Surprising Science

# In One Universe, the Cat Dies

How the uncertain fate of a fictional tabby gave us the multiverse.

Did you know it was the fate of a solitary (imaginary) kitty that led quantum theoreticians—in particular one Hugh Everett III—to the idea of multiple universes? The theoretical cat belonged to Austrian physicist Erwin Schrödinger. Physics professor Jim Kakalios of the University of Minnesota tells the story in this video.

Schrödinger’s Cat is a famous mind experiment that goes like this. Schrödinger imagined a box into which have been placed three things:

2. a sealed bottle of poisoned gas

3. a cat

The isotope has a half-life of an hour, which means that 50% of the time, the pretend isotope will decay in an hour, releasing an imaginary alpha particle that breaks the bottle and kills the fictional cat. 50% the time, that kitty will be fine.

After one hour, you’ll open the box to learn the fate of the cat. The mathematical way to describe the cat before you look is an average of a living kitty and a dead kitty, which makes no sense really, zombies notwithstanding.

To avoid such a nonsensical idea, Schrödinger posited that until you look, the cat exists simultaneously in two quantum states, one living and one dead. It’s only upon opening the box and looking that one or the other state comes true. (Is this different than believing you don’t have cancer until the doctor tells you you do?)

Anyway, this was too much for Everett to buy, and he proposed instead that when the box is closed, the two possible outcomes split off into two quantum states, each in its own parallel universe, one where the cat lives and one where the cat dies. In a different situation with more than two possible outcomes, each outcome would exist in its own quantum state, and thus its own universe—and so bam, welcome to the multiverse. Everett suggested that when we open the box and look, all that really happens is we learn which of the two living/dead cat universes we happen to be in.

Which makes so much more sense?

On the other hand, it’s nothing any devoted Fringe fan doesn’t know.

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