Is the multiverse cosmology or metaphysics?

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

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.

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.
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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.

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).

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

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.

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.

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.

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.

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.

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

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

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 accomodate 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

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

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

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.

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.

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.

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.

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

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

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

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

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

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 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.

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.

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 accomodates the data, but the original model is dead.

Ken G

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 this statement demonstrates quite fully to anyone who understands Popper that you have completely missed his entire purpose.


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

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

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.

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*.

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* 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.

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.

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.

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.....

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

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

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.
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.
True, but a model that makes only obvious predictions is not much of a model either.
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.
Excellent-- so do they make any risky predictions? If so, what? If not, why the heck not?
Well, this thread is about the multiverse concept.

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.

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).
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.

twofish-quant

And I'd argue that it isn't as long as the theory factory creates mathematically unambiguous predictions, and excludes some possibilities.

The reason that I believe this is that this is the situation in most "ordinary physics." We don't know how supernova work. We know that they exist. If someone can create a "logical chain" from "physical principles" to "known outcome" this is useful, because the fact that each part of the chain is "logically strong" keeps you from making up anything that you want.

It gets a bit harder with the early universe because you don't have basic "physical premises" to anchor the chain, but with more observations we can "anchor that chain."

No it doesn't. String theory makes the prediction that force coupling constants are quantum mechanically *random*. They are determined by vacuum energy and that is a value which an undetermined value until you have a wave function collapse. Note that string theory doesn't say that *all physical constants are random*, merely the force coupling constants. Within string theory, planck's constant and the speed of light are going to be the same in all parts of the multiverse, and you can use things like the fact that the speed of light is the same everywhere to infer things about the other universes.

But that's not what is going on.....

That comes out as a rather unambiguous prediction of string theory. A lot of people take it to mean that string theory is wrong. That's why there is a lot of work in alternative quantum gravity models like loop quantum gravity.

One nice thing about LGC is that it *doesn't* produce the types of multiverses that string theory does. You have one universe. The fact that LGC *doesn't* produce multiverses (or at least the types that string theory does) is one reason people like LGC.

Also, the types of multiverses that string theory produces are different than what inflationary cosmology produces, and those are different from the multiverses that the MWI of quantum mechanics produces.

Nothing at all to do with metaphysics. If you start with the assumptions of string theory, multiple universes pops out. If you don't like that, then the answer is simple. Reject string theory. It's not a metaphysical claim. If you start out with the string theory assumptions, you get multiple universes.

Yes it is. For example, cosmological measurements of distant supernova Ia assume that their brightness is constant. We have no theoretical model for this, and we really have no clue why this is true. If someone came up with a model of SN Ia that explained *why* supernova Ia have the same brightness, this would give us more or less confidence in that assumption. Now you can "fix" this problem by coming up with other independent measurements that have nothing to do with SN Ia. You can look at gamma ray bursters or use the Tully-Fisher relationship.

The SNIa results seem to be holding up, but we have a problem since we don't know how Tully-Fisher or GRB work. Having something work with three unknown assumptions that are independent gives you more confidence that you haven't messed anything up, but it's still something to worry about.

Also for things we can't explain the obvious. Take a tube of water. If you pump in the water fast enough, it will turn turbulent. It's something that is trivial to measure, but we do not know how to calculate the exact Reynold's number at which a fluid will go turbulent. What people do when they simulate aircraft is that they use a model that involves punching in parameters that are experimentally measured.

No it's not. Then you can look at what happens if you set it at two.

And sometimes you take what you can get. When people model turbulence, there are a lot of parameters that people just type in based on experiment. You have this relationship that has a free parameter and you set it based on experiment.

Ohm's law. When you build a circuit, you put in a resistor. To calculate how much stuff you have to use to make the resistor, you have to know the material resistance. Being able to calculate that is beyond current physics, so you just measure it, and you put it into your equations. Ohm's law itself is a semi-empirical observation.

Etheral inflation predicts that the CMB spectrum is gaussian to arbitrarily low scales. OK, if you claim that prediction is not "risky" then what about alternative models that predict that the CMB spectrum becomes non-Gaussian at low scales?

If you claim that predicting A is not good enough because it's non-risky, and then you claim that predicting not-A is not good enough because it's non-risky. Then I can't help you.

Also you can't *force* a theory to make the predictions you want to make. What keeps physics from "rationalizing" things is math. The math *forces* you to make some conclusions.

A lot of the ways that people "compromise" outside of physics don't work in physics. I claim 50, someone else claims 100. It's not going to work to just say 75.

To a lot of people this would seem like useless navel-gazing. One thing that you need to do physics is precise definitions, because without precise definitions, you can't figure out precise logical consequences, and if you can't do that, then you can't do physics.

That's not the purpose of interpretations of quantum mechanics. The purpose of physics is to make statements that have observable and testable consequences. If different interpretations of QM don't have testable consequences then it's irrelevant from a physics standpoint, you can just do the numbers, and make up something random to explain them.

The reason that physicists are interested in different interpretations is that it's not clear that they don't have some sort of observable consequence, and if you have different consequences, you can do an experiment (which I play to do on my 150th birthday. I think I'll wear a cat suit before zapping myself with gamma rays).

This is important for the early universe, because a lot of the ways that people avoid conflict between different interpretations won't work at the start of the universe. You can show that interaction with the environment will give MWI the same outcome as Copenhagen under "ordinary" situations. But what happens if there is no environment to interact with?

For the Bohm interpretation to work, then every particle in the universe has to match books with every other particle in the universe. However, suppose inflation is right. Then suppose the "bookkeepping" principle goes out to 1000 trillion light years. Then you'll be able to see it in some experiment. You have an electron with a probability distribution function. Does that PDF get "cut off" at the length of the observable universe.

In physics, it's not metaphysical. It's to come up with experiments (sometimes thought experiments) to figure out what happens. I create a coherent wave function, part of it falls into a black hole, now what happens? (Too bad we don't have a black hole nearby to find out.)

If we get into the subjective, I don't see the point in arguing about this. You believe what you want to believe. I believe what I believe, and there is no reason to change each others minds. I don't care what your metaphysical beliefs are. At least for the purposes of this thread, I just care that you get the physics right. If you believe in multiverses, fine. If you think they are non-sense also fine. If you believe that the world is 6000 years because God said so, I'm not going to try to change your mind.

The *only* reason I'm arguing with you is that you are getting the physics wrong.

I don't care how you define metaphysics. The trouble is that if you include things in metaphysics which are also in the realm of physics, then you have a potential or actual conflict when rules collide.

Not true. Mathematical statements are scientifically unverifiable. 2+2=4 is a mathematical assertion that cannot be scientifically verified. Many religions take as dogma that statements about God are not scientifically verifiable. "Do curious dreams squeam furiously?" is not a scientifically verifiable statement because it has unclear meaning.

Also unicorns are a special case, because there is no law of physics or biology that prevents unicorns from existing, and if the biochemists are right, there is a DNA sequence that will generate unicorns. Warp engines on the other hand, conflict with known laws of physics.

Since you yourself say that the definitions are subjective, then what's wrong with my definition other than it puts metaphysicists out of work? This is the problem with you have unclear definitions, which is that people can change them at will.

Ken G

Right, in "ordinary physics" you are not inventing new unknown physics to explain a supernova, you are trying to see how already known physics might do it. There it suffices to try models until one works-- because the "risky predictions" were already made and tested, when justifying all the physics you are putting into your model.
No, the chain never gets anchored, until there is the risky prediction. That's the difference, the essential issue that separates true discovery from simple rationalization.
String theory makes that prediction? So I can do an experiment, and if it comes out the coupling constants are random, I say string theory worked, and if they don't come out random, I say string theory didn't work? No, not at all, that is not a prediction of string theory, it is a metaphysical interpretation that many associate with string theory (for quite unsubstantiated reasons). But we need a whole new thread to ascertain which claims associated with string theory are scientific, and which ones are metaphysical. A prediction must be able to say experiment A will come out X, if we cannot agree on the meaning of the term "prediction", we are really up the creek!

OK, that's an interesting point, I didn't realize that. But it is still not a prediction of string theory-- it is a concept that one intends to embed in string theory, if and when one ever really does create a theory there. The predictions must stem from that postulate, and they must be risky, and they must be testable. That's why string theory is a very long way from being a scientific theory, it's more of a signpost to a theory-- nobody really knows what that theory is going to end up looking like, if at all.
And yet I would argue that not implying multiverses is no more scientific than implying them-- either way, it ain't nothing until you make risky predictions that a skeptic of your theory would be inclined to expect to fail. If you are cooking up new physics, whether multiverse or not, you better not just be rationalizing!
Yes, they are all different types of multiverses, yet if one of them comes through the front door, it's hard to reject the others. As none of them yet make any verifiable predictions we'd be inclined to not expect, it's still the back door for all of them.
Nah, it's metaphysics. It's just like in Newton's laws, it's metaphysics to say that forces actually cause acceleration. I can reframe Newton's laws and never mention the word "force" even once, ergo, forces are metaphysics in regard to Newton's laws. They are a way to picture the action of a theory, they are not part of the theory, and they do not make claims on existence. Few people who know Hamiltonian mechanics would even claim that "forces exist." Instead, they (not I) might say "the Hamiltonian exists", and the presence of forces is a kind of illusion that stems from the action of the Hamiltonian. It's a lot like the force carriers-- virtual particles. Is the existence of virtual particles not a question of metaphysics? Note, for example, how subjective that question is, when it shows up on this forum!

No, not at all necessary. I can embrace string theory, and reject the existence of multiple universes, as easily as I can embrace quantum mechanics, and reject that all observations give every possible result. It is indeed a metaphysical claim, it is a quintessential metaphysical claim. Do you really think there is no way to frame string theory, getting all the same testable predictions, and not have the existence many universes? I can even frame chaotic inflation that way, it's a piece of cake.

Not if they invented physics to do it, that's the whole point. If they did it with existing physics, already accepted by virtue of other risky predictions, now that's another matter altogether-- with no connection at all to this thread.

And they don't claim it is a theory of what is really happening in turbulence, nor do they make claims on the physical existence of anything in that model! That is an empirical model. If someone says they are making an empirical model of eternal inflation, more power to them, but then it's obviously metaphysics (and rather lamely supported at that) to claim those other universes actually exist!

That is the closest we've come to evidence of an actual risky prediction. But Gaussian seems pretty easy to get-- I'd be much more impressed by a non-Gaussian prediction that worked, that seems a lot more risky to me-- given the mean value theorem.You aren't making any sense now.
Baloney. Most physicists are very interested in interpretations, while holding out not the least bit of hope the distinctions will ever be resolved, certainly not in our lifetimes. Count me in that huge class, for example.

Well, if that's the reason you are arguing, you're wasting your time, because you certainly have not demonstrated that in the least, and I don't believe it for a minute.
Obviously, what's wrong is that this thread uses a particular meaning of "metaphysics", which is the standard one among those who use the term. Perhaps I should have replaced "metaphysics" in the OP with the intended meaning, but it seemed unnecessary at the time-- everyone else on the thread seemed to take the same meaning I did, for example.

yrreg

Forgive me for the interruption, but if I may, about "One thing that you need to do physics is precise definitions, etc.."

Shouldn'the better term in place of "precise definitions," "precise conventions"?



Yrreg

twofish-quant

But there is still "unknown physics". For example, in trying to figure out how supernova work, it's unlikely that we will find out that the speed of light is not constant or that gravity doesn't work through GR. However, putting the pieces together still involves "unknown physics." In the case of inflation, the high energy physics is known well enough so that you are limited in what you can "make up" and so it's a matter of connecting the dots. I don't see how this is much different from supernova or accretion disk jets.

Also, the definition of "risky prediction" is problematic. What's the difference between a "risky prediction" that has already been made with "non-risky prediction" that has already been made.

Yes. It's considered a bad thing that string theory has this characteristic, and people are trying to fix it so that it doesn't.

It's also known from high energy experiments that the effective coupling constants do change at high energies.

Also, if you do an experiment and the vacuum speed of light changes, that would call into question inflation because if the vacuum speed of light does change, then the big problem for inflation disappears. The only logical alternative to inflation (assuming that the big bang actually happened) that anyone has come up with are variable light speed models.

If the FTL neutrino measures had actually confirmed, then *that* would have caused people to question if inflation is necessary.

It's a *prediction* of string theory. If string theory didn't *predict* multiple universes, there would be a lot less interest in them. It's *not* a metaphysical interpretation. If you accept string theory then you end up with multiple universes in much the same way that if you accept the Coprenician model of the solar system, you quickly end up with exoplanets.

I'm not a string theory expert, but this is what the experts tell me. If you don't believe that there are multiverses, then string theory is wrong. This is actually why there is so much work on alternatives to string theory.

They are all scientific.

If string theory predicts A, B, C and multiple universes, and A, B, and C are confirmed, there are no other viable theories that predict A, B, and C, then we wouldn't be crazy in assuming D is true. String theory has not been able to do that, and personally I don't see much hope that it every well. Eternal inflation is much closer to doing that, and we may get to the point where there are enough predictions about the current universe that makes sense that we are willing to accept predictions that are not testable.

The same is true with exoplanets. Once you accept the Coprehenican model of the solar system and Newtonian physics, then you pretty much had to accept that other planets existed even if you couldn't see them.

No. It's a *prediction*. The idea of multiverses isn't something that people added. People started working on string theory in the 1970's and it wasn't until the 1990's, that people realized that string theory would have to predict many, many universes to work. If you reject the concept of multiverses, then you *must* logically reject string theory, which is what a lot of people have done.

To put it crudely as to why, string theory works by symmetry, and in order to have a symmetry that explains the current universe you have to set certain quantities as "quantum mechanically random". If something is quantum mechanically random, that means that different observers in different parts of the universe will "flip the Schroedinger coin" differently and come up with different numbers for those quantities. At that point you explain away the fact that we *don't* see things like the fine structure constant changing in our universe by the fact that some sort of inflation expanded the universe so that the parts that we see are all parts where the "coin got flipped the same way" but that must mean that there are parts that we don't see in which the "coin got flipped differently."

One consequence is that some numbers will be different in different parts of the "multiverse" and some numbers will be the same. Force coupling constants will be different. The speed of light and Planck's constant will be the same. The macroscopic dimensionality of space may be different although the microscopic dimensionality will be the same (i.e. we all live in an 11 dimensional universe, but certain parts may such that more or less than 3 of those dimensions get "blown up").

It's not as if people like the idea of multiverses and added it because it was fun. People generally hate the idea of multiverses, and only add them because the math gives them no choice in the matter. (Also, I'm not a string theorist, so if I've messed up the explanation, corrections are appreciated).

A lot of the interest in things like entropic gravity or loop quantum gravity are to avoid the multiverse problem. In these gravity models, you just care about gravity and QM. You don't care about anything else. Since you don't care about anything else, you don't have to try to fit everything into a symmetry, and since you don't have to fit everything into a single symmetry, you don't get the bad consequences of said symmetry.

One other thing is that the "argument by symmetry" worked beautifully to explain the electroweak force, once people got that done in the early-1970's, it wasn't a crazy idea to try to use it to try to explain everything. Nature had other ideas. The math is hard, so it took about two decades for people to realize that string theory *requires* multiverses to work.

It's a theory. It may be a bad or incorrect theory, but it's a theory. It's a theory because it makes predictions. These may be untestable or silly predictions, but they are predictions. Right now they look silly enough so that the "cool kids" are trying to work in other directions, but science is hard. One thing about string theory is that you can spend two decades arguing about something before you reach a conclusion that it's not going to work, but for things outside of physics, you can argue for centuries and not come to a conclusion.

There is a linguistic ambiguity here. When you say "I don't like hot chocolate" that may either mean "I dislike hot chocolate" or "I have no opinion about hot chocolate." or even "It may be the case that I dislike hot chocolate or that I have no opinion about hot chocolate, but I'll leave you guessing about which one."

When someone says "X does not imply Y" it's not clear whether that means "X implies not Y" or "X has no opinion on Y."

This is why physicists talk in math.

Also "implying *anything*" is science in this context. If you have a theory that says nothing, that's useless. If you have a theory that makes meaningful predictions, even if they are stupid and wrong, that's progress.

Why not? Most of "ordinary science" consists of rationalizations, and I don't see anything wrong with that. If you are on the wrong track, then what will eventually happen is that the rationalizations will lead to a model of the universe that is so crazy that it will collapse under it's own weight.

I don't see anything wrong with "rationalization", since most of science works that way.

A lot of the different types of multiverses are logically incompatible so you *must* reject one if you accept another. If you think that the universe works by collapsing black holes generating new universes, that kills eternal inflation (and vice versa), because the two models are logically incompatible.

Well define "cause" and "exist" and the issue disappears.

No. It's not. Define "exist" and then problem goes away. Do numbers "exist"? Does anything outside my head, "exist"? Do I "exist"?

I don't see how the "existence" of "virtual particles" are any more problematic than the "existence" of "financial risk."

Personally, I'd argue that if you want to argue philosophy, it's best to do that with examples that you are familiar with. The problem with talking about "virtual particles" is that people get the facts wrong.

No you can't. (At least that's what the string theorists tell me.)

They are different situations.

If you have one uranium atom and you watch it decay, you *can* create a set of interpretations so that there isn't some "phantom" atom in an alternative universe.

The trouble is that if the decay of atoms is quantum mechanically random then if you have two uranium atoms next to each other then will probably decay at different times. I got two uranium atoms, they decay differently. OK.

So what happens is that if force coupling constants are quantum mechanically random (and string theory says they are), then when they get set up at the early universe, you are going to end up with different values for different parts of the universe. You can "fix" the problem by blowing up the universe so that we can see only one value, but that creates "other parts of the universe" with different values.

This problem isn't quite as bad with eternal inflation, because you can assume that whatever causes the force constants to be what they are have already caused the numbers to be set.

I'm not an expert in string theory, so I can't say no way. The people I know say that it's hard and they don't know if they can do it. (Again, any corrections from people that know string theory better than I can are appreciated).

I'd like to see you try, because that will help me to explain chaotic inflation to you.

The thing about eternal inflation is that you start with a scalar potential, cause it to inflate the universe, see when it stops inflating, and you'll find that for many scalar potentials, it doesn't.

Now you can come up with scalar potentials in which inflation does stop, so it's possible (but hard) to have inflation without eternal expansion.

But....

The thing about scalar potentials though is that they aren't some random "fudge factor." Once you have a specific scalar potential, then you plug that into your Langrangian for QCD, and that gives you an equation that you can use to calculate lots and lots of things, like the mass of the proton or quark interactions. If you change the scalar potentials then those numbers change.

So you are claiming that you can take any model with eternal inflation and turn it into a model without multiverses with exactly the same predictions.

I claim, you can't without coming up with something close to Last Tuesday-ism

That's not rule in the case of inflation. Inflation comes out of grand unification theories, which is semi-known physics.

Define "really happening" and "physical existence." If you have turbulence, you have waves and eddies which most certainly "physically exists". If you have a theory of turbulence that doesn't describe what is "really happening" then this seems a bit silly.

No more than it is to claim that exoplanets exist or to claim that Paris France exists even though I've never been there. Just because I can't see it doesn't mean that it's not there.

In the case of eternal inflation, the idea of "other universes" is something of a misnomer. The idea is that the total universe is much much larger than we can directly observe, and talking about "other universes" is like talking about "other galaxies." I've never been to Paris, but I can figure out that it exists. Talking about parts of the universe we can't see isn't much different from talking about places in the world that I can't go.

One reason that I tried to avoid getting into philosophy is that I thought that we could *avoid* the philosophical issues once I established *why* cosmologists are talking about multiverses. You take the physical laws as we think they exist or that could reasonably exist, and you end up with large parts of the universe that we can't directly see.

It's not.

There's a difference between "not in our lifetime" and "totally impossible." Also showing whether or not those interpretations are the same or different is what makes it interesting. If it were shown that it doesn't make a difference, then it doesn't make a difference.

The other thing is that we are making some progress. There's been a lot of work on "decoherence" and it's pretty clear that "naive Copenhagen" isn't going to work. The problem with "naive Copenhagen" is that you collapse the wave function by waving a magic wand, but there are lots of systems in which it's not obvious when you should wave that wand.

I've just spend a few pages trying to explain why you've got your physics wrong. If you aren't listening then maybe someone else will. If no one is listening, then it's a waste of my time.