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Does a Time Travel Simulation Resolve the “Grandfather Paradox”?

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That’s the one where the girl travels back in time and kills her grandpa, so how did she get born, so as to travel back in time?

One solution from Scientific American:

Instead of a human being traversing a CTC [closed timelike curve] to kill her ancestor, imagine that a fundamental particle goes back in time to flip a switch on the particle-generating machine that created it. If the particle flips the switch, the machine emits a particle—the particle—back into the CTC; if the switch isn’t flipped, the machine emits nothing. In this scenario there is no a priori deterministic certainty to the particle’s emission, only a distribution of probabilities. Deutsch’s insight was to postulate self-consistency in the quantum realm, to insist that any particle entering one end of a CTC must emerge at the other end with identical properties. Therefore, a particle emitted by the machine with a probability of one half would enter the CTC and come out the other end to flip the switch with a probability of one half, imbuing itself at birth with a probability of one half of going back to flip the switch. If the particle were a person, she would be born with a one-half probability of killing her grandfather, giving her grandfather a one-half probability of escaping death at her hands—good enough in probabilistic terms to close the causative loop and escape the paradox. Strange though it may be, this solution is in keeping with the known laws of quantum mechanics.

Another solution from same:

Deutsch’s model isn’t the only one around, however. In 2009 Seth Lloyd, a theorist at Massachusetts Institute of Technology, proposed an alternative, less radical model of CTCs that resolves the grandfather paradox using quantum teleportation and a technique called post-selection, rather than Deutsch’s quantum self-consistency. With Canadian collaborators, Lloyd went on to perform successful laboratory simulations of his model in 2011. “Deutsch’s theory has a weird effect of destroying correlations,” Lloyd says. “That is, a time traveler who emerges from a Deutschian CTC enters a universe that has nothing to do with the one she exited in the future. By contrast, post-selected CTCs preserve correlations, so that the time traveler returns to the same universe that she remembers in the past.”


Lloyd, though, readily admits the speculative nature of CTCs. “I have no idea which model is really right. Probably both of them are wrong,” he says. More.

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Maybe this 2010 Seth Lloyd lecture hasn’t actually happened yet:

5 Replies to “Does a Time Travel Simulation Resolve the “Grandfather Paradox”?

  1. 1
    bornagain77 says:

    Of related note to time travel paradoxes, Godel’s critique of General Relativity was along that line,,,

    Excerpt: Gödel’s personal God is under no obligation to behave in a predictable orderly fashion, and Gödel produced what may be the most damaging critique of general relativity. In a Festschrift, (a book honoring Einstein), for Einstein’s seventieth birthday in 1949, Gödel demonstrated the possibility of a special case in which, as Palle Yourgrau described the result, “the large-scale geometry of the world is so warped that there exist space-time curves that bend back on themselves so far that they close; that is, they return to their starting point.” This means that “a highly accelerated spaceship journey along such a closed path, or world line, could only be described as time travel.” In fact, “Gödel worked out the length and time for the journey, as well as the exact speed and fuel requirements.” Gödel, of course, did not actually believe in time travel, but he understood his paper to undermine the Einsteinian worldview from within.

    Of note to Godel’s critique of time travel being possible in General Relativity, the following study, through a fairly ingenious thought experiment, challenged the assumption of length contraction as being valid for ‘photon clocks’. In doing so, they cleared up some loose ends in relativity concerning time’s relation to space. Loose ends that had been ample fodder for much of the speculation of time travel being possible in relativity:

    Physicists continue work to abolish time as fourth dimension of space – April 2012
    Excerpt: “The rate of photon clocks in faster inertial systems will not slow down with regard to the photon clocks in a rest inertial system because the speed of light is constant in all inertial systems,” he said. “The rate of atom clocks will slow down because the ‘relativity’ of physical phenomena starts at the scale of pi mesons.”
    He also explained that, without length contraction, time dilation exists but in a different way than usually thought. “Time dilatation exists not in the sense that time as a fourth dimension of space dilates and as a result the clock rate is slower,” he explained. “Time dilatation simply means that, in a faster inertial system, the velocity of change slows down and this is valid for all observers.,, Our research confirms Gödel’s vision: time is not a physical dimension of space through which one could travel into the past or future.”

    As to the Deutsch’s speculations, personally, I take anything David Deutsch speculates about with a grain of salt. He is the one who wrote a book that gave an ounce of respectability to the many worlds interpretation of quantum mechanics…

    Here is, in my personal view, a excellent mini-overview of the many empirical problems with the Many Worlds Interpretation:

    The Parallel Universes of David Deutsch
    (As argued for in Deutsch’s book The Fabric of Reality) – A Critque by Henry R. Sturman
    Excerpt: 1. The whole argument rests on the untestable, and therefore invalid, assumption that a photon goes through one of the four slits when a four slit interference pattern emerges. In particular, Deutsch’s argument seems to rest on the hidden assumption that non-locality is impossible (see below), while he does not present any arguments for this assumption.
    2. Deutsch fails to explain an essential fact of the slit experiments, that the interference pattern disappears when we measure which slit the photon goes through. This fact is evidence against the existence of shadow photons rather than evidence for it.
    3. Deutsch fails to invalidate the alternative standard single universe explanation of the slit experiments.
    4. Deutsch fails to explain the structure of the interference patterns.
    5. Deutsch’s argument against his critics that their theory makes use of imaginary things which have an effect on real things, is based on a straw man.

    A bit deeper look at the fallacies inherent in the Many Worlds Interpretation is here:

    A Critique of the Many Worlds Interpretation – (Inspiring Philosophy – 2014) – video

    as well,,

    Is Shor’s algorithm a demonstration of the many worlds interpretation?
    Excerpt: David Deutsch is very fond of pointing out Shor’s integer factorization algorithm is a demonstration of the many worlds interpretation. As he often asked, where else did all the exponentially many combinations happen?
    Are there any other alternative interpretations of quantum mechanics which can explain Shor’s algorithm, and the Deutsch-Jozsa and Simon’s algorithm?
    ,,, this argument is totally wrong for a simple reason: the real Universe – our Universe – is a quantum system, not a classical system. So it is normal for quantum systems in a single Universe to behave just like the quantum computer running Shor’s algorithm. On the contrary, if we only use the classical computers, we exponentially slow down the computer relatively to what it could do. In this sense, Deutsch’s “argument” shows that the many-worlds interpretation is just another psychological aid for the people who can’t resist to incorrectly think about our world as being a classical world of a sort.,,,
    There is one more lethal conceptual problem with the “many worlds” explanation of the Shor’s algorithm’s speed: the whole quantum computer’s calculation has to proceed in a completely coherent way and you’re not allowed to imagine that the world splits into “many worlds” as long as things are coherent i.e. before the qubits are measured. Only when the measurement is completed – e.g. at the end of the Shor’s algorithm calculation – you’re allowed to imagine that the worlds split. But it’s too late because by that moment, the whole calculation has already been done in a single (quantum) world, without any help from the parallel worlds.
    (Many more excellent answers are on the site)

    Deutsch also claims that the ‘particle interfering with itself’ is another proof for many worlds, but the notion that particles intefere with themselves in the double slit was proven to be wrong by Stapp when he was a Jr. in college:

    A Conversation with Henry Stapp, Ryan Cochrane – March 2014
    Excerpt: As a junior in college, at the University of Michigan, (around 1950), I carried out, during Easter vacation a double-slit experiment where the photons were, on average, 1 km apart, and verified that effect was not due (to) different photons interfering with one another.
    Henry Stapp – Physicist

    If anyone is interested in how Dr. Stapp accomplished the preceding experiment, I e-mailed him and this was his reponse,

    The experiment was meant only to inform myself, and there was never any thought of publication, although I saved for many years the glass slides with the two photographic images, one below the other, of the two double-slit patterns.
    The U of M optics lab featured a double slit experiment. My modified version was not very ingenious: the lab had some calibrated color filters. I merely placed a stack of filters between the light source and the rest of the experiment, so that, using the stated absorption coefficients of the filters, the light was attenuated to an intensity that amounted to an average distance of 1km between photons, whose coherence length was supposed to be about a meter.
    The run lasted ten days. The two interference patterns, one just above the other, were, to my eye, indistinguishable. The “crazy” quantum mechanical prediction was apparently correct! Something very, very interesting was afoot.
    – Henry Stapp – Physicist

    Many Worlds also carries some ‘heavy baggage’ to put it mildly. One funny thing is, even in the many worlds hypothesis, that immortality is possible, even inevitible.,,, Atheists just can’t seem to catch a break anywhere in quantum mechanics even when they try to force fit it into their materialistic worldview. 🙂

    10 Mind-Bending Implications of the Many Worlds Theory – February 2013

  2. 2
    Mapou says:

    Time travel is pure unmitigated crackpottery. It is a direct consequence of Einstein’s spacetime theories. What is amazing is that the proponents of time travel are not even aware of the fact that nothing can move in Einstein’s spacetime, by definition! This is the reason that Sir Karl Popper called spacetime, “Einstein’s block universe (in which, too, nothing ever happens, since everything is, four-dimensionally speaking, determined and laid down from the beginning).” Source: Conjectures and Refutations.

    It gets even uglier. The many-world interpretation of the quantum physics requires the existence of time travel and that other gem of crackpot science called superposition (cat is both dead and alive). So-called “quantum computing” (of which David Deutsch is the most visible guru) is based on all that nonsense.

    The fact that time travel is taken seriously by the likes of Stephen Hawking and other physics luminaries is a sign that modern science has fully transcended democracy and placed itself above the criticism of the public who pays their salaries. They are the new high priests of the modern elitist, chicken feather voodoo religion. As Paul Feyerabend wrote in Against Method, “the most stupid procedures and the most laughable result in their domain are surrounded with an aura of excellence. It is time to cut them down to size and to give them a lower position in society.”

  3. 3
    Popperian says:

    The Everettian interpretation of quantum mechanics is what we end up with if we take quantum mechanics seriously. It’s implied in the very theory itself.

    You don’t have to add anything to quantum mechanics to get the Many Worlds theory.

    From this post by Sean Carroll.

    To see why objections along these lines are wrong-headed, let’s first think about classical mechanics rather than quantum mechanics. And let’s start with one universe: some collection of particles and fields and what have you, in some particular arrangement in space. Classical mechanics describes such a universe as a point in phase space — the collection of all positions and velocities of each particle or field.

    What if, for some perverse reason, we wanted to describe two copies of such a universe (perhaps with some tiny difference between them, like an awake cat rather than a sleeping one)? We would have to double the size of phase space — create a mathematical structure that is large enough to describe both universes at once. In classical mechanics, then, it’s quite a bit of work to accommodate extra universes, and you better have a good reason to justify putting in that work. (Inflationary cosmology seems to do it, by implicitly assuming that phase space is already infinitely big.)

    That is not what happens in quantum mechanics. The capacity for describing multiple universes is automatically there. We don’t have to add anything.


    In any formulation of quantum mechanics, the apparatus starts in a “ready” state, which is a way of saying “it hasn’t yet looked at the thing it’s going to observe” (i.e., the particle). More specifically, the apparatus is not entangled with the particle; their two states are independent of each other. So the quantum state of the particle+apparatus system starts out like this:

    (“spin is up” + “spin is down” ; apparatus says “ready”) (1)

    The particle is in a superposition, but the apparatus is not. According to the textbook view, when the apparatus observes the particle, the quantum state collapses onto one of two possibilities:

    (“spin is up”; apparatus says “up”)


    (“spin is down”; apparatus says “down”).

    When and how such collapse actually occurs is a bit vague — a huge problem with the textbook approach — but let’s not dig into that right now.

    But there is clearly another possibility. If the particle can be in a superposition of two states, then so can the apparatus. So nothing stops us from writing down a state of the form

    (spin is up ; apparatus says “up”)
    + (spin is down ; apparatus says “down”). (2)

    The plus sign here is crucial. This is not a state representing one alternative or the other, as in the textbook view; it’s a superposition of both possibilities. In this kind of state, the spin of the particle is entangled with the readout of the apparatus.

    What would it be like to live in a world with the kind of quantum state we have written in (2)? It might seem a bit unrealistic at first glance; after all, when we observe real-world quantum systems it always feels like we see one outcome or the other. We never think that we ourselves are in a superposition of having achieved different measurement outcomes.

    This is where the magic of decoherence comes in. (Everett himself actually had a clever argument that didn’t use decoherence explicitly, but we’ll take a more modern view.) I won’t go into the details here, but the basic idea isn’t too difficult. There are more things in the universe than our particle and the measuring apparatus; there is the rest of the Earth, and for that matter everything in outer space. That stuff — group it all together and call it the “environment” — has a quantum state also. We expect the apparatus to quickly become entangled with the environment, if only because photons and air molecules in the environment will keep bumping into the apparatus. As a result, even though a state of this form is in a superposition, the two different pieces (one with the particle spin-up, one with the particle spin-down) will never be able to interfere with each other. Interference (different parts of the wave function canceling each other out) demands a precise alignment of the quantum states, and once we lose information into the environment that becomes impossible. That’s decoherence.

    Once our quantum superposition involves macroscopic systems with many degrees of freedom that become entangled with an even-larger environment, the different terms in that superposition proceed to evolve completely independently of each other. It is as if they have become distinct worlds — because they have. We wouldn’t think of our pre-measurement state (1) as describing two different worlds; it’s just one world, in which the particle is in a superposition. But (2) has two worlds in it. The difference is that we can imagine undoing the superposition in (1) by carefully manipulating the particle, but in (2) the difference between the two branches has diffused into the environment and is lost there forever.

    All of this exposition is building up to the following point: in order to describe a quantum state that includes two non-interacting “worlds” as in (2), we didn’t have to add anything at all to our description of the universe, unlike the classical case. All of the ingredients were already there!

    Our only assumption was that the apparatus obeys the rules of quantum mechanics just as much as the particle does, which seems to be an extremely mild assumption if we think quantum mechanics is the correct theory of reality. Given that, we know that the particle can be in “spin-up” or “spin-down” states, and we also know that the apparatus can be in “ready” or “measured spin-up” or “measured spin-down” states. And if that’s true, the quantum state has the built-in ability to describe superpositions of non-interacting worlds. Not only did we not need to add anything to make it possible, we had no choice in the matter. The potential for multiple worlds is always there in the quantum state, whether you like it or not.

    However, you must add something to QM. Namely, that observers are immune from the wave function.

  4. 4
    Popperian says:

    To elaborate on why this is ad-hoc…

    The conclusion, therefore, is that multiple worlds automatically occur in quantum mechanics. They are an inevitable part of the formalism. The only remaining question is: what are you going to do about it? There are three popular strategies on the market: anger, denial, and acceptance.

    The “anger” strategy says “I hate the idea of multiple worlds with such a white-hot passion that I will change the rules of quantum mechanics in order to avoid them.” And people do this! In the four options listed here, both dynamical-collapse theories and hidden-variable theories are straightforward alterations of the conventional picture of quantum mechanics. In dynamical collapse, we change the evolution equation, by adding some explicitly stochastic probability of collapse. In hidden variables, we keep the Schrödinger equation intact, but add new variables — hidden ones, which we know must be explicitly non-local. Of course there is currently zero empirical evidence for these rather ad hoc modifications of the formalism, but hey, you never know.

  5. 5
    bornagain77 says:

    Quoting Sean Carrol, (especially since I do not respect his opinion), does not alleviate to falsification that contextuality dealt to Many Worlds:

    A Critique of the Many Worlds Interpretation

    Moreover, you have to deny the reality of your very own conscious mind (the one thing your can be most sure about), and also deny your free will, to accept many worlds, and in doing so thus forfeit any claim to reason you might have had.

    See Plantinga EEAN

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