Re: Radio Photons...

On Wed, 20 Jun 2007, Benj wrote:

Does anyone know of any experimental evidence giving the direct
observation of radio waves consisting of "photons"?

Does anyone know of any experimental evidence giving the direct observation of visible light waves consisting of "photons"? Basically, what do you mean by "direct"?

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.

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.

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.

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.

--
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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