At what point does an RF photon change to DC?

[Beamin]

Since RF is a photon and DC is bumping of electrons what happens at the transition at really low frequencies? If you have something at 1hz its a photon with very long wave length but what happens at 0.1hz? 0.00001hz? or one cycle for 13.8 billion years? Technically its not DC and therefore must be a photon?

[IanB]

You ask some really strange questions. You said: "Since RF is a photon and DC is bumping of electrons" -- but since this statement is inaccurate, the rest of the question really doesn't follow.

[Beamin]

You ask some really strange questions. You said: "Since RF is a photon and DC is bumping of electrons" -- but since this statement is inaccurate, the rest of the question really doesn't follow.

Its more of a thought experiment. Whats the lowest frequency you can have before it turns into DC? DC is electrons jumping from atom to atom down a wire. Put AC down that same wire and those jumping electrons start to give off photons. DC doesn't give off photons. So where is the transition? One the other end the highest you can oscillate a photon would be one plank length very high energy gamma rays that would probably have other properties at that energy. .

[IanB]

DC doesn't give off photons.

What about the filament of an incandescent light bulb?

[Doc Daneeka]

Our 'laws' for electricity eg at DC are just one model for what's happening. Models have a range of conditions where they are valid and elsewhere they aren't. You're kind of asking where does the model of electromagnetism as photons stop being valid and our DC model valid? It's not clear cut though. You can always model even ideal DC as photons if you take into account every electron - when they 'bump' into eachother they have a 'force' on each other by exchanging photons.

Don't take models literally, and also don't try to make models from analogy!

Our models of photons are accurate everywhere all the time (as far as we know so far). Our model for DC is only accurate when we think of current as a current density etc (not even considering individual electrons - we imagine current density as the averge result of a huge number of drifing electrons but beyond that we have noting to say about it - thats where the analogy breaks down)

It's not as if one model stops and another starts - one is a refinement of the other

[Beamin]

DC doesn't give off photons.

What about the filament of an incandescent light bulb?

That would be black body radiation from heat. So not generated by oscillations of AC current flowing and giving off " EMF waves". 

[helius]

All electrical and magnetic phenomena involve the action of photons, as they are the force carriers of the electric field. The radiation emitted from a bulb and from a long-wave antenna are the same: the only difference is that the photons from the light bulb have greater momentum, as it is proportional to their wave frequency. The current oscillations are still causing their emission in the light bulb, but instead of a radio-frequency, they oscillate at hundreds of terahertz—not from the input voltage, but from thermal motion.

Another way of answering is to pose a different question: at what point does an RF photon become light? And the answer is that there is a transitional band of energy with properties of both radio and light, called millimeter waves. At the low-frequency end, 0 Hz is a limit that photons can approach but never quite reach, as they would, by the Planck-Einstein relation, have zero energy, which is not allowed. The slogan of ultra-wide-band techniques, "DC To Daylight", must be understood as having DC as an exclusive, not inclusive, lower bound.

[T3sl4co1l]

Wave-particle fallacy.

A photon cannot have zero frequency; it would have zero energy and therefore would not exist.

But zero frequency can only exist for all time.

The known universe has only existed for a few attohertz.

There is no such thing as strictly zero-Hz DC.

No, DC is a convention, referring to the lowest frequencies in a circuit, usually the frequency band that's applying device bias.  Often, this band is exclusive of the AC signal, which lies in the middle (pass) band.  But often they overlap, as you can have DC coupled amplifiers which are therefore biased with overlapping ("DC") frequencies.



Tim

[Brumby]

Our 'laws' for electricity eg at DC are just one model for what's happening. Models have a range of conditions where they are valid and elsewhere they aren't.

This is the closest anyone has come (in my mind) to addressing the fundamental issue of the question.

You're kind of asking where does the model of electromagnetism as photons stop being valid and our DC model valid?

I wouldn't say "kind of".  I would say that it is exactly what is being asked.

To take this question further, I would be travelling down a path that heads towards quantum mechanics and field theory.  If we then consider that what we call a photon is actually a localised disturbance in the electric and magnetic fields, then the limitations of the macro models that we use everyday become more apparent.

[Brumby]

The definition of "DC" is also something to look at.

If my computer is on 12 hours a day and off 12 hours a day - is the 12V rail (1) DC .... or (2) a sorry looking AC square wave with 6V offset and a frequency of 11.574µHz ?

[tom66]

The definition of "DC" is also something to look at.

If my computer is on 12 hours a day and off 12 hours a day - is the 12V rail (1) DC .... or (2) a sorry looking AC square wave with 6V offset and a frequency of 11.574µHz ?

It's a sorry looking square wave but we round that down to DC for most practical purposes.

I think when you start describing the on time of a square wave in seconds then frequency is not a useful measurement any more.  I would also describe it as DC that changes slowly, even though that's a contradiction.

[T3sl4co1l]

This is exactly why "DC" is contextual.  A seismic wave filter might be quite low frequency, whereas an RF amp might call 100s of MHz "DC".

Tim

[EEVblog]

Our model for DC is only accurate when we think of current as a current density etc (not even considering individual electrons - we imagine current density as the averge result of a huge number of drifing electrons but beyond that we have noting to say about it - thats where the analogy breaks down)

You could also view it as the lowest "AC" frequencies are just slight variations in that average result.

[Vtile]

DC is iquilibrium state. Every time you turn imaginary switch or disturb the equilibrium with outside force (ie. by moving the object in earth magnetic field or keeping it in place, but moon disturbs the magnetic equilibrium) or any other form be it hard or soft turn on or off the equilibrium is lost for x amount of time as you introduce a transition state of dV/dt or dI/dt. We also do consider that in this non-equilibrium state it is possible to have DC where energy is constant, moving in one direction and superimposed AC state mixed to it (AC or transition is working against and/or with the direction of steady energy flow). That kind of situation is ie. rectified sinewave that many times is described as pulsating or variable direct current. In reality the equilibrium is constantly disturbed, but in such a small scale that only physicist (or voltnuts) will worry about it

Also to be noted that frequenzy is independent from amplitude.

... Just kind of opening a bit of the swamp of terminology. Not trying to go to particle physics.

[Z80]

Your original question is a little poorly worded, but I think you are asking what is the lowest frequency electromagnetic wave is it possible to have which is an interesting question!  Well as said already not zero, so how low can you go in photon energy?  Unfortunately I'm not a nobel prize winning physicist, but a quick web search would suggest that there is no lower limit to photon energy emitted by a free electron, so infinitely low (but not zero) would be my guess.

[Vtile]

Interesting indeed. Also might be good to remember that all our units which we use to describe your surroundings are artificial, but scientifically agreed statics for now. In a few hundred years ago we still described the world with different kind of elements air and fire etc. and that were the scientific truth then. No one can guarantee that there will not be any further revolutions in those, when our general knowledge and understanding evolves. Scientific truth is per se only a social norm that also happens to allow predictions with that socially accepted norm.

edit. sorry I'm distracting  this to somewhere at direction of philosophy.

[Kleinstein]

Using the photon picture is implying a quantum mechanical view. Using the more like DC and also classical RF description is using classical electrodynamics.  AFAIK classical electrodynamics is compatible with quantum mechanics. So the classical electrodynamics is just a good approximation, that usually works well at low frequencies and allows for a much easier calculation. QM is supposed to be more accurate, but the math is so complicated that it is not really practical in many cases and would thus need approximate calculations. Classical theories can be seen as such approximations, though they are historically older.

However there can be exotic cases where even rather low frequency EM field should be treated in a QM way. One such example is "zero" field NMR - here a proton interacts with the earths magnetic field at radio frequencies in the low kHz range.

[Yansi]

The question should be: And when does the photon turn into photonicinduction?   

[hermit]

When the energy level drops below what is required to keep it at the speed of light.

Yes I made that up, but I like it and I'm sticking to it.   

Code: [Select]

      e? +  e+ ? ? +  ?


 

[CopperCone]

What is the lowest energy photon in free space?

During the transition from the hot compressed universe to a more relaxed one, what happened when the first large photons began to form? Are they on the edge of the boundaries of the universe? I.e. they gotta be old due to the lack of large radiators in the contemporary  universe compared to what could have been during the denser state.

A upper bound on wavelength might be the comparable to the dimensions of the universe by a few orders of magnitude?

During the hot dense phase, was there metastable low frequency radiators present ?

What are the largest ELF radiators present in the current universe?

Or is it the opposite, with almost no ELF forming in the early stages due to extreme temperature? I thought maybe as it cooled you might have some kind of massive lighting strikes inside of it.

[helius]

The photons emitted when the universe cooled from a plasma to its present state are what is called the microwave background. They are everywhere in the universe, not "on the edge", because the universe has no boundary. As light-speed particles, photons do not age: none are "older" than any others, although they may have traveled a long distance, emitted far in the past.

Electromagnetic waves do have a minimum frequency: inherent oscillations of the electrons in a plasma block lower-frequency waves from propagating. This is the reason that low frequency radiation cannot be detected from the Earth's surface, as the ionosphere oscillates at around 10 MHz. There is also plasma in the interstellar medium, which is much less dense. It oscillates at around 300 Hz: ELF waves below this frequency cannot travel far through the universe.

[CopperCone]

oh, so because the early universe in some duration was plasma, we can rule out it making ELF? I figured CMBR had a bell curve distribution focused around the microwave spectrum.

are there any cosmic sources of ELF? have any measurements been made?

[T3sl4co1l]

Try more basic, more conceptual, than that:

For a ~DC photon to be a reasonable concept, there must be a persistent bias or structure in the universe giving rise to it, for the better part of the age of the universe.

It also must be, not just that there was a source in the past, and will be a sink arbitrarily in the future, but that it's still present and ongoing, or at least was very recently (~billions of years).

Such a field could manifest as an electromagnetic bias in the universe.  It could be as basic as the cosmological constant, or dark energy: a pressure that permeates space, not really interacting with anything directly, but indirectly through its energy density, its effect on spacetime curvature.

That does assume some sort of origin for such a phenomenon, like charge imbalance.  Which is exceedingly unlikely to be the case (all observations point to a damned neutral universe).

But again, wave-particle fallacy.

The photon particle is just the quantized manifestation of the EM field.  It is the Fourier transform (frequency domain) representation of the transient waveform.  The definition of the FT is frequencies (sine waves) that exist for all times.  There is nothing necessarily causal or realistic about frequency domain, it's just another tool we use to work with these sorts of problems.  (And, as it turns out, most physical processes exhibit some sort of frequency dependency, or explicit frequency levels -- energy levels -- as a result of quantization.  So it is a very useful tool indeed!)

Speaking about frequencies over time is tricky at best.  It's much better to fall back to a transient model in that case.  The Schroedinger equation is just a differential equation, meaning, for given boundary conditions, it has some sort of solution; and if the boundary conditions are suitable, then that solution has quantized energy levels, and therefore frequencies and wavelengths.  You don't need to solve it in this way.  You can solve it the same as any other difference equation, like SPICE does: by integrating a small timestep at a time.  On the upside, you can solve for chaotic and non-analytic* conditions: you just keep on stepping, until you divide by zero or something!

*Though, they might be divergent, and attempts to make those divergent forms converge, may not be physically realistic.  (Or maybe they'll be too realistic**.)  Note that chaotic behavior is bounded, like a sine wave, and tends to be cyclical, but, the cycle rate is unbounded, so it cannot be analyzed with a mere Fourier transform.

**QED is solved analytically (and at a deeper level, not for given boundary conditions), so it's not by analogy with a transient solution.  But it does encounter infinite divergent sums.  These can be forced through (the series of partial sums diverges, but the infinite series can still be assigned a finite value, using renormalization), with the result being the most accurate physical theory we know to date.  So, that's fun, huh?

So the takeaway point is this: photons are just another conceptual tool, like the FT.  They are not a useful tool at very low frequencies.  They're not inapplicable, but trying to force meaning from such a view is a stretch at best.  One must always keep in mind that the physical laws must be consistent.  If classical E&M suffices to explain an observed field, then one gains little insight from repeating the same exercise in a higher (quantum) domain.

Tim

[Beamin]

Try more basic, more conceptual, than that:

For a ~DC photon to be a reasonable concept, there must be a persistent bias or structure in the universe giving rise to it, for the better part of the age of the universe.

It also must be, not just that there was a source in the past, and will be a sink arbitrarily in the future, but that it's still present and ongoing, or at least was very recently (~billions of years).

Such a field could manifest as an electromagnetic bias in the universe.  It could be as basic as the cosmological constant, or dark energy: a pressure that permeates space, not really interacting with anything directly, but indirectly through its energy density, its effect on spacetime curvature.

That does assume some sort of origin for such a phenomenon, like charge imbalance.  Which is exceedingly unlikely to be the case (all observations point to a damned neutral universe).

But again, wave-particle fallacy.

The photon particle is just the quantized manifestation of the EM field.  It is the Fourier transform (frequency domain) representation of the transient waveform.  The definition of the FT is frequencies (sine waves) that exist for all times.  There is nothing necessarily causal or realistic about frequency domain, it's just another tool we use to work with these sorts of problems.  (And, as it turns out, most physical processes exhibit some sort of frequency dependency, or explicit frequency levels -- energy levels -- as a result of quantization.  So it is a very useful tool indeed!)

Speaking about frequencies over time is tricky at best.  It's much better to fall back to a transient model in that case.  The Schroedinger equation is just a differential equation, meaning, for given boundary conditions, it has some sort of solution; and if the boundary conditions are suitable, then that solution has quantized energy levels, and therefore frequencies and wavelengths.  You don't need to solve it in this way.  You can solve it the same as any other difference equation, like SPICE does: by integrating a small timestep at a time.  On the upside, you can solve for chaotic and non-analytic* conditions: you just keep on stepping, until you divide by zero or something!

*Though, they might be divergent, and attempts to make those divergent forms converge, may not be physically realistic.  (Or maybe they'll be too realistic**.)  Note that chaotic behavior is bounded, like a sine wave, and tends to be cyclical, but, the cycle rate is unbounded, so it cannot be analyzed with a mere Fourier transform.

**QED is solved analytically (and at a deeper level, not for given boundary conditions), so it's not by analogy with a transient solution.  But it does encounter infinite divergent sums.  These can be forced through (the series of partial sums diverges, but the infinite series can still be assigned a finite value, using renormalization), with the result being the most accurate physical theory we know to date.  So, that's fun, huh?

So the takeaway point is this: photons are just another conceptual tool, like the FT.  They are not a useful tool at very low frequencies.  They're not inapplicable, but trying to force meaning from such a view is a stretch at best.  One must always keep in mind that the physical laws must be consistent.  If classical E&M suffices to explain an observed field, then one gains little insight from repeating the same exercise in a higher (quantum) domain.

Tim

I see where you are coming from but in my question the photon is a real wave/particle "thing" moving at the speed of light with a certain energy/wavelength and existing for some time (our time since it doesn't experience time). So when we slow the oscillation down to 1Hz a photon(s) is (are) given off. When we slow down to once an hour are there photons continually given off like a beam of photons? Or do the photons only manifest the instance the polarity switches? Also if the Hz is low enough how would the photons know they are really slow AC and to be given off? Like you have a 0.0000001Hz wave and you decide to shut off the experiment before the first oscillation then its just DC. So are the photons given off when it switches? How does the first photon of the first cycle know what wavelength to oscillate at? It can't just be some arbitrary wavelength it has to be 300,000,000,000,000 meters long (I might be off by a few zeros). But the second oscillation hasn't happened and may never happen. My brain experiment is breaking down without more knowledge about photon emission.   

[Brumby]

The point being made is that considering a photon to be a real "thing", discrete and self-contained, becomes a problem when you start stepping into the territory that you are trying to explore.

The concept of a photon as we commonly understand it is a useful tool that works pretty well in the day-to-day world.  It is, essentially, a "macroscopic" equivalent that allows for handling the phenomenon in practical situations, a fact further enhanced by some maths that support and describe related interactions.

What this concept does not do well is explain the actual physical mechanism involved.  The best explanation that I have encountered is the one I mentioned before - that a photon is a localised disturbance in the E&M fields.

I found this graphic - which shows how I visualise this (Yes, it's only a 2D "field".  It's not showing both the Electric and Magnetic components and the progress does show signs of dispersion.  It's not perfect, but it's the best I've found so far, OK?)


I equate the wavelength of the photon to the width of the disturbance (wave).  If you follow this down the path of longer and longer wavelengths, you will end up with very broad changes that seem almost smooth - but there is no true transition, no discrete change that can be pointed to.  This model allows for the frequency to get infinitely lower - and we can get to the point of asking ... well, what is DC?  This question has already been addressed, so I won't go into that again - but in terms of the above graphic, DC could be seen as the level of the field being raised across a broad region.

Now, this level may go up or down in time - but if it does, it's not going to do it across the entire field at the same time (remember, this field extends through the entire universe), so there will be some "localised" nature in those changes (even if that "locality" is the size of a planet).  Here we start getting back into the photon mindset - but, again, there is no hard and fast definition as to when this "change" occurs.


The best answer I can give to the original question is that the only way to provide any sort of an answer is to define an arbitrary point where the change occurs.

There are two major problems with doing that, though.  The first is that there will never be universal consensus on where that line should be drawn.  There is no physics which makes for a natural point of differentiation.

The second is - that such a differentiation does not reflect any real distinction between what physically happens on one side of any such line and what happens on the other.  Since this is the direction I felt was coming from your question, then all I can say is ... there is no answer - other than an arbitrary one.