# Longest photon wavelength observed?

1. Oct 2, 2008

### Loren Booda

What has been the longest photon wavelength actually observed?

2. Oct 2, 2008

### vanesch

Staff Emeritus
If you mean in its classical manifestation of EM wavelength, it can be pretty long I guess (km ?).

3. Oct 3, 2008

### Staff: Mentor

150 kHz is routinely used for radio communication (Deutschlandfunk in Donebach). That gives 2 km for commercial applications. ELF used for communications with submarines is in the tens of hertz range, so we are talking tens of thousands km - practical use once again.

Last edited: Oct 3, 2008
4. Oct 3, 2008

### Phrak

How far across is the Earth? I get 16 Hz from one side to the other, following the curvature of the surface.

Any signal at about 16 Hz or less transmitted in Singapore and picked-up in Indiana could be called Coulombic or magnetic coupling--rather than a transmitted photon.

To sufficiently answer your question, we should know if any of the radio telescopes recieve signals at under 16 Hz.

Maybe I'm way off track. I don't know anything about the usual frequency bands of stuff.

5. Oct 4, 2008

### Crosson

Phrak is on the right track, according to pg 7 of Jackson 3rd edition which contains a discussion on the experimental lower bound for the mass of the photon and refers to Schuman resonances in the earth-ionosphere cavity that have frequencies as low as 8 Hz.

6. Oct 4, 2008

### RandallB

But does it say anything about what has actually been observed or detected.

7. Oct 4, 2008

### Crosson

Yes, from the context of the discussion on that page it is clear that Jackson is citing the 8 Hz Shumann resonances as the lowest frequency electromagnetic waves that have been detected to date.

8. Oct 4, 2008

### Phrak

A lot of intesting points here.

Apparently longwave radio telescopes recieve in the 150 - 280 KHz range that Borek was talking about. About one thousand miles.

Crosson. The Shumann resonances are new to me. And it begs the question, what other planetary bodies, the sun or stars radiate at a characteristic frequency about equal to their diameter? And if they radiate, how would you detect it?

But my point was that it's not conclusive to talk about recieving a photon that from a source that is less than a few wavelengths distant, so that the Shumann resonances would not be conculsive as far as the detection of propagating waves.

Last edited: Oct 4, 2008
9. Oct 4, 2008

### lightarrow

Anyway, we shouldn't mix the concept of photon with that of EM waves; having been able to detect very lof frequency EM waves, doesn't imply that we have experimentally proved the existence of photons of such frequency.

10. Oct 4, 2008

### Phrak

Why not?

11. Oct 5, 2008

### lightarrow

Because to say to have detected photons you have to show photoelectric effect, Compton effect, photon antibunching, ecc., not simply a resonance in your electronic circuit.

12. Oct 5, 2008

### RandallB

So the LONGEST so far is about 150 Khz (about 2 km).

13. Oct 5, 2008

### Count Iblis

Ok, so we should make a superconducting LC circuit with some resonance frequency omega, which has energy levels (n+1/2) hbar omega. You can connect an antenna to it and let it absorb one quantum of the electromagnetic field of frequency omega.

For this to work, the temperature of the circuit must be low enough:

$$T\lesssim \frac{\hbar\omega}{k_{\text{b}}}$$

Or we can say that:

$$\omega\gtrsim \frac{k_{\text{b}}T}{\hbar}$$

For T = 10^(-10)K, you find that the frequency is 2 Hz.

So, it would be a Tour de Force to detect single photons of a few hertz.

14. Oct 5, 2008

### Phrak

Fair enough, lightarrow. Everyone seems to have their own criterion.

15. Oct 5, 2008

### Staff: Mentor

No. As I have already posted ELF (Extra Low Frequency) signals were used for transmission. I believe I remember frequencies used were in the range of 50 Hz (sorry, I can be wrong), which gives wavelength of 6000 km.

16. Oct 5, 2008

### lightarrow

Ok, but AFAIK, also a semi-classical theory (EM wave treated classically - material treated quantistically) could explain the effect, so that effect wouldn't be a conclusive prove that EM radiation of those frequencies is made of photons.