Are Photons Actually Infinitely Small Particles?

[ThomasT]

It seems to me that until an agreed definition of what is meant by 'size' when referring to a photon, then the debate is as meaningless as 'does god exist'.

Being a quantum object if we are going to apply a classical concept like size to a photon we really should agree on what we mean by size before saying what this size is.


that's up to your definition of size. That's what I'm getting at. You have to first contend that a photon is a wave front for your purposes of defining what size is. Then if someone else has an equally valid definition of what size means to them but their definition is different to yours then the two of you would be talking over each other while both being right by your own definitions.

Ok, you've said what I wanted to say but was afraid to say because I was thinking that it might look ignorant. But the way you put it doesn't seem ignorant, it seems like a reasonable consideration. At least that's my current assessment until/unless a qm guru tells us that it's ignorant, and why.

 

[fizzle]

That is definitely not the model of a photon that is generally accepted.

Wel then, what is the correct photon model for a radio wave? Is it one photon or a huge number of them?

 

[sophiecentaur]

Wel then, what is the correct photon model for a radio wave? Is it one photon or a huge number of them?

For a power flux of PWatts, the number of photons would be P/hf (a large number, which gets larger as the frequency gets lower).

Why should the photon model be different for different frequencies of em? The time taken for a particular photon to interact with a suitable charge system (receiver) could involve several periods of oscillation. The number would be determined by the actual system in question (the Q of the resonant circuit, in the case of a radio receiver). Would you say
Your picture of a wave as consisting of a density modulated 'cloud' of photons, with peak density at the extremes of field strength and none at the zero crossings is a bit limiting. If each photon were to interact separately (as with an incoherent beam arriving in a gas of isolated atoms or molecules) you would have a totally opposite situation to what happens when a coherent beam of RF arrives at an antenna and where the photons all 'add up' in phase to produce a sinusoidal output voltage.
Basically I am saying, as has been said many times, in different ways on this thread, that there is just not a suitable model for the photon that is just based on classical ideas. You are stuck with QM and that excludes any simple models. Just like Feynman says in that movie earlier on in the thread. You are doomed to failure if you try.

 

[edguy99]

Actually, radio waves emitted from an antenna are best modeled by a coherent state of the electromagnetic field in which the number of photons is not specified, but obeys a Poisson distribution.

I agree, many waves are not modeled by photons. Water waves, waves on the surface of the sun and radio waves emitted from an antenna (Ultra high frequency as modeled here).

That said, the hydrogen line (21 centimeter line) occurs in that exact range at 1420mhz. Its discovery led to the Hydrogen maser.

Consider the question of how "big" is a photon as it relates to this picture:

The photons are created one by one, are filtered a bit and fly into a microwave cavity that could be some multiple of 21cm big.

It seems pretty clear you need a skinny little thing that has some sort time of variance on a regular basis over time and space to account for the waves that form in the microwave cavity.

This picture suggests that a model of a photon 3fm wide that varies in length from 3fm to 21 cm would complete the picture and you would see some diffraction as the particles entered the cavity and a natural resonance at 1420mhz (21 cm) inside the cavity.

 

[Q-reeus]

...The photons are created one by one, are filtered a bit and fly into a microwave cavity that could be some multiple of 21cm big...

edguy99 - Have a closer read maybe of that Wiki article. It's not photons but atomic hydrogen (red dots in the diagram) that enters the bulb/cavity. Microwave oscillations within the cavity are generated by an external power source in the passive variant, or internally in the active variant (presumably energized via recombination of atomic to molecular hydrogen).

 

[edguy99]

edguy99 - Have a closer read maybe of that Wiki article. It's not photons but atomic hydrogen (red dots in the diagram) that enters the bulb/cavity. Microwave oscillations within the cavity are generated by an external power source in the passive variant, or internally in the active variant (presumably energized via recombination of atomic to molecular hydrogen).

Thank you for the correction, too may wiki searches in too short a time.

 

[Q-reeus]

Thank you for the correction, too may wiki searches in too short a time.

No problem - happens to me all the time - which speaking of is well past my bed time!

 

[fizzle]

Basically I am saying, as has been said many times, in different ways on this thread, that there is just not a suitable model for the photon that is just based on classical ideas. You are stuck with QM and that excludes any simple models. Just like Feynman says in that movie earlier on in the thread. You are doomed to failure if you try.

-deleted reply-

Edit: The thread appears to have been started with the intent to say, yet again, that everyone is "doomed to failure" if they don't accept the status quo. Carry on.

 

[sophiecentaur]

Why should you think that our existing models (e.g. the classical ones or SR) would be sufficient to explain any new thing that is discovered? The Victorians were of that opinion, at the time and I think you'll agree that their ideas were not sufficient.
It may be 'comforting' to have the world presented in old, familiar ways but don't we need to progress?

[edit: I am not saying that QM is sufficient for everything. Of course it's not. My problem is with people who still want stuff to be explained in nice concrete models which pre-date even QM. That's what's doomed to failure.]

 

[fizzle]

Why should you think that our existing models (e.g. the classical ones or SR) would be sufficient to explain any new thing that is discovered?

I don't.

The Victorians were of that opinion, at the time and I think you'll agree that their ideas were not sufficient. It may be 'comforting' to have the world presented in old, familiar ways but don't we need to progress?

Don't be condescending ("comforting"). The Victorians looked through EM-colored glasses, attempting to model everything as EM waves. Today we look through QM-colored glasses, attempting to model everything, no matter how tortured, as QM whatevers. Also, QM is old ... almost 100 years old. It's nothing new.

 

[sophiecentaur]

We're not disagreeing then.

There are many times in the history of Maths where people just had to make these jumps. They couldn't't demand a cuddly way round the square root of minus one. I keep reading demands for the equivalent thing in physics. It has to be 'mechanical'.

 

[fizzle]

We're not disagreeing then.

There are many times in the history of Maths where people just had to make these jumps. They couldn't't demand a cuddly way round the square root of minus one. I keep reading demands for the equivalent thing in physics. It has to be 'mechanical'.

I couldn't disagree more. If something goes from place to place, it has to traverse the intermediate space. If you create a theory that doesn't have that as a fundamental part, then your theory is either tracking fictitious quantities or your theory is simply a higher level calculation shortcut.

 

[sophiecentaur]

it has to traverse the intermediate space.

That is making a huge assumption - based on familiar concepts about space and time. All you can say is that you have observed something leaving and arriving somewhere else. Just like with the two slits experiment, you have no idea how it got there because you need to observe it on its journey to know that and this would involve altering the experiment.

What you say doesn't necessarily 'stand to reason' - it's just a familiar, "comforting" way of thinking about it which mostly produces correct predictions and that's all.

 

[fizzle]

That is making a huge assumption - based on familiar concepts about space and time. All you can say is that you have observed something leaving and arriving somewhere else. Just like with the two slits experiment, you have no idea how it got there because you need to observe it on its journey to know that and this would involve altering the experiment.

When you see the double slit experiment with water waves, do you doubt that the waves went from place to place? No, because the observation tools are high enough quality to see the intermediate steps. You apparently want to revert to action-at-a-distance simply because we haven't constructed adequate measuring devices. Who's making the "huge assumption" now?

What you say doesn't necessarily 'stand to reason' - it's just a familiar, "comforting" way of thinking about it which mostly produces correct predictions and that's all.

My "comfort" with it is no more trivial than your "comfort" with the currently accepted theory.

 

[harrylin]

I couldn't disagree more. If something goes from place to place, it has to traverse the intermediate space. If you create a theory that doesn't have that as a fundamental part, then your theory is either tracking fictitious quantities or your theory is simply a higher level calculation shortcut.

Indeed, modern theories of physics such as QM (and yes, it's getting old now) are merely a mathematical description of observations, lacking a physical model; one may call them "mathematical" theories of physics.

 

[sophiecentaur]

When you see the double slit experiment with water waves, do you doubt that the waves went from place to place? No, because the observation tools are high enough quality to see the intermediate steps. You apparently want to revert to action-at-a-distance simply because we haven't constructed adequate measuring devices. Who's making the "huge assumption" now?

I don't doubt that the waves, in general terms, 'went from place to place' because I can actually observe the process, which is based on statistical analysis of a large number of 'events' and which fits the straightforward Maths of wave propagation. But I cannot be absolutely sure that their motion is a totally smooth and continuous one because I cannot observe at a very small level. They could be going in a series of small jumps (or even big ones), for all I (or you) can tell

In the case of a photon, I cannot tell anything about its motion except that I have assumed it to have left the source in my experiment and it can be measured / detected just once on its journey. I have no idea 'what it was doing' in between and neither can I predict what it would have done if I hadn't observed it. I can't even be totally sure that the photon I have detected even came from my source. However, I do 'accept' that it follows a general statistical trend, because of the history of measurements of such phenomena.

So, in both cases, the microscopic and macroscopic, big assumptions are made if we say that there is a direct correspondence between the mathematical model we have used and what we have observed. I say that the 'comfort' is in feeling that there is, in fact, a correspondence. Tomorrow morning, someone may come up with a good reason to shake that faith and I shall not be upset. Just interested and probably very confused!

My "comfort" with it is no more trivial than your "comfort" with the currently accepted theory.

I have a problems with that statement. Firstly, I should like to know what 'currently accepted theory' you claim that I adhere to. Secondly, I am not clear about what theory you actually favour.

My view is that no theory is likely to be sufficient, ultimately, but I take comfort in the ability of many theories to predict stuff 'well enough' to make things like computers and TVs work.

 

[Q-reeus]

As a mathematical tool the Feynman path integral approach which posits a particle/photon simultaneously takes all possible paths between A (emission) and B (detection/absorption) obviously works. Handily all those paths but the intuitively expected 'straight' ones (depending on whether wave or particle properties are observed) cancel out to high probability. Few surely believe that literally all paths are actually taken at once. The position that there may be literally nothing between A and B is another thing again. Gravitational deflection of light has an easy intuitive explanation assuming continuous particle/wave traversal. There is a sensible explanation here when nothing in-between is considered possible?

 

[Cthugha]

A photon is definitely not a localized em wave or "packet". For ordinary optical frequencies the required electric field strength is 10+ orders of magnitude too high.

This is plain wrong by the way. Ever heard of single photons on demand or single photon turnstile devices?

If something goes from place to place, it has to traverse the intermediate space. If you create a theory that doesn't have that as a fundamental part, then your theory is either tracking fictitious quantities or your theory is simply a higher level calculation shortcut.

Well, only if you are wearing 19th century glasses. Also that statement is ill defined until you state what sorts of phenomena are covered by "something". Do you mean only classical particles? Do you also mean more abstract entities like fields and their energy density? What about the cases where just information goes from A to B like in quantum teleportation? Does the information also have to traverse the intermediate space?

 

[sophiecentaur]

@Q-reeus
Feynman was absolutely great with his Physics. He never 'believed' anything and never actually (afaik) made definite statements about reality. He was as slippery as an eel when challenged about such matters and always made statements which were loaded with caveats about the assumptions involved. He was a 'good lad' with the totally right attitude but has been treated a bit like Monty Python's Brian in that many people take, as gospel, statements he made with the greatest of care. They then over-interpret them and quote them way out of context.

 

[Q-reeus]

...He was as slippery as an eel when challenged about such matters and always made statements which were loaded with caveats about the assumptions involved...

And crabby at times too - based on a recent free showing! But I will agree with the sentiment it's all pretty pointless arguing this or that unless one's pet idea can lead to experimentally verifiable 'new truth'.

 

[sophiecentaur]

I have this theory about the Moon and Cheese . . . . . .

 

[Q-reeus]

I have this theory about the Moon and Cheese . . . . . .

Forget it - sure to be full of (Swiss variety) holes!

 

[Cthugha]

I have this theory about the Moon and Cheese . . . . . .

Oh, this has already been published. See Wallace, Gromit, et al., "A Grand Day Out", Aardman Animations (1989)
or have a look at the summary of this research article at
http://www.wallaceandgromit.com/films/granddayout/about.html

 

[harrylin]

[..] Well, only if you are wearing 19th century glasses. [..]

That sounds like a typical 20th century reply.

Does the information also have to traverse the intermediate space?

Do you have a model in which something goes from A to B without traversing space in between? If so, what model is that?

 

[Cthugha]

That sounds like a typical 20th century reply.

Indeed. My 21st century glasses are being repaired right now!

Do you have a model in which something goes from A to B without traversing space in between? If so, what model is that?

Well, that depends on the somethings. There are lots of experiments on quantum teleportation which get states from A to B using entanglement. Even highly non-classical states have been teleported this year. This can be described in a non-local manner. Now the "somethings" come into play. There is no information exchange associated with that teleport as we all know, so what qualifies as a something? And which went from A to B? The state of the light field? The information carried? The photons? And if we are discussing photons are we discussing its probability amplitude or the energy density of the associated field? The spatial dependence of both can be very different. Which quantity is the important one in tunneling? That kind of question is too fishy to give a clear answer while it is about "something".

One of the first things I learned when writing papers is that it is a very bad idea to claim "XXXX has to" unless you have absolutely rock solid evidence that every other possibility is completely ruled out. These claims can kill papers. In fact, I do not propose any specific model, but as long as there are tenable models involving nonlocal influences or similar stuff, I would not dare to declare that "everything" has to go through all space in between.

 

[hbjon]

I can go along with most of that.
SO why is it that the Photon is treated by all and sundry as something with the same sort of 'reality' as a cannon ball? It seems to me that it only serves to confuse. Isn't it time to make it more plain to the World that photons are not like that at all?
How many times do we read that the Photoelectric Effect 'proves' that photons are particles?
Q reeus made the comment a few posts ago. All the photoelectric effect shows is that E =hf and that energy interactions with em waves are Quantised. Can't we, as the relatively well-informed, do the World a favour and start putting things a bit more accurately?

In photoelectric effect, does the cannonball(s) splatter energy on the nucleus, causing the electrons to lose some of their hold on the atom? Doesn't a direct hit force the electron to move towards a more electronegative atom? Help me here. My memory does't serve me. The photon originates from the nucleus or the electron in qm?

 

[sophiecentaur]

In photoelectric effect, does the cannonball(s) splatter energy on the nucleus, causing the electrons to lose some of their hold on the atom? Doesn't a direct hit force the electron to move towards a more electronegative atom? Help me here. My memory does't serve me. The photon originates from the nucleus or the electron in qm?

The photon interacts with the whole system and causes the electron to depart.