The many-worlds interpretation of QM is a fully Objective (realist) and deterministic metaphysics.  Many-worlds says that reality consists of a giant multiverse of 'alternative universes' where every possible quantum outcome is realized.  Human observers are only directly aware of one particular branch of the multiverse.  So according to this interpretation the wave function is a complete description of reality (the wave function never 'collapses') and the evolution of the wave function is also completely deterministic.  See 'The Fabric of Reality' by David Deutsch for a detailed defense.  I would say that many-worlds is in fact the only interpretation fully consistent with an objective metaphysics and epistemology.

This latest experiment if confined would provide a welcome refutation of the mystical/subjectivist Copenhagen Interpretation.  But I do not believe that it refutes many-worlds.

(Edited by Marc Geddes on 8/04, 8:29pm)

(Edited by Marc Geddes on 8/04, 8:30pm)

There is indirect evidence for Many-Worlds.

The theory of QM has been tested and found to correspond to things that scientists can directly observe.

Since QM is giving correct predictions about observables, this is reason to trust it on the non-Observables as well.

If you take a realist picture of reality and you assume that QM is the right theory, the maths has to be referring to things that actually exist in reality.  And the simplest interpretation of the maths is that its referring to multiple alternative universes corresponding to all possible qauntum outcomes.  That is the 'picture of reality' the maths is giving us.

The Transactional Interpretation is intriguing, and worth looking into, but the advantage of Many-Worlds is that it helps to clear up philosophical paradoxs about reasoning and free-will.  For instance the phrase 'I could have done something different this morning' (counter-factual) is actually philosophically incoherent without the existence of an actual concrete place where an alternative version of you did do something different.  Many-Worlds neatly accounts for this by saying that that alternative version of you really does exist in the other quantum branch.

Many-Worlds also clears up philosophical epistemological problems.  According to Bayesian/Popperian reasoning, we have to consider multiple possibilities to which we assign different probabilities.  But to what could these probabilities be referring?  For instance what sense does it make to say that George Bush has a 55% chance of re-election, unless the exact same election was 're-run' many times?  Many-Worlds has a simple answer.  The probablities are referring to the frequency of events in all the alternative universe.

Recall what I said about epistemology in the other thread.  Scientists formulate rational hypotheses and then judge them on the basis of their explanatory power.  'explanatory power' means how well a hypotheses explains reality - there can be many critera for this.
Examples:  Coherence, Consistency with other knowledge, ability to make new predictions, Occam's razor and many more).  The hypotheses with the most explanatory power is taken to be the one most likely to be true.  If this hypothesis uses Non-obervable phenomena, then there is sufficient indirect evidence for believing that these non-observables exist.  There is no need to directly observe something to prove its existence.

Direct observations are only one kind of evidence contributing to explanatory power.  I gave others (coherence etc etc).  Scientists actually believe in all kinds of non-observables.  Examples:  Dark energy, Quarks, Space-Time manifold, Magnetic field etc etc.

If a mathematical theory suggests something, true that doesn't prove with certainty that it exists, but it does provide evidence that a non-observable has a high probablity of existing.

The criterea for judging hypotheses doesn't have to be physical.  See what I said above.  There are numerous examples of mathematical theories suggesting non-observable things which much later turned out to be observable after all.

Much more often than not the maths theory turns out to right.  A famous example of this is the theory of Atoms.  No one could observe them until very recently and for a long time philosophers argued that Atoms didn't exist because they were just 'theory'.  But now we can actually directly observe atoms.  So the maths was right all along. 

A realist picture of reality demands that we believe what our best theories are telling us.  Just because we can't directly observe the alternative universes right now, doesn't mean that science won't find out how to directly observe them at some point in the future.  Again, an Objectivist metaphysics demands that we accept our best theories as providing an accurate picture of reality until proven otherwise.

Your statement:  'if I had acted differently than I had, my experience suggests that another causal chain of events would have happened, resulting in a different outcome?' is meaningless if only one universe exists.  If only one timeline exists there was only one possible chain of events.

The problem with probabilities that you've outlined was exactly my point!  Indeed you can't apply the probablitistic approach to anything that doesn't involve repeated frequencies of events.  That's a reason for believing in Many-Worlds, because with multiple alternative universes, you can then say (for instance) that Bush wins with a frequency of 55% (55% of all the universes have Bush winning). 

The fact of the matter is that people do assign probablities to events in the real-world.  The 55% figure for a Bush win I gave you actually comes from the odds offered by current 'futures markets' (where people place bets on various events).

There is no way to make sense of these probabilities without the Many-Worlds theory.

In 55 out of 100 contexts similar to the present circumstance, this was the outcome. (The estimate of the exact probability here varies based upon the precise criterion of “similar” chosen by a specific estimator.)

55 out of 100 people (with some criterion of authority) believe this will be the outcome.

If the situation were hypothetically allowed to unfold 100 times, this would be the outcome 55 times.

First and foremost, a theory must fit the facts.  We're not talking about a table of numbers here, were talking about a theory, some mathematical system with a unifying aspect (i.e., all data fit can be predicted by some mathematical scheme).  If such a theory does not fit the facts, it's wrong.  Period.  Internal consistency and consistency with other knowledge are great and all, but it is meaningless if it isn't in accordance with reality.  Only after you have a collection of theories which fit the facts more or less equally do you begin to consider other criteria.

Just because some scientists believe in some phenomena does not imply its existence.  This is an appeal to authority.  Again, the aether was postulated because scientists reified the "light as wave" idea and needed a medium to make this picture make sense.  Michaelson-Morley found no evidence of the aether and so it was rightly rejected.  Turns out the Theory of Relativity explained relativistic contractions with fewer assumptions anyway.

Some specific points:  Dark matter is just like the aether was-- we're postulating a mystery substance because the galaxies don't quite behave like they should according to our current theories.  Some scientists think that our problems will be solved by sweeping our problems away under the banner of "Dark Matter;" it doesn't mean they are justified to do so.  We have indirect evidence for quarks (particle jets).  Space time manifolds and magnetic fields are mathematical devices used to make predictions, nothing more.

Yes, but they "turned out to be observable" because we observed them, right?  That means they weren't really unobservable, just unobserved.

Ancient astronomers weren't right about epicycles, early 20th century scientists weren't right about the aether, etc.  Hundreds of unified theories of physics have been published, and not one of them has been found to be acceptable.

Agreed.  However, until I hear of an experiment which purports to show the existence of these alternate universes, I consider it an unlikely proposal, at best.

I have a memory.  I have the ability to reason.  I have observed that my actions begin causal effects, some of which I can predict.  Hence, I can reason that in the past, had I acted differently, the state of things now would have been different.  There being only one chain of events does not rule out reasoning whether that chain could have been different.

My problem was that you are taking the concept of a probability and improperly applying it to events which have no reason to be associated with a probability.  You then rightly reason that in order for an event to have a probability associated with it, it must have been the subject of repeated, controlled observation, and from there you postulate the existence of alternate universes to reify these other hypothetical scenarios into a bunch of "hypothetical scenario universes" which literally exist somewhere.  It would seem that these "alternate universes" are a product of your desire to assign every event a probability, not the other way around.

A bandwagon appeal.  Just because a group of people would like to bet on the election does not mean that their prediction has any special weight.  Even if it did, somehow, there would be no way to know it, since the 2004 Bush/Kerry election is an event which will happen only once, so we have no way to know whether this "futures market" poll of yours is valid or not.

I agree.  That's why you shouldn't claim those probabilities.

Nate T.

There are plenty of ways to give probabilities an exact epistemological meaning, even in cases where you aren't able to do explicit repeated experimentation. Just a few examples I can think of immediately:

• In 55 out of 100 contexts similar to the present circumstance, this was the outcome. (The estimate of the exact probability here varies based upon the precise criterion of “similar�Echosen by a specific estimator.)
  
  

  55 out of 100 people (with some criterion of authority) believe this will be the outcome.
  
  

  If the situation were hypothetically allowed to unfold 100 times, this would be the outcome 55 times.

• Nate did a good job of rebutting the middle interpretation.  And see what I said above explaining why I don't think the first or third interpreations are coherent.  Without Many-Worlds, if we demand an Objective metaphysics there is only one fully deterministic time-line.  So phrases such as 'contexts similar to' and 'hypothetically' (referring to counterfactuals) cannot be made sense of.  Either you have to give up Objective metaphysics or you have to accept Many-Worlds.
  
  

My question (to Mark) is basically this: by what mechanism does our consciousness follow one and only one path through the multitude of universes described by the Many Worlds interpretation? It seems to me that the Many Worlds interpretation just skirts around the collapse of the wave function by introducing a collapse of the human consciousness.

Tessa

How does this follow?  As I've mentioned before, I'm not denying the existence of the consequences of a correct theory, just the existence of the analogies used to describe the theory.

It's not that the "space-time manifold" is the explanation.  It's that "the data, when plugged into a mathematical scheme involving a manifold (which you could think of as space and time) results in consequences which correspond to reality."  You shouldn't assume that the magnetic field or any such thing exists merely because invoking it is useful for describing reality-- that's reification.

And we mustn't forget that the picture is just that-- a picture.

Within the context of all the knowledge to hand , a rational person has to accept the best available theory as accurately describing things that are actually out there (i.e Objective reality) until proven otherwise, or else you are simply flatly denying Objectivist metaphysics.  To say that that the best theory is 'just a device' is to say that according to everything you currently know, it's all just a construct in your head and/or your reasoning is all wrong.

Objective reality is the reality we directly perceive, everything else is an attempt to explain the nature of that directly perceived physical world. The pictures, models, and metaphors of science are only to help us "grasp" those aspects of the perceived physical world that are otherwise hard to "imagine."

Regi old fellow,

If all we ever ascribed reality to was what we could directly perceive, man would never have come out of the caves.  All they would have ever believed in was what their senses immediately conveyed to them.  The whole basis of science and philosophy is the idea that there is deeper reality behind the one we currently perceive, and we can use indirect observation and reasoning to determine what this reality is.  The purpose of science and philosophy is to understand and explain *reality*, not restrict our conception of reality to things we can directly observe.

Here's a very simple example which conveys the point: Ask someone who has never been to Egypt what the Great Pyramids look like.  Chances are they know.  But how?  After all, they have never directly perceived the pyramids with their senses.  Instead they know through *pictures* they saw in books or on T.V.  But should this disqualify their knowledge?  Of course not.  They are justified in believing that there is a *correspondence* between the pictures of pyramids and the pyramids themselves because of their rational *understanding* of where the pictures came from.

Similarly, scientists do not need to directly perceive something to ascribe reality to their conceptions.  If they have indirect evidence combined with a causal rational understanding, they are entitled to believe that there is at least *some* degree of correspondence between their model of reality and reality itself.

Just because entities are not directly observable now, doesn't mean that science won't find a way to observe them directly at some future time.  The famous example here is the atom.  Philosophical arguments for these existed from the time of the Greeks, but logical positivists refused to believe in atoms because no one could directly observe them.  They ridiculed scientists, saying that 'atoms' were 'just a device' and it was 'reification' to ascribe reality to these 'unobservables'.  But lo and beyond, eventually someone invented a device (the Scanning Tunneling Microscope) which let scientists directly observe atoms.  If scientists had believed the positivists, progress in chemistry would have ground to a stand-still far before this.  Another example is the instance of viruses.  No one could observe them at the time they were proposed either and the positivists said that they therefore didn't exist.    If the positivists had had their way, no medicinal progress would ever have been made. 

It is the grossest of subjectivism, claiming the "real" world is the world as I picture it in my head, not the world we actually directly perceive. It is the denial of Objectivism.

Regi

You've betrayed a total lack of understanding about the basic principles of rationality Regi.  We sure as heck don't know what will turn out to be observable or not, so we shouldn't made advance judgements about what is and is not a part of concrete reality.  the nature of reality is what we should be trying to find out, not limit in advance.  It is the grossest of subjectivism to claim that "real" world is simply the world you can directly perceive with your senses at the moment.  

Who is the most of a subjectivist:   the 5 year old who thinks that the whole of reality is composed only of the things he can immediately see before him in his cot, or the 5 year old smart enough to use rudimentary reasoning to determine that there is a much wider reality beyond the one he can currently observe from his cot?

So, what is the ontological status of kinetic energy, potential energy, and action?  Are they "out there in reality"?  Or are they mathematical concepts that make the solution of some classical mechanics problems easier?  Or is there another choice?

Thanks,
Glenn