Has this idea been explored? Dark matter as matter in parallel universes...

DavidMcC

Chalnoth, I understand that dark matter shows no sign of gravitational interaction even with itself, but, as I said in my previous post, in my cosmology, that can be because bits of it aren't really as close as they they seem in the LQG hyperspace.

BTW, it was richarddawkins.net/forum to be more precise. I can't believe there's another "Chalnoth" interested in cosmology!

Chalnoth

Presumably you meant non-gravitational interaction. But then, if the dark matter wasn't that close to itself, it wouldn't be close to us either, and thus we wouldn't feel its effects very much.

Misericorde

It doesn't exactly bowl me over than a name from Star Trek would be something you find cropping up in more than one forum related to science in general.

I keep hearing in this thread about dark matter as a product of universes other than our own, but even if possible (and twofish-quant has made that sensible), doesn't Occam's Razor cut far too close for that? I'd want to see native matter eliminated through process and trial before looking to other realities.

DavidMcC

I don't see how "Occam's razor" can be defeat the right kind of multiverse - on the contrary. The problem of conventional, one big bang cosmology are very severe, in that nothing makes sense. Dark matter is but one little aspect of it. Slight parity violation in particle physics, the appearance of fine tuning of the universe for life (the values of the fine structure constant and strong interaction), the variability of the cosmological "constant", inflation, are all better explained by the right kind of multiverse (ie a "fecund universe" type once proposed by Smolin, but I don't want to go through that yet again, there are now so many threads on this , it gets tiresome). I suspect two things : (1) Smolin was too soon with this hypothesis, so that there wasn't enough evidence for it at the time, and (2) there were many rubbish versions of the multiverse, which do, indeed, just complicate things.
EDIT: Also, a single big bang is highly improbable if big bangs are a natural process. Therefore, we only DETECT one with light - the one we're in.

Chalnoth

Yes, Occam's Razor actually prefers a multiverse, even if we don't look at any data. This simply comes down to the fact that within mathematics, it is much easier to get a class of objects than it is to get a single member of that class. So it is a unique universe, not a multiverse, that needs evidence to support it. And the fact is that all of the evidence that has any relevance to this question at all so far points more to a multiverse than it does to a unique universe.

Misericorde

Let me rephrase: the notion of an infinite number of universes is not subject to Occam's Razor if you believe in eternal inflation, or a multiverse of many isolated pockets. It does cut finely (I think) in the case of QM interpretations such as MWI where it's just another variable to remove the "artifact" of collapse.

Chalnoth

In the MWI, there are actually fewer assumptions than in other interpretations. The MWI just assumes the wavefunction dynamics of quantum mechanics, full stop. Other interpretations assume some sort of collapse or other mechanism that looks like collapse. MWI just looks at the wavefunction dynamics and notes that the appearance of collapse is already there, no extra bells and whistles required.

This is just one example (among many) of why a multiverse is preferred over a unique universe.

twofish-quant

In the interest of relating everything to observations, let me point out that a lot of the interest in parallel universes was inspired by recent observations of extra solar planets. It turns out that when we look at extra solar planets, that hot jupiters are very common, but we don't see them in our own solar system, which makes people wonder why our solar system seems to be specially tuned for life, and of course the answer is obvious. If our solar system had a hot jupiter, we wouldn't be here, and it turns out that the orbits of Jupiter and Saturn are specially tuned so that they are well-behaved.....

The other problem is that it turns out that in standard particle physics theories none of the fundamental constants are uniquely determined. The value of the fine structure constant at "zero energy" happens to be because the universe settled at this energy state after symmetry breaking happened.

So this got people thinking about how this applies to the whole universe. I know one scientist that wrote a critical paper on the possibility of multi-universes and he mentions this explicitly as one of the things that got him thinking about it.

DavidMcC

Twofish-quant, I thought all the other observational reasons for suspecting a multiverse are actually rather more convincing than mere exoplanet tendencies, which have nothing much to do with the issue, as far as I'm concerned.
However, I agree about your second para.

Misericorde

MWI has fewer than other interpretations except the only one that has any evidence to support it: shut up and calculate. I don't understand why people think they're going to form a valid ontology based on a theory which is not a "final" theory. Occam's Razor cuts closer to the bone than an attempt to rationalize why we don't recognize quantum behaviour in everyday life (dead/alive cats).

I'd add, if we're going to be willing to boil everything down to imponderables such as "before" the BB, or an infinite multiverse, eternal inflation, and so on... well... why not just say that this universe is all that there is, period. What's outside of the universe? Well, it's possibly as salient as asking for a before the BB; it's just not something human beings can contemplate; eternity, nothingness, a lack of time.

We wisely ignore those issues as being beyond the scope of physics, or at least not falsifiable or verifiable, right? Well, my view is that using the data we have now to even wonder about the 'why' of our existence and the ability of a universe or multiverse to support wondering organisms is predicated on failure. We have imperfect theories, but the benefit of them is the technology and progress they allow; I suppose if a final theory were developed then moving on to ontology would make sense, but right now it's hubris.

DavidMcC

It's a tricky point, Chalnoth. Maybe gravity simply has a short range in hyperspace, so we only feel the gravity from those bits that are very close in it, and by the same token, those bits are isolated from each other.

Chalnoth

Well, the difficulty here is that this is almost certainly a selection effect. Hot Jupiters have high masses and short orbital periods, and so are the easiest to observe. The fact that we see a lot of them seems to indicate more that planets in general are likely to be very common. In fact, given that within our own solar system, every one of the more massive planets has a plethora of moons argues rather strongly that nearly every star out there will similarly have a number of planets (though obviously the more violent star systems, such as, say, tight-orbiting binaries, may end up destroying any planets that form).

Well, Jupiter and Saturn couldn't exist if their orbits weren't well-behaved, because our solar system is some 4.5 billion years old. Anything that wasn't well-behaved that was that massive would have collided with something over the billions of years our solar system has been around.

Chalnoth

That doesn't help.

And by the way, in order to explain our own observations of gravity, gravity in a higher-dimensional space would have to be stronger than it is in our 3+1 dimensions, not weaker.

Chalnoth

You mean the Copenhagen interpretation? Sorry, but that one has been falsified by observations of quantum decoherence.

Misericorde

I mean exactly what I said: "shut up and calculate", not worry about collapse unless decoherence is the focus on your study. The solution to a falsified theory is not to invent countless interpretations of a theory, a majority of which are not falsifiable even if they are elegant. The solution is to calculate and work toward a greater understanding of what is right and wrong in the current formulation, observing and recording, and creating a NEW THEORY.

Clearly GR and QM have holes in them, but they also predict nature with astonishing fidelity. To assume that doesn't mean we have an infinite series of half-steps to a final theory however, again, seems like hubris. Shut up and calculate, and fine tune until we get so close it makes no difference, or we're all dust. It's good to identify the ontological problems uncovered, and some areas such as DCQE experiments demand explanation. I do not believe that interpretation of existing theories is of value in finding that however, but rather only new theories which are MORE predictive and explanatory. The rest is very admirable, but still very clearly intellectual masturbation.

Chalnoth

Sounds like a hand-wavy excuse to stop thinking about it to me.

My entire point was that absent evidence to discern which way to go, we should always consider the explanation with the fewest assumptions to be the most likely. That's MWI. And it turns out that this has been confirmed by observations of quantum decoherence. To claim that there is still some "real" collapse besides simple decoherence is an unjustified and unjustifiable claim and should be considered nonsensical.

Misericorde

There's nothing at all wrong with thinking about it, but there's thinking about it, and forming armed camps; I see far more of the latter.