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Q & A: Radio-frequency photons

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Most recent answer: 09/14/2011
Q:
All discussions of photons in physics texts assume, for convenience of illustration, that they are at at light wave frequencies. But photons span the entire EM spectrum. Doesn't that suggest that each 7mhz photon wave packet that I transmit from my ham antenna, is a quanta 40 meters in length?
- David Werdegar (age 72)
Naperville, IL USA
A:
It's true that the wave from your antenna can be described in terms of photons. Lots of them. Each photon at that 7 MHz frequency has about 5x10-19 Joules of energy.  For each Watt of power, you're putting out about 2x1018 photons/sec. Even per each period of the wave that's about 3x1011 photons. In practice, what that large number means is that the quantum nature of the wave becomes unimportant. The quantum graininess is tiny on the scale of the fields involved.

But I think you may want some fundamental picture of how the photons play into this. That's a little tricky without studying some quantum mechanics. The sort of things one might think- that the wave is put together out of a bunch of photon parts, each with some size and shape- are false. For example, any state with a definite number of photons has an expected value of zero for its electric and magnetic fields at any time. Radio transmitters don't work that way- they're designed to put out known fields oscillating in time- so the waves they put out have only approximately defined numbers of photons.  Weird but true.

Mike W.

(published on 09/14/2011)

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