Is the Moon there when Nobody Looks?
Testing the Copenhagen interpretation of quantum theory
Note that, according to
this idea, the procedure R occurs spontaneously in an entirely
objective way, independent of any human intervention.
for testing the Copenhagen interpretation of quantum theory is the
modification of an
which has been conducted by Leonard Mandel of the University of Rochester:
Yet even this deliberately abstract language contains some misleading
implications. One is that measurement requires direct physical
intervention. Physicists often explain the uncertainty principle in
this way: in measuring the position of a quantum entity, one
inevitably knocks it off its course, losing information about its
direction and about its phase, the relative position of its crests
Most experiments do in fact involve intrusive
measurements. For example, blocking one path or the other or moving
detectors close to the slits obviously disturbs the photon`s passage
in the two-slit experiments, as does placing a detector along one
route of the delayed-choice experiment. But an experiment done last
year (1991) by Mandel`s team at the University of Rochester shows
that a photon can be forced to switch from wavelike to particlelike
behavior by something much more subtle than direct intervention.
experiment relies on a parametric down-converter, an unusual lens
that splits a photon of a given energy into two photons whose energy
is half as great. Although the device was developed in the 1960s,
the Rochester group pioneered its use in tests of quantum mechanics.
In the experiment, a laser fires light at a beam splitter. Reflected
photons are directed to one down-converter, and transmitted photons
go to another down-converter. Each down-converter splits any photon
impinging on it into two lower-frequency photon, called the signal
and the other called the idler. The two down-converters are arranged
so that the two idler beams merge into a single beam. Mirrors steer
the overlapping idlers to one detector and the two signal beams to a
This design does not permit an observer to
tell which way any single photon went after encountering the beam
splitter. Each photon therefore goes both right and left at the beam
splitter, like a wave, and passes through both down-converters,
producing two signal wavelets and two idler wavelets. The signal
wavelets generate an interference pattern at their detector. The
pattern is revealed by gradually lengthening the distance that
signals from one down-converter must go to reach the detector. The
rate of detection then rises and falls as the crests and through of
the interfering wavelets shift in relation to each other.. going in
and out of phase.
Now comes the odd part. The signal photons and
the idler photons, once emitted by the down-converters, never again
cross paths; they proceed to their respective detectors
independently of each other. Never less, simply by blocking the path
of one set of idler photons, the researchers destroy the
interference pattern of the signal photons.
What has changed?
The answer is that the observer`s potential knowledge has changed. He
can now determine which route the signal photons took to their
detector by comparing their arrival times with those of the
remaining, unblocked idlers. The original photon can no longer go
both ways at the beam splitter, like a wave, but must either bounce
off or pass through, like a particle.
The comparison of arrival
times need not actual be performed to destroy the interference
pattern. The mere „threat“ of obtaining information about
which way the photon traveled, Mandel explains, forces it to travel
only one route. „The quantum state reflects not only what we
know about the system but what is in principle knowable,“
Information rather than direct intervention destroys wavelike behavior
in an experiment done at the University of Rochester. A laser fires
photons past a half-silvered mirror, or beam splitter, to two
down-converters, labeled 1 and 2. These convert each incident photon
into two lower-enery photons, called signals and idlers. Because the
signal detector cannot tell how the signals arrived, each signal
takes both routes, like a wave, generating an interference pattern
at the signal detector. But the pattern can be destroyed merely by
blocking idlers from down-converter 1 (dotted line). The
reason is that each signal`s path can now be retraced; simultaneous
detection of a signal an idler indicates that both came from a photon
reflected by the beam splitter into down-converter 2.
I want to know:
Does an interference pattern occur in the signal detector even if the path
from idler 1 to idler 2 has only been made distinguishable after the
signal detector has registered the photons?
In order to answer this question, another experimental construction would be better
suited than the one Mandel used. First, Mandel’s results should simply be
perceived in another manner:
By blocking the path (broken line) the interference pattern in the signal detector
is prevented. This correspond’s to the construction of Mandel’s experiment.
The difference to Mandel’s experiment is minor:
Once again, a half-silvered mirror splits a laser beam into two partial beams,
each of which strikes a parametric converter. Once again, both idlers are united,
but this time via beam splitter 2 such that after penetrating the beam splitter,
it can no longer be determined from which of the two converters (1 or 2 ) it has come.
Consequently, both signal beams penetrating beam splitter 3 must generate an
interference pattern in the signal detector.
This corresponds to Mandel’s experiment:
The path of the two idler photons was made “unrecognizable”, whereupon
the signal photons interfere with each other. If one blocks the path of the idler
photon coming from converter 1, then the interference pattern may no longer appear
in the signal detector.
But somewhat more problematic is prediction of the outcome of the experiment, if
one switches signal and idler photons in his view , more precisely as follows:
The interference pattern in the signal detector is dependent on the interference
pattern in the idler detector. According to the Copenhagen interpretation, its
existence is supposed to be just as uncertain as the death of Schrödinger’s
cat until the interference pattern was measured at the idler detector.
An interference pattern has been generated in the idler detector. From this the
conclusion emerges that the path of the two signal photons has been rendered
unrecognizable by joining them in beam splitter 2, i.e. after penetrating beam
splitters 2 and 3, it can no longer be determined from which converter a photon
registered in an idler detector has come.
Accordingly, it must come from both converters as a wave and form an interference
pattern in the signal detector, just as both idlers formed an interference
pattern in the idler detector. And, more precisely, for the very reason that no
information can be gained on which converter it came from.
On the other hand, however, no interference pattern may form in the signal
detector, if idler 1 is blocked (broken line). In that case, to wit, each signal
photo can be traced back:
Registration of an idler photon at a specific time after registration of a signal
photon indicates that both come from one photon which has taken the path from beam
splitter 1 to converter 2.
The results of the experiment are predicted differently by the two interpretations:
Copenhagen Interpretation of quantum theory:
function describes our knowledge of the world.
objective collapse / decoherence / Bohm:
Quantum theory whithout an observer.
collapse of the wave function is triggered exclusively by measurement, i.e.
by the modified knowledge of the observer.
The wave function of Schrödinger’s cat only collapses through measurement
into the states of “dead” or “alive” and not merely by means
of the photon detector in the container which triggers the cat’s killing
As long as there no measurement by a human observer, it is by no means
unchangeably certain if an interference pattern has formed in the signal detector
(SD) or not. It is rather that the wave function for this exists in a
certain superimposition of the states of “interference pattern” and
“non-interference pattern” only until someone looks
An interference pattern must form in the SD of the photon’s path
cannot be traced back (if the path from idler 1 is not blocked).
Assume that no measurement takes place on which path the photon has taken
(whether via K 1 or K 2). Then the interference pattern in the idler detector is
proof that any information about the photon’s path has been erased.
For this reason, an interference pattern must occur in the SD since no
measurement has taken place on which path the photon has taken (this is proven
by the interference pattern in the idler detector ID).
Consequently, an observer must find an interference pattern in the SD
if he looks for it there after an interference pattern has formed in the ID.
The collapse of the wave function is an objective physical event that occurs
independently of any human measurement.
In the signal detector (SD) either an interference pattern really
formed or did not really form, which means that this event obtains
and can no longer be retroactively changed.
The cat is really “dead”
or really “alive” - this is not changed by the act of measurement.
Assume that in the signal detector an interference pattern comes about. Then one
would block the path from idler 1 after the interference pattern has formed
in the SD (this can be delayed at will by appropriately selecting the
location for mirrors 1 and 2).
But in this case no interference pattern may formed in the SD,
since from the arrival times of the signal photons at the SD and of the
idler photons at the idler detector (ID) conclusions can be drawn about
the path of the photon from the beam splitter (via converter 1 or
converter 2). The arrival of an idler photon at ID at a precisely defined
time after arrival of a signal photon at SD means more precisely that both
come from one photon which has taken its path from the beam splitter via K 2.
Since in this case, no interference pattern may be present in
the SD, an interference pattern may not form in any case in the
According to Penrose, collapse of the wave function is an objective
event and can, after it happens, not be reversed after the act of measurement.
An interference pattern in the SD presupposes that the photon’s path
(via converter 1 or converter 2) is unknown and remains unknown!!!!
Now, however, the mere threat of a later measurement of the path suffices
to preclude the formation of any interference pattern in the SD. Should
one really form an interference pattern in the SD and should the experimenter
subsequently decide to measure the path of the idler photon, then in that
way the path of the signal photons would become known, for which reason the
information of an interference pattern in the SD would no longer possible.
But this is a contradiction.
Thus in no case may an interference pattern be formed in the SD,
since the signal photons, if they superimpose themselves, cannot „know“
if their path is not going to be measured after
With this, one possesses a reliable experimental criterion in order to decide
between the Copenhagen interpretation of quantum theory and the model of an
objective/real collapse of the wave function.
Either an interference pattern comes about at the SD or not.
In the first case, the Copenhagen interpretation has been definitively proven.
For then the interference pattern in the SD can be eliminated after it has
formed (i.e. the collapse of the wave function is an act of measurement and is
only triggered by it).
In the second case, on the other hand, this is proof of the objectivity of the
wave function’s collapse and of an independent reality independent of measurement.
By means of this experiment, a decision can be made about the Copenhagen
interpretation of quantum theory!