Quantum Bayesianism Explained by Its Founder
quantamagazine.org
quantamagazine.org
That doesn't deny an objective reality out there, it's just saying that we have different information about what it's out there and, therefore, we arrive to different conclusions.
I don't understand how that perspective is different from just "objective Bayesianism" with different priors.
I'd like to hear him explain the single particle double slit experiment from this QBism perspective.
Eg, a coin flip is deterministic if you know all the forces involved (airflow, force of flip, exact distribution of mass of the coin, etc). But since we are usually ignorant of all that, instead we model it as a bernoulli trial.
You could be right, That is what this sounds like to me though. According to the model there is a 50% chance the coin will land on heads, until you flip it.
"Quantum measurement is nothing more, and nothing less, than a refinement and a readjustment of one’s initial state of belief. [...] Let us look at two limiting cases of efficient measurements. In the first, we imagine an observer whose initial belief structure ρ = |ψ⟩⟨ψ| is a maximally sharp state of belief. By this account, no measurement whatsoever can refine it. [...] The only state change that can come about from a measurement must be purely of the mental-readjustment sort: We learn nothing new; we just change what we can predict as a consequence of the side effects of our experimental intervention. That is to say, there is a sense in which the measurement is solely disturbance."
https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf
I suppose that is what I don't understand. What does it mean in practice? Probabilities work independently from gambling attitudes, but they work the same for everybody.
This gets interesting if you think of the past as a collection of "states", such as boundary conditions, and the area of the triangle as some "function" over the states. Think: hash function with some subset of a file as the input.
Now, imagine there are two points, one for each observer. Two scientists, say. There's one at the origin, call him Alex ("A") and another scientist in his "past light cone", call him Bob ("B"). Their states are like the hash function, and the inputs are the boundary conditions, like different but overlapping subsets of a file. They'll disagree because they don't all see the same input bits.
From the perspective of Alex, Bob is in his past, and Bob has only experienced a subset of the boundary conditions. In the physical universe, this is the big bang. Alex can, in mathematical principle, but maybe not a physical one, predict everything Bob will say about any experiment Bob will do. But Alex also knows that Bob is missing some information, the greater part of Alex's past light cone not shared by Bob. So Alex can make predictions that Bob cannot possibly predict. The situation is inherently, fundamentally asymmetric!
Here's the rub though: from Bob's perspective, Alex is in his past, and it is Bob that knows the entire state of the universe from the big bang leading up to his here & now, and it is Alex that is ignorant of a "slice" of it.
So what's going on?
The reality is that there are 4 observers: {Alex Prime, Alex's Bob}, and {Bob Prime, Bob's Alex}.
We colloquially talk about them as if there are only 2 observers, but the more accurate perspective is that there are 2 observers in 2 universes, and they can never completely agree on all experiments, not even in principle. They have different inputs, different boundary conditions, and different predictions.
This is not possible, if B is in the past lightcone of A. The relative time ordering can only vary if the two observers/events are spacelike separated — in which case the each have access to events the other doesn’t.
Colloquially, we think that Alex and Bob are the same persons from each others' perspective.
Physically, this is not true. They're different. There's Alex and Bob-in-Alex's past, and also Bob and Alex-in-Bob's past. Four "people".
If helps, use symbols: A(A), A(B), B(B), B(A).
Anyway, the prior is updated after every observation, so if it was a wrong prior, it should be less important in the result at every observation.
The prior is only negligible in infinite sample sizes. It could still have a large effect given 100s or 1000s of observations. For example, if the prior variance is miss specified by an order of magnitude (not uncommon).
For those who are curious, here's Jaynes' view on the subject:
Edit: Going through recent posts of this guy, I would also like to state that anything non physics related on his blog is crazy right-wing babble.
https://news.ycombinator.com/item?id=9714618
Same article but published by Wired.
[1]. https://www.bloomsbury.com/us/ontology-and-metaontology-9781...
Let's say you're in a game. You can create a physics of the game world based on how that game behaves. However, the game world doesn't exist, as such. It's the result of a pattern produced by a computer, interpreted by our minds, which fills in the blanks and makes the game world feel more real than it is.
It makes sense that an account of existence should either include the substructure (in this case, the computer) or the observer (in this case, the mind doing the filling-in), or possibly both. On the other hand, a description of the behavior of the system is much simpler.
Another example: simulation hypothesis people sometimes talk about the possibility of "breaking out" of the simulation, as if the simulation is less somehow less true than whatever is behind it.
But from a predicting-observed-phenomenon perspective, what you describe by physics exists. So the distinction between the rules of physics that apply in a certain domain because you're in a simulation, and the rules of physics that apply in another domain once you find a way to break out, is entirely artificial. That people don't see it that way seems to indicate that they mean something else by existence.
EPR paradox demonstrates that observers can know each other experience.
That's what Bayesianists think, but not frequentists. (As a frequency, probability is very much real. And, what would be the point of talking about something that "does not exist," anyway?)