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Title: Physics/Relativity/Time Travel - Quantum Time Travel Kip Thorne, wormholes, superposition, time translation and stasis field.

The_Relativistic_Asynchronous_Machine Design of a hypothetic machine for travel into the future by Tilmann Schneider.

Rotating_Universes_and_Time_Traveling Thesis, Mach's principle, Einstein's general relativity, equation and diagram.

Rubak_com__Is_Time_Travel_Possible Article examining this controversial situation. Read and add your response.

Some_Ramifications_of_Time_Travel Introduction, scenario for discussion, next 500,000 years, after 500,000 years, features of time travel, constraints on travel to past, operational considerations and conclusions.

Space-Time_Physics_and_the_Future_of_Time_Travel Black holes, wormholes, time dilation factor, gravity, graphs and equations.

Stardrive Super cosmos, Hawking's nutty universe, physics of exotic propulsion, destiny matrix and the world crystal.

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Taking_the_Cosmic_Shortcut The future, Einstein's special theory of relativity, the past, theory of gravitation, black holes, wormholes, negative energy and links.

TIME_LOOPS An interview with Paul Davies, paradox, wormhole, black hole, Newtonian mechanics and quantum computations.

The_Time_Machine Einstein's theory, special relativity, communication faster than light, quantum cosmology, interview with Carl Sagan and links.

Time,_Time_Travel_&_Traversable_Wormholes Links to articles and sites about the possibility and impossibility of time travel.

Time_Travel Simple explanation, black hole containment device, mathematics, energy equation, chaos and super string theories, power of the crystal, FAQ and links.

Time_Travel_-_A_Discussion Hyperspace, space-time continuum, black holes, wormholes, parallel universes, quarks, tachyons, quantum and super string theories.

Time_Travel_-_Fact_or_Fiction The science fiction paradigm, conservation laws, general relativity, paradoxes, tachyons and graphs.

Time_Travel_-_The_Grandmother_Paradox Discusses the possibility of associated paradoxes from a philosophical rather than purely scientific viewpoint, Mobius strip, bibliography and links.

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Time_Travel-NA Time Travel-NA is a small website set up by irish students to see if time travel is possible. Contained in the site is a small list of the different methods of time travel.

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Scientific_American__How_to_Build_a_Time_Machine It wouldn't be easy, but it might be possible Article by Paul Davies (September, 2002)

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Allied_Electronics_Data_Book A near complete reproduction of the famous Allied Book.

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Alternate View Column AV-45AnalogScience Fiction & Fact Magazine"The Alternate View" columns of John G. Cramer Previous Column

Quantum Time Travel

by John G. CramerAlternate View Column AV-45Keywords: quantum, time,travel,uncertainty, stasis, accelerated aging,interferometryPublished in the April-1991 issue ofAnalog Science Fiction & Fact Magazine;This column was written and submitted 9/30/90 and is copyrighted© 1990, John G. Cramer. All rights reserved.No part may be reproduced in any form withoutthe explicit permission of the author.This page now has an access count of:The territory of time travel has, from the days of H. G. Wells to themid-1980's, been the exclusive province of writers of science fiction andfantasy. SF critics have even argued that time travel stories are soscientifically unlikely that they should be considered fantasy, not sciencefiction.In the last few years, however, a few theoretical physicists have been musclingin on the time travel game. The first assault was mounted by a group ofgeneral relativity theorists led by Kip Thorne of CalTech. Thorne's groupdemonstrated how space-time wormholes could be stabilized and used fortrans-time communication and time travel. See my Alternate View columns in[Analog-June'89] and[Analog-May'90] for references and discussions ofthis work.) This column describes the latest physics foray into the timetravel business, which has come from the direction of quantum mechanics.Yakir Aharonov and his group at the University of South Carolina have for anumber of years been "testing the envelope" of quantum mechanics. Their workhas involved probing and poking at the foundations of orthodox quantummechanics in search of weaknesses, soft spots, and unexpected predictions. Thelatest of these is contained in a paper entitled "Superposition of TimeEvolutions of Quantum System and a Quantum Time Translation Machine" by Y.Aharonov, J. Anandan, S. Popescu, and L. Vaidman (AAPV) that was published inthe June 18, 1990 issue of Physical Review Letters. In essence, theAAPV paper describes how an aspect of quantum mechanics called statesuperposition can be used to translate objects inside a closed and isolatedsystem into the future or the past. The paper, in other words, describes how,in theory, to build a quantum mechanical time machine.To understand what Prof. Aharonov and his colleagues are proposing, we have todelve a bit into quantum mechanical superposition. To take a simple example,consider a half-silvered mirror. This is a piece of glass that has had justenough reflective material so that exactly half the light striking it at 45deg.incidence goes straight through the glass and the other half bounces from thereflective surface at a right angle. If a single photon of light encountersthis half-silvered mirror, there is a 50% chance that it will be transmitted(pass through) and a 50% chance that it will be reflected.One would think that the photon must do one thing or the other, but quantummechanics tells us that it can do both. This can be verifiedexperimentally. If we provide some extra mirrors to make two paths so that,after travelling some distance, a photon that was transmitted comes back to thesame path as a photon that was reflected, we can observe "quantuminterference". The quantum mechanical wave describing the transmitted photonadds or subtracts from the wave for the reflected photon. If they subtractcompletely, the photon waves on the two paths cancel each other and no photonscan be observed at the exit. If they add, the photons waves on the two pathshelp each other and can be observed. The two quantum states of the particle,"photon reflected" and "photon transmitted", are superimposed, with definiteobservable consequences. Quantum superposition is not limited to photons,which are somewhat special because they have no rest mass. It works equallywell for massive particles and has been demonstrated using very slow neutrons,for example.The mathematics of state superposition is special because quantum mechanicsdescribes each state function as a complex variable that has a real part and an"imaginary" part (that behaves like the square root of -1). Each state has itsown quantum mechanical "phase", the angle in the complex plane that its complexstate function makes with the real number axis. The result of superimposingseveral states is determined by the quantum phases of the states. Somecombinations of phases produce cancellation while others producereinforcement.The point of departure of the AAPV calculation from standard quantum mechanicsis that it applies the principle of superposition to the time evolutionsof a group of similar quantum states. This is not normally done in quantummechanics, but it should be OK. It seems to be consistent with both theformalism of the theory and its usual interpretation, provided the states areisolated from measurement and outside interaction while evolving.The application of superposition in this way, however, has unexpected andsurprising consequences. Superposition can be considered the quantummechanical form of averaging. Common sense tells us that when several valuesare averaged, the resulting average value must fall within the range ofvariations of the averaged values. AAPV, however, demonstrate that insuperimposing a group of carefully prepared time-evolving states, they canproduce a measured result that is far larger (or smaller) than the value thatwould have resulted from any of the states taken separately. This is aconsequence of the fact that in quantum mechanics the "averaged" states arecomplex (real plus imaginary) space-time functions with complexweighting factors (not real functions with real weighting factors).The AAPV paper illustrates this point with an example. They consider asuperposition of a number of time-evolving states that differ from one anotherbecause of the size of the force that is acting to produce the change withtime. The authors show that the effective force which results from thesuperposition of a set of time-evolving states may occasionally, under theright conditions, be far larger than any of the individual forces acting withinthe individual states. If ten thousand such states are superimposed, a forceamplification of 100 is in principle possible through this process. The downside of the procedure, however, is that with the superposition of ten thousandidentical systems, only one of these experiences this amplified force. Or, toput it another way, the 100-fold amplified force is observed only about once in10,000 tries. This makes the effect observable, at least in principle, but itmay not be particularly useful as a force amplifier.The AAPV paper goes on to consider the application of this new quantumprinciple to time translation. The states they consider evolve with differentelapsed times due to the time dilation effects of a gravitational field ingeneral relativity. They arrange each superimposed system within a hollowmassive shell with a quantum-uncertain radius, so that it experiences timedilation but no other gravitational effects (acceleration, tidal forces, etc.).They show that the superposition of a group of these time-evolving systems canresult in a net time displacement which is quite different (positive ornegative) from the elapsed time experienced by the external observer of thesystem. Thus, the system inside the shell had been translated forward orbackward in time with respect to the observer.The AAVP authors are careful to point out that the gravitation time translationdevice which they describe is a thought experiment only and is not feasible intoday's physics laboratory. They also emphasize the probabilistic aspect ofthe time translation. Since the weightings that produce a large timetranslation occur only rarely and at random, they point out that "for anysignificantly long time journey in time the probability to obtain this outcomeis extremely small."For SF readers and writers, the question raised by this new result of quantummechanics is the relation between the AAPV "time translation machine" and thefamiliar "time machines" and other devices of SF literature. It is not, forseveral reasons, a conventional SF time machine. The AAVP machine cannot beused to materialize a traveller or a message in some arbitrary point in thepast or the future. What it does is modify the way a system within the machineexperiences the passage of time. We could place a system in the AAVP machine,run it, and with some probability remove a system that has experienced a largepositive time displacement. This could speed up the decay of a long-livedradioactive source from many years to a few seconds, suggesting a radical newway of dealing with radioactive waste. A barrel of wine or scotch might alsobe "aged" to increase its value and quality.If we could select for zero time displacement, this would in effect produce a"stasis field" of the kind Larry Niven has used in his fiction. We couldfreeze the internal system with no time change for the period it was isolated,while time in the external world moved forward at the usual rate.But perhaps the most interesting use of the AAVP machine would be to producenegative time displacements. It is not difficult to advance the time evolutionof a system it we are patient. We have only to wait while it evolves or ages.But reversing its time evolution is not within our capabilities. The secondlaw of thermodynamics, the inexorable increase in the entropy of a system withtime, prevents such reversals. Yet the AAVP machine appears to offer thepossibility of doing just that. A system, placed in an AAVP machine selectedfor negative time displacement, would evolve in reverse and grow younger. Anaged experimental mouse, placed in a negative time displacement AAVP device,should grow younger. A clock should run backwards. A dead mouse (or person)might even be resurrected.Another possibility worth considering is backwards-in-time communication.Communicating forward in time is no real problem. Office file cabinets arefull of forward-in-time communications. But backwards-in-time communication,or foreseeing the future, is something that we don't know how to do, (except inthe National Enquirer). But suppose that we use a large number of very smallidentical systems which are capable of containing an encoded message in theirstructure. Chain-like protein molecules would serve for this purpose. Weencode them with a blank message, place them in the AAVP machine, and set it toselect for large positive (future) time displacement. After operation on allthe systems, we scan them and then place them in cold storage for a time, afterwhich we systematically modify all of them to contain an encoded message to besent to the past. Most of the systems have not undergone any change in themachine. But a few have undergone a large positive time displacement. Thesefew should contain the future message before to was encoded into thegroup, and could be decoded in the past, before the systems were changed.Thus, a message might be sent to the past using this peculiar aspect of quantummechanics.Can the AAVP effect really be demonstrated in the laboratory? Can it be usedto freeze, advance, and reverse time? Can it raise the dead? Can it be usedto communicate with the past? I would have to say that I'm not sure. The AAVPpaper is very new, and it has not been completely to the full scrutiny andcriticism of the physics community. There may be holes in the logic of thepaper or additional effects that must be considered in stating the implicationsof its mathematics.What is clear is that its implications are quite remarkable. The AAVP workserves to remind us once again of the intrinsic weirdness of quantum mechanics,and shows us that there are aspects of its weirdness that we do not yet fullyunderstand or appreciate.Reference:AAVP Effect:"Superposition of Time Evolutions of Quantum System anda Quantum Time Translation Machine", Yakir Aharonov, Jeeva Anandan, SanduPopescu, and Lev Vaidman, Physical Review Letters 64, 2965 (1990). Previous Column

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issue index.This page was created by John G. Crameron 7/12/96.

Kip	Thorne,	wormholes,	superposition,
time	translation	and	stasis	field.

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