Editing Time travel (section)

== Time travel in physics ==
Some solutions to Einstein's equations for [[general relativity]] suggest that suitable geometries of [[spacetime]] or specific types of motion in [[space]] might allow time travel into the past and future if these geometries or motions were possible.<ref name="Thorne1994">{{cite book|last=Thorne|first=Kip S.|author-link=Kip Thorne|title=Black Holes and Time Warps|publisher=W. W. Norton|year=1994|isbn=978-0-393-31276-8|title-link=Black Holes and Time Warps}}</ref>{{rp|499}} In technical papers, [[physicist]]s discuss the possibility of [[closed timelike curve]]s, which are [[world line]]s that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, such as [[Gödel metric#Closed timelike curves|Gödel spacetime]], but the physical plausibility of these solutions is uncertain.<ref>{{Cite journal |last1=Ringbauer |first1=Martin |last2=Broome |first2=Matthew A. |last3=Myers |first3=Casey R. |last4=White |first4=Andrew G. |last5=Ralph |first5=Timothy C. |date=2014-06-19 |title=Experimental simulation of closed timelike curves |url=https://www.nature.com/articles/ncomms5145 |journal=Nature Communications |language=en |volume=5 |issue=1 |pages=4145 |doi=10.1038/ncomms5145 |pmid=24942489 |issn=2041-1723|arxiv=1501.05014 |bibcode=2014NatCo...5.4145R }}</ref>

Any theory that would allow backward time travel would introduce potential problems of [[Causality (physics)|causality]].<ref name="Bolonkin">{{cite book|title=Universe, Human Immortality and Future Human Evaluation|first1=Alexander|last1=Bolonkin|publisher=Elsevier|year=2011|isbn=978-0-12-415810-8|page=32|url=https://books.google.com/books?id=IFZWTf93KwgC|access-date=2017-03-26|archive-date=2023-03-24|archive-url=https://web.archive.org/web/20230324181808/https://books.google.com/books?id=IFZWTf93KwgC|url-status=live}} [https://books.google.com/books?id=IFZWTf93KwgC&pg=PA32 Extract of page 32] {{Webarchive|url=https://web.archive.org/web/20230324181801/https://books.google.com/books?id=IFZWTf93KwgC&pg=PA32 |date=2023-03-24 }}</ref> The classic example of a problem involving causality is the "[[grandfather paradox]]," which postulates travelling to the past and intervening in the conception of one's ancestors (causing the death of an ancestor before conception being frequently cited). Some physicists, such as Novikov and Deutsch, suggested that these sorts of [[temporal paradox]]es can be avoided through the [[Novikov self-consistency principle]] or a variation of the [[many-worlds interpretation]] with interacting worlds.<ref name="Everett MWI" />

=== General relativity ===
Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling [[faster-than-light|faster than the speed of light]], such as [[cosmic string]]s, traversable [[wormhole]]s, and [[Alcubierre drive]]s.<ref>{{cite web|url=http://ccrg.rit.edu/files/FasterThanLight.pdf|title=Warp Drives, Wormholes, and Black Holes|author=Miguel Alcubierre|date=June 29, 2012|access-date=January 25, 2017|archive-date=March 18, 2016|archive-url=https://web.archive.org/web/20160318223348/http://ccrg.rit.edu/files/FasterThanLight.pdf|url-status=dead}}</ref><ref name="Gott">{{cite book|author=J. Richard Gott|title=Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time|url=https://books.google.com/books?id=3QBgCgAAQBAJ&pg=PT45|date=25 August 2015|publisher=HMH|isbn=978-0-547-52657-7|page=33|access-date=3 February 2018|archive-date=24 March 2023|archive-url=https://web.archive.org/web/20230324181808/https://books.google.com/books?id=3QBgCgAAQBAJ&pg=PT45|url-status=live}}</ref>{{rp|33–130}} The theory of [[general relativity]] does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from [[semiclassical gravity]] suggest that when [[quantum mechanics|quantum]] effects are incorporated into general relativity, these loopholes may be closed.<ref>{{cite book|arxiv=gr-qc/0204022|last=Visser|first=Matt|title=The quantum physics of chronology protection|url=https://archive.org/details/arxiv-gr-qc0204022|year=2002|bibcode=2003ftpc.book..161V}}</ref> These semiclassical arguments led [[Stephen Hawking]] to formulate the [[chronology protection conjecture]], suggesting that the fundamental laws of nature prevent time travel,<ref name="chronology protection">{{cite journal|first=Stephen|last=Hawking|author-link=Stephen Hawking|title=Chronology protection conjecture|journal=Physical Review D|volume=46|year=1992|issue=2|pages=603–611|doi=10.1103/PhysRevD.46.603|pmid=10014972|bibcode=1992PhRvD..46..603H|url=http://thelifeofpsi.com/wp-content/uploads/2014/09/Hawking-1992.pdf|archive-url=https://web.archive.org/web/20150227141021/http://thelifeofpsi.com/wp-content/uploads/2014/09/Hawking-1992.pdf|archive-date=2015-02-27}}</ref> but physicists cannot come to a definitive judgment on the issue without a theory of [[quantum gravity]] to join quantum mechanics and general relativity into a completely unified theory.<ref name="sagan-nova">{{cite web|url=https://www.pbs.org/wgbh/nova/time/sagan.html|title=Carl Sagan Ponders Time Travel|work=NOVA|date=December 10, 1999|publisher=[[PBS]]|access-date=April 26, 2017|archive-date=July 15, 2019|archive-url=https://web.archive.org/web/20190715000440/https://www.pbs.org/wgbh/nova/time/sagan.html|url-status=live}}</ref><ref name="futureofspacetime">{{cite book|last1=Hawking|first1=Stephen|author-link=Stephen Hawking|last2=Thorne|first2=Kip|author-link2=Kip Thorne|last3=Novikov|first3=Igor|author-link3=Igor Dmitriyevich Novikov|last4=Ferris|first4=Timothy|author-link4=Timothy Ferris|last5=Lightman|first5=Alan|author-link5=Alan Lightman|title=The Future of Spacetime|publisher=W. W. Norton|year=2002|isbn=978-0-393-02022-9|url=https://books.google.com/books?id=LlVcB7rz4mkC&pg=PA750}}</ref>{{rp|150}}

==== Different spacetime geometries ====
The theory of [[general relativity]] describes the universe under a system of [[Einstein field equations|field equations]] that determine the [[Metric (general relativity)|metric]], or distance function, of spacetime. There exist exact solutions to these equations that include [[closed time-like curve]]s, which are [[world line]]s that intersect themselves; some point in the causal future of the world line is also in its causal past, a situation that can be described as time travel. Such a solution was first proposed by [[Kurt Gödel]], a solution known as the [[Gödel metric]], but his (and others') solution requires the universe to have physical characteristics that it does not appear to have,<ref name="Thorne1994"/>{{rp|499}} such as [[Mach's principle|rotation]] and lack of [[Hubble expansion]]. Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched.<ref name="Hawking">S. W. Hawking, ''Introductory note to 1949 and 1952'' in Kurt Gödel, ''Collected works'', Volume II (S. Feferman et al., eds).</ref>

==== Wormholes ====
{{main|Wormhole}}
Wormholes are a hypothetical warped spacetime permitted by the [[Einstein field equations]] of general relativity.<ref name="Visser1996">{{cite book|last=Visser|first=Matt|author-link=Matt Visser|title=Lorentzian Wormholes|publisher=Springer-Verlag|year=1996|isbn=978-1-56396-653-8}}</ref>{{rp|100}} A proposed time-travel machine using a [[Wormhole#Traversable wormholes|traversable wormhole]] would hypothetically work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced [[Vehicle propulsion|propulsion system]], and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both these methods, [[time dilation]] causes the end of the wormhole that has been moved to have aged less, or become "younger", than the stationary end as seen by an external observer; however, time connects differently ''through'' the wormhole than ''outside'' it, so that [[Synchronization|synchronized]] clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.<ref name="Thorne1994" />{{rp|502}} This means that an observer entering the "younger" end would exit the "older" end at a time when it was the same age as the "younger" end, effectively going back in time as seen by an observer from the outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine;<ref name="Thorne1994" />{{rp|503}} in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backward in time.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with [[negative energy]], often referred to as "[[exotic matter]]". More technically, the wormhole spacetime requires a distribution of energy that violates various [[energy condition]]s, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition,<ref name="Visser1996" />{{rp|101}} and many physicists believe that the required negative energy may actually be possible due to the [[Casimir effect]] in quantum physics.<ref name="casimir">{{cite web|url=http://www.npl.washington.edu/av/altvw69.html|title=NASA Goes FTL Part 1: Wormhole Physics|work=Analog Science Fiction & Fact Magazine|year=1994|access-date=December 2, 2006|last1=Cramer|first1=John G.|author-link=John G. Cramer|archive-url=https://web.archive.org/web/20060627211046/http://www.npl.washington.edu/av/altvw69.html <!-- Bot retrieved archive -->|archive-date=June 27, 2006}}</ref> Although early calculations suggested that a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.<ref name="negative energy">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|author2=Sayan Kar|author3=Naresh Dadhich|title=Traversable wormholes with arbitrarily small energy condition violations|journal=[[Physical Review Letters]]|volume=90|year=2003|issue=20|pages=201102.1–201102.4|doi=10.1103/PhysRevLett.90.201102|arxiv=gr-qc/0301003|bibcode=2003PhRvL..90t1102V|pmid=12785880|s2cid=8813962}}</ref>

In 1993, [[Matt Visser]] argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.<ref name="visser_1">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|title=From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture|journal=Physical Review D|volume=47|year=1993|issue=2|pages=554–565|doi=10.1103/PhysRevD.47.554|pmid=10015609|arxiv=hep-th/9202090|bibcode=1993PhRvD..47..554V|s2cid=16830951}}</ref> Because of this, the two mouths could not be brought close enough for [[Causality (physics)|causality]] violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "[[Roman ring]]" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.<ref name="visser_2">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|title=Traversable wormholes: the Roman ring|journal=Physical Review D|volume=55|year=1997|issue=8|pages=5212–5214|doi=10.1103/PhysRevD.55.5212|arxiv=gr-qc/9702043|bibcode=1997PhRvD..55.5212V|s2cid=2869291}}</ref>

==== Other approaches based on general relativity ====
Another approach involves a dense spinning cylinder usually referred to as a [[Tipler cylinder]], a GR solution discovered by [[Willem Jacob van Stockum]]<ref name="stockum">{{cite journal|first=Willem Jacob|last=van Stockum|author-link=Willem Jacob van Stockum|url=http://www-lorentz.leidenuniv.nl/history/stockum/Proc_R_Soc_Edinb_57_135_1937.jpg|title=The Gravitational Field of a Distribution of Particles Rotating about an Axis of Symmetry|year=1936|journal=Proceedings of the Royal Society of Edinburgh|url-status=dead|archive-url=https://web.archive.org/web/20080819215608/http://www-lorentz.leidenuniv.nl/history/stockum/Proc_R_Soc_Edinb_57_135_1937.jpg|archive-date=2008-08-19}}</ref> in 1936 and [[Kornel Lanczos]]<ref name="lanczos">{{cite journal|first=Kornel|last=Lanczos|author-link=Kornel Lanczos|doi=10.1023/A:1010277120072|title=On a Stationary Cosmology in the Sense of Einstein's Theory of Gravitation|year=1924 <!--republished in 1997-->|journal=General Relativity and Gravitation|publisher=Springland Netherlands|volume=29|issue=3|pages=363–399|s2cid=116891680}}</ref> in 1924, but not recognized as allowing closed timelike curves<ref name="Earman">{{cite book|last=Earman|first=John|title=Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes|publisher=Oxford University Press|year=1995|isbn=978-0-19-509591-3|bibcode=1995bcws.book.....E}}</ref>{{rp|21}} until an analysis by [[Frank Tipler]] in 1974.<ref name="tipler">{{cite journal|first=Frank J|last=Tipler|author-link=Frank J. Tipler|title=Rotating Cylinders and the Possibility of Global Causality Violation|journal=Physical Review D|volume=9|year=1974|issue=8|page=2203|doi=10.1103/PhysRevD.9.2203|bibcode=1974PhRvD...9.2203T|s2cid=17524515}}</ref> If a cylinder is infinitely long and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. 

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the [[weak energy condition]] is satisfied, meaning that the region contains no matter with negative energy density ([[exotic matter]]). Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough,<ref name="Earman" />{{rp|169}} he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."<ref name="futureofspacetime" />{{rp|96}} This result comes from Hawking's 1992 paper on the [[chronology protection conjecture]], which Hawking states as "The laws of physics do not allow the appearance of closed timelike curves."<ref name="chronology protection" />

=== Quantum physics ===
{{main|Quantum mechanics of time travel}}
==== No-communication theorem ====
When a signal is sent from one location and received at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of [[Relativity of simultaneity|simultaneity]] in the theory of relativity show that all reference frames agree that the transmission-event happened before the reception-event. When the signal travels faster than light, it is received ''before'' it is sent, in all reference frames.<ref name="Jarrell">{{cite web|url=http://www.physics.uc.edu/~jarrell/COURSES/ELECTRODYNAMICS/Chap11/chap11.pdf|title=The Special Theory of Relativity|access-date=October 27, 2006|last1=Jarrell|first1=Mark|pages=7–11|archive-url=https://web.archive.org/web/20060913173236/http://www.physics.uc.edu/~jarrell/COURSES/ELECTRODYNAMICS/Chap11/chap11.pdf <!-- Bot retrieved archive -->|archive-date=September 13, 2006}}</ref> The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a [[tachyonic antitelephone]].<ref>{{cite journal|last=Kowalczyński|first=Jerzy|date=January 1984|title=Critical comments on the discussion about tachyonic causal paradoxes and on the concept of superluminal reference frame|journal=[[International Journal of Theoretical Physics]]|publisher=[[Springer Science+Business Media]]|volume=23|issue=1|pages=27–60|doi=10.1007/BF02080670|bibcode=1984IJTP...23...27K|s2cid=121316135}}</ref>

Quantum-mechanical phenomena such as [[quantum teleportation]], the [[EPR paradox]], or [[quantum entanglement]] might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the [[Bohm interpretation]] presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.<ref name="Bohm">{{cite web|url=http://plato.stanford.edu/entries/qm-bohm/|title=Bohmian Mechanics|date=March 27, 2017|access-date=April 26, 2017|last1=Goldstein|first1=Sheldon|archive-date=January 12, 2012|archive-url=https://web.archive.org/web/20120112030926/http://plato.stanford.edu/entries/qm-bohm/|url-status=live}}</ref> This effect was referred to as "[[action at a distance (physics)#spooky action at a distance|spooky action at a distance]]" by Einstein.

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern [[Quantum field theory|quantum field theories]], and therefore modern theories do not allow for time travel or [[Superluminal communication|FTL communication]]. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used.<ref name="Nielsen and Chuang">{{cite book|last1=Nielsen|last2=Chuang|first1=Michael|first2=Isaac|title=Quantum Computation and Quantum Information|url=https://archive.org/details/quantumcomputati00niel_056|url-access=limited|publisher=Cambridge|year=2000|page=[https://archive.org/details/quantumcomputati00niel_056/page/n55 28]|isbn=978-0-521-63235-5}}</ref> The [[no-communication theorem]] also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.

==== Interacting many-worlds interpretation ====
A variation of [[Hugh Everett]]'s [[many-worlds interpretation]] (MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel.<ref>{{citation|title=Time Travel and Modern Physics|date=December 23, 2009|author1=Frank Arntzenius|author2=Tim Maudlin|url=http://plato.stanford.edu/entries/time-travel-phys/|encyclopedia=Stanford Encyclopedia of Philosophy|access-date=August 9, 2005|archive-date=May 25, 2011|archive-url=https://web.archive.org/web/20110525025650/http://plato.stanford.edu/entries/time-travel-phys/|url-status=live}}</ref> The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories.<ref name="many-worlds">{{cite web|url=http://plato.stanford.edu/entries/qm-manyworlds/|title=Many-Worlds Interpretation of Quantum Mechanics|date=January 17, 2014|access-date=April 26, 2017|last1=Vaidman|first1=Lev|archive-date=December 9, 2019|archive-url=https://web.archive.org/web/20191209220612/http://plato.stanford.edu/entries/qm-manyworlds/|url-status=live}}</ref> However, some variations allow different universes to interact. This concept is most often used in science-fiction, but some physicists such as [[David Deutsch]] have suggested that a time traveler should end up in a different history than the one he started from.<ref name="deutsch">{{cite journal|last=Deutsch|first=David|author-link=David Deutsch|year=1991|title=Quantum mechanics near closed timelike lines|journal=Physical Review D|volume=44|issue=10|pages=3197–3217|doi=10.1103/PhysRevD.44.3197|pmid=10013776|bibcode=1991PhRvD..44.3197D|s2cid=38691795|url=http://pdfs.semanticscholar.org/8e99/3e3e9b0952198a51ed99c9c0af3a31f433df.pdf|archive-url=https://web.archive.org/web/20190228075930/http://pdfs.semanticscholar.org/8e99/3e3e9b0952198a51ed99c9c0af3a31f433df.pdf|url-status=dead|archive-date=2019-02-28}}</ref><ref>{{citation|author=Pieter Kok|title=Time Travel Explained: Quantum Mechanics to the Rescue?|url=https://www.youtube.com/watch?v=uz9eLjO2BrA |archive-url=https://ghostarchive.org/varchive/youtube/20211211/uz9eLjO2BrA| archive-date=2021-12-11 |url-status=live|date=February 3, 2013}}{{cbignore}}</ref> On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one.<ref name="Hawking warp"/> The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI". Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.<ref name="Everett MWI">{{cite journal|last=Everett|first=Allen|title=Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics|journal=Physical Review D|volume=69|issue=124023|pages=124023|year=2004|doi=10.1103/PhysRevD.69.124023|arxiv=gr-qc/0410035|bibcode=2004PhRvD..69l4023E|s2cid=18597824}}</ref>

=== Experimental results ===
Certain experiments carried out give the impression of reversed [[causality]], but fail to show it under closer examination.

The [[delayed-choice quantum eraser]] experiment performed by [[Marlan Scully]] involves pairs of [[Quantum entanglement|entangled]] [[photon]]s that are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the [[double-slit experiment]]. Depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an [[Interference (wave propagation)|interference pattern]] is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can be observed only after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, only by gathering classical information from the entire system; thus causality is preserved.<ref name=Greene2004>{{cite book|last=Greene|first=Brian|title=The Fabric of the Cosmos|url=https://archive.org/details/fabricofcosmossp00gree|url-access=registration|year=2004|publisher=Alfred A. Knopf|isbn=978-0-375-41288-2|pages=[https://archive.org/details/fabricofcosmossp00gree/page/197 197–199]}}</ref>

The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry, but a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see [[Fourier analysis]]), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light,<ref name="gauthier">{{cite news|last=Wright|first=Laura|title=Score Another Win for Albert Einstein|magazine=[[Discover (magazine)|Discover]]|date=November 6, 2003|url=http://discovermagazine.com/2003/nov/score-another-win-for-einstein1106|access-date=October 21, 2009|archive-date=June 12, 2018|archive-url=https://web.archive.org/web/20180612193137/http://discovermagazine.com/2003/nov/score-another-win-for-einstein1106|url-status=live}}</ref> so this experiment is understood not to violate causality either.

The physicists [[Günter Nimtz]] and Alfons Stahlhofen, of the [[University of Koblenz]], claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which [[microwave]] photons traveled "instantaneously" between a pair of prisms that had been moved up to {{convert|3|ft|m|abbr=on}} apart, using a phenomenon known as [[quantum tunneling]]. Nimtz told ''[[New Scientist]]'' magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. [[Aephraim M. Steinberg]], a quantum optics expert at the [[University of Toronto]], Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.<ref name="nimtz">{{cite news|last=Anderson|first=Mark|title=Light seems to defy its own speed limit|magazine=[[New Scientist]]|volume=195|issue=2617|page=10|date=August 18–24, 2007|url=https://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php|access-date=2018-09-18|archive-date=2018-06-12|archive-url=https://web.archive.org/web/20180612142609/https://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php|url-status=live}}</ref>

[[Shengwang Du]] claims in a peer-reviewed journal to have observed single photons' [[Precursor (physics)|precursor]]s, saying that they travel no faster than ''[[Speed of light|c]]'' in a vacuum. His experiment involved [[slow light]] as well as passing light through a vacuum. He generated two single [[photon]]s, passing one through [[rubidium]] atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at ''c'' in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than ''c'' and, thus, no possibility of violating causality.<ref>{{citation|url=http://www.ust.hk/eng/news/press_20110719-893.html|publisher=The Hong Kong University of Science & Technology|access-date=September 5, 2011|title=HKUST Professors Prove Single Photons Do Not Exceed the Speed of Light|date=July 17, 2011|archive-date=February 25, 2012|archive-url=https://web.archive.org/web/20120225022608/http://www.ust.hk/eng/news/press_20110719-893.html|url-status=live}}</ref>

=== Absence of time travelers from the future ===
Many have argued that the absence of time travelers from the future demonstrates that such technology will never be developed, suggesting that it is impossible. This is analogous to the [[Fermi paradox]] related to the absence of evidence of extraterrestrial life. As the absence of extraterrestrial visitors does not categorically ''prove'' they do not exist, so the absence of time travelers fails to prove time travel is physically impossible; it might be that time travel is physically possible but is never developed or is cautiously used. [[Carl Sagan]] once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers.<ref name="sagan-nova" /> Some versions of general relativity suggest that time travel might only be possible in a region of [[spacetime]] that is warped a certain way,{{clarify|date=April 2022}} and hence time travelers would not be able to travel back to earlier regions in spacetime, before this region existed. [[Stephen Hawking]] stated that this would explain why the world has not already been overrun by "tourists from the future".<ref name="Hawking warp">{{cite web|url=https://www.hawking.org.uk/in-words/lectures/space-and-time-warps|title=Space and Time Warps|year=1999|access-date=September 23, 2020|last1=Hawking|first1=Stephen|archive-date=October 31, 2020|archive-url=https://web.archive.org/web/20201031050328/https://www.hawking.org.uk/in-words/lectures/space-and-time-warps|url-status=live}}</ref>

[[File:WelcomeKrononauts Artforum Jan1980 p.90 800x600.png|thumb|Advertisement placed in a 1980 edition of ''[[Artforum]]'', advertising the Krononauts event]]
Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's [[Destination Day]], [[Massachusetts Institute of Technology|MIT]]'s [[Time Traveler Convention]] and Stephen Hawking's [[Hawking's time traveller party|Reception For Time Travellers]] heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet.<ref>{{citation|url=https://www.wired.com/2005/05/time-travelers-welcome-at-mit/|title=Time Travelers Welcome at MIT|author=Mark Baard|date=September 5, 2005|publisher=[[Wired (magazine)|Wired]]|access-date=June 18, 2018|archive-date=June 18, 2018|archive-url=https://web.archive.org/web/20180618125948/https://www.wired.com/2005/05/time-travelers-welcome-at-mit/|url-status=live}}</ref><ref>{{cite news |title=Stephen Hawking service: Possibility of time travellers 'can't be excluded' |url=https://www.bbc.co.uk/news/uk-england-cambridgeshire-44073903 |access-date=18 October 2024 |work=BBC News |date=12 May 2018}}</ref> In 1982, a group in [[Baltimore]], [[Maryland]], identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future.<ref>{{cite news|last=Franklin|first=Ben A.|date=March 11, 1982|url=https://select.nytimes.com/gst/abstract.html?res=F70E13FD395F0C728DDDAA0894DA484D81|title=The night the planets were aligned with Baltimore lunacy|archive-url=https://web.archive.org/web/20081206170526/http://select.nytimes.com/gst/abstract.html?res=F70E13FD395F0C728DDDAA0894DA484D81|archive-date=2008-12-06|work=[[The New York Times]]}}</ref><ref>"Welcome the People from the Future. March 9, 1982". Ad in ''[[Artforum]]'' p. 90.</ref>

These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so far—no time travelers are known to have attended either event. Some versions of the [[many-worlds interpretation]] can be used to suggest that future humans have traveled back in time, but have traveled back to the meeting time and place in a [[Multiverse|parallel universe]].<ref>{{cite journal|author1=Jaume Garriga|author2=Alexander Vilenkin|doi=10.1103/PhysRevD.64.043511|year=2001|volume=64|issue=4|page=043511|journal=Phys. Rev. D|arxiv=gr-qc/0102010|bibcode=2001PhRvD..64d3511G|title=Many worlds in one|s2cid=119000743}}</ref>