160:(where Thorne taught), arrived at a solution to the problem, that lays out the same elements as the solution Feynman and Wheeler termed the "glancing blow" solution, to evade inconsistencies arising from causality loops. In the revised scenario, the ball from the future emerges at a different angle than the one that generates the paradox, and delivers its younger self a glancing blow instead of knocking it completely away from the wormhole. This blow alters its trajectory by just the right degree, meaning it will travel back in time with the angle required to deliver its younger self the necessary glancing blow. Echeverria and Klinkhammer actually found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each situation. Later analysis by Thorne and
310:
result. Given an appropriate negative delay something else is possible: the result of each iteration of the function is brought back in time to serve as the "first" approximation. As soon as the machine is activated, a so-called "fixed-point" of F, an input which produces an identical output, usually signaling a perfect answer, appears (by an extraordinary coincidence!) immediately and steadily. If the iteration does not converge, that is, if F has no fixed point, the computer outputs and inputs will shut down or hover in an unlikely intermediate state.
258:
younger self if you travel back in time. You can coexist, take yourself out for a beer, celebrate your birthday together, but somehow circumstances will dictate that you cannot behave in a way that leads to a paradox in time. Novikov supports this point of view with another argument: physics already restricts your free will every day. You may will yourself to fly or to walk through a concrete wall, but gravity and condensed-matter physics dictate that you cannot. Why, Novikov asks, is the consistency restriction placed on a time traveler any different?
121:
249:
change the past. Such change is incompatible with the principle of self-consistency. Consequently, any being who went through a wormhole and tried to change the past would be prevented by physical law from making the change; i.e., the "free will" of the being would be constrained. Although this constraint has a more global character than constraints on free will that follow from the standard, local laws of physics, it is not obvious to us that this constraint is more severe than those imposed by standard physical law.
225:, a self-evident truth that cannot possibly be false. However, the Novikov self-consistency principle is intended to go beyond just the statement that history must be consistent, making the additional nontrivial assumption that the universe obeys the same local laws of physics in situations involving time travel that it does in regions of space-time that lack closed timelike curves. This is clarified in the above-mentioned "Cauchy problem in spacetimes with closed timelike curves", where the authors write:
129:
230:
self-consistency is intended to rule out such behavior. It insists that local physics is governed by the same types of physical laws as we deal with in the absence of CTCs: the laws that entail self-consistent single valuedness for the fields. In essence, the principle of self-consistency is a principle of no new physics. If one is inclined from the outset to ignore or discount the possibility of new physics, then one will regard self-consistency as a trivial principle.
348:, even if it is not built up by quantum systems. The authors conclude that hence, Deutsch's condition is not specific to quantum physics, nor does it depend on the quantum nature of a physical system so that it can be fulfilled. In consequence, Tolksdorf and Verch argue that Deutsch's condition is not sufficiently specific to allow statements about time travel scenarios or their hypothetical realization by quantum physics.
197:, such a sum over histories produces unique, self-consistent probabilities for the outcomes of all sets of subsequent measurements. ... We suspect, more generally, that for any quantum system in a classical wormhole spacetime with a stable Cauchy horizon, the sum over all self-consistent histories will give unique, self-consistent probabilities for the outcomes of all sets of measurements that one might choose to make.
559:(1979), Novikov's comment on the issue is rendered by translator M. M. Basko as "The close of time curves does not necessarily imply a violation of causality, since the events along such a closed line may be all 'self-adjusted'—they all affect one another through the closed cycle and follow one another in a self-consistent way."
180:
initial trajectory they analyzed—can be seen as problematic, since classically there seems to be no way to decide which extension the laws of physics will choose. To get around this difficulty, Thorne and
Klinkhammer analyzed the billiard ball scenario using quantum mechanics, performing a quantum-mechanical sum over histories (
88:
argued that they cannot entail this type of causality violation: events on a CTC are already guaranteed to be self-consistent, Novikov argued; they influence each other around a closed curve in a self-adjusted, cyclical, self-consistent way. The other authors recently have arrived at the same viewpoint.
451:, centers heavily on the question whether the past can be changed or not. Some of the characters in the plot are driven to change it, whereas others, who have already tried doing so in vain, have resigned themselves to come to the conclusion that the Novikov self-consistency principle seemingly applies.
335:
problems in polynomial time. Deutsch shows that quantum computation with a negative delay—backwards time travel—produces only self-consistent solutions, and the chronology-violating region imposes constraints that are not apparent through classical reasoning. Researchers published in 2014
309:
Make a computing box that accepts an input, which represents an approximate solution to some problem, and produces an output that is an improved approximation. Conventionally you would apply such a computation repeatedly a finite number of times, and then settle for the better, but still approximate,
257:
According to the consistency conjecture, any complex interpersonal interactions must work themselves out self-consistently so that there is no paradox. That is the resolution. This means, if taken literally, that if time machines exist, there can be no free will. You cannot will yourself to kill your
87:
The only type of causality violation that the authors would find unacceptable is that embodied in the science-fiction concept of going backward in time and killing one's younger self ("changing the past"). Some years ago one of us (Novikov) briefly considered the possibility that CTCs might exist and
179:
Even if self-consistent extensions can be found for arbitrary initial conditions outside the Cauchy horizon, the finding that there can be multiple distinct self-consistent extensions for the same initial condition—indeed, Echeverria et al. found an infinite number of consistent extensions for every
116:
could lead to closed timelike curves, and unlike previous CTC-containing solutions, it did not require unrealistic conditions for the universe as a whole. After discussions with the lead author of the 1990 paper, John
Friedman, they convinced themselves that time travel need not lead to unresolvable
171:
initial conditions for the billiard ball for which there were no self-consistent extensions, but were unable to do so. Thus, it is plausible that there exist self-consistent extensions for every possible initial trajectory, although this has not been proven. This only applies to initial conditions
188:
The simplest way to impose the principle of self-consistency in quantum mechanics (in a classical space-time) is by a sum-over-histories formulation in which one includes all those, and only those, histories that are self-consistent. It turns out that, at least formally (modulo such issues as the
248:
If CTCs are allowed, and if the above vision of theoretical physics' accommodation with them turns out to be more or less correct, then what will this imply about the philosophical notion of free will for humans and other intelligent beings? It certainly will imply that intelligent beings cannot
472:
follows the
Novikov self-consistency principle. Two of the main characters can travel backwards in time by jumping into a pond, but they are unable to change anything in the past. All of their actions become part of history, and they actually end up causing the tragic events they were trying to
229:
That the principle of self-consistency is not totally tautological becomes clear when one considers the following alternative: The laws of physics might permit CTCs; and when CTCs occur, they might trigger new kinds of local physics which we have not previously met. ... The principle of
336:
a simulation in which they claim to have validated
Deutsch's model with photons. However, it was shown in an article by Tolksdorf and Verch that Deutsch's self-consistency condition can be fulfilled to arbitrary precision in any quantum system described according to relativistic
148:
at an angle such that, if it continues along its path, it will exit in the past at just the right angle to collide with its earlier self, knocking it off track and preventing it from entering the wormhole in the first place. Thorne would refer to this scenario as
112:, and Ulvi Yurtsever, who in 1988 had stirred up renewed interest in the subject of time travel in general relativity with their paper "Wormholes, Time Machines, and the Weak Energy Condition", which showed that a new general relativity solution known as a
99:
This principle allows one to build a local solution to the equations of physics only if that local solution can be extended to a part of a (not necessarily unique) global solution, which is well defined throughout the nonsingular regions of the space-time.
79:. Novikov discussed the possibility of closed timelike curves (CTCs) in books he wrote in 1975 and 1983, offering the opinion that only self-consistent trips back in time would be permitted. In a 1990 paper by Novikov and several others, "
418:: In Eliezer Yudkowsky's exposition on rationality, framed as a piece of Harry Potter fanfiction, Harry attempts to use his Time Turner to influence the past and comes to the conclusion that the Novikov self-consistency principle applies.
239:
The assumptions of the self-consistency principle can be extended to hypothetical scenarios involving intelligent time travelers as well as unintelligent objects such as billiard balls. The authors of "Cauchy problem in spacetimes with
293:
and relies upon the self-consistency principle to force the sent result to be correct, provided the machine can reliably receive information from the future and provided the algorithm and the underlying mechanism are
184:) using only the consistent extensions, and found that this resulted in a well-defined probability for each consistent extension. The authors of "Cauchy problem in spacetimes with closed timelike curves" write:
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is largely based on the notion that the possibility of time travel tempts the characters to try change the past, which only leads them to cause the events they were trying to prevent in the first place.
340:
even on spacetimes which do not admit closed timelike curves, casting doubts on whether
Deutsch's model is really characteristic of quantum processes simulating closed timelike curves in the sense of
176:. This could mean that the Novikov self-consistency principle does not actually place any constraints on systems outside of the region of space-time where time travel is possible, only inside it.
281:, is a hypothetical system of computation that exploits the Novikov self-consistency principle to compute answers much faster than possible with the standard model of
923:
1122:
Ringbauer, Martin; Broome, Matthew A.; Myers, Casey R.; White, Andrew G.; Ralph, Timothy C. (19 June 2014). "Experimental simulation of closed timelike curves".
167:
Echeverria, Klinkhammer, and Thorne published a paper discussing these results in 1991; in addition, they reported that they had tried to see if they could find
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The
Novikov consistency principle assumes certain conditions about what sort of time travel is possible. Specifically, it assumes either that there is only one
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illustrated that for certain initial trajectories of the billiard ball, there could actually be an infinite number of self-consistent solutions.
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wrote them a letter arguing that one could avoid the issue of free will by employing a potentially paradoxical thought experiment involving a
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Tolksdorf, Juergen; Verch, Rainer (2018). "Quantum physics, fields and closed timelike curves: The D-CTC condition in quantum field theory".
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Tolksdorf, Juergen; Verch, Rainer (2021). "The D-CTC condition is generically fulfilled in classical (non-quantum) statistical systems".
459:(2019): A video game involving time travel which does not follow the principle, causing a game over if the player experiments to test it.
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298:. An incorrect result or no result can still be produced if the time travel mechanism or algorithm are not guaranteed to be accurate.
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the only solutions to the laws of physics that can occur locally in the real
Universe are those which are globally self-consistent.
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55:. The principle asserts that if an event exists that would cause a paradox or any "change" to the past whatsoever, then the
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Friedman, John; Michael Morris; Igor
Novikov; Fernando Echeverria; Gunnar Klinkhammer; Kip Thorne; Ulvi Yurtsever (1990).
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Given these assumptions, the constraint that time travel must not lead to inconsistent outcomes could be seen merely as a
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and path integrals. In particular, the path integral is over single-valued fields, leading to self-consistent histories.
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1332:(27 January 2011). "Closed Timelike Curves via Postselection: Theory and Experimental Test of Consistency".
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389:. The anomaly returns and sends it back into the present, before it has a chance to affect the outcome.
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See note 10 on p. 42 of
Friedman et al., "Cauchy problem in space-times with closed timelike curves"
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sent back in time through a wormhole. In
Polchinski's scenario, the billiard ball is fired into the
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Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes
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Upon considering the scenario, Fernando Echeverria and Gunnar Klinkhammer, two students at
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Physicists have long known that some solutions to the theory of general relativity contain
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Assertion that time-travel paradoxes are impossible, as paradox-causing events cannot occur
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1412:(2011). "The quantum mechanics of time travel through post-selected teleportation".
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later extended this result to show that the model could also be used to solve
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Cosmic Catastrophes: Exploding Stars, Black Holes, and Mapping the Universe
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378:(1980): A science-fiction time-travel movie in which the aircraft carrier
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explores the interplay between free will and self-consistent time-travel.
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showed in 1991 that this model of computation could solve NP problems in
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1057:"Closed Timelike Curves Make Quantum and Classical Computing Equivalent"
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Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction
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410:(2009): Cited by Makise Kurisu during her presentation on time travel.
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Similarly, physicist and astronomer J. Craig Wheeler concludes that:
210:, or that any alternative timelines (such as those postulated by the
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753:"Classical Electrodynamics in Terms of Direct Interparticle Action"
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paradoxes, regardless of the object sent through the wormhole.
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in spacetimes with closed timelike curves", the authors state:
1111:
from the original on 2022-10-09 – via scottaaronson.com.
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from the original on 2022-10-09 – via scottaaronson.com.
898:(2nd ed.). Cambridge University Press. pp. 294–295.
43:
in the mid-1980s. Novikov intended it to solve the problem of
1477:, which also addresses the Novikov self-consistency principle
792:
Echeverria, Fernando; Gunnar Klinkhammer; Kip Thorne (1991).
289:. In this system, a computer sends a result of a computation
244:" commented on the issue in the paper's conclusion, writing:
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of that event is zero. It would thus be impossible to create
314:
47:, which is theoretically permitted in certain solutions of
576:"Cauchy problem in spacetimes with closed timelike curves"
1121:
623:"Wormholes, Time Machines, and the Weak Energy Condition"
684:
Black Holes and Time Warps: Einstein's Outrageous Legacy
621:
Thorne, Kip; Michael Morris; Ulvi Yurtsever (1988).
555:(1983), which was a translation of his Russian book
681:
234:
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385:passes through a wormhole back to the eve of the
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1481:Einstein Physics prevent paradoxical time travel
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356:An alternative proposal was later presented by
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953:"Quantum mechanics near closed timelike lines"
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873:. American Institute of Physics. p. 508.
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104:Among the co-authors of this 1990 paper were
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1469:Notion of the Past & Can We Change It?
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132:Echeverria and Klinkhammer's resolution
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395:The Merchant and the Alchemist's Gate
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426:: A science-fiction novel series by
91:We shall embody this viewpoint in a
25:Novikov self-consistency conjecture
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21:Novikov self-consistency principle
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1027:10.1038/scientificamerican0308-62
1000:"The Limits of Quantum Computers"
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519:Quantum mechanics of time travel
494:Chronology protection conjecture
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235:Implications for time travelers
51:that contain what are known as
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1486:Time Travel and Modern Physics
1364:10.1103/PhysRevLett.106.040403
998:Aaronson, Scott (March 2008).
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136:By way of response, physicist
93:principle of self-consistency,
33:law of conservation of history
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499:Cosmic censorship hypothesis
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924:"Time Travel and Computing"
848:. Oxford University Press.
652:10.1103/PhysRevLett.61.1446
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473:prevent in the first place.
430:that applies the principle.
305:algorithm. Moravec states:
273:Time-loop logic, coined by
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1444:10.1103/PhysRevD.84.025007
1268:10.1007/s10701-021-00496-z
894:Wheeler, J. Craig (2007).
727:Black Holes and Time Warps
509:Many-worlds interpretation
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212:many-worlds interpretation
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1215:10.1007/s00220-017-2943-5
778:10.1103/RevModPhys.21.425
757:Reviews of Modern Physics
688:. W. W. Norton. pp.
553:Evolution of the Universe
41:Igor Dmitriyevich Novikov
977:10.1103/PhysRevD.44.3197
818:10.1103/PhysRevD.44.1077
600:10.1103/PhysRevD.42.1915
447:(2016): A video game by
283:computational complexity
45:paradoxes in time travel
1334:Physical Review Letters
951:Deutsch, David (1991).
869:Nahin, Paul J. (1999).
680:Thorne, Kip S. (1994).
631:Physical Review Letters
551:On p. 169 of Novikov's
462:All time travel in the
301:A simple example is an
75:—for example the
1330:Steinberg, Aephraim M.
1238:Foundations of Physics
1094:10.1098/rspa.2008.0350
504:The chicken or the egg
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291:backwards through time
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242:closed timelike curves
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218:) are not accessible.
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124:"Polchinski's paradox"
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53:closed timelike curves
1406:Giovannetti, Vittorio
1302:Giovannetti, Vittorio
1124:Nature Communications
844:Earman, John (1995).
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352:Lloyd's prescription
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151:Polchinski's paradox
114:traversable wormhole
23:, also known as the
1471:– speech by Novikov
1436:2011PhRvD..84b5007L
1402:Garcia-Patron, Raul
1356:2011PhRvL.106d0403L
1298:Garcia-Patron, Raul
1260:2021FoPh...51...93T
1207:2018CMaPh.357..319T
1146:2014NatCo...5.4145R
1086:2009RSPSA.465..631A
1019:2008SciAm.298c..62A
1007:Scientific American
969:1991PhRvD..44.3197D
810:1991PhRvD..44.1077E
769:1949RvMP...21..425W
644:1988PhRvL..61.1446M
592:1990PhRvD..42.1915F
514:Grandfather paradox
489:Causality (physics)
433:The Netflix series
375:The Final Countdown
1511:Temporal paradoxes
1310:Pirandola, Stefano
1154:10.1038/ncomms5145
368:In popular culture
342:general relativity
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95:which states that
49:general relativity
1414:Physical Review D
1322:Soudagar, Yasaman
1070:(2102): 631–647.
963:(10): 3197–3217.
957:Physical Review D
798:Physical Review D
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638:(13): 1446–1449.
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1506:Conjectures
1501:Time travel
1394:Lloyd, Seth
1290:Lloyd, Seth
524:Time viewer
484:Causal loop
456:Outer Wilds
407:Steins;Gate
360:based upon
202:Assumptions
193:before the
153:" in 1994.
110:Mike Morris
57:probability
29:Larry Niven
1495:Categories
1251:1912.02301
1244:(93): 93.
1198:1609.01496
1137:1501.05014
934:2008-07-28
530:References
423:Orthogonal
400:Ted Chiang
398:(2007) by
392:The story
358:Seth Lloyd
319:Physicist
275:roboticist
106:Kip Thorne
1427:1007.2615
1347:1005.2219
1276:208637445
1223:253751446
1077:0808.2669
428:Greg Egan
269:Time loop
223:tautology
37:principle
1452:15972766
1380:18442086
1372:21405310
1170:12779043
1162:24942489
1130:: 4145.
1106:Archived
1039:Archived
1035:18357822
985:10013776
922:(1991).
826:10013968
724:(1994).
664:Archived
660:10038800
608:10013039
478:See also
208:timeline
146:wormhole
1432:Bibcode
1352:Bibcode
1256:Bibcode
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1142:Bibcode
1082:Bibcode
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158:Caltech
67:History
35:, is a
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327:, and
285:using
1448:S2CID
1422:arXiv
1376:S2CID
1342:arXiv
1272:S2CID
1246:arXiv
1219:S2CID
1193:arXiv
1166:S2CID
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604:PMID
436:Dark
380:USS
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