Time Travel/Headscratchers

Wait. What if there is someone who's already built a time machine in the future, and may time travel back to this time?
It gets even scarier considering how the inventor may be you. A future version of you may suddenly appear in your living room without any warning, or worse, may be stalking you right now. Also, how will we track the date when time travel gets invented?
- By asking the time traveller, of course!
Has anyone ever considered the physical implications of time travel? When a system travels back in time, they generally appear out of the blue in a bright flash. Doesn't that mean that something just appeared from absolutely nothing? For the duration of the time travellers's visit, doesn't that mean that the amount of matter and energy in the universe is the same plus a couple of dudes and their luggage? Also, what happens to the stuff in the spot that the travelers appeared in? Does the time-traveling system start at a point and expand out? If it does, what happens if the system appeared, say, in the middle of a mountain made of titanium? In air, a person's body is easily able to withstand the pressure against it and move the air out of the way. Inside of rock or metal, though, the corpse is the weaker body, and, well, what happens when a human body appears in a space smaller than it and tries to expand?
- Presumably, yes, it would add mass/energy to the other point in time/space, but it depends on the time machine (or whatever they used). It could create a vacuum where they left and "push" matter out of the way where they arrive. They could switch places (and times) with the matter in the other time period. It could be a 2-dimensional portal that lets things travel freely between the two points in space/time.
- Any realistic depiction of time travel requires a time machine to already exist in the past if you want to travel backward, something like a landing pad. For example, there's the idea of using a wormhole with one end older than the other. This allows you to go back in time by going through young end and coming out the older end back when it was the age that the young end is now, and you can go back even further by repeating the journey, but you could never go back to before the formation of the wormhole. Another way is to have an enormously powerful computer that can predict the future and materialize objects and people; it can snatch people out of the future, but no one can go back to before the computer was built, so there is always a destination time machine of some sort to somehow take care of issues of mass, energy, and space.
A couple things about Stable Time Loop just bugs me. If the past is already changed, and thusly there is no way to change the past, but the protagonist really, really wanted to change the past, what's to stop them from just trying again?
- Nothing. They could try as many times and as hard as they wanted to, but in a true Stable Time Loop they would simply never succeed.
- If you would have done something that would have cause a paradox, a different Stable Time Loop would form to stop you, e.g., someone from the future has Already Changed The Past in a way that stops you.
  - Stable time loops don't "form", though. The basic idea behind them is that history can not be changed, period. That means that in any fiction where a Stable Time Loop exists, it has always existed and always will.
  - With stable time loops, you sort of have to give up the idea of free will. That doesn't mean you're not free to try to change the past again and again, but however many times you try, those attempts have "already happened", and there's nothing you can do that won't already be in the history books. It's the price a time traveler in that sort of story pays for going into the past: everything they did or "will do" in the past is already set in stone in the present and any attempt they'll make to change it also happened the first time around (in stories like Oedipus, it's the attempt to change things that actually made things happen the way they did).
    - And as Howard Taylor said, if you've ever wanted to gouge your eyes out after reading yet another Stable Time Loop story, now you know why.
    - It's not so much free-will is sacrificed as the universe conspires to make things happen irrespective of what you do. Try to stop JFK from being shot? An officer will delay you before you can get to the scene. It's a shield to the most truly horrifying aspects of a Stable Time Loop. Most writers are not willing to admit humans are puppets of fate, and that even our thoughts are already scripted. The only instance I've seen that explored is in Dr. Manhattan's nihilistic speech on Mars in Watchmen.
- I think the main difficulty people have when trying to comprehend Stable Time Loops is that they tend to think of the same event happening multiple times and always working out the same way, whereas what is really happening (at least in a "hard" Stable Time Loop) is that the event happens once but if you follow the loop you will pass through it multiple times. Imagine you are on a circular road and you see a tree by the roadside. You then drive around a 360 degree circuit and see an absolutely identical tree by the roadside. You keep on driving and every 360 degree cycle you see a tree that looks exactly like the first one. Should you be surprised that all these trees look exactly the same? No, because they're all the same tree, you just keep moving around in space and coming back to it. Similarly, in a setting that uses an immutable timeline each moment only occurs the once, but you can potentially move backwards and forwards to experience it multiple times.
Why is it referred to as the Grandfather Paradox? I mean, killing your father or mother before they had you would have a similar effect. Why go back more than one generation?
- Complete guess but it could be the 'Grandfather' bit makes people think of a Grandfather Clock? And so the association with time.
- Probably to avoid confusion; if you just used the example of killing your father, you would then need to specify that you hadn't been born yet. The point of the name is for it to be clear that you haven't. I mean, even going back one year and injuring your father would create a paradox since it would change the series of events leading to your time travel, but time travel stories only like to handle the obvious paradoxes.
  - Ah, but the same rule applies for the Grandfather Paradox anyway. You must specify that the death took place before your mother/father was born, or it doesn't work.
- Because Rene Barjavel wrote a story in which it was killing the grandfather, and we derived the name from that. Just consider that story the Trope Namer, if it makes you feel any better.
An interesting point that I've never heard anyone bring up before: technically speaking, since time travel is only movement in the 4th dimension and not the other three, anytime anyone goes backwards or forwards in time, they would end up in the middle of outer space. Why? Because firstly, the Earth revolves around the sun continuously, so a day in either direction would equal many thousands of miles between you and Earth. Secondly, the Sun revolves around the center of the galaxy, so even if you went back an exact year, you would be stuck in space, far from the solar system. So, unless The time travel allows more than just 4-d motion, time travel will kill you!
- But don't forget that every movement is a movement only when it's related to something else. For example, When I'm travelling in a bus, I could say that I'm moving in relation to the road, but I'm not moving in relation to the bus. Same thing with any time traveller and Earth/Sun/Wilky Way. Movement is relative, as Einstein pointed out.
  - Nitpick: in relativity, movement is relative only in what are called "inertial frames." You're in an inertial frame if you're coasting at a constant speed and direction; in that case, you won't be able to tell whether you're moving or everything else is moving around you. You also can't build any experiment that will conclusively prove one way or the other which it is. But the rotating Earth is an accelerating frame of reference- you're not just coasting, you're spinning around in big circles. So on Earth it is possible to design an experiment to prove that you aren't standing still. It's called a Foucault pendulum.
    - That only proves that you are experiencing an acceleration. If you were sitting still and the entire universe were rotating around you, you'd get the same effect.
- I think Piers Anthony brings it up in his Incarnations of Immortality series, in the book about the incarnation of time.
- Strontium Dog from Two Thousand AD uses the 'movement in time, not in space' idea quite effectively with 'time grenades' that jump their target 10 minutes into the future, by which point the planet they were on has moved and they die in space.
- My science could be completely wrong here, but if you leap out of a moving car, you'll roll along the road in the direction the car was moving, so...same basic principle, perhaps?
  - Except if the car is turning (orbiting) when you jump, the car will continue to turn while you go straight.
    - But of course turning and orbiting are very different things. When I jump out of a turning car, I go one way and the car goes another way. When I jump off of an orbiting planet, I stay with the planet. It's not just because the planet's gravity sucks me back in; no matter how far I get from the planet, the gravity of the sun still applies, bending my path into an orbit. It works just as though being in an orbit had changed what it means to go "straight".
- It's one of the Acceptable Breaks From Reality. The main reason that you never see it mentioned is that it would mean a lot more technobabble to explain how a computer is able to compensate for the various rotations and revolutions, and it would make the time machine a de facto teleportation device.
  - Time and Relative Dimensions In Space. If the device is capable of teleporting through one dimension, I don't see why it's such a stretch to assume that it's teleporting through the other three as well. And according to Relativity Theory, any faster-than-light teleportation already is de facto time travel.
    - If I understand correctly, there's a plot device that'd be able to allow FTL without allowing time travel, or at least while avoiding the possibility of paradoxes. If FTL uses a mean that has a fixed inertial frame of reference, then time paradoxes can be avoided. In some frame of reference, it may appear to be time travel, but they couldn't exploit it to go back in time, as to do so would require FTL which would require reverting to the fixed frame of reference, in which they cannot go back to cancel the previous action. This however has no physical backing, it'd be just a completely arbitrary plot device.
What just bugs me about Time Travel is how brain-damaging the discussions end up being. Exploding heads are not unheard of.
- This troper's hoping that it's impossible in real life, because if they discovered it, the universe would be broken in no time.
  - It is this troper's firm belief that if time travel is ever invented, every point in time that one could potentially be time traveled to would have the time bit of the time-space continuum ripped up, resulting in an apocalypse. However, any point that cannot be time traveled to would be fine, until it became a point able to be time traveled to. If that didn't make much sense, neither does Time Travel.
  - Time-travel is only brain-hurting when it's written poorly. There are plenty of time-travel stories where everything makes sense.
What bugs me is that most versions of time-travel are total game-breakers, but hardly anyone figures out how to exploit it. Bill and Ted figured it out, but supposedly smarter characters don't. I'll give the original Terminator a pass because nobody (except possibly the Connors) knew the rules - until the sequels retconned it so that Skynet was more-or-less right all along.
My Own Grampa. How does it work? I mean, let's assume person A has all different genes, none from "himself". So he goes past, becomes his own grampa/dad, resulting that his genes pass down the line. Now, his father (or himself), carries the same genes, from which half are from his mother. His dad (or himself) has sex with his mother, passing down genes. Now, since half the genes come from each parent, doesn't that mean that with each cycle the mothers gene become more and more dominant, eventually resulting a clone of his mother?
- Sam Hughes actually went to the trouble of mapping out the genetics of such a sequence (using Futurama as his example) and came to the common-sense-defying conclusion that, in order to prevent that from happening, the father and the mother would need to be close relatives, making their relationship incest, allowing the son to go back in time and become his own grandfather.
  - But they always are. If person A is his own grandfather person's A father married his grandmother. So it is incest is part of the scheme without any additional conditions.
- My Own Grampa only really makes sense with an immutable timeline, which means that going through multiple "cycles" doesn't change anything. The guy gets approximately 25% of his DNA from each of his three non-paradoxical grandparents, with the remaining 25% forming an ontological paradox as it is essentially copied from itself.
  - The remaining 25% can be thought as result of fixed point applied to genome. If it is the only sequence that in given circumstances could be copied - problem solved.
  - Perhaps the half of his genes that the father passed on to the time traveler son happened to be the exact half of his genes he got from the time traveler grandpa. 25% of the genes making it 2 generations is just the average.
- Stable Time Loop That's the only way I see it working. The time traveler is his grandfather and always was. His parents would also have always had to have been insestuous from what has been said above.
My Bazillion Alternate Future Selves And Me or the "giant pile of phone boxes" end of the world scenario. Why is that never brought up?
To clarify, I am talking about works of fiction that acknowledge both time travel and the multiverse. Presumably, if a man of the future decides to travel back in time, countless slightly different versions of him must have the same idea. And since all those futures diverge from the same past...
- The many worlds version of time travel tends to work in such a way that when you travel back in time, you create a new timeline which diverges from the old one at the time that your time machine shows up, while leaving the original timeline time machine-free (thus helpfully avoiding any kind of Grandfather Paradox since you don't change your own past). Presumably all your alternate selves create their own timelines.
- Fair enough, but it still bugs me that any difference you see in the new timeline is a result of your actions, while all events you are not involved in happen exactly like you remember them. If the new timeline continues to split over and over on its own, the characters shouldn't be that good at foretelling.
- What bugs me about the whole "you just cause a new timeline to diverge" concept are the following two points.

1. 1. First, in a larger context you're still simply changing history -- only now it's the history of the larger multiverse, where presumably that new timeline didn't exist in the past 'before' the 'new' past created by your actions. (This also assumes that 'before' and 'after' are even words that make sense when applied to time itself, of course.)
  2. And second, if you can create a new timeline where none was before, and both it and your original one now happily exist side-by-side...well, then you effectively just created an entire new universe out of nothing, right? So where did all the matter and energy needed to perform this amazing feat come from?
    You can, of course, get around both these issues by deciding that all timelines are already pre-set and that time travelers, rather than actually creating any new ones, just move along the ones that are already there...but then you're back in Stable Time Loop country.

- You can also flip the perspective. All matter and energy have past, future and alternate states. There is no new energy involved.
- In a truly infinite multiverse, every instant that any future self would have attempted to go to would have anywhere between zero and uncountably many versions of that self. However, one time traveler is more interesting (in the long run of a story) than zero successful arrivers and seven hundred septillion successful (and immediately squished) arrivers, and is easier to write a compelling story about than a few arrivers, at least in most cases (since the more complex a story becomes, the more likely it is to have passed the point of diminishing returns). As for a story where the point is several of the same time traveler conflicting with itself, it has happened in fiction (at least twice, if I recall correctly, usually as a Tomato Surprise), and is faster to run up against the earlier-mentioned law of diminishing returns (as the time traveler needs a reason to go back, and providing too much distraction in the form of multiples of the same traveler can cause the reader to not only lose the suspension of disbelief, but also to get fed up with the whole thing).
My personal gripe is that the Trope Namer for Timey-Wimey Ball (the Doctor Who episode Blink) isn't actually an example - it's a Stable Time Loop.
- And made even weirder in that Doctor Who probably has more actual examples than any other series.
- Worse: in the mini-episode Time Crash, he uses the term again... about another Stable Time Loop. It could very well be the true meaning of "timey wimey" in the mind of the writers.