Re: Light

RHRRC wrote...

Is light a particle or a wave?

Yes or perhaps no!
Depends entirely how you look at it. Look for a wave and you'll find a
wave
but look for particles and that's what you'll find!
A really honest physicist should admit that we don't really have a clue
but
wave/particle is the best model that we can come up with and, bizarre
though
it seems, it works!

Just for any students looking in ...... any physicist with even a mere
passing of the topic has no problem understanding the concepts of waves
and particles.
They are not exclusive but are alternatives of observation. There is no
dichotomy at all.

The wave/particle description of light is the perfect example of a
dichotomy - "Division (esp. sharply defined) into two"!

The individual concepts of waves and particles are indeed simple to grasp
but few, if any, physicists could truly and honestly claim to understand the
concept of "alternatives of observation" as you put it, and there are still
ongoing fierce debates about the physical interpretation of quantum
phenomena. Depending upon the way that you observe it light (or indeed any
matter) behaves AS a particle or AS a wave and these are extremely useful
models for describing its behaviour. However they are analogies arising
from a desperate human need to visualize mathematics and this does not imply
that light is ever actually one or the other.

According to the "Copenhagen Interpretation", with which most people have
some familiarity, light behaves either as a particle or a wave and there is
a discontinuous "collapse" of the wavefunction when its position is
"observed". However if you pursue this concept as reality you will rapidly
end up in philosophically deep water and find yourself asking questions such
as "Will light always be a wave if no-one is around to observe it?", "If a
mouse observes a photon will its wavefunction collapse?" and ultimately have
to ask whether the Universe only exists because there are intelligent beings
around to observe it.

The Copenhagen Interpretation is only one of several (at least eight)
alternative interpretations of Quantum behaviour. Unfortunately, with the
possible exception of "Quantum Decoherence" (about which theory I know very
little) they all introduce difficult to accept concepts such as collapsing
wavefunctions, non-deterministic behaviour, multiple universes and hidden
variables.

Heisenberg formulated quantum theory in the form of Matrix Mechanics which
fully described the behaviour of light etc and was extremely reluctant to be
party to any physical visualization of what is going on at the time of the
Copenhagen discussions. In fact in a letter to Bohr in 1926 he wrote:

"The more I think about the physical portion of Schrödinger's theory, the
more repulsive I find it...What Schrödinger writes about the visualizability
of his theory 'is probably not quite right,' in other words it's crap."

On the other hand Schrödinger wrote (rather more politely) in the same year:

"I knew of [Heisenberg's] theory, of course, but I felt discouraged, not to
say repelled, by the methods of transcendental algebra, which appeared
difficult to me, and by the lack of visualizability."

A famous and much quoted comment by Feynman (in 1964) suggested the ultimate
futility of trying to visualize a model of quantum mechanics:

"...I think I can safely say that nobody understands quantum mechanics. ...
Do not keep saying to yourself, if you can possibly avoid it "But how can it
be like that?" because you will go down the drain into a blind alley from
which nobody has yet escaped. Nobody knows how it can be like that."

Perhaps it's summed up in a comment attributed to Niels Bohr himself:

"If anyone says he can think about quantum problems without getting giddy,
that only shows that he has not understood the first thing about them".

So when the father of the quantum theory admitted to such difficulties in
getting to grips with the concepts "any physicist with even a mere passing
of the topic" who claims that he "has no problem understanding the concepts
of waves and particles" must be very sadly deluded!

David


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