This episode is sponsored by Skillshare I had a really good time travel joke to startthe episode with, but it turned out none of you liked it.
So today we start off our sixth year hereon Science and Futurism with Isaac Arthur, and I am your host, Isaac Arthur.
As we travel from 2019 to 2020, Time travelseemed like a good topic, and I figured we’d start by discussing why we don’t think itcan work under known physics, then talk about what the implications would be if it turnedout we were wrong.
Of course[a] the thing I always take as thebiggest proof time travel isn’t possible is that nobody is offering package deals fortemporal vacations.
If time travel were possible we should see folks from the future and thepast everywhere, and every when, and not just a few secretive folks with a time machinelike we see so often in science fiction.
People don’t really seem to look much at what theextended and expanded use of such a technology would be.
While that may be forgivable whena story contains such technology having just been invented, so everything revolves arounda prototype or its first use, this doesn’t really work with Time Travel since its beinginvented and used at any point means it’s basically been invented and used at all pointsin time.
If you managed to invent time travel, you ought to be able to walk out of the machinea moment later and be welcomed by all your admirers and descendants from when you hoppedback in time to, say, cure smallpox or similar.
Needless to say, a lot depends on how timeworks, which depends a lot on what time actually is, and honestly we still don’t really know.
We’ll try to look at how time travel might function, or not function, under a few ofthe main concepts folks have offered for explaining time later in the episode.
First though, let’s talk about the issue with time travel under modern relativity, and then under modern quantum mechanics, where they don’t quite match.
In relativity, time is a flat linear thing plodding on at a set rate in the same wayplanets are flat; which is to say they aren’t, unless you build one that way, but they usuallylook flat, or fairly flat, at low altitudes, get up high and it doesn’t look flat atall.
Space-time, on the other hand, looks rather flat at low gravities and accelerations, but when those are high it doesn’t look flat at all.
On a curved planet you also can only see so far because of the horizon, and somethingmuch the same applies to space-time curvature.
The ‘edge’ of the Universe is actuallyour cosmological horizon and as far as we can see, but we’re pretty sure it keepsgoing on a long way after that.
As we look further from Earth in any direction we seeobjects receding away from us at faster and faster speeds, eventually so fast they canno longer be seen because space expands between us and them faster than light can cover thenew space being added.
Or so we assume, we obviously can’t seethose places, but if it didn’t we’d actually be at the center of our Universe.
And indeedwe are when we normally reference the Universe as that’s shorthand for the Observable Universe, which practically speaking is all there is since we can never see or speak to or travelto the rest, without some sort of faster than light technology.
The problem is that the faster you go, the more energy you need to get to that speedand the more you need for each increment higher.
More and more of your total energy is of thekinetic or potential variety, rather than mass-energy, that E=mc² thing.
The more yourtotal energy is not mass, the slower time is passing for you.
If you look at some objectthat appears as stationary relative to you, like say a coffee cup you’re holding, allof its energy is just mass energy.
Almost all anyway, there’s a little heat energyin that warm coffee which is trivial compared to its mass energy, which is the rough equivalentof a dozen megaton hydrogen bombs, hence why coffee is so great for giving you energy presumably.
If you were to add an equal amount of kinetic energy, or speed, to that coffee cup, it wouldnow have half of its mass energy in the form of kinetic energy, zipping along at 86% ofthe speed of light.
Since only half of its energy is now mass energy, it will experiencehalf as much time as would a stationary object.
If we were to double the cup’s kinetic energy, with only one-third of its energy being mass energy and two-thirds of its energy is kinetic, and the cup would experience time at one-third of a stationary object – and it would be rocketingalong at 94% of the speed of light.
It’s impossible to actually reach the speedof light because it takes more and more energy – kinetic energy – for each percent closerto light speed you go.
It took the same energy to get from 86% of light speed to 94% as itdid from starting at rest and up to 86%, and in fact it takes 7 times that to get to 99%, 22 times that to get to 99.
9%, and 70 times that to get to 99.
So we say you can’tget to light speed because you’d need infinite energy to do that.
But I think this sometimes gives people the wrong impression that you could actually hitlight speed if you just had some phenomenal power source.
Mass is essentially what experiencestime and things without mass, like photons or gravitons, experience none.
All objectssplit their energy between motion through space and motion through time.
How much timeyou have pass is all about what portion of your total energy is your mass energy, go99.
99% of light speed and time runs at a 70th the normal speed, and I think it’s easierconceptually to ask how you’d ever eliminate all your mass entirely, infinite power ornot, as that’s the only way you can move at light speed.
Time slows down until you hit light speed, at which point it stops entirely, and as aresult photons and gravitons and other light-speed particles don’t experience time, whereasultra-low mass particles like neutrinos, which move at nearly light speed, experience verylittle time.
If you could go faster than light, then by that relativistic equation, you wouldbe experiencing time not just very slowed down but running backwards.
Again that’sthe usual explanation and I imagine most of you have heard it before, for my part I thinkit’s a bit easier to ask first, how would you ever remove all your mass to be goinglight speed, and second, how you would somehow have a negative mass to be going backwardin time.
See the Things Which Will Never Exist episode for discussion of negative mass andother negative quantities.
It is not coincidental that virtually everyproposed type of faster than light travel relies on negative mass or energy, which wehave not yet detected or created a single particle of, contrary to some grossly inaccuratelytitled popular science articles.
For this same reason, we always point out that almostall these methods open the door to time travel.
When it comes to normal matter at least, wecould not move so much as a single atom at the speed of light if we used every bit ofenergy in the entire Universe, and we’d need more than infinity to make it move backwardin time.
We might get around these issues somehow oneday, but hopefully it’s a bit clearer why FTL and time travel so often get phrased asan impossibility not just some technical issue that can be solved by sheer determination.
It’s not like traveling to the Moon, it’s like making 2+2=5, and when we say this orthat theory might allow FTL or time travel, what we usually means is that the theory doesn’texplicitly forbid it thus far.
One of those methods that does get kickedaround is using a wormhole, and those require the use of negative mass or energy, and actuallyonly let you travel back in time to when the wormhole was created, a result of closed time-likecurves or CTCs.
This is a worldline, or path through space-time, that leads eventuallyback to the exact same coordinates in space and time.
Think of it like any curved pathyou might walk down and end up coming back to where you started, like a circular runningtrack, only you also come back to when you started.
Now assuming we had this option invented, somehow, it might make for some interestingcivilizations.
You can’t go back and see the dinosaurs or end some war in ancient historybefore it began, but you can go back to the moment it was invented and built.
Isaac Asimovuses this sort of time travel in his novel The End of Eternity, which happens to be myfavorite book by him.
There, a group of time travelers called the Eternals, who live ina pocket of space-time, and can travel anywhere inside time between when the device was inventedtill the end of the Sun, and use the death throes of the sun dying as their power sourcefor time travel.
The book spends a fair amount of time lookingat how that impacts civilization, since they are not secret time travelers and actuallyengage in trade between centuries.
Indeed that was the original purpose and they laterbegan trying to change events to engineer better futures by making very tiny and calculatedchanges, like sabotaging someone’s vehicle not to start up before they’d have goneon to attend some major event that shaped them into being a dictator or similar.
Youcan see our episode, the Butterfly Effect, for an idea just how small you could makea change and expect it to have world-altering effects a few centuries down the road.
Tryingto predict those in terms of an outcome is far harder of course.
We’ll discuss that scenario for a civilization, one that can travel back to the time suchtravel got invented but not before, in a bit.
First though, I mentioned quantum mechanicsis a bit different than relativity for time travel and it actually offers us many pathwaysthat could either allow time travel or just sort of trick us into thinking we had.
Oneof those is the notion that you aren’t traveling to your own past but one very similar, outof the multiverse concept of the Many-Worlds Interpretation of Quantum Mechanics.
We’renot going to get into the weeds of how Quantum permits, or might permit, time-travel, buthere you’re not actually traveling to your own past so you don’t have to worry aboutthe Grandfather Paradox, because it’s not your past you’re changing, just somethingvery close to it.
The grandfather paradox is where you go backin time and accidentally kill your grandfather, I assume accidentally anyway, and as a resultare never born and can’t travel back in time to kill him.
Needless to say that’san extreme case but it’s the notion that something you do in the past would cause younot to be able or interested in doing the trip after you made that change.
You travelback in time to save JFK from being assassinated and now you no longer have a motivation todo so.
Against that we have the self-consistencyprinciple, which is that you’d inevitably have events between when you went back to, and when you left from, play out so as to make the trip still happen.
Conceptually thisis like water evaporating from the ocean and falling down as rain on land.
That water cantake any number of paths back down to the ocean again but still gets there.
For instance, you go back to next week with today’s winning lottery numbers but write yourself a notesaying you need to make that trip.
It’s not that any given action is predestined tohappen the same, just that only those various sequences of events that lead back to causingyou to get into that time machine actually continue to exist while all the others essentiallypoof out as hazy possibilities that can’t exist.
The raindrop can fall many places andfollow many courses to flow to the same place and time.
Many timelines are possible butonly those that lead to a self-consistent time travel event can be stable and exist.
That one works a bit better under the Copenhagen Interpretation of Quantum, which is the Schrodinger’sCat version where it’s alive and dead simultaneously till observed, as opposed to Many Worlds wherein one universe it’s alive and in another it’s dead, and you just don’t know whichone you’re in till you opened the box and checked.
As an analogy to self-consistency, events are in quantum flux till observed, like with the cat, at which point only onepossibility has happened and the others wink out, and only a self-consistent time travelevent could be observed.
This presumably results in no option for paradoxes, always a problemwith time travel, as no paradoxical sequence will not wink out.
This paradox issue is a sore spot in some science fiction franchises.
What exactly wasthe point of the crew of Voyager going back in time to fix histories they didn’t like, when it was made clear elsewhere in the franchise that this wouldn’t change their historyat all, only create a different history for someone else to enjoy, one among infinity?Another, more interesting possibility for paradox avoidance is that as the Many Worldsbranch off from one another, some branches are pruned.
Only the branches that containno violations of the laws of physics or logic remain on the tree.
And so, if you did tryto kill your grandfather, then starting at the moment of his near death, the universewould branch into the histories where you succeeded and the histories where you failed, and only the latter would survive.
What you would experience would be frustrating failureafter failure, because no matter how bizarre or contrived a coincidence was required toprevent you from killing him, only the histories in which you failed and he produced your fatherwould be left.
If this is true, there’d be no reason for time travelers to worry aboutcausing a paradox, because the universe absolutely would not let them.
And indeed you can argue it can’t possibly be your true past anyway because the Universewe’re in has a set amount of matter and energy and if someone from next year justarrived here now, they’d be adding mass and energy, mass and energy that is interactingwith stuff like producing gravity.
So even if we transported you back in time to somesealed underground bunker that little extra mass is slightly altering the Earth’s spin, how much gravity it exerts on the Moon, and so on, changing the future, even if apparentlyvery little.
Little effects can have massive long term changes, again see the ButterflyEffect, but from a multiverse perspective it’s a different universe if even a singleatom is not exactly where it otherwise would have been.
Conservation of mass and energy isn’t the entirety of it, you can’t get around thisby just exchanging equal masses of material between two times, they’d have to be identical, the same state, to switch and not produce changes to the state of the Universe, andthrowing something backward in time to replace something identical in every way, which wouldinclude positions and state of memories in a brain or harddrive, is not meaningful timetravel.
So you couldn’t just teleport data from tomorrow back to today on your harddriveso it told you tomorrow’s lottery numbers, even though you wouldn’t seem to have changedthe mass or energy of the universe.
And indeed that sort of quantum teleportation, changingthe states on bits on a harddrive, is one of those possible FTL or time travel notionsthat gets kicked around and probably would not work.
Another ‘cheat’ approach to quantum time travel is that even though the odds of itare beyond minuscule, there’s always a chance of things assembling effectively at random.
So you could have a bunch of atoms randomly assemble into a person with memories of beingfrom the 22nd century.
Those odds, while again vanishingly small, are still larger than thetotal number of universe combinations in a multi-verse where everything that can happenwill happen, so if Many Worlds Interpretation is correct, there are universes where someoneidentical to you right now has emerged in the time and place your grandfather and grandmotherfirst met and interrupted their date raving about being transported to the wrong dateand causing them not to meet and fall in love.
No grandfather paradox results from this though.
There’s also one where you think you’ve invented time travel, blow yourself up turningthe machine on, and a copy of yourself identical to the moment you threw the switch appearsat the intended destination time and place, smugly convinced the machine worked.
See theInfinite Improbability Issues episode for more craziness associated with Multiverses, but while that technically isn’t time travel, it effectively is for all practical purposesexcept the paradox, and of course not being something you can really choose to do.
Okay, that’s the not-too-quick overview of the mechanics of how time travel and paradoxesmight or might not work, let’s consider some scenarios for those if they did work.
What sort of civilization gets produced? First, if the multiverse approaches work, and lets you just travel there, adding mass and energy to some place when you arrive, this becomes really good news because it means the number of possible Universes is now infinite, rather than just very large.
Many-worlds generally assumes there’s a Universe for every singlepossible combination and arrangement of each particle, that’s a huge number but not infinitebecause the mass of the Universe is assumed to be finite, at least the pocket we callthe Observable Universe.
If I can add an atom to that, I now createeven more universes.
One might argue it’s still not actually infinite since you canonly put so much matter in the Universe without collapsing it into a black hole, but as theUniverse expands that amount increases.
You also get FTL travel out of it, since you canjump back in time, move in a direction, jump back again, move again, and so on until youare at a destination, like the Edge of the Observable Universe.
So the number of possiblerealities you could visit and colonize is either infinite or so large it makes no differenceand you never have to worry about the Heat Death of the Universe because you could addor subtract mass and energy and entropy from any given time, or just move to a differentone.
Under more classic time travel you can avoidheat death too, by invading your own past.
If the world was about to end you could jumpback to when it was younger and colonize that, and it’s presumably fine to do so sinceyou can’t do it if paradoxes are a problem as you’d erase the civilization that producedyou and your time travel device.
At the cosmic level you just jump back toa billion years after the Big Bang every time you run low on material.
You might be wonderingif that proves time travel can’t happen since clearly no one did that, but logicallythat doesn’t quite hold up as it’s a bit of a tautology, circular reasoning, whichis kind of appropriate for contemplating circular bits of time I suppose.
If the universe permitsno paradox of this type, time travel isn’t possible so it doesn’t matter, but if itdoes it’s because you are getting new realities out of it, and there will be realities forevery possible scenario including a reality that never had time travelers arrive in itprior to time travel being invented there, we’d just be in such a Universe.
Or onewhere the footprint was minimal.
Of course you also have the Self-Consistencyangle as an option too, you jump back in time and colonize the Early Universe but the patternof possible futures leading to us only allows the ones where we’d be ignorant that occurred, like they died off or grew very slowly and distantly from us so we hadn’t noticed thesetemporal colonists yet.
Tenses are rather confusing in time traveltoo, how do you refer to your own future that is your world’s past or vice versa? I oftenget a bit confused when writing scripts on the show just because I often refer to thingslike last week’s video, also discussing problems with Time, which came out December26th, except that it is not January 2, 2020 as I write this, it’s October 28, 2019.
When I finish writing this script and upload it for the crew to edit, I’ll get to workon making the video for November 7, Cybersecurity, it can get rather confusing at times and yesterday, Sunday October 27th, I was doing a live chat session after our livestream on our Discordserver and started referencing people to our Space Pirates episode for a detailed explanationof what I was discussing at that moment, only that episode hadn’t come out yet.
I’llalso sometimes reference episodes that don’t exist, because some bit of material got removedfrom the draft for time constraints or to improve the flow of the episode, and I forgot.
Those unused scripts or versions of scripts are similar to timelines that never happened, due to changing time, and presumably there are ones where those episodes were made.
That’s probably a small hint at the sort of tense confusion time travel civilizationswould get.
How do you refer to something you plan to do yesterday, or that you alreadydid tomorrow? If this sort of travel is common and non-paradoxical, because it’s a sequenceof multiverses, how do you phrase an explanation to your friends when you come into the officein the morning sporting much longer hair and beard and say you just came out of a timelinewhere your wife died in a car accident so you had to skip back and fix that, and howit was a very nice funeral and then the two of you went on a vacation afterwards becauseit was a bit stressful.
How about when a longer-bearded copy of yourco-worker Bob walks in and say it’s going to be a busy day, and then one with an evenlonger beard comes in and says, “Good news everyone, tomorrow we closed the deal withour new client, but we all need to do some extra overtime last weekend”.
It would tend to seem like cohesive timelines would utterly breakdown too, and you mightget weirder day-to-day options like dating apps that let you swipe forward or back, insteadof left or right, to find the girl or boy of your dreams from a different century andsit down on your first date and see what your future would look like.
Now in fiction we often get some sort of rules preventing tons of time travel, concepts likeinertia of future events or changes requiring more effort the further back you seek to makethem.
But often it’s actual legal rules, and that’s problematic because it doesn’treally seem enforceable to have temporal police.
Someone just goes back in time and preventsthem being formed.
If they’ve got some widget that prevents such things happening, somesort of time shield, then they can presumably build a bigger one that protects their wholecivilization too and folks can go do as they please elsewhere.
I get a time travel device and nothing is stopping me from taking some classes in Latinand Roman History, grabbing lots of textbooks and schematics and modern widgets, and jumpingback to Caesar’s day and setting myself up as Emperor, or jumping forward to the futureto get some life extension treatments and awesome cybernetic enhancements and then jumpingback to the past to set myself up an immortal leader.
If you do have that infinite numberof timelines option, this might be a better alternative to virtual realities where youcan be whatever you please as it might be cheaper and easier and more real-feeling tojust hurl folks to other timelines.
There should always be one where the only differencewith your normal history was that someone identical to you popped up at a given timeand place, at which point it diverges.
We usually say technologies like FTL and timetravel only exacerbate the Fermi Paradox, the big question of where all the aliens are, because it now means you have to contemplate aliens from a much broader region of spaceand also ones from the future jumping into the early universe.
But this is one alternative, that nobody spreads out to the stars because they just colonize earlier parallel historiesof their own world, and we just happen to live in a timeline where that hasn’t happened.
Confusing stuff that’s giving me a headache, so I’m going to close out here and get towork on the Cybersecurity episode for November 7, 2019, which I already finished as I recordthis on November 1st, 2019, but first let me wish you an excellent 2020, now that 2019is over.
When contemplating time travel, or fantasizingabout it, which I imagine I’m not alone in doing, one common problem is all thosemissing skills you’d need to actually have to live back then, whenever then was.
We havea very skilled based society compared to prior eras but they had a lot of skills we generallylack, so if you just popped up in some ancient kingdom you’d have a rough time survivingwithout knowing the language or how to do things like grind grain or swing a sword around.
But we do live in a far more skill-based society than in olden times and it’s always handyto be acquiring more skills.
Plus it’s nice to just have them, even when they aren’tmeant for career purposes they offer fun new hobbies and a feeling of confidence that youcan get stuff done.
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So that will wrap our show up for the day, but we’ll be back next week to our alien civilizations series to take a look at alienconspiracies and aliens who conspire, and ask what sort of motives they might have fora given conspiracy.
The week after that we’ll return to the Moon, to look at what industriesmight arise there and help us develop an interplanetary society, in Moon: Industrial Complex.
If you want alerts when those and other episodes come out, make sure to subscribe to the channel, and if you’d like to help support future episodes, visit our website IsaacArthur.
netto donate to the show or look over our inventory of over 200 episodes or our awesome SFIA merchandise.
Until next time, thanks for watching, and have a great year!.