CosmicVariance: Rules for Time Travelers

[RJN]

http://mblogs.discovermagazine.com/cosm ... travelers/

But time travel isn’t magic; it may or may not be allowed by the laws of physics — we don’t know them well enough to be sure — but we do know enough to say that if time travel were possible, certain rules would have to be obeyed. And sometimes it’s more interesting to play by the rules. So if you wanted to create a fictional world involving travel through time, here are 10+1 rules by which you should try to play.

[bystander]

Quantum Time Machine Solves Grandfather Paradox
Technology Review | the physics arXiv blog | 19 July 2010

A new kind of time travel based on quantum teleportation gets around the paradoxes that have plagued other time machines, say physicists.

Of all the weird consequences of quantum mechanics, one of the strangest is the notion of postselection: the ability to trigger a computation that automatically disregards certain results.

Here's an example: suppose you have a long, tortuous expression in which there are a frighteningly large number of variables. The question you want answering is which combination of variables makes the expression logically true. And the conventional way to solve it is by brute force: try every combination of variable until you find one that works. That's hard.

Postselection, however, makes the solution easy to find. Simply allow the variables to take any value at random and then postselect on the condition that the answer must be true. This automatically disregards any wrong'uns that come up.

Postselection is controversial because it leads to all kinds of fantastical predictions about the power of quantum computers. Nobody is quite sure if these kinds of computations are possible or how to achieve them but quantum mechanics seems to allow them.

Now postselection gets even weirder thanks to some new ideas put forward by Seth Lloyd at the Massachusetts Institute of Technology and a few buddies. They say that if you combine postselection with another strange quantum behaviour called teleportation and you can build a time machine.

Before we look at how this idea works, a quick reminder about quantum teleportation. This uses the phenomenon of entanglement to reproduce in one point in space a quantum state that previously existed at another point in space.

Lloyd and cos idea is to use postselection to make this process happen in reverse. Postselection ensures that only a certain type of state can be teleported. This immediately places a limit on the state the original particle must have been in before it was teleported. In effect, the state of this particle has travelled back in time.

What's amazing about this time machine is that it is not plagued by the usual paradoxes of time travel, such as the grandfather paradox, in which a particle travels back in time and some how prevents itself from existing in the first place.

Lloyd's time machine gets around this because of the probabilistic nature of quantum mechanics: anything that this time machine allows can also happen with finite probability anyway, thanks to these probabilistic laws.

Another interesting feature of this machine is that it does not require any of the distortions of spacetime that traditional time machines rely on. In these, the fabric of spacetime has to be ruthlessly twisted in a way that allows the time travel to occur. These conditions may exist in the universe's extreme environments such as inside black holes but probably not anywhere else.

The fact that similar time machines may also be possible when quantum mechanics is pushed to its limits suggests an avenue that may prove fruitful in uniting this disparate areas of science. "Our hope is that this theory may prove useful in formulating a quantum theory of gravity," say Lloyd and buddies.

So where might their time machine be built. That's a tricky question too. Postselection can only occur if quantum mechanics is nonlinear, something that seems possible in theory but has never been observed in practice. All the evidence so far is that quantum mechanics is linear. In fact some theorists propose that the seemingly impossible things that postselection allows is a kind of proof that quantum mechanics must be linear.

However, if nonlinear behaviour is allowed, time travel will be possible wherever it takes place. As Lloyd and co say: "It is possible for particles (and, in principle, people) to tunnel from the future to the past. "

Fire up the Delorean.

The quantum mechanics of time travel through post-selected teleportation

• arXiv.org > quant-ph > arXiv:1007.2615 > 15 Jul 2010 (v1), 19 Jul 2010 (v2)

Taming time travel
Science News | 20 July 2010

New work solves paradoxes by making the impossible impossible

Novelists and screenwriters know that time travel can be accomplished in all sorts of ways: a supercharged DeLorean, Hermione’s small watch and, most recently, a spacetime-bending hot tub have allowed fictional heroes to jump between past and future.

But physicists know that time travel is more than just a compelling plot device — it’s a serious prediction of Einstein’s general relativity equations. In a new study posted online July 15, researchers led by Seth Lloyd at MIT analyze how some of the quirks and peculiarities of real-life time travel might play out. This particular kind of time travel evades some of its most paradoxical predictions, Lloyd says.

[swainy]

Interesting post Bystander:

So they think they can use entanglement. I would love to know how it works. I do have lectures on quantum mechanics but i just get lost. I have tried guys.

tc

[neufer]

So, like Schrödinger's cat, Gramps is half dead and half alive.

(Where are those health care reform death panels when you really need them?)

[orin stepanek]

I think that the best time machine is film; video; and memories. Oh and photography

[Orca]

If you think that time doesn't run slower in some situations, try taking a 3 1/2 hour lecture on SQL queries..

[geckzilla]

But did you learn how to create the most convoluted sql query possible? Requires at least 6 joins, a few subqueries, the creation of a temporary table, and then figure out how to sort it by some arbitrary means the client didn't tell you about before hand...

[rstevenson]

By an astounding coincidence, we seem to have had the same client.

Rob

[Beyond]

But did you learn how to create the most convoluted sql query possible? Requires at least 6 joins, a few subqueries, the creation of a temporary table, and then figure out how to sort it by some arbitrary means the client didn't tell you about before hand...

Geckzilla----Please, do not try and explain to me what you just said!!!!

[orin stepanek]

Just think of all the things we could do if we had a time machine that could take us back into time. We could avoid the mistakes we made. We could relive wonderful times. We could watch ourselves be born. You heard the old saying; Boy if I knew then what I know now;---Well! We could go back and live in a much slower time. We could change history. We could cause some people not to be born. We could manipulate the stock market. The list could go on and on! I think it is great that we can't go back in time.

[StarstruckKid]

Wouldn't this make the theory that there's only one electron in the universe possible???

[Henning Makholm]

Wouldn't this make the theory that there's only one electron in the universe possible???

Not really. It seems that even under the most charitable interpretation what "travels back in time" here is information, not material particles.

In any case, Wheeler's speculation that there might be only one electron (with the multitude we observe being simply different events during its time-traveling history) belongs more in the realm of "vivid and colorful mind gymnastics" than in that of "theories with meaningfully observable consequences".

[StarstruckKid]

It seems that even under the most charitable interpretation what "travels back in time" here is information, not material particles.

Pardon my too-sparing use of smilies!

However, since 'information' means literally 'that which gives form to', consider one theoretical concept of a transporter, in which the object being 'transported' is scanned, broken down into its constituent particles (which are then discarded or saved for incoming objects), and the object at the other end is reconstituted based on the information received (sort of like Scotty in the episode with the Dyson sphere) (remind me to stay out of that thing ), would it not be possible to send Capt. Kirk, or even multiple Capt. Kirks, back to past eras in need of dashing starship captains, or that the extreme conditions at the End of the Universe could break that electron down into its quarks and return it to the Big Bang for its next trip through existence?

Or not...

[bystander]

Electrons are not made of quarks. Electrons, like quarks, are elementary particles.

[StarstruckKid]

'scuse. me, I was speaking poetically, not rigorously... substitute 'component parts' for 'quarks'. To quote the Wikipedia article 'Electron', "[the electron] has no known components or substructure..." (emphasis mine).

Also, let me respectfully point out that everything we think we know is actually only our brains' best attempt to understand the information we get through our senses. There is always that small chance that we have it completely wrong... which is what keeps it interesting, IMHO.

[Henning Makholm]

:oops: 'scuse. me, I was speaking poetically, not rigorously... substitute 'component parts' for 'quarks'. To quote the Wikipedia article 'Electron', "[the electron] has no known components or substructure..."

Even if the electron had components it could be assembled from, it wouldn't work. One of the things that made Wheeler's idea slightly more interesting than just idle speculation is that it would "explain" why all electrons we observe appear to have exactly the same mass, charge and so forth. A priori one would assume that if a lot of electrons were created at the Big Bang (by some unknown mechanism), there would be certain "manufacturing tolerances" that led different electrons to have slightly different masses, but all observations support the hypothesis that they are indeed exactly identical. Under Wheeler's idea, this would be because they are in fact all the same electron, which just happens to have the mass and charge it has.

The idea breaks down because in the real universe there are a lot more electrons than positrons, so the "single electron" could not get back in time often enough to play the rôles of all the electrons we observe. If you want to solve this by sending the electron back in time by "blueprint transfer", that just punts the original problem back to the next level: Why are all the electron-constituent particles (whatever they might be) similar enough that they can be used to build identical electrons?

(The idea also breaks down because the weak force can create electrons "at run time" from neutrino and a W-. So there are more electron-worldline ends in the world than the two a single world-electron could have. You could still imagine "only a single lepton", but that is not nearly as attractive, because then you still have to wonder why each time the world-lepton converts from neutrino to electron, it swells in mass and charge by precisely the same amount).

[StarstruckKid]

Why would I assume a priori that all electrons aren't identical? If it's an elementary particle, then one might as well have to explain why there is no variation in the integer '1', also an elementary 'particle'. Same goes for any constituent particles, if they were to exist.

As one of my professors once said, "You can't paint an electron green". If electrons stop being identical, a lot of theories and calculations are going to need a lot of adjustment, aren't they? Where does that leave us?

Seems to me the concept of 'manufacturing tolerances' is a purely human one, born of our existence in a macro world. That's not where electrons exist, so to speak. My physicist friend can be inclined to take issue with the idea that there is any such thing as a 'physical' particle of any kind, but that's another plate of mullet entirely.

[StarstruckKid]

Maybe I should retract my assertion about the immutability of integers, to wit:

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7 + 4 = 9!

[bystander]

This topic is in the forum Open Space, so the 4 is part of the 7, not in addition to.

[Henning Makholm]

As one of my professors once said, "You can't paint an electron green". If electrons stop being identical, a lot of theories and calculations are going to need a lot of adjustment, aren't they? Where does that leave us?

In trouble. Which is why it would be nice to know why they're all identical, as opposed to just knowing that they're identical.

My explanation is partially inspired by the following passage (arguing for another solution than Wheeler's suggestion) from A. Zee, Quantum Field Theory in a Nutshell, chapter II.4:

Quantum statistics, one of the most subtle concepts in physics, rests on the fact that in the quantum world, all elementary particles and hence all atoms, are absolutely identical to, and thus indistinguishable from, one other. It should be recognized as a triumph of quantum field theory that it is able to explain absolute identity and indistinguishability easily and naturally. Every electron in the universe is an excitation in one and the same electron field psi. Otherwise, one might be able to imagine that the electrons we now know came off an assembly line somewhere in the early universe and could all be slightly different owing to some negligence in the manufacturing process.

[Beyond]

Starstruckkid, did your professer ever tell you why you couldn't paint an electron green?

[StarstruckKid]

beyond wrote:

Starstruckkid, did your professer ever tell you why you couldn't paint an electron green?

Because then it would be distinguishable from all the others, and that doesn't happen. Also couldn't find a small enough paintbrush, not as silly an answer as it may first seem.

More to the point: you can't paint an electron green because an electron can't be "green". It can drop to a lower energy state and "emit" a "green" photon, but it exists "below" where "green" exists, you can't paint "green" on it. It's too "small".

More subtle point: "green" is a product of our brains and perceptions; it doesn't exist outside of that. There is no such thing as "green" in the physical world, only a range of wavelengths/energies which, when they impinge on our eyes, induce a response that we call "green".

If a man says something and a woman isn't there to hear it, is he still wrong?

[StarstruckKid]

...some negligence in the manufacturing process.

Which would impose a macro human concept where it doesn't apply... maybe even imply a "Creator", i.e. "Intelligent (though negligent) Design(er)".

Of course we're presuming some sort of cosmic uniformity... until somebody brings back (teleports?) a jar full, we have no way to be absolutely certain an Andromedan electron is indistinguishable from a Milky Wayvian one, do we?

[Henning Makholm]

Of course we're presuming some sort of cosmic uniformity... until somebody brings back (teleports?) a jar full, we have no way to be absolutely certain an Andromedan electron is indistinguishable from a Milky Wayvian one, do we?

If they were too different from the electrons we get around here, atomic spectra would be different in ways that would be easily observable from afar.

[Beyond]

If a man says something and a woman isn't there to hear it, is he still wrong?

Yes!! Because she WILL find out!