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Time Travel in Science Fiction has multiple variants to cope with its paradoxes such as the Grandfather Paradox, the most famous of them being

  • the closed time loop, where anything that seems to modify the past turns out to already have been part of the past
  • the parallel universe theory, where there basically is no paradox since the time travelling creates a new timeline existing in parallel with the original one
  • the Back To The Future vanishing act, where the time travellers are basically slowly replaced by the new futures travellers, should they still be existing and travelling back in time, otherwise simply vanishing (this sentence should actually require some new grammar to better express the "will have would happening"-ish stuff)

To my knowledge, Star Trek's time travel usually obeys the multiverse theory, and another question's answer lists some post-TOS episodes where the conservation/loss of memories is treated. But my question concerns the Temporal Prime Directive (TPD):

When the TPD was established, were there any precedence cases known where the past was modified, and therefore, is the (pre-2009) universe a future that only came to be due to some time travelling?

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    I don't think ST is nearly as consistent in it's use of multiverse theory as you think. In most episodes it's impossible to tell if the "alternate future" is another universe or an aborted timeline. In others, it's clearly the latter: e.g. in "City on the Edge of Forever" when Bones changes the past it changes the present for the rest of the crew. – KutuluMike Jul 10 '15 at 16:52
  • @MichaelEdenfield "I hate temporal mechanics." – Izkata Jul 10 '15 at 23:56
  • I don't feel the wording on the headline question is clear. (And I think you should be asking whether the writers know what kind of time travel they have, not Starfleet.) – ThePopMachine Jul 11 '15 at 1:06
  • @ThePopMachine The wording is certainly optimizable, but I do want to know about Starfleet, not the Writers - why didn't they just say "Screw that, whatever rewritten future you return to is the one you're in anyway"? – Zommuter Jul 11 '15 at 13:26
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Yes, for example in Star Trek IV: The Voyage Home Mr Scott gives a 20th century scientist the formula for transparent aluminium, which history in the main Star Trek time stream does say was invented by that scientist.

  • I remember that scene - but wasn't that more of a tongue-in-cheek reply to McCoy à la "He {now will have} invented it"? stating that no one in the potentially slightly altered future will be able to prove him wrong? – Zommuter Jul 10 '15 at 16:53
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    @Zommuter: It's unclear which it is in the movie. The novelization makes it explicit that he was the inventor all along, if you consider that canonical... – Micah Jul 10 '15 at 16:57
  • @Micah I haven't read that, how canonical is that generally considered? Does the novelization state anything about Scotty at least influencing the date of invention, or whether the invention wouldn't have occured without this time travel? – Zommuter Jul 10 '15 at 17:00
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    As far at the creation of transparent aluminum would be concerned, back in the future they return to, the scientist who they gave the information to will have ALWAYS been its inventor. Since Scotty didn't have a tricorder with the information, he was making a guess that the change, if it occurred would be both minor and relatively harmless to the future they returned to. – Thaddeus Howze Jul 10 '15 at 21:48
  • One can of course query why Scotty didn't have a tricorder with the information. Right now, in 2015, I have an offline copy of Wikipedia on my phone that can be used without a network connection, and in fact includes information on transparent aluminium. So by the time of Star Trek all tricorders would surely include vast databases of information. – Mike Scott Jul 11 '15 at 5:56
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For a less-ambiguous example, in the first half of the Voyager two-parter Future's End, Janeway and Chakotay come to the conclusion you're asking about:

(Both reading from a Chronowerx Industries computer)

Janeway: Starling's computer designs were inspired by technology from the timeship. He introduced the very first isograted circuit in 1969. Two years after Braxton's ship crash-landed.

Chakotay: And every few years, there's been an equally revolutionary advance in computers, all from Chronowerx Industries, all based on Starling's crude understanding of 29th-century technology.

Janeway: Are you thinking what I'm thinking, Chakotay?

Chakotay: I wish I weren't.

Janeway: The computer age of the late 20th century-

Chakotay: -shouldn't have happened.

Janeway: But it did, and it's part of our history. All because of that time ship.

The point where the timeline was modified happened long before the Temporal Prime Directive was established, but the ship that went back in time (to Voyager's period, and then accidentally sent both even further back) was from centuries after it was established.

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If you have multiple timelines, you have therefore multiple Starfleets (one per timeline) and is quite possible that each Starfleet might have a different knowledge of the other Starfleets; for example one Starfleet might know that they have one type of time-travel while another would have been unable to answer that question.

Futhermore, the Grandfather Paradox is not really a paradox. When you do time travel in the past, you use energy to travel back in time and the paradox can then simply be seen as a way of converting energy into a particular form matter using information provided by the time-travel machine.

Think of it like a teleporter where you would make a second copy of someone (or a changed copy if you manipule the information) by re-using the information used for the first transfert.; therefore creating a new person from "nothing" by converting energy into mass. Of course, you can argue that this can be possible or not depending on the way that the teleporter is working but in the case of time-travelling, all you need to know is that you are injecting energy into the time-travel machine each time you use it.

Each "present" will be different but this simply would reflect the loss of mass that had to be converted into energy for each time travel into the past (or the futur). The grandfather paradox exists only if your time-travel machine only need a little amount of energy to work and because this machine could use its own supply of matter as a source of energy, the total amount doesn't necessarily equate the amount of direct energy that you are putting into it.

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