# Can someone explain Quantum Bayesianism to me?
**URL:** https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966
**Category:** Factual Questions
**Created:** [May 21, 2013, 10:42pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966 "2013-05-21T22:42:11Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [May 21, 2013, 10:42pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/1 "2013-05-21T22:42:11Z")
The latest issue of Scientific American had an article on it, and I’m not sure I even understand what it’s claiming. Superficially it sounds almost like solipsism. About all I understand is that it claims that DeBroglie wave functions are subjective on the part of the observer.
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### Author:  [@Trinopus](https://boards.straightdope.com/u/Trinopus)
#### Post date: [May 22, 2013, 12:35am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/2 "2013-05-22T00:35:14Z")
THANK YOU for posting this! I read the same article, and came away with a great big “Huh?”
How does this new (?) approach resolve the double-slit experiment? Aren’t we “really” seeing interference fringes? Those surely aren’t just in our heads.
The approach may be valid, but the article needed a lot more care in explanation!
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [May 22, 2013, 3:10am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/3 "2013-05-22T03:10:43Z")
This is definitely one for **Half Man Half Wit** , but this is my limited understanding of it:
The basic idea, I believe, is to frame quantum theory as if it were a guide for an observer to bet on the outcome of experiments (of course this is more of an analogy as betting does not actually comes into it). It shares with the Copenhagen interpretation the idea that the important information about a system is not the quantum state itself, but the probabilities of getting different results, but goes further and tries (I say try because it hasn’t been proven that the method it uses works in all cases) to frame the formalism of quantum mechanics in terms of probability, so that the quantum state is merely a useful calculational tool that can in principle be ignored completely (though each allowable probability distribution will be associated with a mixed or pure quantum state).
Obviously there is a heavy emphasis on probability in the approach and probabilities can be subjective. For example let’s say a red and a black dice are thrown and there are two gamblers. Due to a brief slip by the croupier, one gambler knows that one of the dice is a six (but not which one) and the dice are fair, the other simply knows the dice are fair. The probability that the red dice was a six for the first gambler is 6/11, but for the second gambler the probability is 1/6.
QB similarly says that the probabilities as to the outcome of the same experiment may not be the same for two different observers, as for example one observer may’ve conducted a previous experiment which alters their knowledge of the system. As each probability distribution is associated with a quantum state, this makes the quantum state subjective too. In particular this tackles the Wigner’s friend gedanken experiment in a consistent way that doesn’t invoke consciousness, multiple Universes or fundamentally alter quantum theory by invoking spontaneous collapse.
It may seem like solipsism, but a proponent of QB would argue that quantum theory s an observercentric theory as it’s predictions are the probabilities of different results of an experiment from the point of view of an observer, which from a subjectivist Bayesian p.o.v. would be interpreted as the strength of belief of the observer as to each different result. They would also say that given this, you would not necessarily expect to derive the concept of an observer from quantum theory itself.
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### Author:  [@Vitalis\_Kissmeoff](https://boards.straightdope.com/u/Vitalis_Kissmeoff)
#### Post date: [May 22, 2013, 10:02am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/4 "2013-05-22T10:02:44Z")
> [@Asympotically\_fat](#):
>
> This is definitely one for **Half Man Half Wit** , but this is my limited understanding of it:
>
> The basic idea, I believe, is to frame quantum theory as if it were a guide for an observer to bet on the outcome of experiments (of course this is more of an analogy as betting does not actually comes into it). It shares with the Copenhagen interpretation the idea that the important information about a system is not the quantum state itself, but the probabilities of getting different results, but goes further and tries (I say try because it hasn’t been proven that the method it uses works in all cases) to frame the formalism of quantum mechanics in terms of probability, so that the quantum state is merely a useful calculational tool that can in principle be ignored completely (though each allowable probability distribution will be associated with a mixed or pure quantum state).
>
> Obviously there is a heavy emphasis on probability in the approach and probabilities can be subjective. For example let’s say a red and a black dice are thrown and there are two gamblers. Due to a brief slip by the croupier, one gambler knows that one of the dice is a six (but not which one) and the dice are fair, the other simply knows the dice are fair. The probability that the red dice was a six for the first gambler is 6/11, but for the second gambler the probability is 1/6.
>
> QB similarly says that the probabilities as to the outcome of the same experiment may not be the same for two different observers, as for example one observer may’ve conducted a previous experiment which alters their knowledge of the system. As each probability distribution is associated with a quantum state, this makes the quantum state subjective too. In particular this tackles the Wigner’s friend gedanken experiment in a consistent way that doesn’t invoke consciousness, multiple Universes or fundamentally alter quantum theory by invoking spontaneous collapse.
>
> It may seem like solipsism, but a proponent of QB would argue that quantum theory s an observercentric theory as it’s predictions are the probabilities of different results of an experiment from the point of view of an observer, which from a subjectivist Bayesian p.o.v. would be interpreted as the strength of belief of the observer as to each different result. They would also say that given this, you would not necessarily expect to derive the concept of an observer from quantum theory itself.
Okay, thanks for making it perfectly clear.
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [May 22, 2013, 3:16pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/5 "2013-05-22T15:16:06Z")
OK, I’ve gotten this far: Bayesian statistics is one particular school/philosophy of interpreting probability. Quantum mechanics involves the probability of certain measurement outcomes being observed. QB is what you get when you apply the tenets of Bayesian statistics to Quantum mechanics.
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### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [May 22, 2013, 4:05pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/6 "2013-05-22T16:05:12Z")
Well, I can’t say I’m an expert, but the broad outline—adding to what **Asymptotically Fat** has already said—is roughly the following:
Probability, in the classical case, can be interpreted in various ways. The most widespread approach is perhaps the frequentist one: probabilities are simply the limit of relative frequencies of certain occurrences. That way, the sentence ‘a fair coin comes up heads with probability 1/2’ means nothing else but that on a sufficiently large number of throws, about half of them comes up heads.
But another interpretation is related to knowledge about something. As a way to illustrate this, one often uses metaphors related to placing bets on outcomes—given what you know about the system, what sorts of bets should you accept? If you believe a coin to be fair, for instance, anything but an even money bet would be stupid; but if you have observed a series of coin throws, and have a reason to believe that it’s biased (say, heads appears more often), you’d be justified in placing higher bets on heads.
Now, quantum mechanics is a fundamentally probabilistic theory: for any given occurrence, it only gives us a certain probability (which may be one). Quantum Bayesianists interpret this as meaning that the quantum state or the wave function, i.e. the mathematical object we use to calculate these probabilities (via a prescription known as Born’s rule), is itself just an encapsulation of our knowledge about the world, and not in itself a kind of ‘thing’ in the world—wave functions don’t describe any real, physical wave (thus, the interpretation is of a kind known as ‘epistemic’, as opposed to the ‘ontic’ kind that take the wave function to be, in some sense, ‘real’). Note that this does not mean that the observed effects aren’t real: after all, those are what we calculate the probabilities of. It’s just an instrumentalist or operationalist take on these phenomena: quantum mechanics doesn’t tell us what happens to produce them, it just provides a recipe to calculate what we should expect to observe.
Specifically, quantum Bayesianism postulates a set of ‘Bureau of Standards’-measurements, and aims to interpret the quantum state as giving the probabilities of their outcomes (though as **Asymptotically Fat** notes above, it’s not known—though strongly conjectured—whether these special measurements exist in all cases). So, the quantum state is nothing but a list (Schrödinger called it ‘a catalog of our knowledge’) giving the probabilities of making certain observations, i.e. of the standard measurements yielding certain values.
The charm (to its proponents) of this interpretation is the potential solution to several philosophical problems quantum mechanics brings with itself, among them the infamous measurement problem: the question how an indefinite quantum state suddenly upon measurement produces a definite outcome. On a quantum Bayesian interpretation, this is no problem at all: it just encapsulates the fact that when you learn something new, you have to adjust your beliefs. More accurately, in Bayesian inference, you start with ascribing some probability distribution to the system you are considering. Then, upon receiving new data, you change—‘update’—that distribution to reflect this new data (I’ve described this somewhat in detail in [this rather lengthy OP](http://boards.straightdope.com/sdmb/showthread.php?t=632703)). This is a process that is discontinuous and instantaneous, and as such, looks just like the infamous ‘wave function collapse’.
So, to summarize, quantum mechanics is a calculus to make reasonable predictions about measurement outcomes; whenever we learn new stuff, we change our beliefs, and thus, future predictions.
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [May 23, 2013, 3:47pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/7 "2013-05-23T15:47:27Z")
So… is this a theory within physics, or is it a philosophical standpoint about the nature of human knowledge? It doesn’t seem to actually predict anything new. I mean, I could adopt a nihilistic pose and say that since existence is meaningless and ultimately nothing matters, my take on the meaning of quantum physics would be “don’t worry about it”. That’s not particularly useful.
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### Author:  [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)
#### Post date: [May 23, 2013, 4:06pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/8 "2013-05-23T16:06:09Z")
As a non-expert, this sounds like a different statistical framework for hidden variable theories. How does it differ? I thought that local hidden variables have been essentially disproven.
(Being a biologist who wallows in uncertainty and hunches I have a fondness for Bayesian approaches. But I really only know just enough to be dangerous…)
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### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [May 23, 2013, 5:19pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/9 "2013-05-23T17:19:47Z")
> [@Lumpy](#):
>
> So… is this a theory within physics, or is it a philosophical standpoint about the nature of human knowledge? It doesn’t seem to actually predict anything new. I mean, I could adopt a nihilistic pose and say that since existence is meaningless and ultimately nothing matters, my take on the meaning of quantum physics would be “don’t worry about it”. That’s not particularly useful.
It’s an interpretation, so no, it doesn’t predict anything new. It’s a way to cope with the problems of quantum mechanics, which its proponents claim is more sensible than alternative approaches. Of course, that’ll depend on taste somewhat.
> [@lazybratsche](#):
>
> As a non-expert, this sounds like a different statistical framework for hidden variable theories. How does it differ? I thought that local hidden variables have been essentially disproven.
Quantum Bayesianism doesn’t make any kind of ontological statement in itself, i.e. it carries no commitment regarding what actually goes on. To the quantum Bayesianist, what’s real is ultimately just the ‘click’ of a detector (or lack thereof), and quantum mechanics is just a formal tool to predict the statistics of these clicks. There might be some more fundamental theory we don’t know yet (in which case it won’t be a local realistic theory, as you note), but that question is simply left open. If there is some philosophical tendency inherent in the framework, it’s probably roughly the view that absent of measurement, there simply is no fact of the matter regarding ‘what’s really going on’.
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [May 23, 2013, 5:26pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/10 "2013-05-23T17:26:03Z")
> [@Lumpy](#):
>
> OK, I’ve gotten this far: Bayesian statistics is one particular school/philosophy of interpreting probability. Quantum mechanics involves the probability of certain measurement outcomes being observed. QB is what you get when you apply the tenets of Bayesian statistics to Quantum mechanics.
The basic idea is to take the rules of Bayesian interpretation along with a philosophically subjectivist interpretation of those rules and then to add further rules that are derived from the formalism of quantum mechanics.
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### Author:  [@deltasigma](https://boards.straightdope.com/u/deltasigma)
#### Post date: [May 23, 2013, 5:58pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/11 "2013-05-23T17:58:01Z")
Something human accessible (no offense) from [physics world]([http://boards.straightdope.com/sdmb/In](http://boards.straightdope.com/sdmb/In) less than 100 seconds, Daniel Mortlock ponders whether the quantum wavefunction could be more than a mathematical function.).
> [@](#):
>
> In less than 100 seconds, Daniel Mortlock ponders whether the quantum wavefunction could be more than a mathematical function.
Spoiler - that’s sort of a fib, but still worth watching.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 23, 2013, 6:14pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/12 "2013-05-23T18:14:05Z")
Just as a general-purpose reminder, deciding between different interpretations of quantum mechanics is a philosophical matter, not a scientific one. All of the mainstream interpretations produce exactly the same measurable outcomes, and therefore no measurement can prove nor disprove any of them. Since it doesn’t change anything scientifically-speaking, most physicists prefer not to hold to any particular interpretation at all, in what’s sometimes called the “shut up and do the calculations” interpretation.
Now, sometimes thinking about a particular interpretation makes it easier to figure out how to set up a particular calculation (the same calculations can be done in any interpretation; sometimes it’s just easier than others), and in such a case a physicist might temporarily make use of that interpretation, but then might just as easily switch to some other interpretation for some other problem.
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### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [May 23, 2013, 6:57pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/13 "2013-05-23T18:57:41Z")
> [@Chronos](#):
>
> Now, sometimes thinking about a particular interpretation makes it easier to figure out how to set up a particular calculation (the same calculations can be done in any interpretation; sometimes it’s just easier than others), and in such a case a physicist might temporarily make use of that interpretation, but then might just as easily switch to some other interpretation for some other problem.
Sometimes, also interpretations may suggest or facilitate new technical advances—quantum Bayesianism has been quite fruitful in that regard, providing a quantum analogue to the de Finetti theorem and introducing the notion of symmetric informationally complete positive operator-valued measures (SIC-POVMs), the aforementioned ‘bureau of standards’-measurements whose properties are interesting beyond just this use.
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### Author:  [@Trinopus](https://boards.straightdope.com/u/Trinopus)
#### Post date: [May 24, 2013, 12:07am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/14 "2013-05-24T00:07:32Z")
Well, by gum, I’m accepting the Many Worlds interpretation.
* * *
Well, by gum, I’m rejecting the Many Worlds interpretation.
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### Author:  [@deltasigma](https://boards.straightdope.com/u/deltasigma)
#### Post date: [May 25, 2013, 11:13pm UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/15 "2013-05-25T23:13:10Z")
Here’s an [interesting experiment](http://phys.org/news/2013-05-hydrogen-atoms-magnifying-glass.html) that apparently creates a 2d visual representation of a hydrogen atom’s electronic states.
> [@](#):
>
> To describe the microscopic properties of matter and its interaction with the external world, quantum mechanics uses wave functions, whose structure and time dependence is governed by the Schrödinger equation. In atoms, electronic wave functions describe - among other things - charge distributions existing on length-scales that are many orders of magnitude removed from our daily experience. In physics laboratories, experimental observations of charge distributions are usually precluded by the fact that the process of taking a measurement changes a wave function and selects one of its many possible realizations. For this reason, physicists usually know the shape of charge distributions through calculations that are shown in textbooks. That is to say, until now. An international team coordinated by researchers from the Max Born Institute has succeeded in building a microscope that allows magnifying the wave function of excited electronic states of the hydrogen atom by a factor of more than twenty-thousand, leading to a situation where the nodal structure of these electronic states can be visualized on a two-dimensional detector.
[Orig. article](http://physics.aps.org/articles/v6/58)
Wouldn’t this sort of thing tend to undercut more subjective interpretations?
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [May 26, 2013, 12:41am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/16 "2013-05-26T00:41:49Z")
> [@deltasigma](#):
>
> Here’s an [interesting experiment](http://phys.org/news/2013-05-hydrogen-atoms-magnifying-glass.html) that apparently creates a 2d visual representation of a hydrogen atom’s electronic states.
> [Orig. article](http://physics.aps.org/articles/v6/58)
>
> Wouldn’t this sort of thing tend to undercut more subjective interpretations?
Generally speaking interpretations of QM do not differ in what they predict for the outcome of experiments as they are interpretations of what the formalism of QM predicts for the outcome of experiments. These results aren’t any different in that respect.
What you are seeing in the images in the first link is the electron density of the screen of a detector (i.e. the number of electrons hitting the screen per unit area) which, due to the experimental set-up, corresponds to a 2-D cross-section of the square-modulus of the wavefunction of an electron in a hydrogen atom.
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### Author:  [@deltasigma](https://boards.straightdope.com/u/deltasigma)
#### Post date: [May 26, 2013, 1:03am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/17 "2013-05-26T01:03:46Z")
Well, as the article describes it more specifically:
> [@](#):
>
> They predicted that projecting the electrons onto a two-dimensional detector placed perpendicularly to the static electric field would allow the experimental measurement of interference patterns directly reflecting the nodal structure of the [electronic wave](http://phys.org/tags/electronic+wave/) function. The fact that this is so, is due to the special status of hydrogen as nature’s only single-electron atom. Due to this circumstance, the hydrogen wave functions can be written as the product of two wave functions that describe how the wave function changes as a function of two, so-called “parabolic coordinates”, which are linear combinations of the distance of the electron from the H+ nucleus “r”, and the displacement of the electron along the electric field axis “z”. Importantly, the shape of the two parabolic wave functions is independent of the strength of the static electric field, and therefore stays the same as the electron travels (over a distance of about half a meter, in our experimental realization!!) from the place where the ionization takes place to the two-dimensional detector.
So aren’t you actually “seeing” the wave function in some meaningful sense?
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [May 26, 2013, 1:41am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/18 "2013-05-26T01:41:04Z")
> [@deltasigma](#):
>
> Well, as the article describes it more specifically: So aren’t you actually “seeing” the wave function in some meaningful sense?
No more meaningful than I put it: i.e. you are seeing something that is a representation of a cross-section the square-modulus of the (time-independent) wavefunction - which is not surprising at all as the square-modulus of the wavefunction of a single particle is the probability density for a position measurement on that particle.
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### Author:  [@deltasigma](https://boards.straightdope.com/u/deltasigma)
#### Post date: [May 26, 2013, 1:54am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/19 "2013-05-26T01:54:36Z")
> [@Asympotically\_fat](#):
>
> No more meaningful than I put it: i.e. you are seeing something that is a representation of a cross-section the square-modulus of the (time-independent) wavefunction - which is not surprising at all as the square-modulus of the wavefunction of a single particle is the probability density for a position measurement on that particle.
Uh, ok. I’m sure that couldn’t be any clearer, although I seemed to have no problem understanding the article. Odd.
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### Author:  [@deltasigma](https://boards.straightdope.com/u/deltasigma)
#### Post date: [May 26, 2013, 3:15am UTC](https://boards.straightdope.com/t/can-someone-explain-quantum-bayesianism-to-me/658966/20 "2013-05-26T03:15:56Z")
For anyone who’s interested, [Physics World](http://physicsworld.com/cws/article/news/2013/may/23/quantum-microscope-peers-into-the-hydrogen-atom) has a much better article on this which I just got to (sorry). It’s not easy reading but they do try to explain things, for example:
> [@](#):
>
> The wavefunction is a central tenet of quantum theory – put simply, it contains the maximum knowledge that is available about the state of a quantum system. More specifically, the wavefunction is the solution to the Schrödinger equation. The square of the wavefunction describes the probability of where exactly a particle might be located at a given time. Although it features prominently in quantum theory, directly measuring or observing the wavefunction is no easy task, as any direct observation destroys the wavefunction before it can be fully observed.
>
> In the past, “Rydberg wavepacket” experiments have tried to observe the wavefunction using ultrafast laser pulses. In these experiments, the atoms are in a superposition of their highly excited “Rydberg states”. These experiments show that the periodic electron orbitals around nuclei are described by coherent superpositions of quantum-mechanical stationary states. The wavefunction of each of these states is a standing wave with a nodal pattern (a “node” is where there is zero probability of finding an electron) that reflects the quantum numbers of the state. While previous experiments have attempted to capture the elusive wavefunction or the nodal patterns, the methods used were not successful. Direct observation of the nodal structure of a single atom being most difficult to achieve.
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