Despite its counter-intuitive nature, the case for wave particle
duality appears, at face value, strong and convincing: The concept was
born on twin pillars of hypothesis and experiment.
When we review the history of the problem of describing the behavior
of light, we see that the model went from corpuscular, to wave then finally,
a composite: duality.
Some important steps were:
Early on, Isaac Newton found that it was possible to model many properties
of light with ray optics and proposed that light be of a corpuscular nature.
In the 19th century, this idea was overturned because of the need to account
for the observation of diffraction and interference patterns.
From there, light was shown to be a form of electromagnetic radiation and
could be described with the electromagnetic wave theory developed by James
Clark Maxwell.
At that point, the notion of light as a wave phenomenon looked to
a closed case. However, new discoveries demanded that a corpuscular approach
be revisited.
Max Planck's model for thermal radiation required radiation to be emitted
and absorbed in discrete quantities related to a constant "h" now called
Planck's constant.
Early this century in 1905, Einstein postulated that experimental observation
of photo-electricity provided convincing evidence that light interacted
with matter as though it consisted of distinct particles. His findings
showed that light exchanges energy (e) with matter in discrete amounts
related to the frequency (f) of the light by the formula e = hf where h,
again, is Planck's constant.
Observations of atomic spectra showed atoms exchanged only particular frequencies
of light. In 1915, the Danish physicist Neils Bohr developed a simple model
of atomic states. The model involved electrons orbiting the nucleus in
stable orbits that only occurred at discrete energies. He showed that the
allowed orbits were characterized by an orbital angular momentum (L) that
satisfied the relationship L=nh/2pi . Where n is an integer and h, yet
again, is Planck's constant.
Then there was a major development in 1923, when Louis De Broglie took
the brilliant step of postulating that the relationships that held for
light could be applied to material particles. Working from relativistic
relationships regarding wavelength and momentum, he predicted a universal
relationship (for photons and matter) between a particle's momentum and
its wavelength (L) being:
L = h/P Where
h is Planck's constant and P is the momentum of the particle.
The experimental confirmation followed in 1927, when Davisson and
Germer produced what is taken to be very clear cut experimental confirmation
by detecting the diffraction of electrons by a nickel crystal and then
determining that the required wavelength matched De Broglie's prediction.
Subsequently, many more experiments with diverse particles (including whole
atoms) confirm the relationship predicted by De Broglie.
Finally, Erwin Schrödinger, starting from De Broglie's relationships,
developed his famous wave mechanics that described the states of motion
that particles adopt with respect to one another.
In an effort to reconcile the obvious difficulty in reconciling wave
and particle properties, the Danish Physicist, Neils Bohr proposed the
Copenhagen interpretation. Bohr's interpretation employs the notion of
"complemetarity" in which wave properties and particle properties were
exclusive, in that one cannot simultaneously observe both properties. To
accomodate this duality, the model invokes ad hoc requirements in that
the act of observation "determines" reality and that the "wave function"
"collapses" when the particle is observed.
If you take the model literally, then a particle that appears to be
localised in that it travels in straight lines and impacts in a very small
area (less than a millionth of a meter for a photon or an electron) also
has a distributed wave-like presence that acts in domain that extends over
many meters. Nevertheless, the concept is incorporated most current
quantum interpretations because there is a clear set of mathematical relationships
and the observations match those predictions.
For example:
-
Particles, both of matter as well as those of light (photons), always "show
up" in measurements as discrete, corpuscular entities.
Phenomena such
as the photoelectric effect show that light always delivers discrete localised
packets of energy as though it is made up of a stream of distinct particles.
-
Particles also clearly exhibit patterns of interaction and scattering that
look to be identical to the patterns that waves produce when propagating
and interacting. Beams of photons or particles (eg. electrons) produce
scattering patterns that are identical to the interference and diffraction
patterns made by travelling waves.
As far as I can tell, this way of relating to these dual types of behavior
underpins all orthodox, and most unorthodox, interpretations of quantum
theory in this one respect:
All contain the notion that particles possess wave-like properties,
or are associated with a sort of "wave function", that propagates with
the particle (light or matter) and plays an active part in their interactions.
This assumption runs deep, even within statistical interpretations of quantum
theory (along the lines of Max Born's where explicit references to "waves"
are avoided) there is still an underlying presumption that wave processes
such as interference and diffraction are the cause of the observed behaviors.
Computer generated image of a Twin slit "interference" pattern.
In the laser experiment, the general pattern looks exactly like the kind
of interference pattern that would be created if the light were made up
of waves, except for one notable feature. Waves are distributed and continuous
and it would be reasonable to expect that an image generated by waves to
be continuous. This image, on the other hand, is built up from tiny specks,
the impacts of individual photons.
When one of the slits is obscured then the fine pattern of bars disappears
and the broader background of the pattern remains as shown below. The form
of this "single slit" pattern is also exactly like the pattern made by
a wave process. The process is called diffraction, the process by
which a short segment of wave breaks up and spreads as it leaves a single
slit. Note that the twin slit pattern above is superimposed on this "single
slit" pattern when both slits are opened.
Computer generated single slit pattern
To sum up, particles/photons scatter into patterns that look exactly like
patterns produced by non-local waves and yet they always retain their local,
discrete nature when detected.
Furthermore, the "interference" patterns are not caused by one photon
going through one slit "interfering" with another going through the other
slit. The interference and diffraction patterns are a result of effects
that apply to individual particles/photons. When the beam is dimmed to
the point that photons traverse the slits one at a time, the pattern remains
unaltered.
If a wave process is responsible then it would be as
though each photon "interferes and diffracts" by, and of, itself. However,
I think that there is a viable alternative and that is, that the particles
are simply being scattered by a local quantum process that occurs as the
particles interact with the slit screen.
