Photons . . . ?
AES
photon concept; and the newsgroup has suddenly gone dark on the whole
subject.
I hope my remarks were not too intemperate? -- I was hoping for some
replies and continued debate.
Salmon Egg
siegman-07A9B3...@news.stanford.edu, "AES"
<sie...@stanford.edu> wrote:
Did you look at the Feynman lectures? IIRC he claims that photons are the
only real things and goes on to show how to calculate their propagation to
the extent that it could be calculated. He, was at the time, being the
contrarian.
Bill
-- Ferme le Bush
redbelly
Hello Tony.
I've re-read you're previous post. If I have the right one:
http://groups.google.com/group/sci.optics/browse_frm/thread/c7206754b05062b7/25e0c7881e15c82b?lnk=st&q=&rnum=13&hl=en#25e0c7881e15c82b
The crux of your argument seems to be that we cannot detect the
existence of a single photon and still keep it intact, therefore we
should not think of photons as "objects" or at least as "well-defined
objects".
But doing some googling gives a description of a 1999 experiment by
Serge Haroche's group where this has actually been done:
http://physicsweb.org/articles/news/3/7/11
Atoms are passed through a cavity which might contain a photon. The
photon and atom interact in a way that changes the phase information of
the atom's wavefunction without destroying the photon. The change in
phase of the atom is detected using interference techniques.
My own view is that the quantization of energy in an E-M field
necesitates the concept of photons. It's when people stretch this to
say that light can behave like a particle, and identify a photon as a
particle of light, that the disagreements arise.
If we just say a photon is a unit of energy that may be added to or
taken away from some given field configuration, and left it at that,
there wouldn't be any disagreement. But then again I could be wrong!
:-)
Mark
Phil Hobbs
I haven't shut up, I was just on vacation. You guys aren't that lucky. ;)
I've been known to count photons, and to use them for bookkeeping--it's
dramatically easier, e.g. to use conservation of momentum and energy to
show that of the beams leaving an acousto-optic cell, the one that gets
bent in the direction of acoustic propagation gets upshifted and the one
bent away gets downshifted--each photon having notionally absorbed or
emitted a phonon.
This is a useful shortcut--similar to the use of the second law of
thermodynamics to show that you can't make a solar furnace hotter than
the Sun. But you can show the same thing classically, in both cases.
The photon problem is basically ontological. What do we mean when we
say 'x exists'? Existence isn't a property of a thing--there's nothing
you can point to that merely exists, and nothing more. It's clearly the
case that the electromagnetic field is quantized; but what do we mean by
saying that the field quanta exist? Systems with at least one can
sometimes, under very specially prepared circumstances, be distinguished
from systems with none. We can detect their destruction, e.g. in
photodetection. But we can't take one out and look at it.
Existing things come in certain categories, e.g. material objects,
mental ideas, and (possibly, depending on one's philosophical
commitments) spirits. Then there are accidental properties, such as the
colour of my eyes--they could be a different colour while still
remaining eyes.
We can't study a photon in isolation, and if the system of which it is
an excitation changes, it changes as well. Thus I'd want to describe a
photon as a property, like the blueness of my eyes, rather than a thing.
On that basis, an electron is a thing.
The reason that this matters is that reasoning with concepts that aren't
well-defined is like trying to climb a mudpile--you can exert yourself
all you want, but you don't get anywhere because you can't get any
traction. The mathematical technology of physics allows us to make
correct inferences of a complexity that would otherwise be impossible,
but without care, we can lose track of what we know, how we know it, and
what it means--that is, of the connection of knowledge to human life.
The foundations of science are not themselves scientific questions.
Physicists in general make terrible philosophers--Newton and Jeans and
Schrodinger and even Einstein being cautionary examples. (I'm not an
exception either, but perhaps I can recognize my limitations.)
Cheers,
Phil Hobbs
Helpful person
It's about time that people begin to understand that it is impossible
to fully describe a photon or wave packet (or any other "particle") by
comparing it to our everyday experiences. Our environment does not
have adequate analogies.
Bits of light are described mathematically. Their properties are what
they are. If it gives one an illusion of understanding, by all means
label them how you like. But please don't try to thrust your (not you
Phil) models on everyone else.
After all, it is mathematically possible to fully describe the solar
system with all the planets and the sun revolving arround the earth.
However, it makes us feel comfortable to use simpler equations that
have the planets revolve arround the sun.
Please visit my web site at www.richardfisher.com