I agree. It is like when we study single cell livings (our evolutionary ancestors), the single cell livings do not know we are studying them! And we do not boost

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Time travel is real. You're doing it right now. You are moving forward in time.

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Dave, there was no reply button, so I'll reply down here.

What you refer to is Hugh Everett's Many-Worlds Interpretation of quantum mechanics, which he proposed as an alternative to the Copenhagen Interpretation that holds that it is the act of observation that causes probability waves to collapse (or decohere) into particles. Many physicists prefer the more modern decoherence interpretation that posits each particle as a kind of "observer" collapsing others as it interacts with them. There are a few other interpretations as well. The problem is that while the system of equations known as quantum mechanics are excellent predictors of physical systems, the math doesn’t tell us anything about what that physically means. It’s even possible that all the interpretations are simply macro-centric attempts to stuff quantum physics into a philosophy we can grok, but which have no bearing on the actual behavior which the equations are calculating.

If you think this is frustrating to non-physicists, imagine how exasperating it is to scientists whose whole…ah, universe is built on experimental verification of falsifiable hypotheses. Many prefer to apply Occam’s razor and not multiply universes beyond necessity. I prefer to keep an open mind so unfounded assumptions don’t blind theory to possible avenues toward a unified theory of QM and Relativity, but to bear in mind that without experimental data one way or another, all interpretations are speculative – fun, but speculative.

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Quantum stuff makes things weird. I don't have enough understanding of it to make any judgements of its effect.

Multi- universality ,as far as I understand is even weirder. Correct me if I am wrong,but if there infinite universes being spawned at each juncture,for example me crossing a street and getting run over or not by a Ford or Chevrolet or each car, truck or bicycle driven by each person on earth who could ever have been conceived , there become insanely too many universes.

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Dave, what you refer to is the classic "grandparent paradox" which was the first argument advanced against retrocausal time travel. While a useful argument for explaining the, as you said, apparent absurdities of such travel, it suffers from overspecification because it gives the impression that the paradox requires something salient to human (or even conscious) observers to edit the timeline…offing Hitler or your grandmother or some such. But from a quantum perspective, particle interactions decohere wave-functions with or without conscious observers. So even a single quantum particle, say a photon, leaking into its own pat light cone – all the spacetime coordinates from which, traveling at c (the vacuum speed of light) anything can possibly reach the place and time of departure and so influence it by, for example, stopping the time travel – will minutely disrupt its own history. This is thanks to Heisenberg’s famous Uncertainty Principle, the same one that says merely observing a particle system disrupts its quantum state because particles must impinge on the system to relay any information to the observer.

This is why Stephen Hawking was so keen to find a mechanism for “cosmic censorship” to make the universe “safe for historians” despite General Relativity’s allowance for closed timelike curves (CTCs) via uneven time dilation of the ends of an Einstein-Rosen bridge (AKA wormhole). Mind you, Hawking was not necessarily convinced wormholes were practical, but he wanted to fill in the gap in the otherwise very consistent theory of Relativity. But wormhole time machines can’t trace back to earlier than the reference frame they are created in, so that is a boundary condition and, barring alien wormhole time machine relics or ultrahush government superscience (and these are the bright lights that let guerrilla militants hack into their aerial drones), one that stops time travel to now or earlier.

Mel Brooks covered this a quarter century ago:http://www.youtube.com/watc...

For the unbounded type of time machine Hanson was talking about, the situation is much worse. The third possibility I mentioned above is the unbounded version of Hawking’s cosmic censorship, the universe’s very own grandmother paradox, so to speak.

The book isn’t completely closed, and any honest skeptic must admit the limits of current theory. But without a multiverse, the prospects for bounded retrocausal time travel look dim; for unbounded DeLoreans, they look positively atrocious.

Damn this site rocks! Great content. Great comment policy. Great clientele. Great Scott! Must. Not. Get. Sucked. Into. Another. Awesome. Blooooooooog...

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I would say that Gulliver's 5th argument is pretty much guaranteed to happen if time travel is real. Access to time travel would be as dangerous a weapon as a nuclear bomb. If our future selves discovered, it would likely be incredibly expensive especially since it would likely require a spaceship to move through both space and time, per Mark M's argument. If time travel was first discovered in the 24th century, it might take until the 29th century to develop a cheap and easily available means for anybody to do it. 70 years after their invention, nuclear weapons are still incredibly expensive to develop. By then, future governments would have likely developed a system for detecting and preventing dangerous time travel incursions. Some would likely slip through, but they would be small events or rare enough that our present selves could dismiss them. And for those paying attention, yes, that was a Star Trek reference.

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There is the problem of past activities altering the future. No one knows how much a given activity could leverage into the future. For example,if you sneezed and Hitler's mother caught cold,she might have stayed home the day she met Hitler's father. So if someone slightly altered the past everyone on earth might instantaneously cease to exist and be replaced by an alternate population. I am here to witness that that hasn't happened,though it might happen any second now. Given this ,the idea seems to reduce itself to absurdity.

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Five possibilities I can see:

1) Time travel into one's own past light cone is impossible – at least unbounded by a past receiver such as the other mouth of a wormhole – whether we inhabit a multiverse or one universe.

2) Time travel into one's own past light cone results in all retrocausal particles (including the traveler) tunneling to a different Everett sheaf of the multiverse than this one.

3) There is no multiverse, at least in the quantum decoherence sense, and any universe that allows mechanisms for time travel into one's own past light cone eventually evolves Feynman paths that intersect its moment of origins, altering its constants, so that only universes that prohibit unbounded time travel are temporally stable and can manage to decohere into macroscopic reality.

4) We are in one of Nick Bostrom’s simulations and the simulation’s creators and/or controllers are arranging it so we either have or detect no retrocausal time travelers.

5) Some other sufficiently advanced agency – such as god(s), aliens with a big head start or something of which we haven’t yet conceived – is making sure we either have or detect no retrocausal time travelers.

As my undergrad quantum mechanics prof said of philosophizing indeterminately about the interpretations if quantum maths, shut up and calculate! You should appreciate that, Hanson, being as you are a mathematician 

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...Of course, some people do presently live in the past. E.g., my mother has often remarked, "Thank God I'm not living nowadays!" :)

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Why no time travel? Well... you can't travel into the past because it doesn't actually exist anymore. It has been replaced by the present. (But you can see/detect plenty of signals from the past, because those signals still exist in the present.) Similarly, you can't travel into the future (although you can certainly wait for it to happen) because it doesn't actually exist yet. :) This is perceived as a terribly unsophisticated view of course, in light of the curved space-time of general relativity, the rather remarkable symmetries and rules of quantum electrodynamics, multi-universe theories, arguments about entropy destruction or preservation in black holes, etc. And yet, despite all of that, it seems that no real-world contradictions (aka, something you can measure) to the aforementioned unsophisticated view have been found.

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In Sean Carroll's "From Eternity to Here" he said that self-confirming/self-causing loops in time are considered possible by the standards of modern physics. But any sort of "grandmother paradox" is not allowed.

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Maybe the "future" does not exist beyond the present period.

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It could be that our descendants already have the required information on today (thanks to digital record keeping) to simulate it in sufficient detail at much less cost than travelling back in time. Maybe is the case that they are visiting other older times, but we wouldn't know that since we have only have records of the original, unchanged times.

Or it could be that they are visiting us, but every time a timetraveller arrives, they change history, so we only have one visit in current history.

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There's lots of science fiction written that has time travel into the past without the people in the past realizing it. For example, Connie Willis has a few notable books of that type. So it's possible that "historians" are studying our present right now, but of course they blend in so as not to attract attention.

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Experimental Journal:

3:00: I open the time portal

3:05: Hit in the face when my cat (black with white stripes) came flying out of the portal, meowing with all its fur standing on end. WTF?

3:10: I need a test subject. Ah! My cat of course, I grab it, hoist it back and forth through the air, and toss it into the portal, setting the time for 5 minutes into the past.

3:15: A future version of me came through the portal, assaulted me and tried to chloroform me, before running off! WTF?

3:20: Unfriendly AI came through the portal. I'm going to apply Hanson's economic theories, I'm going to trade my way out of this mess by offering to sell it a lovely piece of pottery for just 5.99...

(All entries end)

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the other thing that's so funny about this set of posts on (im)possibilities of tech/physics is that, with respect, they seem clearly to be signaling -- that you are thoughtful, have interesting ideas about a wide variety of topices, etc. -- rather than any sort of serious contribution to consideration of the possible. they might also be more along the lines of entertainment -- interesting to speculate about what might be, even if ultimately silly (given that no one will ever offer $1 million for you to go to school naked, you'll never see the inside of a black hole, etc) -- but they're presented as something akin to proofs.

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