# What is a quanta?

• B
I am very confused. Here are a couple of facts I cant connect :
Quanta is the Delta of values of energy.
Quanta is a packet of energy.
a packet doesn't really stop at a finite distance from its peak.
The Energy of a wave is the total energy of its quantas.

After some more research I have more specific questions :
what is the relationship between a wave and a photon? is every photon a peak? is every peak the sum of a couple photons which each represents a quanta?

Orodruin
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You will not be able to make a classical analogue of what photons are. Electromagnetism is essentially as difficult as it gets when it comes to quantisation. A classical electromagnetic wave is not consisting of any fixed number of photons. The classical limit is provided by so-called coherent states, which is a superposition of states with different numbers of photons.

The main message is that photons are not "little balls of light".

bhobba and mfb
That I understand, and because I know this it confuses me more.
Ill be more organized :
Is the energy of the wave is its intensity(which means total value over a specific time if I understand right)?
are quantas levels of energy or the difference between the possible levels of energy? _folllow-up(if first one is true)_: is the total energy of the wave the sum of the different quanta?
in many explanations people talk of a wave having a quanta multiple of photons, how photons relate to this?

mfb
Mentor
Is the energy of the wave is its intensity(which means total value over a specific time if I understand right)?
Energy and intensity are different things.
are quantas levels of energy or the difference between the possible levels of energy?
What exactly do you call "quanta"? While "is quantized" is used frequently, I rarely see that word as subject. Do you mean photons? The energy of photons emitted by an atom (for example) corresponds to the energy difference between two levels of that atom.
is the total energy of the wave the sum of the different quanta?
For many waves, that sum is not even well-defined.

1oldman2
firstly, I would like to mention my appreciation for both of your comments.
Now lets get to my questions :
If the energy of a wave is not its intensity, than what is it? and how do you convert it to other stuff?
some sources I read used photons as examples to quanta. Some said its the difference of wave levels. And some said the Energy of a wave is the sum of its quanta's energy. Now its really starting to sound like photons.
The main things that confuse me are the quanta/photon-wave relation(it sounds like photons are parts of waves instead of having waves describing them which is what I thought in the beginning) and what the wave function describes.
does it all relate to superposition and having many frequencies at the same time?

Sorry for being so ignorant. I am 15 and stupidly volunteered to do a presentation on this topic.

mfb
Mentor
If the energy of a wave is not its intensity, than what is it?
Intensity is power per area. Asking if energy is "inside it" does not make sense. It is a different physical quantity.
it sounds like photons are parts of waves instead of having waves describing them which is what I thought in the beginning
Forget about waves if you want to consider photons. That doesn't work well together because it is very unintuitive.
does it all relate to superposition and having many frequencies at the same time?
Superposition is often relevant, different frequencies not necessarily.

Drakkith
Staff Emeritus
A greatly simplified explanation is that a photon is a quanta of energy associated with an electromagnetic wave. This just means that when an EM wave interacts with matter it does not do so in a "smooth" manner. As the matter absorbs the energy of the EM wave it does so in "chunks" or "packets", with each packet of energy being deposited all at once. We call these packets "photons". This is important because prior to the development of Quantum Physics, all waves were thought to deposit energy in a smooth, continuous manner.

It also turns out that the energy level of electrons within atoms is also quantized. This means that the energy levels cannot smoothly vary from one value to another. Instead they can only be certain amounts, corresponding to certain orbitals in a manner that's much too complicated to explain in detail. Imagine if a spacecraft orbiting the Earth couldn't choose any arbitrary orbital height, but had to be in one of only a few possibilities. Perhaps they could only choose ones that were 1.1x the height of the last orbit, or ones whose height in miles were multiples of 100. These orbits would be quantized.

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Imager and bhobba
I think I am starting to understand this.

What I understand is that the waves describe the path of the photon to the interaction which happens all at once becouse its a quanta. The energy of the photon is related to its frequency. Are different interactions/photons derived from the same wave or does every atom has its own wave?

vanhees71
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Forget photons at your stage of your studies. They are by far the most complicated case you can think of, because they are massless and thus do not even have a position observable in the strict sense.

Start with non-relativistic quantum theory. A nice introduction is given in Suskind's "Theoretical Minimum".

mfb
Drakkith
Staff Emeritus
I agree with the others that you should probably not think about photons at this stage in your education, but that obviously doesn't help you if you're doing a report on the subject. If you're going to talk about photons, just keep it simple. Paraphrase my earlier post if you need to. Don't worry about the details of how photons relate to EM waves other than the fact that they are quantized energy packets.

We seem to have covered photons and electron energy levels in an atom so far. What are some other examples of quantization that would help the OP in their presentation?

I think an explanation of how qunatization of electrons and photons are connected can help. As the first one is levels of energy and the second one is the absorbtion of energy. I see that they both have quantization - they "jump" from one possible value to another; but is this the only connection or is there something I am missing? If so, its a bit blurred. And how do electrons act to photons in frequencies that dont allow them to jump? Do the photons "bounce" instantaneously or is there a chain of reactions happening?

If I am right you said not to think about the wave-partivle duality so the questions above arent supposed to be as complicated.

mfb
Mentor
The energy levels are not properties of the electrons, they are properties of the atom, molecule, or the whole material, depending on what you consider.
And how do electrons act to photons in frequencies that dont allow them to jump?
Not at all. Examples are air and glass for visible wavelengths: the light just passes through.

Drakkith
Staff Emeritus
I see that they both have quantization - they "jump" from one possible value to another; but is this the only connection or is there something I am missing?

The only connection is that it takes energy to make an electron jump to another energy state and a photon is one method of providing that energy. A collision between two atoms or molecules is another method.

vanhees71
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I agree with the others that you should probably not think about photons at this stage in your education, but that obviously doesn't help you if you're doing a report on the subject. If you're going to talk about photons, just keep it simple. Paraphrase my earlier post if you need to. Don't worry about the details of how photons relate to EM waves other than the fact that they are quantized energy packets.

We seem to have covered photons and electron energy levels in an atom so far. What are some other examples of quantization that would help the OP in their presentation?
Ok, if you are forced to report on photons, then you should do it right and start with the quantization of the electromagnetic field. The most simple way is to give up manifest Lorentz invariance and work in the radiation gauge of the free electromagnetic field and then quantize the theory in terms of the two physical degrees of freedom (e.g., in the momentum helicity basis of single-photon modes).

This approach is very nicely explained in Landau&Lifshitz, vol. IV.

Drakkith
Staff Emeritus
Ok, if you are forced to report on photons, then you should do it right and start with the quantization of the electromagnetic field. The most simple way is to give up manifest Lorentz invariance and work in the radiation gauge of the free electromagnetic field and then quantize the theory in terms of the two physical degrees of freedom (e.g., in the momentum helicity basis of single-photon modes).

This approach is very nicely explained in Landau&Lifshitz, vol. IV.

I think that may a bit too advanced for the OP.

vanhees71
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It's not too advanced. It's the minimum level of sophistication to start defining what a photon actually is. One must simplify things as much as possible but not more (free quote by A. Einstein).

Drakkith
Staff Emeritus
It's not too advanced. It's the minimum level of sophistication to start defining what a photon actually is. One must simplify things as much as possible but not more (free quote by A. Einstein).

Based off of what the OP has already written, and the fact that this thread is marked "B", I'm going to disagree.

vanhees71
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Then don't even think about photons! Period. Start with non-relativistic QT and study the harmonic oscillator thoroughly. That's a much better preparation for tackle the photon issue than learning "old-fashioned quantum theory", which imho shoud be banned from the physics curriculum at any level anyway. It has it's place in a history-of-physics lecture but not in a physics lecture.

Orodruin
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It's not too advanced. It's the minimum level of sophistication to start defining what a photon actually is. One must simplify things as much as possible but not more (free quote by A. Einstein).
The OP is 15 years old.
I am 15 and stupidly volunteered to do a presentation on this topic.
Any presentation on that level is going to have to be essentially popular science with all its merits and flaws.

mfb
There is no completely correct metaphor for photon/quanta, but I know one which close enough.
The fields can be imagined as a tub of water. The quantum is an amount that can upload in it or also pick out of it at once.
Easy to see that the extra volume of water will spread out wavelike.

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DrChinese
Gold Member
There is no completely correct metaphor for photon/quanta, but I know one which close enough.
The fields can be imagined as a tub of water. ...

That is pretty far on the useless side of an explanation, especially given the OP.

As you are new, I will simply say that posts like you have been making are at the edge of forum rules. I would ask you to be considerate of those, and if you have something to add, keep it relevant and useful.

weirdoguy
Khashishi
A dynamical system, such as an atom, can be in various energy states. Quanta are the differences in energy between some energy states. For an ideal harmonic oscillator, the differences in energy are the same size, so they act like particles. In many materials, the atoms act kind of like little springs, and sound can travel through the material. On the quantum scale, the sound travels by passing small quanta of energy (called phonons) from spring to spring.

Space itself acts like a dynamical system which can be excited. The differences in energy levels of space appear as various particles, such as photons. The main difference between a quasiparticle like a phonon and a particle like a photon is that a quasiparticle is an excitation of some material substance whereas a particle is an excitation of space itself.

Drakkith
Staff Emeritus
Space itself acts like a dynamical system which can be excited. The differences in energy levels of space appear as various particles, such as photons. The main difference between a quasiparticle like a phonon and a particle like a photon is that a quasiparticle is an excitation of some material substance whereas a particle is an excitation of space itself.

I believe particles are excitations of fields within space, not of "space itself".

Khashishi
I considered saying fields within space but decided that it was a completely artificial distinction. A field is a value at each point in space, so we could simply consider it to be a property of space. Maybe it's a different "interpretation" but not really a different model.

Drakkith
Staff Emeritus
I considered saying fields within space but decided that it was a completely artificial distinction. A field is a value at each point in space, so we could simply consider it to be a property of space. Maybe it's a different "interpretation" but not really a different model.

You can have that opinion if you'd like, but as far as I know there is a very real distinction between "space itself" and fields within space when it comes to mainstream science, and attempting to explain things as being properties of space instead of fields is both confusing to those trying to learn science and incorrect.

vanhees71
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A "quantum" has a very well defined meaning in relativistic QFT. It's a one-particle Fock-space state of a (asymptotic) free quantum field. There is no simpler explanation or "metphor" for it than that!

weirdoguy and Mentz114
This is an analogy I find useful for starters.

Imagine a material made of Democrit's atoms. When you try to divide it, you will soon encounter indivisible building blocks.
Now imagine an ideal "philosophical" fluid. You can divide it infinitely into droplets of arbitrary small size. You can also merge droplets into bigger ones.

Now imagine a fluid you can divide, but there is a minimal possible droplet. I.e. due to surface tension. You can divide the fluid and merge droplets, but there is a minimal droplet you can't divide further. This is a quantum :).
This material shares some properties of atomic and non-atomic ones. There is a minimal indivisible building block, just as with atoms. But the building blocks can be merged, as with fluid drops.

Don't drag this picture too far. It's just a food for imagination.

Drakkith
Staff Emeritus