Re: Let's fight

Windy wrote:
NashtOn wrote:
Light is comprised of photons whether you want to treat them as waves or
particles in your calculations. In the case of vision I suspect they
would necessarily be viewed as particles.

No, it's not. When it behaves exclusively as a wave, there are no
photons involved.

In general there are two kinds of waves that
must be distinguished when one talks about
the quantum mechanics of the electromagnetic
field. First there are probability waves, corresponding
to the many photon wavefunction. Second, there
may be classical waves in some limits, corresponding
to the field strengths in the classical Maxwell
equations.

We know that the quantum theory underlies
the classical theory, and that the classical
theory is a limit, in some appropriate sense,
of the quantum theory.

So it is reasonable to say that there are still photons
involved, even when a purely wavelike description
of some optical phenomenon (based for example
on the classical Maxwell equations) is quite
adequate for the treatment of some particular
optical phenomenon in question.

The underlying, second quantized theory describing
the behaviour of light is called quantum electrodynamics.

Quantum electrodynamics can be arrived at by starting
from the Maxwell equations for the classical electromagnetic
field. One then couples the electromagnetic field to the
electron field, using the minimal coupling prescription.

The electron fields have spin 1/2, which at the
classical level means that they are four component
spinor fields.

One then uses some procedure to quantize the field
theory, for example canonical quantization can be
employed, with a particular choice of gauge. Gauge
fixing and gauge invariance of the final quantum
theory are extremely technical issues, but the
upshot is that they can be preserved provided
proper care is taken.

Having done all of that, it emerges that the
basic physical degrees of freedom entering into
the quantum theory of light are two basic kinds
of particles: photons and electrons. For the
most part, physical situations of interest can
be described with only small corrections to this
picture, which are handled by a perturbative
expansion. There are some exceptions, however,
which are not strictly perturbative, such as positronium,
which is a bound state of electron and positron
that is predicted by the theory. So positroniums
are in a sense also a degree of freedom of the
underlying quantum theory. But in most situations
of interest in biological phenomena, there will
be no positrons present, so this is not a major
bar to the description of everything in terms
of photons and electrons.

Of course, one also needs to add in neutrons
and protons bound into nuclei to be able include
atoms and molecules in the quantum theory.

The description of the binding of nuclei is beyond
the scope of quantum electrodynamics, but
again this is not a major bar to the description
using electrons, photons, nuclei, and atoms (bound
states of nuclei and electrons), and molecules
(bound states of atoms) as the basic degrees of
freedom that are important for low energy physics.

The field theory then, in effect, allows you
in principle to calculate what the overall
wavefunction is for a set of interacting photons,
electrons, atoms and, in principle, molecules too.

Note that the word `wave' here refers to a wave
of probability ... not to a classical wave such as
is described in the classical Maxwell equations.

In an appropriate limit, however, it is possible to
show that a quantum wavefunction (a coherent state)
can be constructed for a large number of photons that
in effect behaves as a classical wave obeying the
Maxwell equations for the field strengths. And thus
it is possible to say that there are photons involved
even in cases where light behaves as a wave. They
are not directly observed, but theoretically they
are present.

Here, of course, the word wave is used in a quite
different sense. It is not a probability wave.

Now in the case of a phenomenon involving
a single photon, of course, you are not in
a classical limit.

In this case a photon theoretically can be
said to behave as a *probability* wave, before
it is observed, and as a particle when it is
observed. So, for example, interference
phenomena can indeed be observed using
single photons.

This is the wave-particle duality that you're
referring to, and which confuses the issue.

Where you seen to have a difficult time is defining a
natural phenomenon by the manner in which it behaves.
As for vision and particles, the receptors in your retina react to
light. Light can behave as a wave *and* a particle simultaneously, or it
can behave as a wave or a photon exclusively.
What depolarizes the receptors in a retina is energy. Last time I
checked, energy can also be carried via waves.

Last time I checked, the retina only detected a very narrow spectrum of
radiation and not "energy waves" in general.


True.

But the whole retina can be used to sense phenomena
of the classical interference of light in some special
situations ... like for example, when you stare at a
bright streetlight at night with your eyes nearly closed.

If you look carefully, you can sometimes see crossed
horizontal bands of light extending far from the
streetlight itself which exhibit periodic bright
and dark fringes (at least I have observed this
on many occasions ;->).

I think this phenomenon may be due to diffraction
off of the eyelashes. Another case is when you
see the speckled effect in the case that laser light is
reflected off of something into your eyes. The very
high coherence of laser light makes it possible for
your eye to directly sense the spatial interference
pattern set up by the laser.

If vision could not be thought of as detection of particles, it would
not make any sense to ask "Can the human eye detect a single photon?".


Absolutely right.

http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

OTOH we say that human eyes can detect certain "wavelengths" of light,
but these can also be thought of as energy levels of photons. The
signal is primarily triggered by the absorption of light (a photon) by
the rhodopsin molecule, "depolarization of receptors" follows later, if
by that you mean the signal transduction of nerve cells.

Possibly of interest:
"How photons start vision"
http://www.cellbio.wustl.edu/faculty/huettner/baylor.pdf


The physics of vision is a fascinating subject.

Cheers!

David

.

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