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Say that at some point of history in the far future, time travel becomes possible. That means that from then on anybody could travel in time. If that is possible, doesn't this mean that our present, past and future would be filled with time travelers over the years?

For example say that a time traveler from 2100 comes to 2016. We now have 1 time traveler apparition in 2016. In 2106 somebody else has the idea to visit 2016 so we would have 2 time traveller apparitions in 2016. Over the years all the people who will ever have the idea to visit 2016 will be added to the apparitions. Won't that create a 2016 (or any other year) filled with time traveler apparitions that would be too hard to be hidden?

What I am asking is: is any time travel (suppose that both forward and back in time is invented) possible, without making the planet a, huge and obvious to the people of the past, 'tourist attraction' or operation field for swarms of time travellers who will ever want to be there for any reason?

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closed as unclear what you're asking by AndreiROM, Hohmannfan, John Dallman, Thucydides, Josh King Oct 28 '16 at 18:03

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    $\begingroup$ It entirely depends on how your time travel mechanic works. I can think of two 'possible' ways : time is already written until the end so yes you would end up with History filled with time travelers and since everything is written is could not be otherwise (the creation of the time travel device is conditional to them going in the past which is itself conditional to the machine being created). On the other hand, you could have that when a time traveler goes back in time they actually create an alternate reality, time is not already set here, so no paradox and each traveler gets their reality $\endgroup$ – Riff Oct 28 '16 at 12:11
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    $\begingroup$ @Nicolas the 2nd one doesn't work. If that were the case we'd still see time travelers be common unless we were one of the first few alternate reality. It's the same problem the Simulation or Real universe thing runs into. If it is the case that time travel with alternate realities were real/simulated universe existed it is far more likely we're the one of the alternate realities with multiple time travelers/simulated than the opposite. $\endgroup$ – Durakken Oct 28 '16 at 12:16
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    $\begingroup$ God put a stop to time travel after the first chronotourist took a selfie at the Crucifixion. $\endgroup$ – EvilSnack Oct 28 '16 at 13:24
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    $\begingroup$ Possible duplicate of If time travel is possible in the future, no matter how distant, why haven't they come back to tell us? $\endgroup$ – RudolfJelin Oct 28 '16 at 14:31
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    $\begingroup$ @EvilSnack that reminds me of a story I read where all the spectators at the Crucifixion turned out to be time travelers. $\endgroup$ – stannius Oct 28 '16 at 16:29
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This is a classic philosophical question. You can probably find all sorts of interesting discussion of it around the internet, so I'll just talk about my view. I'll largely ignore travel into the future, because travelling into the future is something we already know how to do - just build a spaceship, get on board and accelerate as close to the speed of light as you can manage and let time dilation do the work. Building such a spaceship is left as an exercise for the reader.

Certainly, the lack of time travellers from the future has been seen as an indication that time travel will never be invented. Douglas Adams wrote in one of the Hitchhiker's Guide To The Galaxy books or radio episodes (or both), something to the effect of "time travel, by its very nature, is invented in all eras simultaneously". So either there is time travel, or there isn't, but you can't have a point where time travel is invented because once it's been invented people can travel back in time and someone in the past is going to figure it out and it just carries on from there.

That only works, though, if you look at one particular model of how time travel works. In Adams' work, time travel technology allows the user to move freely backwards and forwards in time, from the beginning of the universe (where they built a restaurant called the Big Bang Burger Chef), to the end of the universe (where you can dine at Milliway's, the Restaurant at the End of the Universe). But it doesn't necessarily have to work like that.

In the television series Quantum Leap, a time travel device is invented which only allows the user to travel into the past within their own lifetime. Mutation of history is entirely possible in the show's model, and indeed that's primarily what the main character engages in for largely altruistic reasons. In addition, the "leaper" actually swaps places with a targeted person in the past, taking on their appearance during the leap and basically impersonating them.

So in this model, not only is time travel limited to a human lifespan before its invention, time travellers are effectively undetectable (provided they impersonate people sufficiently well). Obviously this is a storytelling device to give the show interesting scenarios each week (they had a lot of fun sending Sam into people with no legs, pregnant women etc) but it does provide an answer to your question.

There are other such limitations. Rather than the lifetime of the traveller, the machine may only be able to enable time travel within a time period where the machine itself exists - so you can never travel back to before the machine was invented (or forward to after it was destroyed, if it allows you to travel forward).

This is shown in several things, but the film Primer stands out as an example of this for me. In that film, the time machine has to be built, turned on and left. Later on, you can get inside it and wait the same amount of time before getting out, and you'll emerge just after it was built (it only travels backwards at the same speed you usually go forward, so travelling back two months takes two months of sitting in an airtight box). So in that world, again, once the machine is invented you might see time travellers everywhere, but never beforehand, and because you travel back at 1 second per second you're severely limited by both your lifespan, your ability to cope with the experience of the journey, and the availability of a machine which someone else isn't already using and which retains power and integrity for the necessary time.

There are also the numerous possibilities of multiple timelines, such as the idea that travelling backwards in time immediately makes a new timeline which isn't the same as the one you originally came from, and that this limits the appearance of time travellers somewhat because they don't all go back to the same one and neither do they all come from the same future timeline.

Of course, there is also the possibility that there is a fair bit of time travel activity but that various people try to cover it up, regulate it, police it and possibly utilise more time travel to prevent the first time travel incident from ever occurring. This model is much in evidence in the Star Trek: Department of Temporal Investigations books, where a variety of "uptime" agencies are seen intervening to mitigate, cover up or outright erase from history various time travel incidents. However, in that setting many people are aware that time travel is possible and that it happens sometimes. They're just usually sworn to secrecy if it happens, or their memories are altered, or they end up dead.

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  • $\begingroup$ I was also influenced by primer and asked this question. Great answer. $\endgroup$ – papakias Oct 28 '16 at 15:28
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    $\begingroup$ Another possibility is that time travel is very costly, and/or its cost is proportional with the distance in time to travel, maybe even exponentially so. $\endgroup$ – UncleZeiv Oct 28 '16 at 16:10
  • $\begingroup$ Good possibility! I hadn't encountered that one before, or I surely would have included it. Certainly anything along the lines of tech which swaps spatial and temporal axes to allow you to fly backwards in time in a spaceship would have an element of this to it. $\endgroup$ – Matthew Walton Oct 31 '16 at 11:27
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Depends on your time travel model - if you have a branching model, where each time hop creates a new branch, then there isn't an issue. Each branch has a fixed number of time hops, so doesn't change - if anyone extra hops back, they end up in a different, new branch, so the original branch isn't affected, and the new branch has always had one more hop than the original. So you can have a branch with no hops, and branches with lots of hops - but from the point of view of people in that branch, it's always been the same; you don't see history changing.

If you have a single timeline model, I like Larry Niven's solution: If time travel is ever invented, someone sometime will use it to meddle, changing the timeline. This will keep happening as long as time travel exists - until someone meddles so that time travel is never invented, which is the only stable state.

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There is a time travel model where the time travellers can be sent back into time to a fixed time. Basically, it is a wormhole in time domain. This model do not allow wormhole to be opened to the past, but to the future. Wormhole can be traversed in either direction or from future to past. With this model and ability to reach lightspeed (should be trivial by now) allows someone to reach any time zone between his time zone to the creation of time travel. This means, there will be no time travellers until the invention of time travel.

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    $\begingroup$ +1, that is exactly what I was going to say. As a side note, this is the method of time-travel used in the movie "primer". $\endgroup$ – DJMcMayhem Oct 28 '16 at 17:07
  • $\begingroup$ This is probably the time travel used by the doctor too. $\endgroup$ – Cem Kalyoncu Oct 28 '16 at 19:14
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Time is big, and the future is much larger than the past. If took the current world population and sent each person to a random point in time of stars and they then lived for a century, for less than 1% of that time would there be a time traveller alive in the universe.

So you might get everyone wanting to visit 2016, but they may be a trillion other years to visit.

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  • $\begingroup$ If there are far more humans in the future then you will get plenty of travelers just due to the trillions of people. If their aren't that many people then clearly their are far more humans now than at almost any other point in time. If time travelers are interested in people that makes 2016 special. $\endgroup$ – Donald Hobson Oct 28 '16 at 21:33
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First, for there to be a lot of time travellers, time travel would not only have to possible, it would have to be (relatively) cheap. As a parallel, space travel is certainly possible, but space is not (currently) filled with space travelling humans. (Google gives the largest number as 13: http://www.space.com/6503-population-space-historic-high-13.html )

Second, having a large number of time travellers implicitly assumes a long future. Suppose time travel is invented in 2100, but due to global warming humans (at least those who haven't escaped into the past :-)) go extinct by 2200. Then a relative handful of time travellers are spread over hundreds of millions of years of habitable past.

PS: A scenario where we're all (mostly) descended from time travellers. Time travel is invented just before the global extinction, the developers and their friends escape to Europe ca. 50K years ago, and so displace the Neandertal population...

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  • $\begingroup$ I agree about the long future hypothesis. And I also like your Post Script! $\endgroup$ – papakias Oct 28 '16 at 18:31
  • $\begingroup$ In addition to time travelers being our anscestors, if they're anything like us, they were probably responsible for the mass extinction(s) millions of years ago (tongue in cheek)... $\endgroup$ – David Schwartz Oct 28 '16 at 18:47
  • $\begingroup$ NO if you were a time traveler would you want to sit in a neolithic swamp? Now is probably near the top of the list of options for most time travelers. Also fixing global warming is MUCH easier than time travel. $\endgroup$ – Donald Hobson Oct 28 '16 at 21:36
  • $\begingroup$ @Donald Hobson: Why do you think the Neolithic has nothing to offer but swamps? The terrain & climate were as varied as today. I certainly would prefer to escape to the Neolithic (given a reasonably-sized colonization project) than to live in this overcrowded world. Second, if fixing global warming is easy, why isn't it being done? You'd have to get most of the denialists to agree, whereas a time colony would select only those interested in going. (Alas, I find that someone had the idea already: see Julian May's Pleistocene Exile" series.) $\endgroup$ – jamesqf Oct 29 '16 at 4:45
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You may want to look at this paper. Quote from the Conclusion of this paper:

"According to our model, if you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you. In other words, while you are aware of the past, you cannot change it. No matter how unlikely the events are that could have led to your present circumstances, once they have actually occurred, they cannot be changed. Your trip would set up resonances that are consistent with the future that has already unfolded."

This does not necessarily answer your question directly, but from it follows that maybe there will be or has been or will have been a year in which there is a huge influx of time travellers, which will cause future time travellers to know about this historic event, making them interested in seeing what was up back then. On the other hand, it's not possible for these time travellers to slowly accumulate "over the iterations" so to say, because they will always have been there.

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A very possible reason we haven't seen any time travelers yet, is a time machine can only "bring back" and so you can't travel to before the machine was invented. The movie Primer used this, and this style of time travel can be a bit hard to wrap your head around...

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  • $\begingroup$ No kidding-- it seems like no matter how many times I watch it, I still have trouble explaining to myself exactly what happened... $\endgroup$ – David Schwartz Oct 28 '16 at 18:25
  • $\begingroup$ @DavidSchwartz I also had trouble understanging what is going on until I saw this diagram upload.wikimedia.org/wikipedia/commons/thumb/8/84/… $\endgroup$ – papakias Oct 28 '16 at 18:35

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