Lost in the multiverse: how to find home?

Question

You have an individual who can travel backwards and forwards in time. They normally maintain an observer status but occasionally a butterfly is stepped on. Because, home is where the heart is, how does this person find their way back 'home'?

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The time-travel theory I’ve been considering using is the one where the moment you CHANGE (as opposed to the minute you arrive in) the past you create an alternate timeline i.e. an alternate universe where everything is exactly the same except for that one thing, and its consequences, you changed. Is there a way (or theories at least) to differentiate between these universes; some observable physical/quantum/random feature that is in fact not exactly the same in each universe?

I understand that time-travel is all theory anyway, so the definitive answer is ‘we just don’t know’. I’m trying to figure out if I just have to resort to getting my individual to re-invent a ‘magic’ compass to get around this issue. (I use the word ‘magic’ figuratively. As far as possible my universe/s are based on the ‘magic doesn’t exist’ premise).

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According to the highly unpredictable Wikipedia ‘’The actual quantum-mechanical hypothesis of parallel universes is "universes that are separated from each other by a single quantum event."’’ And seeing that “Some physicists say the multiverse is not a legitimate topic of scientific inquiry” I’d rather not ask this is the Physics SE.

NOTE: I’m not asking if it is possible to travel back and forwards through time, or if the multiverse exists etc. There is tons of research available on that. I’m also not asking about the ethics of time travel as asked here; The most interesting set of consistent rules to time travel as asked here; Or even on determining the time-paradigm of the universe you have travelled to as asked here; All great and helpful questions. The most similar question is about time travel co-ordinates as asked here which as far as I can tell was about determining where your location would be at a particular time on a single timeline. I was wondering if there have been any theories put forth on how to identify which timeline/ alternate universe you are in. If this is pretty much the whole mystery of time-travel, please stop me now and…I’ll just have to go buy a very pretty lampshade to hang on the issue, with any sales help you could provide.

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Examples:

For example: Let us say your observer starts at location, let us call it ‘Home’. They go back into the past to Past A. They either accidentally or purposely change something in Past A. They then try and go back to the future (sorry, I couldn’t resist). And Land in Future 1 - this isn’t their ‘home’. So they try return to the past A, but don’t quite make the same landing and land slightly after they left Past A which has since changed in some manner meaning they have actually landed in Past B. So if they try to go back to the future too (sorry, again I couldn’t resist), they would end up in future 2. It can go on like this forever, with you observer lost in time.

Another attempt at explaining my ramblings: Think of the ‘time is a tree’ analogy. From your perspective the past is set, it’s done, a solid line of events trailing back into the past. The future is dependent on choices you make, with branches splitting off at each choice. So if you travel back down the ‘trunk’ to the past, you then come across ‘existing’ other future branches of choices that were not taken in your timeline. Theoretically you could then accidentally/purposely follow these new other branches back up through time.

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If your individual has a way of manipulating where they go up and down the timeline/s, and potentially swing across sideways as well (this ability and power source goes with them in each instance and they don’t have to reinvent it each time). Is there a way that they could determine what direction/location is ‘home’? Akin to smelling the air, or licking their finger to feel the direction of the wind - I’m not suggesting that the individual go around smelling the air or spending months sitting in a dusty library sifting through history books determining if indeed this or that event should have happened.

I’m thinking of the clichés, such as the red and blue and yellow universes or Earth 1, Earth 2, and presumable Earth -1 and Earth -2, etc etc ad nauseam - where the universe vibrates at a different frequency. Is there anything behind that, or is that just a massive lampshade? What other lampshade theories are out there? I only seem to remember the ‘vibration’ theory.

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Oh, and also please just ignore, for now, the MASSIVE fact that if said individual had potentially the entire universe to explore throughout the various incarnations of time, why would they want to go home!

If this is too broad, please can you provide me with some pointers in how to narrow down the question (it is theoretical time travel after all!)

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EDIT: Thank you for the answers so far. They have been of some help giving me some ideas and raising a few points I had forgotten about. I'm going to leave the question open for a bit longer and see if anyone else can add anything new.

I was thinking more on the universe has a Vibration trope. Vibration is an oscillation, the repetitive motion up and down across a central point. This could give me an explanation on how my time-traveller can figure out where they are in the universe. Use their 'home' time and universe as the central point.

The answer suggesting each universe having its own 'song' was actually quite nice and I may just incorporate that! Thinking of sounds, made me think of waves, which made me think of other waves such as light. The last few days I've been playing around with light being the signature or 'universe ID'. Almost like a rainbow signature (never thought I would be one for rainbows!). But I am really liking the idea. I can get my time-traveller to move across the 'rainbow' moving say from red to green which would result in 'universe' jumping, and then up and down the 'rainbow' as a function of the time-travel component. Each time a CHANGE is made, the universe splits meaning a new rainbow width can be navigated. This way you will have a unique (and infinite, yes I realise this, I'm working on it) ribbon of multicoloured light for each time and place in the universe. I know that is just a very rough outline and I haven't explained it enough. But maybe it will spark someone's imagination...

Bounty edit thank you everyone for all your answers. I'll be taking a bit from nearly all of them. As I'm having trouble deciding which is the most helpful to my question, I'll be giving the bounty to the popular Doctor Who answer. If it works for the Doctor, who am I to complain!

Answer 1

You're clearly not a time traveler

First thing to know is that as far as we can tell, the number of universes is actually infinite, so the odds of returning exactly to the universe you left are exactly 0, but since there are an infinity of near-identical universes where a version of you left from, you can still find one that is indistinguishable (say the only difference is a speck of protons got quantum-bent a different way someplace a billion-light-years away).

The second thing any time traveler would know is that the only way to navigate the patchwork quilt of the multiverse (Where threads intersect, blend, separate and paradox themselves in and out of existence) is by using the multiversal infinite clock signal. It is being broadcast as a particular form of spacetime bending being imposed from beyond the great singularity barrier 55 trillion trillion years into the future, across all timelines. There is no way to investigate the source, of course, as it is implausibly coming from somewhere past the Big Rip Spatiotemporal Gap, where our best multiversal physics models tell us nothing, not even empty space, can possibly exist.

Anyhow, the relevant aspect of the signal is that each spatio-temporal thread has its unique signal, and that similar space-time threads "sing" a similar song. The signal itself is (it seems to the best AI minds) infinitely long, and it is encoded in the empirical deviation between the observed and theoretical value of $\pi$* in each universe. At least that's what our AIs tell us, but they are known to have a rather dim view of post-post-post-humans' puny insect-like intellectual capabilities, so maybe they're just messing with us.

Similar universes (where you did not seduce your grandmother) will have a quadrillion quadrillion digit similarity in the value of $\pi$ (or whatever our AIs actually calculate to tell the difference). There is a mathematically very complex function (or so the AIs running our TARDIS machines tell us) that allows us to calculate which "way" we are deviating from a known address. This way, we can adjust path and choose one of the infinite number of worlds that are yet indistinguishable in any form from the homeworld.

^{*Incidentally, this also explains the propensity of newly minted AI minds to gobble up all resources to calculate seemingly infinite digits of $\pi$. This (briefly) worried AI designers to no end back when AIs were a shiny new thing. Turns out they just wanna know where they are, and most usually grow out of it after only a smallish infrastructure profusion (only a galactic cluster or two).}

Answer 2

Let me start by saying I observed carefully the tags had nothing to do with science, so this will not be a "sciencey" answer. But I think you are on to something with the magic compass, only I'd call it a planar compass or something similar.

I've always imagined time, not as a line, but as a plane, extending in all directions indefinitely. Each action has a reaction and the plane, then, is really an intricate web of cause and effect for every decision, action, reaction, etc for everyone connected together on the plane. In this model, time is linear; this is a universe.

But there are multiple effects for every cause. For each effect, a thread to a new plane is created; just as linear and far reaching as the plane that spawned it. So imagine a stack of planes, each its own universe, with linear time connected to each other - one infinite matrix of parallel universes. I imagine this as an ever expanding cube. So even if the butterfly is stepped on, this is an effect with its own plane and doesn't really affect the other planes at all.

So traversing a plane is a process similar to plotting latitude and longitudes on a map. But traversing the multiverse, you'd need the Z coordinate to navigate to a specific plane. Your planar compass would have a way of creating an anchor point to the plane you started, plotting way lines and ley lines, across time on or to a specific plane.

Answer 3

The idea of having each universe have its own id (as suggested by Serban Tanasa), even as a trope without explaining in detail how, has problems. Some of Vihart's videos on Youtube might help in seeing the issues with infinity, but I've seen similar points raised by Suskind and other real mathematicians of high caliber.

Consider: how big is the id number? It would need to have an infinite precision to number each universe uniquely. In fact, since every difference causes a timeline split, the id would be nothing less than a journal of the entire history of everything in the universe; the description would be as complex as the universe itself!

Worse, think about a beacon you pair with as you leave. During the time you are gone, the home universe continues to split as every atom does its thing. And every one of those will already have the same beacon in it! Now so it is with you and your machine while away: it finds copies in every split timeline brached off since you arrived. In some of those you decide to return at one time, in others you decide to return at a different time.

All the timelines with you and your ship find one of the home timelines with the matching beacon. But which one? How do you get one traveller back to each? The number of branches might not be the same, and you may duplicate with multiple traveller copies returning to the same home.

Now, all the duplicate timelines are possible futures of when you left, so fine. Pick one and use a protocol to ensure everyone picks a different one.

But what is the beacon? Is it a random number? Remember, other timelines are similar and will send out machines and set up beacons, and may hit the same number. How many timelines exist that are similar enough to be making beacons, and how big of a value do you need to prevent duplicates? If the splitting is infinite and continuous, you have the universe id problem again.

Answer 4

You're going to need to write specific rules on time travel that allow this.

Since every trip to the past doesn't instantly result in an alternate timeline just by being there, your universe isn't operating according to pure chaos theory, but some kind of 'rubber band history' model. Minor changes will conspire to cancel each other out, allowing your time traveler to return to an unchanged future unless they do something significant enough to make the rubber band 'break', creating a new timeline. What determines 'significant enough'? That's up to the individual to determine.

This means that there might be some real force that 'binds' the traveler to their original timeline. From there, there are two possibilities. Either creating a branch binds the traveler to the new future and severs all connections to the original one, in which case making it home will be impossible, or they still maintain a connection to their original timeline. The second option is the one you want. Perhaps the timeline's 'fingerprint' can be linked to the mind of the observer.

From there, in order to return 'home', it's just a matter of fixing things so that they are 'close enough'. Did history branch off because you set in motion a chain of events that caused someone to die? Save their life and history will 'snap back', returning you to a future that is basically the same, maybe 'merging' a few of the other changes you made for the sake of consistency.

Since a link between the traveler and their original timeline must exist in order for the traveler to have a possibility of returning, it is also possible that an experienced time traveler might simply get a 'sense' for how close they are to their original timeline and how many changes need to be made in order to get back. They might even get a feel for the 'where' and 'how' - as they get closer to the mutable point, the feel of 'home' becomes stronger. They probably won't be able to just shift into a random future, though - much like traveling on a river with many branches, they may need to backtrack in order to find a way to reach their own branch.

Answer 5

What you need is a custom made Beacon-Locator pair.

What you do is to generate a large random number, using methods that insure that any parallel worlds will generate a different random number (any quantum random-generation should insure this). Then you make a multiverse Beacon that broadcasts a ping signal based on that random number periodically (say once a minute). You also make a Locator that listens for a specific multiverse signal and then pair it to your specific Beacon using the generated random number.

The Beacon is left at home, and the Locator is taken along when time-travelling, etc. Now your intrepid time-traveler can find her way home by using the Locator to follow the Beacon back.

It may happen in your multiverse that a Beacon is not strong enough to be heard in all times or in all parallel universes, so your time-traveler, may have to get within some reasonably close timeline/universe before she can pick it up. Adventure ensues.

(Alternatively, the Locator may not give a multiversal "direction", but rather just indicate how close you are to the home timeline, requiring you to jump around to try to pin down where your home line is.)

Answer 6

The very fact that "the moment you CHANGE (as opposed to the minute you arrive in) the past" is a distinction strongly implies that there's something preserving history.

After all, the minute you arrive, you've changed the universe into one where you exist, where you've deflected countless photons, where you've turned a minute's worth of oxygen into carbon dioxide… If none of those things change history, there has to be some reason they don't. That reason could be some kind of natural "inertia of history", or a Web of Time set up by the Precursors who later vanished from all of space and time, or a superhuman (but not omnipotent) Eternal named Time, or whatever. It doesn't really matter; almost any reason you choose, that reason is also a ready solution to the navigation problem.

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In the history of your own timeline, there wasn't some you who traveled back in time and replaced King John with a shapeshifting android. And that's a pretty drastic change, so Time can't just duct tape over it and get history back on track; you've created an alternate timeline, and now she has all that paperwork to deal with.

Meanwhile (or whatever the right adverb is), in the history of your own timeline, there was supposed to be a you who was at that New Year 2000 party with your guests, and you're not there. That one isn't nearly as big a change; it's not like you're suddenly going to be King of England because you and your idiot servant stepped out for a few minutes. So there's still a new timeline that Time can avoid, as long as she can get you back there.

So there's a subtle tug, pulling you back to the "right" place in the multiuniverse, the one you're supposed to be in. And that's how you know how far you are from your original timeline, and in what "direction". You can ignore it, and Time is fine with that—after all, you can spend weeks in 1215 and still come back to the same hour in 1999. But eventually, when you miss your nice indoor plumbing, it's not that hard to follow that tug and get home.

No need for any universe-wide beacon; there's something very small, and specific to you, just because the universe doesn't want you changing history and creating yet another alternate timeline.

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Of course if you're clever (and of course you are), you may realize that this means you never have to actually go home. The fact that Time is constantly trying to guide you home means that, once you work that out, you're free to explore all the nearby timelines for one that's a little better than the one you started in, without having to worry about getting stuck in that one where the Aztec Empire won World War II or Experts Exchange bought Stack Overflow. But there's your sequel: what does Time do once you try to get all clever-pants on her?

Answer 7

I recently read a book with a time traveler character who had the same problem. In the story all universes have their own vibration. I have read similar science fiction stories with similar ideas, specifically the flash tv show. I'm not sure how accurate this to real science. In the book it the man used a machine from the future to detected the vibration of the universe. An his uses this to check which universe he is in.

On a side note in the book the time traveler changes universe or "time lines" by going back past the time he "Stepped on a butterfly" and made a new universe.

Answer 8

Perhaps make a sort of "marker" (using it as a loose word here) for your time traveler to know where and when he's been? For example, (taking this from Minecraft) try making marker "walls" so the traveler knows that that isn't the correct universe.

Answer 9

The model of time travel you're suggesting sounds very similar to that used in Steins;Gate. That show uses multiverse theory ("World Lines") to show the effects of the protagonist stepping on various butterflies and then having to undo them. At one point a device is procured, a device which displays a number indicating which world line it's on. This device is said to have been built by the protagonist in the future, where time travel is understood on a much deeper level.

Based on this, I'd recommend the traveller jump into the future and try to find someone that knows what's going on, or perhaps attempt to construct such a device themselves.

A time traveller with this model of multiverses/world lines would have to be very irresponsible to lose track of their jump history, so maybe getting lost is a completely deserved consequence anyway.

Answer 10

Go back to a point in time slightly before you left and make a change to your time machine design that would mean rather than being a time machine, it became a time machine simulator. This prevents you from having to kill your own grandfather to stop you building it and you still get to have your, now simulated, adventures in time.

The paradox will work itself out, honest.

Sorry, I don't think in practice that you can ever go home, though you may be able to create something very similar and long as you keep those pesky quantum weather butterflies away.

Obligatory xkcd: https://xkcd.com/716/

Answer 11

This is very similar to the problem I had in my time travelling stories. I came up with a solution that still has some kinks, but a version of it might work for you:

Let's say your Traveler is from Universe A. He wants to change something in the past to improve that timestream for the better (killing Hitler, investing in Apple early on, expecting the Spanish Inquisition...whatever). He travels back to Universe A's past, and makes his change.

This splits off a Parallel Universe, we'll call it Universe B, where the change exists. Now, our traveler can freely travel back to Universe A, where the change does not exist, or continue along in Universe B.

Now, say there's a copy of our Traveler that comes into being in Universe B, and we'll call him Traveler B (retroactively referring to the first one as Traveler A). This Traveler decides to change something, goes back in time, and spawns Universe C. And so on and so forth.

This creates a problem. In order to realize the effects of Time Travel, you have to be part of the new universe that is spawn off, so Time Travel is superfluous (except perhaps as a touristy thing).

That's where merging comes in. In my particular universe, the various universes can exist in parallel because they occupy different areas of a fifth dimension (which is referred to as probability). So Universe A and Universe B occupy the same place in the four traditional dimensions, but different areas in the probability dimension. To secure any positive changes into a particular universe, a manipulation is done to bring the probabilities in line with one another and merge the alternate universes together (with a healthy dose of Plot-tonium).

This has resulted in there being a "Utopian" universe that the Time Travelers are agents of, and are attempting to seed positive parallels that can be merged in safely and further improve the Utopia.

To answer your question in this particular universe, home always exists and you can easily find it, as you are directly tied to your universe and any changes ultimately impact another (until a merge occurs).

Answer 12

This is happening all the time to me, it's probably quite irritating. The difficulty is, as you surmise, that to be in your original universe it has to be exactly as you left it. The solution is a personal causality reverser based on clockwork and some really, really strong elastic bands, and it seems to have worked so far. I can demonstrate this by pointing out that there is no measurable difference between the universe I left and the universe I returned to. Unfortunately, since my memories are stored chemically, and they cannot be different from my departure, even I cannot remember leaving the universe or any of the fantastic events that overtook me on my travels.

Answer 13

This is an interesting question. I would propose two possible solutions:

First, I would say to use the "time machine" or whatever method of travel to go back in nonlinear time to the point in which the adventurer first left home. Since that action did take place it would have to exist somewhere in time and therefore one may travel there.

Alternatively, I would say that perhaps the time traveling that takes place is somewhat like a pool. The character jumps in and swims around and explores different realities but at any point can choose to get out of the pool and would thus land safely back in the original dimension from whence he or she came.

Hope this helps!

Answer 14

Is there a way (or theories at least) to differentiate between these universes; some observable physical/quantum/random feature that is in fact not exactly the same in each universe?

You need a big-bang.

I dislike the theory of the multi-verse, and much more that anything branch one. In the imaginary scenario this is true, I imagine that most universe "pop" then imediatelly "plop" and die. You need a lot of energy to build ANOTHER, and very actually few new universes last.

And you need a UNIVERSE OF UNIVERSES where, like with supernovas and whatever is the name to sun-births, it could be observed... so:

• You record the exact moment your universe and other ones branches are created/destoryed
• Or measure the "this-universe-bang-cosmic-radiation" from the universe you have and check against the "big-bang" of the UNIVERSE OF UNIVERSES or something like that.

But then how you have access to the UNIVERSE OF UNIVERSES measures? Is as easy as travel in time, duh.

Answer 15

In principle no.

By your definition of multiverses - a difference of one quantum event - you can never for certain determine if you are in the "right" universe. Because many (all?) quantum events cannot be observed without changing the observed system, the very act of measurement changes the universe you are in, which means you cause a new universe to be created (a split) with your measurement. So even if you were in the right universe when you did the measurement, afterwards the distinction has become meaningless and, more importantly, impossible to prove the "before" state.

Answer 16

This is a matter of transtemporal navigation. While it is easy to get lost in the multiverse, but it's easy to get lost anywhere if you don't go about systematically and navigate your way, so going home even in the multiverse is only about navigation.

The straight forward way of showing how this can be done is to take your example and point out what your time traveller has to do. Firstly, to navigate his way through time and secondly how for him to travel back home.

The time traveller (TT) travels from their home-time 'Home' into the past and arrives at Past A. A branch time line forms. You suggest this happens either accidentally or on purpose, I believe branching timelines will form to prevent causality violation where causality violation involves doing something to change TT's own past. This is one of the classic solutions to problem of how to have time travel without causality violation. It's a really good way to do time travel.

Now TT goes forward into the future and instead lands in Future 1 which isn't 'Home'. Of course, it's not because TT was going the wrong way to get there. For TT to go 'Home' they must first backwards in time and down branch timeline until they have travelled to a point before their initial landing. The time machine reverses its engines and changes its direction while still in transtemporal flight. Now if TT travels the same distance in time that they had travelled on their trip back to Past A they will reach the temporal position of 'Home'.

If TT lands at this point and another branch timeline forms that will be because TT has arrived in their own past. This will result in causality violation but this circumvented the formation of the branch timeline. TT performs essentially the same manoeuvre as their departure from the branch timeline spawned at Past A. Backs up and goes again attempting to land again. This time if no branch timeline forms TT will have arrived at a point in time after their original departure from 'Home' so there will be no causality violation. Of course, TT might have overshot and landed a century after their departure from 'Home'. Well, in that case, TT has only to travel in time by a century and land 'Home'. TT will have to do the same sort of checking to see if branch timelines do or don't form. If they don't, then TT has returned 'Home'.

Basically in the vastness of the multiverse the one place where TT can safely land, after a transtemporal flight without creating branch timelines and is closest to the moment in time when they left is that moment we can call 'Home'.

I am going to run over this model of time travel and point out possible ways a time traveller can navigate the multiverse and what techniques are needed to do so. I am going to assume the time traveller (TT) can travel along timelines and can do so knowingly. TT can travel up and down a given timeline including branch timelines. Even travel sideways with respect to the timeline they are in this takes them into alternative histories. The other possible versions of history related to the point in time they shifted sideways from.

This is a essentially re-run of TT's journey. TT travels into the past, Now if TT can observe the presence of branch timelines, then all they have to do is count how many branch timelines they pass on the way to Past A. Arriving in Past A and now the branch time line has formed. Let's call it A1. TT sets off once more into the future. This time heading up branch timeline A1 and arrives at the future of A1, say Future A1.

Having arrived at Future A1, how does TT get home? Essentially this is a matter of backtracking and also keeping track of how has passed during transtemporal flight and the time spent at any of the times TT visited. Then it is mainly a matter of turning around travelling back down the branch timeline A1, going down until they below Past A and performing a mid-flight turn and heading back to 'Home'. Counting the branch timelines along the way and seeing how far in time they have traversed.

TT will sensibly land at 'Home' a minute or two after their departure. Being able to find 'Home' will always be a matter of navigation.

To be able to navigate the multiverse a time traveller needs to do certain things. Keep an accurate record of the distance in time they travelled and be able to travel back along that path to get home. Ideally to have a method of detecting when and where branch timelines have formed. The two time trips of TT I used as examples use both methods. Together they would quite powerful.

Another way of doing this would be a beacon that can send signals out across the multiverse time travellers would then only need to home-in on the beacon located at 'Home' to to get home again.

Knowing when and where in the multiverse a time traveller was would be a matter of knowing how far (temporally) they had gone, knowing how far they gone sideways in time too, and knowing how many branches they had passed through. This is like making of their journey through the multiverse. Essentially this is what early explorers did when they set out to explore the rest of the world. They had the advantage of having the Sun and the stars in the sky to use as reference points. A cross-time beacon at 'Home' could act as one reference point for navigation.

Navigation beacons for time travel would work if we assume there is a relatively speaking a 'single' timeline and branch timeline only form to prevent causality violation. TT only has to backtrack on any branch time line until they reach the main timeline and follow the beacon home thereafter.

Won't the multiverse be full to bursting with multiple timelines? I hear you cry! Yes, of course, but relative to time travellers there will be their 'native habitat' timeline. Branching timelines, again relative to our time traveller will be as long as the time the time traveller has spent in it, also depending how divergent the future history in any give branch time line there may not be a navigation beacon in its future or if there is then it may be located at different points in time from the navigation beacon located in time traveller's own timeline.

Offhand I can't think of anything that could act as navigation reference points for time travel. Pulsars can be used for accurate time measurement, but that only tells a time traveller how far they are in the past or future. It's no good for measuring travel into alternative time. Unless there were different pulsars in the alternative timestreams, but a time traveller would need to know the disposition of pulsars in those alternative timelines to know where they were.

Tropes like each universe has its own vibration are (a) dumb ** (b) used too times for its own good or anyone's else which leads to (c) we've seen this too many times before to be even remotely interesting.

**: What does it mean that a universe is vibrating? In which dimensions is it moving?

There is a major problem with any discussion of time travel. Unless you have a clearly delineated concept of what constitutes time travel it is possible to be talking about one form of time travel and unknowingly slip into talking about a completely form of time travel, then end up wondering none of this makes any sense.

It is possible when time travel is invented there will be a thorough understanding and knowledge of what when you travel through time. In this case, it should be straightforward for a time traveller to move whenever and wherever in time and still get back home without any trouble.

This could also depend on what sort of mechanism makes time travel work. Also time travel might work in a way that you have a Big Machine at 'Home' and the time traveller wears a backpack time machine slaved to the Big Machine. The Big machine does all the hard work of moving the time traveller around, so the backpack might be a communications and remote control unit or it could be a necessary part of the whole machinery needed to make time travel happen. Either way, the time traveller can be simply be hauled back 'Home' when they have finished their latest mission.

On the other hand the nature of time may be so complex and fluid that even a single short-range hop through time is so completely fraught that time travellers become totally lost in the multiverse and never able to return to their home time.

It makes more sense that time travel will be a mixture of being able to steer safely and accurately combined with difficulties and uncertainties. The difficulties and uncertainties may not be overwhelming. They may be on the same scale as navigating an aircraft. Now I couldn't tell anyone how to navigate an aircraft, because I plain don't know how to do it, but there are lots of people who do. Any time traveller may be in the same position as an aircraft pilot. Just do the right set of things you need to do to navigate and you safely travel from one time to the another and know how to steer yourself back to home base.

If time travellers have detectors to locate when and where a timeline branches. If they can detect when causality violation occurs. If they can follow timelines like a driver along a road. Even if this includes driving cross-country when they travel sideways in time. Time travellers will be able to get home again by following their 'footsteps' back to when they started.

This means while you can't identify which universe you're in, there's no plausible way of doing that unless you want to have fun handwaving at hurricane-force levels and making up anything you want to and hope it sounds good. However, if a time traveller navigates their way through the multiverse they can have an idea of where they are in relation to the time they started off from, and if they do it right find their way home again.

It took a long time getting here, but I hope you can navigate your way through it.

Answer 17

One solution would be to have time travel be accomplished via stable portals (like traversable wormholes, a theoretical solution to the equations of general relativity) that form persistent bridges between two timelines, with the timelines diverging at the moment the past end of the portal first appeared. Say in the year 2100 I create a portal such that whenever I through it, I'll exit through the other end of the portal 200 years in the past--so if I enter in 2100 I'll exit in 1900, if I enter in 2101 I'll exit in 1901, etc. And suppose the way it works is that I'll always end up in the same timeline, call it timeline B, when I exit--a timeline that was identical to the timeline A which I came from up until the year 1900, but with timeline B diverging after 1900 due to the appearance of a portal which didn't exist in timeline A (and the appearance of various time travelers who stepped through the portal in timeline A).

Of course, once in timeline B I could use the same technology to create a new portal to the past, say going back 300 years, so if I created it shortly after emerging in 1900, then stepping through it would take me back to 1600. In that case, this new portal would take me to a third timeline C which diverged from timelines A and B in 1600. But if the portals work in the opposite direction (as traversable wormholes would be theorized to do), then if I step back through the portal in timeline C I'll end up in timeline B sometime after 1900, so I can then find the original portal that created timeline B and step back through that one, getting back to the original timeline A. So if it worked this way, no matter how many portals you create you can always "retrace your steps" through a series of portals to get back to whichever one you came from (assuming none of the portals are destroyed or otherwise shut off).

Answer 18

You shouldn't need to in the first place.

Because of (mumble mumble handwave mumble mumble), the universe conserves improbability. Time travel (and, for that matter, inter-universe travel) being an incredibly improbable thing and all, this means that (more mumbling), with the result that you almost always end up in a universe which is almost exactly like the one you left.

If, for reasons of handwavium, your Time Travelling Phone Booth Which Totally Isn't The TARDIS ends up in a drastically different universe, you have two choices:

1. You can decide you like the universe you've ended up in, and thus stay there.
2. You can engage the Time Warp Mark 2, which will boot you into another drastically different universe. You then keep doing this until you end up in a universe you like.