Young's 2-slit experiment for RF/microwave and its implications

yeah, I repeated it with microwaves in a freshman physics lab (X band I think.) two little boxes with square sheet-metal horns one was a sender the other a receiver (it had a strength meter on it)

set up the source and the slits and then move the receiver through the interferance and see the peaks and nulls.

It is the foundation of interferometers and all modern aperture synthesis radio astronomy although the fringes are then sensitivity to the sky brightness distribution. The classic radio astronomy techniques have even been pushed almost up to the optical band with near IR closure phase interferometers being built at several sites.

If you listen to FM radio in the car, and had occasion to stop at a light and land in a null and had to move up a few inches to get the signal back, then you have done the experiment yourself.

Mark

What, I think, would be hard is to do it with single RF "photons".

George H.

Heinrich Hertz demonstrated the optical-like behavior of RF-range waves, in the 1880s.

Yes, but did you have the transmit power turned down enough so that only one photon was getting through at a time?

The single-photon thing is a different issue from the Young experiment. Every multielement antenna is an RF example of wave interference.

Cheers

Phil Hobbs

The single-photon thing is a different issue from the Young experiment. Every multielement antenna is an RF example of wave interference.

Cheers

Phil Hobbs

I disagree. The single-particle (photon or anything else) thing is an extension of the Young experiment, showing that subatomic thingies show both particle and wave behavior, in a hard to separate (much less understand) sort of way.

I was mostly raising the issue because of the difficulty I have wrapping my head around the notion that RF, even microwaves, comes in photons -- yet it must, for quantum theory to be correct.

Yup.

So my favorite example of a single photon RF is optical pumping. Under certain conditions, (one wavelength, circular polarization, B-field.) The probability for an atom to absorb an optical photon, can be switched from zero to "one" by changing the magnetic state of the atom... mostly the magnet state depends on the electron, orbital angular momentum and spin. (the nuclear spin gets mixed up too..) Anyway by changing the electrons Zeeman state the atom is now absorbing.

With "nice" DC magnetic fields you can follow these states down to ~kHz.

Of course it's perfectly valid to say all the quantization is in the atom and not the "photon". (Since we only use atoms to interact with light, it's hard to know for sure.)

George H.

Young and everyone else thought that his experiment proved that light did _ not_ consist of particles.

The detailed behaviour of light is very mysterious, but it's possible to go overboard. It was a radio guy (Robert Hanbury Brown) who got quantum optic s going, essentially by insisting (and later proving) that you could interf ere light from separate sources, like a superhet radio.

Cheers

Phil Hobbs

At ordinary frequencies and ordinary power densities (not much warmer than a supernova, say) photons don't interact with each other. Thus Maxwell's eq uations are the relativistically- and quantum-mechanically correct descript ion of the propagation of light. It's the interactions with matter that are quantized, and show all the weird symmetry behaviour, e.g. generation of e ntangled pairs.

We've been round this particular mulberry bush several times in the last fe w years--the net is that photons don't possess the attributes of a _thing_. They're quantized _properties_: elementary excitations of a normal mode of something else.

Cheers

Phil Hobbs

I am not sure how much multipath is comparable to the double slit problem. Sure, they both have interference patterns.

That's not young's experiment.

the single photon thing would require more elaborate aparatus and lots of liquid helium.

Oh, yes, and much much more. You can make diffraction based optics (zone plates) for the long wavelengths, quite easily. Antenna design is a well-developed application of diffraction principles.

If I had a visible-wavelength laser, and a commercial transmitter (for FM radio or TV... somewhere between 50 and 700ish MHz), could I interfere them and have anything interesting happen? What kind of thing would I shoot them both into to do that?

In theory I should come up with a slightly different color on the output side vs the input, but in practice the shift might be so little that it would be hard to tell. For instance, red light is apparently about 400-484 THz, and the radio transmitter would only be about 0.0007 THz tops. Even moving up to something like a microwave oven or Wi-Fi transmitter only gets me 0.0024 THz.

Matt Roberds

Multipath off a transmitter (or receiver) and a single large conductive wall of material is pretty close to Young's slits for radio. Strictly it is more like Lloyd's mirror to give the right man his fair credit. It is the specific case of multipath for number of paths is exactly two.

The technique was one cheap way to build early radio interferometers with one big aerial on top of a cliff facing out to sea pioneered in Australia - see:

Or in more detail one of the original 1953 papers:

There are a number of kinds of optical modulators. The easiest ones to use are acousto-optic cells, in which the laser beam gets diffracted from a tra velling acoustic wave. You can get a few degrees' scan range for an octave of tuning (generally 50-100 MHz).

They frequency-shift the beam, so you can make heterodyne interferometers. I'm building a 2-D scanning microscope for a customer that way.

Cheers

Phil Hobbs

Well, yes and yes and yes.

But I know that the two-slit, single-photon experiment has been done for visible light (which is fairly easy because you just need a Really Dark room, a Really Dim source, some decent film, and then wait a Really Long time).

I was just curious if anyone's done it with microwaves (presumably in a Really Cold room).