Let me explain... you have a time traveler from the year 10,000. The traveler has a device that has all of recorded history up until 10,000 a.d. in it, like a text book to be read.

If that traveler goes back in time and they make a change to time (say Hitler parents are killed before he's born) the device would reflect the change, right, and so would the travelers memory of events too, right?

How could that be avoided? With handwave tech allowed what would be a mechanism by which changes could be made in time but the device and the travelers memory would remain as they were before they time travelled?

I'm afraid the explanations I've come up with sound a little too "because I said so"; so I'm wondering if there is a Star Treky "tachyon-field" sounding thingamajig that would make for this allowance that maybe has some even remotely feasible reality behind it.

More info: Many people in the year 10,000 have the "recorded time" devices and time travel is a regular matter of course in their time. They study to do it the same way that we go to college for a Masters or PhD. "What do you do for a living?" "Oh, I'm a time traveler." "Oh, how interesting!" It's more specialized than the average occupation of course, like becoming a brain surgeon or something.

But could any of that work if they didn't have a baseline of time? I don't see how unless the device keeps things straight for them.

  • $\begingroup$ Question doesn't appear to have anything to do with worldbuilding. As worded. But rather about some kind of primar-world what-if phenomenon. Any answer regarding avoiding the effects of time travel that requires actual physics or technology will have to be deferred until we even know what the effects of time travel actually are. If there are any effects. $\endgroup$
    – elemtilas
    Jan 30, 2018 at 22:54
  • 3
    $\begingroup$ A nicely written (but not translated and probably never would be translated, hence, irrelevant) modern Russian Sci-Fi book about traveling to the past solves exactly this problem easily. The protagonist is sent to another copy of the world in the past, a fork, so to say. Hence, he does not know the impact of his actions. $\endgroup$ Jan 30, 2018 at 23:04
  • 1
    $\begingroup$ @TimothyAWiseman. Yes, great novel. If I can ever only write one book as good as that... $\endgroup$
    – Len
    Jan 30, 2018 at 23:25
  • 2
    $\begingroup$ It is considered to be good manners to wait at least 24 hours before accepting an answer to allow people from all time zones to post. Questions without accepted answers also tend to attract more attention. $\endgroup$
    – Olga
    Jan 31, 2018 at 0:59
  • 1
    $\begingroup$ @ArtisticPhoenix, I'll let the alternate timeline Len worry about it, since he's rich. THIS Len is most definitely not. $\endgroup$
    – Len
    Feb 1, 2018 at 21:14

4 Answers 4


There are several approaches to time travel in general.

1. Deterministic universe

Everything is set in stone. A time traveller not only is incapable of changing anything but every 'change' of history (from their point of view) is pre-determined.

2. Change-resistant universe

A time-traveller is capable of changing things, however, this produces minimal effects. For example, if Hitler parents are killed and he is never born someone else takes his place. WWII, Holocaust, and so on still happen, just the name of the leader changes. But as the time passes the changes become smaller and smaller until they fully disappear.

3. Forking universe (multiverse)

This is the concept of alternative universes which can be created either by changes introduced by a time traveller or in a more general case by different outcomes of the same events. Potentially, there is an infinite number of universes some of which are almost the same as our own.

4. Single, change-weak universe

In this universe, all changes are 'instant' and do not produce any forks. The effects can be unpredictable and vary in scale. This is the 'butterfly-effect' type of universe.

5. Single universe with time waves

Same as #4 this universe does not fork. However, the changes slowly propagate through time, e.g. time travellers from the 23rd century will be affected earlier than time travellers from the 29th century.

I am highly doubtful if it is even possible for time waves like this to exist, but a similar concept is used, for example, in Legends of Tomorrow.

6. Some other universe

I am sure I forgot about some other general type of time-travel enabled universe :) Please fit it here.

A time traveller does not need any protection in universes #1 (no change is possible) and #3 (the time traveller never belongs to a changed universe, hence does not experience any effects on themselves or their records).

A time traveller from universes #2 and #4 needs to find a way either to be outside of time (time-stasis field, tachyon field, before the time [before the Big Bang] or whatever handwavium is in fashion today) or not to introduce any changes to the history at all. It is especially critical in the universe #4 since the consequences can be catastrophic. However, if #2 (change-resistant) has time waves (as #5) protection can be delayed and there can be a chance to restore history. On the other hand, it is not that important since everything repairs itself anyways.

The #5 universe is probably the trickiest. Depending on a time travellers origination point and a history change point different degrees of protection are needed. It might even be possible to outrun the wave. However, being outside of time is, as usual, the best option.

In recent TV series, Travelers, this problem is somewhat solved by sending only consciousness (no physical matter) in a form of a matrix that gets imposed onto a host's brain. They do not get into many details, but the idea is that a brain of a human is rewired with personality and knowledge of a time traveller. When the timeline changes matrixes do not change because they are already in the past. It still creates some paradoxes, but it is an attempt...

  • $\begingroup$ Thank you so much for these different aspects/concepts. Very helpful! $\endgroup$
    – Len
    Jan 31, 2018 at 17:10

All solutions are handwaving in a manner because time travel may very well violate real world physics and thus have no real solution. With that said, there are a couple of somewhat plausible approaches for use in world building for a fictional reality which preserve the time traveler's memories unchanged.

Copies of the world

The easiest and most believable approach is that time travel actually takes you to an alternate reality that is indistinguishable for your home reality up to the point of the time travel. This can kind of sort of be supported by the many worlds hypothesis of quantum physics (not very well, but its far less hand- waving than the other options). This resolves all paradoxes inherent in time travel and does not affect the traveler's memory at all.

Either the time travel technology or physics itself makes the traveler immune to their own changes.

Since you are using physics that doesn't exist and likely can't exist, its perfectly plausible for you to say that physics itself simply prevents the time traveler from being directly affected by their own changes. This is not only plausible but likely if time travel were real since it is another clean way of resolving the "grand-father" paradox. No further explanation need be given than some vague references to "temporal mechanics."

  • $\begingroup$ Hawking said that he believed time travel might be possible... but at near light speeds. its like solving one problem with just as big problem. I don't believe it possible either, but its fun. $\endgroup$
    – Len
    Jan 31, 2018 at 17:12

if that traveler goes back in time and they make a change to time (say Hitler parents are killed before he's born) the device would reflect the change, right, and so would the travelers memory of events too, right?

DISCLAIMER I am taking this to be a question, which makes it somewhat opinionated as we don't know how time travel works.

My view of time travel, and this is only my view, is actually the opposite. I base this loosely on local causality.

So the travelers device and his memory does not change. Now if he go's back to his time, no one would remember the "real" history to them that was how history was. The only way they would remember is if they were also "unstuck" in time.

The interesting thing here is the safest thing for a time traveler to do then, is to kill their own parents. They would not simply cease to exist, as they are removed from the timeline, local causality can't affect them. Furthermore when they go to the future, the cause is no longer local as it happened in the past. But they would have never existed for anyone in the timeline that was not "unstuck" in time.

This eliminates a lot of "paradoxes" in my mind. They actually are a paradox, but it doesn't really matter because they were out of time when the events happened.

You could argue that they cease to exist when they return, but if that was the case they could have never left to kill their parents, which would also be a paradox.

So in my view of how time works you would have the opposite problem. Where another time traveler could change events and they would realize that history was different, but no one else would. This would also cause issues where their device they rely on is "out of date" where none of the information is correct. Which could cause all kinds of issues for them.

  • $\begingroup$ You just @#$%ed up my mind. I love this concept so much that I must use it! LOL Promise not to sue me and I'll give you credit! Thank you so much. $\endgroup$
    – Len
    Jan 31, 2018 at 17:09
  • $\begingroup$ Glad I could help, I doubt that is a unique view on the topic though. $\endgroup$ Jan 31, 2018 at 17:20
  • $\begingroup$ So you wont sue me? lol $\endgroup$
    – Len
    Jan 31, 2018 at 17:25
  • $\begingroup$ no, I won't. but thanks for the option. $\endgroup$ Jan 31, 2018 at 17:26
  • $\begingroup$ So... your saying Hitler was the time traveler who killed his own parents... man this is like that episode of Red Dwarf where Kennedy assassinated himself! $\endgroup$
    – Josh King
    Jan 31, 2018 at 18:25

If the traveler doesn't need to actually change history then just have them do it from an 'observer only' standpoint. This would be completely fictitious, but then so is time-travel. Recommendations for this are to project the traveler into the past in an incorporeal state that is invisible. This could be something similar to how the Star Trek transporter worked.

If you do actually need the traveler to interact with the world and accidentally (or intentionally) change it for everyone except themselves, then I would not worry about the hand-waving part. The paradox of time travel is that something could exist and not exist at the same time. For example, if the traveler went back and killed his/her own parents, the traveler doesn't exist? Then who killed the traveler's parents?

For your 'baseline' idea, it might be helpful to look at an event horizon. If your future people could put a computer into a black hole and still communicate with it (maybe with quantum entanglement) then it could store the baseline of information. But keep in mind that only relevant people/things to a particular history would be guarded against these effects. Just because your computer knows about Hitler doesn't mean the Holocaust ever existed in the traveler's world after his parents died.


Not the answer you're looking for? Browse other questions tagged .