[url=http://www.newscientist.com/article/mg20827871.400-quantum-time-travel-black-hole-not-required.html]Source (registration required)[/url]
[quote][img]http://www.newscientist.com/data/images/ns/cms/mg20827871.400/mg20827871.400-1_300.jpg[/img]
You don't need to set the universe in a spin to see time travel in action – so what happened when a photon with a quantum gun went back to kill itself?
CHATTING about time travel in a room overlooking a verdant quadrangle at the Massachusetts Institute of Technology seems strangely appropriate. The building dates from 1916 and looks its age: the high ceilings, echoing corridors and musty offices with heavy wooden doors have changed little in that time. If it weren't for a computer screen in the corner, the room's interior could almost date from that time.
The office belongs to Seth Lloyd, one of the world's leading theorists on quantum mechanics. We are talking about a paper he and his colleagues circulated a month or so earlier describing a subtle new twist on time travel. This kind of paper crops up every few years, and usually focuses on some kind of thought experiment that uses logic and reasoning, rather than equipment, to describe how time travel could actually occur for real.
Except, there is always a caveat; the unfortunate time traveller must journey to the edge of a black hole to perform the feat, for example. So there is never an experimental test of the ideas. It is all good fun and usually worth a few column inches, but there is rarely anything tangible to get your teeth into.
Lloyd has come up with a new way of looking at the problem using quantum mechanics, so I am keen to hear about his thought experiment and, of course, to learn about the inevitable caveat. He is telling me how photons could travel back in time when he says something extraordinary. "...and we've done the experiment", he says casually. "You've seen that paper too, haven't you?"
I stare disbelievingly. For a split second, it seems time stops still. "The experiment?" I think. As Lloyd continues, his story becomes even more amazing. Not content with just thinking about sending photons into the past, the experiment is a physical recreation of the famous "grandfather paradox". This is the mind-twisting problem in which a time traveller visits the past and kills his grandfather before his grandmother is even in the picture, meaning the time traveller will never exist and so cannot have travelled into the past to kill his grandfather.
I am stunned. We have been trying to get our heads around time travel for centuries. Now Lloyd and his collaborators have identified a new approach to the problem that opens up the strange world of time travel to experiments.
What is so tantalising about time travel is that there seems to be nothing to prevent it. As far as the laws of physics are concerned, time can run forwards or backwards. But time travel of the kind that Marty McFly gets up to in the movie Back to the Future is a different kettle of fish. It requires an object to go back in time while everything else keeps creeping forward. Still, there is no shortage of ideas about how this might happen.
Most of them focus on the fabric of space-time as the medium of travel. According to Einstein's general theory of relativity, space-time is a kind of substance that can be squeezed and stretched like a giant sheet of rubber. A massive object like a star deforms this fabric causing everything in its vicinity to feel the pull towards it.
Twist space-time enough, however, and strange things can happen. If the sheet becomes folded, for example, regions that are ordinarily far apart suddenly become connected, setting up a loop called a "closed time-like curve" that would allow a time traveller to repeatedly visit the same point in space and time (see diagram).
The first person to show how relativity leads to closed time-like curves was the Austrian mathematician Kurt Gödel. His 1949 blueprint for a time machine required the whole universe to be rotating. Many related ideas have been put on the table since then, ranging from long strands of energy known as cosmic strings to rotating black holes and tunnels through space-time called wormholes.
Quantum physics entered the scene when theorists began asking how a quantum particle would fare if it were to make a leap into the past. In 1991, the British physicist David Deutsch put this question on a firm theoretical footing. He imagined a particle that travels back and destroys its former self, creating a grandfather-type paradox.
Against all expectation, Deutsch went on to solve the paradox. He did so by invoking the "many worlds" interpretation of quantum mechanics in which the instant the particle enters the closed time-like curve, the universe splits in two. In one universe, the particle survives having failed to kill itself; in the other, it is destroyed. By invoking multiple universes, Deutsch cunningly avoids the grandfather paradox and resolves one of the fundamental problems with time travel. But many physicists feel uneasy with the many worlds idea because it spawns an unimaginably huge number of other universes and is unnecessarily complex.
Lloyd and his colleagues have taken a different approach to quantum time travel, using the fact that quantum particles such as photons and electrons are not bound by the arrow of time.
The mathematics of quantum theory says that the quantum state that describes them evolves both forwards and backwards in time. This odd state of affairs has led to some researchers claiming that the normal rules of causality don't apply, so things that happen in a quantum particle's future will affect its past.
One of the first people to show this was John Wheeler at Princeton University. He showed that the classic "double slit" experiment, where an unobserved photon passes through two slits simultaneously, can be affected by a measurement that takes place after the experiment is ostensibly finished.
The wave characteristics of the unobserved photon means it passes through both slits at once. If it is observed as it goes through the slits, experiments have shown that it will take on particle-like qualities and only go through one. Wheeler wanted to know what happens if you delay your decision to look at the wave or particle nature of the photon until long after it has passed through the slits.
He suggested that using a pair of distant telescopes to look back at the slits would also force the photon to take on particle-like properties. This selection of a property after the main part of the experiment is effectively over is known as "post-selection". Post-selection may sound unsettling. However, experiments by Jean-François Roch at the Ecole Normale Supérieure in Cachan, France, and others have shown that post-selection really does change the properties of a photon up to a few nanoseconds into the past.
According to the Copenhagen interpretation of quantum theory, there is no objective reality until a measurement is made. But we are beginning to learn that even that reality may be a moveable feast: the past state of a quantum particle has no more reality than its future state. Which is why post-selection has an effect. In other words, everything is up for grabs. In theory, the post-selection process could even change the entire history of the universe.
Lloyd and Aephraim Steinberg, of the University of Toronto, Canada, say this peculiar property of the quantum world might be the key to a working time machine. Our daily experiences tell us that the conditions given at the beginning of an experiment will determine its outcome. But if quantum particles can't discriminate between things that affect them forward and backward in time, that means specifying a final condition can determine what happens before it. "Mathematically, there's no reason why final conditions can't be 'givens' as well and everything has to follow logically from them," Steinberg says.
Teleportation in time
It is exactly this sort of thinking that led physicists Charles Bennett at IBM Research in Yorktown Heights, New York, and Ben Schumacher at Kenyon College in Gambier, Ohio, to suggest that quantum mechanics could be used to build a time machine by making use of quantum teleportation, a phenomenon that has been demonstrated experimentally countless times. The process exploits a curious quantum property called entanglement, by which two particles, such as photons, become so closely linked that they share the same existence. Entangled particles are special because a measurement on one immediately influences the other, no matter how far away it is.
Now imagine that you want to teleport a third space-travelling particle from A to B. The trick is to make a pair of entangled particles and place one of them at A and one at B, then carry out a set of measurements at both locations. If you do this just right, you can use this "spooky action at distance", as Einstein called it, to ensure that the second particle ends up in a state that is exactly the same as the "space traveller".
In fairness, the traveller hasn't physically moved, but the quantum information that completely describes the traveller has made the trip instead and this allows the second particle at B to take on the traveller's identity.
The curious thing about teleportation is that it occurs instantly. In this process, the quantum information moves from point A to point B, so it is natural to think that the measurements at A set the journey in motion.
But because teleportation happens instantaneously, it is just as valid to think that the measurement at point B triggers the journey, even though it takes place moments later (see diagram). This is post-selection in action and it is a feature that quantum physicists use all the time to do things like quantum computation. It is this ambiguity between cause and effect that Steinberg and Lloyd exploit in their time-travel simulator. "In essence, the time travel is just teleportation," says Steinberg.
Is it an exaggeration to call this time travel? Perhaps - in the same way that quantum teleportation transports a quantum state and not a material object. Yet Lloyd and Steinberg argue that the logic of post-selected teleportation is the same as for time travel, so their experiment is a time travel simulator. And while that may not be as exciting as a time machine that can send us back to the dinosaurs, it is possible to do extraordinary things with it.
The first thing Lloyd and Steinberg's team did was to simulate the grandfather paradox by sending a photon back to kill itself. To do this, the team used teleportation, but with a crucial twist. Conventional quantum teleportation is guaranteed to give you a replica of the state that you intend to send. What Steinberg and Lloyd wanted to know is whether this would work for photons intent on killing themselves with a quantum gun.
To work, their simulator required two additional features: a quantum gun that sometimes fires, and a means for the teleportation itself to fail. The team also decided that, rather than entangle two photons, as often happens in quantum teleportation, they would entangle two attributes of a single photon. The photon's polarisation would represent the photon's "present" and its direction would represent the "past". Next, they gave the photon a quantum gun that can either fire or fail. This device, called a wave plate, can flip the photon's polarisation, or not. Because the photon's polarisation and direction are entangled, giving the photon such a gun affects the "past".
Now, how to ensure that the teleportation sometimes fails? That is easier, as teleportation has an in-built failure mechanism. Unless you make your measurements in a particular way, it only works 25 per cent of the time. So there are four possible outcomes from the team's experiment, depending on the combined status of the teleportation and gun.
When this experiment is done, something interesting happens: every single time the time travel works, the gun fails to go off. And when time travel fails, the gun works. To put this in the language of the grandfather paradox, as long as there is some chance of your gun misfiring and the assassination failing, time travel may work. "You can point the gun but you can't pull the trigger," says Lloyd.
Clearly time travel has some important implications, not least for the nature of free will and our ability to exercise it. Time travellers may be somehow prevented from even thinking about shooting their grandfathers or perhaps they can make the decision to shoot only to be foiled in some other way. "Nature wouldn't care much whether it interferes with my synapses or with the workings of my Colt 45," says Steinberg.
While the idea of a time travel simulator is a jaw-dropper for many people, the work has so far made little impression on quantum physicists. That is because the experiment does exactly what quantum mechanics predicts - there is no controversy over how teleportation works or how bits of quantum information, or qubits, behave. "Sure, it's fun, but everyone knew perfectly well what the outcome was going to be," says Scott Aaronson, a computer scientist also at MIT.
Lloyd and Steinberg have a different take. They say that thinking about time travel in this way raises important questions and provides significant new insights. For a start, it can actually be tested in the lab, unlike Deutsch's closed time-like curves. And while post-selection may feel like cheating, that could just be because we are biased towards a specific direction for time.
While Lloyd and Steinberg's experiment simulates time travel, one question remains. Is it really possible to time travel without black holes?
According to certain formulations of quantum mechanics, the answer is yes. One of the most troubling aspects of quantum mechanics is that it is so incompatible with general relativity. For decades, physicists have tried to combine the two into a quantum theory of gravity and failed. While many physicists lay the blame with relativity, others suspect that the problem is really to do with the incompleteness of quantum mechanics (New Scientist, 21 August, p 33).
This has motivated alternative formulations of quantum mechanics. Aram Harrow, a quantum physicist at the University of Washington in Seattle points out that these can skew quantum mechanical measurements in the same way that post-selection does, meaning that time travel is viable in theory. So far, physicists have no evidence for such new formulations of quantum mechanics, though that is not to say that there aren't places in the universe where they might operate.
So back to the original problem: will time travel be possible? Maybe, but chatting about time travel in a room overlooking a verdant quadrangle at the Massachusetts Institute of Technology seems strangely appropriate. The building dates from 1916...[/quote]
Keep in mind that this isn't [B]"true"[/B] time travel, as
a) only information is being transmitted and
b) it' just that you can't tell whether it's the past that sent the message to the future or that the future sent the message to the past
Wouldn't the grandfather paradox just create a new dimension or multiverse or something when you kill your grandfather, and your grandfather exists in another reality?
This hurt the fuck out of my mind.
This is interesting... What practical applications would this potentially have?
[media]http://www.youtube.com/watch?v=nQCbdLGrlpY[/media]
The permutations were endless..
Uhm, can someone explain to me how the hell the laws of physics allow time to move backwards?
[quote]"Sure, it's fun, but everyone knew perfectly well what the outcome was going to be," says Scott Aaronson, a computer scientist also at MIT.[/quote]
Well fuck your shit
[QUOTE=JgcxCub;26222608]Uhm, can someone explain to me how the hell the laws of physics allow time to move backwards?[/QUOTE]
Time doesn't move backwards. The photon's state can be changed at any place in time.
I can go forward in time.
:downs:
[QUOTE=Canned Induvidual;26222676]I can go forward in time.
:downs:[/QUOTE]
Judging by the progress I've made in life, I can do it significantly slower than usual
This could make one hell of an aimbot. ;)
I think my brain just melted and came out through my noseeeejinsisernvger.....................lööööööööööööööööö
If I read all of that my brain would implode and become a singularity.
[QUOTE=UnidentifiedFlyingTard;26223642]If I read all of that my brain would implode and become a singularity.[/QUOTE]
Read it anyway. Even if you don't understand it you might learn something.
Great scott!
So p. much what you're telling me is if my life ever became really really fucking shit, and I tried to off myself, it would just never work and I'd end up potentially physically maimed for life?
Swell.
[editline]22nd November 2010[/editline]
Or have I misunderstood this and I would die but continue existing in another reality?
[editline]22nd November 2010[/editline]
this is like brainfreeze without the freeze fuck
tl;dr please
In before every Facepuncher comes in and acts like they know what the fuck was just said.
Time does not go back and forward, it changes state. You only perceive it as a time line because of the limitations in this dimension.
TLDR:
Two entangled photons share the same existence, something happens to one, it happens to the other.
Photons don't follow the normal conventions of time.
By using these two properties its potentially possible to send information across time.
They tested the grandfather paradox by setting up a simulation of a photon going back in time to kill itself with a gun, each having a potential to fail. When the teleportation failed, the gun went off, but when it did teleport the gun would never fire.
So essentially if time travel were ever possible and you went to kill your grandfather, something would go wrong. Could be the gun, could be you just cant fire it, you miss, etc.
Thats my understanding of it anyway :)
[QUOTE=Flon22;26224744]TLDR:
Two entangled photons share the same existence, something happens to one, it happens to the other.
Photons don't follow the normal conventions of time.
By using these two properties its potentially possible to send information across time.
They tested the grandfather paradox by setting up a simulation of a photon going back in time to kill itself with a gun, each having a potential to fail. When the teleportation failed, the gun went off, but when it did teleport the gun would never fire.
So essentially if time travel were ever possible and you went to kill your grandfather, something would go wrong. Could be the gun, could be you just cant fire it, you miss, etc.
Thats my understanding of it anyway :)[/QUOTE]
If that's the case I read about this around a month ago...
The very act of going back in time should already cause a change in that reality, you don't even have to kill your grandfather to create a paradox. I don't think that travelling to the past is possible.
[quote]When this experiment is done, something interesting happens: every single time the time travel works, the gun fails to go off. And when time travel fails, the gun works. To put this in the language of the grandfather paradox, as long as there is some chance of your gun misfiring and the assassination failing, time travel may work. "You can point the gun but you can't pull the trigger," says Lloyd.[/quote]
I vaguely remember a Michael Crichton book that dealt with time travel, and I think it basically said the same thing as this. The universe won't let you make a paradox: if you try to kill your grandfather, your gun will jam, someone will see you and stop you, or the time machine will break, etc.
[QUOTE=Flon22;26224744]TLDR:
Two entangled photons share the same existence, something happens to one, it happens to the other.
Photons don't follow the normal conventions of time.
By using these two properties its potentially possible to send information across time.
They tested the grandfather paradox by setting up a simulation of a photon going back in time to kill itself with a gun, each having a potential to fail. When the teleportation failed, the gun went off, but when it did teleport the gun would never fire.
So essentially if time travel were ever possible and you went to kill your grandfather, something would go wrong. Could be the gun, could be you just cant fire it, you miss, etc.
Thats my understanding of it anyway :)[/QUOTE]
This may be an ignorant response, but if a human were to go back in time there is nothing preventing it from killing anything. If the gun fails you can pick up a knife and physical laws can't prevent you. So does that suggest that the grandfather paradox happens differently on a quantum level? Seems so to me.
Time Travel won't work. There are too many variables.
Example: Let's say that tomorrow at 12:00 PM EST, New York will get hit by a large meteor, destroying most of the town. Now, you time travel back and destroy the comet. However, New York is still destroyed by a random missile that some when off course and hit New York. So, you time travel and try to stop that, but then something else destroys New York. Point is that if something is meant to happen, it will.
[QUOTE=Sodisna;26225584]Time Travel won't work. There are too many variables.
Example: Let's say that tomorrow at 12:00 PM EST, New York will get hit by a large meteor, destroying most of the town. Now, you time travel back and destroy the comet. [B]However, New York is still destroyed by a random missile that some when off course and hit New York. So, you time travel and try to stop that, but then something else destroys New York. [I]Point is that if something is meant to happen, it will.[/I][/B][/QUOTE]
I have highlighted the parts that are aimless conjecture
what evidence do you have to support your theory that certain events are set in stone
[QUOTE=DainBramageStudios;26225614]I have highlighted the parts that are aimless conjecture
what evidence do you have to support your theory that certain events are set in stone[/QUOTE]
This.
[QUOTE=EcksDee;26224879]If that's the case I read about this around a month ago...[/QUOTE]
took me a while to get it
[QUOTE=Sodisna;26225584]Time Travel won't work. There are too many variables.
Example: Let's say that tomorrow at 12:00 PM EST, New York will get hit by a large meteor, destroying most of the town. Now, you time travel back and destroy the comet. However, New York is still destroyed by a random missile that some when off course and hit New York. So, you time travel and try to stop that, but then something else destroys New York. Point is that if something is meant to happen, it will.[/QUOTE]
Haha
No
Real Time Traveling works with Sandwiches!
[url]http://store.steampowered.com/app/57800/[/url]
[editline]22nd November 2010[/editline]
Toasted Sandwiches!
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