|Jun11-12, 09:59 PM||#18|
Is the multiverse cosmology or metaphysics?
Since there is no physical evidence at all and none is expected to arise, I say that it is metaphysics and not physical science.
|Jun11-12, 10:05 PM||#19|
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.
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.
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.
|Jun11-12, 10:30 PM||#20|
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.
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.
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.
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.
|Jun11-12, 11:44 PM||#21|
i thought that anthropic question regarding constants of Nature was sufficiently dealt with.
|Jun12-12, 06:08 PM||#22|
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.
|Jun12-12, 06:23 PM||#23|
|Jun13-12, 02:59 AM||#24|
Popper's philosophy of science has nothing to do with the story that I mentioned, but *my personal* philosophy of science does.
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.
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.
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.
|Jun13-12, 03:10 AM||#25|
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.
|Jun13-12, 09:35 AM||#26|
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.
|Jun13-12, 11:00 AM||#27|
One important way is to clearly define the model in ways so that you can make logical deductions from that model.
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.
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.
|Jun13-12, 11:56 AM||#28|
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.
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.
|Jun13-12, 01:19 PM||#29|
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, 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.
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.
And no one has any clue if the interpretations of QM are in fact identical when we have a full theory of gravity.
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.
Now how decoherence works for the entire universe is an interesting question.....
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.
|Jun14-12, 12:01 AM||#30|
|Jun14-12, 01:44 AM||#31|
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."
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.
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.
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.
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.
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.
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.
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.
|Jun14-12, 04:04 PM||#32|
|Jun14-12, 11:18 PM||#33|
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"?
|Jun15-12, 02:53 AM||#34|
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.
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.
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.
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.
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.
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.
I don't see anything wrong with "rationalization", since most of science works that way.
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.
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.
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.
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
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.
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.
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