“collapses” the wave function, so that it can no longer be everywhere, and
is instead somewhere in particular. The observation also collapses the
electron’s ability to go from here to there via all possible routes, and in-
stead selects one route from the possible gazillions.
Wheeler also proposed that nature could illustrate how counterintuitive
quantum mechanics is via a grandiose delayed-choice experiment. In-
stead of a beam splitter in a laboratory, a quasar billions of light-years
away, acting as a gravitational lens, would do what the beam splitter
does—allow the photon to come to Earth by one of two differing paths.
These paths could be focused out in space; if no photon detectors had
been placed along the paths, an interference pattern would result, and if
there were photon detectors in place, the photons would act like particles.
The counterintuitive aspect is that the photon, billions of years ago as it
passed the gravitational lens, appears to have made the “decision” to be-
have as a wave or a particle. Experiments have shown that this decision is
not made by the photon but by the universe—if an observation is made,
the photon acts as a particle; if not, it acts as a wave.
Probability Waves and Observations: A Human Example
Arcane though the idea of probability waves and observations collapsing
them may seem, there is a simple analogue that takes place annually at
every university in the country. Many students enter as undeclared ma-
jors—not certain whether their futures lie in biochemistry, business, or
something else. As a result, they take a diverse assortment of courses,
encouraged by the university’s general education policy of requiring stu-
dents to take courses in a number of disciplines. These students are like
probability waves; their as-yet-unselected majors are a probabilistic amal-
gam of biochemistry, business, and a whole bunch of other alternatives.
At some time, though, the student must select a major, usually done by
conferring with an adviser who tells the student the options available,
what the various majors require, and the career paths they allow (if the
student does not already know), and the student makes his or her choice.
This choice collapses the probability wave, and the student is now a de-
clared major.
You’re Nobody Till Somebody Observes You
A popular song from the 1950s was Dean Martin’s “You’re Nobody ‘Till
Somebody Loves You.” In quantum mechanics, you’re only a probability
wave until someone, or something, observes you. What constitutes an
52 How Math Explains the World