>>LLOYD: So it’s great to be here. Perimeter Institute had me give

a popular lecture last night, but, of course, the reason I’m really here

is to find out what’s going on here because this is the epicentre of quantum information

processing and quantum computing in the world. And so I am really here to learn about stuff.

I also need to apologize to Olaf Dreyer. Last night, when I said that the birds were

quick to escape Hamburg in the wintertime, I would like to point out that they’re also

just as quick to come back in the spring, not merely for the good beer. Alright! I’d like to talk to you about the

Quantum Mechanics of Closed Timelike Curves. Of course, this sounds pretty wacky

and it is wacky because it is the quantum mechanics

of time travel, essentially. In quantum mechanics we are

accustomed to the situation where our intuitions don’t work

out to be true, and when you add in time travel, your intuitions

doubly don’t work out to be true. Now, this may sound kind

of wacky fictiony, and in fact if you look at

the history of time travel there’s a long literature about time travel. Interestingly, for most of it, if you go into

the distant past, it’s about time travel to the future, which actually isn’t that hard because

we’re doingthat right now. In the Mahabharata there’s a great scene where a king goes to visit

Brahma in his beautiful palace and he’s there for a few days

partying like mad. When he comes back countless eons have passed and his entire civilization has

decayed into dust. There’s a beautiful Japanese story called

Urashima Taro where a fisherman, Urashima Taro, frees a sea turtle from a net

and then a few days later a very beautiful turtle princess comes and

invites him to the house of the Sea King. He takes her invitation, and they’re married. It’s a great time and he lives there in great

splendour for three years, but he misses his parents and

so he begs her to let him go back. So she says “Ok, you can go back

and I’m going to give you this box covered with beautiful seashells,

but you must not open the box.” You can already see where this is going. So Urashima Taro comes back,

he lands on the beach of his island. Everything looks strange. He goes to his village,

and his village is no longer there. He finally finds this monument to

his parents and to himself saying that he died 300 years ago. In despair, he says “Well gosh! Maybe if I

open the box I’ll be able to see them again” so he opens the box and this mist

comes out and envelopes him. He feels himself rapidly turning into a very

old man, and then he dies. There’s another, an Irish legend about Finn

McCool, the famous Irish hero, who goes to visit the King of the Fairies

and he stays there for a few days partying like mad, which if you’ve read

the legends of Finn McCool you know he could really party like mad, and he comes back. When he goes away, the

King of the Fairies gives him a magic horse, which he rides back on, and he tells him

‘Whatever you do, don’t get off the horse.’ So what does he do? Of course he gets off the horse!

Just like Urashima Taro, whenever they give you a magic talisman and

tell you not do something, you always do it. So he gets off the horse and as soon as his

foot touches the ground, then he turns into an old man and dies. Right, we detect a pattern here in these stories. So the first story is about

travel into the past. Time travel into the past

shows up in the 1700s, but they don’t really get

the notion of time travel. There’s also Mark Twain’s famous story, A

Connecticut Yankee In King Arthur’s Court, in which this Connecticut engineer is hit

on the head on the job site and he ends up in King Arthur’s court, where

he decides to modernize things and hence wreaks total havoc,

destroys the civilization, et cetera, which is kind of a metaphor

for modern times too. But it’s not until H. G. Wells’ famous story,

The Time Machine, that we see the kind of picture of time travel that we’re familiar

with from movies and books today. In that famous story, there’s a machine

which the time traveller enters and it allows him or her to go backwards in

time to a specific date and then come back to the future,

or back to the present. Now, once these stories got started about

real time travel, people really began to think about the real

contradictions inherent in time travel. You’ve got to be careful in thinking about

time travel because there are a number of paradoxes,

which I’ll describe. I’ll describe how our theory and, I should

say our experiment as well because with Aephraim Steinberg we did an

experiment I’ll describe to you, effectively sending a photon a few billionths

of a second backwards in time. What’s the first thing you would think of

trying to do if you have a photon interacting with its past self?

What would you have it try to do? Would you have it buy its former self a beer,

which would be the nice thing to do? No! Of course, we have it try to kill itself! We have it try to go backwards in time

and kill its former self and then we see what happens. I’ll give you a hint. You know those movies

where they say at the end ‘No animals were harmed in the filming of

this movie’? Well, to say that no photons were harmed in

the course of this experiment would be an exaggeration. So in time travel stories, there are basically

two fundamental paradoxes about time travel. One is the so-called Grandfather Paradox,

where the time traveller goes back in time and, either inadvertently or on purpose, kills

her grandfather before he meets her grandmother so she doesn’t exist so she can’t go backwards

in time so what the hell is that about? How does that work? There are essentially

two resolutions of this paradox. One is that when she does this,

she can kill her grandfather and in doing so she enters an alternate world. So there’s a famous–Is it a Ray Bradbury?–story

called The Sound of Distant Thunder, where the time traveller

goes back to the Jurassic Period, determined not to change anything, but he inadvertently steps on a butterfly.

And because of the famous Butterfly Effect, when he returns to the present

everything is weird. It’s sort of like it was,

but the politicians are different and the language is spelled

in a different fashion, et cetera. So that’s one version in which you enter into

an alternative universe when you come back to the present, and the other is that you can’t do anything

that’s inconsistent with the past. If you look at movies about this, for instance

the famous movie, Back to the Future, not to mention the slightly less famous movie,

Hot Tub Time Machine, which I haven’t seen but I have

had it described to me, is of the type where you go back into the

past, you change the past, and you enter into an alternative future. The other type, which is exemplified

by the movie and the book, Harry Potter and the Prisoner of Azkaban.

Who here has seen this movie? Ok, great! Alright, I’m glad some people have. Don’t you guys ever get out? I know we’re

a little far from town here, but come on! In that movie, Harry and his wizarding buddies

are trying and doing all this stuff, and all kinds of weird crap is happening and they can’t figure out

what the heck is going on. But in the end, they figure out that what’s

happening is that they’ve been interacting with themselves coming backwards in time,

but everything is self-consistent. So things are weird and strange,

but they’re self-consistent. For the Grandfather Paradox,

this would correspond to a situation in which the time traveller is unable to kill

her grandfather no matter what she does. I’m sorry, but I’m going to use you

for this example since you’re sitting in the front row. So she points the gun at her grandfather and

–Blam!–pulls the trigger and– Whoop!–at the last minute a quantum fluctuation

whisks the bullet out of the way. The prevailing theory of closed timelike curves

is due to David Deutsch. The quantum mechanics of closed timelike curves

is due to David Deutsch. This is a theory of the first kind where you

can enter into an alternative universe. The one I’m going to tell you about: projective

closed timelike curves, is of the second kind where you cannot actually

go back and kill your grandfather. Ok? Alright. Are there any questions at this point? Any more time travel stories you want me to

hear about? I’ve been collecting time travel movies. Any good time travel movies you want me to

go and watch?>>AUDIENCE MEMBER: I just have a question.

How would you know you’ve travelled back in time if you can’t interact?>>LLOYD: If you can’t what?>>AUDIENCE MEMBER: If you can’t interact?>>LLOYD: No, you can interact! You can interact.

You cannot cause something to happen, which you know not to be the case. Right? Obviously, this is someone who has

not seen Prisoner of Azkaban. [laughter]>>LLOYD: What’s your problem? So you can interact with the past and make

all kinds of weird things happen, and in the future you will remember that those

weird things happened because you can certainly interact with things

but you can’t go and actually change the past. So you can’t go back to that horrible blind

date that you had when you were fifteen, you can’t go back and undo that. I’m sorry. [laughter]>>LLOYD: Yes?>>AUDIENCE MEMBER: Primer.>>LLOYD: Primer?>>AUDIENCE MEMBER: Primer. That’s the time

travel movie.>>LLOYD: Ah! Yeah, I’ve heard that

this is a really awesome movie. Who here has seen Primer? Yeah, I’ve heard this is really awesome. This

is definitely on my list of things. And there’s this one, Eight Monkeys?

Or Ten Monkeys?>>AUDIENCE MEMBER: Twelve.>>LLOYD: Twelve Monkeys! Twelve Monkeys. Twelve Monkeys, I’m told, with one slight

mishap is of the second kind where the time traveller goes back in time and tries

to change some horrible thing happening and it doesn’t happen.

I’ve heard it’s an awesome movie, too. Yeah?>>AUDIENCE MEMBER: Have you seen

The Time Traveller’s Wife?>>LLOYD: Oh! The book, The Time Traveller’s

Wife. You know I started reading that and, maybe I’m too close to the subject,

I had to stop after fifty pages. Which category is that?>>AUDIENCE MEMBER: The movie is like Azkaban.

It’s very good, but it asks… The Grandfather Paradox is ‘Can you go back

in time and kill your grandfather?’, this asks ‘Can you go back in time and modify

your most intimate relationships?’>>LLOYD: Right. Yeah.>>AUDIENCE MEMBER: Well, I thought it was

a very good movie.>>LLOYD: Excellent. So is this possible to

modify your most…?>>AUDIENCE MEMBER: You gotta go see it.>>LLOYD: Ok, I gotta go see the movie. Ok,

alright. [laughter]>>LLOYD: Yeah?>>LEUNG: So I haven’t seen this one but Charlie

told me about one in Futurama.>>LLOYD: Futurama. See, the problem is that

there are so many of these things that I can’t see them all.

So which type does this fall under?>>LEUNG: Basically, it is about the Grandfather

Paradox again. He managed to kill his grandfather.>>LLOYD: He did manage. Ok.

There you go. Awesome.>>AUDIENCE MEMBER: He became his own grandfather.>>LLOYD: Ah! Ok, this brings up another one.

I always say that if I went back in time and met my grandfather, I really was very

fond of my grandfather, I would certainly buy him a beer. And only if we had too many beers and got

in a fight would I accidentally kill him. There’s another famous story about

the Grandfather Paradox in which the time traveller goes back in time, goes

to a club, meets a beautiful woman, sleeps with her, they do not practice safe sex, which I do

not recommend, then she gets pregnant and in turn she gives birth to his mother.

So he is his own grandfather. That’s another Grandfather Paradox. Thank you for bringing that up. This is the

second major paradox about time travel. Let me explain it in less… Since I talked

about quantum hanky panky last night, I’m uncomfortable being seen as

the resident quantum pornographer. [laughter]>>LLOYD: Hey, if you look at technologies, one of the main things that drives

their introduction is pornography. So if we could come up with

quantum pornography that might be good. Pictures of naked electron–Well, never mind. The second major paradox is what’s called

the Unproved Theorem Paradox. In this paradox the time traveller reads a

cool proof of a theorem in a book, and she goes back in time and she shows the

proof to a mathematician. The mathematician says “Wow! What a cool proof!

I’m going to include it in my book.” The book, of course, is the same book in which

she obtained the proof in the first place. This is actually quite disturbing because

you have a carefully constructed, beautiful proof that came from nowhere.

It was never produced. Actually, this other version of

the grandfather story is, in fact, the Unproved Theorem Paradox in disguise because

if you go back and become your own grandfather, then a quarter of your DNA came from nowhere. It was never subjected to natural selection

or anything like that so, in fact, the problem there is

this Unproved Theorem Paradox. I’m trying to do a lot of research on the

philosophy and things of time travel. I haven’t been able to find time travel paradoxes

which are not variations on these two themes. So any theory on time travel in quantum mechanics

or elsewhere has got to come up with a resolution of these paradoxes, and so I will tell you

what our resolutions of those paradoxes are. But first, let me give a little bit of history. We probably wouldn’t be discussing this at

all if Kurt Gödel, in 1948, hadn’t… Gödel, when he was a famous logician and

when he got to the Institute of Advanced Study Einstein was there, and Gödel and Einstein

were buddies so Gödel learned general relativity. Gödel because he was fond of…

paradoxes he decided that he would say “Wow, maybe it’s possible to have a space-time

that has closed timelike curves in it. So you have this funny space-time manifold

and it could be possible that you have a path that goes backwards and

you end up in the past.” The way this normally looks is the famous

coffee cup picture, so time goes up here. This is in a 1+1 dimensional space-time. Down

here, space is just some big circle, and here is the handle of the coffee cup.

Time is flowing here, but down here when you go around this handle,

time goes like that and you can end up interacting with

yourself in the past. Ok? Gödel space-times are really weird-looking.

They’re these massive clouds of swirling dust. It seems to be important to have rotation

in these space-times to have closed timelike curves, but they do indeed have closed timelike curves.

This is what’s called a timelike wormhole. Lest you think that this is some weird wacky

thing… By the way, Google Images has some

great pictures of Gödel space-times. I’ve been told that Gödel, when he told Einstein

about this, it was sort of a birthday present for Einstein, and Einstein hated it. [laughter] Einstein was not Mr. Paradox Guy.

He didn’t like it. He didn’t like quantum mechanics,

he didn’t like paradoxes. Everything was supposed to be cut and dried.

But not so Gödel. It’s also a fact, when you take any space-time

and it’s rotating sufficiently and there’s a sufficient amount of mass, you’ll

get a closed timelike curve. The interior of a Kerr black hole,

a rotating black hole, inevitably has closed timelike curves in it. Back me up, Olaf or some other gravitational

person here. This is actually a fact. Unless you think “Oh! Who cares what happens

inside a Kerr black hole?” If our space-time itself, if our universe,

is actually over-dense and so it’s collapsing, then it is effectively a black hole. And if there’s even a tiny bit of

net rotation of the whole universe, then there would be closed timelike curves

within our universe. General relativity allows closed timelike

curves and, you could say in some circumstances, it even encourages them to happen.

How do you deal with this? How, in particular, do you deal

with things like the Second Law of Thermodynamics

going around here? What’s the quantum structure

of the quantum states? Can you even have a

quantum Hilbert space picture of what’s going on inside

this closed timelike curve? The answer to that actually is that you can

have a quantum Hilbert space picture but you cannot assign a quantum state, just

to telegraph some of the punches. The next hint of how you might deal with quantum

mechanics of closed timelike curves comes from John Wheeler.

This is not anything he published. Amusingly, this is reported by Feynman in

his Nobel Prize acceptance speech, which is online so you can get it. And you go look at it,

and he starts off with this dry stuff but then he starts talking about his experience

and there’s this place where he says– I’m going to try to paraphrase it in

a Feynman-esque kind of way– he says “Well, Johnny Wheeler called me up

from the Institute and he said ‘I know why all electrons have the same mass.'”

And Feynman said “Really?” And Wheeler then said “Yes, and I also know

why they have the same mass as the positron.” And Feynman said, “Why? Why?”

And Wheeler said, “Well…” By the way, time is always go up in this picture because anything to do with general relativity

has time going up, so get used to it. Wheeler says, “Well, look. Electrons and positrons

are always created in pairs.” e+, e-. “And they’re always destroyed in pairs.” e-, e+. “So we can think that what’s going on is that

there’s just one electron that’s going forward and backward in time. When it’s going forward in time, it’s an electron. When it gets destroyed,

it turns around and becomes a positron. So the reason that they all have the same

mass is because there’s only one of them!” “And the reason why they have opposite charge

but identical charge is that when you do charge-time reversal you take

plus-charge to minus-charge by a famous theorem of quantum field theory:

the CPT theorem.” So then Feynman says, “This was totally crazy,

but I did steal from this the notion that positrons were

electrons going backwards in time.” So in fact, at the heart of

contemporary quantum field theory was the notion that positrons are

electrons going backwards in time, there’s this crazy idea of Wheeler’s that says “Look, there’s only one electron and one positron.” And, of course, if you go and look at Feynman

diagrams you realise why this isn’t really true because you can have

other Feymman diagrams like this and they’re connected by photons

and so there’s probably not only one electron in the universe. Though it’s kind of a nice idea

if there’s only one electron. Our theory, which is with Lorenzo Maccone,

Yutaka Shikano, Raul Garcia-Patron, and then also we have Aephraim Steinberg’s

experimental group at University of Toronto who did the experiment. What I’m going to tell you, you can think

of as essentially the mathematization of this Wheeler notion of positrons

being electrons going backwards in time. It’s going to rely strictly on

entanglement and, indeed, when you create electrons and positrons

in pairs out of the vacuum, their spins are entangled singlet states. And when you destroy them,

the only place they can be destroyed is in entangled singlet states. This is going to be key for how this theory

of time travel works. Let me continue with the theory a little bit

because I think it’s important to know about. I have this Master’s degree in History and

Philosophy of Science from Cambridge and so I think that the history of ideas

is actually quite important to understanding what the next idea

is going to come from. Around 1988, Kip Thorne and Ulvi Yurtsever,

and later in the early 1990s, Jim Hartle and David Politzer looked at

path integral approaches. Path integrals, of course, what you do is

you take a bunch of classical trajectories, you assign them an action, and you sum e^(iS)

over all classical trajectories. What they did is they say, ‘We only take classical

trajectories, classical paths, which are self-consistent.’ In classical mechanics, you don’t have this

many-worlds stuff that David Deutsch advocated, but we’re only

going to take classical paths that are self-consistent so we sum over

self-consistent classical paths. This is a perfectly reasonable idea

and you can formulate it. The problem is that path integrals

are very hard to evaluate so they never got very far with this approach,

but they were able to look at it. Then David Deutsch, in 1990,

came up with this… Maybe I should actually start…

yeah, this is the right order. So in 1990…no, no, no,

this is not the right order. Yeah, it’s the right order. Nah! It’s not the right order. [laughs] I want to especially mention that

Charlie Bennett and Ben Schumacher, Charlie Bennett in particular starting with

the invention of teleportation, which was… When was teleportation? It was around… Do you remember, Debbie? ’94?>>LEUNG: ’92.>>LLOYD: ’92. Yeah, so starting with teleportation,

Charlie Bennett talked about… With teleportation you have–I’ll be kind

of graphically suggestive about this– you have an entangled singlet. And then you make a measurement right here. And then you send

classical information over here. You perform some operation dependent on this

classical information. This is a Bell measurement. And then you do something right here, and

if you have a state psi, you end up getting state psi here. Charlie Bennett always talked about

teleportation as if this part– Where did the information go?– as if the quantum information went here and this is the quantum information going backwards

in time and forwards again. By the way, this idea is at the essence of

what I’m going to tell you about. Unfortunately, Bennett and Schumacher have

never published anything on this. They’ve talked about it for years. They’ve

never really developed, so far as I can tell, any explicit theory about this so you could

also say what we’re doing is developing a theory out of this

metaphorical description that Charlie Bennett has been using

for decades now. Ok, so now let’s get down to it.

This is all history and now it’s going to be math and stuff like that

so are there any more historical questions, comments, or things like that? Were you bored by hearing the history of this?

Maybe you were. That’s ok. Ok. I’m sorry?>>AUDIENCE MEMBER: Which paradox does arbitrage

come in?>>LLOYD: Arbitrage?>>AUDIENCE MEMBER: Yeah, why didn’t you send

that photon half a billionth of a second after the stock market was updated and tell

itself to…>>LLOYD: So that’s consistent with both.

This is like a many-worlds version. This is ‘enter another world.’ And this is ‘same self-consistent world.’ So if you send information about what the

price of the Swiss Franc is going to be back in time and then invest

in the Swiss Franc, as long as the amount of your investment is

sufficiently small that the history of the Swiss Franc

is the same then it’s ok. But if you try to buy all the Swiss Francs,

then it will cause things to go haywire. Then it would fall in this many-worlds version. You can imagine making money off of

time travel in either of these worlds. In this way, you can make a lot more money, in this world, in the Deutsch version, the

many-worlds version. Ok. Ok, so enough of this fun fooling around with

history, fooling around with the past, which is of course what time travel is about. Let me now actually tell you

how these things work. The first thing I’d like to tell you is about

David Deutsch’s theory. So I’m going to review. Who here is familiar with Deutsch’s theory

of closed timelike curves? Yeah, some people are.

So let me tell you how Deutsch– I’ll switch colours. Blue for serious stuff. Let me describe to you what

this Deutsch paper in 1990 does. By the way, even though we think our theory

of time travel is better for reasons I’ll tell you, this is a beautiful

and elegant theory because it’s very hard to formulate

quantum mechanics in these contexts. These path integral approaches

are a good start, but it’s tough to deal with these paradoxes. Let me tell you what Deutsch suggested and

how he dealt with this paradox here. So Deutsch’s–

Oh, I guess I won’t use this one. I’ll throw that one on the ground.

I’ll use this one. Ok. So the way that Deutsch’s theory works is

like this. You have normal, what he called, chronology-respecting

degrees of freedom, and let’s call this the state rho-A. And

then you have the closed timeline curve, and this could be many degrees of freedom.

I’m just going to do it as if it’s two qubits but it could be many different degrees of

freedom. Let’s call this rho-B. And you have some interaction between these

things. Then the question is: How do you make sense of what happens up here? Now interestingly, Deutsch does not tell you

what happens over here. He does not assign this a degree of freedom,

this thing going backwards in time. Which is sort of funny if you think of the

coffee cup picture because there’s something happening in

the handle of the coffee cup, but Deutsch does not give you a state for

this which is already a hint that something is a little fishy. Deutsch’s self-consistency condition

basically says that– Here we have rho-prime of A and

here we have rho-prime– Sorry, I should put it right here so at this

point right here you have rho of AB. No, let’s actually… Yes. So when the thing

comes out…let’s call it rho-prime of AB. When it goes in, it’s in the state

rho-A tensor rho-B. And then what he asks is that the state that

reduced density matrix for the system that comes out of this closed timelike curve

is the state rho-B. Right? You can see why this gives you a

self-consistency condition because it says that the state that enters the

closed timelike curve in the future is the same as the state that emerges from

the closed timelike curve in the past. So this condition is the trace over A, U rho-A

tensor rho-B U-dagger, is equal to rho-B. Ok. This is Deutsch’s self-consistency condition. Alright? So you see this makes sense, right?

He wants the states to be self-consistent and he wants this state to be

the same as this state there. Moreover, this is because this is a superoperator. This is the same as saying if I have some

superoperator which is this interacted with rho-A via U, and then just look at B. We are asking that the state rho-B

be an eigenstate with eigenvalue 1 of this corresponding

superoperator, this process. And because superoperators always have an

eigenstate with eigenvalue 1, such states always exist. Ok? So it’s a nice self-consistency condition.

It looks good. Ok. Now, let’s look at actually what happens then

if we do something where… No, let me just leave it at that. Let me just mention some things that are a little

disturbing about this before I go on. I’ll tell you our theory, and then I will

compare the two of them. There is something a little disturbing here. Deutsch is assuming that when

the time traveller exits from the curve, and the rest of the universe is out there,

that they are in this tensor product state, which means that the time traveller,

when she exits from the curve, is completely uncorrelated with the stuff

outside of the curve. That is, she emerges in a universe where none

of her memories are any good. They don’t correspond to

the universe she sees. That’s a little disturbing already

because that’s certainly not this Urashima Taro kind of time travel or

H. G. Wells kind of time travel. It’s like you end up in this weird place that has nothing whatsoever to do

with what you remember. And the reason for this, of course, is that… I would describe this

in quantum information terms as I would personally prefer a closed timelike

curve to behave like a quantum channel and quantum channels preserve correlations

with the surroundings. But by demanding that only the reduced density

matrix be the same as it enters the curve in the future

and emerges in the past, in this case you are actually erasing all

memories of outside. This is not behaving like a quantum channel,

so this is a bit disturbing. I should say that after we wrote our paper,

we corresponded with Deutsch about this and he said that he found aspects

of his theory unsatisfactory and I believe that this might be one of those

aspects that he found unsatisfactory. Yeah?>>AUDIENCE MEMBER: In this result, is the

superoperator linear?>>LLOYD: Yeah, sure. Any superoperator can

be written as an unitary interaction with an environment when you start out in

the tensor product state. This is whosey-whatsit’s theorem.

I don’t know whose theorem it is. The transformation that B undergoes

when I take U, it interacts with A, and I take the trace over A. This is certainly a legitimate

quantum mechanical transformation. It corresponds to a superoperator; it is linear. So this is a superoperator and all superoperators

have an eigenstate with eigenvalue 1. There is a non-linearity in Deutsch’s theory.

This transformation is linear. There is a non-linearity because what Deutsch

is saying is that the only rho that we can allow are things

that satisfy this. So you can’t put anything in here, you can

only put rhos that satisfy this. Not anything can go through this

closed timelike curve. There’s a non-linearity in the sense that

you’re selecting out of the set of possible states just these

rhos-sub-B from which it can happen. There may be multiple rho-sub-B

for which this is the case. That is, the eigenvalue may be degenerate. In that case, for reasons that I’ll describe

in just a second, Deutsch says “Take the one with maximum entropy.” Actually, I’ll describe it in a second, so

the reason for that is that if you don’t take the rho-sub-B

that has maximum entropy then you immediately run into this

Unproved Theorem Paradox because the Unproved Theorem Paradox

is perfectly self-consistent. You can have the Unproved Theorem

go through this, everything is fine. If you think of this as some classical

transformation and then…that’s bad. Deutsch really doesn’t like that.

If you read his paper, which is a really excellent and interesting paper, he spends a lot of time talking about

this Unproved Theorem Paradox and how much he’s upset by it. So he says

“Ok, you take the maximum entropy: 1.” Alright. So. Let me now contrast this with–

any more questions about this? I’m going to stop talking about

this Deutsch thing right now. There’s more disturbing things as we’ll see

in just a second about this. Actually, I’ll mention one more disturbing

thing which is that Scott Aaronson, John Watrous, and Todd Brun

and a bunch of other people have shown that this is

absurdly computationally powerful. So this non-linearity of selecting out the

particular states allows you to solve anything. Deutsch CTCs allow you to solve anything in PSPACE. Plus computation:

both classical and quantum computation. It says that PSPACE is equal to polynomial

time so you can solve any problem in polynomial space in polynomial time. For

you computational complexity people out there, and I know you’re there because I see you, you know that anything that is

this case is really bad, and if you say that you can do this then computer scientists will immediately

disbelieve that this is possible, all physical evidence aside. That’s actually kind of

a bad thing about this. I should say that the way I got involved in

this is there is a paper from IBM in which, the summer before this last one,

where they objected to this Aaronson result. Is anybody co-author on this paper

here in this room right now? [laughs] Because I want you to step up

and defend it if you are. Anyway, so this caused a big argument. This got us, we found this argument to be

annoying so we decided to work on these closed timelike curves ourselves

and when Charlie Bennett came…I’m sorry?>>LEUNG: What annoys you about the argument?>>LLOYD: What annoys me about the argument?

Because I don’t believe it. That’s why. [laughter]>>LEUNG: The opposing idea or you don’t believe

the argument, you said?>>LLOYD: So the Aaronson paper is correct,

I went through it very carefully. The IBM argument says that in the presence

of closed timelike curves you cannot prepare your computer

in a particular problem state in order to solve that problem. Now, I don’t

know if this argument is correct or not, but the two papers start from

different assumptions and I actually quite distrust

the argument from the IBM paper. Anyway, we brought Charlie Bennett to MIT and we put him and Scott Aaronson in a steel cage

and we had them duke it out. It was inconclusive in the end. I would prefer not to discuss this paper because

I don’t think it’s very illuminating. Sorry, shouldn’t have brought it up. When we looked at this I said to myself, “I actually know of another way of doing

closed timelike curves” because about eight or nine years ago

I worked on this problem of how information escapes from black holes. By the way, this talk has every wacky

possible thing you could imagine. It’s got closed timelike curves, we’re going to have teleportation,

that’s the least wacky of things, and we’re going to have how information

escapes from black holes. So let me review how this model works. I said “There’s something very funny about this”

because there’s another Aaronson paper, which says that quantum computing

plus post-selection is equivalent to solving the computational class of

probabilistic polynomial time, affectionately known as PP. When I told my children that there was

a computational complexity class known as PP, they felt that this was really hysterical. [laughter]>>LLOYD: So because of my work on

escaping from black holes, I happen to know that you can make a theory

of closed timelike curves based on quantum mechanical post-selections,

which I’ll now present to you, which I thought all along was

equivalent to Deutsch’s theory. I should note that Charlie Bennett and Ben

Schumacher, while they’ve been talking about this going

backwards in time in a method that’s very close to what I’m going to describe, they were also not aware that their theory

was different from Deutsch’s. Now, if you look at these two things together, if you can get post-selection and quantum

computing, you can get closed timelike curves, which I’ll show you in just a second, then you’ve proved that PP equals PSPACE,

which would be really news to lots of people. You would have collapsed part of the polynomial

hierarchy, which would be pretty amazing. So the first way we started on

this is that we said “Hey! Look, we can prove that PP equals PSPACE!” If you’ve ever worked on these things you

know that after a weekend of working on it, you realize that you were wrong but it was

fun for about a weekend. The resolution was that

we realised that, in fact, the closed timelike curves via

post-selection are not equivalent to Deutsch’s closed timelike curves. So let me tell you how this works. Let’s look

at first escape from a black hole. How does escape from a black hole work? Ok.

Here again is time going up. Here is space. Here is my picture of a black hole. This is

the event horizon of the black hole. This is the singularity, where everything

gets smushed into nothingness. Note that the singularity of

a black hole is spacelike. It’s not actually pointlike, it’s spacelike

which is kind of interesting. Note also the horizon is lightlike.

Light goes at forty-five degree angles here. You can see why the horizon is lightlike because

if you’re right at the horizon of a black hole

and you send off a beam of light that’s just trying to escape from it at an angle, then this beam of light will just keep rotating

around the black hole. If you want to take a path that hugs the horizon, it’s a lightlike path

so the horizon is a lightlike surface. Here is the picture of what happens. We have this poor innocuous state

falls into the black hole. It can be your favourite evil professor or

something falling into the black hole. And it’s going to get smushed at the horizon,

but wait! Wait! There’s more to this picture! Because outside of the horizon,

Hawking radiation is being created and the way that Hawking radiation works is that–

Let’s do it like this– you have pairs of particles are created

from the vacuum that are created in singlet states because

the only thing you can create if you create something out of nothing

is in a singlet because all conserved quantum numbers have

to be zero. Alright? They’re created as singlets. Part of this vacuum fluctuation has negative

energy and it falls into the black hole, thereby reducing its mass, and the other part has positive energy and

it escapes to infinity, thereby carrying part of the mass of the black

hole off to infinity and that’s how black holes evaporate. Ok. Now, there’s a lot of debate about what

happens about information in black holes, but there’s one mechanism proposed by

Gary Horowitz and Juan Maldacena, two heavyweight string theorist types. The

mechanism is the following… I’m going to describe it in the way that makes

it sound most plausible even though nobody knows if this mechanism

takes place or not, but just go with me for a second. Maybe it’s the case that the only way, just

in the same way that the only way to create something from nothing

is for it to be in a singlet state, maybe the only way you can have it go away

to nothing, like at the singularity, is for it to be destroyed as a singlet state. Suppose that every state that falls in the

black hole gets destroyed as a singlet or projected onto a singlet,

that would be more explicit, suppose that the singularity projects

incoming stuff onto a singlet together with half of

a Hawking radiation pair. Alright? I mean, who knows, right?

The great thing about being a theorist is that we have no idea what happens at

the singularity so let’s just say it’s whatever we want it to be! Experimentalists

are not allowed this kind of leeway. Now, you can say

“Well, what is the state out here?” Aha! We recognize that this is just like teleportation. Ok, here’s a singlet state right here.

Here’s the state psi. But here, we measure and get the singlet. We measure and get the singlet

and what that means, if you are familiar with teleportation which

I know lots of people are, in the singlet state is one where

Alice sends to Bob the information “Whoa! Don’t do anything!

You already have the state psi.” And so I’ll draw this like this

because now we have projection– this is creation of a singlet–

this is projection onto a singlet, and so here’s this nice picture. This is the kind of thing that

Charlie Bennett was fond of drawing. Oh look! The information goes back here,

up here, and out here, right here. Alright? Is everybody ok with this? In this case,

if you project onto a singlet, which is a non-linear operation. It’s like

saying we make a Bell state measurement and we toss out all three quarters of it and

we renormalize the problem. Get this. So the renormalization of

the probability to 1 is non-linear and so this is non-linear quantum mechanics,

which is dangerous. That’s how you can solve

these hard problems using it. But at any rate, the state

escapes from the black hole. Ok? Are people happy with this? Yeah.>>AUDIENCE MEMBER: But wouldn’t we need to

know the Bell state measurement?>>LLOYD: Right, so in ordinary teleportation–

You know, ordinary teleportation. Everybody can do that.

People have been doing this for decades. You know, anybody can do that and you make

a Bell state measurement. Actually, making all the

Bell state measurements is hard. You make the Bell state measurement,

you get two bits of information. Alice sends those two bits to Bob,

and then Bob does something as a function of those bits. So the idea here is that

a non-linear process takes place where instead of having an ordinary

quantum measurement, it– for whatever reason,

ike you’re at the singularity of a black hole– it projects you onto the singlet part.

So it only gives you the singlet part. All the other stuff gets tossed away

and now has probability 0.>>AUDIENCE MEMBER: So only the ones that

make it out were projected onto the singlet.>>LLOYD: Right, only the ones that make it

out were projected on the singlet. This is totally illegal in ordinary quantum

mechanics so that’s a very good question. It’s illegal, so this is something

non-linear and bad. And you can see it’s bad because

already you have things that are propagating faster

than the speed of light, you clearly have violated

the No Cloning Theorem because, of course, this could be back down here. So you’re definitely doing bad things. Yeah.>>AUDIENCE MEMBER: Well, you’ve partly addressed

my question, but what’s unanswered here is what determines the physics of when it

goes back into the future from the outside?>>LLOYD: Interestingly, the back into the

future part… The Back To The Future part: note that one

of the main actors in that is named Christopher Lloyd, no relation I believe.

[laughs] Only distant relation. Interestingly, this part down here is really

the uncontroversial part. That’s just entanglement.

That’s just an entangled singlet state. The controversial part, of course,

is this projection part.>>AUDIENCE MEMBER: Well, sure, but it could

go at any one of those times including earlier in the past.>>LLOYD: Yeah, well, of course. Let me segue,

because I realize that in indulging myself and telling you about history and talking

about time travel movies and stuff like that I’m going to go short on time. As everybody knows–by the way, I try this

out on people I meet on the street who have nothing to do with physics

and they know this– so as everybody knows if you can go faster

than the speed of light, you can go backwards in time.

Everybody knows this. That’s just the way it is. Here is the model for post-selected, or projective,

closed timelike curves. We create a singlet down here. We project–

This is ‘create singlet.’ We project onto a singlet up here and renormalize

probabilities. Renormalize probs to 1. This is all really easy to do

and calculate because it’s just as if you were taking a Bell state measurement

and you say “What’s the conditional probability of

everything else, given I got a singlet?” So all probabilities in this theory are calculated

using the conditional probabilities that you got a singlet here. Here’s this other thing.

Then you have some transformation, and then something else comes out at the end. Ok? So probabilities of events… …Equals conditional probabilities. It’s

a very well-defined theory. The one thing you have to say is

“What is the probability of this as 0?” and the answer then is nothing. It can’t happen. Because conditional probabilities are not

defined if the probability outcome is 0. This just doesn’t happen, which is good

because now you can see already that we are going to always have things

that are self-consistent because in this picture of

closed timelike curves what happens is a valid conditional probability for a sequence

of events in quantum mechanics. It might be hugely improbable if you don’t

do the projection on a singlet, but it’s still possible. So you can’t get

things that are exactly impossible like, for instance, killing your grandfather. Ok. Once you have this… I want again to

give Charlie Bennett and Ben Schumacher credit even though I’ve actually rather had an

annoying time dealing with them over this because they have talked about

things like this for a long time. I know that Raymond told me–

I didn’t know about this paper– that you guys did an experiment here with

NMR to look at this notion of “Oh look! Things are going backwards in time.

Let’s look at what happens with this.” And I want to give them credit for

talking about this for years but I’m not going to give them credit for,

and I wish to express my annoyance at them for, not writing this up as a paper so

we can actually see what they mean because they never wrote it up. It only exists in the form of

four transparencies on a talk of Charlie Bennett. So they have a theory, which is like this,

but it’s not clear what they meant by it. In fact, when we talked to them, they actually

use quite a different language and I’m not sure if we agree on stuff. At any rate, Charlie told me that

they were unaware that their theory was different from Deutsch’s and this theory is definitely different from Deutsch’s

as I’ll now show you. I’ll show you by giving you

a picture of a quantum circuit for our Grandfather Paradox experiment. I’ll show that Deutsch’s theory and our theory

give different results for what happens. I can do this over here.

Let’s do a Grandfather Paradox. The simplest version of a Grandfather Paradox

is something like this. Here’s our closed timelike curve.

Ok. Zero equals dead, one equals alive. We’re just going to switch this over

to having our photon killing itself. If you do a sigma-x, you flip this around

the x-axis right here, then what happens is zero equals dead,

one is equal to alive. And you see if you’re alive here,

you’re dead here. You get turned into dead. If you’re dead here,

you get turned into alive here. So this is the Grandfather Paradox. The very

simplest thing that you can imagine about it. And moreover, in our experiment we’re going

to ask this qubit to declare whether it’s dead or alive. So here we’re

going to measure, up here. So here we measure it, we couple it to two

other qubits via a controlled-NOT, and then we’re going to ask it to declare

if it’s dead or alive. By the way, I think if you look at the Grandfather

Paradox in Charlie Bennett’s notes, again trying to decipher

what they meant by this, it’s a different paradox and this might be why they didn’t figure out that their result

was different from Deutsch’s. We also asked Charlie Bennett to be

a co-author on our paper after we’d done the experiment and discovered

that they’d been doing this before. After this delayed the submission of the paper

by four months while he decided, in the end, that he hadn’t contributed enough

to it. This is the kind of thing that happens in

science. What they hey. You gotta negotiate with people, people get upset if you don’t give them credit,

et cetera. So I’m trying very hard to give them credit

and express my annoyance. [laughs] What happens here?

Well, in Deutsch this is totally ok. Remember, Deutsch always works,

it always gives you something. What is the state that work here? What is

the state where if I pop it in here, it comes around here,

and it’s still the same state?>>AUDIENCE MEMBER: The Hadamard state.>>LLOYD: I’m sorry?>>AUDIENCE MEMBER: The Hadamard state: equal

superposition of zero and one.>>LLOYD: Right, that’s right. That will work.

However, Deutsch asks us to take the maximum entropy state.>>AUDIENCE MEMBER: So a mixture then.>>LLOYD: Right. So, here, for Deutsch, basically–I’ll

call this rho-B again– rho-B is equal to 0.5(|0>>AUDIENCE MEMBER: Sir?>>LLOYD: Yes?>>AUDIENCE MEMBER: What happens if you think

the density matrix is lack of knowledge? It’s actually a pure state somewhere…>>LLOYD: Right, of course, as you can tell

when you have this kind of system all your intuitions about quantum mechanics

and what you’ve been taught, you’ve got to be careful about. If you think of it as a lack of knowledge,

say, from an outside observer, like the person out here monitoring

the situation while we’re here, we’ll say “Well, I don’t know what it was.

Hold it, let me go look over here.” They’ll get statistics up here.

What they’ll find is, basically, in Deutsch’s case if

this one is zero the next on is one, if this one is one the next one is zero. So what it says is “Well, I don’t know if she was alive or dead

when she went in, but, by gum, whatever she was if she was alive

she turned out dead and if she was dead she turned out alive.”

So I think it’s still ok with that. You didn’t know it beforehand, and then you

found out. Yeah?>>AUDIENCE MEMBER: I think if you’re using

Deutsch’s recipe for this problem, then you shouldn’t use CNOTs as

a part of your unitary.>>LLOYD: Yeah, it is. I didn’t derive to

you that this is the maximum entropy state from this, but if you include it with the

CNOTs, take the trace over the CNOT gates, you’ll find that the fully mixed state satisfies

Deutsch’s self-consistency criterion and because it’s pretty clearly

the maximum entropy state– Unless you want to disagree with that, too

–then this is the Deutsch prediction.>>AUDIENCE MEMBER: But zero plus one doesn’t

hold.>>LLOYD: Oh! If you have the CNOTs, yes.

Sorry about that. If I have this, then it’s ok. You’re right. I’m sorry, you’re right. You’re right. I’m sorry, I didn’t mean to. I was unfairly scoffing at you.

You’re exactly right. If you put this in here,

then this state doesn’t work if you’re actually doing

these measurement interactions. Completely correct. Ok. What happened with P-CTCs? Well, what

happens here is that this can never happen. If I think of it without the CNOTs to make

your life easy, you see what happens is that the singlet right

here gets changed into a triplet which has zero overlap with the singlet so

that the projection onto the singlet is zero. In the raw Grandfather Paradox,

it’s an example where it doesn’t happen. On the other hand, what happens–

Let’s suppose that you actually have something which is just e^(i theta sigma-x), so you’re performing a partial rotation

around the x-axis, and e^(-i theta sigma-x)

is equal to cos(theta)*I-i sin(theta sigma-x). What happens then is that this projection

to the triplet knocks out this, so in fact the qubit never gets flipped.

It only behaves like the identity, you just take the identity part, no matter

how small it is, amplify it back up to one, and so what P-CTCs say is that you get

zero, zero, one, one. If the time traveller entered the curve alive,

she exits the curve alive. If she enters the curve dead,

she exits the curve dead. And that’s because these

projective closed timelike curves behave like idealised quantum channels.

They preserve not merely the state of the system when it is a closed timelike curve, they also preserve any legitimate correlations

with variables out here. So if you remember being alive when you enter

the curve, or somebody, maybe if your mother, remembers that you were

alive when you enter the curve she will see you emerge alive in the past.

Ok? This already shows you that Deutsch’s closed

timelike curves are different from these projective closed timelike curves and the main difference is

this quantum channel version. I’ll also tell you I’m out of time, but let

me just tell you what happens with the second Grandfather Paradox,

the Unproved Theorem Paradox. Let me see if I can get this right

This is always tricky. Here’s the closed timelike curve. What happens is the time traveller

reads a theorem in the future… This is a qubit right here. Here’s the theorem. And then, in the past,

she writes the theorem onto this qubit. She tells a mathematician what the theorem

is, and then she goes her merry way. This is the Unproved Theorem Paradox. She needs to be able to read the qubit in

the future and then write it back in the past, and I claim that this is the quantum version

of this Unproved Theorem Paradox. Sorry that I’m rushing through

it a little bit. ‘Read theorem’. And this is ‘tell theorem’. To read the theorem she has to be in the state

zero, so she knows what the theorem is, and then she tells it to the mathematician.

This is the mathematician. This is the time traveller. Now you can ask, ‘What happens here?

What is the state right here?’ Now, remember when Deutsch

does this circuit what happens is he identifies this right here with this

right here, but now any theorem will do. Could be zero, that’s fine,

one will do as well. So he has to take the maximum entropy state

in order to get rid of this paradox. Basically, Deutsch says, ‘Ok, look, it’s gotta

be the maximum entropy state up here so you get a mixture of |0>>LLOYD: Look, Raymond, as I told you tomorrow,

I was going to go over. [laughter]>>LLOYD: Let me summarize. Closed timelike curves are a perfectly legitimate

part of general relativity. Therefore, it’s important to figure out what

happens in them quantum mechanically. Now, they may not be allowed in

our universe or not, we don’t know. Stephen Hawking says no.

He has this chronology protection postulate, which says you can’t have closed timelike

curves. On the other hand, since he doesn’t give any reason for why this

is so– It’s like many of his other statements: ‘It’s just so and, by God,

I believe it to be true!’ We don’t know if this is possible. It’s certainly possible in

the ordinary laws of physics. It’s good then to figure out how quantum mechanics

would work on this. Deutsch proposed this theory.

We propose a separate theory and we believe that Ben Schumacher

and Charlie Bennett, had they actually managed to write the paper

and work out the theory, would have arrived at the same theory. It is different from Deutsch’s theory. It has this nice feature that it can still

be described in Hilbert space, but of course because you have post-selection you cannot uniquely assign a state to the

system as it’s moving along right here. We were also able to show that– this was pretty tough because if you go read

these Politzer and Hartle papers, they’re about path integrals

over Grassmannian variables and it’s been a long time since I did a path

integral over Grassmannian variables– but you can show that they’re equivalent to

Politzer path integral method. Politzer only does it for a single qubit going

backward in time, but they’re equivalent for that in that case so we think that they’re equivalent generically

to these path integral methods. And we performed an experiment and, by gum, we’re going to probably do

this experiment soon as well and if you guys would like to do this experiment

we’ll be happy to collaborate with you to figure out the right way to do it. So, thank you very much. [applause]>>LAFLAMME: We’re a bit late, but if somebody

has a profound question…>>LLOYD: Raymond has a group meeting

right now. Maybe you should go to your group meeting.

[laughs] I’m happy to answer questions now

and also later. Yes.>>LEUNG: It’s more a comment back to your

original objection to…>>LAFLAMME: Speak louder, so…>>LEUNG: Right, so just to comment back

on the paper, the Deutsch response against

Aaronson and Watrous…>>LLOYD: The IBM paper, yeah.>>LEUNG: Yes, I should say it while

the crowd is still here.>>LLOYD: Yeah.>>LEUNG: I think the difference here is just

that we object to the model as trying to compute on a fixed input and we think that a computation

algorithm should work on an arbitrary input that is decided on the spot.>>LLOYD: Right, in the paper… Were you

a co-author on the paper?>>LEUNG: Yes.>>LLOYD: So, in that paper… Actually, I should say I’m still confused

about the paper so my understanding of it, correct me if I’m wrong,

Aaronson and Watrous just said “Hey, suppose we have an input and

we can put an input into this.” We have access to these

closed timelike curves and we’re allowed to put in

any input we want over here. And then we say

“Ok, what kinds of problems can we solve?” The answer is that they can

both solve problems in PSPACE. In your guys’ paper, and now is your chance

to correct me if I’m wrong, says “Hold it! You have to look at how you prepare

this input, and that if you have these non-linear systems you’re no longer

allowed to think of things– A mixture is necessarily… When you apply

something to a mixture, it’s no longer the same as applying it to

the individual components of the mixture and then seeing what happens to

the individual components.” So you say “Well, you have to say

how do you prepare this input and if it’s entangled with

some other state or something then you can’t necessarily prepare that input.”>>LEUNG: Basically, that changes what you

mean by the input and, therefore, the fixed point changes as well.>>LLOYD: Yeah.>>LEUNG: The fixed point doesn’t do anything

for each of the individual components.>>LLOYD: I agree with that, but having witnessed the Scott Aaronson/Charlie Bennett steel cage fight

I have to say that my impression was– as I said, it was kind of a draw–the different

assumptions are Scott says “We can prepare this input and we need to

have this part of the system, and we want to look at what happens,”

whereas you guys say “We look at the whole universe and we ask

what it means to prepare an input then and then if you just have

a mixture of inputs coming in here then you have to redo the calculation again.” I agree with that because the calculations

in your paper are correct about that, but what I don’t agree with,

and I don’t agree with this. I will now come clean and

say I don’t actually agree with it rather than say I don’t know

whether it’s right. I don’t agree that that’s the

correct way to talk about whether you can prepare an input or not.

I don’t think that Scott’s way is wrong. I don’t think that your way is wrong either. They’re both different ways and

each is equally self-consistent. So I, in fact, don’t think that your paper

really refutes Scott’s result. Simply, you choose to have a different definition

of what it means to choose an input.>>LAFLAMME: [speaking over LEUNG] Maybe we

can have a rematch of the Bennett/Aaronson… [laughter]>>LLOYD: I’ve got the pointy hockey stick

here. [laughter]>>LAFLAMME: I could go downstairs and try

to find a cage. [laughter]>>LLOYD: Yeah, we can do it in the clean

room. Oh, no, that’s a bad idea. [laughs]>>LAFLAMME: Let’s thank Seth again!>>LLOYD: Thanks. [applause]

## shemadeitforme says:

His laugh at 9.99! Pack another bowl mate!! Meme It.

## FWM FWM says:

I have made it my hobby for the past 20 years to show why time travel ( back) is impossible.

for 20 years I have been struggling with the following question ….are we free or are we in a movie that has been shot edited and ends in 5 billion years when our sun dies?

only if we are in a movie can time travel back be possible.

simple thought……if person A lives for 30 years ( 30 Year old) and finds or builds a time machine and goes back to the past (before his/her own birth) no one has to witness it, it is never the less a fact that Person A was at a point in time space before person A's birth.

this must mean that person A will be born and LIVE ! for 30 years to be able to go back in time. person A can NEVER die in any way shape or form till person A goes back in time cause it is a fact of time/space that person A was there in the past.

so person A can not die as a baby, can not die as a child, can not die as an adult. this only means that we are in a movie and nothing we do can change what happens in the next 1 second or micro/nano/ angstrom of a second. it is all predetermined not only for this solar system but for the whole universe out there.

## Walter Chavez says:

Another paradox (maybe not the correct word to use) could be that things going into the past will add extra matter and probably energy to ( at least to the past's ) universe. This probably breaks matter creation laws and would be a possibility of filling the universe with matter.

I believe the 'Unproven Theorem Paradox' described at around 13 minutes into the video is also often referred to as a 'Bootstrap Paradox'.

## sparhopper says:

27:17

I really enjoy Seth (it's why I'm here), but In

another world, this is where he accidentally sets himself up to die by his own hand, when later, he slips and falls on this same pen half way through the presentation, that gratefully, we get to hear the end of …I hope. 😉## fivefivevirgo says:

this is the second science related video where someone in the audience mentioned futurerama that dumb stupid cartoon what the hell!!!!

## Heather Tipsword says:

Time Travel in a long-running TV series, of course. Short clip of a good "grandfather paradox" I think you'll like. https://vimeo.com/142048515

## Zach Baldwin says:

A quantum theorist who has an analytical prediction for future ideas… (21:23).

## Be Macabee says:

I need to go back to the time this was presented. We put all money into GMCR on this date. I get a ROTH IRA out for both my Mom and myself. Maybe for Miki also. We go to Mexico for 3 weeks. We have Agnes and John come over and take their stuff. Then we go to the church together and we take on the job with Frici bacsi. I take the chiropractor job. In January another 5000 goes into the Roth IRA. We go together to Ecuador in March and to the DR in April. My Mom goes to Roszke in the summer. Then I rent out Timi's apartment for the summer. I meet Joe and we do not sell the vineyard. We keep the apartment until the following year when we buy an apartment in Budapest.

## Arthur Arcturus says:

too many stupid stories, no patience, bye

## William Ozier says:

Let's say I use a slit to split a photon and I now have two entangled photons. I then send one of them back in time. Then I meddle with one….will that meddling information be sent through time by their entanglement? OR is is possible that time travel would DISentangle the photons?

## Gustav Bodner says:

there is only 1 electron in the universe? — 18:10

## BOOK OfSOULS says:

The Quantum Mechanics of Time Travel

## DisneyCanada says:

The Quantum Mechanics of Time Travel

## Sonic Screwdriver says:

can electrons time travel??

## Jesse Panganiban says:

Tough crowd.

## S3DGamerZone says:

I love the relation between thermal dynamics and the quantum mechanics of time travel, very cool.

## Michael Anderson says:

13 minutes into this and still telling stories.

## Mr. Lowery says:

Couldn't simply stopping someone on the street to ask them for directions throw their time line completely out of whack?

## eXtremeDR says:

So movement slower than light = forward in time while movement faster than light = backward in time.

Does time stand still when moving exactly at the speed of light?

And what happens at a state with no movement at all?

## Paul H says:

Is teleportation possible? YES. With precise mathematical equations which exist, this will give you all the measurable elements required (recipe) to pass through all matter and in multi polynomial states at the same time. This is faster than the speed of light. Clue: Zero is 1 and 1 is Zero. 1n 2n 3n 4n 5n 6n 7n……1-n 2-n 3-n 4-n 5-n 6-n……infinitely. Universe maths not known to man

## IRONMAIDEN says:

StarsDanceTV9 months ago

Dr. Seth Lloyd, an MIT professor and self-described "quantum mechanic," describes the quantum mechanics behind time travel during a guest lecture at the Institute for Quantum Computing, University of Waterloo. Recorded on Nov. 4, 2010, this is the entire lecture entitled "Sending a Photon Backwards in Time."

Dr. Lloyd also sat down for a one-on-one interview at IQC, during which he discusses the weirdness (and beautiful simplicity) of quantum mechanics, and the incredible importance of quantum information research. Excerpts from this interview are below:

Seth Lloyd on Quantum Weirdness:

## Sotallytober says:

I want time travel and I want it right Oh well , that's irrelevant.

## Jack Pullen says:

At 32:28 Could it be that the memories are stored in the field loop not erased. That could also be the marker to exacting point of return from other dimension. The only way in is the only way out.When you return, you remember everything to the point up to leaving and memories of the trip are in the loop field…

## Alan Warrick says:

Hi Dr. Lloyd,

Because we exist in a condition of gravity on the earth which is different than in other sections of space: how can we make a man made black hole or a worm hole – This closed time line curve without that much mass on a smaller scale?

## Wing Man says:

For those interested I had a time travel experience when I was a kid which I am 95% sure was an OOB experience though (which I used to have many of) and not a physical one. I do actually have my doubts regarding physical time travel. Anyway for the open minded here that might trust in my experience the one thing I noticed of importance is the loop. So the scenario was that aged 8 i enter a wormhole that for some unknown reason appears in my bedroom, and travel 15 years in time play on a game boy in a dept store (hadn't been invented yet) go back to being a kid. 15 years later I re-lived that same day as a adult. So here is my hypothesis. To time travel that took place and the information i was exposed to was bi directional. So I took a peek into the future when I was a kid and no doubt could have changed the future by for example persuading the Gameboy maker to call the toy a Gamegadget! In which case 15 years forward would mean i would end on playing on a Gamegadget and not a Gameboy. What blows my mind is how free will is sill possibly in a closed time-like curve!?

## Teapartyla says:

Time travel can only happen one direction because of the expansion rate of the universe. We can travel to the future but not the past.

## Tim Rubin Halcomb says:

Time travel in both directions is very possible. In fact, I know how to do it. It is the reason why my life is being altered right now.

I am very serious! With funding, I can design a machine that is paradox independent, but I need help.

## David Wilkie says:

From the proposition of the universe as a Quantum Computation, ie the reality of existence is a standing wave in which the past and future are every side of now as potential inherent in the incidental structure, the incident connection of the kind modeled in Analog computers, then the central point of time is fixed now and forever. The architecture is implied by cyclic reflection, of which the simplest form/dominant probability, is a spectrum of resonance, part of which Astronomy studies. The flow of information past to future and future to where the past was when reflection of the information now shaped the present, is contained in virtual continuity of the kind that is mathematical.

Time travel is just travel across the combinations of the information spectrum that is the apparent physical structure of the universe, the "architecture".

But don't let any of the process ruin a good story, proving something wrong can reveal what is right some-times, (the probabilistic stability of shaped waves of QM).

## Markus Hooge says:

modern day genious

## Roger Starbuck says:

I think i'll head back to my own time now. lol

## Maz Dela Cerna says:

Glad I found this!

## J H says:

Delayed choice shows that the universe does time-travel constantly, and gives you a template of how you are required to accomplish the same task. The universe is never wrong, it's that simple.

Where the information goes, when it doesn't arrive to destroy itself, and whether it splays evenly throughout the universe, and what effect happens to the universe, in such cases, is still on my mind.

I once had a weird dream, with hockey and quantum mechanics. I had to step away from my computer when the hockey stick came out, as this is exactly how I would have told myself that my future time-travel was/will be successful.

## CHIEF TICKLESBALLSWITHFART says:

Another possibility is that future humans (interdimensional entities), or "GREY" aliens as they're sometimes called went back in time and hooked everyone up to glorified X-boxes. Or better yet, replicated our consciousnesses artificially. Why? To protect earth. Right now, we could be in an exact copy of earth they're in the real world playing us like video games.

## CHIEF TICKLESBALLSWITHFART says:

I have proof that this is a controlled simulated existence. It's so obvious now. Then when you realize it, they call you crazy. Laugh and mock you. They're evil assholes who are evil assholes even when they're trying to be nice.

## CHIEF TICKLESBALLSWITHFART says:

Whatever is controlling everything and everyone like a video game likes long drawn out butterfly effect challenges. It likes doing the least to influence the most. It sits, stalks and controlls complex scenarios and variables in time in the interest of control. That's the game for it. We're puppets in a sick play. Just look at nature. It's cool but repulsive at the same time.

## Scarakus says:

I have a camera that records 10 minutes into the past, but it only goes far back as far as when i first turned in on. The camera is a time machine, and if you have one that continuously records 10 minutes into the past, and I can view any part of that footage, didn't I just travel back in time? I did, but without leaving my frame of time.

## Bradley Richardson says:

Really enjoyed this! Keep them cominggg! 😁😁

## Shawn says:

He lost me at theta1

## 80thiconoclast says:

Awesome class. Was 46:49 an example of time travel?

## Mike Fuller says:

I am a Philosopher and an Essayist on Black Hole Thermodynamics.

## Emre Sahin says:

19:44 that laugh tho

## Sara H says:

Your voice is very familiar

## Star light says:

Co linear, asymmetrical, matrices.

## Laurens du plessis says:

I am a believer in the multiverse therefore the self consist of the multi and has no singularity. with time travel I do not move in a liniar motion and therefor the outcome can never be liniar that means between true and false there will always be more than one answer to the paradox. the now is always determined by the past events towards future posibilities but the now is also flexible for its perception of both future and past can influence an answer. so in short i believe that when you go back and change something one only changes one universe of prediction and if that is so then everything is determined and freewill can never exist and therefore closes any posibilities of creation . in this way of thinking one can imagine that everything that exist, is in a frozen state of posibilities that one can move in certain laws of direction. so even if time travel is posible in this, then one would never be able to hold on to a memory and would only be able to observe events either in future or past. Its like reading a book and turning the pages back and forwards but you would have to be outside the book never to be able to get in or out of it. the editer of the book can never be a character in the book. I want to believe in time travel but the only way i can truely folow a logic outcome is when probilities can become posibilities in a mutitude of universes of in which i can peer through the percepton of being. in this, the memories are all flued and flow through the self, moment by moment never to be true or false to be possible. everything is information but information is also flued and can restructure in endless ways of new information. Can you escape a blackhole? well, even gravity is not completely understood and therefore I would guess it would escape, because do you really understand who that what you call the self, really are? and where you are?

## Iván says:

I haven't finished but I hope somebody mentions Donni darko

## Alien says:

I am sorry but thats not how time travel works… time is not linear.

Math and physics cannot be applied to time travel.. time is all logic…and logic is beyond humanity.

think of time as a massive tree with endless branches…

Using the grandpa example, as most people can relate to it. you go back in time and kill your grandpa…you will still exist in that time stream…that branch of time… in the one you left.. you dont exist in it anymore. unless you are 4th dimensional entity.. but thats a whole new level.

not such thing as a paradox..

## Giovanni Wites Bolzan Assumpção says:

Screw that's 44:50 glitch. Anyone got the fixed video? Or it was a some naked electron that hit the camera man…

## Yusof Sarangani says:

Somebody tell him about Steins;gate.

## Nic Parker says:

He is super great and funny. we think a lot alike.

## tigeruby says:

this is truly exciting & brilliant stuff

## Brijendra Giri says:

i don't want to time travel but everything reverse we easily go past and future. but here is main problem is when we reverse we also who getting the experiment, if he goes long in past he will be erase in time.

## Ms Bunny says:

Haha he made me laugh when he explained about the harry potter prisoners of Azkaban the first thing that popped in my mind was "Honestly Ron how can two people be in two places at once?" LOL

## T Taylor says:

Didn't I see Captain James T. Kirk experience this?

## INNOVATION & INITIATIVE Diy says:

42:02 why only negative Particle fall into Blackhole why not positive and how do we know that , it's reality or theory ??

## INNOVATION & INITIATIVE Diy says:

laughing with barrier haaha hi haa hi haa hi

## gizmo amsterdam says:

he is so funny at 10:08 lol like a looney tune figure..

## Jesus hl Christ says:

Do we trust pony tail guy lol

## Santana Eccles says:

what a dick head

## Eric Pedro says:

Woody the wood pecker? Is that you ?

## Joel Weidenfeld says:

Stupid CLOWN

## Arthur Spina says:

10:00 your'e welcome.

## freeman guess says:

As far as a computer can time travel is based on timezones because of GPS otherwise you cannot reach a site because the certificates are not valid so I'm thinking if you built a time machine it would have to violate the rules

## lee curtis says:

We would probably be invisible if we went back in time. Maybe that's why people see ghosts or know they are around

## lee curtis says:

I am no scientist but if you have an electron and a positron of the same amount, they would cancel themselves out and become a neutron?

## lee curtis says:

I mean no disrespect. He is introducing ideas that I just don't agree with

## Mike O'Lynn says:

Nice fucking ponytail

## Science Revolution says:

Time travel is impossible, because time has no location, time is not a physical place. Where is past? Where is future? Past and future only exist in our imagination.

The universe is existing at forever ongoing now, all past became now, all future will become now.

Why most scientists talk about time travel is possible? Because they cannot think rationally.

They think 1 proton and 1 electron able to form a stable hydrogen atom. But they cannot explain precise mechanism.

There is only one force exists between 1 proton and 1 electron at distance r, the strongest attraction force in the universe F=Ke x pq/rr.

According to physics laws, the two particles must collide under that force. Electron is impossible to wave/cloud/orbit around proton to form a stable atom.

The standard model of atomic structure is total BS, but it is mainstream science.

Do you understand? Do you understand?

## adh0522 says:

i sense he lives a parallel universe where there's no accountability – affectionately known as a college campus…

## ZiplineShazam says:

Dash Rip Rock played Buddy Holly's "Not Fade Away" at The Grant Street Dance Hall in Lafayette, La. in the year 1989 . . .and it was Euphoric.

## SMT4 Productions says:

13:30, Peasing to write

## Richard Payne says:

Are hair follicles small enough to be influenced by quantum mechanics? therefor causing the hair of a person to be in two states of existing and not existing at the same time?

## A Heldenby says:

If the place and time you wanted to travel back in time to didn't contain yourself then whatever thing you think you have travelled back to isn't that past.

## A Heldenby says:

If the place and time you wanted to travel back in time to didn't contain yourself then whatever thing you think you have travelled back to isn't that past.

## Jikook Harling says:

" You're right, I was unfairly scoffing at you" lmao priceless

## KILL BILL says:

This was in 2010. Interstellar(2014) hands was the greatest time travel movie ever!

## ReliaCar Transportation says:

Do you any serious David Deutsch CTC theoreticians know about the Mandela Effect phenomenon? Please do serious, open minded research into the pbhenomenon. Large numbers of people gave experienced effects very similar to what the theory predicts!

## Inglorious Basterd says:

Very interesting and anybody who says they understand quantum mechanics is either deluded or just fucking lying…drink up;)

## Odival Quaresma says:

though crowd huh?

## John Urwin says:

Time Travel BULL

## UltimateBargains says:

Time dilation is not time travel.

The First Law of Thermodynamics prevents time travel.

## David Don says:

Today I completed watching the entire video presentation at the 2nd hr:43min after midnight AM on Saturday 5/11/2019.

The video is strenuously long and sometimes difficult to follow without the video-post's caption function.

I will have to review it in rerun fashion several times so that my *"Attention-Deficit-Disorder" (*My "ADD" was diagnosed some 50 years ago) can acclimate to the video's lecture content.

_______________________________I began reviewing video at 2:02 PM on Saturday 3/23/19.

I paused play at 2min:47sec and will resume watching at a later date.

Video footage duration is 1hr:13min:21sec in recorded length.

_______________________________I am noting my thoughts at 2hrs: 43min after midnight AM on Saturday 5/11/2019 as follows:

The video is strenuously quite long at 1hr:13min:21sec. And it would take a photographic memory to redact the content just to comprehend the lecture's objective.

## Taleb Ali says:

ty cobb attorney wh former sword and shield , john presk the three stooges lawyers of thomas more !.

## Ronald Johnson says:

this guy is so dumb he doesn't realize he doesn't even know what he's talking about. i was almost gonna feel embarrassed for him for making such an ass of himself but i am sure he is acutely aware… i couldn't take any more of his rubbish so i bow out at 27:47/ buh bye

## Keanan Jacobs says:

Who's here after hulks explanation about back to the future.

## HREHAN says:

Sir, Where exactly should the singlet lie on the curve ? 35:00 onwards.

## Adam Warlock says:

Wow He Described Mahabharata 2:03

Great India

Great Hindu

## Adam Warlock says:

Endgame

## lushin burl says:

Why would somone thumbs Down this. Are you scared of quantum?

## Samuelson baker says:

Entanglement is what makes time travel possible . ( on a human level )

## Karla Cath Jefferson says:

Who's here after the Avengers?

## Edward Ongeso says:

Avengers Endgame

## Blvd Ro says:

I find it extremely interesting that I can enjoy this content as much as a world star knockout and my two favorite artists right now are Kevin Gates and Chris Stapleton. We humans are diverse and complex.

## JPQ Stevens says:

9:32 Oh I dunno know maybe…AVENGERS ENDGAME!

## Ron B says:

No. Please, no balding people with pony tails.Its very distracting and actually somewhat disgusting . Not only that, it’s hard to take a person seriously when they do that.

## 123456 78910 says:

Avengers!

## ThothHeart Maat says:

Did you know that you can scream into the past to warn yourself of danger? Sound can travel into the past.. I did it one time. I heard a scream then I walked towards where I heard it then this animal came out from under a bridge at night and I screamed.. if you hear a scream it might be you in the future warning yourself . Do research on this.. but if you hear the scream does that mean you can't avoid going towards the danger or if you do choose to avoid the danger do you then not hear the scream so then not choose to avoid the danger? A paradox..

I even just read an article where some scientists determined a sound they were testing displayed negative time. It instantly traveled faster than light backward in time.

But if you scream into the past to warn yourself of danger in the future and the mere fact that you heard the scream is proof that you will inevitably encounter the danger, you have interacted with yourself in the past and in some way caused yourself to encounter the danger? But you haven't altered the past because it's all one timeline..

## ThothHeart Maat says:

If determinism is true then you would be able to go back into the past and anything you did would just coincide with the natural timeline because you would be unable to control your actions and change anything at all.. your choice to go into the past would be predetermined and anything you did in the past would also be predetermined..

## ThothHeart Maat says:

A lecture on the plot holes of back to the future to answer ant man's question…

## ThothHeart Maat says:

I think it's impossible to interact with yourself in the past because think about time traveling. To travel back in time is to reverse space time so you yourself would follow this reverse path and never meet an alternative you. Hollywood has shown us a bizarre spontaneous transmission version of time travel that's probably bullshit.. we can time travel into the future no problem but the way it works is by distorting time at different locations but nothing is ever teleported anywhere, things just move at a different rate. To travel back in time probably works the same way. But if we follow the instant transmission sound reverse time travel logic then I suppose maybe reverse time travel is the instant transmission type. And if everything in the universe is a sound wave.. even think about that word universe.. one verse.. one sound.. om….

## itsRAWRtime007 says:

At 44:47 he travels in time