How do deterministic Bohmian/Pilot Wave Theories Handle These?

[Dmitry67]

To clarify this issue, can you point me at a set of postulates that define MWI so I can refer to them?

Ha! This is the most interesting thing: while other interpretations have additional postulates (wave, guiding a particle inside, mysterious collapse and special role of measurement, offer and normal wave, etc) MWI does not have any additional postulates. It is a NULL interpretation - there are no any additional assumptions.

(Common misconseption that MWI postulates splitting and the existence of the parralel worlds)

Just take wavefunction, and shut up and calculate. If you wonder what observers would see - use quantum decoherence.

There are many blah blah around MWI - universes et cetera, but parralel worlds is merely a consequence of QD.

 

[DrChinese]

a small quibble, inre: "B. An unbound neutron has a half-life of perhaps 15 minutes."

a single neutron does not have a half-life.

If you mean to imply "a sample of unbound neutrons can be said to possesses a half-life, rather than an individual unbound neutron": I stand corrected.

 

[DrChinese]

A. Even in standard QFT, vacuum fluctuations do NOT produce virtual particles. The vacuum is an eigenstate of the operator of the number of particles, so there are no particle fluctuations in the vacuum. What fluctuates in the vacuum is the value of field. Virtual particles are something different, they are artefact of a specific mathematical method of treating interactions, based on perturbation method. If you use some other method, such as numerical path integration on the lattice, then nothing analogous to virtual particles exists. For more details see e.g. Secs. 9.3. and 9.4. in
http://xxx.lanl.gov/abs/quant-ph/0609163 [Found.Phys.37:1563-1611,2007]

From the paper:

The notion of a “virtual particle” originates only from a specific mathematical method of calculation, called perturbative expansion. In fact, perturbative expansion represented by Feynman diagrams can be introduced even in classical physics [52, 53], but nobody attempts to verbalize these classical Feynman diagrams in terms of classical “virtual” processes. So why such a verbalization is tolerated in quantum physics? The main reason is the fact that the standard interpretation of quantum theory does not offer a clear “canonical” ontological picture of the actual processes in nature, but only provides the probabilities for the final results of measurement outcomes.

As I understood it, the probabilities include terms for many paths (or branches), actually I guess an infinite number. Considering more and more branches (assuming they are appropriately selected) leads to progressively more accurate predictions in QFT. Now, are there virtual particles in these branches? I think you are saying that these paths are a mathematical artifact and do not need to require the existence virtual particles, right?

OK. But there are contributions from some higher order branches that need to be considered, and they represent "something". We can't just ignore them. And I suppose these are usually thought of as virtual particles simply because they are mathematical artifacts (i.e. act mathematically like particles). They are NOT there when the smoke clears, but their presence IS felt. As something of a realist, I would expect you of all people to acknowledge that what appears in the formula must have some counterpart in nature.

So are the paths/histories/branches real? Do virtual particles exist? Nature acts "as if" they do and are. The relevance for BM is: I presume that some non-local effect can be substituted for the effect currently occupied by virtual particles, so that the predictions remain the same between theories.

I guess that makes as much sense as any explanation when I think about it: If non-local particles provide the "hidden" variables, then non-local dynamics could also provide the source of randomness which I refer to as a field.

Am I close?

 

[Count Iblis]

Ha! This is the most interesting thing: while other interpretations have additional postulates (wave, guiding a particle inside, mysterious collapse and special role of measurement, offer and normal wave, etc) MWI does not have any additional postulates. It is a NULL interpretation - there are no any additional assumptions.

(Common misconseption that MWI postulates splitting and the existence of the parralel worlds)

Just take wavefunction, and shut up and calculate. If you wonder what observers would see - use quantum decoherence.

There are many blah blah around MWI - universes et cetera, but parralel worlds is merely a consequence of QD.

You do need to define a preferred basis. E.g. if you choose a basis and then you apply the time evolution operator to your basis states, then the same wave function is interpreted as how the inverse time evolved wave function would be interpreted.

So, the wavefunction describing the universe as it is now with people living on Earth can just as well be reinterpreted as describing the universe as it was billions of years ago.

 

[ueit]

Ha! This is the most interesting thing: while other interpretations have additional postulates (wave, guiding a particle inside, mysterious collapse and special role of measurement, offer and normal wave, etc) MWI does not have any additional postulates. It is a NULL interpretation - there are no any additional assumptions.

I didn't ask for "additional postulates", but for the set of postulates that define MWI.

Just take wavefunction, and shut up and calculate.

You cannot calculate anything from the wavefunction unless you specify what the relationship between this wavefunction and our observation is. For example, in the Copenhagen interpretation (CI) you use Born postulate to calculate the probability of finding a particle in a certain volume of space. As you see I have used words like "particle", "volume of space" that have a very clear meaning here (CI assumes the existence of the classical world). If you only have the wavefunction how are you supposed to recognize in it anything that looks like a 3D world? How do you calculate the probabilities of anything?

If you wonder what observers would see - use quantum decoherence.

What does "observer" mean in MWI? How do I recognize such an entity only looking at a wavefunction?

 

[Dmitry67]

You do need to define a preferred basis.

Exactly. The choice of that basic is arbitrary. It might look as a flaw, but it is not.

When you talk about systems without any observers, like quagma in the begginning of the Big bang, then pure QM is enough.

If you ask "How universe looks for an observer X?" then you calculate the decoherence based on the basis of X.

You don't need a basis until the question is asked, and when it is asked, the question itself contains the basis

 

[Dmitry67]

1 I didn't ask for "additional postulates", but for the set of postulates that define MWI.

2 You cannot calculate anything from the wavefunction unless you specify what the relationship between this wavefunction and our observation is. For example, in the Copenhagen interpretation (CI) you use Born postulate to calculate the probability of finding a particle in a certain volume of space. As you see I have used words like "particle", "volume of space" that have a very clear meaning here (CI assumes the existence of the classical world). If you only have the wavefunction how are you supposed to recognize in it anything that looks like a 3D world? How do you calculate the probabilities of anything?

3 What does "observer" mean in MWI? How do I recognize such an entity only looking at a wavefunction?

1. There are no postulates.
If you still want something, then I can give it in a negative form: you don't need to make any extra assumptions (collapses, particles) to explain the reality. Just pure QM.

2. Check my previous post: classical reality is explained ia QD, and a choice of basic depends on the observer

3. Do other interpretation give a definition for what "human" is? :) it is an interesting question, but shouldn't it be a part, say, of a biology rather then QM?
Observer does not play any special role in MWI. Nor the "measurement devices". They are treated the same as all other systems - microscopic or macroscopic. There is no boundary between QM and classical world. World is quantum on all scales, and our classical view is just an illusion.

 

[Demystifier]

And yes, I know that there is a duality between BI and MWI. But can I ask, what makes BI so attractive to you?

I have already explained it to you, but let me try again. MWI requires frogs, but there is no any mathematical theory of frogs within MWI. On the other hand, the additional equations that BI contains are nothing but a mathematical theory of frogs. Moreover, BI seems to be the simplest possible mathematical theory of frogs consistent with MWI.

Of course, I do not expect you to change your mind and become BI fan. But at least, I would be happy if it would help you to better understand why some people still prefer BI over MWI.

 

[Demystifier]

As I understood it, the probabilities include terms for many paths (or branches), actually I guess an infinite number.

No. Feynman diagrams have nothing to do with paths or branches. To understand this, I suggest you to see a mathematical derivation of Feynman diagrams from first principles. For pedagogical purposes, I suggest you to see this within CLASSICAL (not quantum) mechanics (Refs. 52, 53).

OK. But there are contributions from some higher order branches that need to be considered, and they represent "something". We can't just ignore them. And I suppose these are usually thought of as virtual particles simply because they are mathematical artifacts (i.e. act mathematically like particles). They are NOT there when the smoke clears, but their presence IS felt. As something of a realist, I would expect you of all people to acknowledge that what appears in the formula must have some counterpart in nature.

Am I close?

It seems to me that you are not close at all.

Let me use an analogy. Consider a classical wave, say a wave on the surface of watter. It may have a complicated shape. Mathematically, it can be written as a Fourier sum of plane waves. But does it mean that plane waves are really there, that they are real? Not at all. Alternatively, it can also be written as a Taylor sum. But does it mean that particular terms in the Taylor expansion represent real objects? Again, not at all. All that makes a physical sense is the TOTAL function that describes the real wave. Particular terms in this or that expansion are nothing but a mathematical trick that simplifies calculations.

If it is still not clear to you, let me use an even simpler analogy. Assume that you have 1 apple. You can write it as
1=2+(-1)
Does it mean that you actually have 2 apples and -1 apple? Of course not. All you really have is 1 apple, and that's it.

 

[Ilja]

I can understand how you can define a hidden preferred frame in flat spacetime. It is the same in LET (Lorentz Ether Theory, compatible with SR).

But how can you do the same for the black hole metric? Can you define a single frame common to space inside and outside the horizon?

Yes, I can. I have to modify the theory of gravity for this, but I have done this already before becoming a Bohm supporter. See ilja-schmelzer.de/glet.

The math is easy, essentially GR in harmonic gauge. The gauge-breaking term gives some additional cosmologica terms which stop the collapse at a distance arbitrary small to the Schwarzschild radius.

The idea that one needs a flat spacetime in quantum gravity was even older, it was the first nonstandard idea I had. I have rewritten it now as arXiv:0902.2040, A quantum variant of Einstein's hole argument.

In this sense, I was already prejudiced in favour of some hidden background structure when learning pilot wave theory.

 

[Ilja]

I am not sure what "pure QM" is supposed to be but the orthodox interpretation assumes the existence of a classical world with observers, instruments and all that. In BM you also work on a 3D space background so one does not need to explain it. The issue is different in MWI where all reality is supposed to somehow emerge from a wavefunction. Decoherence does not explain this.

I think the "pure QM" is the thing I have critisized in arXiv:0903.4657, Why pure quantum theory is not enough.

 

[Ilja]

Hum... then all I wanted to say is that, that is not condensed matter physics.

IMHO it is an approximation which is part of condensed matter physics, and sufficiently accurate for the point I want to make.

 

[Ilja]

yes, sure, I am MWI fan.
I don't see any problems with a frog.
the key concept of MWI is that both possibilities are observed.

But my main problem with MWI is that none of the possibilities has anything to do with the wave function. The wave function appears to be some linear combination of them, but every two states |a> and |b> are connected in such a way: |a> = |b> + (|a> - |b>). Thus, this linear decomposition defines no connection at all between the wave function and the particular branch.

 

[Fra]

In this discussion it amazes me how few that at least seems to see this quest for physical science in the larger context of a science in general, and in the context of a theory of scientific inquiry.

...I always felt that QM in general, as opposed to classical physics, had at least a higher ambition of been more a theory of scientific inqiury than classical physics, since it does not just ask for "what the world is like", it focuses on the process of measurement, and thus what we can infer about the world, from measurements. In this sense a measurement theory is one step closer to a theory scientifc inqiury. (The fact that is is not yet finished is nother story)

Those whose still aim for realist views, seems to want to bypass this essential process of inqiury, or process of mesurement and interaction, that I can't help feel must somehow one of the basics of a scientific method.

This realist affinity seems to me at least, to somehow not play well with some of the basic ideas how the process of acquiring knowledge works. The point then is that knowledge can't be treated out of the context, that is how this acqustion or inference process actually works?

This isn't just Bohmian mechanics, it's also realist type of elements contained in many ways of reasoning, advocated be very famous people indeed. My impression is that, at some point these advocates manages to somehow ignore a part of the creative process that I thikn is part of the quest for NEW knowledge (not just attempts to falsify a given hypothesis; but rather creating NEW hypothesis fore testing), and dismiss it to psychology and philosophy. This was also pretty much the way Popper "solved" the problem of induction.

One can see signs of this in several places. Somehow "theory" is pictured without a context. Just like a transendent structure that really doesn't interact with our world. Instead I try to acknowledge that it does, and understand and describe the life of theories.

I haven't figured out hte options, but I have a feeling that if one could ask the right questions, of the view on physics and the view of the philosophy of science, there would be a strong correlation here in that the realist minded would tend to form distinct groups also in the view of science.

Maybe someone else can figure out a suitable poll for that, it would be intersting :)

/Fredrik

 

[Dmitry67]

I have already explained it to you, but let me try again. MWI requires frogs, but there is no any mathematical theory of frogs within MWI.

In the middle ages alchemists believed that the product of a chemical reaction depended not only on the ingridients, but also on the phase of the Moon and the magic spells pronouned during the process.

Now astronomy explains phases of the moon while chemistry explains the reactions.

I see CI, for example, as a modern alchemistry: a description of the QM world based on some magical 'measurement devices' and 'knowledge of an observer about the system'.

MWI get rids of it like modern chemistry get rids of the phases of the Moon. So your question 'but how MWI explains the frogs?' is like 'I see that chemistry works well, but how chemistry explains the phases of the moon?'

For the definition of Human use biology. For the definiton of 'observation' study what the qualia or consciousness is (we have no idea, it is almost no progress in thsi area). But it has absolutely no relation to the QM world (excuse me, Fra :) ) ! Stop saying magical spells (... based on the knoledge of an observer...)

:)

 

[Dmitry67]

Yes, I can. I have to modify the theory of gravity for this, but I have done this already before becoming a Bohm supporter. See ilja-schmelzer.de/glet.

Could you point me to the right place?
So you want to say, that you can define a frame common to the outer and inner space which does not have any singularities?

Also, how the preferred frame is consistent with the Big ang theory? If there is a preferred frame, then there is a (hidden) preferred point in the universe, where preferred frame is in rest to the CMB, right?

 

[Dmitry67]

But my main problem with MWI is that none of the possibilities has anything to do with the wave function. The wave function appears to be some linear combination of them, but every two states |a> and |b> are connected in such a way: |a> = |b> + (|a> - |b>). Thus, this linear decomposition defines no connection at all between the wave function and the particular branch.

yes, I agree with you, but so what? 'branch', like an 'observer', is a meta (nonphysical) structure - some compelx system which can be isolated based on some criteria

P.S. I will read both your articles a little bit later

 

[Dmitry67]

I have a cosmological question to the BM supporters.

What is price per barrel next year?
I mean, it is definitely pre-coded at the time of the Big bang, right :) ? Laplace determinism is valid for any deterministic single=history theory...

 

[Demystifier]

I have a cosmological question to the BM supporters.

What is price per barrel next year?
I mean, it is definitely pre-coded at the time of the Big bang, right :) ? Laplace determinism is valid for any deterministic single=history theory...

I will calculate that when you calculate the probability distribution of prices per barrel next year starting from first principles of QM in the many-world interpretation.

 

[Demystifier]

So your question 'but how MWI explains the frogs?' is like 'I see that chemistry works well, but how chemistry explains the phases of the moon?'

That is a good point. Still, there is a set of physical laws, which are not completely independent, that explain BOTH chemistry and phases of the moon. BI is an attempt to do something similar for BOTH MWI and frogs. Moreover, the encouraging fact is that BI achieves that in a rather simple way.

If you still cannot understand why some people prefer BI over MWI, then you probably don't want to. It is certainly an advantage to be able to understand how other people think. But of course, it is much simpler to say that all other people are crazy, so why bother with their way of thinking anyway ...

 

[Fra]

I see CI, for example, as a modern alchemistry: a description of the QM world based on some magical 'measurement devices' and 'knowledge of an observer about the system'.

MWI get rids of it like modern chemistry get rids of the phases of the Moon. So your question 'but how MWI explains the frogs?' is like 'I see that chemistry works well, but how chemistry explains the phases of the moon?'

My spells doesn't seem to work on Dmitry67, but there is a view that is close to mine, but still slightly more realistic put than I have in mind, and it's sort of an opposite to MWI, that Smolin also mentions in the last chapters in his book "the life of the cosmos".

The last section, which he calls "Einsteins Revenge" mentions a pluralistic universe which instead of multiple universes, considers multiple observers that are interacting. There he suggest that there is no magic separation of observer / observerd, as any separation is valid, any separation is interpreted so that it's the information one side has about the other side. Here he argues differently then I would, and uses a constraint of consistency. In my view this consistency is emergent, and corresponds to a kind of equilibrium.

This is close to view: There are not multiple universes, there are multiple observers :) And furthermore the constantly interact and evolve, and long with them does inferred physical law evolve.

He also argue for the evolving observers, which is really a resolution to the problem of defining the observer. Because no way how you define it you are lead to infinite regress. And unlike Dmitry67 who thinks that this is bad, I think it's good. Because this "infinite regress" is nothing but the evolving and dynamical world we witness.

If you stumble over that book, go right to the last section of "Einsteins Revenge" and see if you find that more convincing. There are also some conceptual arguments what observers always evolve, and that this is intermingled with the process of information updates.

Unlike what standard QM seems to suggest, information prcessing is not just about communication channels. There is also a sink and a source. The nodes, that store and hold information.

Here are I fully with one of Smolins example in that section, that and an observer IS his own memory device. And when an observer makes an observation, he unavoidably changes. The obvious part is that his state of information changes, but I'd add that also his memory hardware itself evolves.

He alse makes some additional arguments why it makes no sense to consider the wavefunction of the universe. It's beacuse it implictly assumes an observer sitting outside of the universe. So it's a non-physical position - it's an answer to a question none asks :) Unless you think God asks questions and that he needs our help ;)

FWIW, it's probably not going to convince you either, but if you get around to it check section 5 of https://www.amazon.com/dp/0195126645/?tag=pfamazon01-20. From how I see it, he doesn't say anything I haven't tried to say, but perhaps he is more convincing.

Also, the one think about Smolins reasoning I think is not so satisfactory is the actual CNS hypothesis of black hole spawning universes. I think his general motivation is justifiable and a lot of his overall reasoning is good, but there might be other ways to technically implement evolving law, than via black holes only.

But Smolin is one of the few physicists I've seen elaborate on this. Even though his general reasoning is hardly new, it's public attempt to application to physics seems so. so even if I don't share his ideas all the way, the general attitude is one of the best I've seen.

Some of the critic on Smolins reasoning here has not really made an impression on me either. Most attacks IMO misses that largest points he tries to convey. and I think the best way to understand it, is to see this also in the context of the scientific method. He is not just talking about "a theory", it is about seeing theory in context of a scientific process. Something that I think several critics seem to not appreciate.

/Fredrik

 

[Dmitry67]

Yes, I can. I have to modify the theory of gravity for this, but I have done this already before becoming a Bohm supporter. See ilja-schmelzer.de/glet.

Ha ha!
This is possible because you don't believe in black holes.
But wait, there are much MORE differences between GLET and GR then 4 items in your list.

jets from AGNs, possibility/impossibility of the reception of the gravitational waves, hypernovas, et cetera et cetera... Such radical theory has much more then 4 predictions different from SR. What is a reason for not giving your theory a chance to win? Put 20-30-100 predictions, the more predictions, the sooner your theory would win LOL

 

[Count Iblis]

What do the people opposed to MWI make of the thought experiment by David Deutsch in which you do a measurement and then forget the result of the measurement but not that the measurement was carried out, in such a way that the original wavefunction of the object is restored?

 

[DrChinese]

1. Feynman diagrams have nothing to do with paths or branches. To understand this, I suggest you to see a mathematical derivation of Feynman diagrams from first principles. For pedagogical purposes, I suggest you to see this within CLASSICAL (not quantum) mechanics (Refs. 52, 53).

2. It seems to me that you are not close at all.

Let me use an analogy. Consider a classical wave, say a wave on the surface of watter. It may have a complicated shape. Mathematically, it can be written as a Fourier sum of plane waves. But does it mean that plane waves are really there, that they are real? Not at all. Alternatively, it can also be written as a Taylor sum. But does it mean that particular terms in the Taylor expansion represent real objects? Again, not at all. All that makes a physical sense is the TOTAL function that describes the real wave. Particular terms in this or that expansion are nothing but a mathematical trick that simplifies calculations.

If it is still not clear to you, let me use an even simpler analogy. Assume that you have 1 apple. You can write it as
1=2+(-1)
Does it mean that you actually have 2 apples and -1 apple? Of course not. All you really have is 1 apple, and that's it.

1. I picked up ref 53 and am looking at that, thanks.

2. That is more or less what I thought I was saying... ...as reflecting your viewpoint. Elsewhere, you also said - which I think puts it nicely:

"The vacuum is an eigenstate of the operator of the number of particles, so there are no particle fluctuations in the vacuum. What fluctuates in the vacuum is the field, not particles."

The part about the field fluctuations was actually part of my original question: we have (what I called powerful) a fluctuating field. Depending on your interpretation, that field may represent the effect of "virtual particles" or may merely a mathematical device which leads us to better solutions.

But to consider your apple example: We see a series like: Results= .8(apple) + .001(apple+pear-2 grapes) + .04(apple+grape-grape) + .01(apple+pear-pear+orange-orange)... and somehow end up with a good answer. Obviously, the success of the path integral approach was that these expansions made mathematical sense. But they once again led to confusion about the underlying meaning. So I believe you are urging us not to consider that as evidence that there are oranges, grapes and pears jumping in and out of existence, and consider that we always* start with 1 apple and end up with 1 apple. It seems to me that those "extra" terms must map to "something non-local" in BM if it is going to explain the terms in the expansion.

(* Of course with higher energy or less stable particles, we don't always end up with the same coming out as in because there are decay modes. Which was one reason I was also asking about weak interactions.)

 

[dx]

What do the people opposed to MWI make of the thought experiment by David Deutsch in which you do a measurement and then forget the result of the measurement but not that the measurement was carried out, in such a way that the original wavefunction of the object is restored?

Could you give a more detailed description? I'm not familiar with the argument. I don't see how us forgetting things can have any effect on the wavefunction.

 

[Doc Al]

I have a cosmological question to the BM supporters.

What is price per barrel next year?
I mean, it is definitely pre-coded at the time of the Big bang, right :) ? Laplace determinism is valid for any deterministic single=history theory...

Why would determinism not apply even more so to MWI? Of course, it doesn't answer what happens in this world, but that's a fundamental issue with MWI, isn't it?

 

[Demystifier]

But to consider your apple example: We see a series like: Results= .8(apple) + .001(apple+pear-2 grapes) + .04(apple+grape-grape) + .01(apple+pear-pear+orange-orange)... and somehow end up with a good answer. Obviously, the success of the path integral approach was that these expansions made mathematical sense. But they once again led to confusion about the underlying meaning. So I believe you are urging us not to consider that as evidence that there are oranges, grapes and pears jumping in and out of existence, and consider that we always* start with 1 apple and end up with 1 apple. It seems to me that those "extra" terms must map to "something non-local" in BM if it is going to explain the terms in the expansion.

Like Feynman diagrams, path integrals are also nothing but a mathematical trick. Path integrals are nothing but a method of calculating the Green function. As you might know, Green functions are a useful mathematical device in solving ANY linear differential equations, including those that appear in classical physics. Thus, you can use the path integral method to solve the CLASSICAL harmonic oscillator or a CLASSICAL wave equation describing water waves.

What is relevant is the wave function. Even for BM this is all what is needed from standard QM. The way you have calculated the wave function (Feynman diagrams, path integrals, theory of special functions, Runge-Kuta method of numerical integration, ...) is completely irrelevant; there is no physics in it.

 

[Dmitry67]

Why would determinism not apply even more so to MWI? Of course, it doesn't answer what happens in this world, but that's a fundamental issue with MWI, isn't it?

Because it is not single history. In MWI has a unique symmetrical symmetry-breaking mechanism. I find it very beautiful.

So, look around. There is Sun, Earth and a void around. It is a symmetry breaking. Matter condensed HERE but not THERE. Why?

In deterministic theory you can not begin from a space with the same density and end with stars and galaxies! because if deterministic theory if 2 regions in space are in the same conditions then their evolution MUST be identical. If ALL regions are in the same state then ALL regions will continue to be in the same state.

In deterministic universe with simple initial conditions no there is no graitational instability: everyhting is in ABSOLUTE equlibrum - forever.

Deterministic theory can not, in princliple, break any symmetry, unless it is pre-coded in the initial conditions.

Now about MWI. Let's say we have 2 regions A and B, with the same density 0.5. So evolution is deterministic, so if we have sometimes after A=0.4, B=0.6 in one branch then we MUST have an opposite A=0.6, B=0.4 in another one.

Do you see how beautiful is it? Symmery is broken in every branch, but the whole universe remains symmetric and deterministic!

 

[Demystifier]

What do the people opposed to MWI make of the thought experiment by David Deutsch in which you do a measurement and then forget the result of the measurement but not that the measurement was carried out, in such a way that the original wavefunction of the object is restored?

From the Bohmian point of view, MWI is in fact correct (in the sense that the wave function does not collapse, making it consistent with the thought experiment above), but is not complete.

 

[Count Iblis]

Could you give a more detailed description? I'm not familiar with the argument.

The argument appeals to artificial intelligence that can be implemented by a quantum computer. So, the first part of the argument is that however the brain works, it is ultimtely formally describable using a finite number of bits. Therefore it can be implemented by a computer and thus also by a quantum computer.

The different branches of the observer correspond to the different projections of the quantum computer in the |0>, |1> basis of the qubits. Suppose that this observer measures the state of a qubit in the |0>, |1> basis. Let's call this qubit a "spin" to avoid confusion with the qubits that are part of the observer.

Then what we can achieve is the following.

1) We start with the spin in state |0>, then we rotate it to
1/sqrt(2) [|0> + |1>]


2) The observer then does a measurement in the |0>, |1> basis, which causes a qubit (that was initiallized to |0>) of his memory to be entangled with the state of the spin. This is performed using the controlled NOT gate. Also another qubit of his memory that was initialized to |0> is flipped to |1>. That qubit detects that a measurement has taken place (but not the result of the measurement).


3) The observer then applies the controlled NOT gate again, reversing the measurement. Then he flips another qubit that was initialized to |0> to |1>, which records the fact that the memory qubit that registered the spin has been erased.


4) At this stage the spin is back in the state 1/sqrt(2) [|0> + |1>]. The observer can verify this by applying the inverse rotation that he appied to the spin at the start, rotating it back to the state |0>. A measurement of the spin by the observer (or some other observer) will yield zero with 100% probability.


Now, the fact that the observer knows that he measured the spin in the |0>, |1> basis when it was rotated to 1/sqrt(2) [|0> + |1>] means that in the CI interpretation, the spin's state should have collapsed to either |0> or |1>. Only one of the branches really exists. Then, applying the inverse rotation won't bring the spin back to the state |0>, instead it will be a mixed state of

1/sqrt(2) [|0> + |1>]

and

1/sqrt(2) [|0> - |1>]

Measuring the spin again in the |0>, |1> basis must thus yield a 50% probability of finding it to be |0>.

 

[Fra]

Could you give a more detailed description? I'm not familiar with the argument. I don't see how us forgetting things can have any effect on the wavefunction.

I'm also interesting in hearing the detailed argument and the supposed problem. Since I'm not into MWI or Deutch I never read anything from him.

Edit: I just noticed that we got a response while I was typing. thanks.

About forgetting information in general though, in my view, this happens all the time for an evolving observer in the form of dissipation of actual history. If all history was retained, the mass of the observer would have to increase. This can happen, as well as that it can loose mass, but for a constant mass observer the evolving observer constantly processes information, remodels it's retains history and dissipates excess. I picture this as a form of radiation that is random (contains no informatin) _from the point of view of the observer_ but which generally does contain information relative to the environment.

/Fredrik

 

[DrChinese]

What is relevant is the wave function. Even for BM this is all what is needed from standard QM. The way you have calculated the wave function (Feynman diagrams, path integrals, theory of special functions, Runge-Kuta method of numerical integration, ...) is completely irrelevant; there is no physics in it.

I wouldn't say there is no physics in it. Excerpted from Wikipedia:

* The Casimir effect, where the ground state of the quantized electromagnetic field causes attraction between a pair of electrically neutral metal plates.

* The van der Waals force, which is partly due to the Casimir effect between two atoms.

* The so-called near field of radio antennas, where the magnetic effects of the current in the antenna wire and the charge effects of the wire's capacitive charge are detectable, but both of which effects disappear with increasing distance from the antenna much more quickly than do the influence of conventional electromagnetic waves, for which E is always equal to cB, and which are composed of real photons.

* The spontaneous emission of a photon during the decay of an excited atom or excited nucleus; such a decay is prohibited by ordinary quantum mechanics and requires the quantization of the electromagnetic field for its explanation.

* Lamb shift of positions of atomic levels.

* The Coulomb force between electric charges. It is caused by exchange of virtual photons. In symmetric 3-dimensional space this exchange results in inverse square law for force.

* The strong nuclear force between quarks - it is the result of interaction of virtual gluons. The residual of this force outside of quark triplets (neutron and proton) holds neutrons and protons together in nuclei, and is due to virtual mesons such as the pi meson and rho meson.

I realize you are saying that these effects are essentially an artifact of the math, but you have to admit that they are not the first things you would guess would come from something that is just a mathematical device. I mean, the virtual particle analogy seems pretty apt given the above. I admit that doesn't prove anything, just pointing out that the popularity of the viewpoint that virtual particles exist might be tied to the phenomena they help to explain.

And always, these "mathematically virtual particles" appear randomly without apparent cause. So that seems to go against the tenor of BM-type theories. If the psi^2 quantum equilibrium hypothesis for initial positions leads to the uncertainty in BM, which would otherwise be determinate, then I would expect another hypothesis to appear to explain additional random behavior of the field.

I don't think it all flows from that single hypothesis; but I could easily be wrong as I am not sufficiently versed in the matter. But maybe you can help on that. Or maybe that is what you have already said and I am too dense to follow. :)

 

[Demystifier]

And always, these "mathematically virtual particles" appear randomly without apparent cause.

This statement does not make any sense to me. Can you write down any equation that determines PROBABILITIES for the random appearance of these objects? I don't think so. Without a probability, it does not make sense to talk about their randomness.

Some physicists like to talk about virtual particles as if they really existed because they want to have some intuition about quantum processes, and yet do not want to cope with interpretations of QM seriously. But in my opinion, this is the worst thing to do. One should either strictly stick to the "shut up and calculate" approach, or think in terms of some of the logically consistent interpretations, such as Copenhagen, Bohm, many world, etc. Virtual particles do not play any serious role in any of these logically consistent interpretations.

 

[Dmitry67]

Demystifier, again, it depends on how you define what is a reality.
Tell me, is a real (non virtual) photon real?
You say, it is real? But do you know that photon is just a mixture of Z0 and anti-Z0?
So what is real, photon, Zo or anti-ZO? :)

 

[WaveJumper]

What about this research in New Scientist, where theorists claim most of the mass of real particles comes from virtual particles? Is this accepted as a valid description of reality?

http://www.newscientist.com/article/dn16095-its-confirmed-matter-is-merely-vacuum-fluctuations.html