Approved For Release 2000/08/08
: CIA-RDP96-00789R002200240001-1
2lZ
Physica B 151 (1988) 339-348
North-Holland, Amsterdam
TESTING SCHRODINGER’S PARADOX WITH A MICHELSON INTERFEROMETER
Evan Harris WALKER
U.S. Army Ballistic Research Laboratory , Aberdeen Proving Ground , Maryland , USA
E.C. MAY, S.J.P. SPOTTISWOODE and T. PIANTANIDA
SRI International , Menlo Park , California , USA
The
Schrodinger
paradox
points
out
that quantum
mechanics
predicts
a
linear
superposition
of
states
even
for
macroscopic objects prior to measurement. However, at the macroscopic level of ordinary objects
it has not been possible
to maintain the phase correlations needed
to demonstrate or disprove the
reality of such a superposition of states
as
opposed to the mixture of states. Without such a quantum “signature”,
this paradoxical prediction of quantum theory
would seem
to have no
testable consequences.
State
vector
collapse
in
that
case becomes
indistinguishable from
a
stochastic ensemble description.
The experiment described here provides a means for testing Schrodingers’ paradox. A Michelson interferometer is used
to
test for the presence of state superposition of a pair of shutters that are placed along the two optical arms of the
interferometer and driven by a beta decay source so that either the first shutter is open and the second closed or vice versa.
The shutters take on the role of the cat in the Schrodinger paradox.
The experiment that we discuss here has been carried out at SRI International. Under the conditions of the experiment,
the results remove the possibility of the existence of macroscopic superposition prior to observation.
1, Introduction
The Schrodinger paradox
is among the oldest
of the puzzles surrounding the interpretation of
quantum
mechanics.
Like
the
Einstein-
Podolsky-Rosen (EPR) paradox
it has engen-
dered a great deal of speculation about our basic
understanding of physical
reality. Also like the
EPR paradox, Schrodinger proposed his paradox
to point out that the
statistical interpretation of
quantum theory must
at some
level
contain
a
flaw, since it implies the reality of a linear super-
position of states even at the macroscopic level -
before observation. Moreover, just as in the case
of the EPR paradox,
the Schrodinger paradox
has long been thought of as an untestable con-
sequence of quantum theory, since
it relates to
the state of a macrosystem just before observa-
tion, a state that we know must approach asym-
ptotically
to
that given by
classical mechanics.
That
is to say, we know that the usual interfer-
ence effects by which we distinguish the presence
of state superposition in atomic processes can be
shown to be too small to observe in the case of
macroscopic
systems.
But
the development
of
Bell’s
theorem
showed
us
how
to
test
the
paradoxical implications of quantum mechanics
that had been pointed out by Einstein, Podolsky
and Rosen
in
1935.
Within
the
limits
of our
experimental setup, we have now done the same
for the Schrodinger paradox.
There
are
important
reasons
for
doing
this
experiment. All of us are quite aware of the fact
that
the
existence
of
a
linear superposition
of
states at the macroscopic level
is quite counter-
intuitive. Nevertheless, no experiment has ever
been done
that has yielded
results contrary
to
the
literal
application of quantum
theory. The
absence of superposition at the macrolevel prior
to observation has not been experimentally de-
monstrated - and
in
fact
it
has
generally been
thought
that such
a
test was not
feasible. This
has led to the developement of various interpre-
tations of quantum mechanics having to do with
the macroscopic reality of quantum states.
A second reason
for carrying out an experi-
ment of the present type
is that
it represents an
efficient way
to
search
for
the
nature of and
Approved For Release 2000/08/08
: CIA-RDP96-00789R002200240001-1
Approved For Release 2000/08/08
: CIA-RDP96-00789R002200240001-1
E.H. Walker et al.
/
Testing Schrodinger' s paradox with Michelson interferometer
340
cause of state vector collapse. We
all know that
the machinery that effects state vector collapse,
whatever
that phrase
actually means, must be
somewhere between the thing observed and the
observer.
Indeed,
this observer-observed dich-
otomy
has
become
a
rather
commonly
used
phrase in discussions of the measurement prob-
lem. But the gap between these two covers a lot
of territory. Moreover, we
also know that the
perponderant opinion
is that the transition from
the pure state to the mixed state probably takes
place at a level above that of the largest coher-
ence that
exists for the system being observed.
But to focus
all our attention at that level at this
stage of the game when we
still know so
little
about what causes state vector collapse may not
be an efficient way to explore the physics invol-
ved.
It may be a more efficient strategy to carry
out
experiments
looking
for
the
existence
of
state superposition at various levels between the
atomic level and that of the macroscopic world.
Most experiments
in
this
field
are designed
to
examine a cut in von Neumann’s chain between
the observer and
the observed
just above
the
level of the basic atomic interaction
itself. Our
experiment goes to the opposite extreme to look
for state superposition immediately prior to ob-
servation at the macroscopic level.
By doing this we are able to deal experimen-
tally
with what has come
to be
a
quite wide-
spread and popular conception of what quantum
mechanics has to say about physical reality. The
Schrodinger paradox has been used to imply an
actual “observer-observed” dichotomy exists as
a fundamental aspect of physical reality, and to
imply that the observer creates his own reality in
the act of observation.
It has been used to raise
such questions as the “Wigner’s friend” paradox
and even
to promote
speculation
that by our
observation we may be creating the Big Bang of
the
universe.
If our
experiment
does
nothing
more
than
lay
such
speculation
to
rest
it
will
have been more than worthwhile.
At
the
other
extreme,
however, we
should
recognize the
possibility
that
it
is through
this
doorway that some phenomena, heretofore not
dealt with by science, may be approached. The
existence of consciousness as a phenomenology
that
lies beyond what we as physicists mean by
distance,
mass,
electric
charge
and
the
other
constructs
of our physical equations cannot be
denied. Consciousness surely arises out of some-
thing that goes on in the brain of each of us, but
yet lies beyond its usual description as a physical
object no matter how complex.
It may be that
if
quantum mechanics does require the observer as
an
essential and
irreducible
aspect
of physical
reality,
then we may
find
its proper
scientific
description to be bound up with an understand-
ing
of how
state
vector collapse comes about.
Whatever the
likelihood
that we
will
find
evi-
dence for this in the experiment we discuss here^
it would seem to be worth the effort to look.
The
Schrodinger paradox
arises because
the
prescription for writing the general state vector
for any system
requires
that one can sum
all
possible component states for an unmeasured or
unobserved system
irrespective of the
scale
of
the system
to be observed. As
a consequence
according to Schrodinger, a cat placed in a box
rigged to release a tranquilizer (out of difference
to the SPCA - and
this writer)
if a beta decay
occurs in a specified interval of time, or not if the
beta decay does not occur, must be represented
by a state vector that is the sum of both possible
outcomes before a measurement
is made on the
system. Using Dirac’s notation
this would give
for the combined state
hP> = I^A> + l«As)
>
(1)
where the subscripts A and S refer to the awake
and
sleep
states
respectively.
Although
this
is
generally regarded to be a preposterous conclu-
sion, there exist no experiments that violate this
or any of the basic premises of quantum mech-
anics. A
definitive experiment
that would
de-
monstrate
that
such
a
superposition
of
states
does not
exist would be
a
significant
if not
a
surprising achievement. On the other hand, since
there
exist no examples
of
a
violation of the
principles
of quantum
mechanics,
it
must
be
considered
a
viable
possibility
that
quantum
mechanics
is valid here as well.
It
is usually thought that one of two possible
Approved For Release 2000/08/08
: CIA-RDP96-00789R002200240001-1
Approved For Release 2000/08/08
: CIA-RDP96-00789R002200240001-1
E.H . Walker et al.
I
Testing Schrodinger' s paradox with Michelson interferometer
341
influences causes
state vector
collapse. One of
these is that something happens during the trans-
ition from the microscopic realm of the system to
the macroscopic realm. Efforts so far to formu-
late such a suggestion have been unsuccessful -
in fact all such proposals that have been reduced
to a mathematical prescription have proved to be
wrong
because
they
have
predicted
results
at
variances
with
experiments
already
conducted.
Of course we know that on measurement state
vector collapse
will occur - or
will have occur-
red.
If we open
the
box, we
will
see
the
cat
either awake or asleep. Some few scientists have
suggested
that the
act of conscious observation
causes state vector collapse. Wigner has pointed
out that such
is a peculiar implication of quan-
tum mechanics,
but he has made no
effort
to
formulate what
this would mean,
if indeed he
takes this possibility seriously. Wheeler has poin-
ted out that Bohr specifically “rejected the term
‘consciousness’ in describing the elemental act of
observation ... he emphasized that no measure-
ment
is a measurement
until
it
is ‘brought to a
close by an irreversible act of amplification and
MIRROR
PHOTOMULTIPLIER
SCARCE
If
AMPLIFIER
II
a PULSE
if
SHAPER
NAL
SCINTILLATOR
_ji
ruin
BISTABLE MULTIVIBRATOR
8 DELAY
CIRCIUTS
T
n_r
<t^
J"
1_TL
i
'
80 MHz
RF
SWITCH
80MHz
80MHz
D C
OSCILLATOR
nr
SWITCH
PHOTOGRAPHIC
EMULSION
Fig.
1. Experimental arrangement for testing the Schrodinger paradox using a Michelson interferometer. The cesium 137 gamma
source provides the random quantum event that triggers the bistable multivibrator (flip-flop) circuit controlling the AO cells so
that in any given state one cell
is always on while the other
is always off.
Approved For Release 2000/08/08
: CIA-RDP96-00789R002200240001-1