Does the Photon Exist? Exploring Its Necessity

[neobaud]

TL;DR Summary

I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

 

[Hill]

It propagates with a final speed, and it transfers energy and momentum.

 

[PeterDonis]

Is there experimental evidence of its existence?

Sure: the fact that whenever you detect sufficiently faint light, you detect it as discrete particles.

My understanding is that it experiences no time or distance.

Your understanding is wrong. A correct statement is that for light, the concept of "proper time" (which is what your "experienced time" corresponds to) is not well-defined. But that does not mean that light does not exist.

Do we use the Photon to just keep things local?

No. We use it because it is a necessary part of the quantum theory of light, because of the experimental results described above.

 

[DrChinese]

There are many reasons we need photons as a mediator of the electromagnetic force. Here is a specific demonstration of the existence of photons:

http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf

 

[PeroK]

TL;DR Summary: I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

The photon is the quantum of the electromagnetic field. In that sense, it is a particle in the standard model like any other.

 

[neobaud]

Sure: the fact that whenever you detect sufficiently faint light, you detect it as discrete particles.Your understanding is wrong. A correct statement is that for light, the concept of "proper time" (which is what your "experienced time" corresponds to) is not well-defined. But that does not mean that light does not exist.No. We use it because it is a necessary part of the quantum theory of light, because of the experimental results described above.

I don't think this is an argument in favor of photons existing is it? You could restate "discrete particles" as "discrete interactions" right? Why do you need the photon?

 

[neobaud]

Maybe I should put it like this. If there was only one electron, would there still be photons? If so how do you prove this?

 

[PeroK]

Maybe I should put it like this. If there was only one electron, would there still be photons? If so how do you prove this?

There isn't only one electron. The question is immaterial.

 

[Lord Jestocost]

TL;DR Summary: I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

In their article “The concept of the photon - revisited”, Ashok Muthukishnan, Marlan O. Scully and M. Suhail Zubairy remark:

In the final portions of the article, we return to the basic questions concerning the nature of light in the context of the wave-particle debate: What is a photon and where is it? To the first question, we answer in the words of Roy Glauber:

"A photon is what a photodetector detects."

To the second question (on the locality of the photon), the answer becomes: “A photon is where the photodetector detects it.”

(from the book "The Nature of Light: What is a Photon?", edited by Chandra Roychoudhuri, A.F. Kracklauer, Kathy Creath, CRC Press)

 

[neobaud]

There isn't only one electron. The question is immaterial.

It's a hypothetical. I am trying to illustrate that it takes more than one particle to make a photon necessary (or at least it seems to me.)

 

[Nugatory]

You could restate "discrete particles" as "discrete interactions" right? Why do you need the photon?

A “particle” is a quantized excitation of a quantum field, and that’s what gives rise to the discrete interactions.

The photon naturally appears in quantum electrodynamics. So either we have photons or there’s some alternative to QED - and no one has ever found any such thing.

 

[PeroK]

It's a hypothetical. I am trying to illustrate that it takes more than one particle to make a photon necessary (or at least it seems to me.)

It's not clear to me what your objection is. The photon is part of Quantum Theory. It's also the most easily detectable particle, as we see by light (EM radiation). Whereas, we don't see by electrons; or neutrinos- which are extremely hard to derect.

So, you are able to read this thanks to the photon!

 

[PeterDonis]

You could restate "discrete particles" as "discrete interactions" right?

So what? The point is that the classical theory of electromagnetism does not predict "discrete interactions". Only the quantum theory of electromagnetism does. "Photon" is simply the name we give to the distinct features of the quantum theory of electromagnetism that lead to that prediction.

 

[PeterDonis]

it takes more than one particle to make a photon necessary (or at least it seems to me.)

Again, so what? It takes more than one particle to make any kind of useful experimental equipment. So it's pointless to ask what things would be like if there were only one particle, since we can't make measurements with just one particle anyway.

 

[topsquark]

TL;DR Summary: I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

I am wondering why the photon is necessary at all. Is there experimental evidence of its existence? My understanding is that it experiences no time or distance. So if this is true why do we need it at all? Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

You could say the same of any gauge boson, or any particle at all, in fact. How far into Physics courses have you gotten to? Do you know anything about the theory of why we propose that photons are necessary?

-Dan

 

[neobaud]

Again, so what? It takes more than one particle to make any kind of useful experimental equipment. So it's pointless to ask what things would be like if there were only one particle, since we can't make measurements with just one particle anyway.

"So what?"

It is just interesting to me. I have always thought of light and photons as being "real" particles. But it is hard for me to get around the fact that you could also think of them as a symbol for the interaction. Something that makes the resulting interaction easier to calculate. They ensure that causality and locality are maintained but are they more than handy bookkeeping tool?

 

[PeroK]

"So what?"

It is just interesting to me. I have always thought of light and photons as being "real" particles. But it is hard for me to get around the fact that you could also think of them as a symbol for the interaction. Something that makes the resulting interaction easier to calculate. They ensure that causality and locality are maintained but are they more than handy bookkeeping tool?

Those are virtual photons. That's not the same thing as "real" photons.

 

[neobaud]

You could say the same of any gauge boson, or any particle at all, in fact. How far into Physics courses have you gotten to? Do you know anything about the theory of why we propose that photons are necessary?

-Dan

I am not a Physicist that is why I am asking on this forum. I took physics 1,2,3 in college.

The way I understand it they are needed to track the path of interactions and maintain causality/locality.

Please enlighten me.

 

[hutchphd]

I think you need to create a set of characteristics that define, for you, a "real" particle. Otherwise this will not likely be a fruitful interaction.

 

[PeterDonis]

I have always thought of light and photons as being "real" particles.

Photons that are detected in experiments are real particles.

it is hard for me to get around the fact that you could also think of them as a symbol for the interaction.

Photons that are detected in experiments aren't a "symbol" for anything. They're real particles.

If you want to understand how quantum field theory uses gauge bosons to model interactions, and the limitations of the "virtual particle" model that you have implicitly referred to several times, you could start by reading these Insights articles:

https://www.physicsforums.com/insights/newideaofquantumfieldtheory-interactingquantumfields/

https://www.physicsforums.com/insights/physics-virtual-particles/

https://www.physicsforums.com/insights/misconceptions-virtual-particles/

https://www.physicsforums.com/insights/what-are-virtual-particles-intro/

 

[DennisN]

It's a hypothetical. I am trying to illustrate that it takes more than one particle to make a photon necessary (or at least it seems to me.)

I'm not sure what you are saying, but if you are implying there has to be an existing corresponding particle (or particle pair?) for a photon to exist, then no:

See e.g. pair annihilation (HyperPhysics); when an electron (matter) and a positron (antimatter) annihilates, they get destroyed and two photons are emitted.

 

[DaveC426913]

It would be an interesting exercise for the OP to describe how interactions would work without photons.

When a star emits light that reaches my eye, or when a photoreceptor fires from UV exposure, how would they explain it? They cannot simply interact, since they are millions of miles apart.How would they explain fluorescence? Or the photo electric effect?

 

[PeterDonis]

When a star emits light that reaches my eye, or when a photoreceptor fires from UV exposure, how would they explain it? They cannot simply interact, since they are millions of miles apart.

How would they explain fluorescence? Or the photo electric effect?

All of the things you describe are examples of real photons being detected. By "interactions" the OP appears to be referring to things like a static electric field, where there are no photons detected, but the presence of the field is shown by observing its effects on the motion of charged objects. QFT, at least in the perturbation theory approximation, deals with such phenomena using virtual photons. (And yes, this is a very narrow, arguably too narrow, definition of what an "interaction" is.)

 

[phinds]

I have always thought of light and photons as being "real" particles.

It is possible that you are thinking of the classical "particle" (that is, a little tiny ball) but what we are talking about here is not a particle in that sense. It is a quantum object, which means that it is an aspect of something that, if measured for particle characteristics, shows particle characteristics but that if measured for wave characteristics, shows wave characteristics.

 

[vanhees71]

The problem with "photons" is that it's explained almost always wrong in popular-science books as being a localized massless particle, and even many introductory QM1 textbooks use this naive-photon pucture of "old quantum theory".

The only consistent description, however, is in terms of relativistic QFT, and there it turns out that photons are not localizable as massive particles are (although in the relativistic context with less accuracy than within non-relatistic QM). What's localized is the photon detector, and all you know, given the state of the em. field, is the detection probability for photons at the position of the detector. The probability distribution is given by the energy density's expectation value of the em. field in the given state.

The next point is that in many cases also the semiclassical approximation is good enough, i.e., you treat only the charged particles quantum-mechanically but keep the em. field classically. That explains the photoelectric effect as well as Compton scattering in leading-order perturbation theory accurately.

The quantization of the em. field and thus photons in the proper, modern sense becomes necessary as soon as quantum fluctuations become relevant effects. The most simple example is the first-principle explanation for spontaneous emission. Another example is the HOM effect:

https://en.wikipedia.org/wiki/Hong–Ou–Mandel_effect

Finally classical em. fields are quantum mechanically described by coherent states of high intensity.

Dimmed down laser light is not a proper single-photon source but a coherent state of low intensity, i.e., it's most probable to detect no em. field at all or a single photon, but with some probability you'll also detect two or more photons. The photon number of a coherent state is Posson distributed.

 

[gentzen]

Why not just say that a particle here interacts with a particle there. Do we use the Photon to just keep things local?

Even classically, we talk about the electromagnetic field instead of saying that a particle here interacts with a particle there. So the question should rather be whether we favor photons over configurations of an electromagnetic field to just keep things local.

I read in "Do We Really Understand Quantum Mechanics?" by Franck Laloë that a version of Bohmian mechanics using field configuration trajectories for the electromagnetic field and particle trajectories for Fermions (with stochastic creation and annihilation events) works actually quite well. One of the main drawbacks of Bohmian mechanics (including this version) is its non-locality, so the answer to the adjusted question about keeping things local could actually be yes, in a certain sense.

 

[PeterDonis]

the question should rather be whether we favor photons over configurations of an electromagnetic field to just keep things local

"Photons" are "configurations of an electromagnetic field" as far as QFT is concerned; they're just different names for the same thing. And QFT is "local" in the sense that measurements at spacelike separated events always commute; that's true whether you use the word "photon" to describe some QFT configurations or not.

 

[Vanadium 50]

You could restate "discrete particles" as "discrete interactions" right? Why do you need the photon?

We could name them giraffes instead of photons. But that name was already taken.

As a famous physicist once asked "What's in a name?"

 

[topsquark]

As a famous physicist once asked "What's in a name?"

I've never heard of this. What was this person's name?

-Dan

 

[gentzen]

"Photons" are "configurations of an electromagnetic field" as far as QFT is concerned; they're just different names for the same thing.

Is this a special property of "photons"? Or are all bosons "configurations of some suitable field"? And what about fermions?

At least for Bohmian mechanics, using field configuration trajectories for fermions apparently doesn't work well:

Similar methods may be applied to other bosonic fields. ... Introducing Bohmian variables for fields associated with anticommuting operators is more complicated than for bosons ... one then obtains a description of reality where bosons and fermions are treated in a different way, the former having Bohmian field variables and the latter only position variables

 

[PeterDonis]

Is this a special property of "photons"?

Only in the sense of which field.

Or are all bosons "configurations of some suitable field"?

Yes.

And what about fermions?

Yes.

(Note that all of the above answers are given in the context of QFT.)

At least for Bohmian mechanics

Which is not QFT. It is either an interpretation of non-relativistic QM, or (in its more ambitious formulations) an attempt to extend that interpretation into an actual competing theory, which, however, is still non-relativistic, and is therefore considered a non-starter by most physicists (though not all--at least one PF regular, @Demystifier, has published papers defending the view that Lorentz invariance is only an emergent symmetry and that we will end up finding that there is an underlying theory that works more like non-relativistic Bohmian mechanics).

 

[Dale]

Why do you need the photon?

Without it momentum and energy are not locally conserved. That would be problematic

 

[vanhees71]

Even classically, we talk about the electromagnetic field instead of saying that a particle here interacts with a particle there. So the question should rather be whether we favor photons over configurations of an electromagnetic field to just keep things local.

I read in "Do We Really Understand Quantum Mechanics?" by Franck Laloë that a version of Bohmian mechanics using field configuration trajectories for the electromagnetic field and particle trajectories for Fermions (with stochastic creation and annihilation events) works actually quite well. One of the main drawbacks of Bohmian mechanics (including this version) is its non-locality, so the answer to the adjusted question about keeping things local could actually be yes, in a certain sense.

This is really important. The point of introducing the field-point of view about interactions is the causality structure of relativistic spacetime, which leads to the fact that space-like separated events cannot be in a cause-effect relation. Opertionally that implies that there's no propagation of causal effects faster than light.

The field-point of view realizes this notion of the relativistic causality structure in the simple way that all interactions are local, i.e., the entire dynamics of a relativsitic (quantum) system is described in terms of local (quantum) fields. There's no notion of point particles on a foundational point of view.

It's easy to understand, why this solves the problem of action-at-a-distance theories as are the paradigm of Newtonian mechanics: For action-at-a-distance models for point particles within relativistic theory it's impossible that the momentum-conservation law may hold, but it should hold since Minkowski space of special relativity assumes the symmetry of the dynamical laws under spatial translations, and the corresponding conserved quantum theory a la Noether is momentum. In a local (Q)FT that's no problem since the fields are themselves dynamical degrees of freedom which carry momentum (as well as energy and angular momentum), and the conservation laws hold locally.

A formal way to see this is the fact that demanding that there's a relativistic Hamiltonian theory of classical point particles leads to the conclusion that this can only be fulfilled for non-interacting point particles.

The particle aspect comes into the game that in local QFT asymptotic free Fock states describe field excitations that behave in some sense like free relativistic particles. Particularly when preparing a single-particle asymptotic free Fock state it can be detected only once in a local interaction with a detector, leaving a "point-like trace" (think, e.g., in terms of a pixel detector, where a single electron always leaves one and only one spot). What's really observable according to QFT are space-time dependent probabilities for detecting "a particle" in this sense, and these probabilities are usually given in terms of expectation values of correlation functions that describe some kind of local density or a current density.

Photons are particularly special. They have no non-relativistic limit. Among other things that's due to the fact that as massless spin-1 particles one cannot construct a position observable from the representation theory of the Poincare group, i.e., photons cannot in any way be completely localized. That's easy to understand in the field picture: If you want to enclose the electromagnetic field within a region of space time all you can do is to create a cavity which is in as good an approximation as possible an ideal conductor, such that all em. radiation is reflected on the walls without energy loss. Then you learn in kindergarden that the corresponding eigenmodes of the electromagnetic field describe fields that are spread out over the entire volume of the cavity. There's no better way you can "localize" the em. field.

For massive particles you have at least a position operator, but even then if you try to localize particles, this you can do also only by somehow "confining them" in a "cavity" like a Penning trap for charged particles. The better you try to localize the particle in such a way the stronger em. fields you must impose, and at some point the involved energy transfers between these fields and the particle are so large that you rather create new particles (in accordance with the conservation laws) like in electron-positron pair creation, and it's impossible to distinguish the original particle from these other particles, particularly if you create particles of the same kind as this one. So even for massive particles, the localizability is constrained even more than within non-relativistic QM, where you also have the Heisenberg uncertainty relation between position and momentum (which you also have in relativistic QFT since the space-translation group is a subgroup of the space-time-symmetry group in both cases, and position operators are defined as obeying the usual commutation relations with the momentum operators, which generate spatial translations).

The problem with de Broglie-Bohm reinterpretations of the quantum formalism indeed is its non-local nature, which is at odds with the very foundations of local relativistic QFTs although there are some attempts in the literature that try do remedy this difficulty.

In my opinion, there's no need for such reinterpretations, because the probabilistic interpretation of the quantum state in the sense of the minimal statistical interpretation (Einstein, Ballentine,...) describes all observations very well, avoiding any confusing, unnecessary philosophical ballast which is just introduce to prevent people to admit that the classical, deterministic worldview suggested by our experience with macroscopic objects, simply is not the way Nature can be adequately described on a fundamental level. It's rather an emergent phenomenon, which is pretty well understood in terms of quantum many-body theory.

 

[neobaud]

Without it momentum and energy are not locally conserved. That would be problematic

Hey Dale. I am wondering why not transfer the energy/momentum between the two particles directly? Why do we need something to travel between them?

 

[phinds]

Why do we need something to travel between them?

Google "Einstein Rings"