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In the Star Trek TNG episode "Times Arrow", parts one and two, recall the following:

  • Data's head was found in an excavation on Earth that dates from the 19th century (it actually is Data's head).
  • Data, Picard, Crusher, Worf, and others go back in time to the 19th century to investigate.
  • Guinan (her 500-year younger self) was living in San Francisco in the 19th century.
  • This is where Guinan first meets Picard, but not where Picard first meets Guinan.
  • It turns out that alien time travelers are sucking energy out of people on Earth from the 19th century.
  • In the process, Data is caught in an altercation which essentially blows his head off.

Here's where things get especially interesting...

  • In an effort to stop these aliens, all of the away team along with Samuel Clemens, except Picard and Guinan end up back in the future to try to destroy the aliens time travel site on their planet.
  • Picard, still in the 19th century finds out from one of the aliens that if the Enterprise in the future fires upon the alien time travel site, the energy from the ship's phasers will only serve to increase the aliens' power.
  • Commander LaForge manages to reassemble the 500 year old Data head, after removing a suspicious piece of metal lodged in one of Data's cranial ports. "How did that get in there?" LaForge asks.
  • We are then taken back in time to see Picard is responsible for putting the metal piece in there. He also enters a binary message into Data's static memory warning the Enterprise not to fire on the alien site.
  • Data (reassembled) receives this message, and the crew make a plan to destroy the alien site by making a tweak to the ship's weapons.
  • Before destroying the site, Samuel Clemens goes back in time and allows Picard to return to the 24th century

Everybody wins! What an awesome episode!

But here's the rub

From Picard's perspective, he simply entered the binary message into Data's head, and then was immediately rescued. I should think that Picard would have had to have lived a lifetime in the 19th century, then died, and then 500 years later, Data's head would be found. At this point, the static memory in Data's head warning the Enterprise would be there legitimately and the episode could continue as normal.

So how does entering the binary message into Data's static memory seemingly transcend the space time continuum and make it to the future?

  • 8
    I hate temporal mechanics... – Jason Baker Feb 23 '15 at 21:24
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    Because Data's head exists in both places and at both times. By making a change to his head in 1800, the change is visible in 2300. Picard doesn't need to live and die in the 1800s since he's already created an exit point. – Valorum Feb 23 '15 at 21:27
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    @matrixugly Not if we assume that Time's Arrow is a stable time loop, which we have no evidence to refute (not from my admittedly hazy recollection, at least) – Jason Baker Feb 23 '15 at 21:36
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    @matrixugly - No, Geordi only discovers the shard when the head is being thoroughly examined. That doesn't mean that it wasn't in there the whole time. Picard would have been aware that the examination hadn't taken place before he left the ship meaning that any internal changes he makes to Data's head wouldn't be discovered until later. That makes it the perfect tool to communicate with the future. – Valorum Feb 23 '15 at 21:38
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    Yes, except that that continued existence (the alternate timeline where picard lived and died in the 19th century) never happened. Or rather it un-happened. – Valorum Feb 23 '15 at 21:47
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Welcome to the Grandfather Paradox. Start with the Wikipedia article.

There are at least four ways to handle this:

1) Picard's actions form a self-determining loop. That is, the origins of his actions are contained entirely with the loop. The classic along these lines is Heinlein's All You Zombies.

2) When Picard altered Data's memory, he created a parallel universe (timeline) in which the data exists/existed/will exist. When the team returned from the past, they entered this new timeline. In the old timeline, they never returned.

3) When Picard altered Data's memory, he created a history which overwrote the original history which caused him to go back in time. The original history, in which Data's memory was not modified, ceases to "exist", as far as the word applies.

4) There's no problem. The away team went back in time without having performed a detailed examination of Data's head. So it had always been modified, and Picard's actions had always occurred. Again, as far as the word "always" applies.

An excellent discussion of the peculiarities of time travel (from a purely speculative viewpoint, of course) is Larry Niven's The Theory and Practice of Time Travel

  • This page and this one are also good for those interested in discussion of different fictional models of time travel. – Hypnosifl Feb 24 '15 at 0:28
  • More information on option number one can be found under Novikov self-consistency principle – Deltharis Feb 24 '15 at 12:29
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    For a story-telling point of view, option #2 isn't satisfying, because this means that the villains always wind up winning (in some timeline). – user11521 Feb 10 '17 at 16:29
  • @Michael - In some cases that is very satisfying. – WhatRoughBeast Feb 11 '17 at 13:32
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    It's #4. All of the content in the episode already happened before the enterprise started sending people into the past. Guinan has a knowledge of all of these events and knows Picard will make it back. "If you don't go, we'll never meet." Also, Data's head was found before they sent anyone back in time with damage that lines up exactly with the body they later recovered. This is additional proof that the events always played out this way in the past before the episode began. – Kalamane Jan 23 at 16:57
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Data's head begins life with Data's body. At some point, the whole body goes back in time. The head is severed and left behind. Picard (also left behind) plays with it to encode a message. Picard is retrieved by his colleagues in the future. 500 years later, the head is found again and brought back on board the Enterprise. It's plugged back into the body (which took a "shortcut" by not being left behind). Data relays the message, and the crew retrieve Picard.

There's only one path that the head took. It can be a little difficult to get your head around (lol) simply because (from his perspective) Picard was rescued centuries before his SOS message would be received, and because Data's head and body got back to the 24th Century via extremely different means. The head took the long way around. But Picard's fiddling with it didn't need to create an alternate timeline, or bypass a paradox, or make Picard wait (?) or anything like that. It was simply a man playing with his buddy's head.

            olden days                     23rd century       
t ════════════════════════════════════════════════════════════→

    Guinan ┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄→

    Clements ┄┄┄┄┄┄┄┄┄┄ †
                                       Data  Team (ex. Data)
                                         ↓      ↓
       ╭─────────────────────────────────╯      │
       │   ╭────────────────────────────────────╯
       ↓   │
       ¦   ↓
       ¦   ╰┄ Team (ex. Picard)
       ¦        ¦
       ¦        ╰───────────────────────────────────∎┄┄┄┄╮
       ¦                                                 ↓
       ╰┄┄┄┄→ Data's body                                ¦
                ↓                                        ¦
                ├───────────────────────────────────∎┄┄┄┄┤
                ¦                                        ¦
              Data's                                     ¦
            head (with message)                          ¦
                ¦                                        ¦
                ├┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄⁕┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄⁕
                ¦             Head discovered            ¦
                ¦                               Data recombined
                ¦                                Msg discovered
             Picard                                      ¦
                ¦                                        ╰┄╮
                ╰┄┄┄╮                                      ¦
                    ¦                                      ¦
                    ¦                                  Rescue
                    ¦                                   team
                    ¦                                      ↓
                    ¦ ╭────────────────────────────────────╯
                    ¦ │
                    ¦ │
                    ¦ │
                    ╰┄↓
                    Picard and
                    rescue team
                         │
                         ╰───────────────────────────────────∎┄┄┄→

Where solid lines (─) indicate unnatural time travel, and dotted lines (┄) indicate the more conventional way of simply sitting there until years have passed. Rightwards is absolute chronology; downwards is subjective chronology (i.e. events in the order portrayed in the episode and experienced by the characters).

You can see that Data's head, message an' all, just makes it to the future in the usual manner. Picard and the rescue team and Data's body are jumping around all over the place, making it more confusing, but the head is the one part of the puzzle that can't possibly cause anything more than the predestination paradox incited by its being found in the 23rd Century right as the episode's narrative begins.

The key salient point is that Data's head was always there, in San Francisco, with a message encoded in it. Since the beginning of the series, and since way before that. It's just that we (and the crew) didn't know that until the events of this episode (which ironically caused the whole issue in the first place!).

I should think that Picard would have had to have lived a lifetime in the 19th century, then died, and then 500 years later, Data's head would be found.

That would be a paradox. Picard did a thing. That thing had an effect: his crewmates obtaining that information. That he was rescued immediately thereafter is an effect of their time travelling. If he somehow also experienced a timeline in which he just sat there for years because his message had not got through and his crewmates no longer decided to go back in time to rescue him, then that would be a problem.

So how does entering the binary message into Data's static memory seemingly transcend the space time continuum and make it to the future?

When you write a message on a piece of paper, it's still there the next day. That's the normal flow of information through the passage of time. Presumably you are not transcending the space-time continuum in order to allow your message to make it to the future. :)

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picard doesn't have to live a life in the past, he only has to have placed his butterfly for events to continue to unfold as they should. Once the message is received in the future, and the aliens are stopped, picard can go back to the future at any time. The artifacts of being in the past, although lucky to have made it as far as that point in the future are already predetermined as a past event from picard's perspective. I.e. he already knew they found data's head and thus the vehicle for his message to the future was secured.

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