Is the multiverse cosmology or metaphysics?

[Ken G]

There have been a few threads of late on the multiverse concept in cosmology, and whether it can be viewed as a viable, albeit currently underconstrained cosmological theory that is leading us to demonstrably correct discoveries about our universe, or if it is essentially a fairly arbitrary metaphysical conviction that is masquerading as science. I'd like to advance the latter thesis, and central to my argument is the Popperian stance that if, as Feynman said, science should be a way to keep us from fooling ourselves, then we need theories that make "risky" predictions-- predictions that, were we to be skeptical of the theory, we would expect to fail. A theory that only makes predictions that no one can expect to fail, even if they discount the theory, is more like a technique for performing rationalizations than it is a technique for making predictions.

To allow proponents to attack that thesis, I will offer an analogy. I hold that if my thesis is flawed, then its flaws should be identifiable with flaws in this analogy.

Let's say you see five cards in front of you, and you are told the rules of a new game, called phyzbin, a game that you have never played before. You pick up the hand, and see you have 2, 4, 9, jack, queen. You know this is an extremely special hand in phyzbin, called a phyzlaw, because the rules of phyzbin select this hand to be the best possible-- there is a significant selection effect that has occurred here, sometimes expressed that this hand is "finely tuned" in regard to the rules of phyzbin. We need to attribute a source to this selection, we cannot accept it was "just chance" because the odds are too low. But there are many ways to attribute that selection, including:

1) I was selected from a huge set of people playing the game (the people are analogous to the multiverse). There is something special about me, among all those people, such that I get the phyzlaw. Perhaps it is a requirement for me to be who I am, my existence is predicated on being the one who has such a hand. In anthropic thinking, we might say that only the phyzlaw is consistent with me being intelligent. Whatever is the mechanism, the key point is that I have been selected, and that resolves the "fine tuning problem." Instead of asking what is special about the hand, we ask what is special about me such that the hand appears more generic. The other hands must really exist, such that my hand can be selected from them all, because the selection applies after the hands are dealt, it is a selection on me, not on the hands. This seems like a very close analogy to multiverse thinking, and it is certainly one possible "explanation" of the fine tuning. But it is hardly the only possibility.

2a) The rules of phyzbin were selected after ony one hand was dealt, such that a phyzlaw would be that hand. Of course we know that any hand is incredibly unlikely if you deal from 52 cards, so the specialness of the hand only appears if we add some additional constraint that sorts all the hypothetical hands into "generic" and "special" classes. The rules of phyzbin produce that sorting, but if the rules are themselves selected, such that the hand is made to be special by those rules, then we again have no fine tuning problem. The question now shifts from what is special about the hand, to what is special about the rules such that the hand could be viewed as generic. I would argue that this view is analogous to a view of physics that says laws are not fundamental to the universe, they are what we infer in our efforts to understand the universe. Hence, the laws come from us, so it is perfectly natural that the laws share our own special attributes. If there is something about us that says we must have hands like 2-4-9-j-q, then the "rules of the game" that we infer will naturally make a phyzlaw seem like a generic hand, and "poof" goes the fine tuning problem.

2b) The deck contains only 2s, 4s, 9s, js, and qs, so we are simply making a wrong assumption about the possible hands when we conclude a phyzlaw is a special hand. This is similar to #2a, but distinguishes the rules of the game from the elements that make up the game, a distinction we can make if we like but might not need to.

3) The deal was rigged to give us a phyzlaw, perhaps because someone in power wanted us to win. This is a preferred choice of religious people.

Now, as scientists we can probably dispense with #3, not because we know it's wrong, but because it's not in the scientific toolkit. And #2b is a kind of variation on the theme of #2a, so I see the fundamental distinction as being between #1 and #2a, and even then, only once we have established that the "fine tuning problem" really is an issue that science needs to tackle, on grounds that tackling it will help us make "risky" predictions. I would argue two points that undercut the idea that the multiverse is not just metaphysics:
i) There is no scientific way to distinguish between #1 and #2, the distinction is about how we interpret science, and is therefore essentially metaphysical.
ii) Even if we choose a mainstream interpretation that adjudicates for #1 over #2, it is still essentially an issue of pedagogy, because it does not help us use our perspective to make "risky" predictions that could satisfy a skeptic. What's more, being skeptical is a crucial part of science, because it is exactly that element that, as Feynman put it, helps us "avoid fooling ourselves, given that we are the easiest people to fool."

 

[marcus]

You might be interested by some Pirsa video talks you get if you google
"pirsa challenges"
If you google that you get http://pirsa.org/C11008
which is a menu of Perimeter Institute Recorded Seminar Archive (pirsa) videos of talks given at the 2011 conference on Challenges for Early Universe Cosmology.

The first talk on the list is by Neil Turok, the director of Perimeter, who was the main organizer of the conference. It is an overview of what he thinks is wrong with Eternal Inflation. The next talk on the list is by Alan Guth.

A lot of the discussion at that conference revolved around issues of "fine-tuning" and unlikelihood that the mechanisms imagined would actually get it right, get what we see.

I can't say that this directly connects with the most important issues you raise in your post like the responsibilities implicit in carrying on the scientific tradition, and how do we address the curious question of "why these laws, instead of others?"

But the videos (at least Turok's) do address the excuses that people give for the "multiverse" idea based on a widespread fixed idea of "eternal inflation". Several sides of the controversy are presented. So it might be of interest. And you may very likely be aware of the conference and have watched some talks already!

 

[Ken G]

Those do sound like very interesting videos indeed, thank you I have not yet seen them. And for the moderators: I meant this thread to be in "cosmology", so if you think that is a more appropriate place for it, please move it wherever you feel it belongs.

 

[marcus]

Here's what I find most ridiculous about MV talk, and also incidentally most common.

People put blinders on themselves and decide that inflation can only have occurred in a certain way:
1. initiated by a random fluctuation in a field that is everywhere and so subject to being re-ignited by another random fluctuation anywhere, in multiple locations, or

2. inflation is always occurring everywhere except in isolated patches where a random fluctuation (analogous to a particle decay process) causes it to stop and degenerate into slow expansion with more mundane matter fields present.

"Random fluctuation" is the key deus ex machina for people who think along those lines. And it always leads them to imagine a MV.

I find it ridiculous because of the blinders. Inflation may very well have occurred, yet not have been triggered by a random fluctuation.

 

[marcus]

Those do sound like very interesting videos indeed, thank you I have not yet seen them. And for the moderators: I meant this thread to be in "cosmology", so if you think that is a more appropriate place for it, please move it wherever you feel it belongs.

I'm glad the videos interest you! I really only found Turok's talk memorable, though I disagree with him strongly on some points. Disagree but have to acknowledge his stature and general excellence.

It makes sense to me to consider your thread as primarily cosmology. The MV contoversy is about the most active widespread clamor in cosmology at present, I think. But it does not matter which forum (Astrophysics or Cosmology) you have the thread. People will find it, I think, equally well, and contribute to it if they wish to.

 

[Chronos]

Multiverse strikes me as the rationalist version of creationism. Even if you accept the multiverse hypothesis, it fails to explain anything. I trust we already knew we do not live in a universe with laws that preclude our existence, so claiming there are billions of such universes without a shred of evidence seems utterly pointless.

 

[phinds]

Multiverse strikes me as the rationalist version of creationism. Even if you accept the multiverse hypothesis, it fails to explain anything. I trust we already knew we do not live in a universe with laws that preclude our existence, so claiming there are billions of such universes without a shred of evidence seems utterly pointless.

Well said. I agree.

 

[Ken G]

I find it ridiculous because of the blinders. Inflation may very well have occurred, yet not have been triggered by a random fluctuation.

Yes, we tend to ascribe randomness to everything we cannot explain. As such, the argument could be made, if the observations warrant it, that "eternal inflation" involving random fluctuations is our best interpretation of inflationary models. But interpretations are already more metaphysical than scientific, because even processes that seem random, and may be interpreted as random, could actually have some underlying rhyme or reason that we simply have not been able to figure out. That doesn't make randomness a bad model, it makes it a bad way to assert that we have scientific knowledge of the existence of the multiverse.

 

[Ken G]

Multiverse strikes me as the rationalist version of creationism.

That's pretty stinging, but I worry about that too. It's probably not quite fair, because the standards of logic applied by creationists are vastly below those applied by multiversers! But there can be some similarities too.

Even if you accept the multiverse hypothesis, it fails to explain anything.

This is a challenge that I haven't really seen the multiverse camp meet. It speaks to exactly what is an "explanation" in science, so anyone who would like to take the opposite tack, that the multiverse idea does "explain", must first give a concise interpretation of what they think constitutes a scientific explanation. I think that's the place where we will meet Popper (and the importance of falsifiability)-- there are a wide variety of standards to put any "explanation" to, and I feel we must do better than simply mean "a story that we use to achieve cognitive resonance that is consistent with any observations we might actually have", because that limited definition is also met by virtually every creation myth, in the time that the creation myth was widely held. What makes something a scientific explanation, and not a generally accepted myth?

I trust we already knew we do not live in a universe with laws that preclude our existence, so claiming there are billions of such universes without a shred of evidence seems utterly pointless.

This speaks to the dubiousness of "anthropic explanations." I hope someone will accept the challenge of presenting the case that this is an unfair critique of those types of "explanations," again by starting with their criteria for what a scientific explanation is.

 

[twofish-quant]

There have been a few threads of late on the multiverse concept in cosmology, and whether it can be viewed as a viable, albeit currently underconstrained cosmological theory that is leading us to demonstrably correct discoveries about our universe, or if it is essentially a fairly arbitrary metaphysical conviction that is masquerading as science.

It's not so much the "multiverse concept" but rather certain specific multiverse scenarios. It's not obvious how you could disprove "multiverses in general" but it's easy to disprove/support *specific multiverse scenarios*.

As a trivial example, if you have a multiverse scenario that requires all skies to be green, it's wrong because the sky is blue. You can also do this deductively. It may be (and it turns out to be the case) that certain physical theories that we think are true for other reasons, create multiverses.

Let's say you see five cards in front of you, and you are told the rules of a new game, called phyzbin, a game that you have never played before. You pick up the hand, and see you have 2, 4, 9, jack, queen. You know this is an extremely special hand in phyzbin, called a phyzlaw, because the rules of phyzbin select this hand to be the best possible-- there is a significant selection effect that has occurred here, sometimes expressed that this hand is "finely tuned" in regard to the rules of phyzbin. We need to attribute a source to this selection, we cannot accept it was "just chance" because the odds are too low.

That's not how theoreticians think. We can establish that *either* there is fine tuning *or* there are multiple deals. Once you've narrowed the possibilities to two, then you can figure out the consequences of those two.

i) There is no scientific way to distinguish between #1 and #2, the distinction is about how we interpret science, and is therefore essentially metaphysical

At that point your analogy breaks down for the early universe. There are indeed ways of distinguishing between multiverse scenarios. CMB radiation. It's also possible to deal with the problem from the theory end. For example, it is possible that someone will come up with a compelling theory whose consequence is multiple universes.

It's more relevant for quantum mechanics, where people have come up with different "interpretations", which as far as anyone has been able to figure out, are mathematical identical. There is *one* experiment that has been proposed that would confirm the multi-world interpretation of quantum mechanics (quantum suicide), which I'll do on my 150th birthday.

The other thing is that QM has this interesting quirk. Even if you don't accept the MWI as being literal, because it is mathematically equivalent to every other interpretation, it turns out to be much easier to do calculations. For example, in order to calculate the behavior of the electron, what you can do is to assume that there are an infinite number of electrons each in its own universe and then sum up the results. You can also do this with factoring algorithms. One way of thinking about quantum computing is to assume that there is a separate calculation in a parallel universe, and then you sum of the results in the end.

Whether they are *really* parallel universes is a metaphysical question (as far as we can tell, but I'll find out on my 150th birthday). However, if you think of this as a "calculation trick" then it's definitely physics and not meta-physics.

 

[twofish-quant]

This speaks to the dubiousness of "anthropic explanations." I hope someone will accept the challenge of presenting the case that this is an unfair critique of those types of "explanations," again by starting with their criteria for what a scientific explanation is.

I think the problem is talking about "multiverse" as a "camp." You have to restrict yourself to *specific multiverse scenarios* at which point you are in the realm of the testable. For example, *eternal inflation* is certainly a testable and falsifiable theory, since it predicts the curvature of the universe, and it also makes predictions for the scalar potential of particles at GUT energies.

One thing about "multiverse" thinking is that it follows from one of the outcomes of HEP. The current thinking about HEP is that physical constants are the result of what the vacuum energy is, and that number could be anything.

Also, anthropic claims also are falsifiable. For example, if you claim as Weinberg does that the fine structure constant may be what it is for anthropic reasons, this can be disproved by showing that that you can change the fine structure constant and still come up with intelligent life. One thing that is interesting is that it's not *obvious* that changing the fine structure constant *would* result in no intelligent life, and the statement that it is non-trivial. Even if it is true, then you run into the interesting question of "how much" you have to change FSC to get no life.

It would be *very* interesting for example, if it turns out that the "anthropic tolerance" for some physical constant is some order of magnitude higher than the current observational value.

For example pick, some constant X that isn't well known. Say Boltzmann's constant or the value of G. Come up with some "anthropic criteria". If this criteria constrains the value of the constant beyond what can be observed, then it's physics. Whether it's a "calculation trick" or something rule is another issue, and not a new one.

 

[twofish-quant]

Also relevant as far as trying to falsify an anthropic constraint is this paper

http://adsabs.harvard.edu/abs/1987PhRvL..59.2607W

It shows how you can demonstrate that the anthropic principle *doesn't* hold. In particular, it argues that there are no strong anthropic constraints on the cosmological constant.

 

[Ken G]

It's not so much the "multiverse concept" but rather certain specific multiverse scenarios. It's not obvious how you could disprove "multiverses in general" but it's easy to disprove/support *specific multiverse scenarios*.

Really? Example please, of a specific multiverse scenario that is easy to support. Please note that "support" does not mean "find data consistent with", but rather, "find data that satisfies a risky prediction made by that theory." Also, please note it is not at all "risky" to suggest a theory that is actually a thousand theories, one of which can fit any of a thousand observational outcomes. That is not called "risk", it is called "rigging the dice."

As a trivial example, if you have a multiverse scenario that requires all skies to be green, it's wrong because the sky is blue.

Sure, but so what? Here's a theory I just thought of-- the universe is an ocean filled with algae. Now let's consider the ramifications of this theory-- the sky should be green. Hey, I just came up with a theory that I can falsify, I must be doing science! No, it's not science if I already know the sky is not green, I am doing science if I can take a theory that is consistent with everything already known, which then makes risky predictions. This is the Popper criterion, and this is the standard that the multiverse does not seem to be living up to. You certainly have not provided any evidence that it is, not with this effort anyway.

You can also do this deductively. It may be (and it turns out to be the case) that certain physical theories that we think are true for other reasons, create multiverses.

So the creation of the multiverse is the risky prediction of the theories you have in mind? That won't cut it, the risky prediction must be amenable to testing.

That's not how theoreticians think.

Correction, that's not how Adler thought, or Marx or Freud. They were theoreticians too. But it is how theoreticians should think, if they are paying any attention to Popper.

We can establish that *either* there is fine tuning *or* there are multiple deals. Once you've narrowed the possibilities to two, then you can figure out the consequences of those two.

That's why I framed the more clear-cut analogy above, to clarify this kind of issue. If you could frame your argument in the context of that analogy, I think it would be clearer what claims you are actually making, this seems too vague.

At that point your analogy breaks down for the early universe. There are indeed ways of distinguishing between multiverse scenarios. CMB radiation.

But that's just the problem-- to escape trap (i), you fall right into trap (ii). I'm not surprised you can distinguish between multiverse scenarios, but that by itself is no kind of evidence for the multiverse. That is the "rigged dice" situation in a nutshell-- just being able to pick a theory after the fact is rationalization! A good theory predicts before the fact of the observational outcome, and if it is only chosen after the fact, then it must predict some new fact, in advance of that observation. What multiverse scenario is currently doing that? This doesn't make it wrong, but it raises a red flag that we really have no idea it is right. It fails Feynman's standard of helping us not fool ourselves.

There is *one* experiment that has been proposed that would confirm the multi-world interpretation of quantum mechanics (quantum suicide), which I'll do on my 150th birthday.

Quantum suicide is basically a joke, I doubt even Tegmark takes it seriously. You won't test it on your 150th birthday, because you won't have one, but I'll agree that it is a "risky" prediction that you will. The problem is, it doesn't count as falsifiable, for you'd have to be dead to falsify it, and I'm sure you see the paradox there. What's worse, if quantum suicide is really true, you have an absolutely dismal few thousand years ahead of you, I wouldn't wish that on a monster (remember, quantum suicide claims you won't die, but it makes no claims you won't be infirm, in pain, immobilized, semi-conscious, or any of the other certain miseries awaiting a >150 year old person). Anyone who is >150 years old is welcome to believe in the multiverse-- so long as the rest of us can be skeptical of it.

The other thing is that QM has this interesting quirk. Even if you don't accept the MWI as being literal, because it is mathematically equivalent to every other interpretation, it turns out to be much easier to do calculations.

I'm not asking if a multiverse picture makes it easier to do calculations, I'm asking if it delivers on its claims to be scientific evidence for the existence of the multiverse. Image charges make calculations easier too.

One way of thinking about quantum computing is to assume that there is a separate calculation in a parallel universe, and then you sum of the results in the end.

Or I can find the amplitude of a two-slit experiment by summing the amplitudes of each slit. So what? There is no reason to claim this means there is a universe where the particle goes through one slit, and a universe where it goes through another. Why are you even summing the results in the end if you are calling them different universes? Sounds like the same universe to me.

Whether they are *really* parallel universes is a metaphysical question (as far as we can tell, but I'll find out on my 150th birthday). However, if you think of this as a "calculation trick" then it's definitely physics and not meta-physics.

Again, if eternal-inflation cosmologists want to frame the multiverse as their favorite computational trick, who is going to object? They are free to motivate the calculations any way they want, it's a free world. That begs the question of this thread.

 

[twofish-quant]

Really? Example please, of a specific multiverse scenario that is easy to support.

Easy to support is harder. Easy to refute is much easier. But cosmological natural selection. If in fact pulsar masses were were Smolin calcuated them to be, that would be impressive.

Sure, but so what? Here's a theory I just thought of-- the universe is an ocean filled with algae. Now let's consider the ramifications of this theory-- the sky should be green. Hey, I just came up with a theory that I can falsify, I must be doing science! No

Yes. You are.

It's not science if I already know the sky is not green, I am doing science if I can take a theory that is consistent with everything already known, which then makes risky predictions.

Eyes roll... You are doing it again. Telling scientists what is and isn't science.

The problem with your approach is that you end up talking philosophy and not talking about data. So far in this conversation we haven't been talking much about data. We haven't been talking much about theoretical predictions. We've been talking too much philosophy and too little physics.

The thing about physics is that it's non-trivial to go from model -> prediction. A lot of science involves taking a model and figuring out the predictions from it.

But it is how theoreticians should think, if they are paying any attention to Popper.

And if I'm a theoretician that thinks that Popper is wrong about certain things, and do things in a way that's different from the way that Popper thinks I should do them, are you going to have my Ph.D. revoked?

This is actually the big problem that I have with your posts. If you want to do cosmology your way, that's fine. But when you start saying that people that do cosmology differently are "pseudo-scientists" or "incompetent" then those are fighting words.

Or I can find the amplitude of a two-slit experiment by summing the amplitudes of each slit. So what? There is no reason to claim this means there is a universe where the particle goes through one slit, and a universe where it goes through another. Why are you even summing the results in the end if you are calling them different universes? Sounds like the same universe to me.

Right, and there are interpretations of QM in which this does not happen. The trouble with these interpretation is that you end up having to exchange information back and forth, whereas "sum over all universes" you don't.

 

[Ken G]

Eyes roll... You are doing it again. Telling scientists what is and isn't science.

What I am actually doing is laying out a challenge: to demonstrate that the multiverse concept passes Popper's standard for a good scientific theory in the making. So far, nothing you've said has met that standard, which is what I am pointing out. You are arguing why you don't think it needs to. So then, you do not think that it does meet Popper's standard?

So far in this conversation we haven't been talking much about data. We haven't been talking much about theoretical predictions. We've been talking too much philosophy and too little physics.

The absence of physics is entirely the point.

The thing about physics is that it's non-trivial to go from model -> prediction. A lot of science involves taking a model and figuring out the predictions from it.

So, you don't think the multiverse meets Popper's standard but you think the jury is still out on whether or not it ever will?

And if I'm a theoretician that thinks that Popper is wrong about certain things, and do things in a way that's different from the way that Popper thinks I should do them, are you going to have my Ph.D. revoked?

No, I just want to know if you can argue that the multiverse actually does meet the Popper standard. I am concluding you do not think it does. Does anyone think that cosmological multiverse models meet Popper's standard (falsifiability by virtue of making predictions we would not expect to be true if we were skeptical of that theory), or shall we take it as established that they do not?

 

[yrreg]

I notice how everyone eschews coming to concur on what it is to explain something and what is a scientific explanation, which is an invitation from the owner of this thread, Ken G.

#1 Ken G
This seems like a very close analogy to multiverse thinking, and it is certainly one possible "explanation" of the fine tuning.

#6 Chronos
Even if you accept the multiverse hypothesis, it fails to explain anything.

#7 phinds

Even if you accept the multiverse hypothesis, it fails to explain anything.

Well said. I agree.

#8 Ken G
Yes, we tend to ascribe randomness to everything we cannot explain.

#9 Ken G
This is a challenge that I haven't really seen the multiverse camp meet. It speaks to exactly what is an "explanation" in science, so anyone who would like to take the opposite tack, that the multiverse idea does "explain", must first give a concise interpretation of what they think constitutes a scientific explanation. I think that's the place where we will meet Popper (and the importance of falsifiability)-- there are a wide variety of standards to put any "explanation" to, and I feel we must do better than simply mean "a story that we use to achieve cognitive resonance that is consistent with any observations we might actually have", because that limited definition is also met by virtually every creation myth, in the time that the creation myth was widely held. What makes something a scientific explanation, and not a generally accepted myth?

I trust we already knew we do not live in a universe with laws that preclude our existence, so claiming there are billions of such universes without a shred of evidence seems utterly pointless.

This speaks to the dubiousness of "anthropic explanations." I hope someone will accept the challenge of presenting the case that this is an unfair critique of those types of, "explanations" again by starting with their criteria for what a scientific explanation is.
Even if you accept the multiverse hypothesis, it fails to explain anything. This is a challenge that I haven't really seen the multiverse camp meet. It speaks to exactly what is an "explanation" in science, so anyone who would like to take the opposite tack, that the multiverse idea does "explain", must first give a concise interpretation of what they think constitutes a scientific explanation.

#11 twofish-quant

Originally Posted by Ken G
This speaks to the dubiousness of "anthropic explanations." I hope someone will accept the challenge of presenting the case that this is an unfair critique of those types of "explanations," again by starting with their criteria for what a scientific explanation is.

I think the problem is talking about "multiverse" as a "camp."
​

=================


That is sadly what I always observe with scientists that they prefer to not come to concurrence on the terms they are using, but they discuss endlessly among themselves, and their readers are psychologically convinced that they are talking scientifically on scientific explanations.

But when people like myself look for a concurred on concept of what is an explanation and specifically what is a scientific explanation, there is none.



So, physicists-cosmologists like Hawking and Stenger and Krauss and others, they of the universe from nothing "explanation" of the universe, will you people please first agree among yourselves what is an explanation and specifically what is a scientific explanation?

Or even just people who are scientifically savant and expound on the ideas of Hawking and company. please do concur on what is a scientific explanation.

Wanted: a generally accepted glossary of scientific cosmology with the entry among other entries of what is a scientific explanation.



Yrreg

 

[Chronos]

Langauge is clearly the weak link in science. Words are inherently squishy which is why 'mathematics is the language of science' - or at least a better alternative. But even mathematics is vulnerable to unwarranted assumptions and necessarily presumes all relevant variables have been properly considered. This is the source of many disagreements in the science community. My position is mathematically valid results are not necessarily physically meaningful. Were this not true, observational evidence would be unnecessary.

 

[ImaLooser]

Since there is no physical evidence at all and none is expected to arise, I say that it is metaphysics and not physical science.

 

[twofish-quant]

What I am actually doing is laying out a challenge: to demonstrate that the multiverse concept passes Popper's standard for a good scientific theory in the making.

I don't think that the "multiverse concept" as you describe it *is* a good scientific theory. Whether it meets Popper's standards is irrelevant. It doesn't meet mine. So it's not necessary to discuss my disagree with Popper.

So far, nothing you've said has met that standard, which is what I am pointing out. You are arguing why you don't think it needs to.

This is what happens when you argue about ten things at once. I don't like Popper's philosophy. The fact that I don't like Popper's philosophy is largely irrelevant to my thoughts on "multiverse theories."

The "multiverse concept" as *you* have presented it is not a well posed hypothesis. It's too vague and because of it's vagueness, it's rather untestable. Now I have seem papers that talk about multiverses that are interesting. Either the present specific scenarios which are either testable (i.e. eternal inflation) or potentially testable or they point out some non-obvious facts (i.e. that dimensionality is critical for forming stable gravitational objects).

The trouble with this discussion is that it's too abstract. The "multiverse concept" is something of a strawman.
.

So, you don't think the multiverse meets Popper's standard but you think the jury is still out on whether or not it ever will?

Which multiverse?

The jury is still out with eternal inflation. Smolin's cosmological natural selection made one specific and interesting prediction that failed. The many-world interpretation of QM is mathematically identical to Copenhagen, but I'll know whether it's true eventually, so I'm not in a hurry.

No, I just want to know if you can argue that the multiverse actually does meet the Popper standard.

The multiverse as *you have defined it* (or rather failed to define it) is not a proper scientific hypothesis because it's too vague. Now if you want to discuss a *particular multiverse scenario* then that's different. For example, eternal inflation *does* make a risky prediction (i.e. lambda=1 and the CMB will be gaussian to arbitrarily small scales).

Does anyone think that cosmological multiverse models meet Popper's standard (falsifiability by virtue of making predictions we would not expect to be true if we were skeptical of that theory), or shall we take it as established that they do not?

Which model?

Smolin's CNS obviously did. He calculated an upper bound for pulsar masses.

Quantum suicide also does. The prediction that I will be immortal and that when I zap myself with gamma rays on my 150th birthday I won't die, is a risky one.

 

[twofish-quant]

I notice how everyone eschews coming to concur on what it is to explain something and what is a scientific explanation, which is an invitation from the owner of this thread.

That's because a lot of this consider it to be navel gazing and irrelevant in most situations. It's also because we may not come to an agreement.

That is sadly what I always observe with scientists that they prefer to not come to concurrence on the terms they are using, but they discuss endlessly among themselves, and their readers are psychologically convinced that they are talking scientifically on scientific explanations.

We aren't talking science. We are talking philosophy. Since we aren't talking science, I'm rapidly losing interest in this discussion. One difference between science arguments and philosophy arguments is that science arguments tend to come to a conclusions, whereas we can argue forever about philosophy and never agree.

I was trying to turn this into a discussion about science (partly to illustrate what a science argument looks like) but I seemed to have failed.

But when people like myself look for a concurred on concept of what is an explanation and specifically what is a scientific explanation, there is none.

It's like asking for a definition of what constitutes a good joke. I can show you examples of good jokes. I can show you examples of things that aren't funny. If you ask me for a *definition* of a good joke, then this isn't going to work.

Also it's like asking a bird to explain how he flies. He probably couldn't tell you, because he probably has never thought deeply about the question.

So, physicists-cosmologists like Hawking and Stenger and Krauss and others, they of the universe from nothing "explanation" of the universe, will you people please first agree among yourselves what is an explanation and specifically what is a scientific explanation?

I think that Krauss is pretty awful. One thing about Krauss is that the popular talks that he gives look *nothing* like his professional work. There's a Jekyll-Hyde aspect, because he writes excellent professional papers, but then he gives talks on youtube that are loony and misleading.

Or even just people who are scientifically savant and expound on the ideas of Hawking and company. please do concur on what is a scientific explanation.

I'm practical. I wake in a morning. The car doesn't start. I try a few things and conclude that the fuel injector is broken. That's a "scientific explanation." If you use the same sort of reasoning to figure out the big bang, that's doing "science."

Now I can also conclude that my fuel injector is broken because God hates me. That's not a scientific explanation. It' may be true that God hates me, but it doesn't explain why God would punish me by breaking the fuel injector rather than the alternator or making me trip over the rug.

Also, if I invite a Muslim, a Jew, a Southern Baptist, a Mormon, and an atheist, and we pop the hood of the car, we can agree that yes, the fuel injector is broken. If we start getting into philosophy about why the fuel injector is broken, we'll never fix the car.

Same for the big bang. I ask why deuterium abundances are what they are, and I can come up with a "mechanical answer." If I argue that "God did it" that may be true, but it doesn't explain why God chose to create a universe with a higher concentration of deuterium. Yes, God loves me, but He can love me equally well with different concentrations of deuterium.

Wanted: a generally accepted glossary of scientific cosmology with the entry among other entries of what is a scientific explanation.

See above.

Also it's really important, because there are lots of scientists that give popular talks that I think are actively ***misleading***. Krauss is one that I have particular problems with, since mixing science and non-science is bad enough. Getting his science *wrong* while doing it is worse.

One problem is that people with science backgrounds like me who are really careful about not mixing their professional and personal views, don't make as interesting viral videos. I happen to be a Buddhist, but I try to separate that from my "science hat" so I'm not going to make virial youtube videos about how science proves Buddhism (it doesn't).

If you have some specific questions, feel free to ask.

 

[rbj]

One thing that is interesting is that it's not *obvious* that changing the fine structure constant *would* result in no intelligent life, and the statement that it is non-trivial. Even if it is true, then you run into the interesting question of "how much" you have to change FSC to get no life.

i thought that there were some limits to how far the FSC could vary so that the nuclear fusion in stars was both hot enough so that there was enough energy to cook up the necessary elements and to supply sufficient energy in sunlight to make photosynthesis "worth it" and that the burn rate was not so hot so that the Sun would burn out long before any complex life had time to evolve. i don't remember, but i thought it was something like 2%.

i thought that anthropic question regarding constants of Nature was sufficiently dealt with.

 

[Ken G]

Also, if I invite a Muslim, a Jew, a Southern Baptist, a Mormon, and an atheist, and we pop the hood of the car, we can agree that yes, the fuel injector is broken. If we start getting into philosophy about why the fuel injector is broken, we'll never fix the car.

It's best if you don't talk about philosophy, because whenever you do, you demonstrate that you have no idea what philosophy is or what it is for. For example, the question of this thread is the type of question that the philosophy of science was invented to explore. It's a kind of overlap region, where you need the scientists to do the science, but the philosophers are handy to help keep the scientists from fooling themselves.

As a case in point, I will note that both Freud and Adler considered themselves to be scientists, and thought they were doing science, but Popper explored the question of what went wrong in their approaches such that what they ended up with, though well accepted by a cadre of other professionals who thought similarly that they were doing science, was not such good science after all. So no, it has nothing at all to do with God, or religion, or even questions like why the injector doesn't work. Those are all in your head, an absurd caricature of the philosophy of science. It has to do with the simple question of, does the fact that some cadre of theorists can rationalize their theories if they imagine a multiverse, or is a multiverse in fact a demonstrably necessary postulate in any successful theory of modern cosmology. That is not just a scientific issue, it is very much a philosophy of science issue. In my experience, most bashers of philosophy have not the least understanding of what it is.

Your views on cosmology models, on the other hand, are quite knowledgeable and helpful to the question of the thread. As yet, however, you have not met the challenge that could clearly adjudicate in favor of science over metaphysics for the multiverse: identifying a prediction that the multiverse view makes that anyone who is skeptical of the multiverse should not expect to be a true prediction. This prediction must satisfy three additional criteria, however, to make sure we are not just fooling ourselves:
1) it must not come from a "factory" of possible contradicting predictions, all of which would make sense in some small variation of the multiverse picture, that together span pretty much all possible outcomes, and
2) it must be something we did not already know when the multiverse model was built to accommodate it, and
3) it must actually stem from the multiverse picture, and not just the obviously correct weak anthropic statement that any candidate theory we make must be consistent with the fact that we exist.
A successful prediction like that would clearly constitute scientific evidence that the multiverse actually exists, making the multiverse clearly more than an inherently metaphysical stance that is merely consistent with the existing data, such as the wide array of interpretations of quantum mechanics. If I promote my interpretation of quantum mechanics on the grounds that it can provide a plausible account of a two-slit experiment, for example, that doesn't mean my account is not primarily metaphysical-- as most people would agree, no matter what they think about your fuel injector, that interpretations of quantum mechanics are metaphysical until such a time as they can be embedded in some new theory that incorporates them as necessary postulates.

 

[Ken G]

My position is mathematically valid results are not necessarily physically meaningful. Were this not true, observational evidence would be unnecessary.

It sounds like you are cautioning against an overly rationalistic approach, where we regard theories as true simply because they admit to a concise mathematical interpretation. People who adopt the rationalistic stance often make claims like "God is a mathematician", or Tegmark's idea that any self-consistent mathematical structure is always a type of physical universe. I think it is clear enough that the multiverse stems from a rationalistic approach to cosmology, whereas it sounds like you, and I would wager most mainstream astronomers, prefer a more empirically supportable view, where the claims we make are traced directly back to particular observations that appear to support those claims (like, observing the other universes, which is something of a trick).

 

[twofish-quant]

It's best if you don't talk about philosophy, because whenever you do, you demonstrate that you have no idea what philosophy is or what it is for. For example, the question of this thread is the type of question that the philosophy of science was invented to explore.

Exactly, and I'm trying to state what *I* think science is, which is quite different from what Popper thinks science is.

As a case in point, I will note that both Freud and Adler considered themselves to be scientists, and thought they were doing science, but Popper explored the question of what went wrong in their approaches such that what they ended up with, though well accepted by a cadre of other professionals who thought similarly that they were doing science, was not such good science after all.

But Popper takes it as given that Freudian psychoanalysis isn't science. I'm not sure that it isn't, so I can't take that as the starting point. In order to convince me that Popper is right, you first have to convince me that Freud and Marx weren't doing "science", which isn't a given.

So no, it has nothing at all to do with God, or religion, or even questions like why the injector doesn't work. Those are all in your head, an absurd caricature of the philosophy of science.

I was trying to explain *MY* philosophy of science, and how is quite different from Popper. I'm not asking you to necessarily accept my philosophy of science, but I think it might be interesting to understand.

Popper's philosophy of science has nothing to do with the story that I mentioned, but *my personal* philosophy of science does.

It has to do with the simple question of, does the fact that some cadre of theorists can rationalize their theories if they imagine a multiverse, or is a multiverse in fact a demonstrably necessary postulate in any successful theory of modern cosmology.

That depends on the math. The thing about issues of philosophy is that they are hard, sometime impossible to resolve, so before getting to the point were we argue about philosophy, it's probably a good idea to do the math, to figure out if it's really an issue or not.

Part of it is that physicists have different personal philosophies of science. If someone is a Popperian, then that's fine. What I object to is the idea that the Popperian view is that *only* legitimate view of science.

As yet, however, you have not met the challenge that could clearly adjudicate in favor of science over metaphysics for the multiverse: identifying a prediction that the multiverse view makes that anyone who is skeptical of the multiverse should not expect to be a true prediction.

The "multiverse view" as you put it is too ill-posed to be a true scientific hypothesis. In order to have a well-posed scientific hypothesis, you need to have something that is specific enough to make specific predictions. As you have stated it, the "multiverse view" is not specific enough to be a scientific theory. It's too vague.

That doesn't make that any model involving multiverses is not physics. Asking whether or not the universe has something to do with multiverses, is like asking whether it has something to do with "gas". It's too vague.

Now we can talk about *specific theories that create multiverses*. That's something different.

This prediction must satisfy three additional criteria, however, to make sure we are not just fooling ourselves:
1) it must not come from a "factory" of possible contradicting predictions, all of which would make sense in some small variation of the multiverse picture, that together span pretty much all possible outcomes, and
2) it must be something we did not already know when the multiverse model was built to accommodate it, and
3) it must actually stem from the multiverse picture, and not just the obviously correct weak anthropic statement that any candidate theory we make must be consistent with the fact that we exist.

Those are silly rules.

They are silly because they lead to silly outcomes when you talk about balloons or lasers or fuel injectors.

I've got a better idea to avoid "fooling ourselves." Come up with specific theories that make specific predictions. Once you have specific predictions, this reduces the number of logically possible premises, and describe the situation without favoring one premise.

 

[twofish-quant]

It sounds like you are cautioning against an overly rationalistic approach, where we regard theories as true simply because they admit to a concise mathematical interpretation.

We'll if it's a concise mathematical interpretation that happens not fit observations, then we have a problem.

I think it is clear enough that the multiverse stems from a rationalistic approach to cosmology, whereas it sounds like you, and I would wager most mainstream astronomers, prefer a more empirically supportable view, where the claims we make are traced directly back to particular observations that appear to support those claims (like, observing the other universes, which is something of a trick).

I hate the term "mainstream astronomers." Different astronomers have different philosophical approaches, and it's hard to argue that one is more "mainstream". A lot also depends on the field. It's been my experience that HEP theorists are more into Platonic ideals than solid state people. The fact that I do computational stuff rather than pen and paper stuff changes my philosophy since I don't regard "mathematical beauty" particularly highly.

The other thing (and this is why I mentioned throwing a Jew, a Muslim, a Buddhist, an atheist, and a Satanist at something) is that if you have people with different philosophical viewpoints agree on something, then what they agree on is more likely to be "philosophy independent" in which case there's no point in arguing philosophy.

One other way of keeping scientists from "fooling themselves" is to have scientists with *different* and perhaps *fundamentally different* philosophies, on the idea that if physicists with different philosophies agree on something, then it's something that isn't a big problem.

 

[Ken G]

One other way of keeping scientists from "fooling themselves" is to have scientists with *different* and perhaps *fundamentally different* philosophies, on the idea that if physicists with different philosophies agree on something, then it's something that isn't a big problem.

Yes, I agree-- and this is where you keep incorrectly arguing that I am saying we should all share a single philosophy. That is not my argument-- my argument is that the multiverse concept is primarily metaphysics, so it is perfectly obvious that people who have a rationalistic philosophical bent will favor it. I'm fine if people want to choose their own philosophy-- I'm merely pointing out that that is what they have done!

I'm also saying that Popper outlined for those who would choose such a metaphysical stance, a clear path for how to tell the difference between a good physics theory, and a rationalization that is more a reflection of a personal philosophy. No one on this thread has made it very far down that path of differentiation, though we have seen evidence that a path like that does exist in principle-- it just hasn't been followed very far or very well. Instead, we hear only reasons why you don't think you need to follow that path, because you have a different philosophy of what science is. I know that, you are a rationalist, that is obvious-- and connects in a clear way with the question of whether the multiverse is physics or metaphysics: rationalists always have a much harder time making that distinction.

So when you say that we know we have good science when people with very different philosophies agree on it, I say, exactly! So now you are beginning to see the problem with the multiverse: the people who believe in it are invariably highly rationalistic thinkers, whereas "mainstream astronomers" (a term you may dislike but it is perfectly descriptive) tend to be empiricists, so they choose a different philosophy, and they are generally not at all convinced about the multiverse (witness the above posts in this very thread). So by your own logic, expressed just above, we can conclude the multiverse of eternal inflation is currently not one of those things that people of different philosophies agree on, so it is metaphysical, so you are hoisted on your own petard.

 

[twofish-quant]

That is not my argument-- my argument is that the multiverse concept is primarily metaphysics, so it is perfectly obvious that people who have a rationalistic philosophical bent will favor it.

My point is that the "multiverse concept" as you have described it has very little to do with what professional scientists talk about.

I'm also saying that Popper outlined for those who would choose such a metaphysical stance, a clear path for how to tell the difference between a good physics theory, and a rationalization that is more a reflection of a personal philosophy.

And I'm telling you that Popper's rule make no sense as they lead to people labelling "ordinary science" as personal philosophy. It's also unnecessary as there are other tools, both philosophical and sociological that keep things from going in the direct of non-sense.

One important way is to clearly define the model in ways so that you can make logical deductions from that model.

I know that, you are a rationalist, that is obvious-- and connects in a clear way with the question of whether the multiverse is physics or metaphysics: rationalists always have a much harder time making that distinction.

I think you are assuming things that I don't believe.

For example, I haven't found any multiverse models that I find particularly compelling. If you were presenting arguments that multiverse models were *false*, I wouldn't be arguing with you. The trouble is that you are presenting arguments that anything with a multiverse is outside the bounds of science which clearly isn't true. Smolin's cosmological natural selection and eternal inflation are examples of well-posed hypothesis that include a multiverse. CNS has been falsified.

So now you are beginning to see the problem with the multiverse: the people who believe in it are invariably highly rationalistic thinkers, whereas "mainstream astronomers" (a term you may dislike but it is perfectly descriptive) tend to be empiricists, so they choose a different philosophy, and they are generally not at all convinced about the multiverse (witness the above posts in this very thread).

I think you misunderstand the scientific process. Just because a theorist talks about something doesn't mean that they *believe* it. I'm not particularly convinced that a multiverse exists. No one I know is strongly convinced that it exists. If you ask people if they think it's there, you'll find people that say yes, but you'll also find that the depth of belief is not very great.

So by your own logic, expressed just above, we can conclude the multiverse of eternal inflation is currently not one of those things that people of different philosophies agree on, so it is metaphysical, so you are hoisted on your own petard.

I think you are misrepresenting the situation. I think if you put a bunch of astrophysicists together, you'll end up with substantial agreement as to the consequences of eternal inflation, and what the observations say and don't say. *At the current time*, we do not have enough observational data to force a conclusion, so at that point if you ask for an opinion, you'll get different answers, and some of these are likely to be the result of "personal bias" because we don't have enough information. As for me, when there is no data, I tend to say "I don't know" and if you force me to give you an answer I'll flip a coin.

Now if you can come up with an argument that we will *NEVER* be able to determine whether eternal inflation is true or not, then yes at that point it *is* metaphysics, but we aren't anywhere close to that point yet. We are still in the realm of physics because there are a set of possible observations that would support eternal inflation, and a set of alterative observations that would disprove it. I can tell you that if I saw observations X, Y, and Z, I would support the eternal inflation model, and if I say observations A, B, and C, I would consider it to be dead.

(X is omega_k < 1e-5, gaussian CMB to arbitrary low scales, and some particle physics prediction that has nothing to do with cosmology (say a correct prediction of the proton lifetime)). Dead would be some curvature, non-gaussian CMB. You could also kill it theoretically by showing some fundamental inconsistency.

The other thing that is that I've provided an example of a multiverse model is pretty clearly *NOT* metaphysics. Smolin's cosmological natural selection model predicted that there would be no neutron stars over 1.7 solar masses. We found one at 2.0. The original model is dead. Now it's possible that he might be able to come up with a *new* model which accommodates the data, but the original model is dead.

 

[Ken G]

My point is that the "multiverse concept" as you have described it has very little to do with what professional scientists talk about.

That's quite a weak objection. The "multiverse concept" as referred to in this thread is quite simple, it is:
The claim that there actually exists very very many other universes, with different cosmological parameters and possibly even different physical constants (depends on who you ask), that are not directly observable but are logically inferrable from what we do observe.

I hardly find that a controversial description, and if you prefer you can use the equivalent definition given by the Wiki on "multiverse:"
"The multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including the historical universe we consistently experience) that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them."

Of course, if you mean something different than that, you are welcome to start your own thread using your own meaning.

And I'm telling you that Popper's rule make no sense as they lead to people labelling "ordinary science" as personal philosophy.

And I'm telling you that this statement demonstrates quite fully to anyone who understands Popper that you have completely missed his entire purpose.

The trouble is that you are presenting arguments that anything with a multiverse is outside the bounds of science which clearly isn't true. Smolin's cosmological natural selection and eternal inflation are examples of well-posed hypothesis that include a multiverse. CNS has been falsified.

You are oversimplifying the issue considerably. I have already agreed that any specific multiverse model can be considered good science if it is used to generate a risky prediction (if that same prediction cannot also be made without including the multiverse as a postulate in the theory). This is the whole point of invoking Popper. I also agree that Smolin's model, which involved a multiverse, was a good scientific model, and it was falsified. So we agree that the multiverse concept is capable of producing scientific models, the question is, does the multiverse concept currently yield a valid scientific model rather than just a factory for creating rationalizations of what is already known?

If your argument is going to hinge on the fact that every effort so far to use the multiverse concept (which many feel is a metaphysical position) to generate a specific theory (which is falsifiable and is good science) has failed to generate one that works, but that the idea holds up nicely in general terms as long as we don't use it to actually generate a specific scientific theory that makes risky and falsifiable predictions, I hardly see that as a good argument that the multiverse concept isn't metaphysics.

I think if you put a bunch of astrophysicists together, you'll end up with substantial agreement as to the consequences of eternal inflation, and what the observations say and don't say.

I think most will view the general multiverse concept as metaphysics, do you doubt that?

Now if you can come up with an argument that we will *NEVER* be able to determine whether eternal inflation is true or not, then yes at that point it *is* metaphysics, but we aren't anywhere close to that point yet.

No, you are mistaken about the definition of metaphysics-- that is no kind of requirement. Don't you think deBroglie-Bohm interpretation, or many worlds, are metaphysics? Most people do, even though they would never try to prove it is impossible to distinguish those pictures scientifically. The requirement is on the person who says it is science, the default position is that it is metaphysics. Just as for the existence of unicorns-- can you prove those are impossible too?

 

[twofish-quant]

The claim that there actually exists very very many other universes, with different cosmological parameters and possibly even different physical constants (depends on who you ask), that are not directly observable but are logically inferrable from what we do observe.

This is not well posed enough to form a proper scientific hypothesis.

In the professional literature, people do not talk about multiverses in general, but rather about specific multiverses that usually exist as a logical consequence of candidate theories about how our own universe works.

For example, in most eternal inflation scenarios, you do *not* end up with the universes having different cosmological parameters or different physical constants.

The universes that have different cosmological parameters are usually those associated with string theory. However, the basic assumption in those theories are that "string theory is correct everywhere." One problem with string theory is that you end up with different universes with different physical constants. A lot of people take this as evidence that string theory is wrong.

If you just talk about multiverses without any *meat*, then it's just idle and useless speculation, which doesn't happen in professional papers.

You are oversimplifying the issue considerably. I have already agreed that any specific multiverse model can be considered good science if it is used to generate a risky prediction (if that same prediction cannot also be made without including the multiverse as a postulate in the theory).

Also getting a specific multiverse model to make a *non-risky* prediction is extremely difficult. Even getting to the point where a model generates *obvious* predictions is a good thing, because a model that generates *obvious* predictions is better than one that makes *no predictions*.

So we agree that the multiverse concept is capable of producing scientific models, the question is, does the multiverse concept currently yield a valid scientific model rather than just a factory for creating rationalizations of what is already known?

If you could get the "multiverse concept" to turn into a "theory factory" that would be a good thing. If you can show that all multiverse models can be specified by one number, that would be progress, and if you can show that in order to work that number has to be 2 then you've just rejected all models in which the number is not 2.

If your argument is going to hinge on the fact that every effort so far to use the multiverse concept (which many feel is a metaphysical position) to generate a specific theory (which is falsifiable and is good science) has failed to generate one that works, but that the idea holds up nicely in general terms as long as we don't use it to actually generate a specific scientific theory that makes risky and falsifiable predictions, I hardly see that as a good argument that the multiverse concept isn't metaphysics.

*If* I don't believe this.

Also, there are multiverse models that haven't been falsified (yet).

The "multiverse concept" as you describe it may indeed be philosophy and useless philosophy, but it bears no resemblance to the way that professional theorists approach the problem.

I think most will view the general multiverse concept as metaphysics, do you doubt that?

The trouble with answering that question is that saying "multiverse ideas are metaphysics" and be easily misinterpreted as saying that "all theories that invoke a multiverse are metaphysics."

Also there is a huge problem with popularization of science, but that's another issue. One reason that I am careful about my statements is that I don't want to say things that can be easily misinterpreted. Saying "multiverses are not science" can be misinterpreted.

No, you are mistaken about the definition of metaphysics-- that is no kind of requirement. Don't you think deBroglie-Bohm interpretation, or many worlds, are metaphysics?

Unknown. If it can be proved that it is impossible to distinguish between different interpretations of QM, then it's metaphysics. The trouble is that no one has mathematically proved this, and there is at least one proposed experiment (quantum suicide) that should lead to very different results if other QM universes exist.

And no one has any clue if the interpretations of QM are in fact identical when we have a full theory of gravity.

Most people do, even though they would never try to prove it is impossible to distinguish those pictures scientifically.

Why should I care what most people do? Most people aren't scientists. Trying to mathematically prove that something is possible/impossible is a very important part of science.

Also it turns out that it matters. The standard Copenhagen intepretation turns out to be "incomplete" as it does not include decoherence which turns out to be important for quantum computers.

http://en.wikipedia.org/wiki/Quantum_decoherence

Now how decoherence works for the entire universe is an interesting question...

The requirement is on the person who says it is science, the default position is that it is metaphysics.

Who made up this rule, and why should I accept it?

Just as for the existence of unicorns-- can you prove those are impossible too?

No, I can't, because unicorns *aren't* impossible. I'm pretty certain that you can breed unicorns if you take a horse and tinker with it's DNA in the right way.

I can show that there aren't any unicorns living in Time Square. Now, there may be a bred of animals on Tau Ceti IV that look very much like a unicorn, but I can't say anything about Tau Ceti IV.

 

[Ken G]

In the professional literature, people do not talk about multiverses in general, but rather about specific multiverses that usually exist as a logical consequence of candidate theories about how our own universe works.

Yes, I realize there is a factory for spitting out multiverse models for people to talk about, which are capable of fitting essentially anything. That was one of the points I made, it's a prescription for rationalization until a model is generated that makes risky predictions that weren't already verified before the model was built to rationalize them. Until such a risky prediction is made and verified, no one skeptical of it needs to take the multiverse idea seriously as a scientific proposition, rather than a factory for rationalization.

The universes that have different cosmological parameters are usually those associated with string theory. However, the basic assumption in those theories are that "string theory is correct everywhere." One problem with string theory is that you end up with different universes with different physical constants. A lot of people take this as evidence that string theory is wrong.

That's not even a ramification of string theory, that's a ramification of the fact that string theory leaves open the question of what the parameters could be. It is the ultimate rationalistic stretch to assert that just because the theory allows a lot of different possibilities, that every one of those possibilities has to be another existing universe. Talk about a purely metaphysical stance! Even eternal inflation can claim to be somewhat less blatantly metaphysical that that string theory claim.

Also getting a specific multiverse model to make a *non-risky* prediction is extremely difficult. Even getting to the point where a model generates *obvious* predictions is a good thing, because a model that generates *obvious* predictions is better than one that makes *no predictions*.

True, but a model that makes only obvious predictions is not much of a model either.

If you could get the "multiverse concept" to turn into a "theory factory" that would be a good thing. If you can show that all multiverse models can be specified by one number, that would be progress, and if you can show that in order to work that number has to be 2 then you've just rejected all models in which the number is not 2.

That's a good thing only if it leads to risky predictions. If the "2" is needed to get obviously correct predictions, and that's all you get, then the model is still garbage anyway.

Also, there are multiverse models that haven't been falsified (yet).

Excellent-- so do they make any risky predictions? If so, what? If not, why the heck not?

The "multiverse concept" as you describe it may indeed be philosophy and useless philosophy, but it bears no resemblance to the way that professional theorists approach the problem.

Well, this thread is about the multiverse concept.

Also there is a huge problem with popularization of science, but that's another issue. One reason that I am careful about my statements is that I don't want to say things that can be easily misinterpreted. Saying "multiverses are not science" can be misinterpreted.

The title of the thread is certainly intended to be polarizing-- the truth may lie somewhere in between. That's OK, that's what we are investigating.

If it can be proved that it is impossible to distinguish between different interpretations of QM, then it's metaphysics.

I do not think you will find this is a typical requirement that people place on metaphysics. Metaphysics is notoriously difficult to define, and it can mean a lot of different things in different philosophical applications, but in regard to physics, it seems clear enough that metaphysics is not the laws themselves, it is the interpretation of the meaning of the laws, what we will choose to take as their implications. As such, metaphysics tells us what we have accomplished when we arrive at a law, but it is also highly subjective. Since this is also the purpose of interpretations of quantum mechanics, I would certainly say that those interpretations are metaphysics, as long as they are not themselves laws of physics. (They could be laws in some future theory, but none of them are as yet.) Hence, there is no requirement to prove that they could not be laws in some other theory-- it suffices to address their current function (which is metaphysical).

No, I can't, because unicorns *aren't* impossible.

Yes I know-- that was the whole point. It was you who made the claim that we have to prove something scientifically unverifiable before it could be metaphysics-- I'm pointing out that nothing can ever be proven to be scientifically unverifiable, not even unicorns. By your nonstandard definition, metaphysics would be the empty set, which would make it easy to answer the title question by default, but not terribly informative.