On Friday, May 26, 2017 at 2:21:37 PM UTC+10, Brent wrote: > > > > On 5/25/2017 8:36 PM, Pierz Newton-John wrote: > > Is something up with Everything List - your reply is not on the site and I’m > seeing this business with “reply to David 4” etc…? > > > On 26 May 2017, at 12:29 pm, Brent Meeker <mee...@verizon.net> <javascript:> > wrote: > > > > On 5/25/2017 6:30 PM, Pierz wrote: > > Recently I've been studying a lot of history, and I've often thought about > how, according to special relativity, you can translate time into space and > vice versa, and therefore how from a different perspective we can think of > the past as distant in space rather than time: my childhood being 40 light > years away, rather than 40 years for instance. I can visualise my own body as > a sort of long, four dimensional tendril through spacetime, of which I only > ever see a three-dimensional cross-section. This is the block universe idea > of course. What occurred to me recently was that the past, in any physical > theory I know of, is "locked down". There is only a single history consistent > with the present (ignoring the microscopic ambiguities of quantum > interference effects > > I think that is assuming a lot. Consider the biverse model of cosmogony - > then the past "forks" just like the future. > > When I’ve asked physicists this question, I’ve been told that a single past > is the general assumption. IIRC, there may be ambiguous histories very close > to the Big Bang (Hawking?), but that’s not really relevant. Maybe that’s the > biverse cosmology you refer to (googling it didn’t help). > > > In the biverse model the universe inflates into the past as well as the > future (as defined by us), thus maintaining time symmetry. Here's a > another, more worked out version of the idea: > > Spontaneous Inflation and the Origin of the Arrow of Time > Sean M. Carroll > <https://arxiv.org/find/hep-th/1/au:+Carroll_S/0/1/0/all/0/1>, Jennifer > Chen <https://arxiv.org/find/hep-th/1/au:+Chen_J/0/1/0/all/0/1> > (Submitted on 27 Oct 2004) > > We suggest that spontaneous eternal inflation can provide a natural > explanation for the thermodynamic arrow of time, and discuss the underlying > assumptions and consequences of this view. In the absence of inflation, we > argue that systems coupled to gravity usually evolve asymptotically to the > vacuum, which is the only natural state in a thermodynamic sense. In the > presence of a small positive vacuum energy and an appropriate inflaton > field, the de Sitter vacuum is unstable to the spontaneous onset of > inflation at a higher energy scale. Starting from de Sitter, inflation can > increase the total entropy of the universe without bound, creating > universes similar to ours in the process.* An important consequence of > this picture is that inflation occurs asymptotically both forwards and > backwards in time, implying a universe that is (statistically) > time-symmetric on ultra-large scales.* > > Comments: 36 pages > Subjects: High Energy Physics - Theory (hep-th); Astrophysics (astro-ph); > General Relativity and Quantum Cosmology (gr-qc) > Report number: EFI-2004-33 > Cite as: arXiv:hep-th/0410270 <https://arxiv.org/abs/hep-th/0410270> > (or arXiv:hep-th/0410270v1 <https://arxiv.org/abs/hep-th/0410270v1> for > this version) > > > ), but the present is consistent with multiple futures. However, we know that > "now" - and therefore the division into past and future - is an artifact of > mind with no physical reality, a "quale". So therefore, if the past is > singular, so is the future, and seen from "outside", every quantum event, > whether "future" or "past" from any particular fame of reference, is in fact > completely determined in its outcome, even though it is also random in the > sense there is no way of explaining why it is the way it is, beyond the > description provided by Born rule probabilities. Is that not weird, if not > downright absurd? What is this "necessity" that dictates that this particular > subset of all the possible quantum events was selected as the way things are? > > If there were such a "necessity" that would be a deterministic theory and > inconsistent with the Born rule...and observation. > > Well it’s not inconsistent with observation because if such a thing were > true, there’d still be no way an observer inside the system would know what > the predetermined outcome was going to be. Doesn’t mean I like the idea > though, obviously. > > Somehow the idea of the future being indeterminate but the past fixed seems > palatable because it accords with our subjective experience, but really it is > incoherent as soon as we acknowledge that the past-future distinction is not > physically meaningful. > > But it is meaningful. Entropy increases in the future direction. We remember > and record the past but not the future. > > The arrow of time is physically meaningful, not the idea of “now”, which is > the reference point for determining what is future or past. No event belongs > intrinsically to the past or future, it is a relative concept for some > conscious observer. > > > > Would this mean that if we could run the big bang over again from the same > initial conditions, it would always go exactly the same way? That is absurd, > as it would mean there is something prebuilt as it were into the laws of > physics that dictates that only this particular world history is permitted, > for no reason at all. > > That would be t'Hooft's superdeterminism. > > I’ll look it up. But sounds daft :) > > But if it could go a different way, that is equally absurd because it implies > that variance is allowed at the level of entire universes, but not at the > level of individual quantum events within those universes. What can resolve > this paradox? Perhaps I'm cheating by imagining different possible universes > in a universe where only one is real and allowed, but who can seriously > countenance such a cosmology of absurdity where everything "just is"? > > Of course if MWI is correct, then the paradox is resolved, because there is > only a single past in the sense that there is only a single shortest path > from any limb of a tree to its base, and there is no need for some principle > of arbitrary necessity to dictate that all quantum events only have one > possible outcome. For me this is a powerful argument in favour of many > worlds, yet it's not one I've heard before. Any comments? > > It's a pretty good argument...and one I've heard (and even thought of myself) > before. Does it really solve a problem that a collapse of the wave function > doesn't solve? > > Well I believe some experiments (currently impracticable) have been proposed > to test it (can’t recall details but they involve undoing a series of quantum > interactions). And of course Deutsch argues that a sufficiently complex > quantum computer proves it - because where are the informational resources > required for the computation “kept” in a single universe? > > > On the other hand for a quantum computer to work, all the "wrong" branches > of computation have to null out by destructive interference - which sort of > implies they must be in the same world. > I don't see that - destructive interference is the merging of branches, so universes that are briefly different (thus carrying information used in the computation) re-merge, and all the universes end up coming back together with the same result. But anyway, I know not everyone agrees with the argument. Merely making the point that the jury is still very much out on whether MWI "solves a problem" that collapse of the wavefunction doesn't. Apart from the "problem" of collapse being conceptually obnoxious.
> > But anyway I’m not an engineer whose interest in physics is practical :) I’m > interested in truth (whatever that means), and although I agree that the map > is not the territory, I also agree with Deutsch’s argument that scientific > theories are explanations, and good explanations are preferable to bad ones, > even if their utility is not immediately apparent. After all, Copernican > cosmology did not initially provide better predictions than the > much-finessed, epicycle-ridden geocentric model. > > > But the Copernican model was replaced by the model of Kepler and Newton > and then Einstein. So have we arrived at "the truth"? > Not the full truth, in all likelihood, especially given the contradictions with QM. But that doesn't mean Copernicus wasn't closer to the truth than Ptolemy, Einstein than Newton. And the key point here is that we (presumably) really and truly believe that the earth goes round the sun, and it's not "just a model". And if MWI is right, there really and truly are the other branches. They're not just a model either. That's true or it isn't, just like the question of whether the earth goes round the sun, or there are other galaxies. > ISTM that "true" has different meanings in mathematics and Platonism vs > physics and empiricism. In logic and mathematics it's just a marker, like > "t" in LISP, that is propagated through propositions by the rules of > inference and the meaning of words. It has no relation the truth of > observations except for our application. > Well I agree. Truth in physical sciences relates to some (rather notoriously difficult to define) concordance between an abstract model and the physical world, a concordance which is always somewhat approximate. The metaphor of a map is apposite. It can be grossly inaccurate, like world maps from the 15th century, or fairly to very accurate, like a map made a year ago that might miss some small recent changes. I'm not claiming absolute truth like 1+1=2 for MWI, but arguments like the above do tend to persuade me there's a good chance it is right in the sense that Copernicus was right about the earth's relationship to the sun. > > Brent > > Brent > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
