What about the lowest frequency?

[LiuKang]

Hi. How much can elektromagnetic wave be forced to aproach into zero frequency? I asked to teachers but they didn't understand or I didn't explain well.. :| For example is 0.001 hz radio wave possible? I don't ask what capable of the electronic circuits to make. I ask the capability of the electromagnetic environment in space. Regards.

 

[Ahmed Ragab]

To answer this question I suppose you need to recall what is an electromagnetic wave the first place.

I used to think of it this way, any dc current passing through a wire will result is static electric and magnetic fields. Upon any sudden change or even gradual change of this current one should expect that the static fields will change too, i.e. will vary with time, and any time varying field will naturally result in an electromagnetic wave.

It is well known and can be proved by using a DC battery of high current ( a car battery for example) and connect it to a load so that it drains high current, and then just disconnect the battery suddenly using any switch.

I guess understanding this theory will only say that an EM wave can have any frequency no matter how low it is.

The point here, is what kind of medium will this wave propagate in?

In our study of waveguide's for example we learned that there is a cut-off frequency for the waveguide depending on it's dimensions and material of direlectric stuffed inside. No frequency less than this cut-off freq. can pass through.
I guess the same thing will apply to any medium you're talking about, and if you may consider free space as a waveguide of dimensions ----> infinity, then you will find that any frequency -----> zero may propagate through it.

Salam.
===========
Ahmed M. Ragab

 

[lambtron]

The answer to your question is:

The minimum frequency that can propagate through infinite free space is zero Hertz.

This is a limit that can only be approached, however. It can never be realized in practice because although free space is enormous, it is finite.

 

[LiuKang]

Thanks for your answers.

To answer this question I suppose you need to recall what is an electromagnetic wave the first place.

You are so right. I just remembered that the electromagnetic waves (as its name) is composite of electrostatic and magnetic fields which go one after the other. The answer is in it I suppose. Thus its definition has a meaning for every frequency except 0? I only had a dream of a dc wave 8O :roll: Thanks, regards.

 

[rautio]

Some nice discussion above. Adding a little bit, it just occured to me that I have been creating an EM wave with a frequency of 1 day per cycle for the last few years!

My printer has a power supply plugged into a power strip. I turn it off at night and on in the morning. Thus the ouptut of the power supply goes from 0 V to 12 VDC and back once per day. OK, it is a square wave, not a sine wave. But all that means is that there are a lot of harmonics.

That square wave propagates from the power supply to the more-or-less well matched load, the printer, on a coaxial cable.

Now, getting it to radiate at that low frequency would be a problem. I would require a dipole about 1/2 light-day long. That is about 100 times the distance from the earth to the sun. Think I will pass on that challenge.

So, Maxwell's equations work fine at low frequency, and even at zero frequency (electro- and magneto-statics). But Maxwell's eqautions fail spectacularly for...LOW POWER!!!

How can that be? Please feel free to take a guess. When someone asks for the answer I will give it, unless of course someone posts the right answer first. (If you have seen my Maxwell presenation, you know the answer, so please don't post it in that case.)

Hint...the failure gets worse at higher frequencies.

 

[lagrange]

hi Rautio

I suppose it fails because we enter in the quantum theory (photons ..)

 

[Ahmed Ragab]

How does that happen ? Why do you say that Maxwell's equations fail at Low power and how does that occur?

 

[rautio]

Hi Lagrange -- You hit it! When you can start counting photons, Maxwell's equations fail completely. It is usually not much of a concern for us microwave types, but you can start getting into "quantum noise" for comm systems operating in the millimeter range.

Remember the double slit experiment? Shine a laser on a double slit and you get a nice interference pattern. What happens if you launch only one photon at a time through the slits? It must go through one slit or the other, right? So there should be no interference pattern, right? Let enough photons go through (one at a time) and the interference pattern is still there! How can that possibly be???

Well, maybe each photon goes through both slits at the same time. Put something in place to detect which slit each photon goes through and we verify that each photon goes through one or the other, not both. But now the interference pattern is gone!

Ahmed -- If this is interesting, get a copy of Richard Feynman's, "QED, The Strange Theory of Matter and Light". wonderful reading. From your post, I think you will really like this book.

(The above double slit experiment was actually done with electrons, but it is the same idea.)

 

[lagrange]

yes Rautio I remember the double slit experiment.

More than 20 years ago I attended a first level course in quantum mechanics.
After that I never had the time to study more in detail this subject because my job is too far from it
and now I can remember only a small portion of what I have learned at that time.

Nevertheless I remeber that the first time I encountered the double slit experiment I found it
very interesting but also very shocking. At that time I was let to think that the mathematical
formulation of the quantum theory is correct but its phisical interpretation is wrong
(or at least that something is missing from it).

Now after many years I hav'nt any new information which can make a better picture
and I am still a little bit disappointed when I try to explain the oddites of the interfering
particles (or photons).

 

[madengr]

You can also watch this excellent online Feynman lecture. It's about 6 hours total. Pretty much the same topics as QED.

**broken link removed**

It's interesting when you think of driving an antenna with the current of a single electron. I guess a single photon is emitted and the antenna pattern is really a probability distribution as to what direction that photon will go. I guess this is what is holding back quatum encryption over RF links since you can't guarantee (as much) the photon will get there as with fiber, not to mention tyring to detect a single photon from background noise.