During a recent conference on cosmic frontiers, University of California, Davis, professor Andreas Albrecht made a provocative statement: “Every

Brownian motion is a Schrödinger’s Cat.” Technically, it was part of a broader talk on implications for a multiverse contained in various models of inflation in the early universe—based in turn on a recent technical paper. But Albrecht’s colorful phrasing prompted me to ponder more deeply

the conventional wisdom about the difference between classical and

quantum probabilities. Because what he’s really saying is that all

classical probabilities emerge from quantum probabilities.

The

connection between the microscopic quantum world and the macroscopic

classical world can be seen in Brownian motion, the random jittery

movements of small particles suspended in a liquid or gas. It gets

its name from the 19th-century

botanist Robert Brown, who noticed [pdf] that pollen grains floating in water under a microscope seemed to

move randomly of their own accord. That’s because a grain of pollen

is 250,000 times larger than the water molecules, which jiggle from

thermal motion. Even though they can’t be seen with the naked eye,

they are colliding with the much larger pollen grains constantly,

from all directions. You’d think these collisions would all cancel

out in the end, but because they are random, there are always tiny

imbalances at any given time—slightly more molecules pushing the

grain to the right than pushing to the left.

This

has implications for probabilistic predictions. Classically speaking,

if you flip a coin, you would say that there is a 50/50 chance it

will land heads or tails, simply because you don’t have enough

information about the many factors that could influence its landing

position—the rate of spin, height, a slight gust of wind. In

principle, you could always acquire more information to refine your

prediction so that it is more accurate. The classical probability is

a way of quantifying our ignorance, if you will. Furthermore, you

know that one side is definitely heads, and the other is definitely

tails before the coin lands. So there is zero probability that it

will be both heads and tails at the same time.

Quantum

physics is fundamentally different: There’s no further information

to acquire beyond the stated probability. In the quantum world,

things can hover in a fuzzy, nebulous cloud of probability that

encompasses all potential states: heads

and tails, particle and wave.

Things become definite only when an observation forces them to settle

on a specific outcome. Technically, the coin exists in an

indeterminate state of both heads and tails until it lands.

But

it would be silly to think this applies to the macroscale world of

everyday life, right? That was the whole point of Schrödinger’s

cat, the famous thought experiment devised by Erwin Schrödinger to

illustrate the absurdity of the quantum realm. I’ll let Sheldon

Cooper of *The
Big Bang Theory*

explain:

That’s

why Albrecht’s declaration caught my attention: He maintains that

this kind of quantum probability works at every scale, from a simple

coin toss to predicting the weather. Albrecht and his UC Davis

co-author, Daniel Phillips, employed a billiard ball analogy to

illustrate this—or rather, molecules in a gas that collide with

each other like billiard balls. They show that the uncertainty of

such a system increases with every collision, and when that

uncertainty becomes large enough, it’s quantum effects that become

the dominant factor in the outcome—not classical mechanics.

Size

matters when it comes to the number of collisions needed to hit that

threshold. For an actual game of billiards, it takes just eight

collisions between billiard balls for quantum uncertainty to

dominate; it takes 25 if we’re talking about bumper cars. But it

only takes a single collision between molecules in water or air to

make the uncertainty large enough that what’s going on at the

quantum level impacts the macroscale properties of the system.

It’s

a kind of chaotic system, in which the tiny fluctuations at the

quantum scale—the equivalent of a butterfly flapping its wings in,

say, Africa—become amplified via countless molecular interactions,

until they collectively manage to have an impact on the

macroscale—the proverbial tornado in Kansas. Ergo, flipping a coin

is the probabilistic equivalent to Schrödinger’s cat: The coin’s

final state cannot be predicted until it has actually been flipped.

“It

is very likely that all serious probabilities, be it a coin landing

heads-up or a child being female, are manifestations of quantum

chanciness,” King’s College London philosopher David Papineau told Physics World* *when

asked about Albrecht’s and Phillips’ work. “Indeed we have devices, such as Geiger counters, that show how big

results are often caused by chancy micro-events.”

So,

if “all successful applications of probability to describe nature

can be traced to quantum origins,” as Albrecht and Phillips

maintain, that means that even when we think we’re using classical

probabilities, deep down, it’s really the quantum world calling the

shots. We are opening the box on Schrödinger’s cat every time we

flip a coin or check the weather, and countless other times during

every day.

*Jennifer Ouellette is a science writer and the author of *The Calculus Diaries* and the forthcoming *Me, Myself and Why: Searching for the Science of Self*. Follow her on Twitter @JenLucPiquant.*