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> I've never heard a good explanation for what a photon "is", or any particle for that matter. That's because physicists don't know either.

The aim of many/most physicists is to predict rather than to know. Biologists, Chemists, Engineers can agree all they want but their theories aren't tested to 5-sigma (I'm sure some are but that's not as good of a metaphor). We have extremely accurate and predictive models of how photons behave (Quantum Field Theory not solely Quantum Mechanics).

You're arguably right, we don't really know - however, we can only know what we can see with our eyes. There may be some all-encompassing structure hiding at any energy level we haven't yet reached but the explanation given is as good as it gets in terms of explainable what we have observed in nature.

As to Quantum Mechanics, the only place you'll find Physicists disagreeing about the "core concepts" is on the absolute fringes of what can be considered science. You will find Physicists arguing about the interpretation of those concepts but it's not like they don't work. As to that interpretation, there is disagreement but I think it is fair to say that very few physicists actually think about it - i.e. "Shut up and calculate" is a common outlook.

> PS: Compare this to biologists, chemists, or engineers. They all 100% agree on all core concepts.

Biologists certainly don't. What is a species? Is Linnean taxonomy useful or merely confusing and obsolete? What is a gene?

> I've never heard a good explanation for what a photon "is", or any particle for that matter.

What would such an explanation look like, and why do you think that there is such an explanation?

In many cases when people ask what something "is", they want a category defined in terms of categories they are already familiar with. We can build up a set of observations and build a formal model to summarize them whose terms we can map to the results of various observations. We name the various parts of our formal model for convenience. But there is no requirement that the model be able to satisfy queries that depend on things that don't exist in the domain of the model.

> Basic things like: "Are the photons quantised or is it just the photon interaction with atomic orbitals that's quantised?" I've personally asked this question and had working physicists confidently state that either the former or latter is true, and the other is false.

Sure, because you're not asking a question about anything observable, so it's a question of how people put names on the various internal parts of their model. It doesn't matter if people agree on that, because when anyone sits down to actually use the formal model to model something, vague naming of internal parts of it is irrelevant.

So I think the right answer is: stop looking at the photon! There's a charged thing wiggling somewhere, and the interaction between two charged things that wiggle is quantized. If I have an electron at one point making small oscillations and another electron at another point, I can calculate the transition probability of the first electron oscillating with one less quantum and the second one oscaillating with one more, or vice versa.

> Biologists certainly don't. What is a species? Is Linnean taxonomy useful or merely confusing and obsolete? What is a gene?

These are human definitions for classifications, and biologists agree on that. However, if you make a statement that "two groups of animals that can never reproduce successfully with each other are separate species" then nobody will argue that. Similarly, nobody argues about what the mechanism is for species reproduction success or failure. Everyone knows how chromosomes and DNA works. There's no debate.

Every chemist uses the same periodic table with the same elements. Etc...

> What would such an explanation look like

Equations that can be evaluated numerically that shows the behaviour of the EM field, in 4D spacetime, capable of demonstrating interactions with charged particles. That's all. That's not asking much.

> why do you think that there is such an explanation?

Because it's a thing that happens. There are atoms that have electrons that electromagnetically interact with light and change orbits.

Quantum Mechanics provides a highly simplified model of this in a very narrow use-case. That's not wrong, or mathematically invalid. But it's not the whole picture, and isn't a valid source from which to answer questions like "what is a photon".

That's not describing the nature of light in the real physical world, it describes the nature of an abstraction in a simplified model. The photon exists in the equations. That's why it's so hard to visualise and "pin down" with a simple description. It's a property of the model, not the world.

> We can build up a set of observations and build a formal model to summarize them whose terms we can map to the results of various observations.

The models in this case are tiny subsets of all physical behaviour and generally only cover certain categories of scalar observables. E.g.: "The detector went off or not". Hence the cartoon diagrams and focus on the observables covered by the model.

> But there is no requirement that the model be able to satisfy queries that depend on things that don't exist in the domain of the model.

I agree. But photons are a thing that exist in the model. What I'm saying is that the photons exist only in the model.

> Sure, because you're not asking a question about anything observable

See, you made the same mistake as everyone else! Atoms aren't the only things that exist! There are free particles in vacuum chambers, particle accelerators, outer space, etc... that aren't atoms. There's a device called a "free electron laser" that has a continuously adjustable output frequency!

> There's a charged thing wiggling somewhere, and the interaction between two charged things that wiggle is quantized.

Only if they're in potential wells. Even then it's not entirely clear if all interactions are quantised.

> I can calculate the transition probability

That output is a scalar. The electron is in an orbital though, right? With a defined angular momentum, position in space, and everything. Even if you abandon the concept of point particles, it has a bunch of properties. Now show me all of those properties changing as it absorbs the photon.

Classic QM can't, because it doesn't even try. It focuses on the narrow case of "near resonance" light and then handwaves over how the photons actually change their orbitals. Cartoon diagrams and statistics. That's fine if you're designing a laser or something, but that's not what is physically happening with a single electron around a single atom.

"Equations that can be evaluated numerically that shows the behavior of the EM field..."

it's called the Standard Model. You seem far too hung up on the deficiencies of QM, which have been well known for over half a century, so I'm not really sure the point you're trying to make. Every physicist knows that QM is incomplete so whether or not their forced conceptualization of that model agree or not is not really that interesting of a phenomenon.

Also, your comparison to chemistry and biology seem misguided. Chemistry, and the periodic table, are built on our understanding of physics and the constituent particles in each atom. Furthermore, comparing the difference in conceptual descriptions of QM to "but every chemist uses the periodic table" is ridiculous. Every physicist agrees on the accepted values for known physical constants, they all agree what the known fundamental particles are, they all agree what the known fundamental forces are... etc. It seems like you're bothered by the lack of conceptual guidance from physics but that isn't the point of the field. The point is to develop models that make highly accurate predictions about the physical world, from the unimaginably small to the unimaginably large (and they're the most accurate scientific models known to man!).

How you define a species is actually fascinating and there are different ways to do it, depending on what you know about an animal: https://www.youtube.com/watch?v=9fOfFlMe6ek

> These are human definitions for classifications, and biologists agree on that.

I'm a biologist, and I don't agree.

> However, if you make a statement that "two groups of animals that can never reproduce successfully with each other are separate species" then nobody will argue that.

You're assuming animals with two sexes, and even there things like ring species throw a wrench into that.

> What I'm saying is that the photons exist only in the model. and > See, you made the same mistake as everyone else! Atoms aren't the only things that exist! There are free particles in vacuum chambers, particle accelerators, outer space, etc... that aren't atoms. There's a device called a "free electron laser" that has a continuously adjustable output frequency!

But those particles exist to us only as the results of measurements. We cannot impute a category to them that is definable in terms of familiar categories. They are no more or less real than the photon. I don't personally dislike the term "particle" for them, but "vacuum excitation" or similar just doesn't have the same cachet as "particle," and usually carry other undesirable baggage. Even "vacuum" carries problematic classical baggage.

> The electron is in an orbital though, right? With a defined angular momentum, position in space, and everything. Even if you abandon the concept of point particles, it has a bunch of properties. Now show me all of those properties changing as it absorbs the photon.

No, you can't impute properties like angular momentum to a state outside of measurement. We're used to thinking that we can because of classical systems where we take our sequence of measurements of a phenomenon over time and interpolate them infinitely finely and assume that as we do so the measurements converge to a continuous function. But since that limit doesn't exist for quantum mechanical phenomena, you can only speak in terms of measurement outcomes.

Plus, even if you could impute a property before measurement, you could only impute them to pure states. Almost all quantum states are not pure states.

So we can't show you those properties changing. All we can speak of is the probable outcomes of sets of measurements.

> That's fine if you're designing a laser or something, but that's not what is physically happening with a single electron around a single atom.

Is what you consider "what is physically happening" something theoretically observable?

> Even then it's not entirely clear if all interactions are quantised.

Of course. That's why continuous spectra are a thing.

> I'm a biologist, and I don't agree.

I picked a bad example. Well, no, actually, a good one, but an example that does not lend well to online debate. And then I failed to make myself clear.

I already know about all of the corner cases, such as ring species, the fuzziness you get during speciation, sub-species, the constant reclassification of taxa, etc..

What I was trying to get at is that "species", like "photon" is a human word. A label for an abstraction that's useful.

No working biologist (sans a few creationist crazies) would disagree about that vast, vast majority of what makes a species a species. The chromosomes, the interbreeding capability, the genetics, DNA, etc... Where they disagree is on the finer points of classifications, such as whether two very similar specimens are a species, sub-species, or just a "trait" within a species. Those are just human labels, like the dewy decimal code. They're not the thing. They don't disagree that the specimens exist, that their DNA is different, etc...

Quantum theorists disagree with each other on things comparable to the existence of species. As in, some will say that there is such a thing as wave function collapse, others will say that this is nonsense and there's no such thing. They're not disagreeing on exactly when wave function occurs in some obscure corner case. No, they disagree on the existence of the concept itself.

This is a crucial distinction.

No two chemists will ever disagree that H2O is water or not. Ever. There's just no such argument, and hasn't been for well over a century.

In theoretical physics, specifically quantum mechanics and its cousins, there still is debate at this level, and shows no signs of resolution.

> But those particles exist to us only as the results of measurements...<snip>... you can only speak in terms of measurement outcomes.

That is a complex debate that's been had in the past: is it meaningful to speak of something existing if it cannot be measured, even in principle?

The issue with that is that sometimes people just state that certain categories of experiments are impossible, and then it turns out that oops.. no, it was possible, just really hard to do. This is my favourite example: https://tf.boulder.nist.gov/general/pdf/2723.pdf

Note the quote at the end: "Some observations can be made on the existence of ‘blinders,’both in theory and in experiment. Some theorists were accustomed to seeing problems in terms of the tools which had previously proven useful for describing experiments with large numbers of atoms, such as the optical Bloch equations. Their intuition seemed to fail for the case of single atoms, and they were unprepared to accept the phenomenon of macroscopic quantum jumps."

This is particularly common in the QM field, especially in relation to single particles or atoms. The tools are optimised for ensembles, so... people seem to forget that single particles exist. The experiments involving single interactions are crazy difficult, so some people will simply state that they're impossible, and it's difficult to argue with that when there are so few experimental results available to counter that.

Note that GR had some similarly "hard" experiments, such as the detection of gravitational waves. That doesn't mean that people seriously said that talking about gravitational waves was meaningless.

> Of course. That's why continuous spectra are a thing.

Okay, so you're on the photons-are-not-always-quantised side. I agree with you, but other people don't.

This is a fundamental disagreement much like Hinduism and Islam, to pick two random religions. Sure, there are silly people saying things like "all religions are equally valid", but.. no, no they are not. Some of them are just local church traditions, some are full on sects, and others aren't even vaguely compatible.

"Quantum theorists disagree with each other on things comparable to the existence of species. As in, some will say that there is such a thing as wave function collapse, others will say that this is nonsense and there's no such thing. They're not disagreeing on exactly when wave function occurs in some obscure corner case. No, they disagree on the existence of the concept itself."

This is an extremely uncharitable analogy. Physics is not a framework to guide your conceptual understanding of the world. It is human nature to try and conceptualize everything but there are limits to this and the interesting areas of physics operate beyond those limits. You can not conceptualize quantum interactions in a way that is meaningful because that process requires forcing our intuition (based on the macroscopic/classical world) into a space where it does not belong. This is why we create mathematical models to "describe" these types of systems and their interactions. To emphasize this, consider how illogical it is to try and "visualize" the fundamental building block that all visualizations are built with? There are visualizations of the corresponding mathematical models but they only provide insight into certain properties, they do not "show" you what a photon interaction looks like because the concept of physical visualization fundamentally doesn't make sense at the quantum level.

I mean, that's because the language of physics is mathematics. If you ask vague questions with inherently undefined terms, you shouldn't be surprised if you get different answers! I bet chefs wouldn't agree on whether a taco is a sandwich, either, but that doesn't mean they don't know what they're doing.

Furthermore, as someone with a lot of experience thinking about photons in many different limiting cases, your objections don't make sense to me, because in general, absolute statements like "There are no X" in physics don't make sense. For almost any value of X, X can be a useful way of thinking in some limit or other, and whether or not you want to invoke X depends on the calculation you're doing. When you say there is no X and everybody who thinks otherwise is wrong, you're just inviting a debate over semantics, which we physicists are naturally not interested in.

> I mean, that's because the language of physics is mathematics.

But it is not a study of it! Mathematics is a tool, not the thing being studied.

This is lost on some people.

There are properties of the real world that are studied, and there are properties of the mathematical models that are used.

They are not interchangeable.

Similarly, saying that the "this specific mathematics is all there is, just understand that" when a simple verbal statement can be made that is not covered by the mathematical model doesn't mean that the statement is somehow invalid, it means the model is incomplete or narrow in scope.

There is this downright perverse notion in QM that the models are what reality is. If the model can't answer something, then the question is meaningless, etc...

Meanwhile the model has assumptions and simplifications such as linearisation, no GR or even no SR, and so forth.

My point is simply this: The mathematics of QM is not equipped to answer fundamental questions like "what is a particle" or "what is a photon".

> whether or not you want to invoke X depends on the calculation you're doing.

I'm not debating that the simplified models aren't useful, widely applicable, numerically useful, make predictions, etc... I'm simply stating that physical reality behaves in single, definite way.

> Similarly, saying that the "this specific mathematics is all there is, just understand that" when a simple verbal statement can be made that is not covered by the mathematical model doesn't mean that the statement is somehow invalid, it means the model is incomplete or narrow in scope.

Not necessarily. It may mean that the verbal statement invokes a language game to define a category that has rules incompatible with the rules of the formal model.

> My point is simply this: The mathematics of QM is not equipped to answer fundamental questions like "what is a particle" or "what is a photon".

Sure it is. A particle in QM is something that obeys the formal structure laid out. Dijkstra had some very biting things to say about being unable to cope with true novelty because you're trapped in reasoning by analogy (EWD1036).

At a risk of reiterating the same argument: You keep agreeing with me and perhaps not realising it.

> incompatible with the rules of the formal model.

Yes. I agree! In many cases this is just means that the statement is "out-of-scope" for the model. That's fine. That however has no bearing on the validity of the statement, unless the model is all-encompassing. Quantum Mechanics is not all-encompassing, it is not a TOE, and this is well known. It's just that people forget this sometimes.

> A particle in QM

Again, you're agreeing with me. The QM model has a notion of particles that is both a mathematical abstraction and a simplification. It's not what the physical world truly does. It cannot "explain" particles.

> obeys the formal structure laid out

So if I were to write a computer program with things in it that obey the same rules, are these things suddenly real, physical particles just like electrons? How about phonons? Holes? Anti-particles?

Particles are abstractions. They're labels humans assign to certain mathematical structures in certain models. Some of them, like leptons, are real things that exist whether or not they're named. Some of them aren't quite as real. Photons are questionable.

Working physicists were surveyed and asked if they think photons are "just a useful abstraction" for orbital interactions or if they're truly real, in the sense that individual photons travel through space like little point particles. They didn't all agree.

Before you argue that I'm wrong somehow, first figure out why theoretical physicists don't agree about such a basic concept.

> Dijkstra had some very biting things

Dijsktra should have known better than to say that, because the converse of this is that it's easy to lose touch with reality by getting too deep in the weeds of abstract algebra.

"But it is not a study of it! Mathematics is a tool, not the thing being studied."

Who knows? Mathematics turns out to such an acute tool for understanding physics. One might be inclined to say that we live in a mathematical reality. The quoted statement is more a statement of philosophy than of fact.

One might be inclined to say this if one doesn't understand the difference between physical nature and our consciousness of physical nature.

Mathematics does not exist outside of human minds. It is a method for measuring and describing the world around us that happens to lend itself to the study of its own conceptual structure.

Nothing in nature follows mathematical laws. Protons don't consult logarithm tables before they move through space. Particles don't consult statistics textbooks before they "decide" if it's time to decay. They act according to their nature, and that's all.

> Ask any working QM researcher to clearly show how "a single photon" interacts with an electron in an orbital and they will without a doubt draw you a cartoon diagram. Never in the history of theoretical physics has anyone, ever, anywhere made a numerical visualisation of this.

I personally have made a simulation of this. It would be called a semi-classical approximation, in that it's a simulation of a Schrodinger equation-described electron in a hydrogen (Coulomb) potential interacting with a classical EM wave. However I suspect that's how nature works anyway.

> However I suspect that's how nature works anyway.

Uh, could you clarify what you mean here? Are you saying you don't beleive that the electromagnetic field is quantized?

Yeah pretty much. (I say this not from a position of profound knowledge or anything like that, just that I'm not familiar with a good argument for why it is, not that I have looked into it in great detail. My knowledge of QFT is very basic)

Ah, fair enough. I was worried you might be professing some sort of expertise while claiming that.

I can assure you that whatever may be true of the electromagnetic field, it is not classical. We know that to about the same degree of certainty that we know that atoms aren't classical.

It's just that when considering atoms in an electromagnetic field, there are often situations where you can ignore the quantum mechanics of the field.

So what would you say is the best evidence that the EM field is quantised?

Examples abound, but I’d say my favourite is the electron g-factor which is an example of one of the most precise agreements between theory and experiment ever achieved by humans and it’s calculation relies heavily on the quantum mechanics of the electromagnetic field.

But there’s lot of other, more pedestrian examples. For example, you can (and people have) generated entangled pairs of photons and then performed measurements on this entangled pairs that violate the Bell inequalities, meaning that their correlation is not classical.

Looking back on this, I feel I should also mention that there's not any single piece of evidence that should be bullet-proof convincing evidence for the EM field's quantum nature, rather it's the totality of many many many different, independent results exploring various quantum mechanical properties of electromagnetism (and its high energy unification with the weak nuclear force), all of which converge on the basically undeniable fact that electromagnetism is a quantum mechanical phenomenon.

Of course, this isn't the full story. There is physics beyond the standard model. But whatever that physics is, it's description needs to reproduce quantum electrodynamics in the appropriate limit.

I should also mention that it's not even mathematically consistent to have a classical field like electromagnetism coupled to quantum matter. It's fine in certain approximation schemes, but the breakdown and limitations of such an approach is well documented.

I'd love to see the video of this, or even a still picture.

If you have some time, please upload it to YouTube or something, because it would be the only such video on there.

It would have significant educational value.

Ok yeah I'll dig up the code and capture a video of it. I'm currently working on a Dirac equation simulation which should give a more accurate picture of an electron.

> There are no photons in free space.

Um ... I'd suggest rewording that, as this wording is incorrect. Cosmic background photons would likely want to have a word with you on this as well.

It is true that what a photon is in a physical sense is poorly understood (double slit experiment, etc.). The goal of physics is to try to narrow this range, so that we understand more, and have a smaller null space of "here be dragons and photons" that we don't quite grasp.

If you can accept analogies, it would be reasonable to assume that photons are the free space analogs of phonons. Phonons are excitons of an underlying system, with stress/strain tensors. Phonons interact with physical particles, scattering them. Phonon-like things are responsible for Cooper pair formation (effectively a Bose Einstein condensate of two electrons which are fermions, into a pair of fermions with net integer spin). Cooper pairs are responsible for superconductivity. I am not sure if phonons (which I believe are bosons) would be considered particles mediatiating interactions in solids, but I am guessing a good/strong case could be made, along those lines.

The analogy may be apt. But the question that immediately arises is, given that phonons are excitons of an underlying material, is there something analogous for electromagnetic waves?

Up to the very early 1900's, they called this thing the "aether".

Transverse waves (oscillating perpendicular to velocity vector of travel) of phonons are called optical modes. Longitudinal waves (think like sound waves, compression and rarefaction) are called acoustic. I don't know if we have a longitudinal equivalent in photons.

There are many questions remaining. We don't completely understand QM, up to, and including if QM is the right theory. These questions and tests are good. The more questions a theory answers correctly, the less likely it is to be incorrect. As Feynman noted, you only need one conclusive irrefutable result running at odds with a theory to render the theory defunct.

Even with that said, some "wrong" theories (like the "law" of Newtonian gravity) are immensely useful in practical matters, as long as you pay particularly careful attention to range of applicability. Feel happy you don't normally need to pull out full 10-D general relativity to go to the moon. Newtonian mechanics plus corrections works well enough (for now). This will change the farther we want to go.

This all said, there are many open questions in physics, including numerous things that you hear about every day. Anyone claiming, ever, in any field, that the "science is settled" is either not a scientist, a poor scientist, or someone with an agenda. Science is never settled. Good science is looking at data late at night in your office when you should be home with your family, and saying "hmmm, that's funny ..."

(recovering computational theoretical physicist here, who used to quantum mechanically simulate semiconductors on supercomputers ... quite inception-movie like)