# Photon economy of scale

1. Jan 31, 2016

### tim9000

In my undergraduate days I remember learning about the double slit experiment etc. in quantum mechanics.

And I'm only now starting to learn about the Standard model. So there are these fields; higgs, electro-magnetic, strong and weak nuclear. So if a photon is a perturbation in the electro-magnetic field, and it sort of bounces around traveling in accordance with probability. Then take a photon that is a much longer wavelength than visible light, like a radio signal, well I'm in my car, you're in your car, and we're both listening to the same radio station, well we're both hearing the same song. Does that mean that the same photon we're listening bounced around both our antennas, or the tower projecting the radiation output many distinct photons? So in which case the theoretical maximum people that can listen to the radio station is the number of electrons in the station antenna which were jostling about?

2. Jan 31, 2016

### Orodruin

Staff Emeritus
No, you are not listening to "the same photon".

3. Jan 31, 2016

### tim9000

Yeah I started to kind of realise that when I was typing it, thanks for the clarity.
So does that mean that the theoretical maximum people that can listen to the radio station (each with their own receiver) is the number of electrons in the station antenna which were jostling about?

4. Jan 31, 2016

### mathman

There is no theoretical maximum. Wave model applies here, not particle.

5. Feb 1, 2016

### black hole 123

when ur radio receives the photon, it must "collapse and behave like a particle. OP's question is very good

i think maybe because each radio player has a minimum size, even if the earth is filled with radio players its still not enough to absorb all the photons? because if u had plank sized radio players filling the earth then im pretty sure some of them wont be able to listen to the radio. is this right?

6. Feb 1, 2016

### Staff: Mentor

This has nothing to do with the number of electrons in the antenna.

If you know the total power of the emission, you can calculate a maximal number of antennas that can pick up the signal (with sufficient clarity to make music out of it). This number will exceed the population of Earth by several orders of magnitude.

7. Feb 1, 2016

### mathman

This maximum is essentially a function of the space occupied by each receiver, and not by the number of receivers. A receiver picking up a signal does not affect the power available to a receiver at a different location.

8. Feb 1, 2016

### Staff: Mentor

It does - especially for receivers behind it. The effect is negligible for typical radio transmissions, but it is not zero.
And you cannot get an arbitrary number of receivers by increasing the distance, as you also have to increase the receiver size and efficiency then. Not relevant for typical radio transmissions on Earth, but it fundamentally limits the number of receivers you can have.

9. Feb 2, 2016

### mathman

I was thinking of receivers at the same distance from the source. Any one receiver should not effect the power at another receiver. I'll agree if one receiver is behind another, the receiver behind will have some signal loss.

10. Feb 2, 2016

### Staff: Mentor

Well, there can be an influence. But the general limit still applies: the number of receivers that can get a meaningful signal is finite. You need some minimal power (and while antennas and photons rarely mix well, here the concept is useful - you have to absorb some photons), and the overall power is limited.

11. Feb 3, 2016

### tim9000

What I was saying was that there would be a finite amount of photons emited from the antenna (such as the amount of electrons jostling about) so there must be a finite amount of receivers of the signal. I know photons travel as waves, but they mast arrive as distinct particles exciting the receiving radio.
Exactly what I was getting at.

12. Feb 3, 2016

### tim9000

I'm a bit confused, so what do you mean by meaningful signal? Do you mean like it is re-emitted from the receiver? Because I was imagining it is either received or not, not so much picking up a 'signal' as an electron is excited or it isn't. Like when you say 'power' are you talking like poynting vector or something?

13. Feb 3, 2016

### Orodruin

Staff Emeritus
You are thinking about photons in a way which is far too classical. The classical EM wave is more akin to a coherent quantum state and as such does not have a definite number of photons.

14. Feb 3, 2016

### tim9000

I'm not saying we can know the number of photons, but I very VERY much doubt more and less photons pop in and out of existence, I assume the amount emitted and the amount that make contact with matter after emission are the same number.
I don't have the best understanding/recollection of quantum mechanics, but I did take it, and I don't think my personal internal model here has conflicted with heisenberg's uncertainty principle.

15. Feb 3, 2016

### Orodruin

Staff Emeritus
You would be wrong. Again, your view is far too classical.

In order to properly understand photons you not only need an understanding of quantum mechanics, you need a (relatively advanced) understanding of quantum field theory.

16. Feb 3, 2016

### tim9000

Well I don't know how well I understand quantum field theory, I suspect not well. But when I did 'Quantum mechanics and semi conductor physics', to try to draw a similar analogy/example: So an electron has a wave function, much bigger than a photon (or something, due to having rest mass), when they fire a single electron at the double slit, it was never more than a single electron that landed at a point (according to probability) on the plate (as the probability function collapsed due to observation). I appreciate that you can't know where a particle is exactly, at a fixed instant in time, so I understand there are practical limitations to this, but theoretically speaking: So are you saying that the wave function of a photon and an electron are so vastly different that if I fire a single photon out of a laser, that is to say, if I have a single electron that I excite then emit a single photon, fired at the double slit, that more than one photon might land on the plate?

Last edited: Feb 3, 2016
17. Feb 3, 2016

### Orodruin

Staff Emeritus
The photon does not have a wave function. The quantisation of the electromagnetic field is significantly more complicated than the quantum mechanics of a single electron.

18. Feb 3, 2016

### Orodruin

Staff Emeritus
Also, a laser is a coherent phenomenon. You cannot fire a single photon and still call it a laser.

19. Feb 3, 2016

### tim9000

I find that very interesting, and surprising, because as far as I was aware the difference in speed light takes traveling through media was due to do with probability....something like when you shine a laser through a block of perspex and it slows down, because....ok I've forgotten the reason, something to do with the probability change of having to navigate through all the matter...

20. Feb 3, 2016

### tim9000

Yeah I wrote laser before I decided to make my example exciting a single electron, I do appreciate your criticism of my understanding, but that was just nit-picking :p

21. Feb 3, 2016

### Staff: Mentor

Clear enough to hear music instead of noise. There is no sharp minimal signal strength for that, but there are signal strengths where you can hear music, and signal strengths where you cannot.

Orodruin: all that is right, but does it really help here? It does not change the main point of the thread.

22. Feb 3, 2016

### Orodruin

Staff Emeritus
I think there are two separate issues here, the first being the one you address: "How many people can listen to the radio?"
The second one is the fundamental concept of how photons work, which caused the OP to display a misunderstanding in why there would be such a limitation. This is the issue I am trying to address.

23. Feb 3, 2016

### Staff: Mentor

A problem involving radio signals and antennae like this one is best understood using classical electrodynamics. The radio station is emitting electromagnetic radiation, so the electrical and magnetics fields in the region around it are oscillating. These oscillating fields induce a voltage and current in the antenna which the circuitry in the radio receiver detects and amplifies.

Quantum mechanics and photons only come into the picture because energy is transferred between an electromagnetic field (produced by the radio station) and matter (the antenna) in discrete increments so at a small enough scale you can't think in terms of a continuously varying electrical field. However, this effect is completely indetectable with an antenna so we don't worry about it, just as the weatherman reports how many centimeters of snow fell instead of counting individual snowflakes.

24. Feb 3, 2016

### Jilang

Is this not how they arrive at audience counts of radio stations? By measuring how much power the antenna requires?

25. Feb 3, 2016

### Orodruin

Staff Emeritus
No, most of the power is radiated away into oblivion.