you were born but now exist in one that you don't, simple as that. Also weird timeloops like in the movie "Predestination" wouldn't be possible, which is good because they don't make sense.

My favorite time travel story has got to be Chrono Trigger from the SNES. Not only was it a game that was so instrumental in defining how games would develop over the next 10-15 years, it was also my first introduction to some of the story beat concepts that would become popular later in the rise of lovecraftian horror. Granted, CT was SUPER light on those themes, and it and it's time travel were kind of blanketed in more kid friendly stuff and the veneer of magic and fantasy, but that was what made it more accessible for young me to be honest lol.

Is it possible causality is totally bunk? Like just something we perceive and changing reality happens all the time.

Honestly, I have seen a great many models of time travel, from "you already changed the past", to "Time is sentient and wants you to obey fate, so expect your doomed splinter selves to die unless you make some crazy plan that makes it seem like you already changed the past" (Homestuck), to "You can time travel inside 5 minutes, and changes perpetuate in waves" (Achron, an RTS game), "constrained conditions but consequence-free changes once you do, bar some thematic convergence" (Ghost Trick) and dozens more. All I can say is that I would be a STRONG advocate to not allow time machines to exist. Especially if we end up playing 5D Chess with Multiverse Time Travel.

Seeing this I immediately thought oh whole episode based on Kang the Conqueror hoho this is his entire bag! I loved the notion light is the speed of causality. This addresses a lot of my concerns like a bubble of sped up time would be bombarded with centuries worth of photons. I always thought it strange to walk into bubbles of slowed and sped up time because your limbs that enter first would wither from the lag it takes for the rest of you to enter. And I’m glad someone finally talks about time being altered just from you entering due to minute displacements and gravity, it was always stupid to think you just needed to not actively avoid messing with time when just your presence would alter things! Thanks for the video Isaac!

Technically it tends to delete individuals who have the knowledge of how to do time travel or FTL by exploding them and anything around them at roughly the point in time where they are set on the path of eventually being able to understand the technology. Since this generally happens when said tech is operated, there's a good chance that it simply explodes, but the explosion travels all the way back along the frame of reference for the closest intelligent individual until that frame diverges from how the tech came to be activated.

Regarding interesting imetravel stories, I have a book recommendation: Menschen wie Götter (Humans like Gods) by Sergei Snegow. It is a really interesting soviet SciFi book, that sadly was never translated into english. So if you can read german or russian give it a try. It is interesting to see how diffrent SciFi was on the other side of the iron curtain.

@27:30 this just made me think of weaponizing everyday items to turn them into bombs just from being placed into another time

In an alternative timeline, Issac Arthur is named Arthur Issac.

Hey Isaac love your videos. Have you seen the movie Tenet? Or even better Predestination? Two great time travel movies.

One of my favorit time villains was the Observers from Fringe.

Hey at 4:34 the Stargate example (bottom-right) of rapid aging is due to nanobots artificially aging him and not a time thing. I only bring this up because Stargate actually has some good examples of using faster/slower time fields. In one episode of SG-1, Season 2 Episode 15 "A Matter of Time," it's even time dilation due to a black hole, and it shows all kinds of problems with communication and other stuff because of it.

I'm late for Arthursday glad I made it 👍

27:34 I saw someone explain this multiple-iteration corrective loop problem with a Mobius strip. You kill your grandfather, which leads to you not existing, allowing the next grandfather to be alive to make you so you can kill him again. (I don't really remember at this point.)

Interesting as per usual.

Ah yes some good ear fuel for the start of my day. Thank you!

One thing that isn't mentioned in time travel is earth movement. Counting only rotation around sun 30 km pre second going back in time 1 day should move you more them 2.5 million kilometers into space 😁

Outer Wilds was cool Using a supernova *as the power source to time travel to before it happened* was AWESOME

Another superb episode as always Isaac. Another confusing topic and concept well explained for cave man me.

Bro EVERY major technological advancement was "dubious" to those who lived before it became the norm. I don't put anything past us given a long enough timeline.

Re: Universe hostile to time travel. Niven did it earlier, although he wrapped the whole thing into a short story. A civilization losing an interstellar war gets desperate, and one crazy idea that makes it to the top of the chain is that xenoarchaeologists have, throughout their history, discovered some weird artifacts that look like they might be the beginnings of time travel devices, and date, as near as they can tell, _exactly_ to the end of that civilization by way of natural disaster. So, the theory is that if anyone tries to build a time machine that maths out, the universe stabilizes in a state where _something_ prevented the machine's completion. Whether it's 'the universe' acting, a collapsing time loop, or a divinity isn't gone into. So, they devise a cunning plan: 'leak' a time machine plan to their enemy in the hopes that they'll try to build it and the paradox will win the war for them! Even if it's a coincidence and/or not an actual time machine, building an enormous cylinder than spins at relativistic speeds* will waste resources that otherwise would go into the war. No sooner do they agree to this plan, however, than they receive word that the war is already over: their own sun has just begun going nova. * I've never seen this particular time machine referenced anywhere else; it involves charting a path around the cylinder that ends before it starts. I'd guess that someone has figured out an error in the math, or it requires some impossible material, or maybe it's practically useless since I suspect you can't use it to travel to a time where the device doesn't already exist.

Some things will probably be forever sci fi. Backwards time travel and ftl travel are 2 of those.

Nice animation in the intro. I am learning blender now and doing stuff like that but not that good yet.

I try to like all ur videos , I hope I didn't miss one

Currently readong The Abyss Beyond Dreams by Peter Hamilton, so fun to see it being featured on this video.

Theist version of shoulder of giants saying. "If I see far it is because I know which giants to climb."

No mention of Niven's Law? Proposed by Larry Niven in an essay in I think "All the Myriad Ways", it states "In any universe where time travel is possible, time travel will never be invented." The argument is pretty straightforward. Universes can be broken into two types: ones where time travel is not possible, and ones where it is. Obviously, in the first type we don't get time travel so we can discard those. We can also divide the set of universes where time travel is possible into two: ones that have "historical inertia" as you called it, and ones that don't. For those that have historical inertia, what is the simplest, lowest energy reaction that corrects for all the other changes caused by time travel? No time machines! So we can rule that set of universes out, there will be no time travel there. So finally, we get to universes where there is no historical inertia. In those, time travel will get invented, and will get used. And timelines will get changed, willy nilly all over the place. This will continue, until... we land in a timeline where time machines were never invented. And there we stay since by definition you can't get out of that one.

I thought that I was smart and educated but listening to this Awesome Video makes me feel confused and lacking about this subject but maybe it is just that complicated...

Damn. Time travel gets you….riled up. 😂 It makes my head hurt too bruh. But the fact that space/time seem to be so closely connected if not the same damn thing, makes me think there’s probably some really weird shit that “time” can do that whenever we discover it, will blow our minds. Space at least is somewhat tangible, time is much more ethereal…incorporeal….to me.

When I was in high school I wrote a short story describing how humanity got rid of entropy by just pumping it into another universe and called it the entropy conditioner. This ended up in a brutal war between us and the other universe, where our side had no technology to travel to their side and their side had technology to travel to ours. Because we were too lazy to invent the actual techniques to travel there we could just pull energy from them. Anyway one of the tropes of the series was the government mandated that everyone increased their energy usage to decrease the amount of energy the other universe had available to it. Anyway the surrogate universe eventually lost all of its available energy... But there was a lone survivor who had the ability to transfer between universes and so he went to a second universe and describe the process to build an entropy conditioner focused on our universe exclusively. Since we had largely grown hedonistic and didn't really invent a whole lot of stuff this effectively trapped us in a universe that was dying and there was no escape. In the end the lone survivor of universe b lived a life of luxury in universe c as being heralded as the savior of the universe. The brief 20 seconds of the intro of your new video here Isaac, reminded me of this whole silly thing I had written something like 24 years ago.

There was a short lived series starring Kevin McKidd, in which a man who could travel through time had inadvertently altered his meeting with his wife so that she had no idea who he was.

The title immediately reminded me of the book " this is how you lose the time war" by Amar El-Mohtar