# Electromagnetic wave and quantum mechanics [duplicate]

I'm very new to physics. I studied and read about quantum mechanics and what the assumptions are (wave particle duality, uncertainty principle, observation, wave function collapse, etc.), but I also have been reading about electromagnetism.

1. The oscillating electromagnetic wave, why is the wave oscillating between electric and magnetic waves? Is the photon creating the electric field and then it is creating the magnetic field? How specifically does the wave works in simple terms? Why don't the photon or the electron in the wave just go outside the wave and what is holding them together to form a wave? This is more classical electromagnetic wave question.

2. When reading about quantum mechanics, there is always talk of uncertainty of finding the electron, but it seems to contradict the electromagnetic wave? The wave is in predictable places and is oscillating between electric and magnetic field (nicely)? How is the nice fluid electromagnetic radiation wave interpreted in quantum mechanics? Is there any contradiction? In other words I don't see the craziness of quantum mechanics being seen or applied to electromagnetic wave? Am I missing something?

Thank you for your answer. I'm just trying to understand the classical electrodynamics in quantum mechanical uncertainty-way. I'm looking for a simple answer for quite a newbie and any pointers for further reading of the fundamentals of electromagnetic wave being reconciled with the quantum superposition.

Quantum mechanics is the reality of things,not classical mechanics(including EM waves). Having this in mind, the photon is not a different thing from the EM wave. They are the same entities. The craziness of quantum mechanics(well one of them) is the wave-particle duality. It's not that the photon travels with the electromagnetic wave, it is the EM wave(as well as it is a photon). It has a double identity of a particle and a wave(so you can't ask why does the photon don't leave the wave).

The uncertainty comes from this duality. The photon is massless but has momentum which is defined by its wavelength. So, a wave with a perfectly defined wavelength(say f(x)=sinx - this is not a good function as experts will say,but i will use it because your background may be of high school mathematics ) is a photon with a certain value of momentum. But if the particle is also that wave, can we actually know where it is? No, its position is defined(loosely speaking as i understand you have no mathematical background of quantum mechanics) by that wave(which is spread out), so it is not well-defined(we can not say precisely that the particle is at point A for instance).
Now if you see it the other way around, if the wave is a sum of waves (superposition principle - check it in Wikipedia), we can have a wave that is a "spike" , which means that the photon's position is pretty well-defined. But if we know its position, then it means that we don't precisely know its momentum(remember that momentum has to do with the wavelength of the wave and here our wave is a sum of waves, which mean we have many different wavelengths, so no definite momentum). Thus, the uncertainty comes from the fact that you can't precisely know both position and momentum of a photon at the same time(you have a limit on how much you can know,and that limit is not a limit from our measuring devices or our experiments, we will not get better at it because nature-its laws- forbid it).

As for "why is the wave oscillating between electric and magnetic waves"(the oscillation here is between electric and magnetic fields, not waves), check https://en.wikipedia.org/wiki/Electromagnetic_radiation as it is a very good source to get a good visualization and has all the information that you might want to know at your level, and bear in mind that it is all classical explanation(in the link i mean).

NOTE: if you want to properly learn some quantum mechanics without mind-breaking maths(there are physics subjects with more difficult math, but i think at your level you should learn QM without difficult math as i see that you have gone for a qualitative approach-but you can't really learn QM qualitatively,this is why i will give you a source that also has maths in it) you should read the Feynman Lectures Volume 3 (for Quantum mechanics). I also think that there is a volume which has electromagnetic waves in it.

Sorry if i did not address everything that you asked. If i forgot something, just ask in the comment section.

• Thanks your answer. Couple of total dumb questions. Please check me on each one of these. 1. Electromagnetic wave is created by a photon, because the photon is affecting the electromagnetic field and thus creating the electromagnetic wave. 2. Photon is an actual object with direction and momentum (vector) 3. Because of wave-particle duality, we basically see wave pattern in electromagnetic wave because electron is behaving as a wave when traveling through space. Wave (up and down) behavior is central to everything so that is why we see oscillation because "wave" just exists in earth. – iefpw Aug 15 '15 at 10:32
• 1)The electromagnetic wave IS the photon..But to view the EM wave classically(which is different than quantum mechanically) see the wikipedia link that i have provided you with. The wave is essentially due to Maxwell's equations. The link provides all the information than you might want. – TheQuantumMan Aug 15 '15 at 16:42
• 2)Yes, it has both vectors(note that the uncertainty that i explained applies to each dimension separately). – TheQuantumMan Aug 15 '15 at 16:43
• 3) I did not understand what you are trying to say, although i think you did not get my point. Please read my answer again – TheQuantumMan Aug 15 '15 at 16:46

The oscillating electromagnetic wave, why is the wave oscillating between electric and magnetic waves?

I'll focus my answer on electromagnetic radiation. Light - a small part of electromagnetic spectrum - could be produced by accelerating electrons. The easiest way is to take a thin current carrying wire. The electrons inside the wire will bounce on the inner structure and will be accelerated and decelerated. During this accelerations and decelerations the electron emit photons. To be clear, the straight movement of electrons is not enough for photon emission. This emission of EM radiation is chaotic and some thing to say about the photons inner mechanism is not possible.

But there is a special case for EM radiation. In an antenna rod a electric generator periodically push and pull the electrons inside the rod. This varying electric field induces a magnetic field around the rod and surprisingly this magnetic field is able to induce the next electric field outside the rod in free space. And so on. In vacuum the spread of this radiation has the velocity of c approx. 300,000 km/s.

Coming back to the point that every EM radiation is a composition of photons from accelerated particles such like electrons, what can be concluded from radio wave? Every single photon has to have the same sequence of magnetic to electric field and back.

• Time variations of electric fields are related to spatial variations of magnetic fields. And time variations of magnetic fields are related to spatial variations of electric fields. It is not electric fields creating magnetic fields that create electric fields because it it is spatial variations that related to time variations so you need the spatial variation. – Timaeus Aug 15 '15 at 15:30
• Holger, see electromagnetic radiation on Wikipedia: "the curl operator on one side of these equations results in first-order spatial derivatives of the wave solution, while the time-derivative on the other side of the equations, which gives the other field, is first order in time”. If it was an ocean wave and you were in a canoe, E denotes the slope of your canoe and B denotes the rate of change of slope. There's aren't two waves present creating each other, just one: an electromagnetic wave. – John Duffield Aug 15 '15 at 16:45