94
R. Barrett and P. P. Delsanto
Okay, you say, then let us see if we can find a way to detect which slit it is
passing through. Suppose we remove the photographic plate, and replace it
with two telescopes, one directed at the first slit and the other at the second.
If a photon passes through the first slit, a flash of light will be detected by
the first telescope; vice versa, a photon passing through the second slit will be
detected by the second telescope. We keep the beam intensity low enough to
ensure that only one photon is in flight at any particular moment. With this
setup, we are successful. Half of the photons are observed to pass through the
first slit and half through the second, exactly as if we were looking at a stream
of bullets.
In other words, whether the light beam behaves as a wave or a stream of
particles seems to depend on what apparatus we are placing in its passage
to detect it. Any attempt to localise which slit the photon passes through
destroys the interference pattern.
There is one further twist we can add to our thought experiment. Let us set
up an experiment similar to that of Fig. 5.4, but construct it so that we can
randomly choose whether to use a photographic plate (i.e. a wave detector)
or telescopes (photon detectors), and are able to replace one choice by the
other very quickly. We also lengthen the time of flight between the slits and
the detectors so that there is time for us to make our choice after the photon
has already passed through the slit(s) and is on its way to the detectors.
This experiment, which is known as the Wheeler Delayed Choice Experiment, has been performed for a variety of quantum particles (some as large as
whole atoms [6]) with results that destroy the last vestiges of Common Sense.
If we observe the slits with a telescope, we can detect which slit the particle
passed through; if we put the photographic plate in place, an interference
pattern will build up, which is indicative that the particle passed through
both slits. However, the particle had already passed through the slit(s) before
we made our decision which detector to use. How did the wave/particle know
which choice we were going to make before we knew it ourselves?
O, that way madness lies; let me shun that; No more of that [7].
We shall discuss the Wheeler Delayed Choice Experiment further in
Chap. 12 (Part 3).
R. Barrett and P. P. Delsanto
Okay, you say, then let us see if we can find a way to detect which slit it is
passing through. Suppose we remove the photographic plate, and replace it
with two telescopes, one directed at the first slit and the other at the second.
If a photon passes through the first slit, a flash of light will be detected by
the first telescope; vice versa, a photon passing through the second slit will be
detected by the second telescope. We keep the beam intensity low enough to
ensure that only one photon is in flight at any particular moment. With this
setup, we are successful. Half of the photons are observed to pass through the
first slit and half through the second, exactly as if we were looking at a stream
of bullets.
In other words, whether the light beam behaves as a wave or a stream of
particles seems to depend on what apparatus we are placing in its passage
to detect it. Any attempt to localise which slit the photon passes through
destroys the interference pattern.
There is one further twist we can add to our thought experiment. Let us set
up an experiment similar to that of Fig. 5.4, but construct it so that we can
randomly choose whether to use a photographic plate (i.e. a wave detector)
or telescopes (photon detectors), and are able to replace one choice by the
other very quickly. We also lengthen the time of flight between the slits and
the detectors so that there is time for us to make our choice after the photon
has already passed through the slit(s) and is on its way to the detectors.
This experiment, which is known as the Wheeler Delayed Choice Experiment, has been performed for a variety of quantum particles (some as large as
whole atoms [6]) with results that destroy the last vestiges of Common Sense.
If we observe the slits with a telescope, we can detect which slit the particle
passed through; if we put the photographic plate in place, an interference
pattern will build up, which is indicative that the particle passed through
both slits. However, the particle had already passed through the slit(s) before
we made our decision which detector to use. How did the wave/particle know
which choice we were going to make before we knew it ourselves?
O, that way madness lies; let me shun that; No more of that [7].
We shall discuss the Wheeler Delayed Choice Experiment further in
Chap. 12 (Part 3).