I've already posted it.bapowell said:OK. What evidence, taken at face value or otherwise, can you provide for the existence of mutliple universes?
And I have already posted the very good reasons to believe that other regions of space-time with different low-energy laws of physics are exceedingly likely. Far, far more likely than our universe being unique.Garth said:It is as a scientist that I have a problem. It is not the possibility of these other universes that is the question but their verification, or falsification, by scientific observation and testing.
If you are referring to your post about spontaneous symmetry breaking, such a discovery, in itself, is not direct evidence for the existence for a multiverse because of the additional, untestable assumptions that must be made (I pointed these out in response to your post). If you're referring to your comment regarding quantum mechanics, you make no mention of any direct observational evidence for the other branches of the wavefunction.Chalnoth said:I've already posted it.
I don't see this in your response.bapowell said:If you are referring to your post about spontaneous symmetry breaking, such a discovery, in itself, is not direct evidence for the existence for a multiverse because of the additional, untestable assumptions that must be made (I pointed these out in response to your post).
Except that you have to invoke magic to avoid them. You can invoke magic to avoid anything if you like.bapowell said:If you're referring to your comment regarding quantum mechanics, you make no mention of any direct observational evidence for the other branches of the wavefunction.
But you can't observe these other branches of the wavefunction. I'm not denying their existence; I'm not trying to avoid them. I'm trying to say that you can't observe them which is absolutely necessary for empiricism.Chalnoth said:Except that you have to invoke magic to avoid them. You can invoke magic to avoid anything if you like.
Chalnoth said:Specifically, I object to this statement:
"The anthropic principle makes an enormous number of assumptions"
Except that in reality, it requires more assumptions to assume a unique universe than it does to assume a prolific universe-generation process: you still need a universe-generation process, except now it can only ever occur once. And forcing that universe-generation process to only occur once requires additional and completely unreasonable assumptions.
...
Chalnoth said:That's not even necessary, though, as the nature of quantum mechanics guarantees that all of these different symmetry breaking events are realized right here as different branches of the wavefunction.
Direct observation is not required for anything in science. You can't observe an electron either in the strong sense.bapowell said:But you can't observe these other branches of the wavefunction. I'm not denying their existence; I'm not trying to avoid them. I'm trying to say that you can't observe them which is absolutely necessary for empiricism.
It is automatically simpler because it requires fewer assumptions. Just taking the standard model, for example, it is simpler for the electroweak symmetry breaking to be spontaneous, and thus occur differently in different patches, than it is for it to be forced to take on the particular value we observe. This situation is only likely to get more extreme as we learn more about high-energy physics.marcus said:Chalnoth I think you made a mistake in post #2, right at the start. It doesn't make it simpler to assume a patchwork of regions with different laws/constants.
If you want to get pedantic, the low-entropy initial conditions of our observable universe demand an explanation. Some sort of process is required to produce those low-entropy initial conditions, and there is no reason whatsoever to think that this process happened only once. And even if it did, quantum mechanics still guarantees that all or at least a great many outcomes occur even if it did only happen once.marcus said:Don't know what you mean by "universe-generation process". Could be a shell concealing a fallacy in your thinking. It is not incumbent on me or anyone else to explain why existence exists. The universe exists, our job is to explain aspects of its behavior in testable ways, to arrive at the next deeper layer of explanation--the next deeper layer of testable natural law.
I didn't say it did. But there has been some recent work in fixing the infinities by limiting the calculations to one Hubble horizon. No generality is lost due to the nature of unitary quantum mechanics, but the calculations are now finite and do not suffer from the measure problem that plagues eternal inflation.marcus said:And BTW eternal inflation would not solve that problem either. If it could actually be determined that there really is a patchwork of assorted big bang regions then what "universe-generation process" established the field on which they blossom?
I have no sympathy whatsoever for those that would argue that they need to make up extra dynamics that do nothing but reduce the predictive power of quantum mechanics.salvestrom said:Does it? Unless a consensus is reached on the physical reality of the wavefunction then what happens when that wavefunction is collapsed is entirely debatable.
Chalnoth said:Direct observation is not required for anything in science. You can't observe an electron either in the strong sense.
Right, and many of the models that lead to the multiverse are testable. Real consequences of a multiverse, by the way, have already been posted in this thread.salvestrom said:But testability is. Gotta be able to measure it or an affect you expect it to have on something else you can observe. I think the sciency response is along the lines of: if they do exist they have no causal effect on us and are irrelevant. I think irrelevant is harsh, but if it has no effect on any system, it's not science. It's just cool.
But I can directly detect it's presence, for example, viz. a track in a cloud chamber.Chalnoth said:Direct observation is not required for anything in science. You can't observe an electron either in the strong sense.
But you apparently have just the right amount of sympathy for those that would argue for a vast and wasteful proliferation of alternate universes. I don't see why you don't simply admit that you are following a sense aesthetic that helps guide which physical theories you deem most relevant to the universe. That's not a criticism -- we all do this. But it's ultimately just a hunch, not something you've verified with data.Chalnoth said:I have no sympathy whatsoever for those that would argue that they need to make up extra dynamics that do nothing but reduce the predictive power of quantum mechanics.
No. What you can do is test the predictions of a model of the electron. Many of the models which include a multiverse are testable in the exact same way.bapowell said:But I can directly detect it's presence, for example, viz. a track in a cloud chamber.
This is the normal way that science is done. Every single prediction of a theory cannot possibly be verified. So we test what we can, and the more ways that we can test a theory, the more confident we are in its overall correctness.bapowell said:There is a theory A that makes predictions B, C, and D. I've collected the appropriate data and verified, to some degree of significance, that B and C are true. I have no data to verify D. But, since it is predicted by the same theory that predicts B and C, and since I have adequate data to support predictions B and C, then D is true. Sort of like "true by association." I'm sorry Chalnoth, but I don't buy it. And I don't think Francis Bacon would either. Or really any empiricist for that matter.
And there are many scientists working on creative ways to test for the impact of various multiverse ideas more directly. I don't hold much hope that this will prove to be fruitful. But it does, at least, avoid the useless work of trying to avoid such multiverse hypotheses as a matter of principle.bapowell said:EDIT: That's not to say that such a situation shouldn't compel one to strongly suspect the validity of D. Your allusion to the transitional fossils made earlier is an example. Yes, given the success of evolutionary theory and its sound logical framework, many pieces of which have been verified scientifically, it is especially likely that transitional fossils should exist (and they do, as I think we all know, but sake of argument here.) However, such a strong suspicion does not abdicate the scientist from his responsibility of finding them. Their absence in no way invalides the theory -- an argument I think you suspected I was making. I was not. But they do not become objective reality simply because they really, really should be there.
Why is it wasteful? What does that even mean?bapowell said:But you apparently have just the right amount of sympathy for those that would argue for a vast and wasteful proliferation of alternate universes.
But you still haven't said why you've chosen to accept many-worlds over other interpretations, outside of from what I can tell are purely aesthetic reasons. Unless you have some Bayesian prior on your model space that you aren't telling anyone about.Chalnoth said:This is the normal way that science is done. Every single prediction of a theory cannot possibly be verified. So we test what we can, and the more ways that we can test a theory, the more confident we are in its overall correctness.
bapowell said:But I can directly detect it's presence, for example, viz. a track in a cloud chamber.
It looks like you have your own version of how induction is done, which differs significantly from my view. You seem to be saying the following:
There is a theory A that makes predictions B, C, and D. I've collected the appropriate data and verified, to some degree of significance, that B and C are true. I have no data to verify D. But, since it is predicted by the same theory that predicts B and C, and since I have adequate data to support predictions B and C, then D is true. Sort of like "true by association." I'm sorry Chalnoth, but I don't buy it. And I don't think Francis Bacon would either. Or really any empiricist for that matter.
EDIT: That's not to say that such a situation shouldn't compel one to strongly suspect the validity of D. Your allusion to the transitional fossils made earlier is an example. Yes, given the success of evolutionary theory and its sound logical framework, many pieces of which have been verified scientifically, it is especially likely that transitional fossils should exist (and they do, as I think we all know, but sake of argument here.) However, such a strong suspicion does not abdicate the scientist from his responsibility of finding them. Their absence in no way invalides the theory -- an argument I think you suspected I was making. I was not. But they do not become objective reality simply because they really, really should be there.
1. Many-worlds makes more predictions as to how the universe behaves. Specifically, it makes definite predictions about the nature of collapse. Most other interpretations sweep the nature of collapse under a rug and make no predictions at all about it. Considering that the nature of collapse is becoming more and more important as we try to take advantage of quantum mechanics for computing, this really is an essential feature and can no longer be considered up to personal choice (not that the nature of reality ever was up to personal choice).bapowell said:But you still haven't said why you've chosen to accept many-worlds over other interpretations, outside of from what I can tell are purely aesthetic reasons. Unless you have some Bayesian prior on your model space that you aren't telling anyone about.
EDIT: This discussion has actually been helpful because I understand now what we disagree about. It's not so much about the implied correctness of untested predictions or axioms. It's about your staunch acceptance of a version of quantum mechanics based solely on its relative simplicity that I'm not quite in agreement with. Occam's razor is an indispensable guide for selecting the most favored model out of a bunch; but it does not exclude those models it doesn't select.
This is kind of off-topic, but I just wanted to point out that the upgrade to advanced LIGO is now under construction, and is expected to be up and running somewhere around 2015. The sensitivity is expected to be great enough that it will be guaranteed to detect gravity waves from a number of known sources, barring some unforseen systematic errors.skydivephil said:Take gravity waves for example, although there has been indirect evidence from binary pulsars there has never been a direct detection despite LIGO being operational for something like 10 years (?).
Chalnoth said:2. Discoveries in high energy physics point to the existence of spontaneous symmetry breaking, which would lead to different regions of space-time realizing different low-energy laws of physics.
Chalnoth said:... No matter which way you slice it, multiverse theories require fewer assumptions. It is easier for a theory to be prolific than not. ...
OK. So as I understand it you favor many-worlds due to its parsimony and predictive strength relative to alternatives. This is precisely the way one would go about weighing the relative merits of competing statistical models. But at the end of the day, we are not considering statistical models; we are interpreting the candidate theories as representing objective physical reality. The more complicated, less predictive model may well be correct! Again, these considerations suggest a preference, statistically speaking, for the simpler model. I do not, however, think this is adequate to furnish the kind of certainty and correspondence to objective reality that you are advocating.Chalnoth said:1. Many-worlds makes more predictions as to how the universe behaves. Specifically, it makes definite predictions about the nature of collapse. Most other interpretations sweep the nature of collapse under a rug and make no predictions at all about it. Considering that the nature of collapse is becoming more and more important as we try to take advantage of quantum mechanics for computing, this really is an essential feature and can no longer be considered up to personal choice (not that the nature of reality ever was up to personal choice).
2. Many-worlds makes the fewest assumptions. I don't see how there can possibly be any argument about this point.
Well, there are two ways to look at this. One is that inflation strongly predicts that this and potentially other symmetry breaking events are only local effects, and that they will occur differently in far-away regions. The second is that whatever physical model you have for our early universe, it is highly unlikely that that physical model is a one-off event.martinbn said:Why? I mean, the spontaneous symmetry breaking does not say that there are universes where each vacuum state is realized. Or am I wrong? I just don't see how spontaneous symmetry breaking is related to many universes!
Two points. First, I find these theories highly unlikely, due to the apparent reversal of entropy at the bounce. Second, even if this isn't a problem, there's still no reason whatsoever to believe it's a one-off event. You still have to assume it's a one-off event separately from the physical model.marcus said:I think this is mistaken. (BTW it's a claim you already were asserting in post#2)
The way I slice it, the appropriate question to be asking at this point is how did the "big bang" come about.
How did the expansion begin and why does it have the observed characteristics?
Bounce theories of how this happened seem to depend on fewer assumptions. They simply have the U extend back further in time, and be in a contracting mode. No different laws from those operating now.
Except it is true. There is no possible way to have a unique universe without making that an extra, specific assumption in the theory. This is simply because any physical model of the universe which doesn't explicitly mention other regions of space-time also won't explicitly exclude them. It makes the theory more complex to exclude them. Always.marcus said:Just to be clear, I am addressing the claim of simplicity or fewer assumptions that you made in post #2. It simply is not true.
But theories are never ruled out on account of their complexity. This is where I disagree with your reasoning. You are essentially performing a Bayesian model selection on your space of competing theories. They all satisfy the data equally well, however, some have additional structure than others that make them either less predictive, more complex, or both. The Bayesian evidence disfavors these models, but it does not exclude them! This is an incorrect interpretation of the statistical method.Chalnoth said:It makes the theory more complex to exclude them. Always.
Chalnoth said:Except it is true. There is no possible way to have a unique universe without making that an extra, specific assumption in the theory. This is simply because any physical model of the universe which doesn't explicitly mention other regions of space-time also won't explicitly exclude them. It makes the theory more complex to exclude them. Always.
Chalnoth said:Well, there are two ways to look at this. One is that inflation strongly predicts that this and potentially other symmetry breaking events are only local effects, and that they will occur differently in far-away regions. The second is that whatever physical model you have for our early universe, it is highly unlikely that that physical model is a one-off event.
martinbn said:But is not an answer to my question, where in the symmetry breaking is the need for many universes!