Explaining the Quantum Mystery: Are Parallel Universes the Answer?

[rewebster]

I think a lot of people would agree that they ended up on some very unlikely paths.

http://www.ksdk.com/news/news_article.aspx?storyid=95909


http://www.boston.com/news/odd/articles/2007/04/26/woman_95_to_be_oldest_college_graduate/

 

[Evo]

I think you are taking my metaphor too literally. It is not about life choices it is about pure chance such as whether or not you get struck by lightning during a storm. To keep it simple there are 2 possibilities either you are or you are not. The most LIKELY outcome is you are not but because there is a very slight chance you are the universe needs to split to cover that eventuality and so your double is struck by lightning. Throughout your life there are many similar instances where the universe would split to cover equally unbalanced events and invariably it is your double who suffers the UNLIKELY consequence whereas if the path after the split was truly random in a 2 choice scenario 50% of the time it should be you.

Funny you should choose the lightning scenario. I have been "hit" by lightning, according to a tv program about it, they count people as being indirectly hit through phone lines and plumbing, etc... to have been hit. I've been hit twice and my house hit once. I barely escaped a third hit by jumping into my car just in time, I felt my hair rise up, then a loud boom and saw dirt flying up in the air where the lightning hit the ground near my car. So there.

 

[Moonbear]

I think you are taking my metaphor too literally. It is not about life choices it is about pure chance such as whether or not you get struck by lightning during a storm. To keep it simple there are 2 possibilities either you are or you are not. The most LIKELY outcome is you are not but because there is a very slight chance you are the universe needs to split to cover that eventuality and so your double is struck by lightning.

First, why is it more "likely" that you won't get struck by lightening? You seem to be assigning weight to the probabilities that doesn't necessarily exist.

Throughout your life there are many similar instances where the universe would split to cover equally unbalanced events and invariably it is your double who suffers the UNLIKELY consequence whereas if the path after the split was truly random in a 2 choice scenario 50% of the time it should be you.

Only perhaps because you're one of the "lucky" doubles. What about the people who DID get struck by lightning, or killed in a car accident, or caught in a tornado, or drowned in a tsunami? Their doubles are the "lucky" ones in each of those cases. And, the SAME double doesn't have to have ALL the bad things happen to them. Once a split has happened, every individual along that split doesn't now need to be subject to the choices of every other individual that arose from the same origin. Once you've died in one of the multiple universes, your later homicidal rampage in another universe has no impact on the universe in which you no longer exist, nor does it even impact the one just split off one before the one in which you took a blow to the head leading to the brain damage that led to the possibility of you becoming homicidal. And, just because you survived walking around in a storm without being struck by lightning, and your "double" somewhere else did get struck by lightning, doesn't mean someone in that same universe as your double gets struck by lightning the next time someone in our universe avoids getting struck by lightning while standing under a tree in a storm. And, partly, this is because there is no requirement that every split be only a bifurcation. From what ST and Vanesch have been describing, if there are 5000 places that lightning could strike from a particular cloud to the area where you are standing, there will be 5000 universes, and only in one of those does the lightning hit the spot where you are standing, while in the 4999 other universes, it hits someplace near you, but doesn't actually strike you. So, it's not just 50/50 that it hits you or not, but more like 1/5000 that it hits the precise spot where you're standing. The odds get even longer if you're walking around and more universes are popping up with every step you take, so that the probability of lightning hitting you requires being in the one universe out of millions where you took that step to the left just moments before the lightning hit that spot out of all the other spots it could have hit.

 

[rewebster]

In another universe, MB built an addition on my house in an exchange for me putting 'gravity to electric' panels on her roof.

 

[Astronuc]

In another universe, sheep are the highest order of life, and pigs can fly. The sheep are jealous.

OK - this is more like the GD Universe with which I am comfortable. For a moment, this thread started out seriously.

Back to normal. :Phew!:

 

[EnumaElish]

Art-- I think you are asking "is the totality of all universes a fair place?" That's kind of like asking "is life fair?" and we all know the answer -- or can expect to learn (painfully) at some point.

What took me down this path is the thought that every time a coin is flipped, if it's "heads" in this universe then it's "tails" in another. And the other way around.

Suppose we observe 2/3rds of the outcomes as heads. We would not automatically conclude that the coin is not a random coin. We'd say it's not a fair coin, or the coin is biased. But that's not the same as a non-random coin.

With this biased coin, we get 2 heads out of every 3 tosses, on average. So in an alternative universe our copies are getting 2 tails out of 3 tosses. Are you saying that this makes our universe a special place? Because, I disagree. By your reasoning, folks in the alternative universe have the exact reason for thinking that theirs is a special universe. In some sense that's true, because each universe is unique in some way (heads vs. tails). But in another sense, they are equally mundane.

 

[Chris Hillman]

Don't believe what you read in New Scientist

I am no physicist, but it sounds like this is pretty big. From the last paragraph, it sounds like a hypothesis to me.

The second-hand story you quoted appears to based upon a story (which I haven't seen) in New Scientist, or perhaps upon a press release put out by New Scientist. In either case, you should know that New Scientist, which was once a respectable news magazine, is now regarded by many prominent physicists as sensationalist trash; see for example
http://golem.ph.utexas.edu/category/2006/09/a_plea_to_save_new_scientist.html
http://golem.ph.utexas.edu/category/2007/09/the_virtues_of_american_scient.html#more

The story didn't cite a paper or eprint and it is so vague that without reviewing recent papers by the physicists mentioned in the story, I can't guess what they are talking about. FWIW you would look for something similar to but much more recent than this 2001 eprint by David Deutsch on the multiverse hypothesis:
http://www.arxiv.org/abs/quant-ph/0104033
Deutsch and Everett are well known researchers, but it seems fair to say that the many-universes hypothesis is currently preferred by a very small minority of physicists, and I know of no reason why that might be about to change.

Tentative conclusion: this sounds to me very much like the kind of misleading "buzz" which all too often results when
1. Some university publicist overhypes a fairly ordinary paper by some faculty member,
2. An inflated and vague university press release is picked up by an uncritical news magazine (NS currently being one of the worst offenders).
3. That story is then summarized uncritically by some wire service, and this item is printed in random newspapers experiencing a "slow news day".
4. The crankweb picks up and uncritically repeats this item.
5. Someone writes a Wikipedia article which misleadingly portrays fringe claims as a widely accepted hypothesis or even as established scientific fact.

As a general rule, a good skill to learn is googling for press releases with content similar to what you are reading. Often you will find that some newspaper (or general audience magazine or website) has published with minimal changes something written, not by a scientist, but by a publicity agent working for the employer of a scientist. Needless to say, such stories are never unbiased and are almost always scientifically inaccurate.

 

[vanesch]

it seems fair to say that the many-universes hypothesis is currently preferred by a very small minority of physicists, and I know of no reason why that might be about to change.

As has been mentioned here in this thread before, the value of an idea is not necessarily related to the popularity amongst scientists over it ; especially if it is more a philosophical issue than a genuine scientific (that means: prediction/observation and no "story") idea. Now I know that relativists for one are not the most favorable to MWI ; on the other hand, string theorists usually are. And for 99% of all working physicists, mainly working in more applied branches of physics and engineering, it really doesn't matter, and most of them haven't given it a lot of thought. So depending on your original sample of physicists, you'll find different outcomes for your poll.

That said, I agree 100% with you about Deutsch's hype and overselling.

As I said before, MWI is not an "imported idea", it emerges naturally when you take the linear quantum formalism literally, and its axioms as describing an ontological part of nature, instead of a formalism that allows you to obtain outcomes without any descriptive force. As such, MWI is to quantum mechanics, what the "block spacetime" is to general relativity (and has also its share of philosophical weirdness to it).

 

[Ivan Seeking]

And for 99% of all working physicists, mainly working in more applied branches of physics and engineering, it really doesn't matter, and most of them haven't given it a lot of thought.

Which often seems to be the case with the most interesting questions of all!

Some years ago when I set out to determine the state of the measurement problem - looking for a consensus, if any - I found that most physicists wouldn't talk about it. That is partly how I landed here at PF.

 

[Chris Hillman]

Clarification

As has been mentioned here in this thread before

Clarification: I was trying to introduce into the thread some considerations which hadn't been expressed, not to summarize those which had already been discussed.

the value of an idea is not necessarily related to the popularity amongst scientists over it ; especially if it is more a philosophical issue than a genuine scientific (that means: prediction/observation and no "story") idea.

Not sure I entirely agree with the premise that an idea has a well-defined value but I do agree that the arguments over the foundations of quantum mechanics involve interpretation of mathematics and (therefore?) involve philosophical issues.

As I said before, MWI is not an "imported idea", it emerges naturally when you take the linear quantum formalism literally, and its axioms as describing an ontological part of nature, instead of a formalism that allows you to obtain outcomes without any descriptive force. As such, MWI is to quantum mechanics, what the "block spacetime" is to general relativity (and has also its share of philosophical weirdness to it).

Not sure I agree with the analogy, but agree that things like closed timelike curves in Lorentzian manifolds do present a challenge to the physical interpretation of such spacetimes. It might also be worth mentioning that the dividing line between classical physics and quantum physics has been increasingly blurred as mathematicians have discovered more and more analogies/generalizations unifying various mathematical ideas underlying "quantum" and "classical" physics, and as physicists have found more and more applications of "quantum physics" notions in "classical physics".

Which often seems to be the case with the most interesting questions of all!

I dare say that we've all had similar experiences at one time or another. Resistance from individual scientists to contemplating profound changes can be frustrating if you want to discuss a foundational issue, but is not neccessarily "non-adaptive". In fact, from the perspective of each individual scientist, spending his time/energy wisely makes good sense. Even from the broader perspective of the healthy growth of science itself, it is not clear that this phenomenon of strong resistance to foundational revolutions is a bad thing (dunno whether that is what you were getting at). Imagine the mess if we tried to change the foundations every decade!

 

[Moonbear]

Which often seems to be the case with the most interesting questions of all!

I guess that depends on what you think is interesting. I don't think anyone could stand to stay in any line of research, be it experimental or theoretical, if they didn't think they were studying something of great interest.

Resistance from individual scientists to contemplating profound changes can be frustrating if you want to discuss a foundational issue, but is not neccessarily "non-adaptive".

I think that's a good point. It's easy to lose perspective on that if you're among those trying to introduce something that goes completely against the grain of the "popular" opinion in a field. But, having to overcome that resistance (skepticism even) is what ensures your idea really is a solid one...resistance is fundamental to good science, and one should even be resistant to their own ideas to ensure it is rigorously tested and sound before it gains greater acceptance. If it's really a good idea, you WILL overcome that resistance eventually, and the idea will be accepted. On the other hand, if your critics are right in their doubt, that will be revealed as well when you cannot overcome their criticisms, and it'll fall by the wayside. It's important to remain humble enough to accept that you could be wrong, and to consider your critics' concerns carefully. The worst science I've seen comes from those whose egos have gotten so large, and have become so entrenched in the notion that their theory/model/hypothesis/interpretation is the right one that they are no longer objectively testing it, but simply seeking confirmation of their own opinions while rejecting any evidence to the contrary. When you stop being your own worst critic, it's time to step back and reevaluate your approach and question your objectivity.

So, I'm really glad you brought up that point, because it gave me a chance to get up on my soapbox and give that little lecture to all the budding young scientists around here (and perhaps even to a few old curmudgeons reading along who needed a reminder of this).

 

[Ivan Seeking]

I wasn't suggesting that we adopt a new worldview, I was just asking about the state of the problem and opinions. In the end I found that wrt the measurement problem, for example, there was no consensus, which may be why many scientists resist any discussion. But I think this resistance also goes to the heart of what drives people.

MB, I realized before graduating that what most interests me in physics is not what interests most physicsts. To me the deep and profound questions are by far the most interesting ones. These questions are the reason that I have a degree. They are also the questions that will most likely not be resolved in my lifetime, if ever. However, it was worth every minute of study to get a glimpse into the abyss.

 

[SpaceTiger]

I wasn't suggesting that we adopt a new worldview, I was just asking about the state of the problem and opinions. In the end I found that wrt the measurement problem, for example, there was no consensus, which may be why many scientists resist any discussion.

I suspect it's related to the fact that the proposed solutions are largely untestable. It would perhaps be more a reason for concern if scientists had reached a consensus on this issue. As an astronomer, I would tend to avoid some of the more important questions as well (such as the ultimate fate of the universe, what happened before inflation, etc.) simply because they are too far beyond our grasp at the moment -- I don't trust our current models to be able to answer them. I think it's a credit to the training scientists receive that they don't feel the need to always have immediate answers to sweeping questions.

 

[Chris Hillman]

No experimental test? Then it's not (yet) physics

ditto SpaceTiger, and good point. Part of sound scientific judgement involves "judging the moment" and "choosing your struggles wisely". I might mention "analog gravity" as something extremely novel which nonetheless seems to me within reach for young researchers just starting out, and which would be very important if its predictions can be experimentally confirmed.

 

[Ivan Seeking]

Yep. What interested me most were questions that don't generate grant money or careers.

However, this doesn't mean that those questions should be ignored or forgotten. And I think it is true that orthodoxy can stifle valid discussions of the deepest issues. I once had a physics professor close her door so that she could freely discuss her views on such matters, and I was quite struck by this.

 

[SpaceTiger]

Yep. What interested me most were questions that don't generate grant money or careers.

However, this doesn't mean that those questions should be ignored or forgotten.

For questions that are purely philosophical, it's not clear that the scientific community should even be involved. Questions that are just way out of the reach of current theory, on the other hand, may turn out to be trivial or irrelevant in the later stages of scientific development. I think there's an understandable level of suspicion towards scientists who put too much weight on these broad questions that have religious or philosophical implications. These people often have a lot of baggage that can skew their judgement on issues worthy of actual scientific investigation. An example that comes to mind is the steady state universe/big bang debate, in which certain followers of the former continued their crusade well after the majority of the scientific community had been convinced of the latter. I can only speculate as to their actual motivations, but I can say we don't see such passionate resistance to theories of stellar evolution or orbital dynamics,

The questions shouldn't be completely ignored, of course, but I think it's understandable that they're often avoided. We can't disprove the MWI (just for example), so it shouldn't be treated as false, but on the other hand, it's not clear that an extended public debate (within the scientific community, I mean) would be productive.

 

[Evo]

Chris, Vanesch and ST, I find this all very enlightening.

What do you think of Max Tegmark at MIT?

Are multiverse theories testable?
Is it all just philosophy?
On Nov 3, 2003, at 14:38, Walter H.G. Lewin wrote:
Q: Is there a way, at least in principle, that the existence of multiple universes as described by you in Sci Am, can be experimentally verified or falsified? If not, as several of my colleagues have pointed out, it falls in the realm of religion and philosophy, but not physics.
A: Absolutely! The key point, which I emphasize in that article, is that a fundamental physical theory can be testable and falsifiable even if it contains certain entities that you cannot observe. To be testable and falsifiable, it merely nees to predict at least one thing that we can observe. A good example is the theory of eternal inflation, where our Hubble volume constitutes only an infinitesimal fraction of all space. Since this theory makes the firm prediction that Omega = 1 to an accuracy of order 10^{-5}, this model (and all those level I parallel universes with it) would have been ruled out if we had measured say Omega=0.70+0.02. Instead, our latest constraints in astro-ph/0310723 are Omega=1.01+-0.02.

http://space.mit.edu/home/tegmark/

Sorry, this is the watered down layman's discussion, which is all that I can understand.

Of course a lot of this is old, from when I first became interested in MWI, but that was when String Theory was going strong as well. I'd like to hear your opinions on where we've come in the last 4 years on MWI. Has interest in it dwindled as it seems to have with String? Or am I misunderstanding where String Theory is going? The interest in quantum computing is picking up, and it ties in strongly with MWI, doesn't it? Having the opportunity to learn from you guys is awesome.

 

[SpaceTiger]

Evo, he's referring to a different multiverse theory. Notice he uses the phrase "level I parallel universes", referring to an open multiverse predicted by inflation. The MWI is actually a level 3 multiverse by his classification and would not be testable by the methods he's referring to.

 

[Evo]

Evo, he's referring to a different multiverse theory. Notice he uses the phrase "level I parallel universes", referring to an open multiverse predicted by inflation. The MWI is actually a level 3 multiverse by his classification and would not be testable by the methods he's referring to.

Yes, that's correct he does say Level III is not testable.

So, is MWI at Level III and IV? Ah, it's levels II-IV?

Remember, I am at the Scientific American level of understanding, be kind.

 

[SpaceTiger]

Yes, that's correct he does say Level III is not testable.

So, is MWI at Level III and IV?

I think he puts it at level III, but I haven't read the paper. Max gets away with this cause he does a lot of useful stuff too.

 

[Evo]

I think he puts it at level III, but I haven't read the paper. Max gets away with this cause he does a lot of useful stuff too.

That's too funny.

Isn't that how it goes? As long as you're doing something useful, they'll let you play with your pet theories?

How does someone like me sort the wheat from the chaff?

 

[SpaceTiger]

How does someone like me sort the wheat from the chaff?

There's nothing wrong with his multiverse classification, it's just not very useful. Even the multiverse predicted by inflation is highly speculative, I would say, since it assumes that we've a full grasp of the physical limits of the theory. I'm afraid I can't give a very helpful description beyond that, however.

It's difficult to near impossible for a novice to judge the plausibility of the results reported in the media. They're very often wrong, misleading, or irrelevant in mainstream astronomy. Even an astronomer wouldn't be able to give you a useful judgement on the majority of the things reported, since they will only be fully aware of the events related to their own work. It's a shame that there isn't more control over what actually gets reported. Universities will allow an article on anything that interests the journalists, and the researchers usually don't mind because they get free publicity.

 

[Chris Hillman]

Shut a door, start a conspiracy theory?

Yep. What interested me most were questions that don't generate grant money or careers.

Well, call me Dr. Pangloss, but it seems to me that genuine scientific success always generates grants and students, so isn't this simply a lamentation of personal failure to make sufficient scientific progress toward solving your favorite enigma?

However, this doesn't mean that those questions should be ignored or forgotten.

We might offer different answers to the question "what are the deepest enigmas in science?", but whatever our respective lists might be, no-one is suggesting that we should collectively ignore/forget deep questions!

And I think it is true that orthodoxy can stifle valid discussions of the deepest issues. I once had a physics professor close her door so that she could freely discuss her views on such matters, and I was quite struck by this.

I think you're making too much of that incident. Maybe she realized that with rising passions you and she were disturbing her neighbors?

I have a different interpretation: raising the bar when it comes to revolutionary change by adopting some proposed solution to some fundamental enigma internalizes and intensifies discussion and debate. Those obsessed with their own pet theory "solving" the enigma spend far more time in internal critiques than they might with humdrum science (or else become the butt of a jest by Kibo*). This is a good thing and should be encouraged, since it ensures that when scientifically useful "revolutionary" solutions do emerge, they are accompanied by spectacularly well argued and carefully expressed arguments. Einstein 1905 is a good example, in which overwhelmingly persuasive arguments from a scientific unknown quickly captured the attention and indeed the support of the leaders of physics (well, Planck and friends; those who couldn't make the transition to relativistic physics were unceremoniously deposited in the dustbin of science, which was also a good thing).

*You know, the Guy that Runs the Internet.

I happen to think that as a rule, the best papers are singly authored (is Christine Dantas hereabouts, by any chance?), and if so, this simply reinforces my suspicion that laments about the alleged "discouragement of open discourse" concerning some fundamental enigma reduce upon closer examination to the confession "no Einstein I".

(I assume we are discussing something like the interpretation of QM; if not maybe this is one time when you should not disabuse me! Also, I stress that I am assuming that we all belong to the group of individuals who have struggled and failed by dint of our own efforts to resolve some deep enigma of science, whatever humdrum successes we may have enjoyed.)

IOW, scientific revolutions, unlike political revolutions, are best accomplished by lone pioneers, in private. After that it is simply a matter of publishing the enunciation of the new marching orders.

To toss another idea into the mix: with the exponential growth of the hierarchical structure of mathematics, which I like to define as the art of reliable reasoning about simple phenomena, and which is the foundation of science, humans are becoming obsolete as the agents of science. Clearly longer-lived and far more intelligent scientists are needed. If they do not exist they must be invented.

(Some cry COI when I say this.)

 

[Chris Hillman]

Who gets to play in the sandbox of science?

Isn't that how it goes? As long as you're doing something useful, they'll let you play with your pet theories?

Even better, we/they will pay some mind to your musings. This actually makes perfect sense, since doing manifestly useful "ordinary science" (to adopt the phrase of Kuhn) provides an easy check on the continued sanity of someone like Tegmark.

How does someone like me sort the wheat from the chaff?

I think you need to know some minimal number of experts you can canvass.

 

[Evo]

I think you need to know some minimal number of experts you can canvass.

That's why I am glad I'm here. I can't tell you how much my understanding and level of acceptance has changed/increased since coming here. Of course I will always be just a listener, it helps to know what to listen to among all of the noise.

 

[vanesch]

Not sure I agree with the analogy, but agree that things like closed timelike curves in Lorentzian manifolds do present a challenge to the physical interpretation of such spacetimes. It might also be worth mentioning that the dividing line between classical physics and quantum physics has been increasingly blurred as mathematicians have discovered more and more analogies/generalizations unifying various mathematical ideas underlying "quantum" and "classical" physics, and as physicists have found more and more applications of "quantum physics" notions in "classical physics".

I was simply pointing out that assuming the "physical, ontological existence" of a spacetime manifold is to GR what the "physical, ontological existence" of statespace and the state vector is to quantum theory. So in as much as you give a genuine existence to spacetime in GR - as opposed to just a mathematical trick to calculate observations in GR - you can give a genuine existence to quantum states in quantum theory, as opposed to consider that it is just a calculational trick to calculate probabilities of outcomes.

You don't need closed timelike curves or anything: the very assumption of a physical existence of spacetime is enough. You give an ontological status to the axiomatically introduced basic mathematical structure in GR. If you do the same in QM, you arrive at giving an ontological status to what's commonly called "the wavefunction", and if you don't do anything else, you have an ontology like MWI.

A question in MWI like "why do I only observe 1 world" is then equivalent to the GR-spacetime interpretation as "why do I only observe 1 spacelike slice". You can really push the two analogies quite far. "Is there really someone else in another universe who looks a lot like me but had another outcome ?" versus "Does Julius Caesar have some physical existence somewhere in another slice of spacetime ?"

This is BTW where I find some utility of the MWI view. In as much as contemplating a genuine existence of a physical spacetime helps one understand GR (whether this spacetime "really" exists or not, and is only a mathematical trick) in the same way, taking on an MWI view on quantum theory gives one some "intuition" for it, whether superposition 'really' exists or not.

 

[Astronuc]

classical physics and quantum physics has been increasingly blurred as mathematicians have discovered more and more analogies/generalizations unifying various mathematical ideas underlying "quantum" and "classical" physics, and as physicists have found more and more applications of "quantum physics" notions in "classical physics".

I think therein lies part of the problem, which is the belief that the models we contruct and the mathematics reflect the reality, or for some, are the reality. The mathematics and models provides a description as best as we can, but its not the reality, and at best its an approximation.

I do predictive analysis and I know the limitations of the math and models (and computational systems), and also the limitations of the inputs (I.C and B.C), and we often find exceptions and nonlinearities, some of which become significant in time, while others are trivial and insignificant. In some cases, we simply construct better models and get better predictions, and being a few percent off in 4-6 years is pretty darn good.

The greatest challenge (and greatest pressure) is predictive failure analysis.

The foundation of predictive analysis is a robust model and the experimental work (testability) involving both separate and integral experiments upon which individual models and the integral models are based.

Is there anything in astronomy or QM that requires MWI in order to make some model/mathematics fit what is observed in this universe - or rather what is observed locally? And then if so, does the fact that MWI is required to make a model work mean that something is wrong with the model?

 

[Chris Hillman]

Seeking clarification

I was simply pointing out that assuming the "physical, ontological existence" of a spacetime manifold is to GR what the "physical, ontological existence" of statespace and the state vector is to quantum theory. So in as much as you give a genuine existence to spacetime in GR - as opposed to just a mathematical trick to calculate observations in GR - you can give a genuine existence to quantum states in quantum theory, as opposed to consider that it is just a calculational trick to calculate probabilities of outcomes.

OK, I misunderstood.

You give an ontological status to the axiomatically introduced basic mathematical structure in GR.

Actually, I don't, and have often posted warning against that attitude!

At least, assuming I am correct in assuming that by saying "you give an ontological status to the axiomatically introduced basic mathematical structure in GR" you mean "you believe that spacetime exists in Nature and really has the structure of a Lorentzian manifold". If you meant only that when working with gtr I behave as if I believe this when interpreting results of some computation in a Lorentzian manifold, then I am probably guilty as charged

A question in MWI like "why do I only observe 1 world" is then equivalent to the GR-spacetime interpretation as "why do I only observe 1 spacelike slice". You can really push the two analogies quite far.

Sorry, I don't understand what you mean by asking (in the context of Lorentzian manifolds interpreted as a model spacetime, I take it) "why do I only observe 1 spacelike slice"? And what does this have to do with "blocks"? (I understood you to be using that term as it is used in the gtr literature, e.g. in discussing various "blocks" making up a Carter-Penrose diagram depicting the conformal structure of some Lorentzian manifold.)

 

[Chris Hillman]

Hi Astronuc,

I think you might have misunderstood my remark, which was tangential to this discussion. I was simply saying that upon adopting a more sophisticated view of the underlying mathematics, the old distinction between "classical physics math" and "quantum physics math" becomes blurred.

part of the problem, which is the belief that the models we contruct and the mathematics reflect the reality, or for some, are the reality. The mathematics and models provides a description as best as we can, but its not the reality, and at best its an approximation.

That's what I've always said whenever the subject has come up; in fact, in my experience not understanding this is one indication that my correspondent doesn't really understand what theoretical physics is all about.

Sorry for any confusion my choice of words may have caused!

 

[vanesch]

Actually, I don't, and have often posted warning against that attitude!

At least, assuming I am correct in assuming that by saying "you give an ontological status to the axiomatically introduced basic mathematical structure in GR" you mean "you believe that spacetime exists in Nature and really has the structure of a Lorentzian manifold". If you meant only that when working with gtr I behave as if I believe this when interpreting results of some computation in a Lorentzian manifold, then I am probably guilty as charged

Oh, that's interesting ! I didn't know that. The few relativists I know take - as far as I understood them - the spacetime manifold as "real" (ontological). It was in that respect that I considered MWI versus the spacetime manifold in GR. If you do not take that manifold for real - even for the sake of getting an intuition for the way GR (as a theory) behaves - then I fully understand how you cannot take MWI seriously, even for the sake of getting a better feeling of how QM works...

And what does this have to do with "blocks"? (I understood you to be using that term as it is used in the gtr literature, e.g. in discussing various "blocks" making up a Carter-Penrose diagram depicting the conformal structure of some Lorentzian manifold.)

No, not at all, I'm not that sophisticated (although I vaguely understand what you are alluding to). I was alluding to the "static spacetime manifold block universe" which, I thought, was used in interpretational issues with GR - but given your earlier paragraph, this point is moot.

 

[Chris Hillman]

The few relativists I know take - as far as I understood them - the spacetime manifold as "real" (ontological).

Are you sure about that? Did you ever politely but firmly interrogate them about their ontological attitude toward spacetime models? I would be quite amazed if any specialists in gtr truly believe that our universe is literally a Lorentzian manifold. C.f. "quantum foam" and all that.

(If you assiduously Google for my posts to UseNet and elsewhere years ago, you can probably verify that in previous comments I have noted a rare emotional outburst by Chandrasekhar in which he seemed to say that he was awed by the realization that the exterior of a black hole in Nature is literally a Kerr spacetime [sic]. Since he was an expert on perturbations, he can't possibly have believed any such thing, but since he is dead, I can't ask what he did mean, so I think it best we shrug helplessly and move on.)

No, not at all, I'm not that sophisticated (although I vaguely understand what you are alluding to). I was alluding to the "static spacetime manifold block universe" which, I thought, was used in interpretational issues with GR - but given your earlier paragraph, this point is moot.

I'll go out on a limb and guess that you often read hep-th papers but rarely read gr-qc papers. I, OTH, often read gr-qc papers but rarely read hep-th papers. I will guess further that whatever you read (perhaps in a section discussing some aspect of the "philosophy of spacetime"?) about "block universe" might refer to a decomposition of a Lorentzian four-manifold as a disjoint union of infinitely many spacelike hyperslices (Riemannian three-manifolds). If so, I still don't understand the question "why do I observe only one hyperslice?"

 

[Ivan Seeking]

Well, call me Dr. Pangloss, but it seems to me that genuine scientific success always generates grants and students, so isn't this simply a lamentation of personal failure to make sufficient scientific progress toward solving your favorite enigma?

I was too smart to go far enough to fail.

 

[Ivan Seeking]

For questions that are purely philosophical, it's not clear that the scientific community should even be involved.

I hardly think this applies. For example, the measurement problem is a problem of physics, not philosphy.

In fact I think this demonstrates the problem that I observed. When we don't have an answer, call it philosophy. I was talking about foundational problems in physics, and by the end of your post, you made this about religion.

 

[SpaceTiger]

I hardly think this applies. For example, the measurement problem is a problem of physics, not philosphy.

In fact I think this demonstrates the problem that I observed. When we don't have an answer, call it philosophy. I was talking about foundational problems in physics, and by the end of your post, you made this about religion.

I think you may have a bit of a persecution complex, here. I never said that the measurement problem was a religious issue, nor did I say that it was a matter of pure philosophy. The example I used was primarily in astrophysics and was meant to help explain the attitude of scientists towards "sweeping" questions.

I would, however, say that if the "interpretations" of the measurement problem are not producing any new predictions or ways of distinguishing them, scientists would be right to avoid expending a great deal of energy studying the topic. I am not a theoretical physicist, so I don't know the extent to which this is the case, but my impression is that we haven't seen much progress in the last 50 years or so.

The other point I was making was that people in the scientific community that spend a great deal of time worrying about things of this nature are often working with religious or philosophical baggage and are therefore viewed with suspicion. In that regard, I was trying to explain to you why this is sometimes a "closed door" topic of discussion. It is not a conspiracy to enforce scientific dogma, as you seem to be making it out to be.