Editing Time travel (section)

=== 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>