This episode is sponsored by Skillshare I had a really good time travel joke to start
the episode with, but it turned out none of you liked it. So today we start off our sixth year here
on Science and Futurism with Isaac Arthur, and I am your host, Isaac Arthur. As we travel from 2019 to 2020, Time travel
seemed like a good topic, and I figured weâd start by discussing why we donât think it
can work under known physics, then talk about what the implications would be if it turned
out we were wrong. Of course[a] the thing I always take as the
biggest proof time travel isnât possible is that nobody is offering package deals for
temporal vacations. If time travel were possible we should see folks from the future and the
past everywhere, and every when, and not just a few secretive folks with a time machine
like we see so often in science fiction. People donât really seem to look much at what the
extended and expanded use of such a technology would be. While that may be forgivable when
a story contains such technology having just been invented, so everything revolves around
a prototype or its first use, this doesnât really work with Time Travel since its being
invented and used at any point means itâs basically been invented and used at all points
in time. If you managed to invent time travel, you ought to be able to walk out of the machine
a moment later and be welcomed by all your admirers and descendants from when you hopped
back in time to, say, cure smallpox or similar. Needless to say, a lot depends on how time
works, which depends a lot on what time actually is, and honestly we still donât really know.
Weâll try to look at how time travel might function, or not function, under a few of
the main concepts folks have offered for explaining time later in the episode.
First though, letâs talk about the issue with time travel under modern relativity,
and then under modern quantum mechanics, where they donât quite match.
In relativity, time is a flat linear thing plodding on at a set rate in the same way
planets are flat; which is to say they arenât, unless you build one that way, but they usually
look flat, or fairly flat, at low altitudes, get up high and it doesnât look flat at
all. Space-time, on the other hand, looks rather flat at low gravities and accelerations,
but when those are high it doesnât look flat at all.
On a curved planet you also can only see so far because of the horizon, and something
much the same applies to space-time curvature. The âedgeâ of the Universe is actually
our cosmological horizon and as far as we can see, but weâre pretty sure it keeps
going on a long way after that. As we look further from Earth in any direction we see
objects receding away from us at faster and faster speeds, eventually so fast they can
no longer be seen because space expands between us and them faster than light can cover the
new space being added. Or so we assume, we obviously canât see
those places, but if it didnât weâd actually be at the center of our Universe. And indeed
we are when we normally reference the Universe as thatâs shorthand for the Observable Universe,
which practically speaking is all there is since we can never see or speak to or travel
to 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 speed
and the more you need for each increment higher. More and more of your total energy is of the
kinetic or potential variety, rather than mass-energy, that E=mc² thing. The more your
total energy is not mass, the slower time is passing for you. If you look at some object
that appears as stationary relative to you, like say a coffee cup youâre holding, all
of its energy is just mass energy. Almost all anyway, thereâs a little heat energy
in that warm coffee which is trivial compared to its mass energy, which is the rough equivalent
of 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 would
now have half of its mass energy in the form of kinetic energy, zipping along at 86% of
the speed of light. Since only half of its energy is now mass energy, it will experience
half as much time as would a stationary object. If we were to double the cupâs 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 rocketing
along at 94% of the speed of light. Itâs impossible to actually reach the speed
of light because it takes more and more energy - kinetic energy - for each percent closer
to light speed you go. It took the same energy to get from 86% of light speed to 94% as it
did 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 99.99%. So we say you canât
get to light speed because youâd need infinite energy to do that.
But I think this sometimes gives people the wrong impression that you could actually hit
light speed if you just had some phenomenal power source. Mass is essentially what experiences
time and things without mass, like photons or gravitons, experience none. All objects
split their energy between motion through space and motion through time. How much time
you have pass is all about what portion of your total energy is your mass energy, go
99.99% of light speed and time runs at a 70th the normal speed, and I think itâs easier
conceptually to ask how youâd ever eliminate all your mass entirely, infinite power or
not, as thatâs 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 a
result photons and gravitons and other light-speed particles donât experience time, whereas
ultra-low mass particles like neutrinos, which move at nearly light speed, experience very
little time. If you could go faster than light, then by that relativistic equation, you would
be experiencing time not just very slowed down but running backwards. Again thatâs
the usual explanation and I imagine most of you have heard it before, for my part I think
itâs a bit easier to ask first, how would you ever remove all your mass to be going
light speed, and second, how you would somehow have a negative mass to be going backward
in time. See the Things Which Will Never Exist episode for discussion of negative mass and
other negative quantities. It is not coincidental that virtually every
proposed type of faster than light travel relies on negative mass or energy, which we
have not yet detected or created a single particle of, contrary to some grossly inaccurately
titled popular science articles. For this same reason, we always point out that almost
all these methods open the door to time travel. When it comes to normal matter at least, we
could not move so much as a single atom at the speed of light if we used every bit of
energy in the entire Universe, and weâd need more than infinity to make it move backward
in time. We might get around these issues somehow one
day, but hopefully itâs a bit clearer why FTL and time travel so often get phrased as
an impossibility not just some technical issue that can be solved by sheer determination.
Itâs not like traveling to the Moon, itâs like making 2+2=5, and when we say this or
that theory might allow FTL or time travel, what we usually means is that the theory doesnât
explicitly forbid it thus far. One of those methods that does get kicked
around is using a wormhole, and those require the use of negative mass or energy, and actually
only let you travel back in time to when the wormhole was created, a result of closed time-like
curves or CTCs. This is a worldline, or path through space-time, that leads eventually
back to the exact same coordinates in space and time. Think of it like any curved path
you might walk down and end up coming back to where you started, like a circular running
track, only you also come back to when you started.
Now assuming we had this option invented, somehow, it might make for some interesting
civilizations. You canât go back and see the dinosaurs or end some war in ancient history
before it began, but you can go back to the moment it was invented and built. Isaac Asimov
uses this sort of time travel in his novel The End of Eternity, which happens to be my
favorite book by him. There, a group of time travelers called the Eternals, who live in
a pocket of space-time, and can travel anywhere inside time between when the device was invented
till the end of the Sun, and use the death throes of the sun dying as their power source
for time travel. The book spends a fair amount of time looking
at how that impacts civilization, since they are not secret time travelers and actually
engage in trade between centuries. Indeed that was the original purpose and they later
began trying to change events to engineer better futures by making very tiny and calculated
changes, like sabotaging someoneâs vehicle not to start up before theyâd have gone
on to attend some major event that shaped them into being a dictator or similar. You
can see our episode, the Butterfly Effect, for an idea just how small you could make
a change and expect it to have world-altering effects a few centuries down the road. Trying
to predict those in terms of an outcome is far harder of course.
Weâll discuss that scenario for a civilization, one that can travel back to the time such
travel got invented but not before, in a bit. First though, I mentioned quantum mechanics
is a bit different than relativity for time travel and it actually offers us many pathways
that could either allow time travel or just sort of trick us into thinking we had. One
of those is the notion that you arenât traveling to your own past but one very similar, out
of the multiverse concept of the Many-Worlds Interpretation of Quantum Mechanics. Weâre
not going to get into the weeds of how Quantum permits, or might permit, time-travel, but
here youâre not actually traveling to your own past so you donât have to worry about
the Grandfather Paradox, because itâs not your past youâre changing, just something
very close to it. The grandfather paradox is where you go back
in time and accidentally kill your grandfather, I assume accidentally anyway, and as a result
are never born and canât travel back in time to kill him. Needless to say thatâs
an extreme case but itâs the notion that something you do in the past would cause you
not to be able or interested in doing the trip after you made that change. You travel
back in time to save JFK from being assassinated and now you no longer have a motivation to
do so. Against that we have the self-consistency
principle, which is that youâd 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 this
is like water evaporating from the ocean and falling down as rain on land. That water can
take any number of paths back down to the ocean again but still gets there. For instance,
you go back to next week with todayâs winning lottery numbers but write yourself a note
saying you need to make that trip. Itâs not that any given action is predestined to
happen the same, just that only those various sequences of events that lead back to causing
you to get into that time machine actually continue to exist while all the others essentially
poof out as hazy possibilities that canât exist. The raindrop can fall many places and
follow many courses to flow to the same place and time. Many timelines are possible but
only 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 Schrodingerâs
Cat version where itâs alive and dead simultaneously till observed, as opposed to Many Worlds where
in one universe itâs alive and in another itâs dead, and you just donât know which
one youâre 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 one
possibility has happened and the others wink out, and only a self-consistent time travel
event could be observed. This presumably results in no option for paradoxes, always a problem
with time travel, as no paradoxical sequence will not wink out.
This paradox issue is a sore spot in some science fiction franchises. What exactly was
the point of the crew of Voyager going back in time to fix histories they didnât like,
when it was made clear elsewhere in the franchise that this wouldnât change their history
at 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 Worlds
branch off from one another, some branches are pruned. Only the branches that contain
no violations of the laws of physics or logic remain on the tree. And so, if you did try
to kill your grandfather, then starting at the moment of his near death, the universe
would 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 failure
after failure, because no matter how bizarre or contrived a coincidence was required to
prevent you from killing him, only the histories in which you failed and he produced your father
would be left. If this is true, thereâd be no reason for time travelers to worry about
causing a paradox, because the universe absolutely would not let them.
And indeed you can argue it canât possibly be your true past anyway because the Universe
weâre in has a set amount of matter and energy and if someone from next year just
arrived here now, theyâd be adding mass and energy, mass and energy that is interacting
with stuff like producing gravity. So even if we transported you back in time to some
sealed underground bunker that little extra mass is slightly altering the Earthâs spin,
how much gravity it exerts on the Moon, and so on, changing the future, even if apparently
very little. Little effects can have massive long term changes, again see the Butterfly
Effect, but from a multiverse perspective itâs a different universe if even a single
atom is not exactly where it otherwise would have been.
Conservation of mass and energy isnât the entirety of it, you canât get around this
by just exchanging equal masses of material between two times, theyâd have to be identical,
the same state, to switch and not produce changes to the state of the Universe, and
throwing something backward in time to replace something identical in every way, which would
include positions and state of memories in a brain or harddrive, is not meaningful time
travel. So you couldnât just teleport data from tomorrow back to today on your harddrive
so it told you tomorrowâs lottery numbers, even though you wouldnât seem to have changed
the mass or energy of the universe. And indeed that sort of quantum teleportation, changing
the states on bits on a harddrive, is one of those possible FTL or time travel notions
that gets kicked around and probably would not work.
Another âcheatâ approach to quantum time travel is that even though the odds of it
are beyond minuscule, thereâs 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 being
from the 22nd century. Those odds, while again vanishingly small, are still larger than the
total number of universe combinations in a multi-verse where everything that can happen
will happen, so if Many Worlds Interpretation is correct, there are universes where someone
identical to you right now has emerged in the time and place your grandfather and grandmother
first met and interrupted their date raving about being transported to the wrong date
and causing them not to meet and fall in love. No grandfather paradox results from this though.
Thereâs also one where you think youâve invented time travel, blow yourself up turning
the machine on, and a copy of yourself identical to the moment you threw the switch appears
at the intended destination time and place, smugly convinced the machine worked. See the
Infinite Improbability Issues episode for more craziness associated with Multiverses,
but while that technically isnât time travel, it effectively is for all practical purposes
except the paradox, and of course not being something you can really choose to do.
Okay, thatâs the not-too-quick overview of the mechanics of how time travel and paradoxes
might or might not work, letâs 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 thereâs a Universe for every single
possible combination and arrangement of each particle, thatâs a huge number but not infinite
because the mass of the Universe is assumed to be finite, at least the pocket we call
the Observable Universe. If I can add an atom to that, I now create
even more universes. One might argue itâs still not actually infinite since you can
only put so much matter in the Universe without collapsing it into a black hole, but as the
Universe expands that amount increases. You also get FTL travel out of it, since you can
jump back in time, move in a direction, jump back again, move again, and so on until you
are at a destination, like the Edge of the Observable Universe. So the number of possible
realities you could visit and colonize is either infinite or so large it makes no difference
and you never have to worry about the Heat Death of the Universe because you could add
or subtract mass and energy and entropy from any given time, or just move to a different
one. Under more classic time travel you can avoid
heat death too, by invading your own past. If the world was about to end you could jump
back to when it was younger and colonize that, and itâs presumably fine to do so since
you canât do it if paradoxes are a problem as youâd erase the civilization that produced
you and your time travel device. At the cosmic level you just jump back to
a billion years after the Big Bang every time you run low on material. You might be wondering
if that proves time travel canât happen since clearly no one did that, but logically
that doesnât quite hold up as itâs a bit of a tautology, circular reasoning, which
is kind of appropriate for contemplating circular bits of time I suppose. If the universe permits
no paradox of this type, time travel isnât possible so it doesnât matter, but if it
does itâs because you are getting new realities out of it, and there will be realities for
every possible scenario including a reality that never had time travelers arrive in it
prior to time travel being invented there, weâd just be in such a Universe. Or one
where the footprint was minimal. Of course you also have the Self-Consistency
angle as an option too, you jump back in time and colonize the Early Universe but the pattern
of possible futures leading to us only allows the ones where weâd be ignorant that occurred,
like they died off or grew very slowly and distantly from us so we hadnât noticed these
temporal colonists yet. Tenses are rather confusing in time travel
too, how do you refer to your own future that is your worldâs past or vice versa? I often
get a bit confused when writing scripts on the show just because I often refer to things
like last weekâs video, also discussing problems with Time, which came out December
26th, except that it is not January 2, 2020 as I write this, itâs October 28, 2019.
When I finish writing this script and upload it for the crew to edit, Iâll get to work
on 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 Discord
server and started referencing people to our Space Pirates episode for a detailed explanation
of what I was discussing at that moment, only that episode hadnât come out yet. Iâll
also sometimes reference episodes that donât exist, because some bit of material got removed
from 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.
Thatâs probably a small hint at the sort of tense confusion time travel civilizations
would get. How do you refer to something you plan to do yesterday, or that you already
did tomorrow? If this sort of travel is common and non-paradoxical, because itâs a sequence
of multiverses, how do you phrase an explanation to your friends when you come into the office
in the morning sporting much longer hair and beard and say you just came out of a timeline
where your wife died in a car accident so you had to skip back and fix that, and how
it was a very nice funeral and then the two of you went on a vacation afterwards because
it was a bit stressful. How about when a longer-bearded copy of your
co-worker Bob walks in and say itâs going to be a busy day, and then one with an even
longer beard comes in and says, âGood news everyone, tomorrow we closed the deal with
our new client, but we all need to do some extra overtime last weekendâ.
It would tend to seem like cohesive timelines would utterly breakdown too, and you might
get weirder day-to-day options like dating apps that let you swipe forward or back, instead
of left or right, to find the girl or boy of your dreams from a different century and
sit 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 like
inertia of future events or changes requiring more effort the further back you seek to make
them. But often itâs actual legal rules, and thatâs problematic because it doesnât
really seem enforceable to have temporal police. Someone just goes back in time and prevents
them being formed. If theyâve got some widget that prevents such things happening, some
sort of time shield, then they can presumably build a bigger one that protects their whole
civilization 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 Latin
and Roman History, grabbing lots of textbooks and schematics and modern widgets, and jumping
back to Caesarâs day and setting myself up as Emperor, or jumping forward to the future
to get some life extension treatments and awesome cybernetic enhancements and then jumping
back to the past to set myself up an immortal leader. If you do have that infinite number
of timelines option, this might be a better alternative to virtual realities where you
can be whatever you please as it might be cheaper and easier and more real-feeling to
just hurl folks to other timelines. There should always be one where the only difference
with your normal history was that someone identical to you popped up at a given time
and place, at which point it diverges. We usually say technologies like FTL and time
travel 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 space
and 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 histories
of their own world, and we just happen to live in a timeline where that hasnât happened.
Confusing stuff thatâs giving me a headache, so Iâm going to close out here and get to
work on the Cybersecurity episode for November 7, 2019, which I already finished as I record
this on November 1st, 2019, but first let me wish you an excellent 2020, now that 2019
is over. When contemplating time travel, or fantasizing
about it, which I imagine Iâm not alone in doing, one common problem is all those
missing skills youâd need to actually have to live back then, whenever then was. We have
a very skilled based society compared to prior eras but they had a lot of skills we generally
lack, so if you just popped up in some ancient kingdom youâd have a rough time surviving
without 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 itâs always handy
to be acquiring more skills. Plus itâs nice to just have them, even when they arenât
meant for career purposes they offer fun new hobbies and a feeling of confidence that you
can get stuff done. Thatâs why I like Skillshare. Skillshare
is an online learning community with thousands pf classes in a wide range of topics like
design, business, technology and many more. Even just those focusing on productivity or
time management, like Thomas Frank of College Info Geekâs course, Productivity Masterclass.
A premium membership is less than $10 a month and gives you access to that course and all
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today. So that will wrap our show up for the day,
but weâll be back next week to our alien civilizations series to take a look at alien
conspiracies and aliens who conspire, and ask what sort of motives they might have for
a given conspiracy. The week after that weâll return to the Moon, to look at what industries
might 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 youâd like to help support future episodes, visit our website IsaacArthur.net
to 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!
Hey as a sidenote, does anyone know if/how it is possible to do polls in sub-reddits? I'd like to do a topic poll here for a change, particularly considering how badly I tend to neglect our SFIA reddit community compared to the Facebook group.
So... If one COULD go back in time to change the past, would that risk a paradox or would that create a second universe? And if so, what happens to the previous universe?
Yes this was covered in the video but GOD Time Travel is confusing and I could use a little help. lol