Philosophy

# Quantum Bayesianism Says You Can't Spell Reality Without "I"

The traditional view of quantum physics says that very tiny particles exist in many states at once until an observer measures them, which makes them choose just one state. But according to a tantalizing and incredibly controversial interpretation, nothing actually exists in many states at once — the observer just isn't sure which state it's in. Once he makes a measurement, then he knows. Conceived by quantum theorist Christopher Fuchs, this observer-centric view of quantum mechanics is called Quantum Bayesianism, or QBism for short.

## To Be Or Not To QB

If you think about the quantum world as a deck of cards, the traditional (Copenhagen) interpretation would go like this: when the dealer puts a card face down, that card is simultaneously all 52 cards at once. In traditional quantum mechanics, that's a concept known as superposition, and the mathematical formula that describes all of those states is called a wavefunction. Only when you pick that card up and look at it does it "choose" its identity (also known as "collapsing the wavefunction").

Likewise, if all the cards are dealt so that you know that only the Ace of Spades and the Queen of Hearts remain, and you are dealt the Ace of Spades, that changes the state of the remaining card so it becomes the Queen of Hearts. That relationship where the state of one card changes the state of the other is a concept called entanglement.

Here's how QBism sees that same card game. When the dealer puts a card face down, a player knows that the card has a 1 in 52 chance of being any one of the cards in the deck; a 1 in 4 chance of being any one of the suits, and a 1 in 2 chance of being red or black. According to QBism, that's all a wavefunction  a description of probabilities. When he turns the card over, instead of the card "choosing" to be, say, the Ace of Spades, the player just updates his knowledge: that card now has a 100 percent chance of being the Ace of Spades. The wavefunction is updated, not collapsed. Likewise, when only two face-down cards are left on the table, each simply has a 1 in 2 chance of being either card — they're only related by probability, not some spooky quantum phenomenon. According to QBism, quantum mechanics is just probabilities of reality, not reality itself.

## You Got Your Probabilities In My Reality!

Because that's actually how cards work, this simplified version of QBism sounds pretty airtight. But QBism takes a unique view of probability: it's more a description of an observer's uncertainty than the chances that one thing is actually correct. That's why the main objection to QBism is that it's all about subjective personal experience, so it seems to say that reality only exists in the mind of the observer, and two different people can have two different realities. Science is objective, critics say. Reality is reality, whether there's an observer or not.

But, as N. David Mermin writes in Nature, this accusation is unfair: "Although I cannot enter your mind to experience your own private perceptions, you can affect my perceptions through language. When I converse with you or read your books and articles in Nature, I plausibly conclude that you are a perceiving being rather like myself, and infer features of your experience. This is how we can arrive at a common understanding of our external worlds, in spite of the privacy of our individual experiences."

QBism does raise questions of its own, though. If quantum mechanics doesn't describe an external reality, what does? As Fuchs tells it, that may be the wrong question. "Usually we think of the universe as this rigid thing that can't be changed," he told Quanta Magazine. "Instead, methodologically we should assume just the opposite: that the universe is before us so that we can shape it, that it can be changed, and that it will push back on us. We'll understand our limits by noticing how much it pushes back on us."

#### QBism Through the Wormhole

##### QBism Through the Wormhole
– Christopher Fuchs
Ashley Hamer