What is the Grandfather Paradox? | Neil deGrasse Tyson Explains… 

Where would you go if you had the ability to travel back in time?

Everyone is so scared of changing the past a little because it would change the present, yet no one cares changing the present even though it'll change the future! 🤯

“The road you take to avoid fate, is often the one that leads you to it” … one of my favourite quotes

That "stepping on a butterfly" story mentioned toward the end of this video came from the excellent Ray Bradbury short story "A Sound of Thunder" that was first published in the early 50's which was, in my opinion, one of the greatest sci-fi short stories every written.

I always thought that if you can travel back in time, you can easily prevent or change things from happening, but then that creates a parallel universe where both outcomes are possible and you stay in the one that you changed.

The thing with "Supreme Strange," as the alternate version of Doctor Strange in "What If?" is referred to colloquially, is that he "lost his heart instead of his hands." In the main universe, he has horrible injuries to his hands in a car accident. In this alternate timeline, he convinces Christine Palmer to go to the award dinner with him (in the movie, she turns him down). Instead of the horrible hand injuries, he loses Christine in the accident. He spends his fortune researching time travel instead of experimental surgeries. He still ends up in Kamar-Taj and becomes a Master of the Mystic Arts, but he is definitely the *worst* person to entrust with the Time Stone. He goes back in time and tries to save Christine over and over, but because her death is what caused him to encounter the Time Stone in the first place, undoing her death undoes his ability to undo her death: Paradox! He finally accrues enough power to warp reality itself to his will by consuming the lives and powers of numerous extradimensional entities and even an alternate version of himself. But when he uses that power to save Christine, the laws of space and time snap under the pressure. He bent reality beyond its tensile limit. His universe, and every being in it other than himself and a small sphere he is able to preserve, dissolves into nothingness. He is (until the Watcher recruits him and brings him into the Multiverse) doomed to exist within a crystal sphere a few meters in radius, forever. His entire universe is essentially the saddest snow globe ever conceived.

I just imagine Neil DeGrasse Tyson shaking me awake, then says "Did you know..." **cue video**

Two things I've always thought might vastly change time travel the way it is usually depicted: 1. If from now to end of the universe, time travel is invented, could we possibly already be living in a world that is the final product of every change made by a traveler? 2. If changes in a timeline did cause it to branch and become a different universe, then if you changed your own ancestry by accident you would still exist. Only the new universe you would be compromised.

You probably hear this all the time, but… you make learning fun. Coming from someone who hates to learn after listening to you I can’t get enough of wanting to learn more about our solar system and planters and and all the wild exciting and even scariest things about our galaxy. so mesmerizing and you help make it that mesmerizing

The old Simpson's Halloween show where Homer time travels is the best "I wish I wish I hadn't stepped on that fish". Regarding the paradoxes, what about the many worlds theory creating different branches of timelines.... Your arrival in the past would create a different universe... not affecting the one you came from. Just thinking out loud... great episode!!!

The Netflix show “Dark” presented the best time travel story i ever saw.

Groundhog Day also gives us a good example of inevitability/destiny. The little subplot there of Phil Connors (Bill Murray) helping out the old homeless man numerous times - getting him meals, bringing him in out of the cold, taking him to the hospital, etc. - but no matter what he does the old man dies each time. I think I remember the filmmakers touching on this and what they wanted to show is that no matter how much Phil knew and how much he could predict and control having relieved the same day over and over again, that he still wasn't omnipotent. Some things were beyond his control & power no matter how hard he tried or how many scenarios he played out. Comforting thought in a way...

A comic with an appreciation of science and an aptitude for it plus an astrophysicist with a great sense of humor equals me hooked. And thank you Mr Tyson for clarifying that it need not be called the grandfather paradox.

I’ve always been of the mindset that “what is meant to be will be”. That everything that has happened or will happen is that way because of AND in spite of time travel effects. If I interact with someone/something in the past, it doesn’t change the future, because it’s my action that sets that future thing into motion.

The banana peel scenario reminded me of the star wars ep.3 scenario with Anakin when he had a dream of the future with Padme and tried hard to prevent it, going so far as to joining the dark side, but it turned out that him doing all that, was the reason why she died.

I like the idea that every time something changes in the timeline, it doesn’t change the future of the original timeline, it just creates a parallel branching timeline

I love how they can make the most serious and complicated things so simple and funny.

5:00 The final bits of Steins;Gate. Keep trying to save someone from dying, but they just die in a different way each time...at the same point in time...every time. 7:10 Literally the basis of Steins;Gate. lol

I'm just watching this before I go to work. I've never laughed so much and learned so much at the same time🤣 You guys rock. You have made my day 😊

I don't think I can ever get tired of the concept of time travel

Sending messages back in time is basically the Plot of Stien's Gate. That game also deals with the concept of "attractor fields" which are macro scale events that can only be changed if you alter the past enough to escape the pull of them. It's kind of like some kind of causality gravity. Grandfather paradox only really matters in a universe with a single flow of time. In a multiverse kind of scenario you can go back in time, which will automatically create a new branching point, and anything you do creates a new branch to a new future. In that situation it becomes basically impossible for you to ever return to the universe you left. If you broke ancestry, it wouldn't matter because the reality where you didn't and were born would still exist. Nothing gets broken, you just create and follow a new time stream into a new separate universe.

Futurama had a grandfather paradox episode.

The molecular displacement thing is possibly the closest I’ve come to explaining my reservations about time travel. Though my understanding is that it’s probably not possible to begin with.

8:10 sometimes referred to as a "stable time loop" or "a break in rule #0 of time travel" depending on whether or not you intended to become the origin point (another example is the Song of Storms from Legend of Zelda Ocarina of Time, in which the time travel was more "sending your consciousness into the past/future" than physically traveling... And another would be the Goron Vase from Legend of Zelda Oracle of Ages, which despite being more physical here, is just as impossible as the aforementioned song).

The “inevitable scenario.” I believe that was the main problem in butterfly effect. I wish they would’ve talked about the series “Dark”. Would love to hear how they feel about it. The ancestry paradox was crazy in there.

It’s funny, because there is also a short story called “the sound of thunder“ that ends the same way… A guy stepped on a butterfly and it is on his boot… Also, he comes back and the Nazis won the war… So, Chuck made a joke, but he actually predicted the outcome of this 40 year old short story… It is actually a really good read! I encourage everybody to check it out

I agree that the more interesting approach to time travel stories is that events in the past can't be changed. Because the past has already happened, if at some future point time travel becomes possible and someone travels into the past, they were _already in the past,_ so nothing can be changed.

Love it. I have so many time-travel models and theories. Alternative mechanics to predestination and ripple effects. The multi-timeline is a great one, but there are scenarios that break that in some interesting ways.

About the concept NDT mentions at 5:00, I think the Series "Outlander" plays with this pretty well. Great series with an interesting take on time traveling.

Would love to hear your take on time travel creating alternate time loops, kinda like in the TV show 'Dark'. I thought that was a refreshing take on it. Or in 'Travellers' where they send consciousness back in time into the bodies of people moments before they were meant to die. Or even something like 'Interstellar' where we essentially can only be an observer and not interact/ghostly like interactions. Would love to hear these thoughts 😊

Hi Neil! I believe, that if time travel ever is possible, it will be in a sort of "read only" mode. You would be an invisible viewer, with no ability to interact or affect anything. I suppose that's essentially what we're doing when we look up at the stars. 😀 If you instead could travel back and affect things, physics would either prevent a paradox from ever happening, as you more or less said with eventuality, or physics would just completely break down. Or, perhaps, if a paradox is created when traveling back in time, physics might only affect the traveler by instantly correcting the paradox or creating an alternative universe. Fun to think about, maddening to come up with a solution. 🤣 CHEERS! EDIT (7/15/22): People keep wanting to think of a Time Machine or Time Travel as being a more physical experience, of course how movies represent it, and I believe that the laws of physics simply will not allow any form of Time Travel to be a fully physical experience. Time Travel = Time Viewing. It will have to be some sort of virtual, or READ ONLY, experience. Something constructed out of inspecting the light, from the desired time, that has left the area of interest. Slap a telescope on Pluto, point it at Earth, record the footage, send it back to Earth, and you are now "Time Viewing" from 4.7 Hours in the past (+ another 4.7 Hours old sending back the data). If that telescope were powerful enough to record the planet in Uber Ultra MEGA HD [version 99.0], like Google Satellite view as a playable video, and you wanted to know where someone was 4 hours ago, BINGO, you can now get that info. Now push that telescope a light year away from earth, or further, and you now have the ability of Time Viewing further in the past, though no further back than when the telescope was implemented. One rebuttal was "that light has already passed that telescope by", referring to being able to go/view even further back into the past, which is an excellent point. But... Scientists don't believe you can travel faster than light, this is what people think prevents us from being able to view further back into the past in this scenario, and I agree with that speed limit. However, Scientists do believe it may be possible to travel through wormholes or bend space somehow that instantly, more or less teleports you, to other areas in the universe. You're not traveling through space, you're bending space to find a shortcut, to get around the speed limit. With the technology of traveling through wormholes, and being able to precisely inspect light from hundreds, thousands, hopefully millions of light years away, we will one day be able to inspect the light that left Earth when the City of Atlantis may have existed (12,000 light years away), when Dinosaurs roamed (66 million ly away, finally answering the question of how they died; we're pretty confident it was an asteroid impact, but what a sight that would be), or possibly even when the earth was formed (4.5 billion ly away). Even without the precise view using our "UberScope", the instant we discover wormhole, or similar, travel, we've essentially also invented "Time Viewing" just looking out a porthole. The sun may only be a spec, but that speck of light you're seeing from X light years away is from X years ago. Keep this in mind when you're doing things visible from a birds eye view! Someday one of your descendants might see you. 🤣 But, who knows, maybe there'll be another gadget that somehow detects photons through objects (a sort of X-Ray vision), then there's no escape. Granted these would have to be EXTREMELY high tech gadgets, but what's the limit to our technology? Only TIME... will tell. I think Harry Potter's trip into the Pensive is the best representation of "Time Travel/Viewing". It's nothing more than just VR experience, really, but you can see how Harry manages to view the past without affecting anything in the past (the observer effect), but with the information of the past he has the ability to change the future. CHEERS to you and thanks to everyone that's contributed below.

Neil basically describing the premise of stines gate was a great addition to my day

Hey! Loved this one! I wish you could do an episode on the many-worlds interpretation of quantum physics 🥰🥰🥰

Back in the 1980s - and believe me I read tons of Science Fiction in magazines to anthologies - I read a time travel story, possibly in Omni magazine, where a Rescue time traveler went back to find out what happened to a previous time traveler. In short, the time traveler he found wasn't the one he searched for. This "First" time traveler couldn't return because he inadvertently made a change that stopped him from being born. So the machine he traveled in couldn't return to First traveler's time, because that "time" doesn't exist. Why? Because time isn't simply a number like the arbitrarily designated year of 2022. For his time machine, time exists as a coordinate and that didn't exist for Future First. That's all the detail I want to give in case someone else remembers the story. I don't want to spoil it for others. Still, I like the idea that we are too insignificant to the universe for cosmic time to "concern" itself over whether we go back and create a paradox for ourselves or not. So Physics doesn't stop First or create the Samara conundrum. First didn't Marty McFly fade or disappear. He simply kept living in the coordinate he arrived at - unable to return to the coordinate he came from. Rescue traveler has to figure out if he can solve the problem for First traveler.

I love that Chuck watched the “What If ?” Series also 😂🔥

Aside from forgetting the name Back To The Future, Chuck was on point with the jokes and the pop culture references. I thought of the What If? episode with Doctor Strange too

Great conversation! Loved this one

In the movie Final Destination, I think there is the idea that even if you do somehow prevent certain events that were supposed to happen from happening, fate will eventually find you and have its way.

"A Sound of Thunder" the short story by Rad Bradbury where a time traveler tourist steps off the path and upon returning home finds the world has changed.

Pretty sure in H.G Wells book the answer to why he couldn't save her was that his grief from her death is what motivated him to build the time machine so if he saved her he would not had made the machine hence he couldn't go back in time to save her. Meaning he created his own paradox in that moment that it forces the timeline to always flow in that direction in order for him to build his time machine. I find time travel to be a fascinating concept but to ensure you can't mess with the timeliness I would love to build a Tachyon Telescope and Binoculars giving us a window to look into the past and observe our history unfold. That would be amazing.

I wish they would do an episode diving more into Marvel's multiverse theory which states that if you travel back in time, you're not traveling back in time, you're just creating an alternate timeline. I wonder what Neil's take would be on this theory, and how close it would be to the real thing if we ever created a way to time travel.

I gotta say, time travel is just around the corner for us. This new hypothetical particle seems like a worthy component in it's making!

Neil debunked his own debunking of Terminator. He said, "all the machines have to do is disrupt any part of Sarah's genetic line." But the actions of the machines is the catalyst that creates the revolutionary leader. So could the machines use time travel to prevent themselves from inventing time travel, thereby creating a divergent timeline where Kyle Reese was never sent back. Despite the advantages of the machines they failed because they focused on changing external variables when they could have looked inward and realized they had the technological lynchpin that facilitated the very premise. Or......would this divergent timeline spawn 'John Connor' machine counterpart to prevent the machine saboteur from preventing the invention of time travel so the machine race can defeat the human rebellion?

"Edge Of Tomorrow" - Excellent way to perceive time travelling

Working on a story featuring time travel. So glad one of the greatest minds of our time can break it down this simply

I loved a moment in one of Isaac Asimov's novels where time agents would prevent negative events, at one point preventing a massive interplanetary war simply by _moving a bottle_ from one storage room shelf to another, causing a momentary delay. My favourite time travel novel is _The Man Who Folded Himself_ by David Gerrold, in which the first person narrator is given a Time-Belt. He learns not only how to observe history, but due to infinite timelines, he literally can "edit" events to such an extent he can forget the original. He even finds he can strangle _himself as an infant,_ and yet not vanish from existence... he'd just created a new timeline in which he'd not existed. Even your referencing Ray Bradbury's _A Sound of Thunder,_ from which we get the Butterfly Effect, the guy's mistake of falling from the designated path causing the dead butterfly demonstrates an alternate timeline.

I've always heard the inevitability if time described as a stream or river. The river (representing time) flows along like it's supposed to. You can interact with it in limited ways. For example, you can throw a stone into the river. This will cause ripples and a splash but the river itself will still continue on mostly unchanged. I'm not sure I agree with it but it's an interesting analogy.

Chuck Nice makes this video so much better. Great comic ❤️🤙🏻

I love listening to Neil. He's a breath of fresh air

Love you Neil! Keep up the amazing work!

#StarTalk So Neil, as I’m listening I came across a question. In a previous show you talked about the universe expanding at a faster and faster rate, faster than light. In this episode you stated that traveling faster than light you can go back in time. Does this mean as our universe and all galaxies are expanding then we are seeing the formation of everything in reverse?

Man I wish this dude was my science teacher. Something about having this knowledge come from a black man hits different. Being mixed myself it’s joyful to see this level of knowledge. Praise to you bro.

The smartphone “time machine” (sending messages in the past) example, is amazing to hear from Neil say it’s possible after watching steins gate. 🤯🤯🤯

There's another even more obvious paradox in both the Time Machine and in The Terminator, though Wells got it right -- if the protagonist travelled back in time because his wife died, so to save his wife would be to rewrite the history in which his reason to travel back in time ceases to exist. So in any timeline where he travels back in time, his wife is already dead. The Terminator has the same problem. If the terminator succeeds in killing its reason for being sent back in time, it won't be sent back. And T2 repeated that -- Sarah Connor could not stop Judgement Day, because it's now her past... Her son would no exist of judgement day didn't happen, and if judgement day doesn't happen, she doesn't get warned about it, so she wouldn't stop it, therefore allowing it to happen. It seems like they figured that out for T3 though 😁

If we go back in time, we assume the earth is in a different position in the universe, so the most likely scenario is that we just materialize in space somewhere and suffocate immediately. Or rematerialize in the center of the earth? Or at 30,000 feet in the air? Time is one thing, but space (location) is another confounder...

"A Sound of Thunder" by Ray Bradbury is the story Neil mentioned at the end. A short story about time travel and "The Butterfly Effect" and it's basically the exact story Chuck described.

Thank you both! Always learn and laugh 😆

The tachyon enhanced smartphone and banana peel message alert that Neil talked about perfectly describes the plot of Stein's Gate, a very popular sci-fi anime series. The plot is about changing word lines and also employs the idea of minor changes and their effects on inevitable events in the timeline that Neil also mentioned

Great episode as always. You guys should really consider adding movie spoiler alerts though!

Great as usual. As mentioned in another thread, could you please do a video on pseudoparticles? TIA.

This is a great paradox except you've already defeated the paradox by going back in time. The fact you exist prior to your birth literally means you would still exist if you killed your grandfather before you would've been born. There is only really 2 options here, one being simply you exist in a different timeline that was created by your actions, or two being reality itself would come undone before peoples very eyes. There is no way you suddenly just fade out of existence like Back To The Future. The truth is once you time travel you've stepped out of time so there for the interactions within time would not apply to you anymore. This is also saying our understanding of time is correct and not completely wrong. We take time as passage of moments or chain of events, where as it's really a measurement of distance time as it's explained began with the big bang expansion etc. The only real time is how far particles have traveled since the initial expansion. So it might actually be theoretically possible to warp time and space but it may never be possible to go back to specific moments because that isn't what time even is.

What if every moment in time, the universe is splitting into an infinite number of alternate timelines? That would mean that any person travelling back in time would have to arrive at a point before a divergence, which would mean that from his own point of view, all the random variables are reset. So I might flip a coin, see it come up heads, then go back in time and watch myself flip the same coin, but have it come up tails the second time around.

I’m glad he brought up the what if series cause that’s exactly what I was thinking about when they were talking about the Time Machine

The movie you're referring to regarding the different baby being born was called "It's about time". One of my favourite time travel series was in "The Flip Side of Dominic Hide" released in 1980.

This is just one scenario in the grander cause and effect paradox, the biggest being that if you travel back in time to prevent something from happening, you negate the reason for you travelling in in the first place. To me, this is then ultimate proof of parallel universes. If time travel exists, there has to be parallel universes to prevent these self cancelling loops from happening.

2013 movie 'About Time' with Domhnall Gleeson, Rachel McAdams and Bill Nighy is a movie that very much touches on the 'what if you change anything prior to your child being born' aspect Neil talks about. Do catch this movie as it is very much a time travel movie with humor and touching romance. One of the best I've seen of the time travel genre... And I love this genre! :) Great video guys!

I have never had so much fun watching a science video. You guys make science fun.GREAT!

Actually in “ The time machine” The head of the Morlocks, which is the old wise being Explains to the professor the reason his fiancé kept dying is because he would’ve never spent the rest of his time and drive building the time machine had she not died so the fact that he has the time machine and keeps trying to save her he cannot.

What determines what is a major or minor historical event in the context of time travel?

Really REALLY enjoy these chats 🥰🤣😂

Sometimes, I think of the C&E Paradox (cause and effect), where the Effect actually happens before the Cause. The age old question: what came first, the chicken or the egg? A perfect example is The Terminator. In the movie John Conner, Skynet, and the Terminators already exist in Kyle Reese’s reality, but essentially they are the effect and the cause at the same time. Skynet sends a Terminator to 1984 to prevent the birth of John Conner, who in the future miraculously knows how to fight the machines. In the process of sending the Terminator back in time Skynet actually set that future in motion, ensuring both the birth of John Conner and it’s own creation.

Another time travel paradox: All the atoms in your body were somewhere else before they were in you. Tomorrow there will be some new ones, and some of the old ones will be gone, there is constant change. Travel far enough back in time, to before you were conceived, and every one of those atoms will be somewhere else. Since atoms cannot be in two places at once, even travelling back a week or a month will mean some parts of your body fading away, and the further you go the less of you will remain, until you're completely gone. Exactly the same applies to the machine you travel in - just a short time earlier, all of its atoms would be still be in minerals in the ground or whatever. I'm not buying a ticket for that!

I think the movie you are referring to regarding stepping on a butterfly is 'A sound of thunder'. But the original butterfly effect was a chaos theory derived by Edward Norton Lorenz in 1961 after simulating a weather pattern on a computer. The original story depicted a seagulls wing creating a storm, but was changed to a butterfly and a hurricane in later years for poetic effect.

Dr Who similarly did something like "inevitability scenarios" called fixed points. They were somewhere between this and essentially landmarks for time travelers, but if you somehow manage to refute one has severe personal and universal consequences.

Just a quick note about "forces of time stopping specific things from happening" ... Stephen King wrote a really cool book about that idea called '11/22/63', in which a man tries to go back and prevent JFK from being assassinated and all the problems that this man encounters. Another interesting series of books dealing with time travel and its inherent problems is Simon Hawke's "Time Wars" series. In which he takes actual events from history and introduces future soldiers. It is a very difficult storyline to explain, but I will try to be very brief: Basically in the far future, if any country/company/opposing ideologies have a dispute, rather than fighting with each other, you hire "time soldiers" to go back in time to any battle and whoever has the most people come back to the current time is the winner. Those 'time soldiers' don't necessarily have to fight each other, they could be on the same side, or in two completely different battles. Very interesting read.

Am I the only one who died laughing when Neil said “make the train come later?” 😂

What if when you time travel, it sort of "removes" you from the timeline and you can't be affected if anything ever changes? Could be interesting.

Wow I never felt like I know Tyson really appreciated this video 👍

Excellent stuff as always. 👍 Predestination (2014) now that’s weird!

Fry went back in time and accidentally killed his grandfather, then slept with his grandmother, so he had that brain thing making him immune to the brain spawn attack

Since no one has actually invented time travel, as we've seen depicted in film and shows, we really DON'T know exactly how it would work...BUT I've always been intrigued by the, what you now termed, Ancestral Paradox lol But my REAL question is about branched timeline time travel, like the one discussed in Avengers: Endgame, and the show Loki, where you go back in time, but instead of affecting the past of the future you came from, you, instead, create an alternative timeline that deviates from the one you came from, however, the events that happened in your original timeline still happen the same way. Do you think that THAT type of time travel is possible? Where going back in time doesn't change anything in your timeline, but creates an alternative one instead. That could technically and effectively get around the whole Ancestral Paradox

This reminds me of the story Thrice Upon a Time by James P. Morgan. Great story that uses the idea of sending information back in time to try and prevent a disaster. 🤘🧙‍♂️🤘

Now I get it my friend!!! For the most part. Got me thinking 🤔 alwayz

Fun and interesting.

Unless multiple worlds is correct. In which case, you couldn't change the past of your universe, but you could go back and create a separate universe that leads to a different future.

Chuck mentioning "What If?" Super! 😊

my issue with time travel is due to the particles in the universe. right now no matter what arrangements of particles there are we have all of them here right now. if you go back in time you introduce duplicates of them particles. would that break the universe due to having additional particles. would that throw an imbalance into the universe having too many of the same ones there? its like overfilling a bucket of water. if you add more will it overflow or burst the container

I would love to see a serious Terminator movie where they try to scientifically try to foil the future from happening using crazy antics like introducing Sara Conner to Chuck or having the terminator learn some exceptional coffee making skills and get a job at a coffee shop where Sara and Kyle are gonna be right before they conceive just so they can order a second cup. That’d be hilarious!

A Sound of thunder. 2005 film about a futuristic time safari where, in a trip to the past, a butterfly is stepped on by accident, causing a ripple effect in time. Great film.

The book is A Sound of Thunder by Ray Bradbury. Later made into a film. Read it as a project in school. I think it was my first ever sci-fi book.

I really want Neil to watch a Netflix show named " DARK ". This show takes time travel to a whole another level( messes with every time travel paradox that ever exist). Someone please make him watch that show and record it. I wanna see his reaction 😅

If we consider travel back in time as possible, then Novikov's principle of self-consistency would be the only solution to the grandfather paradox to have a logical coherence, as the multiverse-based solution generates grandfather paradoxes in turn.

We need to understand the deeper underlyings of this paradox. The two begin to discuss it towards the end of their discussion, my favorite example being the one when the text message. The anime Steins;Gate has journeyed into this aspect of time travel. And also, the inevitability of constantly causing the outcome in which you are trying to prevent. So bravo to you two for talking about time travel in this sense, I really enjoy the idea of it. I imagine at times my thoughts today are able to be pulled from the future up to the boundary of infinity, sometimes I wonder if it can go beyond the boundary. Anyway, if that’s the case, it does feel like there’s some time confederation suppressing these ideas. My favorite easily testable example is. Learning about Physics to try to counteract nuclear warfare. The learning of Physics LEADING to nuclear warfare. You two should do a review on SpinLaunch by the way I’m not sure that you have. It’s an interesting concept.

Hi Neil! GREAT as always! Could you be thinking of Men in Black 3? Griffin Is at the baseball game & says that the guy who caught the last out "would have been born Claira a typist if his parents hadn't had an extra glass of wine with dinner".

I always liked how Stargate SG-1 did this with Moebius. Where the time travelers of the alternate timeline go back and fix the timeline to revert to original.

Since time travel is theoretical at this "time" there is no way to determine if separate timelines exist. Time may not simply be linear; it may be better described as branching. Each timeline breaks away from the original creating multiple timelines with each result. Thus, it may be possible to go back in time and prevent your grandparents from ever meeting each other but your existence is unchanged because your timeline remains or remained in place. You only change or changed another branch of possible timelines.

solid gold. I love it.

Here's my thought on travelling back in time: The very act of being present in the past, causes so many changes that it ultimately leads to the person not existing or having travelled back in time. For example, if I invented a way to time travel and appeared just 5 seconds before, my very appearance to my 5-second-ago self would have resulted in him being unable to start the time travel experiment exactly on the dot. Which then means that I never travelled back 5 seconds ago, ultimately causing me repeat the time travel experiment on the dot again. Which would mean that at that precise moment I triggered the experiment, I am simultaneously not there (cuz I travelled back in time) and still there (cuz I created a condition that prevented me from travelling back in time)

Excellent incite on a amazing topic, movies seem to get it wrong,they say don’t or do change the past ,when it is already written. Love listening to the explanation from both of you!

05:30 - Tyson mentions that it would be cool for there to be a fiction regarding time travel where big events can't be changed. There's a Twilight Zone audio drama episode where someone tries to go back to stop the assassination of Lincoln and the major event cannot be changed, just the small details.