On Sat, May 1, 2010 at 1:43 PM, Rex Allen <rexallen...@gmail.com> wrote:
> On Thu, Apr 29, 2010 at 10:58 PM, Jesse Mazer <laserma...@gmail.com> > wrote: > > > > I think you've got the argument wrong. > > I think you're wrong about my getting the argument wrong. :) > I suppose it depends what you mean by "the argument". It is possible you could find *some* mainstream scientist who seriously considers the possibility that all our historical records of a low-entropy past are wrong or that we are actually Boltzmann brains with false memories, but for any of the physicists I have read who have brought up these ideas, like Sean Carroll and Brian Greene, it is completely clear to me that they only consider these to be reductio ad absurdum arguments, not that they actually think these are likely to be true. If you disagree, I suggest you haven't actually read these authors very carefully, or haven't really understood what you read. > > > Carroll discusses this in his book "From Eternity to Here" > > From Eternity To Here, Pg. 182 (my comments follow the quote): > > "Cognitive Instability > > I know from experience that not everyone is convinced by this > argument. One stumbling block is the crucial assertion that what we > start with is knowledge of our present macrostate, including some > small-scale details about a photograph or a history book or a memory > lurking in our brains. Although it seems like a fairly innocent > assumption, we have an intuitive feeling that we don't know something > only about the present; we *know* something about the past, because we > see it, in a way that we don't see the future. Cosmology is a good > example, just because the speed of light plays an important role, and > we have a palpable sense of "looking at an event in the past." When > we try to reconstruct the history of the universe it's tempting to > look at (for example) the cosmic microwave background and say, "I can > *see* what the universe was like almost 14 billion years ago; I don't > have to appeal to any fancy Past Hypothesis to reason my way into > drawing any conclusions." > > That's not right. When we look at the cosmic microwave background (or > light from any other distant source, or a photograph of any purported > past event), we're not looking at the past. We're observing what > certain photons are doing right here and now. When we scan our radio > telescope across the sky and observe a bath of radiation at about 2.7 > Kelvin that is very close to uniform in every direction, we've learned > something about the radiation passing through our *present* location, > which we then need to extrapolate backward to infer something about > the past. It's conceivable that this uniform radiation came from a > past that was actually highly non-uniform, but from which a set of > finely tuned conspiracies between temperatures and Doppler shifts and > gravitational effects produced a very smooth-looking set of photons > arriving at us today. You may say that's very unlikely, but the > time-reverse of that is exactly what we would expect if we took a > typical microstate within our present macrostate and evolved it toward > a Big Crunch. The truth is, we don't have any more direct empirical > access to the past than we have to the future, unless we allow > ourselves to assume a Past Hypothesis. > > Indeed, the Past Hypothesis is more than just "allowed"; it's > completely necessary, if we hope to tell a sensible story about the > universe. Imagine that we simply refused to invoke such an idea and > stuck solely with the data given to us by our current macrostate, > including the state of our brains and our photographs and our history > books. We would then predict with strong probability that the past as > well as the future was a high-entropy state, and that all of the > low-entropy features of our present condition arose as random > fluctuations. That sounds bad enough, but the reality is worse. > Under such circumstances, among the things that randomly fluctuated > into existence are all of the pieces of information we traditionally > use to justify our understanding of the laws of physics, or for that > matter all of the mental states (or written-down arguments) we > traditionally use to justify mathematics and logic and the scientific > method. Such assumptions, in other words, give us absolutely no > reason to believe that we have justified anything, including those > assumptions themselves. > > David Albert has referred to such a conundrum as *cognitive > instability* - the condition that we face when a set of assumptions > undermines the reasons we might have used to justify those very > assumptions. It is a kind of helplessness that can't be escaped > without reaching beyond the present moment. Without the Past > Hypothesis, we simply can't tell any intelligible story about the > world; so we seem to be stuck with it, or stuck with trying to find a > theory that actually explains it." > Did you actually read the whole book? If you did, I don't see how you can have missed the fact that Carroll expresses complete confidence that the Past Hypothesis is actually *true*, that the universe did in fact start out in a very low-entropy state shortly after the Big Bang and that our memories of a lower-entropy past aren't all completely false. For example, on p. 222 explicitly says that he considers the idea that we represent a random fluctuation from a high-entropy state to be a reductio ad absurdum, and then expresses confidence that we are *not* Boltzmann brains: 'So the reductio ad absurdum of this scenario is that the overwhelming majority of intelligences in this multiverse will be lonely, disembodied brains, who fluctuate gradually out of the surrounding chaos and then gradually dissolve back into it. Such sad creatures have been dubbed "Boltzmann brains" by Andreas Albrecht and Lorenzo Sorbo. You and I are not Boltzmann brains--we are what one might call "ordinary observers," who did not fluctuate all by ourselves from the surrounding equilibrium, but evolved gradually from an earlier state of very low entropy. So the hypothesis that our universe is a random fluctuation around an equilibrium state in an eternal spacetime seems to be falsified.' Then on p. 223 he explains in more detail why we can be confident we aren't Boltzmann brains: because the level of order we experience is far greater than what the vast majority of possible Boltzmann brains should be predicted to experience (though he does bring up the possibility that our experience of an orderly environment could just be a hallucination). Finally in chapter 14 he gets to his favored explanation for *why* our universe could have started in a low-entropy state: inflation, where a small region of a high-entropy (and basically empty) de Sitter space could blow up into a new "baby universe" that, once it inflated, would be in a very smooth low-entropy state for its size. On p. 358 he says: 'Of course the entropy of the baby universe starts out very small. That might seem like cheating--didn't we go to great lengths to argue that there are many degrees of freedom in our observable universe, and all of them still existed when the universe was young, and if we picked a configuration of them randomly it would be preposterously unlikely to obtain a low-entropy state? All of that is true, but the process of making a baby universe is not one where we choose the configuration of our universe randomly. It's chosen in a very specific way: the configuration that is most likely to emerge as a quantum fluctuation in an empty background spacetime that is able to pinch off and become a disconnected universe. Considered as a whole, the entropy of the multiverse doesn't go down during this process; the initial state is high-entropy de Sitter space, which evolved into high-entropy de Sitter space plus a little extra universe. It's not a fluctuation of an equilibrium configuration into a lower-entropy state, but a leakage of a high-entropy state into one with an even higher entropy overall.' Then on pp. 359-3601 he says that without baby universes, we would expect most observers to be temporary fluctuations (Boltzmann brains) in a high-entropy de Sitter space expanding forever, but that "baby universes change this picture in a crucial way" and allow most observers to be evolved in baby universes that were born in a low-entropy state (a process which nevertheless increases the entropy of the multiverse as a whole, which now has no upper limit and therefore no equilibrium state): 'The prospect of baby universes makes all the difference in the world to the question of the arrow of time. Remember the basic dilemma: The most natural universe to live in is de Sitter space, empty space with a positive vacuum energy, which acts like an eternal box of gas at a fixed temperature. The gas spends most of its time in thermal equilibrium, with rare fluctuations into states of lower entropy. With that kind of setup, we could fairly reliably quantify what kinds of fluctuations there will be, and how often they will happen. Given any particular thing you would want the fluctuation to contain--a person, or a galaxy, or even a hundred billion galaxies--this scenario strongly predicts that most such fluctuations will look like they are in equilibrium, apart from the presence of the fluctuations themselves. Furthermore, most such fluctuations will arise from higher-entropy states, and evolve back into higher-entropy states. So most observers will find themselves alone in the universe, having arisen as random arrangements of molecules out of the surrounding high-entropy gas of particles; likewise most galaxies, and so on. You could potentially fluctuate into something that looks just like the history of our Big Bang cosmology; but the number of observers within such fluctuations is much smaller than the number of observers who are otherwise alone.' 'Baby universes change this picture in a crucial way. Now it's no longer true that the only thing that can happen is a thermal fluctuation away from equilibrium and then back again. A baby universe is a kind of fluctuation, but it's one that never comes back--it grows and cools off, but it doesn't rejoin the original spacetime.' 'What we've done is given the universe a way that it can increase its entropy without limit. In a de Sitter universe, space grows without bound, but the part of space that is visible to any one observer remains finite, and has a finite entropy--the area of the cosmological horizon. Within that space, the fields fluctuate at a fixed temperature that never changes. It's an equilibrium configuration, with every process occurring equally as often as its time-reverse. Once baby universes are added to the game, the system is no longer in equilibrium, for the simple reason that there is no such thing as equilibrium. In the presence of a positive vacuum energy (according to this story), the entropy of the universe never reaches a maximum value and stays there, because there is no maximum value for the entropy of the universe--it can always increase, by creating new universes. That's how the paradox of the Boltzmann-Lucretius scenario can be avoided.' ... 'The real problem with de Sitter space (without baby universes) is that it's almost always in equilibrium--any particular observer sees a thermal bath that lasts forever, with predictable fluctuations. More generally, if there exists any such thing as "equilibrium" in the context of cosmology, it's hard to understand why we don't find the universe in that state. By suggesting there is no such thing as equilibrium, we can avoid this dilemma. It becomes natural to observe entropy increasing, simply because entropy can always increase.' 'This is the scenario suggested by Jennifer Chen and me in 2004. We started by assuming that the universe is eternal--the Big Bang is not the beginning of time--and that de Sitter space was a natural high-entropy state for the universe to be in. That means we can "start" with almost any state you like--pick some favorable distribution of matter and energy throughout space, and let it evolve. We put start in quotation marks because we don't want to prejudice initial conditions over conditions at any other time; respecting the reversibility of the laws of physics, we evolve the state both forward and backward in time. As I've argued here, the natural evolution forward in time is for space to expand and empty out, eventually approaching a de Sitter state. But from there, if we wait long enough, we will see the occasional production of baby universes via quantum fluctuations. These baby universes will expand and inflate, and their false vacuum energy will eventually convert into ordinary matter and radiation, which eventually dilutes away until we achieve de Sitter space once again. >From there, both the original universe and the new universe can give birth to new babies. This process continues forever. In the parts of spacetime that look like de Sitter, the universe is in equilibrium, and there is no arrow of time. But in baby universes, for the time in between the initial birth and the final cooling off, there is a pronounced arrow of time, as the entropy starts near zero and expands to its equilibrium value.' So again, his point is that plenty of observers will arise in these baby universes when their entropy is still low and they haven't evolved into a de Sitter space themselves, and that for those observers, the "Past Hypothesis" that their memories are true and that entropy was low near the beginning of their own universe will be *true*--they are not Boltzmann brains. Brian Greene makes essentially the same point about fluctuations in older universes leading to new baby universes with very low entropy in chapter 11 of "Fabric of the Cosmos". > > ==== > > So it seems to me that physicalism (the proposal that our experiences > are "caused" by an independently existing material world) is riddled > with "cognative instabilities". As is Bruno's mathematical platonism. > And as is any theory that proposes a causal mechanism for conscious > experience. > > There is no sensible story to be told about existence. > > Sean says: "Without the Past Hypothesis, we simply can't tell any > intelligible story about the world" > > I'd go further and say that even with the Past Hypothesis you can't > tell any intelligible story about the world. We *can* say that the > "big bang" theory is consistent with what we observe. But so is a > higher-entropy past. And so is Bruno's AUDA. And so are a lot of > things. > > BUT these things all inevitably lead to more questions. There seem to > be only two possible "final" answers: > > 1) Everything exists. > > 2) Reality is essentially arbitrary. There is no reason why > existence is this way as opposed to some other way. It just is. Even if "everything exists", there is still the possibility of some definite probability distribution on this "everything"--either a probability distribution on all possible universes/computations/mathematical structures, or a probability distribution on all possible observer-moments. It's quite possible that the probability distribution would be such that observers who had *true* memories of a low-entropy past would be much more common than random Boltzmann brains with no memories or false memories. Jesse -- You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-l...@googlegroups.com. To unsubscribe from this group, send email to everything-list+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/everything-list?hl=en.
