Monday, July 2, 2007

The experimental challenge to reality

We continue with our experiment in which particles are directed at
a potential barrier but now, instead of having detectors to tell us
whether a particle has been reflected or transmitted, we have
‘mirrors’ which deflect both sets of particles towards a common
detector. There are many ways of constructing such mirrors, par ticularly if our particles are charged, e.g. if they are electrons, when
we could use suitable electric fields. For this experiment we must
also allow the particles to follow slightly different paths, which can
easily be arranged if there is some degree of variation in the initial
direction. To be specific, we suppose that the source of particles
gives a uniform distribution over some small angle. Then the final
detector must cover a region of space sufficiently large to see particles
following all possible paths. In fact, we split it into several
detectors, denoted by A, B, C, etc, so that we will be able to
observe how the particles are distributed among them. In figure 2
we give a plan of the experiment.

We now do three separate sets of experiments. For the first set
we only have the right-hand mirror. Thus only the particles that are
reflected by the barrier will be able to reach the detectors. When we
have sent N particles, where N is large, the detectors will have
flashed R times. These R flashes will have some particular distribution
among the various detectors.

Next, we repeat these experiments with the right-hand mirror
removed and the left-hand mirror in place. This time only the
transmitted particles will reach the detectors, so, when we have sent
N particles, we will have T flashes. In figure 3 (b) we show a
possible distribution of these among the same five detectors.
For our third set of experiments we have both mirrors in
position. Thus all particles, whether reflected or transmitted by the
barrier, will be detected. When N particles have been sent, there
will have been N flashes. Can we predict the distribution of these
among the various detectors? Surely, we can. We know what
happens to the transmitted particles, e.g. figure 3 (b), and also to
the reflected particles, e.g. figure 3 (a). We also know that the particles
are sent separately so they cannot collide or otherwise get in
each other’s way. We therefore expect to obtain the sum of the two
previous distributions. This is shown in figure 3 (c) for our
example. The world, however, is not in accord with this expectation.
The distribution seen when both mirrors are present is not the
sum of the distributions seen with the two mirrors separately.
Indeed, it is quite possible for some detectors to receive fewer
particles when both mirrors are present than when either one is
present. A typical possible form showing this effect is given in
figure 3 (d).
Can we understand these results? Can we understand, for
example, why there are paths for particles to reach detector B when
either mirror is present but such paths are not available if both
mirrors are present? The only possibility is that in the latter case
each individual particle ‘knows about’, i.e. is influenced by, both
mirrors. This is not compatible with the view of reality, discussed
in the previous section, in which a particle either passes through or
is reflected. On the contrary, the reality suggested by the experiments
of this section is that each particle somehow splits into two
parts, one of which is reflected by one mirror and one by the other.
Such a picture is, however, not compatible with the results of the
detector experiments in which each individual particle is seen to go
one way or the other and never to split into two particles. Thus the
simple pictures of reality suggested by these two sets of experiments
are mutually contradictory.
Clearly we should not accept this perplexing situation without
examing very carefully the steps that have led to it. The first thing
we would want to check is that the experimental results are valid,
Here I have to make an apology. Contrary to what has been implied
in the above discussion, the experiments that have been described
have not actually been done. For a variety of technical reasons no
real experiment can ever be made quite as simple as a ‘thought’
experiment. The apparent incompatibility we have met does occur
in real experiments, but the discussion there would be much more
complicated and the essential features would be harder to see. The
‘results’ of our simple experiments actually come from theory, in
particular from quantum theory, but the success of that theory in
more complicated, real, situations means that we need have no
doubt about regarding them as valid experimental results.
As another possibility for rescuing the picture of reality given in
the previous section, we might ask whether we abandoned it too
readily in the face of the evidence from the mirror experiments. On
examining the argument we see that a key step lay in the statement
that a reflected particle, for example, could not know about the
left-hand mirror. Behind this statement lay the assumption that
objects sumciently separated in space cannot influence each other.
Is this assumption true and, if so, were our mirrors sufficiently well
separated? With regard to the second question one answer is that,
according to quantum mechanics, which provided our results, the
distance is irrelevant. Perhaps more important, however, is the fact
that the irrelevance of the distance scale seems to be experimentally
supported in other situations. The only hope here, then, is to question
the assumption; maybe the belief that objects can be spatially
separated so that they no longer influence each other is false. If this
is so, then it is already a serious criticism of the normal picture of
reality, in which the idea that objects can be localised plays a
crucial role. We shall return to this topic later.
Are there any other alternatives? Certainly some rather bizarre
pggsibilities exist. The ‘decision’ to put the second mirror in place
was made prior to the experiment with two mirrors being performed.
Maybe this process somehow affected the particles used in
the experiment and hence led to the observed results. Alternatively,
it could in some way have affected the first mirror, so that the two
mirrors ‘knew about’ each other and therefore behaved differently.
Such things could be true, but they seem unlikely. We mention
them here to emphasise how completely the results we have
discussed in this chapter violate our basic concept of reality, and
also because they are, in their complexity, in stark contrast to the
elegant simplicity of the quantum theoretical description of these
experiments. It is this description that forms the topic of the next
chapter.

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