Radio waves propagate spherically, how comes the photon moves in a straight line

[CatalinaWOW]

I've searched for Feynman lecture on gratings, and the returned result is about diffraction:  https://www.feynmanlectures.caltech.edu/I_30.html

Is that the lecture, or the situation you were referring to?

Asking because, if that is the lecture then it is not applicable here.  A diffraction grating only works if it is in the way of the photon.  If I put a diffraction grating sideways, 1 meter away from my laser beam, nothing will happen.

No, it was different.  The part I remember is where Feynman started with an angled mirror, and then removing sections.  Does removing a section away from the path of the light have an effect?

It's from "QED: The Strange Theory of Light and Matter".

Finally, there is this possibility: after I tell you something, you just can’t believe it. You can’t accept it. You don’t like it. A little screen comes down and you don’t listen anymore. I’m going to describe to you how Nature is—and if you don’t like it, that’s going to get in the way of your understanding it. It’s a problem that physicists have learned to deal with: They’ve learned to realize that whether they like a theory or they don’t like a theory is not the essential question. Rather, it is whether or not the theory gives predictions that agree with experiment. It is not a question of whether a theory is philosophically delightful, or easy to understand, or perfectly reasonable from the point of view of common sense. The theory of quantum electrodynamics describes Nature as absurd from the point of view of common sense. And it agrees fully with experiment. So I hope you can accept Nature as She is—absurd.

The only caution I would add here is that QED matches with a great number of experiments, but that does not make it reality.   For an example, take the epicycle theories for the observed motion of the planets.  It matched observations for literally centuries, roughly 2000 years.  And was hard to reconcile with common sense. And as we understand the world today it was not reality.

Then a new theory came along and again matched observations for a couple hundred years, Newton's theory of gravity.  Then special and later general relativity.  Now QED.

None of our theories actually is reality, they are just useful descriptions that match our observations.  I suspect QED will not be our last theory.  Whether the next will result in a simplification like what Newton achieved, or will be still more mind bending is beyond my ability to predict.  If Kurzweil was correct none of us may understand it, it may be the product of a second or third generation AI, far beyond human comprehension.  It may even answer the reason why we don't see other intelligences.  They are occupying an aspect of reality beyond our comprehension.

[TimFox]

The geocentric model with epicyclic orbits became more complicated as measurements improved, while the heliocentric model was a far simpler orbital geometry.  An early Jesuitical decision affirmed the theological reason for the geocentric model, but admitted that the Copernican model (with circular orbits) was more useful for astronomical calculations.  The Keplerian model (elliptical orbits with equal area sweeps) was even more accurate when applied to Tycho’s data, but more importantly was used to derive the inverse-square gravitational force law.
That’s how scientific progress is made.

[MrAl]

That's a lot better.  However, the most modern view is not a particle or a classical wave, and the simpler way of stating that is it is neither a particle nor a wave.  The only catch is we have to specify what kind of wave it is, and that's where the probability wave comes into the picture.  It's not a classical wave.

I'm not sure I can explain this in an instant, but it might be said that the water wave could be viewed as a probability wave if we just think of the amplitude as being a matter of probability not a direct measurement that we are usually used to doing.  To take it further, nothing we see has a non-probabilistic nature, it just looks that way because the probability is so low it's almost nonexistent.  "Almost" is the key word though because it is still probabilistic but some of the probability factors are so low that they may not occur for the entire life of the universe.  Pretty nutty, and the really nutty part is that it actually could happen before that, it's just not very likely.

I guess the simpler explanation is that we are constantly projecting our previous knowledge onto the new discoveries.  It takes a contradiction to show us how wrong we might be.

There are no special "probability waves" that require a different kind of wave physics. The same mathematical tools of wave analysis can be applied to all kinds of waves, what changes is what the wave represents and what its amplitude means.

In particular, I would be careful about treating a probability wave as a physical wave that actually exists in space. Probability is a mathematical way of describing a system when we do not have direct access to all of its underlying details.

For example, suppose you secretly choose a number from 0 to 9 and don't tell me which one. You can give me various pieces of indirect information about your choice. From that information, I can construct a probability distribution describing how likely each number is. The more information you give me, the more accurately I can determine that distribution.

However, no matter how much information you give me, I can never know with absolute certainty which number you actually chose. Even if you tell me the number directly, I still cannot be absolutely certain that you are telling me the truth. I have no independent way of checking what number you actually chose.

But that does not mean that you did not choose a definite number and that there is actually a probability distribution inside your head instead of a specific number. The number you chose is perfectly definite, the probability distribution exists in my description of the situation because I do not have direct access to your mind to check it.

I think something similar is worth keeping in mind when talking about probability waves. A probability distribution is not necessarily a physical substance or a new kind of wave. It is simply part of our mathematical description of what we can and cannot know about the system.

I thought I explained all that, but I'll add this............

A probability wave is simply this:
A spread-out pattern that tells you how likely a particle is to be found in different places.
That’s it in one line.

What it actually means, physically...
It’s not a wave of stuff.
It’s not a wave the particle rides on.
It’s a wave whose height at each point corresponds to the probability amplitude.
Squaring that height gives the actual probability of detecting the particle there.
So the wave is the rulebook for where the particle could show up.
[note the particle here is what we refer to which is really the result of the measurement process it does not have to be an actual particle like a speck of dust]

---------------------------------------------------------------------------------------------------------

That's probably the best way to talk about it, but another way to say it could be the engineer-to-engineer version...
A probability wave is the complex-valued state vector whose squared magnitude gives the spatial probability density for a particle’s detection. Its evolution follows a linear differential equation (Schrodinger), and collapse is a stochastic sampling of that density.
In pure signal-processing language:
It’s a distributed complex signal whose power envelope determines where the 'particle' can be absorbed.

---------------------------------------------------------------------------------------------------------

One more angle about the measurement process itself which is what I had also been talking about...

Measurement collapse is not something the photon does.
It is what the detector’s degrees of freedom allow when it absorbs energy.
Collapse is the detector transitioning to a definite state after interacting with the field.

In other words, collapse is the system update that happens when the EM field dumps one quantum of energy into a 'charged' degree of freedom.

Nothing “shrinks,” nothing “localizes,” nothing “jumps.”
The interaction is what becomes localized.

--------------------------------------

What happens here and in other areas of physics is we are looking for simple answers that match what we ALREADY know about reality.  In the case of the waves and particles and stuff like that, we haven't yet completely figured out what reality really is so we are in the middle ground somewhere trying to get from not completely understanding to completely understanding.  The above is about the best we can get right now I think.

[radiolistener]

A probability wave is simply this:
A spread-out pattern that tells you how likely a particle is to be found in different places.

I would make one small but important change here: instead of saying "how likely a particle is to be found in different places", I would say "how likely an interaction between the EM field and matter is to occur at different locations".

I would transform your phrase like this:

Code: [Select]

A probability wave is simply this:
A spread-out pattern that tells you how likely an interaction between the EM field and matter is to occur at different locations.


[MrAl]

A probability wave is simply this:
A spread-out pattern that tells you how likely a particle is to be found in different places.

I would make one small but important change here: instead of saying "how likely a particle is to be found in different places", I would say "how likely an interaction between the EM field and matter is to occur at different locations".

I would transform your phrase like this:

Code: [Select]

A probability wave is simply this:
A spread-out pattern that tells you how likely an interaction between the EM field and matter is to occur at different locations.


That's interesting because I would transform your phrase back into my phrase

The EM field is considered more physical, while the probability wave is considered more mathematical.
You can say it in different ways that's *true*, but once we start talking about probabilities, we end up thinking within the quantum world.

The difference is in the epistemic viewpoint.  The probability wave is what MIGHT happen, the EM field is what DOES happen.  There are different ways to talk about this, and that means it is difficult to exclude one viewpoint with favor to another.  So you can say it ONE way, but you can't exclude saying it another way too.
Your view ADDS to the way we can view this, but does not take anything away from the other viewpoint.

So if we really wanted to "transform" anything we would probably want to transform it into BOTH viewpoints.

[TimFox]

Usually, when we say something "moves", we have observed it at at least two positions, such as watching a vehicle directly or on a motion picture.
However, we don't observe the same photon twice, only when we detect or measure it interacting with a photodiode, photographic film, or other device.

[radiolistener]

The difference is in the epistemic viewpoint.  The probability wave is what MIGHT happen, the EM field is what DOES happen.  There are different ways to talk about this, and that means it is difficult to exclude one viewpoint with favor to another.  So you can say it ONE way, but you can't exclude saying it another way too.
Your view ADDS to the way we can view this, but does not take anything away from the other viewpoint.

So if we really wanted to "transform" anything we would probably want to transform it into BOTH viewpoints.

I agree that different descriptions can be useful, but there is one important point: before we can say that one viewpoint "does not exclude" another, the other viewpoint has to actually be applicable in the first place.

Consider a simple analogy with a car and a thought. A car has a definite velocity. For example it can be traveling at 40 km/h. How would you transform that into the language of a thought? How fast is the thought moving through space?

Likewise a car has definite spatial coordinates and occupies a certain volume at a particular moment in time. How would you transform a thought into that description? What are the spatial coordinates of a thought at a particular moment? What volume does it occupy?

I think the problem is not that we are choosing between two equally valid descriptions and arbitrarily excluding one of them. The problem is that some concepts simply do not have the properties required by the other description.

The same is true for the photon. If position, size, velocity and trajectory are concepts belonging to a classical spacetime description then you cannot simply "transform" a photon into that description and expect all of those properties to exist.

[MrAl]

The difference is in the epistemic viewpoint.  The probability wave is what MIGHT happen, the EM field is what DOES happen.  There are different ways to talk about this, and that means it is difficult to exclude one viewpoint with favor to another.  So you can say it ONE way, but you can't exclude saying it another way too.
Your view ADDS to the way we can view this, but does not take anything away from the other viewpoint.

So if we really wanted to "transform" anything we would probably want to transform it into BOTH viewpoints.

I agree that different descriptions can be useful, but there is one important point: before we can say that one viewpoint "does not exclude" another, the other viewpoint has to actually be applicable in the first place.

Consider a simple analogy with a car and a thought. A car has a definite velocity. For example it can be traveling at 40 km/h. How would you transform that into the language of a thought? How fast is the thought moving through space?

Likewise a car has definite spatial coordinates and occupies a certain volume at a particular moment in time. How would you transform a thought into that description? What are the spatial coordinates of a thought at a particular moment? What volume does it occupy?

I think the problem is not that we are choosing between two equally valid descriptions and arbitrarily excluding one of them. The problem is that some concepts simply do not have the properties required by the other description.

The same is true for the photon. If position, size, velocity and trajectory are concepts belonging to a classical spacetime description then you cannot simply "transform" a photon into that description and expect all of those properties to exist.

Hello,

You seem to want to declare what exists all by yourself.  If that is not true, then maybe you can define how you declare what exists and what does not exist.  For now it seems that you want to believe that one thing exists that maybe favors one of your arguments, and declare that the other does not exist which also favors your argument.  You can explain more if you like though.  In other words, you accept one definition and condemn the other.  It could be that they are both acceptable.  In fact, there are people who naturally accept either or even more.

I do not believe there is any one definition of what actually exists, and you can imagine how deep this gets because we are talking about the edge or ONE of the edges of what we call reality.  Here are a few ideas...

Physical things make us think in terms of interactions.
Experiential things work within our consciousness.
Abstract things follow some logical thought.
Structure works with pattern relations.
Social things are more like a collective behavior.

My favorite example is the "hole in the ground".  This is the simplest example I think.
If we have a piece of land like in the back yard and we dig a 'hole' in the ground with a shovel, the hole is void of any dirt unlike the surrounding soil.
The question is, does the hole exist?
If we say it exists, then why is there nothing there, and why is it made up of only the material that surrounds it.
If we say it does not exist, then why is there material missing from the ground in that one area.

The key to this I think is that we do not start with a definition of what exists, we start with what we find in the world and then try to figure it out from there deciding what exists based on the particular case.  Could it even be that we find that something both exists and does not exist for different cases.

Existence does not seem to be a fundamental property of the universe.  It seems to involve a pattern.  It's a status we assign when the pattern has repetition.  We might say that existence manifests itself as a reliable set of patterns.

[radiolistener]

You seem to want to declare what exists all by yourself.  If that is not true, then maybe you can define how you declare what exists and what does not exist. For now it seems that you want to believe that one thing exists that maybe favors one of your arguments, and declare that the other does not exist which also favors your argument.  You can explain more if you like though.  In other words, you accept one definition and condemn the other.  It could be that they are both acceptable.  In fact, there are people who naturally accept either or even more.

I think you are misunderstanding my point. I was not talking about whether the photon exists or not.

My point is that the concept of a photon is simply not part of the classical description of objects in spacetime. Therefore, asking whether a photon "exists" or "does not exist" within that description is a mistake by definition.

I am not accepting one definition of existence and condemning another. I am saying that we first have to look at what concepts belong to a particular description before asking what exists within it.

My favorite example is the "hole in the ground".  This is the simplest example I think.
If we have a piece of land like in the back yard and we dig a 'hole' in the ground with a shovel, the hole is void of any dirt unlike the surrounding soil.
The question is, does the hole exist?
If we say it exists, then why is there nothing there, and why is it made up of only the material that surrounds it.
If we say it does not exist, then why is there material missing from the ground in that one area.

I think your example of a hole in the ground or perhaps more known as "the hole in a doughnut", and my example of a thought illustrate the same point. A hole exists as a perfectly meaningful concept but it is not a separate physical object occupying space alongside the doughnut. Likewise a thought exists as a meaningful concept but it is not an object occupying some volume of space alongside the person's brain.

The fact that a concept exists in one description does not mean that it is an object in another description where that concept is not part of the model.

So asking for the photon's size, position or trajectory in the classical spacetime description is simply a mistake. The photon is not an object introduced in that spacetime description in the first place, it is a concept belonging to a completely different description.

It is like asking for the spatial coordinates, volume, or trajectory of a thought. The concept of a thought is perfectly meaningful but those are simply not properties of a thought.

The issue is not whether the thought exists or not. The concept of a thought itself is simply not part of the description of objects in spacetime. Therefore a thought is not an object in that description. Treating it as such is the mistake.

[showman]

we know for sure the photon is a lump of energy

Then the next logical question is how much energy? If you're thinking about something like E=hf, then it's wrong, I need some other answer.

In many ways rf-messkopf's answer, perhaps with some caveats, is the only correct one in this thread, since it's the only one that concerns second quantization. But then again it is likely not understandable/unsatisfactory for many. Basically in that case the best advice would be to drop the concept of "photons" altogether. As W. E. Lamb (a Nobel laureate) put it,

At the first of the 1960's Rochester Coherence Conferences, I suggested that a license be required for use of the word "photon", and offered to give such a license to properly qualified people. My records  show that nobody working in Rochester, and very few other people elsewhere, ever took out a license to use the word "photon". It reminds me that there was once a phlogiston theory of heat, which began to go out of style about the time that people at Cambridge University stopped using the corpuscular optics of Newton.

and

It should be apparent from the title of this article that the author does not like the use of the word "photon", which dates from 1926. In his view, there is no such thing as a photon. Only a comedy of errors and historical accidents led to its popularity among physicists and optical scientists. I admit that the word is short and convenient. Its use is also habit forming. Similarly, one might find it convenient to peak of the "aether" or "vacuum" to stand for empty space, even if no such thing existed. There are very good substitute words for "photon", (e.g., "radiation" or "light"), and for "photonics" (e.g., "optics" or "quantum optics").

[CatalinaWOW]

All of this about whether photons have existence or not is important for those trying to understand the ultimate nature of the universe, and even for some working in odd corners of the physical world.  But just as the great majority of us still find Newtonian physics useful for day to day calculation, the photon concept is quite useful in many applications and analysis. 

A more pedestrian example has even found its way into a couple of threads on this forum.  The lumped parameter model for circuit operation nearly completely disdains solving the wave equations that more correctly (and when solvable), accurately describe how electromagnetic fields permeate our devices.  And there have been call outs by some here disdaining those lumped parameter models.  I however would not throw out Spice and its ancestors because it is only a crude and sometimes misleading approximation.

[radiolistener]

All of this about whether photons have existence or not is important for those trying to understand the ultimate nature of the universe, and even for some working in odd corners of the physical world.  But just as the great majority of us still find Newtonian physics useful for day to day calculation, the photon concept is quite useful in many applications and analysis.

If by "existence" you mean whether the concept of a photon exists, then of course it does. The concept of a photon is perfectly meaningful and extremely useful, just as the concept of a thought exists and is useful.

But if by "existence" you mean whether a photon exists as an object in spacetime, then the answer is no. The photon is not an object of the spacetime description. The concept of a photon simply does not apply to the description of objects in spacetime.

It is the same with a thought. A thought certainly exists as a meaningful concept, but it does not exist as an object in spacetime with its own position, size, or trajectory. Those properties simply do not apply to a thought.


Just to be clear, I am not arguing that the photon concept should be discarded. Quite the opposite - it is useful precisely within the description where it applies. The mistake is taking a concept from one description and treating it as an object in another description where that concept is not part of the model.


SPICE is a good example. SPICE is extremely useful, but you cannot meaningfully ask it for the spatial distribution of the electromagnetic field around a circuit node. That concept simply isn't part of the lumped-parameter model. You need a different description, such as a field solver. This does not mean that the electromagnetic field is "unreal" or that SPICE is wrong. It means that different descriptions contain different concepts and quantities.

[studiot]

It is nice to see engineers? with a better handle on things than all too many Physicists.
As far as I can see

Radiolistener and MrAl are saying the same thing , but in different words.

It is worth noting, however, that the phase space in which the wave functions exist have at least one fractal dimension.

[CatalinaWOW]

All of this about whether photons have existence or not is important for those trying to understand the ultimate nature of the universe, and even for some working in odd corners of the physical world.  But just as the great majority of us still find Newtonian physics useful for day to day calculation, the photon concept is quite useful in many applications and analysis.

If by "existence" you mean whether the concept of a photon exists, then of course it does. The concept of a photon is perfectly meaningful and extremely useful, just as the concept of a thought exists and is useful.

But if by "existence" you mean whether a photon exists as an object in spacetime, then the answer is no. The photon is not an object of the spacetime description. The concept of a photon simply does not apply to the description of objects in spacetime.

It is the same with a thought. A thought certainly exists as a meaningful concept, but it does not exist as an object in spacetime with its own position, size, or trajectory. Those properties simply do not apply to a thought.


Just to be clear, I am not arguing that the photon concept should be discarded. Quite the opposite - it is useful precisely within the description where it applies. The mistake is taking a concept from one description and treating it as an object in another description where that concept is not part of the model.


SPICE is a good example. SPICE is extremely useful, but you cannot meaningfully ask it for the spatial distribution of the electromagnetic field around a circuit node. That concept simply isn't part of the lumped-parameter model. You need a different description, such as a field solver. This does not mean that the electromagnetic field is "unreal" or that SPICE is wrong. It means that different descriptions contain different concepts and quantities.

It is beyond my expertise to have an opinion on whether current definitions/descriptions of spacetime are the final answer on what the world "really is".  But I will definitely agree that a photon does not exist in that formalism. 

I guess what I am saying is that the map is not the territory.  And that spacetime and Newtonian physics and lumped parameter models are all maps.  Some more detailed than others, some with fewer areas of discrepancy between observations and the maps.

[SteveThackery]

I guess what I am saying is that the map is not the territory.  And that spacetime and Newtonian physics and lumped parameter models are all maps.  Some more detailed than others, some with fewer areas of discrepancy between observations and the maps.

I think "model" is a better world. All of the mathematics and verbal explanations between them create a "model" of the real world: a model which represents the real world but is not, in itself, the real world or even a complete and perfect description of the real world. We can use this model to make predictions about how the real world will behave. Newton's mathematical models made predictions which appeared to be perfect for over 200 years. Eventually we realised the predictions went awry when objects were moving at a significant proportion of light speed. Hence a new model was developed - Special Relativity. That made even better predictions.

"All models are wrong, but some are useful." Light as a wave; light as particles; both are models. Both make useful predictions. Neither model is at all a complete description of the real world. As it happens, the models appear to be contradictory, but physicists have models which are more sophisticated than "wave or particle" and can incorporate both. It's just that the concepts and the maths are beyond most laypersons' understanding.

So, it's not turtles all the way down, it's models all the way down.

[studiot]

I guess what I am saying is that the map is not the territory.  And that spacetime and Newtonian physics and lumped parameter models are all maps.  Some more detailed than others, some with fewer areas of discrepancy between observations and the maps.

I think "model" is a better world. All of the mathematics and verbal explanations between them create a "model" of the real world: a model which represents the real world but is not, in itself, the real world or even a complete and perfect description of the real world. We can use this model to make predictions about how the real world will behave. Newton's mathematical models made predictions which appeared to be perfect for over 200 years. Eventually we realised the predictions went awry when objects were moving at a significant proportion of light speed. Hence a new model was developed - Special Relativity. That made even better predictions.
"All models are wrong, but some are useful." Light as a wave; light as particles; both are models. Both make useful predictions. Neither model is at all a complete description of the real world. As it happens, the models appear to be contradictory, but physicists have models which are more sophisticated than "wave or particle" and can incorporate both. It's just that the concepts and the maths are beyond most laypersons' understanding.

So, it's not turtles all the way down, it's models all the way down.

Yes I agree the conventional word these days is model.

A couple of thoughts

Is a single molecule of helium a gas ?

So is a single photon - light ?

These days we have been able to isolate (trap) a single ion, molecule and even most recently a single photon (though only for a short time).

[radiolistener]

These days we have been able to isolate (trap) a single ion, molecule and even most recently a single photon (though only for a short time).

A molecule is a physical object in spacetime. It can be localized in space and its motion can be described through spacetime. So saying that a single molecule was trapped for a short time makes sense.

A photon is different. A photon does not exist as an object in spacetime, so saying that a single photon was trapped "for a short time" should not be interpreted as the lifetime of a little object sitting somewhere in space.

[studiot]

These days we have been able to isolate (trap) a single ion, molecule and even most recently a single photon (though only for a short time).

A molecule is a physical object in spacetime. It can be localized in space and its motion can be described through spacetime. So saying that a single molecule was trapped for a short time makes sense.

A photon is different. A photon does not exist as an object in spacetime, so saying that a single photon was trapped "for a short time" should not be interpreted as the lifetime of a little object sitting somewhere in space.

I didn't say anything about lifetime.

However the single photon definitely acts like an object in the French experiment.

It cannot be denied that it has a physical presence, for as long as they could hold it.
Nor can it be said that the photon's lifetime ended when it leaked away.

[MrAl]

You seem to want to declare what exists all by yourself.  If that is not true, then maybe you can define how you declare what exists and what does not exist. For now it seems that you want to believe that one thing exists that maybe favors one of your arguments, and declare that the other does not exist which also favors your argument.  You can explain more if you like though.  In other words, you accept one definition and condemn the other.  It could be that they are both acceptable.  In fact, there are people who naturally accept either or even more.

I think you are misunderstanding my point. I was not talking about whether the photon exists or not.

My point is that the concept of a photon is simply not part of the classical description of objects in spacetime. Therefore, asking whether a photon "exists" or "does not exist" within that description is a mistake by definition.

I am not accepting one definition of existence and condemning another. I am saying that we first have to look at what concepts belong to a particular description before asking what exists within it.

My favorite example is the "hole in the ground".  This is the simplest example I think.
If we have a piece of land like in the back yard and we dig a 'hole' in the ground with a shovel, the hole is void of any dirt unlike the surrounding soil.
The question is, does the hole exist?
If we say it exists, then why is there nothing there, and why is it made up of only the material that surrounds it.
If we say it does not exist, then why is there material missing from the ground in that one area.

I think your example of a hole in the ground or perhaps more known as "the hole in a doughnut", and my example of a thought illustrate the same point. A hole exists as a perfectly meaningful concept but it is not a separate physical object occupying space alongside the doughnut. Likewise a thought exists as a meaningful concept but it is not an object occupying some volume of space alongside the person's brain.

The fact that a concept exists in one description does not mean that it is an object in another description where that concept is not part of the model.

So asking for the photon's size, position or trajectory in the classical spacetime description is simply a mistake. The photon is not an object introduced in that spacetime description in the first place, it is a concept belonging to a completely different description.

It is like asking for the spatial coordinates, volume, or trajectory of a thought. The concept of a thought is perfectly meaningful but those are simply not properties of a thought.

The issue is not whether the thought exists or not. The concept of a thought itself is simply not part of the description of objects in spacetime. Therefore a thought is not an object in that description. Treating it as such is the mistake.

Hi again,

I am not sure what you are saying about a 'thought'.  Who was talking about a thought, or did I say something that sounded like I was talking about a thought?

I think what the bottom line here is that we are all looking for a way to feel comfortable with a theory, and when we get this much into it that is just not possible.  Your comfort seems to come from thinking about things as physical maybe?  For me, a probability is the closest we can get to reality at least for now.  The reason for this is that there simply is nothing else.

We can never know where anything is, we can only know the probability of where it might be.  This gets very deep too.
If we throw a coin over a wall and it lands on the ground where we can't see over the wall, we don't know how the coin landed.  It is heads up or tails up.  We don't know, and we cannot assume that it is either heads or tails (or on edge as some people like to throw in there).  It is in a state of in between.  It's neither heads nor tails, but something else that could be heads, tails, on edge, or something else in between.

Why is that so.  It's because our reality depends hugely on observation.  Everything we see and measure depends on observation, so everything we know is based on observation. We base all of our knowledge on what we have observed in the past.  Nature does not seem to care about how we observe something, it does what it wants to do.  In common knowledge, what we observe is usually what we get, but it's not always like that.  If nature wants to do something or have properties that we can't ever observe, there's not much we can do about it, and it turns out that is what nature does sometimes, unfortunate as it is for us when we try to understand it.
In functional notation this might look like:
WhatWeGet=WhatWeSee(t,x,y,z)
in common knowledge, but nature has another function added:
WhatWeGet=WhatWeSee(t,x,y,z)+WhatNatureDoes(t,x,y,z,...)
Since that second function is not accessible to us, we can never know what it is.

Pretty crazy yes, but then when we observe something we only have so many senses to use in order to observe it, including our sophisticated measuring equipment and techniques.
That means there is no such thing as completely comfortable physics.

[studiot]

My point is that the concept of a photon is simply not part of the classical description of objects in spacetime. Therefore, asking whether a photon "exists" or "does not exist" within that description is a mistake by definition.

Actually photons do exhibit certain spacetime characteristics and this is important because it is these characteristics that answer the original question why a photon travels in straight lines.

But I do agree that things are a lot more complicated than considering some form of corpuscular model, since photon spin produces quantum effects.

[radiolistener]

However the single photon definitely acts like an object in the French experiment.

It cannot be denied that it has a physical presence, for as long as they could hold it.
Nor can it be said that the photon's lifetime ended when it leaked away.

The experiment demonstrates a physical quantum state of the EM field, not the physical presence of a small spacetime object being physically trapped inside the cavity.

The concept of a lifetime, in the sense of a spacetime object existing for a certain period of time, does not apply to a photon.

The fact that the brain can "hold" a thought for some time does not mean that the thought has a physical presence as an object in spacetime. It is a state or process of the brain - not a separate object occupying some volume of spacetime.

Likewise, the fact that a cavity can "hold" a one-photon state does not mean that a little photon object is physically sitting inside the cavity. It is a quantum state of the EM field in the cavity.

[radiolistener]

I am not sure what you are saying about a 'thought'.  Who was talking about a thought, or did I say something that sounded like I was talking about a thought?

I brought up the thought analogy to explain why a photon does not need to have a position, size, or trajectory in spacetime. A thought is a useful analogy because it shows how something can be a perfectly meaningful and useful concept for reasoning, while not existing as a separate object in spacetime.

In other words, I am using a thought as an example of how a concept can be useful without being a spacetime object and why trying to interpret such a concept as an object in spacetime is a mistake.

For me, a probability is the closest we can get to reality at least for now.  The reason for this is that there simply is nothing else.

We can never know where anything is, we can only know the probability of where it might be.  This gets very deep too.
If we throw a coin over a wall and it lands on the ground where we can't see over the wall, we don't know how the coin landed.  It is heads up or tails up.  We don't know, and we cannot assume that it is either heads or tails (or on edge as some people like to throw in there).  It is in a state of in between.  It's neither heads nor tails, but something else that could be heads, tails, on edge, or something else in between.

Why is that so.  It's because our reality depends hugely on observation.  Everything we see and measure depends on observation, so everything we know is based on observation. We base all of our knowledge on what we have observed in the past.  Nature does not seem to care about how we observe something, it does what it wants to do.  In common knowledge, what we observe is usually what we get, but it's not always like that.  If nature wants to do something or have properties that we can't ever observe, there's not much we can do about it, and it turns out that is what nature does sometimes, unfortunate as it is for us when we try to understand it.

I agree that physics does not give us a God's-eye view of reality. But I think there is an important distinction between uncertainty in our knowledge and indeterminacy in the physical system itself.

For example, if a classical coin has fallen behind a wall, the fact that we don't know whether it is heads or tails does not mean that the coin is physically in some intermediate state. Our description is uncertain, but the coin itself is not in an intermediate state.

A quantum state is not a classical spacetime object with a definite position, size, and trajectory. And there is no need to replace every statement about reality with "we only know probabilities".

A model can tell us exactly which concepts and quantities are meaningful within that model. So we don't have to decide what the "ultimate reality" is before deciding whether a particular concept belongs to a particular description.

[studiot]

However the single photon definitely acts like an object in the French experiment.

It cannot be denied that it has a physical presence, for as long as they could hold it.
Nor can it be said that the photon's lifetime ended when it leaked away.

The experiment demonstrates a physical quantum state of the EM field, not the physical presence of a small spacetime object being physically trapped inside the cavity.

The concept of a lifetime, in the sense of a spacetime object existing for a certain period of time, does not apply to a photon.

The fact that the brain can "hold" a thought for some time does not mean that the thought has a physical presence as an object in spacetime. It is a state or process of the brain - not a separate object occupying some volume of spacetime.

Likewise, the fact that a cavity can "hold" a one-photon state does not mean that a little photon object is physically sitting inside the cavity. It is a quantum state of the EM field in the cavity.

Thank you for your response to my comment.

I was not talking about cavity resonators when considering the properties of a photon considered as an object in spacetime (although it partly depends upon one's definition of spacetime).

Photons have no mass and I agree the concept of lifetime is meaningless in their frame.

But photons do carry and can transfer momentum to massive objects.

This is a purely corpuscular property in spacetime.

Unlike the particles in a gas they cannot transfer momentum between each other.


Spin is a dual property which is why I said It also has a foot in the quantum camp.


Jennison's article about electrons is both interesting and thought provoking.

Wireless World  June 1979  R C Jennison

"What is an Electron? A New Model: the Phase-Locked Cavity"

[David Hess]

Unlike the particles in a gas they [photons] cannot transfer momentum between each other.

Sure they can, and the conditions have been created and observed.

https://en.wikipedia.org/wiki/Two-photon_physics
https://atlas.cern/updates/briefing/atlas-observes-light-scattering-light

[radiolistener]

I was not talking about cavity resonators when considering the properties of a photon considered as an object in spacetime (although it partly depends upon one's definition of spacetime).

Photons have no mass and I agree the concept of lifetime is meaningless in their frame.

But photons do carry and can transfer momentum to massive objects.

This is a purely corpuscular property in spacetime.

Unlike the particles in a gas they cannot transfer momentum between each other.

Spin is a dual property which is why I said It also has a foot in the quantum camp.

Imagine floats on the surface of water. The floats can transfer momentum to each other through the water but the water does not thereby become a collection of floats. The water is what mediates the interaction between them.

Likewise, the EM field can carry energy and momentum and transfer momentum to charged matter. That does not make the EM field itself a collection of little corpuscular objects.

And the same applies to molecules in a gas. Molecules can exchange momentum through their interactions, but the EM field mediating those interactions does not thereby become a molecule or a corpuscular object.

So the ability to carry or transfer momentum does not by itself imply that something is a spacetime corpuscle. 


PS: Classical electromagnetism already describes the transport and transfer of energy and momentum by the EM field using the Poynting vector and the Maxwell stress tensor, without introducing photons at all. You don't need QED to describe this.