Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bill S on 31/05/2014 21:50:02
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No one who is familiar with my posting history will be surprised that I have thought quite a bit about the idea of an infinite multiverse. For example, I ask myself if, in an infinite multiverse, there would be a universe at an infinite distance from ours.
My answer has to be “yes”. In fact there must be an infinite number of them.
I then ask if, in principle, I could travel to such a universe. Yes, if it’s there I must, in principle, be able to go there. Unless, of course, the other universes occupy “dimensions” to which we have no access; but, in that case, in what sense can this be considered a multiverse?
If I go there, will I have reached infinity? No, because there will still be universes beyond that, on the same straight line; at least one of which will be infinitely far away.
There’s more!
Not only do we perceive our Universe as being at the centre of this infinite lot of universes; we must physically be at the centre, because universes go to infinity in every direction. The trouble is that every other universe is at the centre.
When, some time ago, we talked of a similar situation involving an infinity of nothing, it was simpler because there is no movement relative to nothing. Now, we have movement relative to universes. However, this is still not motion relative to the infinite multiverse. Where ever we go, we are still at the centre.
Does this begin to sound ridiculous? I think it does. Undoubtedly one could resort to mathematical infinities to find a way round this, but I feel that’s just papering over the cracks.
One could always argue that, outside of maths, infinity is a philosophical concept, and that may well be true, but that must call into question the validity of using the infinite multiverse in physical arguments.
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To nail down this question, you need to pick a multiverse theory. Having done that, the theory should presumably specify the meaning of distance. Only then can you start talking about ideas like distance and the center of a set of multiverses.
For example, we could talk about the many-worlds intepretation of quantum mechanics. In this case, we can't access other universes, so there's no reason to define distance in the sense you use it and the problem doesn't exist. On the other hand, we could define distance to be the number of steps removed from our own universe another one is. In this sense, the idea of center is questionable, since this number would range from 0 steps (within our own universe) to infinite (or a huge number if you prefer not to dabble with infinity).
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JP, you rarely disappoint. :)
Something that looks quite straightforward and logical from my "hitch-hiker" perspective suddenly becomes more complex and needs more thought.
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To nail down this question, you need to pick a multiverse theory
That is not as straightforward as it might seem. A multiverse theory would suggest that we are talking about numerous universes, and such often seems to be the case until the “nailing down” process starts, then the universes can become much more ethereal, or not exist as anything other than a convenient way of thinking about a problem. Experts may well accept this automatically, but how many pop sci books that explain this to non-experts?
Are there, in fact, multiverse theories in which the universes are seen as being real?
I can see that in QM the many worlds concept conveniently provides an explanation that sidesteps a problem that does not need an answer unless one is looking for a reality that might underlie QM, which is probably philosophy.
If the many worlds are just a convenient way of thinking, does the theory actually need a multiplicity of imaginary universes? I ask this because I wonder if the theory would work with, for example, one universe “embedded” in an infinite cosmos in which every path/outcome already existed. If simplicity is of any worth, this explanation would, surely, have merit when compared, for example, with David Deutsch’s trillions of shadow universes, each of which is as solid and real as ours.
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Are there, in fact, multiverse theories in which the universes are seen as being real?
You have to define "real." If you mean physically accessible/observable by us, then I don't think there's a well-accepted multiverse theory for that. You also have the issue that as soon as you can access/observe something, it is arguably part of our universe.
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David Deutsch has argued eloquently that in the double slit experiment trillions of "shadow" photons from trillions of other universes have a physical influence on photons observable in this Universe. I think that must impute a degree of reality to the universes of the multiverse in that theory.
...as soon as you can access/observe something, it is arguably part of our universe.
I guess that would mean that all those other universes, in Deutsch's theory, must be part of our Universe - arguably?
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In many-worlds, all universes branch off at a point in time, which is why they can each influence each other at that instant, but going forward, there can be no communication between universes. I'd argue this makes the independent universes, that are created at the point of measurement. I assume you could also argue that these many universes exist all along and somehow interact at the point of measurement and never again, but that seems somewhat less satisfactory to me. Since it's just an interpretation of the math, you're free to add whatever features you want so long as it agrees with QM.
In general, there's no standard definition of a universe beyond our own, so you'll likely find many alternative definitions of multiverses. There's also no standard definition of what makes a universe separate vs. part of our own. I'd tend to say that if we can interact with it freely or semi-freely by exploiting the laws of physics within our universe, it is part of our own universe. For example, if there are hidden dimensions as in string theory, these should be part of our universe, even though we can't macroscopically "see" them. Or if we live in one slice of a higher dimensional universe, such as in brane cosmology, then this higher dimensional structure is arguably all our universe. If you go more sci-fi, and think about jumping into a black hole to get to another universe, this would probably not be "another universe" since you could traverse the black hole to get to it. I'd tend to lump it into our universe, albeit with some specific laws of how to access it.
In the many-worlds interpretation, there's no way to access these other universes, aside from perhaps their effects on measurement at one instant in time, so I'd argue they are not part of our universe.
But it's a fuzzy line and has the typical lay-science problem of trying to apply a couple of lay terms to complex theories, all of which differ from each other.
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David Deutsch has argued eloquently that in the double slit experiment trillions of "shadow" photons from trillions of other universes have a physical influence on photons observable in this Universe. I think that must impute a degree of reality to the universes of the multiverse in that theory.
You could argue that this is the case in the Everettian many-worlds QM interpretation - the universe you find yourself in owes its existence and particular history to the evolution of the wave function of the multiverse; in that sense it depends on all the other universes.
...as soon as you can access/observe something, it is arguably part of our universe.
I guess that would mean that all those other universes, in Deutsch's theory, must be part of our Universe - arguably?
You can define the universe to include whatever you like from your meta-cosmology, as long as you make it clear and it helps to explain what you want to say.
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In some versions of string theory, there are a large number of parameters that can take on a wide variety of values, without any obvious reason to pick one value above another (except that our universe is one that supports hydrogen-based stars surrounded by silicon-based planets, at least one of which carries carbon-based life).
Some theorists have suggested that many universes may exist, each with their own set of parameters.
This implies that in some of these universes, gravity is much stronger than ours, and the universe will have collapsed to a singularity, while in others with much weaker gravity, space is very cold, matter is very thin, and there are no stars and no visible light. (Not to mention that electromagnetism may be much stronger or weaker than in our universe, which might have equally unpleasant effects on us...)
But there is another barrier, closer to home: common versions of the Big Bang theory seem to suggest that even if you were able to accelerate to within a whisker of the speed of light, you could not reach the edge of our own universe, let alone cross over into some other universe.
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But it's a fuzzy line and has the typical lay-science problem of trying to apply a couple of lay terms to complex theories, all of which differ from each other.
Point taken. I suppose the important thing from the point of view of science is finding models that make "sense" of the maths. Fuzziness then becomes a means of maintaining the sanity of scientists. Only we hitch-hikers need to ask questions like: If a universe is created at a specific moment, where does all the matter and energy come from?
Of course, this is easily countered by: Define "created"; or It's already there.
Why am I chasing these vague concepts round in circles? I suppose the bottom line is that I want to understand why people (scientists, in this case) think the way they do.
By way of a specific example: Does the multiverse have advantages over the idea of an infinite cosmos in which everything that can happen exists already? I'm not just looking for an answer to this; there may well not be one. What I'm really looking for is more like: Why would scientists prefer one over the other?
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This implies that in some of these universes, gravity is much stronger than ours, and the universe will have collapsed to a singularity, while in others with much weaker gravity, space is very cold, matter is very thin, and there are no stars and no visible light. (Not to mention that electromagnetism may be much stronger or weaker than in our universe, which might have equally unpleasant effects on us...)
All this is great if it actually helps someone to visualise something, but beyond that, must be "fairytale physics".
But there is another barrier, closer to home: common versions of the Big Bang theory seem to suggest that even if you were able to accelerate to within a whisker of the speed of light, you could not reach the edge of our own universe, let alone cross over into some other universe.
This seems to assume that other universes are outside/adjacent to ours. This wouldn't work if Deutsch's theory is correct. It would also imply FTL action if the universes are created instantly.
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the universe you find yourself in owes its existence and particular history to the evolution of the wave function of the multiverse; in that sense it depends on all the other universes.
I have difficulty understanding why a multiplicity of universes is either necessary or preferable to a "single" infinite state.
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I have difficulty understanding why a multiplicity of universes is either necessary or preferable to a "single" infinite state.
As mentioned previously, it depends on the multiverse you're talking about. In quantum mechanics, the maths tell us exactly what to expect from an experiment (Schrodinger equation, etc); problem is, although it works better than any theory we've ever had, the results don't make intuitive sense, so a number of interpretations have been developed of what QM means in terms of our image of reality. The Everett 'many worlds' multiverse is the simplest (has fewest awkward problems) interpretation of what the maths is telling us - that the evolution of the Schrodinger wave function for the universe reflects how the universe develops and there's no local wave function collapse; when an interaction occurs for which there are multiple possible outcomes, the outcomes are superposed as separate non-interacting 'layers' or universes, and any observer is also superposed with them, so you get a 'version' of the observer for every outcome, each aware of only the result in their 'layer'. Things like 'spooky action at a distance' when measuring an entangled particle are thus explained as a 'you' discovering which universe he happens to find himself in. It works, but is hard to swallow. My discomfort is that it suggests that everything that can happen does happen in one universe version or other, so there are some pretty weird & unlikely universes out there... A whole new universe isn't created every time, I've heard it described more as a vast number of layers that start off identical and diverge from one another in a branching structure.
Some other multiverse theories are implied by the physics used to describe the development of our own universe. For example, inflation - the incredibly rapid expansion of the universe shortly after the big bang, like a bubble popping out of solution in a soda - is extremely successful in explaining what we see today, and has made predictions that were borne out by observations. The physics that is thought to have triggered inflation implies that it should happen multiple times, as if the top was removed from the bottle of soda and a rush of bubbles forms and continues to form, each one isolated from the others. There are a number of variations on this theme.
If you want to stick with a single state, infinite in extent, then you can divide it up into multiple volumes each the size of the observable universe, and so forever beyond reach of each other. They are all part of the one universe, but might as well be separate universes...
And so-on. They're all a consequence of following up the implications of the physics we use to describe our own universe; some of them may even be falsifiable.
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Dlorde, I have no problem at all with most of what you say. However, I have some reservations about this bit:
If you want to stick with a single state, infinite in extent, then you can divide it up into multiple volumes each the size of the observable universe, and so forever beyond reach of each other. They are all part of the one universe, but might as well be separate universes...
I will avoid any diversion into discussion about the divisibility, or otherwise, of infinity. Assuming that your “multiple volumes” are a possibility, if they are forever beyond the reach of one another, how would information pass from one to another to ensure that the necessary range of outcomes was appropriately distributed?
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... if they are forever beyond the reach of one another, how would information pass from one to another to ensure that the necessary range of outcomes was appropriately distributed?
You've lost me - what necessary range of outcomes? why would anything have to be ensured?
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To invent an unending multitude of scenarios to justify what mathematics is telling us seems to imply a fault in the mathematics. Make it as simple as you can but no simpler.
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To invent an unending multitude of scenarios to justify what mathematics is telling us seems to imply a fault in the mathematics. Make it as simple as you can but no simpler.
I think it's as much a question of of finding new hypotheses as justifying (I presume you mean testing) the mathematics; the results may not always be testable, but they can open new ways of looking at the physics.
Quite a few past discoveries are the result of exploring the implications of the maths behind existing theories - black holes, for example.
The Everett 'Many Worlds' hypothesis is the most literal interpretation of QM, which eschews awkward ideas like wave function collapse, spooky action at a distance, and so-on; it just says don't invent anything, take the Schrodinger equation as a description of the way things are. None of the QM interpretations can avoid the problem that QM is non-classical and horribly unintuitive.
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You've lost me - what necessary range of outcomes? why would anything have to be ensured?
Assume that everything that can happen does happen, but in different universes, that these universes already exist and that they have no contact with one another.
The range of possible outcomes in an experiment carried out in our Universe is A,B,C, & D.
Outcome A occurs in the experiment in our Universe. How does information get to other universes to ensure that outcomes B,C & D occur there?
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Assume that everything that can happen does happen, but in different universes, that these universes already exist and that they have no contact with one another.
The range of possible outcomes in an experiment carried out in our Universe is A,B,C, & D.
Outcome A occurs in the experiment in our Universe. How does information get to other universes to ensure that outcomes B,C & D occur there?
As far as I know, in the 'many worlds' multiverse, the branching superposition of outcomes is described by the evolution of the Schrodinger wave function; how this is supposed to occur, I can't say.
In a universe of (possibly) infinite extent, what will happen in any particular observable volume is not dependent on any other, and (as far as I know) there is no absolute requirement for everything that can happen to happen; although an infinite extent of randomly distributed particles might be expected to have an infinite number of volumes the size of the observable universe with identical distributions of particles. There have been statistical calculations of the average separation distance to be expected between volumes with identical content (a finite, but extremely large distance), but I'm not sure whether these estimates are based simply on duplicate random distributions of particles, or whether they account for the evolution of those distributions into planets, stars, galaxies, etc., which would involve stochasic quantum events which would presumably cause similar starting states to diverge very quickly. I've not looked into this model in detail.
Just my layman's take on what I've heard...
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You've lost me - what necessary range of outcomes? why would anything have to be ensured?
Assume that everything that can happen does happen, but in different universes, that these universes already exist and that they have no contact with one another.
The range of possible outcomes in an experiment carried out in our Universe is A,B,C, & D.
Outcome A occurs in the experiment in our Universe. How does information get to other universes to ensure that outcomes B,C & D occur there?
If there are infinitely many universes, and some probability of each of the outcomes A,B,C & D, then there is no need for any communication between universes. There will be infinitely many universes in which the outcome is A, infinitely many with B, infinitely many with C, and infinitely many with D. The ratio of probabilities of each outcome will determine the ratio of universes with that outcome (I know ratios of infinity is dangerous territory; I define it thus: if one could go to any universe with equal probability, then the probability of ending up in a universe with outcome X is the same as probability of X)
In short: With infinitely large sample size, there is no need for communication/collusion to assure statistical distribution of the sample.
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Our thinking is so locked into our dimensions of space and time that we tend, automatically, to transfer that thinking to the infinite. That works quite well if we are simply using mathematical or quasi-mathematical imagery. We can “do science” without thinking beyond an infinite number of universes, an infinite cosmos composed of an infinite number of divisions, or any other manifestation of the infinite series. Similarly, we can use QM very effectively without trying to think beyond the acknowledgement that it works.
In both cases, perhaps we should leave the “beyonds” to philosophers.
Anyone know a good forum for discussing the philosophy of science. :P
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I know ratios of infinity is dangerous territory
Absolutely! That could take us back to the infinite lottery in which an infinite number of people stake an infinite amount of money, an infinite number of people win infinite jackpots, but the same infinite number of people win nothing – and so on.
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Not knowing well accepted meaning of multiverse it is hard to say whether your multiverse theory can be merged with my multi dimension theory.
As I posted in thread http://www.thenakedscientists.com/forum/index.php?topic=51657.0
is it impossible that we are living in the universe which is further separated by sort of like frequency? Something that is not yet known but it is affecting us all this time.
And in that other dimension, forces that we cannot see or sense can be seen.
so dark matter or dark energy may exist there in different form which is also affecting our current dimension and that is why we are not able to see these energy and matter.