Superluminal communication

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Superluminal communication is the hypothetical process by which one might send information at faster-than-light (FTL) speeds. The scientific consensus is that faster-than-light communication is not possible and to date superluminal communication has not been achieved in any experiment[citation needed].

Some theories and experiments include:

According to the currently accepted theory, three of those four phenomena do not produce superluminal communication, even though they may give that appearance under some conditions. The third, tachyons, arguably do not exist as their existence is hypothetical; even if their existence were to be proven, attempts to quantize them appear to indicate that they may not be used for superluminal communication, because experiments to produce or absorb tachyons cannot be fully controlled.[1]

If wormholes are possible, then ordinary subluminal methods of communication could be sent through them to achieve superluminal transmission speeds. Considering the immense energy that current theories suggest would be required to open a wormhole large enough to pass spacecraft through it may be that only atomic-scale wormholes would be practical to build, limiting their use solely to information transmission. Some theories of wormhole formation would prevent them from ever becoming "timeholes", allowing superluminal communication without the additional complication of allowing communication with the past.[citation needed]

In standard quantum mechanics, it is generally accepted that the no cloning theorem prevents superluminal communication via quantum entanglement alone, leading to the no-communication theorem. Consider the EPR thought experiment, and suppose quantum states could be cloned. Alice could send bits to Bob in the following way:

If Alice wishes to transmit a '0', she measures the spin of her electron in the z direction, collapsing Bob's state to either |z+>B or |z->B. If Alice wishes to transmit a '1', she measures the spin of her electron in the x direction, collapsing Bob's state to either |x+>B or |x->B. Bob creates many copies of his electron's state, and measures the spin of each copy in the z direction. If Alice transmitted a '0', all his measurements will produce the same result; otherwise, his measurements will be split evenly between +1/2 and -1/2. This would allow Alice and Bob to communicate across space-like separations.

However, some authors have pointed out that at least some of the no-communication arguments are tautological, having the limitation on superluminal communication built into the starting assumptions.[2]

Birgit Dopfer's experiment[edit]

Although such communication is prohibited in the thought experiment described above, some argue that superluminal communication could be achieved via quantum entanglement using other methods that don't rely on cloning a quantum system. One suggested method would use an ensemble of entangled particles to transmit information,[3] similar to a type of quantum eraser experiments.[4][5][6]

Birgit Dopfer, a student of Anton Zeilinger's, has performed an experiment which seems to make possible superluminar communication through an unexpected collective behaviour of two beams of entangled photons, one of which passes through a double-slit, utilising the creation of a distance interference pattern as bit 0 and the lack of a distance interference pattern as bit 1 (or vice versa), without any other classical channel.[4][7] Since it is a collective and probabilistic phenomenon, no quantum information about the single particles is cloned and, accordingly, the no cloning theorem remains inviolate. Physicist John G. Cramer at the University of Washington is attempting to replicate Dopfer's experiment and demonstrate whether or not it can produce superluminal communication.[8][9][10][11]

See also[edit]

References[edit]

  1. ^ Feinberg, Gerald (1967). "Possibility of Faster-Than-Light Particles". Physical Review 159 (5): 1089–1105. Bibcode:1967PhRv..159.1089F. doi:10.1103/PhysRev.159.1089. 
  2. ^ Peacock, K.A.; Hepburn, B. (1999). "Begging the Signaling Question: Quantum Signaling and the Dynamics of Multiparticle Systems". Proceedings of the Meeting of the Society of Exact Philosophy. 
  3. ^ Millis, M.G.; Davis, E.W., eds. (2009). Frontiers of Propulsion Science. Progress in astronautics and aeronautics. American Institute of Aeronautics and Astronautics. pp. 509–530. 
  4. ^ a b Strekalov, D.; Sergienko, A.; Klyshko, D.; Shih, Y. (1 May 1995). "Observation of Two-Photon "Ghost" Interference and Diffraction". Physical Review Letters 74 (18): 3600–3603. Bibcode:1995PhRvL..74.3600S. doi:10.1103/PhysRevLett.74.3600. PMID 10058246. 
  5. ^ Dopfer, Birgit (1998). Zwei Experimente zur Interferenz von Zwei-Photonen Zusẗanden (PhD Thesis). Univ. Innsbruck. 
  6. ^ Zeilinger, Anton (1999). "Experiment and the foundations of quantum physics". Reviews of Modern Physics 71 (2): 288–297. Bibcode:1999RvMPS..71..288Z. doi:10.1103/RevModPhys.71.S288. 
  7. ^ The reason why her results are still controversial is the fact that she actually used a classical channel in order to reduce the background noise. It is currently not possible to certainty determine whether the classical channel in such a setup is necessary just to remedy the current lack of technology, or there is something deeper that makes it unavoidable.
  8. ^ Cramer, John G. (April 3, 2010). "Quantum Entanglement, Nonlocality, and Back-In-Time Messages". Norwescon 33. 
  9. ^ Friedlander, Paul. "Experiment". paulfriedlander.com. 
  10. ^ Paulson, Tom (14 November 2006). "Going for a blast into the real past". Seattle Post-Intelligencer. Retrieved 11 July 2011. 
  11. ^ Barry, Patrick (September 30, 2006). "What's done is done… or is it?". New Scientist 191 (2571): 36–39.  (subscription required)