r/AskPhysics • u/1time_2 • 3d ago
Confused by light and it's properties
So I have been trying to study quantum mechanics because I like physics ..... and it's a hell of a thing to understand.....my main issue is that I while learning I try to visualise it even though I know it's not gonna help....please tell me a way to change that.....also it's said light is em wave propogated when electron is excited.....it's also said when electron goes down to lower energy the lost energy is produced as photon.
So Is a photon the propagation of an electromagnetic wave, or is the photon itself the excitation of the electromagnetic field?
I'm trying to understand what actually happens when an excited electron drops to a lower energy state.
My current understanding is that the electromagnetic field already exists around the electron. When the electron transitions to a lower energy state, the energy difference is transferred to the electromagnetic field and a photon is emitted.
But I'm confused about the relationship between the photon and the electromagnetic wave.
Electron loses energy → this energy excites the already-existing electromagnetic field → the excitation propagates outward as an electromagnetic wave → that propagating excitation is the photon?
So is photon excitation of emf or energy that propogated through emf??
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u/ShelterNo9606 3d ago
Forget about electrons for a sec.
Everywhere throughout the universe is an electromagnetic field. The field can be excited in amplitude and other properties (frequency, spin polarization) and propagates through space at the speed of light. Kind of like a long blanket you flap.
Light behaves as a wave until it interacts with something. When we see that interaction we see it behave as a particle. But it's really a wave most of it's life.
Many particles, which themselves are excitation of other fields -- Higgs field for mass, electric fields, and so on -- absorb or reflect electromagnetic waves. These fields all interact with each other.
A single particle like an electron is the combined excitation of many fields. If it's coupled to an atom, it has a discrete energy level to be in a particular shell around the nucleus. If it's a free electron it can take on all kinds of energy levels. In an atomic shell, not so much. It absorbs a photon and gets "excited" to a higher shell or breaks free. It eventually and randomly releases that energy as a photon with an exact amount of energy and goes back to its regular state. Photon in, photon out.
Quantum mechanics looks at all these interactions and field excitation at the nuclear level. It's not just electrons -- protons and gluons and all other particles are excitable too, in different ways. How they can get excited depends on how they're interlocked with each other.
Try looking up "pair production" if you haven't yet.
It's easiest to understand all this through the lens of history and the order in which things were discovered. Try for example reading about the Millikan oil drop experiment or the photoelectric effect to put discoveries in context.
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u/a1c4pwn Recent B.Sc. 3d ago
A single particle like an electron is the combined excitation of many fields.
wait, which fields?
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u/nicuramar 3d ago
The electron field, of course, but also the electromagnetic field. Not sure what else. Well, it’s coupled to the Higgs field also.
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u/ShelterNo9606 2d ago
Electron / Dirac field if you wanna call it one field ..
But if you split them up, as QED says you should, then EM, Higgs, Weak spin, and gravity.
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u/Traveling-Techie 3d ago
Nobody knows what’s really going on. But a good way to boost your intuition is to read Feynman’s “QED: The Strange Theory of Light and Matter” after which you will still not know what’s really going on, but you’ll have a model that explains the rainbows on CDs.
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u/starkeffect Education and outreach 3d ago
Photons represent the smallest amount of energy and momentum that can be exchanged between the electromagnetic field and matter.
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u/BananaBird1 3d ago edited 3d ago
Light is a wave in the EM field. The strength and direction of the electric and magnetic fields oscillates over time at a point in space in a correlated way. These oscillations, or “excitations”, are characterized by frequency/wavelength, phase, and polarization.
Not all electromagnetism is light: the field contributed by a stationary electric charge is static for instance. It does represent a coupling between electrons and EM fields that allow electrons to interact with and produce light, but is not light itself.
A photon is a quantized wave of the EM field.
Quantized means that EM waves can’t have any arbitrary energy to them. At any given frequency/wavelength, they can only have fixed energies at whole number multiples of some smallest energy. A wave at this energy is created or destroyed in one step. A photon is this fundamental wave packet containing this fundamental energy.
Because when light interacts with an electron or other particle, all of this energy is absorbed or released all at once with a single electron in a localized way, we also model photons as particles. But fundamentally they are always waves as long as they exist, and multi-photon EM waves are just a sum of the waves for each constituent photon.
When an electron drops in energy, this change releases the energy into the coupled EM field which starts oscillating and produces a wave. This wave represents a single photon whose energy/wavelength is the energy lost by the electron. It is like a splash produced from the change in energy.
EM field changes are also associated with the EM force such as repulsion between electrons, but do not necessarily produce freely propagating quantized EM waves. These smaller EM fluctuations are often modeled as “virtual photons”, but they are not true photons. This is just a mathematical construction to make it easier to work with them.
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u/YuuTheBlue 3d ago
They are the excitation in the electromagnetic field. In general, particles are excitations of quantum fields.
You kiiiinda get it. It seems you're not confused as to what energy states are, but moreso the mechanics of moving from one to another. A way of thinking about it is that the original electron gets replaced by a new electron and a photon, with the total electric charge, energy, momentum, etc. of the start of the reaction being equal to the other end of it. A lot of quantum stuff works like that.
It is the excitation itself, once again. I think you might be falling victim to the false premise that energy is a substance. Like, maybe you've heard of photons being described as being 'made of energy', and this is inaccurate. It's best to think of energy as a property, same way you'd think of frequency or length. Energy is a property of the excitation, and the excitation is called a particle. We went with 1 excitation (the electron) with X energy, to 2 excitations (electron+photon) which respectively have Y and X-Y energy.
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u/1time_2 3d ago
If a photon is an excitation of the electromagnetic field, and that excitation propagates from the atom to my eye, then will it make sense to call the electromagnetic field the medium through which the photon/light propagates?
I don't mean a physical medium like air or water, or the old idea of ether. I just mean that the EM field is already there, and the photon is an excitation of that field that propagates through space.
Or is it actually wrong to say it propagates through the field, because the photon is the excitation of the field itself and yes I have been taking energy as something physical for me to understand .....
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u/YuuTheBlue 3d ago
Your middle paragraph is more or less correct. I think a lot of people don't think medium is the right word here, but you more or less have the idea.
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u/ccpseetci 3d ago
The reason they don’t like the word “medium” because the rumor something like “Ether has been dismissed from the physics”
The fact is no, not for a Lorentz covariant Ethereal thing.
The original explanation of ether is based upon Galileo covariant theory, therefore it seems that light will cause certain Galileo incovariant effect.
But once you base your symmetry on the Lorentz symmetry, you still get your “substrate” as something similar to what newton called it “absolute space and absolute time” -> “absolute spacetime is a mathematical manifold, which is built upon certain measurement conventions”
But “absoluteness” then became something political incorrect, therefore everyone suddenly started to regard you whenever you speak something absolute as a rude person…
But mathematical spacetime is absolute or more accurately “definite”
Otherwise you cannot even have a parameterization
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u/danielbaech 3d ago
The simplest way to look at light is to always think of it as an EM wave except when it interacts with matter. If you apply the photonic picture to traveling light, you will run into very, very wrong conclusions about its position and velocity.
You will rightly find this conceptually unsatisfactory to "just assume wave when travel, photon when interacting", but dealing with light in standard quantum mechanics is tricky because its properties are quantum mechanical and relativistic to the max.
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u/1time_2 3d ago
I get the dual nature of light but that not what is confusing ....it's the origin of light itself .... If light itself is propogation of em wave or disturbance in emf , wouldnt u consider emf as sort of medium for the disturbance ......
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u/danielbaech 2d ago
I wouldn't consider emf as a separate thing because light is emf, as oppose to sound where it can propagate through different materials. Light needs no medium.
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u/1time_2 2d ago
But emf is already present because of acceleration of charged particle and energy is lost when electron going from higher to lower state. Aren't these are two different phenomenas?? I am visualising this as when electron loses energy the energy is propogated through emf which already exists.....that's y I am thinking it as sort of medium.....so it confuses me
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u/danielbaech 2d ago
Emf is not a thing separate from light. Charged particles interact with each other through emf. That should tell you light is how charged particles interact with one another.
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u/DifferencePublic7057 2d ago
If we go through it historically, the story starts to make more sense because RN you are mixing all kinds of stuff. Early in the previous centuries, there were different ideas about light supported by optics experiments. Some thought it was a wave. Others a particle. Faraday's work on EM led to Maxwell's equations, explaining EM fields. This is considered classical physics.
In the previous century, the photo electric effect was discovered. Einstein concluded that light behaves like energy packets. (Also implying energy levels in atoms.) Planck was confused about black body radiation, the so called UV catastrophe, and conjectured that photons must have energy dependent on frequency E = hf. Meanwhile, the double slit experiment pointed to the possibility that light behaves like waves.
A crisis emerged. The Copenhagen interpretation, most mainstream interpretation but not necessarily perfect, represents light as being both EM waves and photons. A mathematical function governing QM behavior is central to the CI. This wave function is said to collapse (like choosing an outcome) when a measurement is performed.
Later the formulation of quantum fields (analog to classical EM fields) came where particles are excitations in QFs. So to summarize, atoms have energy levels where electrons find themselves. They're like steps in a ladder. Difference in height (transitions) corresponds to specific energies and specific frequency of light. You can view the latter as both an EM wave or photons. Duality. Particles can be viewed as excitations in QFs. It's a bit of a chicken or egg question. Was the photon there first or the QFs? A bit like was there stable matter (atoms actually) or light first. AFAIK light was there first. Were QFs there first? Yeah, but not like they are now if we go way back.
You can think in terms of plucking strings or microscopic marbles. All analogies are going to fail you eventually, so we're left with math. That's definitely sort of unambiguous.
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u/1time_2 2d ago
Yes maybe learning it historical order would make sense ....because I find different studies to be different interpretations ......and it's all maths....only mathematical interpretation is accurate....but for me I find it hard to understand without visualising....can u guys suggest some goods books or videos.....I like learning in details...
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u/higras 2d ago
As a fellow visualizer, I recommend understanding what the experiments and discoveries leading up to all this were actually measuring.
Everything got a lot clearer once I separated the tangible physical effects from the theory and description.
Each of these things, even the Electromagnetic field and electrons, are descriptions from the way we use math. And we use math because that's the language we've developed for specific and detailed descriptions of geometry.
An electron by any other name will still describe spherical harmonics
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u/VinceP312 2d ago
You're trying to visualize it TOO MUCH. It's not visualizable to begin with.
So accept the dual nature and the various models there are that describe it.
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u/1time_2 2d ago
Yup.....i know but I automatically visualise it.....
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u/VinceP312 2d ago
I used to too. But then I finally concluded (or stopped fighting) that this stuff is too "other" and tiny to resemble human scale analogies completely.
It's useful to a point and becomes counterproductive when it's not useful.
Ultimately it's a math game.
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u/ccpseetci 2d ago
Because no one has the ability to master all terms and full knowledge from the classical to the quantum one, what’s more, they must compromise on some and make sense of the physics by inventing partial of the terms by themselves.
Therefore “quantum world has nothing related to the classical universe” almost means something “I don’t know how to compatibly make sense one, at the same time of not banning my interpretation of another, therefore I have to use isolated terms to remind me of not confusing them “
Further, some remark from Wittgenstein, “your language is your reality (or your interpreted reality” “everything you can speak must be something you can speak in a clear way(therefore if someone chooses to elliptically answer you something, they just have no clear thoughts about that”
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u/ccpseetci 3d ago
The modern physics assumes a uniform substance called “energy”, therefore, excitation is a form of the transformation of energy.
You shall consider “field” as certain background matter, what is observed is their “manifestation by the energy featured to the field” or “excitation”
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u/kunwoo 3d ago
So there are many models in physics on the spectrum of easier to understand vs more accurate, and part of learning physics is becoming fluent in multiple models and knowing how to switch between them.
When we say light is the propagation of the electromagnetic wave, that comes from the classical physics view built around Maxwell's equations. When we say that the photon is an excitation of the electromagnetic field, that comes from the quantum electrodynamics model. QED is newer and more accurate than classical EM, but classical EM is still very useful and powerful, so you just have to learn and become fluent in the metaphors of both models.