What happens to a photon after it is absorbed by an antenna?

I recently have read about interception of wireless information, however this mentions that people can intercept the information, and then somehow the recipient also gets the information. Regardless of this context, what happens to the actual photon if it is absorbed by one antenna how can another person receive the same signal? Is it that when the photon is absorbed exciting the electron, the electron will then leap back to the lower energy state causing it to emit another photon? so the antenna acts as a receiver & transmitter? to be honest I'm confused overall in how antennas work.

• Ordinary antennas don't work as quantum objects and there is little to be gained from trying to discuss them as such, since you would be missing 90% of the actually relevant physics. – CuriousOne Sep 11 '14 at 23:59
• This is a wise comment, CuriousOne. I like it. It's also worth mentioning that even though a phenomenon can be described without quantum mechanics doesn't mean some aspects of it can't be. – BMS Sep 12 '14 at 0:01
• That's why I made the distinction about "ordinary" antennas. One can, of course find plenty of quantum physics in there, if one wants to! By making um long antennas out of whiskers or point like quantum dots and cooling them to low temperatures, one can study quantum antennas in great detail and there is a whole cottage industry of physicists who do, but I don't think that's what the OP meant by "antenna" and that is not what happens inside classical receivers, which have to be analyzed as thermodynamic systems (otherwise the received power would be zero!). – CuriousOne Sep 12 '14 at 0:16
• Receiving antennas work because the electromagnetic wave excites EMF in the a antenna wire. It makes the electrons (or charge carriers) in the wire feel force and move. In so doing, energy is removed from the wave. When a photon of light boosts an electron from one orbit to the next, it is the same thing, but on a quantum level. – Mike Dunlavey Sep 12 '14 at 0:18
• You have to clear up in your head the concept of encoding from the concept of carrier. One encodes messages with letters on a paper pamphlet and prints a thousand of them and distributes them. If one pamphlet is destroyed that does not destroy the message in the other pamphlets. An electromagnetic wave can be the medium for encoding messages and it is as if one has printed a zillion of pamphlets. A single photon is like a letter on a pamphlet, carries no meaning or message except its absorption diminishes the intensity of the encoded part at its particular frequency and location in spacetime. – anna v Sep 12 '14 at 4:19

2 Answers

The radio waves or microwaves that are used for communication don't contain just one photon. They contain a bunch. (Maybe someone will do the math for how many photons a standard radio broadcast antenna is producing each second; it'll blow your knee-high off even if you're wearing sandals over them.)

Consider for example a frequency-modulated signal. The information is contained within the frequencies/energies of the outgoing photons. You might remove one photon, but there are many others with the same signal/frequency information also traveling in different directions that you don't intercept.

Thus, intercepting just one photon doesn't destroy these communications. But it does destroy the photon; it is absorbed by the material of the antenna and is gone.

• Well I mean if the antenna were to capture the transmission, what happens to those individual photons. If I understand what happens to an individual photon I could then generalize the transmission when the antenna is receiving the full stream. – AlanZ2223 Sep 11 '14 at 23:58
• The photon interact with electrons in the receiver and vanish; they no longer exist. Are you instead looking for a detailed discussion of how photons interact a receiving antenna (more than what I've done here)? – BMS Sep 12 '14 at 0:04
• Alright, I'll do the math. A bit of searching says radiated power record is 2.5 MW, so let's say 1 MW for a powerful emitter. A 100 MHz (FM band) photon is about 10$^{-25}$ J, so about 10$^{31}$ photons/s. – Kyle Oman Sep 12 '14 at 0:15
• @Kyle: Thanks for the number! I never bothered to calculate the photon energy for FM radio... so for 1uW of receiver power for a useful reception quality, we would still need approx. 1e19 photons/s... That's quite interesting, since it shows just how poor of a receiver a radio is... 1e19 photons per second should be good for the transmission of approx. 1e18bits/s... in reality such an FM radio doesn't do more than maybe 300kbits/s, so it underperforms the physics limit by, at least, 12 orders of magnitude! – CuriousOne Sep 12 '14 at 0:22
• @BMS if you could provide a link for an in depth discussion and theory of antennas I would appreciate it. – AlanZ2223 Sep 12 '14 at 0:57

It is a big misunderstanding that we name radio waves and photons electromagnetic radiation and this masked the different nature of this two phenomenons. A single photon or a serie of photons we get by acceleration of a electron (or the nucleus). This we call emission. A single photon travels as a single object through space until he gets absorbed by an other electron (or nucleus). Photon of a certain energy has a certain wave length and - due to the constant velocity in vacuum - frequency.

Radio waves are a composition of the photon emissions from all the accelerated electrons in the antenna rod. The frequency of the radio waves depends primarily from the generators frequency. But for equal frequencies you get - depends from the generators power and the tuning of the generators frequency with the (more or less) standing waves in the antenna rod - different energies of the radio waves. This is a substantial difference between photon emission with constant ratio of frequency and energy and wide range of this ratio for radio waves.

Even more, radio waves carry energy in packets. This is natural because the usual antenna is made as a rod and the acceleration of the electrons inside the rod is limited by the rod length. The generator has to switch the current direction to give the electrons the chance to accelerate again. Between this accelerations the energy of the radio wave is indeed zero. The photons energy cant be zero, its energy does not disapear at some points in the space. If some mathematics tell something else then it shows that the Maxwell equations have been made for radio waves, Maxwell doesn't know anything about photons.

Radio waves dissipate, a single photon never do so. A single photon can be received only by one object and this object (a electron, a nucleus or the mixed state of more then one electron etc.) has to have certain properties to receive the photon. A radio wave can be received by more than one object and the antenna rod can have any form, can be made from a broad range of materials and last not least can have a tiny length comparing to the emitter rod length. This is because the radio wave is modulated and carries a different number of photons or /and photons with different energy. The sequence of this variations make it possible to transport information from the transmitter antenna to the receiver antenna.