1. Jul 14, 2011

### afims123

Could someone point me in the right direction on how low frequency (< 3 Hz) electromagnetic radiation is detected? I tried googling it, but my searches didn't really work; perhaps I was using the wrong terms?

Another thing: what is the lowest frequency that has been detected, and what limits are there on individual low frequency photon detection as opposed to large numbers of them?

If anyone could point me to some articles or something, that would be great!

2. Jul 15, 2011

### xts

You may directly measure either magnetic or electric field. Take magnetic needdle and look how it oscillates.
If you want to measure it at 50Hz - make a solenoid, connect it to a light bulb and stand close to power line.

It really doesn't matter if the field is caused by electric current flowing in nearby conductor, or if it is flat wave where magnetic field results from oscillations of corresponding E field and vice versa. The measurement technique is the same.

> what is the lowest frequency that has been detected,
0 Hz You may easily measure static magnetic and electric fields.

> and what limits are there on individual low frequency photon detection as opposed to large numbers of them?
At least 3 limitations exist:
- Energy of the single low frequency photon is unmeasureably low;
- Size of the detector: low frequency photon has low momentum, so momentum uncertainity must also be very low, leading to large uncertainity of its position. Size of the detector must be at least comparable to $\lambda$ in order to absorb it with reasonable probability;
- Thermal noise of the background: the detector and its surroundings must be cooled to the temperature low enough to avoid obscuring the measurement by thermal radiation. Even if you cool the detector to 1K, the maximal detectable wavelengths will be of the order of few millimeters.

3. Jul 15, 2011

### Andy Resnick

Most commercial detectors seem to stop below 1 kHz:

http://www.integritydesign.com/ACmeasurement.html

but here are some possible leads-

http://vlf.stanford.edu/people/daniel-golden

4. Jul 15, 2011

### harrylin

The size of a compass needle is quite small, and then at room temperature; and still we detect the slowly changing field... What did I miss?

Thanks,
Harald

5. Jul 15, 2011

### xts

In order to move magnetic needle you must transfer some momentum $p$ to it. Just to fix attention: let's take it needs just one microNewton to be applied for 0.1s to move a needle. $p=10^{-7}kg\cdot m\cdot s^{-1}$. Single photon at 5Hz carries momentum of $p_\nu=h\nu/c=10^{-41} kg\cdot m\cdot s^{-1}$, which means you must absorb $n=p/p_\nu=10^{34}$ photons coming from the same direction. Uncertainity of position of every one of them is bigger than wavelenght $\lambda=10^{8}m$. Let us take it is 100 times bigger (we tune up that 5Hz with 1% accuracy). So uncertainity of average position of $n=10^{34}$ photons is equal to $\frac{100\lambda}{\sqrt{n}}=10^{-7}m$ - still much less than needle length ;)

The same statistical argument applies to thermal noise. It is killing if we count single photons. But as enormous number of photons are emitted/absorbed in all directions, their total effect scales not with their number, but with square root of their number. Their impact on the macroscopic needle movement is as negligible, as the one of thermal gas pressure fluctuations.

6. Jul 15, 2011

### sophiecentaur

There is also the issue of Size. In order to couple the em radiation into a radiation measuring probe, the probe needs to be as large as possible as a fraction of a wavelength. The most efficient antenna would need to be in the order of a half wavelength. This is thousands of km, even at 50Hz - the size for a sub-Hz frequency would be ridiculous. You would, therefore, need to measure E and H fields separately. Furthermore, you can't just increase the gain in order to increase sensitivity. You need to make very long term measurements in order to average out (filter) all the random fluctuations (noise and interference) that you would encounter. This would be a very specialised field of measurement, involving a lot of smart data processing, to achieve the filtering and signal extraction. You could well end up measuring next door's heating switching on and off rather than what you actually wanted to detect.
I'm not surprised you can't find a lot about it!

7. Jul 15, 2011

### xts

About 10 years ago I read an article on an idea to measure few-Hz E-M radiation coming from pulsars. That was a paper by theoretician, and I never find it quoted or responded by experimentalists.

I can't find that article (even Google scholar and arXiv search can't help...), but maybe someone of you is able to find it?

8. Jul 15, 2011

### jambaugh

Detecting ELF wavelengths and below is simplest by measuring the magnetic field component. Typical antennas will be a large ferrite core rod with many windings of wire to couple to the receiver. There are ELF antennas supposedly good down to 0.01Hz offered for sale http://www.stormwise.com/index.html" [Broken], along with circuit plans.

As far as measuring individual photons is concerned, one needs a system with a small enough energy transition to match the energy of a single photon at the desired frequency and large enough in scale to couple to the wavelength with practical strength and then cold enough that thermal fluctuations will cause fewer transitions than the detection of photons, and then a means of measuring the system to see that a transition has occurred.

Considering that the peak frequency for thermal radiation at micro-kelvin temperatures is about 50 or 60 GHz per deg K, and considering that the http://ltl.tkk.fi/wiki/LTL/World_record_in_low_temperatures" for coldest laboratory temperature is 100pico K = 10^ -10 K which has peak frequency in the range of 5 or 6 Hz. Measuring single photon events at the 10Hz range and below is very very very very difficult.

Another point. I doubt you could use the above antenna near any civilization. I suspect passing car's steel frame would perturb the Earth's magnetic field enough to drown out real signals. (But I'm guessing on that.)

Last edited by a moderator: May 5, 2017
9. Jul 15, 2011

### afims123

Thanks for all of the replies!