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
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.
(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
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.
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.
the Autumn of 1926, while Heisenberg was a lecturer at Bohr’s Institute in
Bohr returned to
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, it was canonized as the Standard Interpretation of quantum mechanics, and it has held this somewhat shaky position ever since.
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
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.
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.
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.
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
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
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.
this mean that the
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.
Neils Bohr, Nature 121, 580 (1928).
Neils Bohr, in: Albert Einstein: Philosopher-Scientist, P. A. Schlipp, Ed., Library of Living Philosophers, Evanston, Illinois, (1949).
John G. Cramer, Reviews of Modern Physics 58, 647 (1986); online at http://www.npl. washington.edu/TI
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