Happy Atheist Forum

Just finished it (OK, it was on audible.com, but its kinda like reading).

Hawking (and Mlodinow, I guess) make the multiverse argument from the perspective of Feynman's path of histories approach. I've never heard it explained like that before, and I think maybe I grasp a bit more of it. But here's my difficulty.

It seems clear at this point that we do not live in a universe (or at least our portion of the universe) this is brimming over with life. If there were intelligent beings all over the place (and all over time), I think we'd know about them by now. Therefore, our universe seems to have a fairly strong "non-anthropic" principle at work. Almost everywhere in the universe is hostile to intelligent life.

Hawking's argument is that we find ourselves in a part of the universe (or multiverse, if you prefer) that is suitable to us precisely because if we weren't, we wouldn't be here to wonder about it. As someone else on this board said in a previous post, "A puddle is amazed as how well it fits its pothole" or something like that. But, if every possible history really happened, then why do we find ourselves in a place that's only barely good enough.

As an analogy, consider a science museum. If you've ever spent time in one, you know there are busy days and there are slow days. For argument, let's say that there's a 50/50 split between busy days and slow days. Even though the days are split evenly, the population of the museum is not. Most people will have experienced the museum on a busy day.

Therefore, it would seem, if the many worlds hypothesis holds, that it would be much more likely that we'd find ourselves in an extremely crowded universe positively brimming with life than in one that is only barely suitable, because most intelligent beings would be in those crowded universes. And yet we're not. Just bad luck, or is there something wrong with the many worlds approach?

Quote from: "ablprop"As an analogy, consider a science museum. If you've ever spent time in one, you know there are busy days and there are slow days. For argument, let's say that there's a 50/50 split between busy days and slow days. Even though the days are split evenly, the population of the museum is not. Most people will have experienced the museum on a busy day.

Most =/= all. Some people will not have experienced the museum on a busy day.

Things are the way they are because they are not otherwise.

True, but part of the attraction of Hawking's approach is that it transforms the Strong Anthropic Principle into the Weak Anthropic Principle. It would seem to be special pleading to believe that we live in a universe in which the conditions are just right. If, on the other hand, those conditions are merely one set of an enormous list of just as real conditions, then the coincidence ceases to be so amazing. We are where we are because we can't be where we aren't.

But if we then claim that we're one of the extremely rare species that lives in one of those possible worlds where life is extremely rare, isn't that just special pleading again? I'm not sure it's much better than where we were before with the fine tuning problem.

Does it help if we believe instead that we live in a universe in which the conditions are almost just right?

Maybe. Maybe the answer is that there's a lot more universes where things are almost just right (like ours) than universes where things really are milk and honey all the time. By anaolgy with the museum, there would be many, many more slow days (slow, but not closed) than busy days, and so it could still be that many, many more people experienced the museum on a slow day.

I like that. Let's go with it. Thanks.

Here's one way to look at it: Evolution doesn't produce the "ideal" configuration for any particular life form, but rather only produces what's "good enough" to occupy a particular niche, compete, survive and reproduce. Even so, this process results in what we can only say are amazing and elegant configurations of life. In the same way, our universe  appears to be just "good enough" to allow for the possibility that life can arise. Even so, the particular arrangement of our planet with it's relatively large satellite, orbiting it's star at the proper distance to allow liquid water to exist is an amazing conjunction of circumstances which has produced a thriving infestation of life.

 We don't actually know whether life is relatively common in our universe or not.  There may be a lot of planets out there which are covered with happily thriving pond scum.  It does appear that there is not an abundance of intelligent life, given the failure of SETI so far, but that might only mean that intelligent life is relatively rare in the universe.  However, even if intelligent life only arises once or twice in each galaxy (being rather parsimonious, since there are approximately 400 billion stars in our galaxy), that would mean it has arisen literally billions of times (given that it's estimated there are at least 100 billion galaxies in the universe).  That's not too shabby for a universe that's just "good enough" to harbor life at all.  This is conjecture of course, but what I'm getting at is that we might very well be "surrounded" by intelligent life and yet never encounter it.  The universe is unimaginably immense; there's a lot of room for little varmints like ourselves to rattle around in, never encountering each other at all.

Quote from: "ablprop"Just finished it (OK, it was on audible.com, but its kinda like reading).

I haven't actually gotten around to it yet, but I intend to ASAP.

QuoteHawking (and Mlodinow, I guess) make the multiverse argument from the perspective of Feynman's path of histories approach. I've never heard it explained like that before, and I think maybe I grasp a bit more of it.

The 'path integral' formulation, or 'sum-over-histories' is about the most elegant approach to quantum mechanics available.

QuoteBut here's my difficulty.

It seems clear at this point that we do not live in a universe (or at least our portion of the universe) this is brimming over with life. If there were intelligent beings all over the place (and all over time), I think we'd know about them by now.

That's a very strong statement. In reality, this is almost a perfect statement of the Fermi Paradox, although Fermi himself drilled it down to 'where is everybody?'

In reality, though, it isn't actually a paradox. The reason it isn't a paradox was best summed up by Douglas Adams:

Quote from: "The Tall One In The Bath"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

Think about it like this: Humans have been transmitting radio signals for about 70 years, give or take, and they propagate at c. Our own galaxy alone is 100,000 light years across. What this means is that our earliest signals have made it, less than 1/1,000th across the galactic disc, and that's only on a tiny local scale in cosmic terms. When you factor this in, along with all the other factors that are important, you begin to see why it isn't a paradox. It isn't enough to simply be intelligent. Many think that intelligence is the primary factor in our being a technological organism, but it isn't even the most inportant. Certainly intelligence is important, but in terms of being a technological civilisation, I would consider the most important factor to be opposable thumbs. That may sound daft, but just how would technology have evolved without them? So, you need opposable thumbs, intelligence, and not just intelligence but a particular kind of intelligence, coupled with curiosity and other factors. Given that the window of time that we have actually been able to transmit and receive radio signals (among others, all of which have the same limitation in terms of the speed they propagate) covers only the tiniest portion of our own evolutionary history, let alone the history of the cosmos, there are several scenaria that may explain why we haven't detected any intelligent life.

1. Their signals haven't reached us yet.
2. Their signals have already passed us, long before we were remotely in a position to detect them.
3. They employ transmission methods other than those that we have so far discovered.
4. They haven't discovered any method of transmitting signals over the distances involved.
5. They went extinct longer ago than c x their distance from us.
6. Their signals are unintelligble to us.

These are just several among many reasons why extraterrestrial intelligence hasn't been detected, and the list is extremely long, although most would come under the rubric of one of the headings above.

If an intelligent, technological civilisation exists in Andromeda galaxy right now and is transmitting signals at c, we won't know about it for another 2½ million years.

QuoteTherefore, our universe seems to have a fairly strong "non-anthropic" principle at work. Almost everywhere in the universe is hostile to intelligent life.

Hmmmm. That's a bit of an anthropocentric statement there. In reality, the discovery of many extremophiles in the last couple of centuries demonstrates the danger of our thinking that we know what it takes for life to survive, let alone intelligent life.

QuoteHawking's argument is that we find ourselves in a part of the universe (or multiverse, if you prefer) that is suitable to us precisely because if we weren't, we wouldn't be here to wonder about it. As someone else on this board said in a previous post, "A puddle is amazed as how well it fits its pothole" or something like that. But, if every possible history really happened, then why do we find ourselves in a place that's only barely good enough.

That, again, was Douglas Adams, in a humorous statement of the anthropic principle.

QuoteImagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!' This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it's still frantically hanging on to the notion that everything's going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.'

QuoteAs an analogy, consider a science museum. If you've ever spent time in one, you know there are busy days and there are slow days. For argument, let's say that there's a 50/50 split between busy days and slow days. Even though the days are split evenly, the population of the museum is not. Most people will have experienced the museum on a busy day.

Indeed, but this analogy is flawed, because the museum simply isn't big enough to account for the sheer scale of the cosmos.

QuoteTherefore, it would seem, if the many worlds hypothesis holds, that it would be much more likely that we'd find ourselves in an extremely crowded universe positively brimming with life than in one that is only barely suitable, because most intelligent beings would be in those crowded universes. And yet we're not. Just bad luck, or is there something wrong with the many worlds approach?

[/quote]

Well, I'm not sure what this means exactly. The Many Worlds approach has a very specific meaning in physics, namely one of the interpretations of quantum mechanics (and not a good one, in my opinion, as its failure to be parsimonious is somewhat off the charts). If it's simply talking about the number of occupied worlds there should be, then I think I've covered that above.

Quote from: "ablprop"True, but part of the attraction of Hawking's approach is that it transforms the Strong Anthropic Principle into the Weak Anthropic Principle. It would seem to be special pleading to believe that we live in a universe in which the conditions are just right. If, on the other hand, those conditions are merely one set of an enormous list of just as real conditions, then the coincidence ceases to be so amazing. We are where we are because we can't be where we aren't.

But if we then claim that we're one of the extremely rare species that lives in one of those possible worlds where life is extremely rare, isn't that just special pleading again? I'm not sure it's much better than where we were before with the fine tuning problem.

Well, we do live in a universe in which the conditions are just right, by dint of having evolved in this universe. It is unclear whether life could evolve in a universe with different constants (and that's before you get into whether or not those constants could actually be any different). It would probably be very different, even to the point where we may not recognise it as life, but that doesn't mean it's prohibited. All calculations in this regard are extremely anthropocentric, and simply don't take into account the categorical fact that life could be very different.

In reality, though, there is one scenario that has been worked out that demonstrates that at least one of the things that we take for granted in our universe could be very different and it would actually make life, and in this case I mean carbon-based life, more probable (with due regard to the dangers of atempting to calculate probabilities with a sample set of 1). If the weak nuclear force were removed, it is entirely probable that carbon-based life would be more probable, simply because there would be more stable isotopes of carbon without weak interactions.

Paper here: http://prd.aps.org/abstract/PRD/v74/i3/e035006 (http://prd.aps.org/abstract/PRD/v74/i3/e035006)

Hackenslash, I look forward to reading your reaction to Hawking's book. I agree that we don't yet know how common intelligent life is, but one thing we do know is this. We can easily imagine a universe in which intelligent life is quite apparent. We could imagine a universe in which a 2-billion-year-old civilization has colonized its galaxy with intelligent and essentially immortal robots. That hasn't happened here. If it had, then we wouldn't have to speculate.

Yesssss, but I can easily imagine all sorts of worlds. Some philosophers will tell you that they're all realised in some way under Yablo-conceivability or some such nonsense. I don't put much stock in such ideas.

I am looking forward to the book, though. I was first going to read On The Shoulders Of Giants, which I also haven't gotten around to, because I actually think I'll find it more interesting. My impression of The Grand Design is that it pretty much covers the same ground he's covered already. but makes more explicit the implications for a creator.

I read the book, and found it very informative and entertaining for the most part.

It does give a good overview of science a-z, and puts forth ideas in an easy-to-understand way.

I did feel that the last 10% or so kind of fell apart.
It became more and more conjecture, and less fact/theory/hypothesis.
And although the many-worlds hypothesis can potentially explain a lot, it fails to explain where the laws/rules that govern these ideas arise from.

Just another "Turtles All The Way Down"...

(but read it - still a lot of good stuff in there)
JoeActor

Just finished it and as joe found I thought it got more, and possible too, speculative right towards the end. I did however have no problem with agreeing with the strong anthropic principal if the number of universes comes in at 10 to the 500 possibilities.

I was most impressed by the buckyball interference pattern and the ability to intervene on the sum of histories. Most interesting.

Definitely a very readable and informative book.

In The Lightness of Being Frank Wilczek makes the point that while theologians were arguing back and forth over the nature of God, Galileo was rolling balls down ramps. The theologians have gotten nowhere, but Galileo's simple experiment led to modern science. It would be interesting if a natural extension of Galileo's balls down ramps, the double slit experiment and Feynman's sum over histories explanation of it, now led to the most convincing answer yet to the question of why there is a universe at all. And the theologians keep getting nowhere.

Quote from: "ablprop"In The Lightness of Being Frank Wilczek makes the point that while theologians were arguing back and forth over the nature of God, Galileo was rolling balls down ramps. The theologians have gotten nowhere, but Galileo's simple experiment led to modern science. It would be interesting if a natural extension of Galileo's balls down ramps, the double slit experiment and Feynman's sum over histories explanation of it, now led to the most convincing answer yet to the question of why there is a universe at all. And the theologians keep getting nowhere.

 Can you imagine the Pope trying to get his head around M-Theory?

Finding the true cause of existance, and proving unequivocally that is not deistic/theistic in nature, would just cause warfare to break out. I think it would play out something like this. Those looking for naturalistic cause for existance would mostly just jump on the bandwagon. For a while there would be academic and theological debate (because some percentage of theists would not accept the 'proof')  eventually leading to a schism between the 'Soft God' camp and the 'Hard God' camp. The Soft God camp would simple ignore the evidence while Hard God camp would actively dispute the proof, very much as creationists do today.

However over a period of time, probably 40 to 50 years (a couple of generations), the 'Proof' would become mainstream and the Soft God stance would be eroded away until only the Hard God stance remained. I am of the opinion that when push comes to shove most people are rational enough to be able to determine the truth if exposed to it for long enough. This would be the flash point. How far would the Hard God camp go to make their point felt? I have the feeling that the faithful residuam could get violent as they would have to be demonstrably delusional to be in the Hard God camp in the first place.

Truly frightening. I am most worried about religions like Islam that unabashedly mix with politics to create a situation where disagreement with dogma is dangerous. Not that Christianity wasn't there before, and could be again.

Still, it's a great time to be alive, where instead of just debating "head of a pin" questions we can actually find evidence that can lead to answers. I'm old enough to remember when serious scientists said there are some questions that science will never answer, such as why is there something rather than nothing. While it's true that there will always be this infinite regression of whys, for me what I'm learning now about the beginning of the universe becomes more and more satisfying all the time.

It seems now the question isn't so much, why is there a something rather than a nothing, but why was there this unstable nothing in which something might happen? Still a deep and exciting question, but not nearly so frustrating as the former if you don't have an answer.

Quote from: "ablprop"It would be interesting if a natural extension of Galileo's balls down ramps, the double slit experiment and Feynman's sum over histories explanation of it, now led to the most convincing answer yet to the question of why there is a universe at all.

Not going to happen, and I'm sorry to be pedantic, but the reason it isn't going to happen is that, in science, 'why' is an invalid question. Science only deals in 'how' questions.

Now, usually when I say this, I am met with stiff opposition, so let me pre-empt it here. When a scientist says 'why did that happen?', what he's actually asking is 'what is the mechanism that caused that?', which is not a 'why' question, but a 'how' question. 'Why', implies purpose, which is not a valid assumption in science. Science only deals with mechanisms, which are all 'how' questions.

Incidentally, The Grand Design is going to have to wait for now, as I just started On The Shoulders Of Giants which, I suspect, is going to occupy me for at least a couple of weeks, given how little time I have at the moment.

More opposition. I refuse to cede the why questions. The why questions really are how questions. I find the current explanation for the fine-tuning to be much more satisfying than the old "that's just the way it is" answer. I think we're close to knowing "why" there is something rather than nothing. Of course, there will always be another, "but why should that mechanism exist", but each step leaves me more satisfied than before.

Quote from: "ablprop"More opposition. I refuse to cede the why questions.

Then you aren't likely to get any answers. There's no good reason to suppose that there even is a why.

QuoteThe why questions really are how questions.

Precisely my point, which is why they aren't why questions.
 

QuoteI find the current explanation for the fine-tuning to be much more satisfying than the old "that's just the way it is" answer.

What explanation? More importantly, though, what fine-tuning?

QuoteI think we're close to knowing "why" there is something rather than nothing.

Not remotely, but we already know how, namely by the uncertainty principle, which doesn't allow 'nothing', and means that there has to be something.

QuoteOf course, there will always be another, "but why should that mechanism exist", but each step leaves me more satisfied than before.

This highlights the problem beautifully. It's like when a two-year-old asks 'why?', and to every response you just get anther 'why?'. In reality, aside from the fact that these questions can't actually be supposed to have answers, they are also the surest and quickest route to infinite regress.

Ultimately, this isn't about ceding the 'why' questions, it's about them having zero utility in terms of elucidating reality.

A hairdresser once asked me "why are there butterflies?" One answer is, there are butterflies because there are stars. Tracing the path of carbon from the triple alpha process in stars through the origin and subsequent evolution of life gives one very satisfying answer to the question, even if it doesn't get at the hairdresser's probable initial desire for a "purpose" answer. That's what I mean when I say don't cede the why questions. They are where many people start.

Yet all those answers are answers to how questions, not why questions. You can refuse to cede the why questions, but what you are actually doing is engaging in a fallacy of equivocation, because why questions are a qualitatively different thing to how questions, and science isn't equipped to deal with them. Nor is religion, for that matter, although it certainly asserts answers.

In short, I'm not talking about ceding the why questions, I'm talking about recognising them as invalid and without utility.

But people still ask them. I'm interested in taking humans' natural inclination to ask "why" and show how scientific answers can actually satisfy that curiosity in a fulfilling way. I can't give a purpose answer to "why are there butterflies" because there is no purpose answer, but by walking down the "how" path, I can show that those answers can be ultimately quite satisfying and enriching.

When someone who wants purpose asks, "why is there something rather than nothing?" I can give your answer, that the uncertainty principle won't allow a nothing. Now we can discuss the uncertainty principle, how we humans came to understand it, its various applications to things like the monochromicity of laser beams, and so on. Maybe I didn't get at the original desire for a purpose (because, as far as I can tell, there isn't one), but I did, just maybe, provide a fulfilling substitute. It's fulfilling for me, anyway.

Ah, but if you're not addressing purpose, then you aren't addressing why questions. Simple as that. That's entirely my point.

OK. While I have your attention, I was reading about the Pauli Exclusion Principle in my quantum physics text from college. The authors make a point about a certain kind of fine-tuning. Without the PEP, there would be no chemistry because all electrons would fall to the ground state of atoms and behave like noble gases. Now I am far from an understanding of why the exclusion principle arises for particles with half-integral spin (the text references "deep quantum mechanics", whatever that is), but I wonder if we might state that such a principle is an example of the sort of law that has to exist in order for us to exist (and therefore ask the question)? It caught my attention given our discussion about Hawking's book and whether or not fine-tuning even exists.

Quote from: "ablprop"OK. While I have your attention, I was reading about the Pauli Exclusion Principle in my quantum physics text from college. The authors make a point about a certain kind of fine-tuning. Without the PEP, there would be no chemistry because all electrons would fall to the ground state of atoms and behave like noble gases. Now I am far from an understanding of why the exclusion principle arises for particles with half-integral spin (the text references "deep quantum mechanics", whatever that is), but I wonder if we might state that such a principle is an example of the sort of law that has to exist in order for us to exist (and therefore ask the question)? It caught my attention given our discussion about Hawking's book and whether or not fine-tuning even exists.

Fine tuning implies a Tuner? As in Intelligent Design requires a Designer. But the strong anthropic principle does not require fine tuning but what one might term 'fine slicing' and the PEP 'works' in our particular slice of reality. In the vast majority of multiple reality scenarios the PEP is not within the bounds where it 'works'. Thus we are only here to work out that the PEP needs to be within particular bounds because it is within those bounds. The same would be the case for all the properties that need to be within particular bounds for our universe to be suitably 'tuned' for our existance so we can measure them.

^^ Exactly right. This isn't fine-tuning of the universe for life, it's fine-tuning of life for the universe, and the fine-tuning is all done by dint of it having arisen under these conditions.

When scientists talk about fine-tuning, BTW, they are talking about the fine-tuning of models, not the universe. In other words, certain parameters have to fall within a certain narrow band of values if their models are correct. It's all-too east to view this from the wrong perspective, and to think that it means that the universe has been somehow tweaked to fit these values. A clear example is the horizon problem, in which the inflation of the early cosmos has to fit a certain rate in order to explain the isotropy of the cosmos that we measure today. The reality is that there is no good reason to suppose that any of the values of our constants could actually have any other value. The only reason for exploring other values is that thought experiments are very useful in understanding the make-up of the cosmos.

Most importantly, if the last century has taught us anything, it's that our understanding of what constitutes the conditions for life is very narrow. It may be that changing the value of some of the constants actually makes life more probable. Indeed, there is at least one constant, in the form of one of the four fundamental forces, that could be completely removed and it would mean that even life as we know it would be more probable. Remove the weak nuclear force, and suddenly more stable isotopes of carbon are available, which means that more different carbon chains can be built, making life more likely.

http://link.aps.org/doi/10.1103/PhysRevD.74.035006 (http://link.aps.org/doi/10.1103/PhysRevD.74.035006)

Quote from: "Tank"Fine tuning implies a Tuner?

Not remotely, and I regret if anything I wrote suggested such an idea. I'm in Dawkins' camp, convinced that the idea of a designer only adds complexity and doesn't solve anything. But I do feel that the possibly arbitrary conditions that allow our existence cry out for at least an attempt at explanation. Read on!

Quote from: "hackenslash"When scientists talk about fine-tuning, BTW, they are talking about the fine-tuning of models, not the universe.

I don't understand this distinction, and I've not encountered it (or at least not recognized it) in my other reading. I think there's a deep question here that cries out for an answer one way or another. For instance, at a public lecture I once asked Brian Greene why (ok, I should have said "what's the mechanism for") there are exactly three large space dimensions. What causes the other dimensions, if they exist, to be curled up? Or alternatively, why aren't they all curled up? Greene admitted that no one really knows, though he does suggest some intriguing answers in the final part of The Elegant Universe.

Hawking suggests another alternative. If the existence we know is just one of many, then it could be that the number of large vs. small dimensions is a matter of accident. Planetary orbits would be unstable in dimensions with more or less than three large dimensions, so it is no surprise that we find ourselves in a three-dimensional universe.

I'm not convinced of Hawking's multiverse hypothesis, but I think it is an important question to answer. If he's right, than anyone looking for a deep reason for three big dimensions is wasting her time. There is no deep reason, it's just contingent.

So how is this particular fine-tuning, that there are three large space dimensions and not more or less, part of a model and not of the universe?

Quote from: "ablprop"

Quote from: "Tank"Fine tuning implies a Tuner?

Not remotely, and I regret if anything I wrote suggested such an idea. I'm in Dawkins' camp, convinced that the idea of a designer only adds complexity and doesn't solve anything. But I do feel that the possibly arbitrary conditions that allow our existence cry out for at least an attempt at explanation. Read on!

Sorry, my wording did possibly intimate that I was implying that you were putting forward an argument for a 'Tuner', which was not my intention!

Quote from: "ablprop"I don't understand this distinction,

It's a fairly straightforward distinction. In the models we construct of what we think are the operational paramaters of the cosmos, there are certain parameters that have to fall within a certain narrow range of values if the models are correct. In other words, it could be that those parameters fall outside that range of values, in which case the model is falsified. For example, in the standard inflationary model, inflation has to proceed at a given rate in order to overcome the horizon problem, which is a problem to do with transmission of information (temperature, in this case) without breaching limitations to c, which it must do in order for the inflationary model to overcome the observed isotropy of the cosmos. It could be that the inflation of the cosmos progressed at a rate outside that range of values, in which case the model is wrong, and we have to think again. This doesn't cause any problems for the cosmos, of course, because the cosmos exists, and exists with constants that allow for life, and has the observed isotropy. In short, it's only a problem for the model. This is what cosmologists are talking about when they talk about fine-tuning. They are not suggesting that the cosmos is fine-tuned.

Quoteand I've not encountered it (or at least not recognized it) in my other reading.

You wouldn't necessarily come across it, because fine-tuning arguments only come up in dishonest creationist screeds, and cosmologists don't have much to do with them. Indeed, cosmologists employ a fair bit of language that is open to equivocation because, unlike evolutionary biologists, they aren't used to being confronted by fuckwitted cretinists.

QuoteI think there's a deep question here that cries out for an answer one way or another. For instance, at a public lecture I once asked Brian Greene why (ok, I should have said "what's the mechanism for") there are exactly three large space dimensions. What causes the other dimensions, if they exist, to be curled up? Or alternatively, why aren't they all curled up? Greene admitted that no one really knows, though he does suggest some intriguing answers in the final part of The Elegant Universe.

None of that suggests any kind of fine-tuning, and indeed this question is quite nicely covered by the anthropic principle. If there were more or less extended dimensions, we wouldn't be here to ask the question. The reality, though, is that our understanding of the dimensional manifold is in its infancy, and we don't know nearly as much as we'd like. It may be that there are other extended spatial dimensions, separated from ours by additional, Planck-scale spatial dimensions that are so small we can't perceive them. In fact, that is the basis, loosely speaking, for the Turok/Steinhardt 'brane-worlds' model for cosmic instantiation.

QuoteHawking suggests another alternative. If the existence we know is just one of many, then it could be that the number of large vs. small dimensions is a matter of accident. Planetary orbits would be unstable in dimensions with more or less than three large dimensions, so it is no surprise that we find ourselves in a three-dimensional universe.

Bingo. There's the anthropic principle again. This is not original to Hawking, though. Such proposals have been around for a very long time.

QuoteI'm not convinced of Hawking's multiverse hypothesis, but I think it is an important question to answer.

Well, apart from the fact that, in my opinion, anybody using the word 'multiverse' in a serious scientific discussion should be lined up against the wall and shot, it's a parsimonious proposal. Given a natural mechanism for the instantiation of a cosmos, and given the requisite topology for the dmensional manifold, it would be a bit silly to rule it out, not least because for this to be the only cosmic inflation would require a barrier to it happening again, which is a failure of parsimony, becaus that barrier constitutes an additional entity, which is a violation of Occam's Razor. Pretty simple when you look at it from the correct perspective.

I note that some think that other cosmic instantiations constitute such a violation. Hopefully I've shown the error of this way of thinking.

QuoteIf he's right, than anyone looking for a deep reason for three big dimensions is wasting her time. There is no deep reason, it's just contingent.

There's that word again: 'Reason'.

As for contingent, contingent upon what? I think you might mean here that it's just a brute fact, which is not the same. If that's the case, then I agree. Indeed, that's my own view of the universe, namely that it's simply a brute fact. There are very deep reasons for my thinking this, which I have covered ad nauseum, but I'll be happy to go over them again on request.

QuoteSo how is this particular fine-tuning, that there are three large space dimensions and not more or less, part of a model and not of the universe?

To call it fine-tuning in this context implies that it was tuned. Certainly our existence is contingent upon three extended spatial dimensions, but that's not the same as saying they are fine-tuned.

This was helpful; thank you. I look forward to your report on Shoulders of Giants.