Grandfather Paradox Solved?

RainmanTime

RainmanTime

Super Moderator
Or so sayeth the author! Interesting read, however:

Quantum Time Machine Solves Grandfather Paradox

"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."

As always, it is theory, so caveat emptor! :D

RMT

 
The problem with quantum entanglement is that the moment an entangled particle interacts with the 'classical' world the probability associated with each particle becomes certainty. So entanglement is really a small scale thing, and quite unlikely to be usable in a 'classical' sized object like a time machine.

 
Are there then no macroscopic quantum effects?
Click to expand...
Well, specifically the issue is whether 'quantum superposition' exists on the macroscopic level. Schrodinger tried to argue ( with his famous Schrodinger's cat thought expeiment ) that it does....but not all scientists agree, and it's one of the reasons why quantum mechanics split between the Copenhagen interpretation and the 'many worlds' interpretation.

Quantum teleportation relies on the phenomenon of quantum entanglement. But that in turn relies on the entangled particles being in a state of quantum superposition ( effectively the particles simultaneously exist in multiple states ). That indeterministic state breaks down ( into good solid classical reality ) the moment an 'observation' is made.

Now one might argue ( as Schrodinger did ) that all one has to do is hide the particles....or indeed any object in a quantum state.....from an observer and almost anything can remain in a state of quantum superposition no matter how large. But.....that is precisely why theories of 'quantum decoherence' ( the breakdown of the superposition state ) were developed in which 'the observer' does not need to be a human being, as it seemed rather weird to a lot of scientists that only human 'observers' could decide the quantum states of objects ( the infamous 'observer effect' ).

And that is where this time machine idea starts to fall down. Newer theories of quantum decoherence allow 'the observer' to be almost any classical state that has a deterministic outcome. And common sense almost seems to back this up.....as otherwise most of the rest of the universe would be in a state of superposition...unable to make up it's mind up....and one goes back to a non-Copernican universe.

With a more comprehensive form of decoherence.....entangled particles have a very limited range of travel before interacting with 'the world' and the superposition state breaking down ( without any human observer ). So....tales that one reads of entangled particles travelling light years to send signals would just not be possible.

Some recent experiments HAVE measured entanglement at classical scales...however by classical I think the distance was less than an inch. If entanglement could be measured at significantly larger scales.....one could seriously start to suppose that 'the observer effect' was real...as it would mean only human observers can affect quantum states and we would not need time machines as we'd be God and unknowingly all this time have had immense powers.

But I suspect the recent measurements are about the limit of entanglement......and I don't see how being able to teleport entangled data one inch is going to lead to much of a time machine.

 
Just so everyone knows, the paper is on arXiv ("The quantum mechanics of time travel through post-selected teleportation", arXiv:1007.2615 [quant-ph]--best to look this article up on arXiv via title). What I find interesting is the following quote from its synopsis on the arXiv blog: "Lloyd and cos [co-authors?--my comment] idea is to use postselection to make this process happen in reverse. Postselection ensures that only a certain type of state can be teleported [states can be 'teleported'???--hmmmm--my comment (sounds suspect)]. 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 [states traveling backwards in time?--even more suspect (my comment)]." One finds the origin of this notion, I think, in reference to delayed-choice experiments ("Thus the choice of measuring device could be made after the photon had interacted with the beamsplitter. There was therefore no way the photon could 'know' in advance whether it should pass along both paths to show wave interference effects (both Pockels cells off) or if it should pass along only one of the paths to show localized particle-like properties (one Pockels cell on). From Jim Baggott's book THE MEANING OF QUANTUM THEORY, pg.154). Even more interesting is that in the original paper, Lloyd, et al on page 2 of that paper deem Wheeler's notion of all electrons actually being one electron traveling forward and backward in time as an example of a "non-general relativistic quantum version of time travel" (though if one can actually speak of a particle traveling 'backward in time' that would be an example of 'time travel'--what puzzles me is why 'the electron traveling backward in time' is not in itself just a 'state' of the electron which in itself could not travel anywhere). What seems to get us out of this 'mess' is to hold to Wheeler's dictum, i.e. 'No elementary phenomenon [time included--my comment] is a phenomenon until it is a registered (observed) phenomenon' (Baggott, pg. 156). But then one needs to presume the existence of "registered (observed)' elementary phenomena in order to produce registered (observed) elementary phenomena (registered by measuring instruments). Perhaps there is no such thing really as 'time travel' (as 'time' seems to be a derivative 'classical' concept) only travels in the collective 'shared' consciousness, 'time travel to the past' being appropriate adjustments to the 'memory' of the collective (shared) consciousness. Perhaps 'time' is merely an artifact of limitation and duality....

 
The paper you refer to, "Quantum Information Science" is not on arXiv. However, in reference to the paper under discussion (which is on arXiv), "Quantum Information Science" is an invaluable reference for understanding the paper by Lloyd and Co-authors. The upshot regarding time travel via quantum teleportation as understood by Lloyd is that if you unpack his definition of teleportation via the definition of the rest of the terms in the glossary of "Quantum Information Science" and relate that to his description of quantum teleportation on pg 18 of that paper you find that what passes as 'quantum teleportation' is nothing more than Bob via measurement placing his EPR particle (Alice has another EPR particle so it is the 'state', not the particle that is allegedly 'teleported' (lol)) in the state that the EPR particle of Alice was in (Alice's measurement destroyed ('dematerialized') the state her particle was in) via the recipe given by Alice since according to Lloyd, every particle and every state is everywhere all at once until constructed by measurement (I kid you not--look at the definitions in the glossary and use common sense...). A better paper regarding a quantum mechanical model for time travel and the resolution of the 'Grandfather Paradox' via quantum mechanics is Greenberger and Svozil's "Quantum Theory Looks at Time Travel" (arXiv: quant-ph/0506027). I say it is better because Greenberger and Szozil use in their model beam splitters which relate to delayed choice/Aspect experiments. The delayed choice/Aspect experiments, I think, would be better candidates as a basis for creating 'toy' time travel experiments for the reasons I just stated. Read also Rauscher and Amoroso's "The Physical Implications of Multidimensional Geometries and Measurement", and Rauscher and Targ's "The Speed of Thought: Investigation of a Complex Space-Time Metric to Describe Psychic Phenomena" (to find these look under title on the Web) to see how complex Minkowski space admits the nonlocality associated with the delayed choice/Aspect experiments and how it also allows for instantaneous travel to distant points in both space and time (this would be the Rauscher and Targ paper) without the problems associated with wormholes. I hope folks find these papers helpful and interesting.

 
Yes interesting. Applying quantum mechanics to Boolean algebra might look ok on paper but I question if that would really solve the grandfather paradox. What about the many worlds theory? How would this be used to solve in that situation?

 
Back
Top