What is “Retrocausality”?
In: Physics
You are getting into time travel and to an extent causality loops. Physicists are exploring this. It gets very hairy very fast. Physicists are still at the very very very theoretical stages of exploring this. Whether or not it is even *possible* is still completely unknown. Physicists are at the question of “if we send an electron back in time, is it positive during the trip?” So physicists haven’t gotten to the next paragraph yet.
Normal causality works like dominoes: A knocks down B knocks down C in a nice neat row, one after the other. You can watch it happen and understand it as it goes.
Retrocausality….. describes the idea that an action taken today *could* affect actions that have already taken place in the past.
So knocking down domino C today *could* change how domino A falls yesterday.
Or a slightly different way of thinking about it is: after seeing how domino C falls, I can reach back in time to tweak how domino A falls and thus tweak domino C.
So understandability, changing how domino A falls by tweaking domino C can lead to all sorts of issues.
Yes, this gets beyond messy very fast.
“Back to the Future” shows this concept in a rough sense: hero goes
Back in time and prevents himself from being born and then has to correct it before he is wiped out of the timeline.
You can then get into all sorts of issues with this. It is referred to as “causality loops” (there are many variations on this) Example: If you go back in time and have sexual relations with your mother and she gives birth to you – Who is your father?
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If I may quote a fictional character on the subject: “An excellent reason not to meddle in the natural course of time, wouldn’t you say? Meddling with time is an irrationally, outrageously, catastrophically dangerous and costly business. I encourage you to avoid it at all costs.”
There are two areas of retro-causality that have some relation to physical *theory* (but not experiment, and physicists for the most part are not seriously exploring it).
General relativity as currently stated allows for paths through spacetime that end up at a point in your own past. These paths involve some very exotic geometries or states of matter, neither of which have ever been observed. It is known that GR is only an effective theory, because it has mathematical inconsistencies, and some of these exotic solutions have been heuristically shown to break down when quantum mechanics is incorporated.
Quantum mechanics features some apparently nonlocal effects under some interpretations. Nonlocal on its own just means the effect travels faster than light, but whenever that is the case it’s possible to construct something that apparently has an effect at some point in the past. The issue is that this arises in interpretations of quantum mechanics, and not in the theory itself, which simply predicts probabilities. As such, we can show rigorously that it is not possible to use this form of retrocausality to send signals back in time, even if that interpretation were true.
The rest – movie-style time travel – are simply fiction with no relation to physical theories.
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