Starship Asterisk*

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.

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.

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

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.

geckzilla wrote: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...

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

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

StarstruckKid wrote::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..."

StarstruckKid wrote: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?

Zee wrote: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 wrote:

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

[b]Henning Makholm[/b] wrote:

Zee wrote:...some negligence in the manufacturing process.

StarstruckKid wrote: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?

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