Ken G said:When experiment gives us 100 numbers, and we choose the parameters of our theory to fit those 100 numbers, then we will have 100 reasons to believe our theory."Yeah, my point exactly.
Ken G said:I said above, that may just be (and seems to me is) simply because the multiverse models were flexible enough to accommodate almost any outcome of WMAP.
Right, that's why Popper stressed the need for "risky" predictions.
I have a hard time seeing how multiverse thinking is not similarly flexible, for the simple reason that we have so few a priori constraints on the attributes of the putative "distribution" of universes.
I see nowhere in any of that which requires the existence of a multiverse, any more than playing a single hand of poker in a perfectly intelligent way requires that any other hands of poker have ever existed anywhere else.
That was flawed logic, because you used the largeness of the universe after inflation as a reason to expect inflation, but then assumed a largeness of the universe before inflation as a reason to expect inflation to occur many times.
Not if the curve covers its tracks, which is exactly what a thermal spectrum does, except for the fluctuations-- a much lower-dimensional set of constraints.twofish-quant said:At which point the theorist will ask for several billion dollars for telescopes that will give them 10000 numbers to fit 100 parameters. This is one reason the CMB *spectrum* is so important. You aren't fitting to a number, you are fitting to a curve, which gives you a huge number of numbers to fit to.
Sure, but those would be problems for any cosmology, not just multiverse cosmology. It doesn't single out the multiverse, I wouldn't expect any of those things prior to their being detected. Now, if the multiverse did predict one of those things, then at least it would have been falsifiable. The bottom line is, you really never have any idea if a theory is good until it makes a prediction that you didn't already expect without that theory, and then you test it. When the multiverse idea does that, I will agree it has evidential support. If it never does that, then it will never have evidential support, it will always just be a way to rationalize what we already knew to be true.twofish-quant said:That's not the situation. If you had zero CMB fluctuation, you have a problem. If you have large non-Gaussian CMB fluctuation, you have a problem. If you have unexplained anisotropy, you have a problem. If you have unexplained polarization, you have a problem.
Possibly there could be more details on the models. But I think the real reason you don't see that is they are just too speculative. That's putting it quite mildly, in fact, for the case of the multiverse.This is a teaching issue, because I don't think that most intro astronomy courses for non-major really go into sufficient detail about the empirical evidence for cosmological models. Part of the problem is that to lots of people, they are *boring*.
It was indeed risky to predict a thermal background, and that is a prediction of the Big Bang. Any Big Bang, with our without multiverses. Predicting the fluctuations does not seem to be a risky prediction-- it seems to just be fitting the parameters, knowing that we would be able to fit them given the flexibility to invoke dark matter, dark energy, and any arbitrary scalar potential for the inflation.Predicting the CMB *spectrum* seems to be a very risky prediction.
Which makes the risky prediction that ______? (fill in the blank.)Eternal inflation asserts that the scalar field is such that most areas are still expanding.
This is a natural feature of the inflation phenomenon, it does not adjudicate multiverse existence. I am inclined to disbelieve the multiverse, yet I'm also inclined to believe in a large universe. It's evidence for inflation, not the multiverse.So in the case of eternal inflation, you are *required* to have a large volume outside the observed universe.
Which specific model? What predictions does it make that are going to get tested any time soon? Can you say "the leading candidate model is X, which makes risky prediction Y, which will be tested by experiment Z, so we'll soon have an answer if just wait"? If you can say that, please do, and if you cannot, isn't that trying to tell you something?Again, I think the reason we are even talking metaphysics is that we aren't being specific enough. We aren't talking about *all possible multiverse models*, we are talking about a *specific* model.
Ken G said:Sure, but those would be problems for any cosmology, not just multiverse cosmology. It doesn't single out the multiverse, I wouldn't expect any of those things prior to their being detected. Now, if the multiverse did predict one of those things, then at least it would have been falsifiable.
The bottom line is, you really never have any idea if a theory is good until it makes a prediction that you didn't already expect without that theory, and then you test it.
Possibly there could be more details on the models. But I think the real reason you don't see that is they are just too speculative.
Predicting the fluctuations does not seem to be a risky prediction-- it seems to just be fitting the parameters, knowing that we would be able to fit them given the flexibility to invoke dark matter, dark energy, and any arbitrary scalar potential for the inflation.
Which specific model?
What predictions does it make that are going to get tested any time soon?
Can you say "the leading candidate model is X, which makes risky prediction Y, which will be tested by experiment Z, so we'll soon have an answer if just wait"? If you can say that, please do, and if you cannot, isn't that trying to tell you something?
I guess we've made our cases, there's no point in just repeating. All I want to know is, are you, or are you not, going to answer this question:twofish-quant said:It would rule out certain *types* of multiverse cosmologies. Eternal inflation for example makes pretty specfic predictions about the CMB background.
I didn't ask about inflation, I asked about the multiverse associated with eternal inflation.There is a *particular* power spectrum that inflation predicts, and it's hardly a case of "fitting the parameters" because people made the prediction a few years before COBE went up.
I still await where you show that eternal inflation predicts something we would expect to be untrue if we were skeptical of eternal inflation. I'm skeptical of eternal inflation, and you have not showed me a prediction that I would have expected to fail.Predicting that there will be fluctuations isn't impressive. Predicting the exact spectrum of the fluctuations is.
I see those as predictions of any inflation model that works.Predicts that curvature is < 1e-5 and that CMB is gaussian at small scales.
That is very much the problem. It raises the spectre of rationalization by simply building in enough theoretical degrees of freedom to fit anything necessary, which is exactly what Popper objected to about Freud, Marx, and Adler.I can't say the leading candidate model since there are several hundred inflationary models, all with various predictions.
Yes, but of course the irony is, the only good candidate theory that the multiverse model has been falsified. That means Smolin deserves kudos for the effort, but it is certainly no selling point for the multiverse models that have not been falsified. For them, I await the answer to the above question.Also for an example of a *falsified* multiverse model, look at cosmological natural selection. Smolin made the prediction that there would be no pulsars with more than 1.6 solar mass, and when we found a 2.0 solar mass pulsar, that theory went bye-bye.
Ken G said:Well, that description just does not seem at all accurate to me. That sounds more like what happened to the Big Bang model in the 60s, at which point it became the cornerstone of cosmology and began to be taught as "lesson one" in any introductory cosmology course. I don't see any of that as being true for the multiverse.
Ken G said:I see those as predictions of any inflation model that works.
What is perfectly obvious here is that you are overselling the case for the multiverse. If your argument was valid at all, then we would have a single wisely accepted model of the multiverse, instead, of what we actually have, which is a kind of factory for producing agreement with anything we want. To get some idea of the expanse of this factory, consider:Haelfix said:Anyway, I simply cannot understand why you would think that making a very specific (and at the time an original prediction) --a Harrison-Zeldovich nearly scale invariant primordial spectrum-- with detailed values somehow doesn't count. It is and was about as risky a proposition that you can write down, and quite honestly was not accepted by large portions of the community.
If you believe that, then I guess you missed the whole bit about Popper's challenge to scientists. To summarize, there is no issue with selecting a theory from a range of possibilities, the issue is that one cannot use all the data one has, and can likely expect to get, in the process of choosing, because if one does, one is simply rationalizing that data. If instead one wants to make a discovery about how the universe works, one must take the selected theory, and use it to make a "risky prediction", in reference to data that was not used to select the theory (or may not yet have even been observed, but which can motivate an observation to falsify the theory).Haelfix said:First of all, almost all of high energy physics is a 'theory factory'. The standard model is an example of a theory that was curve fit to experiment, and there are many possible theories that go beyond the standard model where those parameters have not yet been measured. That is very much how science progresses.
I can't help notice that you fell way short of claiming they were not skeptical of the multiverse idea. Are you now claiming that I misrpresented their skepticism, and they would agree with your blanket statements that WMAP data "spectacularly confirms", as you put it, that model?Second of all I happen to know some of those individuals, and I know their opinions are rather more subtle than what can be subsumed in a sentence.
We are talking about the version with cosmological implications, obviously, so that's level 1. All the same, Tegmark might argue that similar kinds of reasoning goes into constructing all those multiverses, so if a "level 1" model ever becomes the mainstream consensus (indeed you have claimed it already is), at the very least this opens the door wide for the other levels (and at least one poster on this very thread has already connected the cosmological multiverse to the many worlds of quantum mechanics).Thirdly, please distinguish between the case for level 1 of the multiverse and level 2-4 (using Tegmarks classification) and indicate which one you are talking about.
That statement neither surprises me in the least, nor contradicts a single thing I said, nor addresses the challenge that I have put for proponents of the multiverse to meet. Now let me tell you a fact. I move among more "rank and file" astronomers (who tend to adopt highly empiricist approaches to science), rather than cosmological theorists (who tend to adopt highly rationalistic approaches to science), and I cannot name even a single one who would be comfortable describing the multiverse idea as "textbook science" to any of their classes, and I would wager that the majority view it is primarily a metaphysical view. That may partly be because they are underinformed, but more likely it is mostly due to the simple fact that a good case has not been made that it isn't primarily metaphysics.If you asked me to take a straw poll of people that I know and work with, I would say (anecdotally) at least one out of 3 people believe in level 1, with mostly undecided remaining, and I don't know many people who still hold out against inflation in general.
And you think those numbers (for chaotic/eternal inflation) would be similar in a general audience of astronomers, say an American Astronomical Society meeting? I strongly doubt it. Cosmological theorists are the most knowledgeable about their models, but they are also the most likely to be taken in by rationalization, which is Popper's whole point.I'll give you a fact. I was present for a rather important conference in Chicago back in 1999, where a lot of famous cosmologists were presenting the case for inflation and what to make of a positive cosmological constant. A straw poll at the conference showed that approximately 60% believed in inflation. Of those that did, approximately 50% believed in some sort of chaotic/eternal inflation.. I daresay the percentages have increased significantly since then.
There's a separate thread on whether the multiverse is physics or metaphysics, and these issues are kind of getting crossed here. I think your explanation of that issue would be quite interesting, but might be more appropriate on that thread.Incidentally, you haven't asked my opinion.. So I will volunteer it. Does it surprise you that my answer is 'I don't know'? There exists a strong theoretical argument against the multiverse that has to do with something called horizon complementarity and I am unsure what to make of it.
I fear you have missed the point-- what you just said is what I am arguing for as well. The arguments in favor of the multiverse almost always have a highly metaphysical quality to them, so they are already philosophy. The challenge presented by Popper is the path to arguing they are not metaphysics, but are rather science, and that's why I find it so significant that this challenge has not been met. In other words, I am not using philosophical arguments to say the multiverse doesn't exist, I'm pointing out the dearth of non-philosophical arguments that the multiverse does exist.But the point is whether the mv exists or not will be decided by a mix of empirical data, and theoretical arguments. Philosophy of the sort you have presented, convinces nobody.
"Pseudoscience" is something very different, had I intended that word I would have used it. The choice is "metaphysics". When people feel the need to change the words to make their case, it always raises a red flag. But again, whether the multiverse is physics or metaphysics is more appropriate for that other thread, there's a lot of crossing over so it's hard to know which arguments were presented in which thread.Worse, the idea that there is no empirical or theoretical support for the MV and that it is pseudo science is basically an absurd proposition that has shown up in laymen topics on the internet, but is decidedly absent from any discussion amongst physicists.
Ken G said:If you believe that, then I guess you missed the whole bit about Popper's challenge to scientists.
To summarize, there is no issue with selecting a theory from a range of possibilities, the issue is that one cannot use all the data one has, and can likely expect to get, in the process of choosing, because if one does, one is simply rationalizing that data.
A theory that never does this is a very poor excuse for a scientific theory, that was Popper's point in the context of theories by Adler, Marx, and Freud, and this point is widely accepted as an important insight into doing good science.
I don't claim categorically that the multiverse never does this, instead I challenge people to show that it does-- and the challenge has gone largely unmet, on this thread and on the one specifically set out to deal with that challenge.
Now let me tell you a fact. I move among more "rank and file" astronomers (who tend to adopt highly empiricist approaches to science), rather than cosmological theorists (who tend to adopt highly rationalistic approaches to science)
2) Even if we are inclined to increase our belief in model X, having observed x, this still does not provide non-metaphysical evidence for the existence of the multiverse because I can simply introduce theory X', which is mathematically identical to X so makes all the same predictions x, but merely refers to our uncertainties about our own universe-- and casts them in terms of some kind of probability distribution, much as quantum mechanics does.
But again, whether the multiverse is physics or metaphysics is more appropriate for that other thread, there's a lot of crossing over so it's hard to know which arguments were presented in which thread.
Haelfix said:Second of all I happen to know some of those individuals, and I know their opinions are rather more subtle than what can be subsumed in a sentence.
If you asked me to take a straw poll of people that I know and work with, I would say (anecdotally) at least one out of 3 people believe in level 1, with mostly undecided remaining, and I don't know many people who still hold out against inflation in general.
Incidentally, you haven't asked my opinion.. So I will volunteer it. Does it surprise you that my answer is 'I don't know'?
There exists a strong theoretical argument against the multiverse that has to do with something called horizon complementarity and I am unsure what to make of it.
Philosophy of the sort you have presented, convinces nobody.
Worse, the idea that there is no empirical or theoretical support for the MV and that it is pseudo science is basically an absurd proposition that has shown up in laymen topics on the internet, but is decidedly absent from any discussion amongst physicists.
All I can tell is not a single thing you've said about Popper's views has seemed even remotely close to Popper's actual views. I know you don't care, you're happy with your misconceptions there.twofish-quant said:We didn't miss it. We just think it's silly.
Of course you would, that's because you still haven't understood Popper at all.Personally, if you had to ask me whether or not Marxism or Freudian psychoanalysis are sciences, I'd say yes.
All you can do is change my arguments so you can refute them. I have never told anyone what their philosophies should be, please quote where you think I did.Again, you are lecturing science to scientists. Rather than telling scientists what their philosophical beliefs *should* be, it might be useful in this discussion to just listen and find out what scientist's philosophical beliefs really are.
Wrong. It is not the job of a scientist to prove that a theory is a bad theory. Rather, it is the job of a scientist to say why a theory is good. You have failed to do that, choosing instead to shoot the messenger who is pointing out to you that you have failed to do that.If you were doing science you would either:
1) state categorically that any theory involving a multiverse model leads to unprovable and untestable conclusions and then back up that claim with a airtight chain of logic, or
2) come up with a specific well-posed multiverse model that we can discuss
Ken G said:Of course you would, that's because you still haven't understood Popper at all.
This is pointless, I'm afraid. You cannot meet the challenge I gave you, all you can do is make excuses for why you cannot meet that challenge.
Wrong. It is not the job of a scientist to prove that a theory is a bad theory. Rather, it is the job of a scientist to say why a theory is good.
You have failed to do that, choosing instead to shoot the messenger who is pointing out to you that you have failed to do that.
Yes, and that's exactly why I would say it is perfectly good science to show what kinds of multiverse theories are ruled out by current observations. Popper would be thrilled.twofish-quant said:If you challenge me to come up with a theory as to how type II supernova work, I can't do it. I tried for seven years, and I couldn't come up with a way to make them go boom. Now, what I was able to do (which got me a Ph.D.) is to show that certain things *won't work*.
twofish-quant said:If you challenge me to come up with a theory as to how type II supernova work, I can't do it. I tried for seven years, and I couldn't come up with a way to make them go boom. Now, what I was able to do (which got me a Ph.D.) is to show that certain things *won't work*.
ibysaiyan said:I think the general consensus is that it's shape is similar to a black hole , hyperbolic. Geometry is dependent on which model universe we are speaking of.. there are open/close systems...
P.S: Open system ( hyperbolic) has the following condition : k<1 , on the other hand a closed system (k>1) would collapse back to a singularity over the passage of time (cyclic universe model).
The torus is homogeneous, finite, and unbounded (although not globally isotropic). It has a flat geometry.denism said:I think that only one of these possibilities fulfills the 3 following conditions for universe: 1) homogeneous, 2) finite and 3) unbounded: the closed version, or 3-sphere.
Am I wrong? thanks in advance
bapowell said:although not globally isotropic
Maybe, but the universe could still exhibit toroidal topology, and it satisfies your three criteria.denism said:as you say, and in addition this form of organization would require some justification because it does not comply with the minimal-sufficiency principle. To my knowledge n-spheres are the highest entropy shapes
bapowell said:Maybe, but the universe could still exhibit toroidal topology, and it satisfies your three criteria.
There is no data supporting any global geometry/topology -- period. But, data does indicate a local universe consistent with flatness, which is evidence consistent with toroidal topology.denism said:OK, I forgot a fourth criterion: minimal sufficiency. In absence of possible verification, no weird proposal of universe shape can be formally rejected, raising the risk of degeneration of this subject. Hence, in an attempt to maintain some rigor in this field, a principle of minimal sufficiency should be observed by rejecting imaginative but supererogatory hypotheses.
I am not aware of data supporting torus
bapowell said:There is no data supporting any global geometry/topology -- period. But, data does indicate a local universe consistent with flatness, which is evidence consistent with toroidal topology.
Could you articulate your minimal sufficiency principle?
EDIT: I'm not advocating for a toroidal universe, just pointing out that it's a perfectly viable possibility.
It has positive and negative principle curvatures, but the Gaussian curvature is zero.denism said:Are you sure that the torus does not cumulate positive and negative curvatures?
They are not -- they are positively curved everywhere.n-spheres are locally flat as well
Right, but none of your criteria are strictly required by observations. We have no data supporting the size (finite vs. infinite) of the universe, whether it is globally compact or bounded, or whether it is globally homogeneous. What we know from observations of the observable universe is that it is approximately homogeneous and close to flat locally. Empirically, all three manifolds of constant curvature -- Euclidean (including toroidal and other flat geometries), spherical, and hyperbolic spaces -- are equally in the running.it is simply making the effort to not introduce hypotheses that are not strictly riquired by new observations.
My feeling is that this curvature is more mathematical than concrete, but it is just a feelingbapowell said:It has positive and negative principle curvatures, but the Gaussian curvature is zero.
bapowell said:They are not -- they are positively curved everywhere.
bapowell said:Right, but none of your criteria are strictly required by observations. We have no data supporting the size (finite vs. infinite) of the universe, whether it is globally compact or bounded, or whether it is globally homogeneous. What we know from observations of the observable universe is that it is approximately homogeneous and close to flat locally. Empirically, all three manifolds of constant curvature -- Euclidean (including toroidal and other flat geometries), spherical, and hyperbolic spaces -- are equally in the running.
denism said:I believed that night would not have been black in an infinite universe? (necessarily forever..)
phinds said:No, that was dealt with conclusively quite some time ago as not being the case. I don't have a reference offhand but I assure you it is true. It has to do with the expansion of the universe. For one thing, it is irrelevant whether the universe is infinite since the only light that reaches us is from the OBSERVABLE universe.
In addition to phinds' response, I'll add that the finite age of the universe also resolves this paradox.denism said:I believed that night would not have been black in an infinite universe? (necessarily forever..)
bapowell said:In addition to phinds' response, I'll add that the finite age of the universe also resolves this paradox.
denism said:something infinite has necessarily always been infinite. The finite age proves that the euclidean 3D space cannot be retained
denism said:OK thanks. The observable universe is the part for which the redshifted wavelengths remain in the visible range?
By finite age, I mean finite time since the big bang. Obviously, if the big bang occurred 13 billion years ago, then there are still CMB photons, as well as starlight, that have simply not had time to reach us yet. This is why the finite age resolves the paradox. It has nothing to do with the size of the spatial geometry.denism said:something infinite has necessarily always been infinite. The finite age proves that the euclidean 3D space cannot be retained
bapowell said:By finite age, I mean finite time since the big bang. Obviously, if the big bang occurred 13 billion years ago, then there are still CMB photons, as well as starlight, that have simply not had time to reach us yet. This is why the finite age resolves the paradox. It has nothing to do with the size of the spatial geometry.
phinds said:Technically, that is exactly correct. In practise, "visible" requires extraordinarily strong telescopes for a long period (see the Hubble Deep Field).
denism said:I asked this question because I read an other interpretation (with which I have some concerns) that the unobservable universe located behind the horizon, corresponds to galaxies for which the recession velocities exceed light speed. Is it erroneous according to you?
phinds said:Not only is it NOT erroneous, it is an understatement in that galaxies IN the observable universe are "now" ("now" gets a bit tricky) are already receding from us FTL. In fact, those at the edge of the OU are receding at about 3c
denism said:I definitely cannot understand this point. My intuitive expectation was that the expansion rate cannot exceed c because it it was the case, everything would disconnect at once, even between the sun and Earth and between your eyes and your screen..
Did you read the problem of the ants on a rubber rope <http://en.wikipedia.org/wiki/Ant_on_a_rubber_rope>[/QUOTE]
The first thing you need to do when studying either cosmology or quantum mechanics it TOTALLY get rid of the concept that your intuition is worth squat. It is not. It's something we all have to get used to.
What you are not understanding is that nothing is traveling FTL in the same reference frame. The universe is expanding. The recession rate has nothing to do with c.
denism said:I definitely cannot understand this point. My intuitive expectation was that the expansion rate cannot exceed c because it it was the case, everything would disconnect at once, even between the sun and Earth and between your eyes and your screen..
Did you read the problem of the ants on a rubber rope <http://en.wikipedia.org/wiki/Ant_on_a_rubber_rope>[/QUOTE]
The expansion is a rate, meaning that the recession velocity increases over distance. The further away something is the faster it recedes. So a galaxy can receded from us at 0.01c that is relatively nearby while another that is very very far away can recede at 2c or 3c or whatever. Note that this also includes light emitted from those galaxies. A galaxy receding from us at 2c that emits light away from us would NOT catch up to the light. In the galaxies frame of reference it is stationary and the light moves at 1c away from it.
