IMHO, time travel (into the past) is fantasy. It is proven to be nigh-impossible, affects causality, there are dozens of resources discussing the vagarities and impossibilities of time-traveling to the past.

Therefore, any worldbuilding involving traveling to the past is hardly based on science.

How putting both tags together make sense? What kind of answers it provokes?

  • $\begingroup$ You can also travel in time... In the future :). $\endgroup$ Jul 1, 2022 at 0:15
  • $\begingroup$ @Tortliena I had to double-check that the Q states "to the past". I already have the power to time-travel into the future eight hours every day when I sleep. I close my eyes and when I open it, a whole night has passed. <poke tongue emoji> $\endgroup$ Jul 1, 2022 at 3:04
  • $\begingroup$ Your question perhaps... But not the tag. If a question asks about making a rehearsal of the "Planet of the Apes", that's time-travelling for the astronaut using science. $\endgroup$ Jul 1, 2022 at 10:47
  • $\begingroup$ Then it would be in another Meta post. $\endgroup$ Jul 4, 2022 at 15:24
  • $\begingroup$ No, because that's what the tag is that determines if it's possible to combine it with science-based, not your question's definition of the tag ^^. If said tag allows time travel to the future and said travel to the future is based on science, then the answer is "yes", regardless of your definition. $\endgroup$ Jul 4, 2022 at 15:30

3 Answers 3


It all depends on what the question asks. If they clearly define the rules for their time travel and are asking for some science based insight about some consequence of their rules then there is no reason for the tags not to make sense.



It does in the sense that humanity has explored the idea and plausibility of time travel mathematically, and that's valid science.

So what's the problem?

IMO, the primary problem is that over the years this Stack has become "physics.se lite" in that people far to consistently demand that questions and answers reflect the reality of the Real World as if that reality, science as we know it today, is the only "science" there will ever be.

It's a weakness in the Stack's culture, meaning that people are taking themselves (and science) a bit too seriously.

So, yes! I believe it's perfectly valid to combine the and tags. I think it's perfectly valid to combine the and tags.

But you should have appropriate expectations...

Because what science we know is based on speculative mathematics — and that mean's it's all you're gonna get. If you pair those tags expecting us to explain how a real and actual time machine would work or how the various paradoxes of time travel really and truly would be resolved...

Well, we'll chuckle first, then do our best, and I'll likely post a link to Advice concerning questions asking HOW to implement a technological procedure or device.


I can easily imagine a question where the time travel bit is only used to set up the initial conditions, and then everything is supposed to happen in as realistic a fashion as possible, while keeping the readers / watchers / players interested and engaged.

As a well-known example of [science-based] world set in motion by [time-travel] consider the wildly successful 1632 shared universe created by Eric Flint and many others.


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