Superluminal communication: Difference between revisions
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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 paradox|EPR thought experiment]], and suppose quantum states could be cloned. Alice could send bits to Bob in the following way: |
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 paradox|EPR thought experiment]], and suppose quantum states could be cloned. Alice could send bits to Bob in the following way: |
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::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'''+><sub>B</sub> or |'''z'''-><sub>B</sub>. 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'''+><sub>B</sub> or |'''x'''-><sub>B</sub>. 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. |
::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'''+><sub>B</sub> or |'''z'''-><sub>B</sub>. 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'''+><sub>B</sub> or |'''x'''-><sub>B</sub>. 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. |
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However, some authors haven pointed that at least some of the no-communication are tautological, having the limitation on superluminal communication built into the starting assumptions. |
However, some authors haven pointed that at least some of the no-communication are tautological, having the limitation on superluminal communication built into the starting assumptions. |
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<ref> |
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{{cite journal |last=Eberhard |first=P. H. |year= 1977|title=Bell's theorem without hidden variables |journal=Il Nuovo Cimento B|volume=38 |issue=1 |pages=75-80 |doi=10.1007/BF02726212}}; |
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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 [[quantum ensemble|ensemble]] of entangled particles to transmit information,<ref>{{cite book |title=Frontiers of Propulsion Science |editor1-last=Millis |editor1-first=M.G. |editor2-last=Davis |editor2-first=E.W. |series=Progress in astronautics and aeronautics |year=2009 |publisher= American Institute of Aeronautics and Astronautics |pages=509–530}}</ref> similar to a type of [[quantum eraser experiment]]s.<ref>{{cite journal |last1=Strekalov |first1=D. V. |last2=Sergienko |first2=A.V. |last3=Klyshko |first3=D.N. |last4=Shih |first4=Y.H. |year=1995 |title=Observation of Two-Photon "Ghost" Interference and Diffraction |journal=Physics Review Letters |volume=75 |issue=18 |pages=3600–3603}}</ref><ref>{{cite book|last=Dopfer|first=Birgit|title=PhD Thesis|year=1998|publisher=Univ. Innsbruck}}</ref><ref>{{cite journal|last=Zeilinger|first=Anton|title=Experiment and the foundations of quantum physics|journal=Rev. Mod. Physics|year=1999|volume=71|pages=288-297}}</ref> As the quantum eraser experiments rely on a classical, subluminal channel for coincidence detection, it is unclear whether superluminal communication would be possible by this method. Physicist [[John G. Cramer]] at the [[University of Washington]] is attempting to replicate one of these experiments and demonstrate whether it can produce superluminal communication or not.<ref>{{cite news |title=Going for a blast into the real past |first=Tom |last=Paulson |url=http://www.seattlepi.com/default/article/Going-for-a-blast-into-the-real-past-1219821.php |newspaper=Seattle Post-Intelligencer |date=14 November 2006 |accessdate=11 July 2011}}</ref><ref>[http://www.npl.washington.edu/sites/default/files/webfiles/annualreports/2011/CENPA-AR2011-7other.pdf UW CENPA Annual Report 2006-7, Article 2.19 "A Test of Quantum Nonlocal Communication", J. G. Cramer, Warren G. Nagourney, and Skander Mzali, April 22, 2007]</ref> |
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{{cite journal |last=Eberhard |first=P. H. |year= 1978|title=Bell’s theorem and the different concepts of locality |journal=Il Nuovo Cimento B |volume=46 |issue=2 |pages=392-419 |doi=10.1007/BF02728628}}; |
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{{cite journal |last1=Ghirardi |first1=G. C. |last2=Rimini |first2=A. |last3=Weber |first3=T. |year=1980 |title=A general argument against superluminal transmission through the quantum mechanical measurement process |journal=Lettere Al Nuovo Cimento |volume=27 |issue=10 |pages=293-298 |doi=10.1007/BF02817189}} |
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</ref> |
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== Superluminal Communication of Electromagnetic waves == |
== Superluminal Communication of Electromagnetic waves == |
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Revision as of 22:57, 11 July 2011
Superluminal communication is the term used to describe the hypothetical process by which one might send information at faster-than-light (FTL) speeds.
Some theories and experiments include:
- Group velocity > c experiments
- Evanescent wave coupling
- Tachyons
- Quantum non-locality
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. As for tachyons, their existence remains 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 haven pointed that at least some of the no-communication are tautological, having the limitation on superluminal communication built into the starting assumptions. [2]
Superluminal Communication of Electromagnetic waves
Surface waves
The quantum of a surface electromagnetic wave is a tachyon whose velocity of motion, equivalent to the energy flow velocity of the wave. can be faster than light speed c in vacuum. [3]
Low frequency electromagnetic fields in good conductors
The dispersion relation of a low frequency electromagnetic field in conductors is equivalent to the energy-momentum equation of a tachyon. Actually, the velocity of such a tachyon just equals that of the energy of the field which is much greater than light speed c= 299,792,458 m/s in vacuum.[4]
References
- ^ Feinberg, Gerald (1967). "Possibility of Faster-Than-Light Particles". Physical Review. 159 (5): 1089–1105. doi:10.1103/PhysRev.159.1089.
- ^ Eberhard, P. H. (1977). "Bell's theorem without hidden variables". Il Nuovo Cimento B. 38 (1): 75–80. doi:10.1007/BF02726212.; Eberhard, P. H. (1978). "Bell's theorem and the different concepts of locality". Il Nuovo Cimento B. 46 (2): 392–419. doi:10.1007/BF02728628.; Ghirardi, G. C.; Rimini, A.; Weber, T. (1980). "A general argument against superluminal transmission through the quantum mechanical measurement process". Lettere Al Nuovo Cimento. 27 (10): 293–298. doi:10.1007/BF02817189.
- ^ Wang, Zhong-Yue (2011). "Superluminal energy transmission in the Goos-Hanchen shift of total reflection". Optics Communications. 284 (7): 1747–1751. Bibcode:2011OptCo.284.1747W. doi:10.1016/j.optcom.2010.12.027.
- ^ Zhong-Yue Wang, Pin-Yu Wang, Yan-Rong Xu (2011). "Crucial experiment to resolve Abraham-Minkowski Controversy". Optik. arXiv:/1103.3559. doi:10.1016/j.ijleo.2010.12.018.
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