Quantum Time Machine Solves Grandfather Paradox

[imadaman]

[release]A new kind of time travel based on quantum teleportation gets around the paradoxes that have plagued other time machines, say physicists.
[img]http://www.technologyreview.com/blog/arxiv/files/44359/Quantum%20time%20travel.png[/img]
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.
[/release]

Fucking [url=http://www.technologyreview.com/blog/arxiv/25494/]sources[/url], how do you post them?

[Robber]

Too bad this won't work because quantum mechanics are probably linear. :frown:

[Ickylevel]

I knew this. That you can go to the past and kill your father without consequences. Then you go back to the present but you are not in the same dimension you came from (you can never go back to were you came from).

[EmperorScorpious]

But I like my grandfather :frown:

[imadaman]

[QUOTE=Ickylevel;23619904]I knew this. That you can go to the past and kill your father without consequences. Then you go back to the present but you are not in the same [b]timeline[/b] you came from (you can never go back to were you came from).[/QUOTE]

:science:

[Mobzor]

Great, now all I need is Dr.Emmet brown and his car..

[Killuah]

I like how the picture has Ket-Vectors of the wavefunction but is totally useles without explaination.

[editline]07:35PM[/editline]

[quote]
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.[/quote]
Explains nothing.

[Garmr89]

Finally the Terminator writer's have a viable excuse for having so many plot holes in their movies.

[Run&Gun12]

I still don't understand.

Guess I'm not destined to be a quantum physician... :sigh:

[Sir Tristan]

[QUOTE=Run&Gun12;23620511]I still don't understand.

Guess I'm not destined to be a quantum physician... :sigh:[/QUOTE]

I lold

quantum physician ahahahahaha

[Dr. Fishtastic]

I know already.Past events can't influence future events before those future events have even happened

[editline]07:43PM[/editline]

[QUOTE=Run&Gun12;23620511]I still don't understand.

Guess I'm not destined to be a quantum physician... :sigh:[/QUOTE]

Physicist

You will never amount to anything

[Zinayzen]

Back to the Future already did it. Doc explained in BttF 2 that when he went back in time and changed a key event, when he went back to the future he was no longer in the same timeline, he was in an alternate timeline that was only created by him changing that event. That is to say, the FIRST timeline stopped existing by the simple act of changing a key event.

--------------------------- Timeline 1

----o---------------------- Event changed

----o This timeline no longer exists because
....|
....------------------------ this timeline now is the present because of that event.

[Occlusion2018]

[QUOTE=Dr. Fishtastic;23620576]I know already.Past events can't influence future events before those future events have even happened[/QUOTE]

You know this from extensive research into theoretical quantum physics do you?

Anyway this is an interesting discovery/theory, thought it creates more questions than it answers.

Also it makes me think that time machines wouldn't be a bad thing (from our timelines perspective), someone couldn't go back and cause some sort of mass death. It also makes time machines completely pointless except for those who went back, as they would only change their own timelines.

[Spartan8907]

[QUOTE=Zinayzen;23621042]Back to the Future already did it. Doc explained in BttF 2 that when he went back in time and changed a key event, when he went back to the future he was no longer in the same timeline, he was in an alternate timeline that was only created by him changing that event. That is to say, the FIRST timeline stopped existing by the simple act of changing a key event.

--------------------------- Timeline 1

----o---------------------- Event changed

----o This timeline no longer exists because
....|
....------------------------ this timeline now is the present because of that event.[/QUOTE]

So, basically Back to the Future minus the disappearing Marty thing going on?

[DainBramageStud]

Someone sum this up for me. Is this Copenhagen or Many-Worlds?

[demoguy08]

Well damn. I'm glad I'm not aiming for a career in mathematics, this is some advanced shit.

[Socram]

Quantum physics are dumb.

Whoops, can't explain it! Must be another dimension!

[Eudoxia]

[QUOTE=Socram;23623707]Quantum physics are dumb.

Whoops, can't explain it! Must be another dimension![/QUOTE]

So you just don't understand what's going on and instead of saying "Well time to read some Wikipedia pages/ask aVoN" you simply disregard everything people with years of education and NO SEX say?

Sounds like Creationist logic.

[DainBramageStud]

[QUOTE=Socram;23623707]Quantum physics are dumb.

Whoops, can't explain it! Must be another dimension![/QUOTE]

Come back when you have a PhD.

Also Strawman Fallacy.

[editline]10:54PM[/editline]

[QUOTE=Eudoxia;23623744]Sounds like Creationist logic.[/QUOTE]

yes because generalisations are fun

[BaconDioxide]

[QUOTE=Socram;23623707]Quantum physics are dumb.

Whoops, can't explain it! Must be another dimension![/QUOTE]

You know quantum theory is one of the most accurate and successful theories that humanity has devised, right

[Fenriswolf]

I'm not so sure I believe this yet. I think I believe the Back to the Future explanation more.

[Dr. Fishtastic]

[QUOTE=Occlusion;23621520]You know this from extensive research into theoretical quantum physics do you?
[/QUOTE]

Read my name

[DainBramageStud]

[QUOTE=Fenriswolf;23624149]I'm not so sure I believe this yet. I think I believe the Back to the Future explanation more.[/QUOTE]

nobody cares if you believe it or not

your belief in it has zero effect on whether it is true or not

your opinion is irrelevent

[Llivavin]

[QUOTE=DainBramageStudios;23624572]nobody cares if you believe it or not

your belief in it has zero effect on whether it is true or not

your opinion is irrelevent[/QUOTE]

This.
It's what is wrong with science today.
(the people taking opinion over fact that is)

[butre]

that's not how quantum time travel works

[Llivavin]

[QUOTE=ButtsexV3;23625113]that's not how quantum time travel works[/QUOTE]

And you would know how?

[David29]

[QUOTE=Ickylevel;23619904]I knew this. That you can go to the past and kill your father without consequences. Then you go back to the present but you are not in the same dimension you came from (you can never go back to were you came from).[/QUOTE]

This.

I don't know why he is being rated dumb because one widely accepted idea is that every action or decision creates a 'split' leading to two (or more) different time-lines branching off from this. So for example, I have a can of cider next to me and I just chose not to take a swig of it. However, if I went back and did drink it, then I would branch off into the alternate time-line.

[imadaman]

long explanation/thing without a reason:

Your best friend pissed you off so much, that you are pointing a gun at his face and thinking of killing him, and you have a choice. Shoot him or not. This is where the realities branch off, in one, you shoot him, and in another, you don't. What ever you do, both happen.

Let's say you spared him. He years later pisses you off so much, that you for some reason decide to go back in time, and shoot him. You take a stance, and ready your sniper and start having thoughts of not shooting him. Here it branches off again, you shoot him, and that creates an entirely different branch where he is dead and the "back then" you wove to avenge him, or you don't do anything, and the normal branches happen. Whatever you do, again, both happen.

Why the fuck did I just write that?

[ZekeTwo]

Is this another one of those things that uses lots of big words and sounds important as fuck but will never actually imply anything

like string theory

[Splurgy_A]

What I got from the article was that if you went back in time, no matter what you attempted you would not succeed in altering history because the quantum time machine would only send back things that could not change the present.
How far off am I? Because I was fine and dandy with thinking "Oh, it must be that you jump into an alternate reality if you change things" until I read the article :saddowns: