QBism

To Be or Not to Be QB

The topic of Fuchs’ Quantum Bayesianism
seems best approached with a Seussian rhythm,
those safe rhyming couplets with which we can tackle
the bonds on our reason which Fuchs will unshackle.

First we must describe just what it is,
and examine in depth this Bayesian biz,
this view on probability which Fuchs has brought out,
this challenge to some great physicists devout
devotion to the notion of odds being real,
facts about factors on Let’s Make a Deal.
Not subjective in nature, especially not here,
with the measurements and observers which we hold so dear.
No, this is the world of actual stuff,
where objective odds reign but strangely enough
Fuchs’explanation appears to hold water,
and here after we’ll find out just whether we ought to
believe it. You see, it’s a succinct explanation,
this subjective answer to our common frustrations,
though there’s a major objection which must be broached,
the argument stating that Fuchs’ approach
requires antirealism. After all, we’re not speaking of things,
just opinion and belief, oh, the confusion that brings!
But there’s no point in rushing, for you’ll see
that Fuchs answers the argument quite handily.
His goal nothing less than to set physics free,
and this essay’s to help you choose to be,
or not to be, QB.

The meat of Fuchs’ paper quite simply consists
of his thesis “Quantum states do not exist”
He goes on to explain that these states we perceive
are constructions of what we would like to believe.
There’s no state A or B at any occasion,
just ideas we have that can match our equations.
Fuchs says the approaches of other incarnations
of linear mathematical quantum speculations
have hinged in the notion of reification,
attaching a thingness to our calculations.
From this we aspired to see just what we’ve got,
in this Fuchs makes clear, we create a blind spot.
A hole in our vision, this making of things,
which hang from our math like puppets on strings.
They bear the appearance of our quickest fixes,
each theorist’s desperate attempt at rolling hard sixes.
We ought to let go of such strange self-deception,
and open ourselves to a brand new conception.
We must abandon for now our metaphysical objectives,
and focus on the notion that probability’s subjective!

Out with the old! Make way for the new!
Let an era of lessened confusion ensue!
This immediately informs us of something quite nifty,
it’s us making superposition fifty-fifty.
It’s not a weirdness in physics but in our belief,
and to those doing hard science that’s quite a relief.
The problems our Bayesianism can end
include Schrodinger’s Cat and Wigner’s good Friend.
It does this by dispelling the idea of states,
a metaphorical clearing of our quantum plates.
They’re thrown out with the scraps,
for there’s no point you see,
asking in what state a system should be.
Don’t ask “Should we include Wigner in our linear equations?”
but “Will his presence help us with our calculations?”
This is an idea to which we already relate
whenever we discuss belief about quantum states.
We’re already ready to take it as read
that there’s some part of it which must be in our heads.

Fie on you, fie! The old school scientist cries,
not ready to let natural metaphysics die.
This quantum use of Bayesian probability
will result in mere subjective futility!
What happened to realism? What happened to stuff?
What happened is Fuchs made it vanish in a puff
of logic wherein he abandons his post,
his scientific mind giving up the ghost.
The physicist must talk about things in the world,
of infinitesimal particles which can be hurled
at great speeds at each other, through boxes and boxes,
while a silent Observer just watches and watches.
And measures of course, though this has proved quite a pickle,
first one at a time, then a flood, then a trickle.
The results don’t always add up in the ways that we like,
but you don’t just walk away when you fall off your bike.
No, you get back on top and you ride it again,
and so physicists do, not with bikes but with brains!
We’ll conquer this thing, you just wait and see,
while the Bayesian’s busy on what bet would be
rational to take on it, given such odds
(the ones his belief sets, not some external god’s).
How crass of the Bayesian to just go ahead
and say superposition is all in our heads.

Now that may seem like quite a damning indictment
but Fuchs meets it head-on with no small excitement.
He’s eager to take on the physical views,
and the state-specific terms they all tend to use.
It’s not about realism, it’s about what we can know,
and when we’re unsure we ought to forgo
any judgement on matters outside our purview
and ought to let additional information accrue.
When we talk about states we are making bets,
on just what kind of data we’ll get.
Will it be white? Will it be black?
Hard or soft? There’s a lack
of good information concerning such things,
and to the classical physicist that really stings.
But the QBist says “Look at what you’re doing!
Your belief is what matters about what state’s ensuing!
Not in the sense that it affects what occurs,
but it affects what you think that we ought to infer!
Focus on the experiments and the math we can get,
and don’t worry about what’s really happening yet.”
And this thinking, Fuchs notes, some have begun to apply,
speaking of quantum information, not the states it implies.
He makes no denial that there’s matter of fact,
only says that right now it’s hard to extract
thetruth of the matter from our mathematics
and attempts to do so create systematic
errors in judgement about what occurs,
leaving us to decide which theory’s preferred.
So while the classicist finds herself stuck with collapse
or many worlds Fuchs says that perhaps
we’re better off sticking with what we know,
and later on looking at where we can go.
To the classicist the QBist can say without apology,
“You thought you were doing metaphysics but it’s really epistemology.”

And now to sum up, for it is the conclusion,
QBism’s stake in our lessened confusion.
Fuchs in his paper carefully points out
that quantum states in themselves are open to doubt.
We can’t really see them, we made up the notion
to describe varying networks of very small motion.
He finds that it’s useless, and quite antiquated,
saying subject and object are interrelated.
One need only read Bayes to see what he means,
and so he proposes a quantum vaccine.
Abandon the thought of probability as objective,
and instead adopt his subjective directive,
talk of belief rather than of quantum states,
and include quantum information in other debates.
The classicist objects, wanting to work with the real,
it’s metaphysics itself that Fuchs tries to steal.
The reduction of odds right down to the subjective
ditches the real, as science it’s defective.
Fuchs disagrees by stating there’s matter of fact,
and that those who cry “Antirealism” overreact.
There’s a certain ambiguity implied by his position,
but it’s needed humility, not scientific sedition.

In short we thought we had awindow,
and strove to make it clearer,
but the Bayesian says that it’s really amirror.