Quantum Erasers
The idea that photons and electrons are probability waves until they are
observed, when they become objects, has been the subject of numerous
experiments. One particular ingenious type of experiment is the quan-
tum eraser experiment, first conceived by Marlon Scully and Kai Druhl
in 2000. Going back to the baseball version of the setup, imagine that
when a photon passes one of the coaches, that coach slaps it on the back
(much like a baseball coach) with an identifying label that enables us to
tell which route the photon takes. When that happens, an observation has
clearly taken place, and the photon acts as a particle—the pattern on the
detector behind second base is the familiar two blobs that characterize
particles.
Now suppose that somehow, just as the labeled photons get to second
base, the labels are removed (exotic as this labeling and unlabeling of
photons might seem, there is a way to do it, but the details are not impor-
tant for this discussion). There is then no evidence of the labeling—the
labels have been erased (hence the term quantum eraser). With no evi-
dence as to which way the photons got to second base, the interference
pattern reemerges.
Bizarre—unquestionably. Surprising—no; this was precisely the result
predicted by Scully and Druhl. We understand what quantum mechanics
is telling us: that photons and electrons are probability waves until they
interact with the universe, and then they are particles. If we cannot deter-
mine that they have interacted with the universe—and that’s what quan-
tum erasing accomplishes—they are probability waves. Among the things
we may never know is why it is this way, and whether it could have been
another way. This is one of the long-range goals of physics: to tell us not
only the way the universe is, but why this is the only way it could be—or
if it could be some other way.
It is a measure of how far we have come technologically that the May
2 007 issue of Scientific American contains an article on how to build your
own quantum eraser.^5 It doesn’t seem very complicated—but whenever
I try to assemble something, I always seem to have parts left over (why
don’t the manufacturers ever ship the right number of parts?). I remem-
ber reading an article on how, just prior to the test of the first atomic
bomb, the physicists were concerned that the explosion might create an
ultradense state of matter known as Lee-Wick matter, the appearance of
which could (at least theoretically) result in the destruction of the uni-
verse. They convinced themselves that if that could happen, it would al-
ready have happened somewhere in the universe. However, they did not
54 How Math Explains the World