Re: Radio Photons...
- From: "Timo A. Nieminen" <timo@xxxxxxxxxxxxxxxxx>
- Date: Mon, 25 Jun 2007 20:24:47 +1000
On Sun, 24 Jun 2007, Benj wrote:
Timo A. Nieminen wrote:
Perhaps not "direct" enough for you, but: NMR, 21 cm hydrogen line,
blackbody spectrum at low temperature (eg microwave background radiation),
sufficiently Doppler-shifted to the red visible light being identical to
radio waves.
Microwave radiation as we've discussed DOES show polarization effects
so really isn't low enough in frequency to be "direct".
So what? RF is polarised. Or are you claiming that radio waves are scalar waves? Polarisation has nothing to do with quantisation; polarisation is a classical phenomenon.
Originally
physicists thought light was waves like radio. But the photoelectric
effect showed that something weird was going on. For one thing the
energy of the "wave" was being absorbed in an extremely short time.
The energy in a continuous wave simply won't act like this.
Experimentally, it does. Quantisation doesn't mean that the _wave_ is discontinuous.
No need to
review the photo-electric data but the bottom line is that light (and
much of the microwave stuff people propose here) is generated by
energy level transitions. EM absorption also results in energy level
transitions.
Technically correct, but note well that the energy levels can be in continua. You don't need to have discrete energy levels to have quantisation - cf blackbody spectrum.
The question that arises is what if I have a RADIO
transmitter that is not using energy level transitions to create EM
waves. Is that the SAME?
Why do you think that a radio transmitter doesn't use energy level transitions? From a quantum picture, the energy level transitions are pretty much the same as free-free transitions giving rise to continuous spectra in visible light.
For EM fields created by RF sources to be somehow fundamentally different from the EM fields created by higher frequency sources would require classical electrodynamics to be fundamentally wrong. Parsimony suggests classical EM + quantisation as the simplest explanation.
What I mean is it is well known that light frequency waves are not
continuous waves in space. They are rather limited pulses of energy
both in time and space.
Why do you say this is well known, since it isn't true, at least in the
sense that you imply? OK, a physically realisable EM wave - optical or
radio frequencies - is spatially and temporally limited, but that has
essentially nothing to do with photons. Wavefunction of X != X.
No , what you say isn't true.
Oh? Do you mean that a physically realisable EM wave can be infinite in space or time? Do you mean that a wavefunction of X _is_ the same as X?
Or are you claiming that energy being exchanged between an EM field and an atom, experimentally observed to occur in < 10^-10 seconds (iirc, and that's a 20 year old number), means that low-photon-number radiation comes in discrete pulses of temporal length of less than 10^-10 seconds? If so, that's simply wrong. Get hold of a decent book on QED and read.
Wavefunctions have nothing to do with
reality. They are PROBABLITY functions. Yes probability tells you
something!
Nothing to do with reality, according to you, it seems.
But it doesn't represent something as hard and fast as an
electric or magnetic field.
Take a monochromatic EM field. What is the energy density? What is the probability of detecting a photon at a particular location? What is the definition of a wavefunction? OK, there's stuff in the literature about why the EM field can't be the wavefunction of a photon, but it clearly tells you what the wavefunction of a photon is. And conversely, photon detection, eg by a CCD, tells you stuff about an EM field.
Radio waves do NOT appear to have the
rapid energy transfer that optical transitions do. Radio waves appear
to operate in the classical sense. So my question is: is that just
apparent or are there actually differences in the two cases.
Given that we know that, if the RF wave is quantised, the quanta are too low energy to be individually observed, then there is little reason to doubt that it is just apparent. There is also the matter of more direct evidence, mentioned previously, eg NMR, 21cm line.
We observe observable effects of radio waves in the high photon number limit. They behave very much like light visible light waves in the high photon number limit. Why suspect a fundamental difference?
Consider an atomic dipole transition. The classical wave associated with
the photon is basically the same as the EM wave emitted by a short dipole
antenna. The photon - including all of the radiated energy - is absorbed
by a receiving atom in one particular direction. The ultimate is a photon
in a (unphysical) plane wave mode. The classical EM fields (effectively
the wavefunction) extend everywhere and everywhen. Look for the photon,
and find it in one spot and time. Try the physically realisable
approximation, and get a similar result. A "point-like" photon doesn't
imply a localised wavefunction.
No but does a non-localized radio wave imply the ability to create
nearly instantaneous "photon" energy transfers from the wave to the
receiver?
Why not? A non-localised visible light wave manages this just fine.
The quantum dot thing sort of implies it does if the
frequency is submillimeter. OK. But that's pretty close to light, no?
No, it isn't close to visible light. Visible light spans a mere octave; orders of magnitude different is not "pretty close".
Recommended reading: Lamb's classic "Anti-photon", App Phys B 60, 1995
iirc, and the special section on photons in Optics and Photonics News a
few years ago.
And oh yeah. Just how would one build a radio transmitter that had an
output of just ONE photon per second? :-)
Antenna and absorber surrounding it.
Well, that is the classic theory for light and that certain works in
that case. But back to the original question. IF one did that, has
anyone observed "photon" interactions (example being the reception of
energy in short periods of time, (much shorter than required for the
absoprtion of energy from the radio wave) with low frequency waves. I
don't think so. But maybe I'm wrong. Which is why I asked. The
quantum dot thing was good, but still doesn't really go far enough
IMHO.
I mean let's lay it right out here. I can generate a low frequency
radio wave with a MECHANICAL device! Is that device making photons?
Inquiring minds want to know!
Take some mechanism by which you can generate RF EM waves, or visible EM waves, or whatever frequency you want. Eg, acceleration of electrons. Consider synchrotron radiation. Perhaps there is joy to be had with Cherenkov radiation, too. Brehmsstrahlung.
So, assuming RF is quantised:
(a) explains the observations
(b) gives a unified framework for EM fields of all frequencies that is well-verified experimentally
Assuming that RF is not quantised, but visible light is:
(a) explains the observations, apart from things such as, again, NMR, 21cm line, etc
(b) demands some kind of magic transition between the two regimes
(c) Doppler shift makes (b) even more problematic.
Perhaps your anti-RF-photon stance is in part due to a lack of appreciation of how far a purely classical description of visible light can go, and how classical EM theory is used in the quantum realm? To repeat: quantised != discontinuous.
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
.
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