Recently I've started reading about Time Travel after watching it in Harry Potter and the Prisoner of Azkaban. There is a line on this subject in the Wikipedia page on 'Causal Loop' i.e.

The term "time loop" is sometimes used to refer to a causal loop, but although they appear similar, causal loops are unchanging and self-originating, whereas time loops are constantly resetting.

The above highlighted idea is present in various other articles as well in slightly different words. However, everywhere it's just this line and then full stop. No explanation. No elaboration.

Can anybody explain what this statement really means?

  • 2
    I'm not sure this is an appropriate question for SF.SE. But I think they're trying to say that a causal loop is a paradox: X caused me to exist, but X only happened because I went back in time to cause it - By His Bootstraps or All You Zombies. Whereas a time loop has no causal implications, it's just the same events eternally resetting: think Groundhog Day. Aug 23, 2019 at 12:13
  • @DanielRoseman Even I was a little skeptical posting the question here but then I thought that this platform deals with questions on Sci-Fi and Time Travel is a widely recognised concept in Fiction so I posted it. However, if you've an idea where I can get the right audience for this question, please do mention. I'll be really thankful. Also, thanks for sharing share your input. It indeed opened some window in my mind to ponder better over this statement..
    – CCCC
    Aug 23, 2019 at 12:22

2 Answers 2


Causal loop - the changes brought forth by the loop create the conditions that cause the loop. Nothing changes, ever.

Time loop - a change in the past will (or at least might) create a different future. Do not, rpt. not kill your grandfather, or you might actually disappear.

  • Eike Pierstorff: Your answer explained me the 'Causal Loop' bit of my question. Daniel Roseman: Your input directed me in the right direction for the 'Time Loop' part and @a4android You gave me the idea to go through the footnotes where I found this-books.google.co.in/books/about/… and it all makes sense now. Thanks a lot everybody. scifi.stackexchange.com never disappoints!!
    – CCCC
    Aug 23, 2019 at 15:12

To quote this footnote, number 29, in the Wikipedia on Causal Loops:

Novikov, Igor (1983). Evolution of the Universe, p. 169: "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."

This suggests that causal loops and time loops are equivalent. The main difference may be that causal loops are strictly causal sequences of events with the events repeating exactly the same sequence without any change. The final causal outcome will be fixed.

A time loop consists of a causal sequence of events that do not strictly repeat events exactly, but preserve the final causal outcome. Effectively the final causal outcome will be fixed, in the sense that it cannot be changed.

Both loops are sufficiently close in kind to each other to almost be equivalent and barely distinguishable from each other. The distinction between them is arbitrary.

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