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This is of course a wild hypothetical question and there is probably no good answer. Suppose a police force obtained a time machine and wanted to use this to obtain evidence to investigate crimes and convict criminals. Would they be able to use this evidence in court? I am thinking about a few ways the time-travelers might obtain potentially tainted evidence:

  1. The officers have obtained a search warrant to search a certain place, perhaps a suspected hideout that a suspect may have used in the course of planning or committing the crime. Currently, there is nothing incriminating in the hideout, but the officers use their time machine to go back to a time, before the warrant was issued, and find incriminating evidence. Would this evidence be admissible? Similarly, could time-traveling officers follow a suspect with no probable cause obtained before the crime if they know from the future that this is a person of interest?
  2. Would anything stop these officers from traveling back in time, watching the crime as it happens, and using either their eyewitness testimony or a videotape as evidence? I'm assuming for now that they're not messing with the past by trying to interfere to stop the crime.

Thank you for entertaining this silly question.

  • 1
    I suggested this gets moved to Worldbuilding, I believe "what if this narrative element exists" questions are on-topic there if they aren't overly broad. – IllusiveBrian Dec 3 at 2:42
  • What if we could read people's minds to see whether they were committing perjury? What if we could see through walls to see evidence that is now "in plain view? What if we could resurrect from the dead and use the murder victim as a witness? This question is nonsense and does not belong here. – Nij Dec 3 at 4:35
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In part, we don't know because there are currently no rules that address certain outcomes, so it will depend on who is on the Supreme Court when the issue is raised. A warrantless search will not be legal beyond current doctrines regarding crime in progress and imminent danger, even if it involves time travel. So you will need a warrant, and you will need probable cause to get it. Currently, if you break into a person's house to discover that there is a body there, you can't get a warrant to legitimize that illegal search. Adding time travel does not change anything. In situation 1, I assume they have probable cause and a warrant but the evidence was destroyed by the time of the search at time T+n. As long as prior time T is still after the crime, a warrant to search at time T would not be a problem. In scenario 2, there is nothing preventing them from stopping the crime or arresting the perpetrators in the act. However, if they travel back in time and break in to a suspect's house in order to witness the crime, that is an illegal search. You could likewise arrest a person before he escapes to the phantom zone, if you have probable cause that he had committed a crime – by the time of the arrest.

Forward time travel poses a more serious challenge, as articulated in Minority Report. It would, or should, be very difficult to issue an arrest warrant at time T based on knowledge of a crime committed at T+n. Because of the arrest, the crime was not committed and there was no probable cause, so there should have been no warrant (oh no, paradox).

  • You mean Minority Report, not Majority Report :) Sometimes the squad in the movie would prevent a crime in progress, so the person would still be guilty of the attempt or conspiracy. I haven't seen the movie in awhile, but people could apparently also be arrested just if the telepaths thought they would commit a crime even if there was no corroborating evidence, which certainly wouldn't fly in reality. – IllusiveBrian Dec 3 at 2:38
  • @user6726 Use civil commitment and you can forget about all of that pesky probable cause, due process crime stuff. Before you know it, "Paradox Lost!" Plus you can keep them locked up forever (oops! I meant to say: until they are cured). Best of all, no need to lose any sleep: SCOTUS says it's not punitive. – Just a guy Dec 3 at 6:24

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