questions about light & radiation

They must have an interesting 3D shape in space, sort of a fuzzy ball but with bands or shells of density, tapering off with distance from the center. Maybe somebody in one of the physics groups could furnish a link to some E/H field density maps that visualize the "shape" of a photon.

Something like this?

Yup, as an opening (or a tunnel/waveguide) gets smaller, the probability of a photon getting through drops radically. But being a photon, either it all bounces off or it all squeezes through.

Somebody said that if quantum mechanics doesn't frighten you, you haven't studied it enough.

John

Something similar. Visible or near-IR laser beams have been converted into x-rays beams.

If you could make a mirror that moves near the speed of light, there would be a very large increase or decrease in a reflected EM wave's wavelength, depending on whether the mirror is moving toward or away from the beam.

Compton scattering off of relativistic electrons in an accelerator is, essentially, EM waves scattering from mirrors moving near light speed. I first heard of it being done in the mid or late 1990's.

For example see (especially the last abstract):

Regards,

Mark

Wow, that really does look like a chunk of a field.

I thought Feynman said something along the lines of if you don't see a problem understanding quantum mechanics, you don't understand it at all.

--
Al in St. Lou

Ah, I get it now. I guess I've "seen the light". Thanks. :-)

Cheers! Rich

I've missed this thread somehow.

Quantised effects apply when radiation comes from intra-atom effects and is limited to a number of fixed levels.

RF radiation in general* is not limited to such fixed levels and can occur at any frequency.

Rf radiation in general* depends upon macro current phenomena and not intra-atomic phenomena.

*masers at the frequencies of intra-atomic effects possibly excepted.

There are non-atomic ways to make EM radiation, like deflecting an electron by a magnetic field. Each generated quantum has a distinct energy/wavelength, but any wavelength can be generated by varying the parameters; there are no discrete levels here, as there are in an atom. A free-electron laser is continuously tunable.

All electromagnetic radiation is the same. It's all quantized. The only difference between AM radio quanta and gamma rays is the wavelength and corresponding energy of the photons.

John

Is there any experimental evidence for that?

Cavity QED experiments performed using microwaves, among other experiments.

--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University

Microwaves cover the frequencies of atomic effects, such as the hydrogen line, where one might expect quantum effects to appear.

What about in the medium wave band, say, from 500kHz to

1500 kHz.....how are photons detected at those frequencies and disambiguated from continuous waves?

Sure. Some of it is indirect. There are many systems where the energy states are not discrete, where the electrons occupy a continuum of energy levels, and are not limited to discrete jumps. However, the frequency of radiation emitted is still proportional to the change in energy. Quantisation of EM radiation has nothing to do with discrete atomic energy levels - that just gives you a discrete line spectrum instead of a (quantised) continuous spectrum.

The spectrum of continuous spectra emitted by hot bodies and cold bodies being the Planck spectrum is a good sign that low and high frequencies are identically quantised. 21cm radiation from an atomic source looks the same to a detector as 21cm radiation from an antenna. The Hanbury Brown-Twiss intensity interferometer works identically for RF and optical frequencies.

It's worth keeping three things in mind about all this:

(a) Photons, as described by our best description of them so far, QED, are _not_ little billiard balls. To call them "particles" is misleading, they have very little in common with our classical idea of "particle". The same equations - the Maxwell equations - describe the behaviour of photons and of classical EM radiation.

(b) There is no experimental evidence _against_ RF EM radiation being quantised. The high-photon-number limit of QED gives us the observed behaviour.

(c) What kind of theory can explain the existence of a "magic wavelength", above which you get quantisation, and below which you don't? A few items that require explanation: why do waves from both atomic sources and antennas behave the same?, since the observed wavelength of the source depends on the motion of the source relative to the observer, how does this affect the magic cut-off wavelength?, why are RF blackbody spectra Planckian?, and why is (most of) the behaviour of quantised EM radiation described by the same Maxwell equations as unquantised EM radiation, while the equations that describe the behaviour of quantised EM also describe all of the observable behaviour of low-frequency EM radiation?

--
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

I think that the disambiguation happens when it's detected. I remember reading somewhere once about the "quantum wave function", which goes in all directions, but when it's detected, the QWF collapses and the whole photon lands in the receiver, whether it be 40-meter antenna or retina. :-)

Cheers! Rich

150 - 1000 MHz

The guy who did (Fred Seeley) this ran the undergraduate physics lab while I was at grad school. It's incredibly simple and a great idea for even a high-school project. The resonator is made from aluminized house insulation panels, available at home improvement stores anywhere.

If you have a transmitter, you could do this yourself. It's a brilliant experiment.

--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University