Time Travel Simulation Resolves “Grandfather Paradox”

http://www.scientificamerican.com/article/time-travel-simulation-resolves-grandfather-paradox/

by markmassie • 12 years ago

154 124 12 years ago

scientificamerican.com

idlewords• 12 years ago

Two nice links for anyone who wants to nerd out about the physics of closed timelike curves:

http://plato.stanford.edu/entries/time-travel-phys/

http://philsci-archive.pitt.edu/1673/1/TMArchive.pdf

aaronbrethorst• 12 years ago

My favorite "layman's" argument for why time travel, as it's been classically described in science fiction, can't exist is that we haven't been overrun with tourists from the future.

randallsquared• 12 years ago

Sure, but CTCs only allow one to travel back to a time when the CTC existed, so I don't think that's a good argument unless there was a CTC-generating device in our neighborhood. Of course, as far as I know, all CTCs require infinities (Tipler cylinders, for example: http://en.wikipedia.org/wiki/Tipler_cylinder ), which is an argument against constructing one...

pyre• 12 years ago

CTCs only allow one to travel back to a time when the CTC existed

So... Quantum Leap rules of time travel then? Oh boy...

th3iedkid• 12 years ago

but the full question being is we = all ?

beachstartup• 12 years ago

and my favorite refutation is that they've been here for a while now.

ghayes2• 12 years ago

E.g. the fellow in this photo from 1940: http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef01347ff60cea...

eridius4• 12 years ago

It strains credulity to believe that we do in fact have time travelers living among us and 100% of them have managed to completely escape detection. For what it's worth, there actually have been serious attempts at determining whether there are time travelers living among us, including at least one study[1] looking at Twitter for evidence of time travelers tweeting

[1] http://www.cnn.com/2014/01/03/tech/innovation/time-travel-tw...

tzs3• 12 years ago

I'm sorry I cannot give a better cite than this, but sometime in the last 20 years there was a terrific short story, probably in Analog (but possibly in Asimov's) about the first time traveller.

His first shock was when he found that the person in the past they selected to visit, Shakespeare, did not seem at all surprised when visited by a time traveller. In fact, he seemed to find it routine. The traveler at one point says he doesn't understand how Shakespeare can already know about time travelers, since he is the first. Shakespeare tells him that he may have been the first to leave, but he wasn't the first to arrive.

Shortly after, many time travelers arrive...but not to see Shakespeare. They are reporters, from all throughout the timeline, coming to try to interview the first time traveller, and Shakespeare, an old hand at dealing with time travelers, steps in to protect the first time traveller and prevent him from being overwhelmed.

atmosx• 12 years ago

"The Merchant of Stratford" by Frank Ramirez (amazing approach, really unique and ingenious).

RyanMcGreal• 12 years ago

Reminds me of this classic: http://www.abyssapexzine.com/wikihistory/

"Everybody kills Hitler on their first trip."

Sukotto• 12 years ago

Looks like this thread is gathering the list of good Time Travel stories. So here's one I like: "Dinosaur Beach" by Keith Laumer. A delightfully twisty and multi-layered novella.

Read: http://www.baenebooks.com/10.1125/baen/0743435273/0743435273...

Reviews: https://www.goodreads.com/book/show/584217.Dinosaur_Beach

rtpg• 12 years ago

I've always found this argument a bit bizarre, because in my mind any time travelling device would work via a pair of machines: a sender and a receiver. So nobody could come to the present simply because there's no receiver yet.

The reason we haven't been overrun by tourists because there's no stop in this time period.

coldtea3• 12 years ago

I've always found this argument a bit bizarre, because in my mind any time travelling device would work via a pair of machines: a sender and a receiver.

Were did you get that impression from? Not most Science Fiction or Physics, because neither postulates such a thing.

inclemnet• 12 years ago

I hate this argument, not because it isn't correct as far as it goes, but because it's very often represented as much more broad than it really is.

That is, yes, we can be reasonably sure that time travel mechanisms whereby tourists definitely could and would become known in our time period will not exist (even this isn't the same as can't exist). But this is a very small sampling of time travel scenarios you could imagine, and I think it's far from the only thing described 'classically' by science fiction.

gambiting• 12 years ago

And my favourite "layman" counter argument to that is that plenty of time travel theories only allow travel as far back as to when the time travel machine was first created.

zrm2• 12 years ago

In the presence of CTCs, quantum mechanics allows one to perform very powerful information-processing tasks, much more than we believe classical or even normal quantum computers could do

That seems like an understatement if I'm understanding it correctly.

Imagine you have a "time machine" and you want to solve some arbitrary computable problem. You try a prospective solution and see if it's correct. If not, you send the next prospective solution back to be tried on the next go around the time loop. Once you arrive at the correct solution, you send that same solution back so that the iteration stops there. Then the loop repeats indefinitely with the correct solution so that the probability of exiting the loop at the correct solution approaches infinity.

It would literally be the end of the world as we know it. P=NP. Forget about quantum cryptography, that would break all public key cryptography, all cryptographic hash functions. It would obsolete algorithmic complexity theory by effectively turning every finite space algorithm into an O(1) algorithm. It would probably bring Strong AI.

But it still couldn't break a one-time pad.

terhechte• 12 years ago

There's a chapter in the brilliant fan fiction "Harry Potter and the Methods of Rationality" where Harry gets access to a Time-Tuner, i.e. a time machine from the Harry Potter universe, and this is the first experiment he envisions and performs.

HPMOR is a really great book.

https://www.fanfiction.net/s/5782108/17/Harry-Potter-and-the...

Semaphor2• 12 years ago

fair warning: This, absolutely awesome and actually well written, fanfiction is by now longer than all the books combined.

ww520• 12 years ago

One-time pad is just a generated key used once. The "time machine" will break it by trying out all keys in the key space in the time loop. If time is not an issue, brute force method pretty much breaks any crypto.

alcari• 12 years ago

One-time pads are still secure because the decryption process will produce every plaintext the same length as the ciphertext, and you'll have no way of knowing which one was correct.

acchow• 12 years ago

You can't "break" a one-time pad because you can't possibly know when you've got the right "key".

tzs3• 12 years ago

Others have explained why that does not work. The way you'd use backward time travel to crack a OTP is to wait until the recipient of the message decrypts it, then your steal the decrypted message, then you go back to the time where you wanted to break the code and give yourself the message.

Another use for backward time travel is data compression. To compress a file, you simply replace its contents with a note saying when you did this. When you later want the file, just pop back to that time and get it.

This also greatly simplifies backup systems.

CocaKoala• 12 years ago

Here's a message encrypted by a one time pad: "AHWAM"

The key is very short, only five letters, so it should be pretty trivial to brute force. Can you tell me what the message is?

baddox2• 12 years ago

What algorithm would you run on this machine to yield strong AI?

zrm2• 12 years ago

First you need a rigorous formal definition of strong AI. Then write a computer program that iterates over all possible computer programs until it finds one that meets the definition.

Or try simulating a human brain.

DennisP2• 12 years ago

You could do AIXI: "The AIXI formalism says roughly to consider all possible computable models of the environment, Bayes-update them on past experiences, and use the resulting updated predictions to model the expected sensory reward of all possible strategies."

http://wiki.lesswrong.com/wiki/AIXI

So you could use the time loop to run through this algorithm.

But what's this about predictions? Why predict when you have a time machine? All you really need is a reward function and this algorithm:

- Get a message from the future with its best-ever reward and how it achieved it.

- Try something else

- When you get to the future, see whether you've done better than the best ever. If so, replace the best ever.

- With each iteration keep a count. If you've reached some maximum number of tries without improving the best score ever, do the best strategy one last time and quit.

It might be tricky figuring out the best span of time to use, and whether you can do mini-loops within bigger loops somehow.

maaku• 12 years ago

Or, instead of receiving the next iteration you end up with a piece of paper with "DON'T MESS WITH TIME" written in your own shaky handwriting.

The space of possible stable outcomes is much larger than just the ones carrying the outcome of the computation.

arethuza• 12 years ago

Or maybe:

"I am the Eschaton; I am not your God. I am descended from you, and exist in your future. Thou shalt not violate causality within my historic light cone. Or else."

http://en.wikipedia.org/wiki/Singularity_Sky

possibilistic• 12 years ago

It wouldn't be a constant energy algorithm though, would it? For each successive optimization iteration, you would have to expend energy. Does that energy get put back with every rewind of the clock? If so, energy is also infinite.

I wonder what the time-energy-complexity trade-offs are?

As an aside, I would gladly pay someone to walk me through the math and physics sometime. (Any grad students in Atlanta are more than welcome to take me up on that offer... I might even try Skype.)

wuliwong4• 12 years ago

Hah, funny, I had a similar question and I'm also in Atlanta. :)

onion2k2• 12 years ago

It would probably bring Strong AI.

A CTC is effectively a computer that can process a loop instantly, regardless of how complicated it is. Given that, we could design an AI machine that could analyse an arbitrary amount of data in any depth without regard to how long it'll take. More than that, everything happens in one 'tick' of whatever is controlling the CTC, so there aren't any issues around asynchronous processing. It just 'goes away'. Such a machine would never have to wait for another process to finish. Strong AI would be pretty much a certainty - we'd be able to brute force any problem.

tjhance• 12 years ago

www.scottaaronson.com/papers/ctc.pdf

Shows that (with or without quantum computing) you can do all of PSPACE using a CTC (in polynomial time) and no more.

mshron• 12 years ago

This is the key result in this conversation; this should be higher. Basically: if we insists on Deutsch's causal fixed point propety of CTCs, they do not give you P=NP, they give you P=PSPACE. That is, all problems which can be solved in polynomial space can now be solved in polynomial time, and vice versa.

So for a time-travel computer to give P=NP or P=O(1), it would have to violate Deutsch's causality criterion. I have almost zero knowledge of either field, but it sounds to me like it makes Deutsch's model more plausible.

rst• 12 years ago

Beyond P=NP: this actually gives you a constant-time solution for any problem where it's possible to enumerate candidate solutions, regardless of how long it takes to do so. So, we'd have ANYTHING=O(1).

Houshalter2• 12 years ago

Only if you are certain there is a correct answer. If there isn't a correct answer, you just cause a paradox. It's also more likely that the machine will spontaneously fail, or the creator will get hit by a bus before he can complete it, etc.

phpnode2• 12 years ago

Using a time-loop in node.js to compute factorials https://gist.github.com/Marak/3967740

wuliwong4• 12 years ago

I wonder if you'd run into some sort of energy problem. Like, this time machine would require some sort of energy and incur some amount of wear and tear on during each pass. The amount of possible solutions to today's cryptography might make this brute force level of effort, even with a time machine, infeasible. I feel like nasty old thermodynamics may preserve our world still. :)

Granted, I'm way way out of my depth here. I have a Ph.D. in physics but I studied nonlinear oscillators, relating them to problems in neuroscience.

vbit• 12 years ago

Say I go back in time. Does it mean I disappear from the time I was in? And I'm in two places at a point in the past (the me from that time and the me from the future)? Now if I want to go back to the time I came from, do I join back at the exact time I left (which means no one would notice my travel), or would I have to travel back to the time I left + the elapsed time I spent in the past? And if I really did return to the exact time I left, does that really mean no one noticed my leave? Because I'm sure when I left time continued and somebody noticed.

afafsd2• 12 years ago

In the form of time travel being talked about, the answers are:

a) Yes, you disappear from the time you were in.

b) Yes, you're in two places at once for the duration between your arrival and your departure

c) Forwards travel in time doesn't cause too many philosophical problems (since we're already doing it) which is why nobody talks about it. But there's no requirement to wind up exactly where you started.

jaza• 12 years ago

Time to dust off my old flux capacitor again...

wuliwong4• 12 years ago

lol, can't believe this gets down-voted. God forbid there is humor on HN!?!

stinos• 12 years ago

Can someone enlighten me: what is this simulator they speak of? It doesn't seem answered anywhere in the article what it is, whether it's software, let alone how it works. Yet the whole article relies on it. Does it act on real protons? Or are they simulated as well?

And, most important, why do they trust a simulation device built by humans who hardly understand the matter completely, when the device is then used to make assumptions about that same matter? Is it somehow proven this simulator is correct? And how did that happen?

jessriedel• 12 years ago

This is a poor article. It is reporting on a simple table top device created by researchers eager to misrepresent their work. It tells us essentially nothing about quantum mechanics or CTCs.

We don't know how quantum mechanics behaves in the presence of CTCs. There are multiple, mutually incompatible proposals for extending QM in this regime that all make identical predictions in normal, causal spacetimes (i.e. the only sorts of spacetimes we have access to). Deutch has one extension of QM, and it suffers from serious internal consistency problems, but there are others.

Since this experiment takes place without CTCs, it can't tell us anything we didn't already know. It's literally just a toy device that is sorta described by the same math as a certain system with CTCs in QM_Deutch would be, if you squint your eyes hard enough. But you could say the same thing about a computer simulating QM_Deutch in software. It tells you nothing about whether the universe actually obeys QM_Deutch in the presence of CTCs ... if they exist at all.

Similarly deceptive bits of "research" in the news often appear that claim to find quantum gravity effects on table-top experiments.

http://www.space.com/5052-black-hole-effect-created-lab.html

In all these cases, the scientists will eagerly over-represent their work to the journalist, and the journalist will eagerly gobble it up and write eye-catching headlines about new breakthroughs in physics, but the heart of the claim will be buried in the article and subtly couched in weasel words. And if you actually try to drive down at it by asking probing questions, the journal and scientist will retreat to much, much less exciting claims and you will never be able to prove they intended otherwise.

elseless• 12 years ago

Scott Aaronson has done some work regarding CTCs and quantum computers: http://www.scottaaronson.com/papers/ctc.pdf

Apparently, in the presence of CTCs, quantum computers are no more powerful than classical ones.

smokel• 12 years ago

Apparently, Stephen Hawking organized a time travel experiment in 2009. There was also the Time Traveler Convention in 2005 [1]. If people keep organizing such events, future time travellers may be at a loss as to which one to attend.

[1] http://web.mit.edu/adorai/timetraveler/

narag• 12 years ago

future time travellers may be at a loss as to which one to attend

Ahem, all of them at once?

coldtea3• 12 years ago

That they can do that, technically, doesn't mean the would.

Just like we don't attend all parties available to us, they would soon get bored.

Dylan16807• 12 years ago

Do you really want to go to 10000 conventions for the same thing?

stevewillows• 12 years ago

The best part of this event was that they turned people away. I feel sorry to those time travellers who didn't go back far enough.

swalsh• 12 years ago

Imagine you're a time traveler from the future, and you've read all about the NSA from 2009. Would you want to risk making your presence known?

What do you think the government would do if they got a hold of a real life time traveler?

3pt14159• 12 years ago

The first one?

u1245562• 12 years ago

We all know no one attended the party, so maybe time travelers just didn't want to disturb that part of history?

DCKing• 12 years ago

Well, the argument is that quantum indeterminism implies quantum time travel is possible, because the grandfather paradox is solved/migitated by indeterminism in which the grandfather paradox is still a valid outcome. However, quantum indeterminism itself is not yet completely undisputed; some theoretical physicists such as Gerard 't Hooft maintain that quantum mechanics can be both deterministic and consistent with current experiments.

This also raises the following question - if quantum mechanics is undeterministic, then the macroscopic world is at least probabilistic. If probabilistic systems allow the possibility time travel on the quantum scale, would the grandfather paradox also not be solved on classical scales? Since it is possible that me killing my grandfather in the past would fail with some very small probability (due to various quantum effects adding up), would the grandfather paradox then not also be solved in this case? I'm not sure if I'm interpreting the article correctly.

axilmar3• 12 years ago

If time travel exists, then it cannot be a simple rewind of the universe's tape. Because if that was the case, then rewinding the tape to a time that your father was not born would mean that you will also not be born, and thus there wouldn't be any time travel, since neither you nor anyone else would be able to witness it.

Thus, in order for time travel to work, it means to rewind the tape and add yourself to the tape at a moment you did not exist before. Thus, if you killed your father, your replica wouldn't be born, but you wouldn't stop to exist.

Therefore, the whole 'grandfather paradox' is not a paradox at all: if time travel exists, then you would simply exist in a universe where a replica of you will exist or not, depending on how events play out.

afro88• 12 years ago

Not so fast - who says you become a "replica" in the first place? Assuming time travel replicates things, then yes, what you said holds true.

But as I understand it you don't replicate, your time just loops back on itself. So looping back to a time before you were born and killing your father would create a paradox.

axilmar3• 12 years ago

You seem to have not understood what I said. I did not say time travel would replicate anything.

You would simply be transferred to the past, without any connection to the outcome of that past.

I.e. Let's say that you are transferred back in the 50s and you go kill your father before you were born. Then nothing will happen, there would be no paradox, you will continue to exist in that universe.

TeMPOraL• 12 years ago

I'm starting to adopt the idea of Scott Aaronson that since we have a lot of evidence suggesting that P != NP, we should treat physic ideas as less probable of being true if they make P=NP as a side effect.

sandycheeks2• 12 years ago

When I was younger I thought a lot about the physics of traveling back in time but I always seemed to hit a brick wall with spatial coordinates.

Where we are in the universe today is very far from where we were in the universe yesterday based on the movement of the earth alone. Add to that the movement of our solar system, galaxy, cluster, supercluster and movements I am not even aware exist and it becomes really far away. Grandpa would probably be light years away from me and my time machine.

Am I missing something here, because I've never heard this mentioned by anyone else?

jdjb• 12 years ago

If you can travel backwards in time (or even just stop time) you can be at any spatial location you desire in the entire universe (since you have essentially infinite speed).

growse• 12 years ago

Moreover, imagine I write down some information, then wait for exactly 6 months for the earth to take me far away from that point in space. If I now travel 6 months minus 1 nanosecond back in time with that piece of paper, but preserve my spacial position, haven't I just 'transmitted' information through space faster than light can?

TheLoneWolfling8• 12 years ago

FTL travel is synonymous with time travel, and vice versa.

Relativity is weird.

kevincennis• 12 years ago

It's been almost 10 years, so I may be mistaken... but I think the director of Primer mentioned this in the DVD commentary.

EDIT: Right, not really part of the plot. I just think I remember him (in the commentary) talking about some of the "trickier" aspects of writing a story about time travel, and I think he mentioned the fact that nobody ever really addresses the positional aspect.

slazaro• 12 years ago

I think the model of time and space in Primer is more consistent because it's a very specific kind of time machine.

If I recall correctly, you'd turn on the machine, wait N hours, go inside the machine, and in the next N hours you'd be going back in time and come out the other end at the moment the machine was turned on. So you would have spent 2*N extra hours, the machine would have always been in the same spot, so there'd be no problem with traveling through space.

There are still problems with the paradoxes, but that's part of the plot of the movie.

sp332• 12 years ago

It wouldn't seem to matter in the movie, since whatever you put in the device would get dragged around with the device as it traveled through space.

rjshade• 12 years ago

"Perhaps! But this problem only comes up if you're not the centre of the Universe."

http://www.qwantz.com/index.php?comic=1394

Steuard2• 12 years ago

It's a valid issue. You wouldn't need to worry about any component of your motion that's "constant velocity" (so, inertial straight-line motion), since all inertial reference frames are equally valid. But you would need to keep track of curving motion (like the rotation of the Earth, orbit around the Sun, Sun circling around the galactic core, etc.).

You might be able to wave your hands and claim that the time machine follows paths back in time along natural world-lines in curved space-time (gravitational geodesics), which will at least get things like orbiting around the Sun right (and galactic motion as well). But I think you'd still need some way to keep the machine's path rotating on the surface of the Earth rather than falling back and forth through the center of the planet.

sandycheeks2• 12 years ago

I thought it might follow backwards in space as well as time but when the book The Physics of Time Reversal by Robert G. Sachs came out in 1987 it made me skeptical of that possibility. I haven't followed the theory of CP violation for many years so this may not be a pertinent point anymore.

Hermel• 12 years ago

Maybe time travel needs some kind of tracks, like a railroad. That would also explain why we don't see any time travelers - they can't visit us as there are no tracks that lead back to 2014.

wuliwong4• 12 years ago

You are not alone! I've thought a lot about how much we move in space while we "sit still" on earth. :)

bithive123• 12 years ago

The time travel show "Seven Days" from the 90s dealt with this -- sometimes the time machine would end up stuck underground or floating out in space.

TheLoneWolfling8• 12 years ago

Most of the time this is handwaved over, for bunches of reasons. Part of it is that there is no one valid frame of reference: you can't just say "this stays stationary" - because what does that even mean? Stationary relative to what?

If you mean "you go wherever inertia would take you in that amount of time, with no acceleration", then if you go backwards a multiple of a year you end up pretty close to Earth (quick approximation: the Sun's acceleration in its orbit in the milky way is ~2*10^-10m/s/s, which works out to ~100km in a year, ~400km in two years, etc.). In actuality you could probably get somewhat better range by letting your velocity build up to intercept Earth-that-was at the target time. (You'd need to take into account a bunch of things for best accuracy: the Earth's orbit is not "perfect", the moon's phase, the acceleration of the Sun, etc, but we know most of that.)

You probably would end up needing to make multiple jumps, pausing in-between to let Earth's (or another planet's, or even the Sun's) gravity well readjust your velocity to what is needed for your next jump.

This means that you'd need to build in re-entry capability into the machine (as you are unlikely to have the precision necessary to be able to jump into atmosphere at a sane velocity), and potentially maneuvering delta-v. Note that with a time machine you lose conservation of momentum, which makes things a lot easier. You can change the direction of your velocity vector by jumping back in time and interacting with your past self. You can potentially use this to get "free" (read: reactionless) velocity out of any gravity well.

bengali3• 12 years ago

whats your reference for coordinates?

Earth Center? Solar Center? Galaxy Center? ... Universe Center?

hmm traveling backwards along the 4th dimension by itself sounds like we are NOT traveling in the other 3.... so where would the atoms (that originated from your banana at 9:25am 3 days ago) go if we merely changed their value on the 4th axis? does their xyz stay locked at the current values or revert back to their previous values.

stan_rogers• 12 years ago

That is a major plot point in Spider Robinson's Callahan's Con. (Look, his first few Callahan stories were charmingly quirky, and I got into the habit of reading them. This is not an endorsement of his larger oeuvre, okay?) In that case, the cosmic microwave background was used as the inertial reference, probabilities were the main problem to solve (we'd be well beyond the relative simplicity of the three-body problem), and little details like delta-v were not merely hand-waved away.

TheLoneWolfling8• 12 years ago

My pet theory:

You create a paradox, it just keeps looping (you go back in time and kill your grandfather -> you don't exist -> you don't build the time machine and go back -> your grandfather lives -> you exist -> you build the time machine etc). But every time around the loop there is a certain amount of actual randomness.

Perhaps that atom decays this time around, or that transistor erroneously conducts due to shot noise.

So all we see is the final "fixed point". The iteration where everything ends up going just right to avoid a paradox - perhaps the gun fired prematurely, or the time machine didn't work, or a passing airplane dropped an engine on you, or...

From the perspective of any time traveler, it ends up being as though the universe is conspiring against you if you try to do anything that would cause a paradox.

DennisP2• 12 years ago

This sounds like the Novikov self-consistency principle.

Say you have a wormhole on a billiard table. It curves around and goes three seconds backward in time. You roll a ball into the wormhole, aimed such that after it exits the wormhole, it will knock its earlier self off the path so that it never enters the wormhole. Paradox.

Except when you try it, instead of emerging along the pathway you aimed, it emerges along a slightly different path, and strikes its earlier self only a glancing blow. And why did it emerge along a different path? Because it was struck a glancing blow.

http://en.wikipedia.org/wiki/Novikov_self-consistency_princi...

TheLoneWolfling8• 12 years ago

I suppose having a superposition of all (self-consistent) loops that collapses into a single result is equivalent to the result of what I said... Hmm.

Thanks for the link!

cscheid• 12 years ago

David Deutsch also proposed a similar resolution for closed timelike curves, which say that if you have a probability distribution of the present, all that's necessary is that when you travel back in time, you get the same probability distribution. In short, if you flip a (true!) random coin to before shooting your grandfather, no paradox exists:

http://journals.aps.org/prd/abstract/10.1103/PhysRevD.44.319...

This is described with more intuition and background in one of (always excellent) Scott Aaronson's lectures: http://www.scottaaronson.com/democritus/lec19.html

Houshalter2• 12 years ago

The problem with this is the universe is no longer causal. Events can happen which cause themselves. It requires you to try every possible universe and then "throw away" the ones that aren't consistent. And if you simulate a universe, doesn't it cause that universe to exist, even if you later decide you don't like it? Who is to say we don't exist in one of the many "throw away" universes vs the one where the particle causes itself to be consistent.

Time Travel Simulation Resolves “Grandfather Paradox”

Related Stories