Time Travel Simulation Resolves “Grandfather Paradox”

&gt; 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<p>That seems like an understatement if I&#x27;m understanding it correctly.<p>Imagine you have a &quot;time machine&quot; and you want to solve some arbitrary computable problem. You try a prospective solution and see if it&#x27;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.<p>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.<p>But it still couldn&#x27;t break a one-time pad.

My pet theory:<p>You create a paradox, it just keeps looping (you go back in time and kill your grandfather -&gt; you don&#x27;t exist -&gt; you don&#x27;t build the time machine and go back -&gt; your grandfather lives -&gt; you exist -&gt; you build the time machine etc). But every time around the loop there is a certain amount of actual randomness.<p>Perhaps that atom decays this time around, or that transistor erroneously conducts due to shot noise.<p>So all we see is the final &quot;fixed point&quot;. The iteration where everything ends up going just right to avoid a paradox - perhaps the gun fired prematurely, or the time machine didn&#x27;t work, or a passing airplane dropped an engine on you, or...<p>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.

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.<p>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.<p>Am I missing something here, because I&#x27;ve never heard this mentioned by anyone else?

Scott Aaronson has done some work regarding CTCs and quantum computers: <a href="http://www.scottaaronson.com/papers/ctc.pdf" rel="nofollow">http:&#x2F;&#x2F;www.scottaaronson.com&#x2F;papers&#x2F;ctc.pdf</a><p>Apparently, in the presence of CTCs, quantum computers are no more powerful than classical ones.

My pet theory:<p>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.<p>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.<p>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.<p>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.

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.<p>[1] <a href="http://web.mit.edu/adorai/timetraveler/" rel="nofollow">http:&#x2F;&#x2F;web.mit.edu&#x2F;adorai&#x2F;timetraveler&#x2F;</a>

My favorite &quot;layman&#x27;s&quot; argument for why time travel, as it&#x27;s been classically described in science fiction, can&#x27;t exist is that we haven&#x27;t been overrun with tourists from the future.

Well, the argument is that quantum indeterminism implies quantum time travel is possible, because the grandfather paradox is solved&#x2F;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 &#x27;t Hooft maintain that quantum mechanics can be both deterministic and consistent with current experiments.<p>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&#x27;m not sure if I&#x27;m interpreting the article correctly.

Two nice links for anyone who wants to nerd out about the physics of closed timelike curves:<p><a href="http://plato.stanford.edu/entries/time-travel-phys/" rel="nofollow">http:&#x2F;&#x2F;plato.stanford.edu&#x2F;entries&#x2F;time-travel-phys&#x2F;</a><p><a href="http://philsci-archive.pitt.edu/1673/1/TMArchive.pdf" rel="nofollow">http:&#x2F;&#x2F;philsci-archive.pitt.edu&#x2F;1673&#x2F;1&#x2F;TMArchive.pdf</a>

If time travel exists, then it cannot be a simple rewind of the universe&#x27;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&#x27;t be any time travel, since neither you nor anyone else would be able to witness it.<p>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&#x27;t be born, but you wouldn&#x27;t stop to exist.<p>Therefore, the whole &#x27;grandfather paradox&#x27; 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.

I&#x27;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.

Say I go back in time. Does it mean I disappear from the time I was in? And I&#x27;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&#x27;m sure when I left time continued and somebody noticed.

Can someone enlighten me: what is this simulator they speak of? It doesn&#x27;t seem answered anywhere in the article what it is, whether it&#x27;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?<p>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?

All hypothetical time machines must also be <i>space</i> 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?

<a href="http://en.wikipedia.org/wiki/Closed_timelike_curve" rel="nofollow">http:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Closed_timelike_curve</a><p>&quot;In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime that is &quot;closed&quot;, returning to its starting point.&quot;<p>Now, go to <a href="http://en.wikipedia.org/wiki/Virtual_particle#Vacuums" rel="nofollow">http:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Virtual_particle#Vacuums</a> and look at the picture &quot;One-loop diagram with fermion propagator&quot;<p>It seems to me that virtual particle pairs satisfy the CTC criterium.

I do not think that there can be a Grandfather Paradox. If one goes back in time, he&#x2F;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&#x2F;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.

I&#x27;ve enjoyed thinking about the following:<p>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 <i>now</i> of being visited by these future time travellers?

It&#x27;s quite possible that the problem with paradoxes stems not from them being &#x27;impossible&#x27;, but from our limited cognitive tools failing to equip us to think about them.<p>Sort of like people in ancient times believing you can&#x27;t predict the motion of the planets because they didn&#x27;t have calculus yet.

I would like to see a unified explanation that describes how our current modes of time travel work.<p>How does the matter that comprises my car and myself successfully get from Palo Alto to The Mission every morning?

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?

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