
Time Travel Simulation Resolves “Grandfather Paradox” - markmassie
http://www.scientificamerican.com/article/time-travel-simulation-resolves-grandfather-paradox/
======
zrm
> 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.

~~~
ww520
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.

~~~
CocaKoala
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?

~~~
kybernetikos
Was the one time pad HDDLC, or perhaps WJHLP or maybe BJENK or IEEOF?

------
TheLoneWolfling
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.

~~~
DennisP
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...](http://en.wikipedia.org/wiki/Novikov_self-
consistency_principle)

~~~
cscheid
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...](http://journals.aps.org/prd/abstract/10.1103/PhysRevD.44.3197)

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

------
sandycheeks
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?

~~~
kevincennis
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
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.

------
elseless
Scott Aaronson has done some work regarding CTCs and quantum computers:
[http://www.scottaaronson.com/papers/ctc.pdf](http://www.scottaaronson.com/papers/ctc.pdf)

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

------
camelNotation
My pet theory:

Quantum mechanics is a statistically based science because we have yet to
discover the constants that would unify it with general relativity, making
this entire thought experiment irrelevant.

Imagine if you are watching 10 parallel strings ripple up and down at various
frequencies. Viewed from the side, it would at first glance appear chaotic,
but by observing the behavior long enough and recording where the activity
occurred over time, you could statistically predict where the strings would be
at any given time.

However, if you were aware of the number of strings you were viewing and the
frequencies at which they were moving, you could formulate a precise theory of
exactly what you were viewing and what you would view at every point in the
future.

That is the current state of physics. Quantum mechanics is seeing without
understanding the rules governing the system, so predictions and observations
are made using statistics instead of constants. Once we discover the
constants, the current theories will be entirely obsolete and articles like
this will become relics.

~~~
ep103
yeah. I remember some ~7 years ago reading about a research team that stated
that they had rewritten many of the fundamental equations of (some branch of)
quantum physics to show that they could be written in a completely
deterministic manner. They said that while such equations weren't likely to be
useful (due to the limitations of our testing technology), it was an important
result, particularly when it comes to understanding the underlying science.
And they were very upset that basically no one paid their result any real
attention.

------
smokel
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/](http://web.mit.edu/adorai/timetraveler/)

~~~
narag
_future time travellers may be at a loss as to which one to attend_

Ahem, all of them at once?

~~~
coldtea
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.

------
aaronbrethorst
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.

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

~~~
andyjohnson0
Looks like it was in Isaac Asimov's Science Fiction, July 1979 [1].

[1] [http://www.abebooks.co.uk/book-search/author/ramirez-
frank-r...](http://www.abebooks.co.uk/book-search/author/ramirez-
frank-r/sortby/3/page-1/)

------
DCKing
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.

------
idlewords
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://plato.stanford.edu/entries/time-travel-phys/)

[http://philsci-archive.pitt.edu/1673/1/TMArchive.pdf](http://philsci-
archive.pitt.edu/1673/1/TMArchive.pdf)

------
axilmar
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
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.

~~~
axilmar
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
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.

------
vbit
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.

~~~
afafsd
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.

------
stinos
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
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](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.

------
phpnode
All hypothetical time machines must also be _space_ machines, i.e. they must
take into account the movement of the planets, the galaxy, the universe to
avoid materializing their passengers in the middle of space or the centre of
the sun. My question is what frame of reference would they use?

~~~
claar
This is exactly what always bothers me in time travel movies.

You never hear about the poor time traveler who got the calculation slightly
wrong and ended up deep in space or inside of a mountain.

~~~
chippy
Only when you find their fossils.

------
im2w1l
[http://en.wikipedia.org/wiki/Closed_timelike_curve](http://en.wikipedia.org/wiki/Closed_timelike_curve)

"In mathematical physics, a closed timelike curve (CTC) is a world line in a
Lorentzian manifold, of a material particle in spacetime that is "closed",
returning to its starting point."

Now, go to
[http://en.wikipedia.org/wiki/Virtual_particle#Vacuums](http://en.wikipedia.org/wiki/Virtual_particle#Vacuums)
and look at the picture "One-loop diagram with fermion propagator"

It seems to me that virtual particle pairs satisfy the CTC criterium.

------
axilmar
I do not think that there can be a Grandfather Paradox. If one goes back in
time, he/she will simply be removed from the state of the universe as it is
now and be added to the state of the universe at the target past date. He/she
could then go on killing their father before they were born, but nothing will
happen to them except not meeting with themselves as babies.

------
chippy
I've enjoyed thinking about the following:

It must be almost impossible to time travel back in time. But it could be
possible, given unlimited time. Thus, given that at some point in the
unlimited future, time travel would occur, how can we increase the chances
_now_ of being visited by these future time travellers?

~~~
testing3212
> But it could be possible, given unlimited time.

Just because time is infinite, does not mean that all things are possible.

My understanding is that the universe is in a bounded random walk. As space-
time moves, the unexplored potential state space increases more quickly than
can be explored.

If there are multiple universes, then each universe could explore one of these
states. So if there are more states than universes, not all things are
possible. But if there are more universes than states, all things would be
possible.

Though there are infinite potential states and infinite universes, I feel like
the potential states expand faster than the number of universes. This means
that our likelihood of living in a universe where something like like time
travel is possible, is incredibly low.

------
onion2k
It's quite possible that the problem with paradoxes stems not from them being
'impossible', but from our limited cognitive tools failing to equip us to
think about them.

Sort of like people in ancient times believing you can't predict the motion of
the planets because they didn't have calculus yet.

~~~
cLeEOGPw
It's also quite possible that the whole idea of time travel seeming probable
stems not from being actually possible, but from human brain cognitive bias of
memory. In our head, past events are almost as real as current events,
therefore creating obvious illusion of possibility of traveling in reality
just like you travel in memories.

------
stevewilhelm
I would like to see a unified explanation that describes how our current modes
of time travel work.

How does the matter that comprises my car and myself successfully get from
Palo Alto to The Mission every morning?

------
u124556
If you had a program that generated a random number, and somehow send a hash
of said number to the past to be used as a salt to generate itself. Would the
new random number be truly random?

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

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

