The Science Book

A PARADIGM SHIFT 233

Schrödinger’s cat
Taken to its logical conclusion, the
Copenhagen interpretation resulted
in a seemingly absurd paradox.
Schrödinger imagined a cat sealed
in a box that contains a vial of
poison linked to a radioactive
source. If the source decays and
emits a particle of radiation, a
mechanism will release a hammer
that breaks the vial of poison.
According to the Copenhagen
interpretation, the radioactive
source remains in its wave function
form (as a so-called superposition
of two possible outcomes) until it
is observed. But if that is the case,
the same has to be said of the cat.

New interpretations
Dissatisfaction with apparent
paradoxes such as Schrödinger’s
cat has spurred scientists to
develop various alternative
interpretations of quantum
mechanics. One of the best known
is the “Many Worlds Interpretation”
put forward in 1956 by American
physicist Hugh Everett III. This
resolved the paradox by suggesting
that during any quantum event, the

universe splits into mutually

unobservable alternate histories

for each of the possible outcomes.

In other words, Schrödinger’s cat

would both live and die.

The “Consistent Histories”

approach addresses the problem

in a rather less radical way, using

complex mathematics to generalize

the Copenhagen interpretation.

This avoids the issues around the

collapse of the wave function, but

instead allows probabilities to be

assigned to various scenarios, or

“histories,” on both a quantum

and classical scale. The approach

accepts that only one of these

histories eventually conforms

to reality, but does not allow

prediction of which outcome

that will be—instead it simply

describes how quantum physics

can give rise to the universe we see

without wave function collapse.

The ensemble, or statistical,

approach is a minimalist

mathematical interpretation that

was favored by Einstein. The

de Broglie–Bohm theory, which

developed from de Broglie’s initial

reaction to the wave equation, is

an attempt at a strictly causal,

rather than probabilistic,

explanation, and postulates the

existence of a hidden “implicate”

order to the universe. The

transactional approach involves

waves traveling both forward

and backward in time.

Perhaps the most intriguing

possibility of all, however, is one

that verges on the theological.

Working in the 1930s, Hungarian-

born mathematician John von

Neumann concluded that the

measurement problem implied

that the entire universe is subject

to an all-encompassing wave

equation known as the universal

wave function, and that it is

constantly collapsing as we

measure its various aspects.

Von Neumann’s colleague and

countryman Eugene Wigner took

the theory and expanded it to

suggest that it was not simply

interaction with large-scale

systems (as in the Copenhagen

interpretation) that caused the

wave function to collapse—it

was the presence of intelligent

consciousness itself. ■

Schrödinger’s thought experiment produces a situation
in which, according to a strict reading of the Copenhagen
interpretation, a cat is both alive and dead at the same time.

A cat inside a sealed box remains
alive as long as a radioactive source
in the box does not decay.

If the source decays, it releases

poison and the cat dies.

We must measure the

system to find out

whether the source has

decayed. Until then, we

must think of the cat as

both dead and alive.

Radioactive

material

Geiger

counter

Hammer

Poison