This
column is about experimental tests of the various interpretations of quantum
mechanics. The question at issue is whether we can perform experiments that can
show whether there is an "observer-created reality" as suggested by
the Copenhagen Interpretation, or a peacock's tail of rapidly branching
alternate universes, as suggested by the Many-Worlds Interpretation, or
forward-backward in time handshakes, as suggested by the Transactional
Interpretation? Until recently, I would have said that this was an impossible
task, but a new experiment has changed my view, and I now believe that the Copenhagen
and Many-Worlds Interpretations (at least as they are usually presented) have
been falsified by experiment.

The
physical theory of quantum mechanics describes the behavior of matter and energy
at the smallest distances. It has been verified by more than 70 years of
experiments, and it is trusted by working physicists and regularly used in the
fields of atomic, nuclear, and particle physics. However, quantum mechanics is
burdened by a dismaying array of alternative and mutually contradictory ways of
interpreting its mathematical formalism. These include the orthodox Copenhagen
Interpretation, the currently fashionable Many Worlds Interpretation, my own
Transactional Interpretation, and a number of others.

Many
(including me) have declared, with almost the certainty of a mathematical
theorem, that it is impossible to distinguish between quantum interpretations
with experimental tests. Reason: all interpretations describe the same
mathematical formalism, and it is the formalism that makes the experimentally
testable predictions. As it turns out, while this "theorem" is not
wrong, it does contain a significant loophole. If an interpretation is not
completely consistent with the mathematical formalism, it can be tested and
indeed falsified. As we will see, that appears to be the situation with the
Copenhagen
and Many-Worlds Interpretations, among many others, while my own Transactional
Interpretation easily survives the experimental test.

The
experiment that appears to falsify these venerable and widely trusted
interpretations of quantum mechanics is the Afshar Experiment. It is a new
quantum test, just performed last year, which demonstrates the presence of
complete interference in an unambiguous "which-way" measurement of the
passage of light photons through a pair of pinholes. But before describing the
Afshar Experiment, let us take a backward look at the Copenhagen Interpretation
and Neils Bohr's famous Principle of Complementarity.

Quantum
mechanics was first formulated independently by Erwin Schrödinger and Werner
Heisenberg in the mid-1920s. Physicists usually have a mental picture of the
underlying mechanisms within theory they are formulating, but Heisenberg had no
such picture of behavior at the atomic level. With amazing intuition and
remarkable good luck, he managed to invent a matrix-based mathematical structure
that agreed with and predicted the data from most atomic physics measurements.
On the other hand, Schrödinger did start from a definite picture in
constructing his quantum wave mechanics. Making an analogy with massless
electromagnetic waves, he constructed a similar wave equation describing
particles (e.g., electrons) with a rest mass. However, it soon was demonstrated
by Bohr and Heisenberg that while Schrödinger's mathematics was valid, his
underlying mass-wave picture was unworkable, and he was forced to abandon it.
The net result was that the new quantum mechanics was left as a theory with no
underlying picture or mechanism. Moreover, its mathematics was saying some quite
bizarre things about how matter and energy behaved at the atomic level, and
there seemed no way of explaining this behavior.

In
the Autumn of 1926, while Heisenberg was a lecturer at Bohr's Institute in Copenhagen, the two men walked the streets of the ancient city almost every day, arguing,
gesturing, and sketching pictures and equations on random scraps of paper, as
they struggled to come to grips with the puzzles and paradoxes that the quantum
formalism presented. How could an object behave as both a particle and a wave?
How could its wave description spread out in all directions, then
"collapse" to a location where it was detected like a bubble that had
been pricked. Did an electron smoothly make the transition from one atomic orbit
to another or did it undergo a "quantum jump", abruptly disappearing
from one orbit and appearing in the other? How could the occurrence of seemingly
random quantum events be predicted?

The
Copenhagen
autumn phased into winter, and no solution was found. In February of 1927, Bohr
went away on a skiing vacation, and while he was gone Heisenberg discovered a
key piece to the puzzle concealed in the mathematics of Schrödinger's wave
mechanics. When one tried to "localize" the position of an electron by
specifying its location more and more precisely, the mathematics required that
the momentum (mass times velocity) of the electron must become less localized
and more uncertain. One had to add more and more wave components with different
momentum values to make the position peak sharper. Knowledge of position and
momentum were like the two ends of a seesaw: lowering one raised the other. The
product of the uncertainties in position and momentum could not be reduced below
a lower limit, which was Planck's constant. The mathematics required that any
attempt to do so must fail. This became the essence of the Heisenberg
Uncertainty Principle, first published in early 1927.

When
Bohr returned to Copenhagen, he was presented with the new idea. At first he was skeptical, because of
problems with Heisenberg's "gamma ray microscope" example used in
the paper, but he finally convinced himself that, bad example or not, the basic idea
was correct. The Uncertainty Principle brought Bohr to a new insight into
quantum behavior. Position and momentum were "complementary," in the
sense that precise knowledge of one excluded knowledge of the other, yet they
were jointly essential for a complete description of quantum events. Bohr
extended the idea of complementary variables to energy and time and to particle
and wave behavior. One must choose either the particle mode, with localized
positions, trajectories, and energy quanta, or the wave mode, with spreading
wave functions, delocalization and interference. The Uncertainty Principle
allowed both descriptions within the same mathematical framework because each
excluded the other. Bohr's Complementarity and Heisenberg's Uncertainty,
along with the statistical interpretation of Schrödinger's wave functions and
the view of the wave function as observer knowledge were all interconnected to
form the new Copenhagen Interpretation.

In
Bohr's words: "… we are presented with a choice of either tracing the
path of the particle, or observing interference effects … we have to do with
a typical example of how the complementary phenomena appear under mutually
exclusive experimental arrangements." In the context of a two-slit welcher weg (which-way) experiment, the Principle of Complementarity
dictates "the observation of an interference pattern and the acquisition of
which-way information are mutually exclusive." By 1927 the Copenhagen
Interpretation was the big news in physics and the subject of well-attended
lectures by Bohr, Born, and Heisenberg. In the next decade, through many more
lectures and demonstrations of the effectiveness of the ideas and despite the
objections of Albert Einstein, the Copenhagen
Interpretation was canonized as the Standard Interpretation
of quantum mechanics, and it has held this somewhat shaky position ever since.

The
Afshar experiment was first performed last year by Shariar S. Afshar and
repeated while he was a Visiting Scientist at Harvard. In
a very subtle way it directly tests the
Copenhagen
assertion that the observation of an interference pattern and the acquisition
of particle path which-way information are mutually exclusive. The experiment
consists of two pinholes in an opaque sheet illuminated by a laser. The light
passing through the pinholes forms an interference pattern, a zebra-stripe set
of maxima and zeroes of light intensity that can be recorded by a digital
camera. The precise locations of the interference minimum positions, the places
where the light intensity goes to zero, are carefully measured and recorded.

Behind
the plane where the interference pattern forms, Afshar places a lens that forms
an image of each pinhole at a second plane. A light flash observed at image #1
on this plane indicates unambiguously that a photon of light has passed through
pinhole #1, and a flash at image #2 similarly indicates that the photon has
passed through pinhole #2. Observation of the photon flashes therefore provides
particle path which-way information, as described by Bohr. According to the
Copenhagen Interpretation, in this situation all wave-mode interference effects
must be excluded.

However,
at this point, Afshar introduces a new element to the experiment. He places one
or more wires at the previously measured positions of the interference minima.
In one such setup, if the wire plane is uniformly illuminated, the wires absorb
about 6% of the light. Then Afshar measures the difference in the light received
at the pinhole images with and without the wires in place.

We
are led by the Copenhagen Interpretation to expect that the positions of the
interference minima should have no particular significance, and that the wires
should intercept 6% of the light they do for uniform illumination. Similarly,
the usual form of the Many Worlds Interpretation of quantum mechanics leads us
to expect 6% interception and no interference, since a photon detected at image
#1 is in one universe while the same photon detected at image #2 is in another
universe, and since the two "worlds" are distinguished by different
physical outcomes, they should not interfere.

However,
what Afshar observes is that the amount of light intercepted by the wires is
very small, consistent with 0% interception. There are still locations of zero
intensity and the wave interference pattern is still present in the which-way
measurement. Wires that are placed at the zero-intensity locations of the
interference minima intercept no light. Thus, it appears that both the
Copenhagen Interpretation and the Many-Worlds Interpretation have been falsified
by experiment.

Does
this mean that the theory of quantum mechanics has also been falsified? No
indeed! The quantum formalism has no problem in predicting the Afshar result. A
simple quantum mechanical calculation using the standard formalism shows that
the wires should intercept only a very small fraction of the light. The problem
encountered by the
Copenhagen
and Many-Worlds Interpretations is that the Afshar Experiment has identified a
situation in which these popular interpretations of quantum mechanics are
inconsistent with the quantum formalism itself.

What
about the Transactional Interpretation, which describes each quantum process as
a handshake between a normal "offer" wave (y)
and a back-in-time advanced "confirmation" wave (y*)?
The offer waves from the laser pass through both pinholes and cancel at the
positions of the zeroes in the interference pattern. Therefore, no transactions
can form at these locations, and the wires can intercept only a very small
amount of light. Thus, the Transactional interpretation is completely consistent
with the results of the Afshar Experiment and with the quantum formalism.

Does
this mean that the
Copenhagen
and Many Worlds Interpretations, having been falsified by experiment, must be
abandoned? Does it mean that the physics community must turn to an
interpretation like the Transactional Interpretation that is consistent with the
Afshar results? Perhaps. I predict that a new generation of "Quantum
Lawyers" will begin to populate the physics literature with arguments
challenging what "is" is and claming that the wounded interpretations
never said that interference should be completely absent in a quantum which-way
measurement. And most practicing physicists who learned the Copenhagen
Interpretation at the knee of an old and beloved professor will not abandon that
mode of thinking, even if it is found to be inconsistent with the formalism and
with experiment.

But
nevertheless, the rules of the game have changed. There is a way of
distinguishing between interpretations of quantum mechanics. It will take some
time for the dust to settle, but I am confident that when it does we will have
interpretations of quantum mechanics that are on a sounder footing than the ones
presently embraced by most of the physics community.

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional
interpretation of quantum mechanics, The Quantum Handshake -
Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is
available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406
or https://www.amazon.com/dp/3319246402.

Alternate View Columns Online: Electronic reprints of 212 or more
"The Alternate View" columns by John G. Cramer published in Analog
between 1984 and the present are currently available online at: http://www.npl.washington.edu/av
.

References:

The
Copenhagen
Interpretation Neils Bohr, Nature121,
580 (1928).
Neils Bohr, in:Albert
Einstein: Philosopher-Scientist, P. A. Schlipp, Ed., Library of Living
Philosophers, Evanston, Illinois, (1949).