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Regarding the Many World Multiverse, everything I've been reading (and searched on SE) seems to assume that each (parallel) universe is, at this moment, the same age.

For my story, I need this to not be true for my time-traveler. By making it not true, is it no longer within the scope of the Level III: Many World (Multiverse) interpretation of quantum mechanics? Is it within the scope of some other theory?

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  • $\begingroup$ ekpyrotic model which is an extension of brane theory claims that the observable universe is created when 2 parallel membranes collide, this collision although never been detected can happen at any moment thus hinting at many new baby universes constantly emerged. $\endgroup$ – user6760 Jan 15 '17 at 3:54
  • $\begingroup$ Any universe that would bare a passing resemblance to ours would be approximately the same age. For universes of different ages, the initial conditions would be likely so different to be unrecognizable from our own, perhaps incompatible with our form of life. $\endgroup$ – Isaac Kotlicky Jan 15 '17 at 14:18
  • $\begingroup$ @user6760 The ekpyrotic model is certainly a part of my thinking, as it may relate to a certain type of multiple time dimension (if I can take liberties to reconcile it to a pseudo-MWI). $\endgroup$ – James Olson Jan 15 '17 at 16:27
  • $\begingroup$ @IsaacKotlicky Exactly what conventional wisdom seems to insist and what I'm hoping to work-around; primarily by finding a plausible explanation that early divergent universes would have no significant reason to be different other than their relative delta t to a constant Prime cosmological t [if I stated that correctly]. $\endgroup$ – James Olson Jan 15 '17 at 16:34
  • $\begingroup$ @JamesOlson how precisely can your characters slip into alternate universes? "Nearby" same t universes would logically be more proximal than same condition but later/earlier t universes. If your characters can target based on physical constants, they stand a chance of reaching similar universes from a different t... $\endgroup$ – Isaac Kotlicky Jan 15 '17 at 17:34
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I don't think there's any reason you couldn't stagger "now" between different worlds, but you may have some explaining to do.

In most multiverse theories, the argument is that anything that could have happened did happen, just in an alternate universe. This includes all sorts of things that could have happened, and the vast majority of timelines look nothing like ours! Consider that there's been billions of years of events happening from the big bang to the present. That's billions of events that could shape the evolution of the human race... or prevent it from occurring at all. There's no way to know the numbers for real, but it's reasonable to assume that the number of universes where humans even exist are vanishingly small compared to the number of universes out there.

This is all fine and dandy for exploring quantum mechanics, but for telling a story it's inconvenient. You don't have thousands of pages to document all the failed and boring timeline jumps that occurred before you found somewhere worth going to. So typically worldbuilders creating a multiverse handwave in an assumption that there's some sense of "nearness." Each universe has some sort of "neighborhood" of similar universes but with some small change. Typically these something that's easy for the reader/viewer to understand such as "Germany won WWII," even though quantum mechanically it's not clear why that would qualify as "near" our universe. But, nontheless we use this nearness or sameness concept to make sense of our multiverse. It brings some sanity to the worlds as we explore them.

You will have to use this process in your own literature to tame the wilds of the multiverse. However, you don't have to use anyone else's definition of nearness. While it is common to assume "two points at the same 'now' are near to each other in the multiverse," that's really a matter of convenience, not a physical law. If you want to stagger "now's," go for it!

The one thing to be aware of when doing this is Sanderson's First Law of Magic, which I quote so often I really need to have some notepad file up with it rather than looking it up every time!

Sanderson’s First Law of Magics: An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic.

You will certainly be resolving conflict with your multiverse theories and whatever approach you develop to jump between universes. The better a reader understands this process, the more you can do with it. In your case, one question to ask yourself is "why isn't it different?" If timeline A is 5 hours behind timeline B, why is it that way? Why is it not timeline A 3 hours ahead of timeline B? Surely that is an equally probable staggering of "now!" If everyone is at the same time, justifying it is easy. If universes are staggered, you have a bit more justification to do. It's certainly not impossible to do, just something to be aware of as you go forward with a slightly less standard approach to multiverse theory.

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  • $\begingroup$ The bulk of your answer explains what we all (should) know and then you repeat/reword my question in the last paragraph. Did I miss something? $\endgroup$ – James Olson Jan 15 '17 at 13:37
  • $\begingroup$ The first part is tearing down assumptions of what a multiverse must look like, because most of those assumptions are really just for convenience purposes. The last paragraph is pointing out that, if you don't choose to follow those assumptions, you're going to have a bit more explaining to do than most authors regarding why travel between universes works the way it does in your story, that's all. $\endgroup$ – Cort Ammon Jan 15 '17 at 15:17
  • $\begingroup$ I suppose technically I didn't answer your actual question, which was about whether what you described was still a Level III many worlds multiverse. I suppose I can answer it here, but the answer is rather boring. "No, what you describe is not consistent with the many worlds interpretation of quantum physics because MWI does not include any way to travel between the different universes and your system requires that." There's also no concept of time travel either. That answer's boring though... go out and write fun things! $\endgroup$ – Cort Ammon Jan 15 '17 at 15:20
  • $\begingroup$ Sorry for the confusion; ignore, "For my story, I need this to not be true for my time-traveler." Now, is what I described (parallel universes of a different cosmological age) consistent with the MWI of quantum physics? $\endgroup$ – James Olson Jan 15 '17 at 15:35
  • $\begingroup$ If you say you can travel between them, are they really "parallel?" in MWI, the universes are permanently separated. You're going to have to play games with information that are not typically played. For example, if you observed a particle as horizontally polarized in one universe, and then went to another universe, you're going to have to reconcile what we can know about the particle in that other universe. I'd argue that adding the idea of travling between universes may completely undo the concept of separating universes in the first place, making the type scales silly. $\endgroup$ – Cort Ammon Jan 15 '17 at 15:39
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Of course there are other theories.

There's even one which is contained within the scope of the one who seem to wish to use: Level II of Tegmarks hierarchy. Which states that within the multiverse, there exists a infinitude of universes having each different properties to our own. Any number of properties could explain why the entire causal chain of a specific universe would be out of sync relative to ours yet be effectively identical other than in that one measure.

If you don't want your "time-traveler" to be able to jump into worlds where the laws of nature are completely different (and where he would immedietely die if he were to go to them) and yet want an explanation for why he can't(or doesn't accidentaly do so), simply handwave the problem away by making whatever method you're using to hop realities only work with universes which are "compatible", that is to say ones with universal constants that are sufficiently similar to the original reality. This seems perfectly reasonable to me.

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  • $\begingroup$ "Any number of properties could explain why the entire causal chain of a specific [parallel] universe would be out of sync relative to ours yet be effectively identical other than in that one measure." Such as? $\endgroup$ – James Olson Jan 15 '17 at 13:39
  • $\begingroup$ Speed of light different? $\endgroup$ – Willk Jan 15 '17 at 15:37
  • $\begingroup$ Are you saying there's a probability that a parallel MWI could have a different constant (ie. c) but be identical in every other way? $\endgroup$ – James Olson Jan 15 '17 at 15:59
  • $\begingroup$ @JamesOlson Not only different constants, but also additional constants. For example there is a probability that a universe has a constant(let's call it "flerb") which simply stops the universe from beginning to expand for 3 years, after which the flerb constant simply stops having an effect on the universe. Yes, it sounds strange but there is no reason why this wouldn't be. $\endgroup$ – AngelPray Jan 15 '17 at 16:08
  • $\begingroup$ @AngelPray Interesting. I have to think about that. $\endgroup$ – James Olson Jan 15 '17 at 16:39
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Short answer:

There are 2 cases: either the universe splits with each measurement, or the 'path of measurements' of each universe is already mapped out, even though it measurement outcomes still appear probabilistic.

In the first case, fundamental constants can be changed, allowing for different universes to appear to be different ages, although this is really a result of changing the (non-fundamental) laws of physics, so things just happen at different rates (if they even happen at all). Alternatively, handwave that the big bang is probabilistic and so the multiverse is constantly splitting into new universes, giving multiple copies of the same universes, only at different times.

In the second case, big bangs can happen at different times, so different universes can be different ages as required.

Long answer:

As details about the various interpretations about quantum mechanics are fairly complicated, let's assume that there was a big bang at a multiverse time $t = 0$ (ignoring all the complicated details that this brings up) and that time travelling from a universe with multiverse time $t$ in that universe will bring you into a different universe, also with multiverse time $t$ in that universe (if not, then you don't even have a problem to begin with). Now, there are two possible ways that things could go using the Many Worlds Interpretation (MWI):

  1. Every time that a measurement (in the quantum sense of measurement), the existing universe splits with everything in the new universes being the same, except for the measurement outcome, so that everything that could happen does happen in some universe within the multiverse, which originated with the big bang
  2. There was a big bang for each universe. The measurement outcomes for everything that will be measured in each universe is already 'determined' in some sense, even though it appears to be probabilistic (a bit like the two state vector formalism of quantum mechanics). Perhaps each black hole contains another universe or somesuch, with the multiverse being the originally created universe, containing all the other universes

Case 2 is straightforward - other universes can be younger or older than our own, depending on when it was created relative to our own.

Case 1 is probably what most people think of when they think of the MWI. There are a couple of ways to solve this:

    1. (Messy)

Things start to get a bit more complicated here, due to time. Here, a universe that only recently split off from our own is going to be almost the same as our own, including having the same time. However, a universe that split off right at the beginning may have different physical constants (actually a level II multiverse and entirely equivalent to level III). Now, if (as an example) you change the speed of light, $c$, things will start travelling through time at a different rate than in our own universe due to relativity, so although the alternate universe will have the same cosmological time, it will appear younger or older, although at the risk of the universe being considerably different to our own. Having said that, the universe is billions of years old and assuming you only want to travel a few hundred years back (or forward), the changes might not need to be that great. Although you're still messing with the fundamental constants of nature, which is messy...

    1. (much neater, although currently even less provable science and requires more handwavium.)

What was the big bang? Well, no-one knows, except that it was extremely hot and that it suddenly expanded. Now, things like entropy would cause this expansion, which suddenly makes the whole process probabilistic. That is, at the time of the 'big bang', the universe splits into a universe where the big bang has occurred and another where the big bang hasn't yet occurred, Despite this, the 'multiverse time' is still somehow increasing (defining time this close to the big bang is an issue) and so within the multiverse, there are an infinite number of big bangs and so there is an infinite number of worlds just like our own, only at a different time according to that universe, even though the multiverse time is the same.

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  • $\begingroup$ "Case 2's "...there are an infinite number of big bangs and so there is an infinite number of worlds just like our own, only at a different time according to that universe, even though the multiverse time is the same." This seems very much what I am wanting for my story but wasn't sure if I could "play with" TSVF to explain this, $\endgroup$ – James Olson Jan 15 '17 at 15:06
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Piers Anthony's novel Orn in one of his early trilogies had a multiverse set-up where different parallel universes existed at different time rates. If you have time travel, there's no reason why there shouldn't be time differences between parallel universes.

In fact, if you were constructing a model to explain time travel a multiverse where parallel universes were at different times (or ages) would work well. Time travel in parallel universes has less problems than most other forms of time travel. No causality violations for starters.

Worrying about whether this concept works in or is contrary to quantum mechanics or any version of a scientific theory about multiverses is irrelevant. Unless you want to publish it as a paper in Physical Review.

If you are building a fictional world, you are allowed to make it work however you want it to; though it does make sense to try and avoid any obvious errors. Time relations or differences between parallel universes is an open question. Science doesn't have an opinion either way or another, therefore, you're free to do whatever you like to make your story work.

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  • $\begingroup$ "Time travel in parallel universes has [fewer] problems than most other forms of time travel." I may have to re-think the wording of my question if others believe "traveling back in time is not = traveling to a parallel universe". $\endgroup$ – James Olson Jan 15 '17 at 15:16
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What is the “same moment” between different branches? If travel is possible between different points in time across branches, does the idea of synchronization even make sense? What kind of observer is saying the same time on different lines are “lined up” or “not lined up”?

In The Proteus Operation, connection between the two ends of an established time link (which will always connect different timelines!) was complicated by the fact that the past end was experiencing time faster than the future end. This would provide a sense in which different timelines share a present; the point at which the rates are the same at each end of the connection.

But the idea of a universal present moment doesn’t even exist in our normal single universe! It makes sense that there is no implicit lining up of timelines, unless you provide reference for what that actually means. And then you can push that meaning in interesting directions to make more surprising consequences or unusual situations.

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  • $\begingroup$ There's actually a thing called cosmological time, defined by the 3D surfaces, $\Sigma$, of constant t in 4D spacetime. These surfaces (which in the FRW metric, evolve with time, giving the different surfaces in the first place) are then threaded together to create what's essentially the time-evolution of the universe, as well as the idea of the cosmological 'present moment' $\endgroup$ – Mithrandir24601 Jan 15 '17 at 12:01
  • $\begingroup$ So you can find points on each timeline that are the same age. You can set your clock and navigate up and down the line. But in what sense are different timelines “experiencing” the same present time at the same (meta?) time? $\endgroup$ – JDługosz Jan 15 '17 at 12:04
  • $\begingroup$ I'm not 100% sure what you're asking, but from the Earth, at a cosmological time $t$, if we observe something at a distance $d$ (I'll assume that it's stationary relative to us), we're observing how it was at that point at a cosmological time $t - \frac{d}{c}$. If you mean different timelines from different universes, then it depends on what the OP wants - I've assumed that he's asking about travelling between universes at the same 'multiverse time' If this doesn't help, I'm happy to discuss in chat $\endgroup$ – Mithrandir24601 Jan 15 '17 at 12:21
  • $\begingroup$ Since he’s engaging in time travel, he’s jumping to different times in different timelines. The same moment exists in different time lines, but the idea of the lines being synchronized just doesn’t have a reasonable meaning. $\endgroup$ – JDługosz Jan 15 '17 at 20:16
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One reason parallel universes are generally "the same age" is that it is often portrayed as similar universes being grouped closely together - variations on a single timeline being "easiest" to reach (usually demarcated by different decisions at critical points) - though this is generally for story-purposes rather than physics, as I understand it (it being more fun to tell stories of really-nearly similar universes, or really, really different ones).

That being said, I have read stories with different universes in different places in their timelines. Sometimes it's a function of events being placed differently - some event happening "soon" or "late" means the timing of effects and aftereffects will be shifted up or down the timeline, rushing or delaying the historical events that shape the setting. Or events, history, can happen slightly closer together, or further apart, so even though the broad strokes look the same, the "now" is placed in somewhat different places because the events are a bit shifted about in time.

Sometimes, it's a function of some really subtle alteration of universal constants - like each second being a quarter or tenth or a nanosecond longer or shorter, which may not make an observable difference in a person's observations, or life, or perhaps even a hundred lives, but over the universe's lifetime will lead to very similarly-developing timelines being in different places in their timelines (or even different ones, though usually you don't need to be meddling in universal constants to just make the universes very different). So the timeline can be off by a few years, or a few hundred, very easily - as long as the parallelism of the timelines being developed justifies the universes being "close together" despite the very fundamental differences in their universal constants.

Or to take it in a completely different direction, one of the theories of the multiverse have new universes branching off at critical decision points - thus the "new" universes are much younger than the ones they branch off of, even if they share the same long history up till that point. In case I misunderstood what you meant when you wanted the universes to be different ages.

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If you want to head into the fantasy side of things, the Fae worlds are generally considered to be running at different time rates.

I see nothing wrong with different universes having different rates of time. None of them would know unless someone were to travel from one to the other and back.

To have any kind of "parallel" universe, the difference in time rate would be very small. After all being 1000 years ahead or behind is a very small fraction of 13B years. The time rate differences would probably not be noticeable (or even measurable) over short periods.

If the time rate difference was something noticeable (like 2x), you would have a case where the Sun had long ago gone nova or, in the other direction, the Sun may be condensing our of interstellar gas.

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