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

[iMo]

..is the statement in the first line on the page 37 (in the above book, replay #111) correct?

[radiolistener]

..is the statement in the first line on the page 37 (in the above book, replay #111) correct?

If you mean the statement "why do photons travel in straight lines or alternatively how does the photon model lead to spherical radiation patterns?", then the question itself is misleading.

A photon is a quantum excitation of the EM field, not a classical object with a trajectory in spacetime. Therefore, the classical concept of a trajectory cannot be applied to it.

To make the question consistent with the physics, I would formulate it as follows: how does the quantum model predict where photons will be detected and why does light propagate in straight lines only under certain conditions?

---
Note: Following showman's comment below, when I say here that a "photon is a quantum excitation of the EM field", I mean the quantized EM field, not the classical EM field. In classical electromagnetism, there is no concept of a photon.

[studiot]

..is the statement in the first line on the page 37 (in the above book, replay #111) correct?

Yes and no.

In a dielectric medium the photons travel at a velocity less than c

If you are measuring the time of travel from entry into the bulk medium to exit from it and dividing that into the distance travelled then yes the statement is correct.

But it depends upon your model.

If your model attributes all the delay to interaction with encountered particles of matter and none to the empty space between those particles then no it is not quite correct.

However we do not actually know which model offers the process that really happens, if any of them do.

[studiot]

..is the statement in the first line on the page 37 (in the above book, replay #111) correct?

If you mean the statement "why do photons travel in straight lines or alternatively how does the photon model lead to spherical radiation patterns?", then the question itself is misleading.

A photon is a quantum excitation of the EM field, not a classical object with a trajectory in spacetime. Therefore, the classical concept of a trajectory cannot be applied to it.

To make the question consistent with the physics, I would formulate it as follows: how does the quantum model predict where photons will be detected and why does light propagate in straight lines only under certain conditions?

I asked you before and you ducked the question.

What EM field  ?

If the photon is an excitation of an EM field the EM field must have been pre-existing.

So where did the pre-existing field come from and what sustains it until the excitation occurs ?

And where did the excitation come from to create the photon ?

It should be remembered that light as a wave is a travelling wave that carries its own medium with it so there is no EM field until the wavefront arrives.

[Marco]

why does light propagate in straight lines only under certain conditions?

The specific condition being never ever.

[showman]

..is the statement in the first line on the page 37 (in the above book, replay #111) correct?

If you mean the statement "why do photons travel in straight lines or alternatively how does the photon model lead to spherical radiation patterns?", then the question itself is misleading.

A photon is a quantum excitation of the EM field, not a classical object with a trajectory in spacetime. Therefore, the classical concept of a trajectory cannot be applied to it.

To make the question consistent with the physics, I would formulate it as follows: how does the quantum model predict where photons will be detected and why does light propagate in straight lines only under certain conditions?

I asked you before and you ducked the question.

What EM field  ?

If the photon is an excitation of an EM field the EM field must have been pre-existing.

So where did the pre-existing field come from and what sustains it until the excitation occurs ?

And where did the excitation come from to create the photon ?

It should be remembered that light as a wave is a travelling wave that carries its own medium with it so there is no EM field until the wavefront arrives.

For the last few pages way too much hand-waving and even some hints of crackpottery or have arisen, so I'll just give some hints to hopefully steer the discussion more on topic.

For example, there are bunch of claims and discussion that can only apply to first quantization, as if photon was a massive non-relativistic particle, but it is not. So it's best to stay away from that.

Then, a field is not "something which has a value". With some simplifications that do not matter much here, a classical field is a mapping/function that assigns a value and quantum field is a mapping that assigns an operator. So there is a difference between a classical field and a specific classical field configuration.

Then also saying something like "photon is a quantum excitation of the EM field, not a classical object" says the same amount of something as it does nothing. In the same way an electron is an excitation of Dirac field, not a classical object. Yet an electron can be in a position eigenstate, whereas a photon cannot. So why keep repeating the field excitation part as it is something unique.

In addition it says "the EM field", but as I already said, there is a large difference between a classical field and quantum field (which "the" is it). A photon is not an excitation, neither classical nor quantum, of a classical EM field. That is complete nonsense, but saying it like that makes many people have unnecessary illusions about what a photon actually is or when or even whether the concept is actually useful.

[radiolistener]

Then also saying something like "photon is a quantum excitation of the EM field, not a classical object" says the same amount of something as it does nothing. In the same way an electron is an excitation of Dirac field, not a classical object. Yet an electron can be in a position eigenstate, whereas a photon cannot. So why keep repeating the field excitation part as it is something unique.

I agree that the fact that a photon is an excitation of a quantum field is not, by itself, a sufficient argument against assigning it a position or a trajectory. I was not using it as such.

My point was that treating a photon as a classical object in spacetime with a definite position and trajectory is a conceptual mistake.

In addition it says "the EM field", but as I already said, there is a large difference between a classical field and quantum field (which "the" is it). A photon is not an excitation, neither classical nor quantum, of a classical EM field. That is complete nonsense, but saying it like that makes many people have unnecessary illusions about what a photon actually is or when or even whether the concept is actually useful.

I never claimed that a photon is a quantum excitation of the classical EM field. I was referring to the quantized EM field. But I agree that not mentioning that I'm talking about the quantized EM field may lead to confusion.

But your wording "A photon is not an excitation, neither classical nor quantum, of a classical EM field. That is complete nonsense" may also make many people have unnecessary illusions about what a photon actually is or when or even whether the concept is actually useful. And it can even make one doubt that a photon is a quantum excitation of the quantized EM field, which is even worse... 

So, the corrected wording is: A photon is precisely a quantum excitation of the quantized EM field. Don't confuse this with the classical EM field: photons are not part of the classical EM field description.

[radiolistener]

I asked you before and you ducked the question.

What EM field  ?

If the photon is an excitation of an EM field the EM field must have been pre-existing.

So where did the pre-existing field come from and what sustains it until the excitation occurs ?

And where did the excitation come from to create the photon ?

It should be remembered that light as a wave is a travelling wave that carries its own medium with it so there is no EM field until the wavefront arrives.

A photon does not require a pre-existing classical EM wave or some physical medium in which to travel. The quantized EM field is a fundamental field and the vacuum is one possible state of that field. A photon is an excitation of that field relative to the vacuum state.

So there is no separate classical field that has to be "sustained" until the photon appears.

In a material medium, light still propagates through the electromagnetic field rather than through a separate physical medium. The EM field interacts with charged particles in the medium, driving them to oscillate, and their oscillations produce secondary radiation. The interference of this radiation with the original field results in phase delays and observable changes in the wavefront.

[iMo]

I think the photon is simply woven into the "fabric of spacetime"-> it is "spacetime" itself.
A photon is massless, and it has no concept of time -> the passage of time does not exist for a photon.
A photon is born and dies in the same moment.
It exists everywhere in the Universe within the same infinitely short interval, from a photon's perspective, the Universe has no size.

We perceive a photon as "propagating" or "moving" because we have mass and we are slow. Therefore, for us, time exists and the Universe is large.

On our spacetime bazaar photon traded his mass for no time and no space, while we traded our mass for space and time. This is how the business works in our Universe, imho..

PS: a photon always "moves" with the "constant speed of light c which is the max speed" because that bazaar "spacetime offering" was/is bundled that way (and the size of that package is 1).

[radiolistener]

why does light propagate in straight lines only under certain conditions?

The specific condition being never ever.

Fair enough

The corrected question would be: How does the quantum model predict where photons will be detected and under what conditions does the propagation of light appear to follow straight lines?

[studiot]

I think the photon is simply woven into the "fabric of spacetime"-> it is "spacetime" itself.
A photon is massless, and it has no concept of time -> the passage of time does not exist for a photon.
A photon is born and dies in the same moment.
It exists everywhere in the Universe within the same infinitely short interval, from a photon's perspective, the Universe has no size.

We perceive a photon as "propagating" or "moving" because we have mass and we are slow. Therefore, for us, time exists and the Universe is large.

On our spacetime bazaar photon traded his mass for no time and no space, while we traded our mass for space and time. This is how the business works in our Universe, imho..

PS: a photon always "moves" with the "constant speed of light c which is the max speed" because that bazaar "spacetime offering" was/is bundled that way (and the size of that package is 1).

It seems I have boobed again and I owe a couple of apologies.

I am new here and struggling a bit with the unfamiliar layout of the site.

In particular I am having trouble with the concept of quoting something before I have read it.

Consequently I inadvertently quoted your post whilst actually replying to radiolistener.

Straight after I was feeling so proud of being able to reply to your post splitting the quote to reveal what I was responding to.

Sorry for the confusion.

[studiot]

I asked you before and you ducked the question.

What EM field  ?

If the photon is an excitation of an EM field the EM field must have been pre-existing.

So where did the pre-existing field come from and what sustains it until the excitation occurs ?

And where did the excitation come from to create the photon ?

It should be remembered that light as a wave is a travelling wave that carries its own medium with it so there is no EM field until the wavefront arrives.

A photon does not require a pre-existing classical EM wave or some physical medium in which to travel. The quantized EM field is a fundamental field and the vacuum is one possible state of that field. A photon is an excitation of that field relative to the vacuum state.

So there is no separate classical field that has to be "sustained" until the photon appears.

In a material medium, light still propagates through the electromagnetic field rather than through a separate physical medium. The EM field interacts with charged particles in the medium, driving them to oscillate, and their oscillations produce secondary radiation. The interference of this radiation with the original field results in phase delays and observable changes in the wavefront.

You appear to be postulating an Everett style universal EM field, for which there is much evidence against.

I do not propose to discuss QM further in this thread as it has nothing to do with the original question.

[studiot]

A photon and a straight line are concepts belonging to different descriptions, so they should not be combined as if a photon were a classical object following a geometric trajectory.

I don't mind the nitpicking, but what concept to use then, other than a straight line?  How would you describe the trajectory of a photon?

I am pretty sure the photon goes in a straight line.  A very straight line, because the stars in the sky looks like sharp dots.  In fact, a photon's trajectory is uncannily straight, if we think about how they manage to fall in the same pixel of a CCD camera, after they traveled for 14 billions light-years.  Made an estimation, out of curiosity:  the ratio between 14 billions light years, the distance to some star, and 1 micrometer, the width of a CCD pixel in my camera, where the photons from that star eventually land, is about 10³². 

$ qalc
> 14billion*300_000km/s*365*24*60*60s/1um

  ((14 × billion) × (300000 kilometers/second) × 365 × 24 × 60 × (60 seconds)) /
  (1 micrometer) =
  1.324512E32

If the photons were to deviate, the stars would have looked like fuzzy blobs of light, the further the star, the foggier it will look.  But it is not like that.  The stars seems to look pretty sharp no matter how far they are.  That's a hint the photons must be traveling very, very, very straight.  Somehow, the errors in the landing coordinates of a photon are smaller than 10^-32, and in fact the landing errors are probably much smaller than that. 


It doesn't matter how large or small a photon is, or what it is, but we know for sure its trajectory is a straight line, and we know for sure the photon is a lump of energy, because we see very faint stars as individuals "blips" of consecutive photons.

Same when you produce single photons by heavily filtering/attenuating the light.  If the attenuation is large enough, a photomultiplier will "blip" only once in a while.  The more we attenuate the light, the rare the blips.  The blips do not become fainter with the attenuation, they only become rarer in time.  This indicates the photon is a localized lump of traveling energy.

A photon is not a marble, but it seems to be mostly at one place at a time, and always moving on a straight trajectory.  Somehow, it manages to stay as a mostly compact lump of energy apparently forever. 


Another aspect, the photon has a physical existence, in the form of a small packet/lump of electromagnetic waves.  The photon is not a probability wave as in quantum mechanics.  All that probability wave-function does not describe a single particle, the wave-function tells about the expected behavior of a population of particles.

To give an example, a herd of buffaloes appears to flow like lava while the herd run along a valley.  That doesn't mean a single buffalo flows, and it doesn't mean each buffalo is made out of lava.

Similar, a photon is not a wave of probabilities, it is an electromagnetic wave.  It exists with 100% certainty at all times, until it bumps into something.  And the photon is not spread everywhere in space, it seems to stay compact and well localized, such that after traveling for billions of years it can still fit in my camera.  It travels in a straight line, it exists as a certitude, not as a probability.  A photon is as physical and as well contained, as you and I.


Every time it comes about understanding the physics of the very small, people start calling quantum mechanics.  Quantum mechanics doesn't help, because it is about imaginary events of a large population of particles, it is not about a single particle.  A particle, or an electromagnetic wave, can be instantiated in the physical world as something with a physical existence.  By contrast with that, probability waves can not be instantiated in a physical way.  You can not instantiate a 20% probability with a single particle.  When you measure a single particle, it can only be 0% or 100%.

To me, the probability wave looks entirely fabricated, an "as if", valid only for large populations, telling more or less only this:  "we see randomness when we look at the very small".  It doesn't tell how it is.  It doesn't even tell if the apparent randomness is caused by noise (as in, we don't have all the details to calculate the exact outcome), or if it is a true-randomness in the mathematical sense (as in, there is no such function that can tell the next value).


Anyways, sorry for the long rant, the question was how comes that a spherical ripple in the electric field, a ripple inflating into all directions, eventually turns into a single photon going straight, into only one direction.

So far, the best hint at this seems to be the hairy ball problem, as goldel_labels was saying, because it tells how a sphere of tangential vectors has to have a special point where the tangential vectors swirl.  Highly likely, an enlarging sphere of tangential vectors might be able to produce at the swirling point a lump of self propagating electromagnetic waves, the photon.

I didn't check if that's how it really is, still struggling to visualize that, but so far this would be the explanation that makes the most sense.

I think RoGeorge has answered his own question better than anyone else so far because the explanation involves a much simpler model and folks are making things far too complicated.

There are two parts to the question.

Firstly the issue of the straight line.
Secondly the issue of photons.

It cannot be denied that there is a model of light we call the photon model, which at its most basic is a corpuscular hypothesis.
We also have a more basic model called 'line of sight'.
So it is quite reasonable to ask do photons travel in a straight (or any other) line and how can this be compatible with spherical radiation patterns.
Whether this combination is actually useful is another matter.

It may be more useful to note that between any two players source (S) and Receiver (R) we can identify a straight line in our ordinary world space.
Fermat's principle of least time and the principle of least action tell us that light follows these paths.
General Relativity identifies these paths with null geodesics which are straight or nearly straight in our world.
If S switches the light source on and off, R  can see the presence and absence of light and can can deduce that at least some of the light pulse has travelled directly to him.
Further experimentation would show that light from the source can also travel in other directions, though not necessarily in a spherical shell.

Introducing the photon model, the first thing to note is just how small an individual photon is,
There are several ways to measure its size but the key point is that they all say it is small.

Consequently the number of photons required to produce the light pulse is very very very large.
A source may be regarded as an aggregate of a large number of microsources, each producing a photon.
And the model does not include a description of the direction any given photon will take leading to an average omnidirectional pattern.
So some of the photons must travel along the direct path between S and R.
What happens to the rest of them is determined by conditions external to the source or microsource or both.

As with geometrical optics, it doesn't matter that some of the light goes elsewhere, we are only interested here in the light that travels along the designated paths.

This model, although lacking in much detail and unable to explain other phenomena, is sufficient to explain the phenomenon you ask about.

[Marco]

How does the quantum model predict where photons will be detected and under what conditions does the propagation of light appear to follow straight lines?

It doesn't predict much of anything at that scale, not enough compute, but philosophically the "shape" of the wave packet gets constrained due to decoherence/entanglement.

[radiolistener]

You appear to be postulating an Everett style universal EM field, for which there is much evidence against.

I do not propose to discuss QM further in this thread as it has nothing to do with the original question.

That is not what I was claiming. A quantized EM field and the vacuum state are standard QED, not an Everett-specific assumption.

[radiolistener]

So it is quite reasonable to ask do photons travel in a straight (or any other) line and how can this be compatible with spherical radiation patterns.
Whether this combination is actually useful is another matter.

The fact that a question can be formulated does not mean that all the concepts used in it necessarily apply to the object being discussed.

Of course, one can ask whether a photon travels along a straight line, just as one can ask how much a thought about the Pythagorean theorem weighs in kilograms. The question is grammatically meaningful, but that doesn't mean the concept being assumed by the question necessarily applies. 

It may be more useful to note that between any two players source (S) and Receiver (R) we can identify a straight line in our ordinary world space.
Fermat's principle of least time and the principle of least action tell us that light follows these paths.
General Relativity identifies these paths with null geodesics which are straight or nearly straight in our world.
If S switches the light source on and off, R  can see the presence and absence of light and can can deduce that at least some of the light pulse has travelled directly to him.
Further experimentation would show that light from the source can also travel in other directions, though not necessarily in a spherical shell.

Introducing the photon model, the first thing to note is just how small an individual photon is,
There are several ways to measure its size but the key point is that they all say it is small.

Consequently the number of photons required to produce the light pulse is very very very large.
A source may be regarded as an aggregate of a large number of microsources, each producing a photon.
And the model does not include a description of the direction any given photon will take leading to an average omnidirectional pattern.
So some of the photons must travel along the direct path between S and R.
What happens to the rest of them is determined by conditions external to the source or microsource or both.

As with geometrical optics, it doesn't matter that some of the light goes elsewhere, we are only interested here in the light that travels along the designated paths.

This model, although lacking in much detail and unable to explain other phenomena, is sufficient to explain the phenomenon you ask about.

What experiment establishes that an individual photon has a definite classical trajectory between emission and detection?

I am also puzzled by the discussion of the "size" of a photon. A photon is not a classical corpuscle in spacetime. Roughly speaking, in QED a photon is a quantum excitation of the EM field and it is not described by classical concepts such as a definite size and trajectory. Therefore, asking "how small an individual photon is" simply does not make sense.

And just as a joke: if we still insist on asking "how small is a photon?", the answer might as well be "infinitely"  

I'm not even sure what "small" would mean here, since there is no obvious classical size to compare it with. 

[MrAl]

I quote this statement you wrote because this is a good illustration of where we diverge from our view of everything:
START QUOTE
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.
END QUOTE

Here is my view of that statement, for better or worse...
First, there is no such thing as a "classical coin".  We have coins, that's it.  That's reality and there is no way around that.  We can talk about a classical coin, but that means we have automatically fallen under some assumptions about reality, when what we are trying to determine all along is what reality is or what we know about it.  We want the bottom-line truth.
This is why I think you are looking too hard for comfort in physics when there is none really.
The point I made about observation is that since EVERYTHING we see and know is based on observation, then we look at everything through a sort of "observation" filter.  Something appears, but we don't see it, we only see it AFTER it passes through that observation window.  It's completely out of view until it passes though, and unfortunately once it passes through we lose detail, detail that we may never get back.

The coin is in a superposition of states behind the wall because we can't observe it.  This comes from a chain of reasoning that is part of quantum physics.  This is really the way reality is.  The only reason we BELIEVE it is in one state or the other is due to PAST observations that told us that "coins always land heads or tails" (excluding the edge case for simplicity).  But that observation set was done in the PAST, and that pushes it right into the statistical average category where we rely on common observations over time.  That does not mean that it is REALLY heads or tails, it just SUGGESTS that it is PROBABLY heads or tails.  We have never seen it in a state other than that, that's what makes us BELIEVE it will be again.  In quantum physics though, the down to earth hard and very uncomfortable truth, is that belief is just a heuristic, although it is a very good one for our common experience and our survival.  The QM statistics says that the probability that it will be in some other state is very, very low, BUT it's NOT ZERO.  Again uncomfortable, but true.

This is not just weird as all heck to us, it's weird as all heck to everyone.

I would draw the line between uncertainty in our knowledge and quantum superposition. A coin being behind a wall does not put it into a quantum superposition simply because we cannot observe it. If the coin has landed heads or tails, our lack of knowledge about which one it is does not by itself establish that its physical state is indefinite.

Returning to the example of the mathematical model of a wave using a rotating vector. The projection of that vector onto an axis can describe the measured real signal extremely well but this does not mean that the signal is physically represented by a rotating vector. It may be a very useful and valid mathematical description but the fact that a model accurately describes the observed behavior does not by itself prove that the model is literally what exists in reality.

So I think we should be careful not to confuse a mathematical model with a statement about what reality ultimately is. Claiming that the mathematical description itself is literally the underlying reality goes beyond what the model and the observations alone establish. Without experimental evidence distinguishing that interpretation from other possibilities that becomes a philosophical claim rather than a scientific conclusion.

We may have several models that work within certain limits but we tend to choose one and treat it as reality. Yet reality may be described by a completely different model that we have not discovered yet. 

Hello again,

Well we've both been hand waving a little and that's why we have not reached a consensus yet.  We can't talk about something with the detail needed to describe it without supplying that detail.  I think it's time to dig a little deeper, and it does get interesting, but it also gets even stranger.  In the process though I think I can provide a reasonable solution that will seem more comfortable for you, although you may be aware of a lot of this already.  I just want to point out a few details that should clear things up a bit more.  I'll quote your statements so that I can reply to them appropriately, then add some notes at the end.

"I would draw the line between uncertainty in our knowledge and quantum superposition. A coin being behind a wall does not put it into a quantum superposition simply because we cannot observe it. If the coin has landed heads or tails, our lack of knowledge about which one it is does not by itself establish that its physical state is indefinite."
Your uncertainty idea is right in the classical sense, but in the quantum sense it is very, very, very, rare, but also not zero I don't think.  This means that even in the quantum world we might see heads or tails, but there is a really huge requirement for that to happen.  We'd have to see all of the quantum states in some sort of symmetry where they interact so as to provide a stable state that looks classical even though it's still quantum.  We do have to get more detailed though to see this.  If we have one quantum object behind the wall I think you would agree that it is in a superposition of states.  But what if we have a LOT of quantum objects behind the wall that interact the way they might if they were part of a small coin.  In this toy model, the quantum states would present an oscillation in amplitude that would keep the whole in a state of superposition.  That one very rare case though is if they could somehow interact with some sort of symmetry.  In that case they could stabilize into LOOKING like either heads or tails, and stay that way indefinitely.  This would fit your scenario where they MUST be heads or tails.  It's incredibly rare though because there are a lot of states and they would all have to act in just the right way, such as their phase relationships.  So it's not that it can NEVER happen, but it is very UNLIKELY, and the more particles, the more unlikely it gets, and I think with every new particle the chance decreases by half (or something like that).  But the uncertainty is not the same as quantum superposition, but then I'm not being uncertain about the superposition.  In fact, I am very certain about the superposition.  We may not know if it is heads or tails in the classical sense, but we not only do not know the state in the quantum sense, it actually has not been determined YET so we could not possibly know it.  The difference is that in the classical sense we ASSUME something, while in the quantum sense we do not assume anything because there is nothing to assume: it is simply not in one state yet (except it may look like it is in that one very very rare case).  So in short, classically we assume something (as we always do based on more common experience) and quantumly we don't.

"The rotating vector analogy..."
I do not think we can use an analogy here because there is no classical analogy.  If it is classical, then it's not quantum, end of story, and the fact that it is quantum is what makes the difference, so here we can't talk classical really and expect it to work like a quantum system.  That will never happen.

"So I think we should be careful not to confuse a mathematical model with a statement about what reality ultimately is. Claiming that the mathematical description itself is literally the underlying reality goes beyond what the model and the observations alone establish. Without experimental evidence distinguishing that interpretation from other possibilities that becomes a philosophical claim rather than a scientific conclusion."
We are not really confusing a math model with reality.  We did not find a math model lurking around in space somewhere and say, "hey let's use this math to find out what reality is". We have observed reality in the strictest sense and then found that it has certain properties that can be DESCRIBED by a logical expression(s), and some of the logic depicts the idea that sometimes we get no immediate solution. That's all it really is really, it's a math expression that shows us about quantum physics.  But that math did not come first, it only came after certain measurements were made and conclusions drawn.  It's sort of like curve fitting but on a much more precise scale that includes no "sum of squared error" because it is completely accurate to any desired precision.  So in short, reality told us what math to use, math didn't tell us what reality was.

"We may have several models that work within certain limits but we tend to choose one and treat it as reality. Yet reality may be described by a completely different model that we have not discovered yet."
Oh I have to agree with that for the most part.  However, quantum physics is different because it describes cases where nature has no solution and never will.  This area is a little shady though because we can't predict the future.  Something could change that changes a lot of what we know about reality.  For example, is there a better cosmological model that can show that quantum physics is not the best way to view nature, that there is something more basic.  That is possible, but the way it looks right now is that a more fundamental theory would simply be able to produce quantum theory as a sub theory.  We can't know for sure of course.  Since we are talking about this though we are concentrating on two models: classical and quantum, as we know them today in the latter part of the year 2026.

Ok so that's the end of the quoting I'd just like to add a few more notes...

I think it's important to note that a quantum system is not the same as a classical system.  It can become a classical system though once decoherence takes place.  When we talk about a quantum system though, we don't mix it with a classical system unless we introduce decoherence, and then we explain how that decoherence came about.  In the case of a set of quantum objects that represent a toy coin, if we place a solid wall between us and the coin we assume that the wall did not interfere with the coin.  If we did, we could not talk about the quantum system anymore because it would probably decohere into a classical system where we would see heads or tails.  If we allow that, then we just reduced everything to classical which is not what we want to do all the time.  In fact, it would have to take place in vacuum and we would have to prevent anything from interfering with it.  If anything did interfere, it would eventually turn it classic either very quickly or after some time.  What does this mean for a coin we find in our pocket.  Unfortunately, that coin already has states that have decohered so we can't even start with that in order to gain understanding of the quantum world.  If somehow it did not go through that decoherence yet, as soon as it reacted with the environment it would decohere, so that does not help either.  This is why we have to think about doing this in a vacuum with no radiation and no random fluctuations, no EM, etc.  This is probably the detail you need in order to accept the idea of a continuous quantum set of states with the very rare case of looking classical even though it stays quantum.

We have to look at the details in order to understand how this can happen.  I think that should make you more comfortable with the whole thing

[CatalinaWOW]

So it is quite reasonable to ask do photons travel in a straight (or any other) line and how can this be compatible with spherical radiation patterns.
Whether this combination is actually useful is another matter.

The fact that a question can be formulated does not mean that all the concepts used in it necessarily apply to the object being discussed.

Of course, one can ask whether a photon travels along a straight line, just as one can ask how much a thought about the Pythagorean theorem weighs in kilograms. The question is grammatically meaningful, but that doesn't mean the concept being assumed by the question necessarily applies. 

It may be more useful to note that between any two players source (S) and Receiver (R) we can identify a straight line in our ordinary world space.
Fermat's principle of least time and the principle of least action tell us that light follows these paths.
General Relativity identifies these paths with null geodesics which are straight or nearly straight in our world.
If S switches the light source on and off, R  can see the presence and absence of light and can can deduce that at least some of the light pulse has travelled directly to him.
Further experimentation would show that light from the source can also travel in other directions, though not necessarily in a spherical shell.

Introducing the photon model, the first thing to note is just how small an individual photon is,
There are several ways to measure its size but the key point is that they all say it is small.

Consequently the number of photons required to produce the light pulse is very very very large.
A source may be regarded as an aggregate of a large number of microsources, each producing a photon.
And the model does not include a description of the direction any given photon will take leading to an average omnidirectional pattern.
So some of the photons must travel along the direct path between S and R.
What happens to the rest of them is determined by conditions external to the source or microsource or both.

As with geometrical optics, it doesn't matter that some of the light goes elsewhere, we are only interested here in the light that travels along the designated paths.

This model, although lacking in much detail and unable to explain other phenomena, is sufficient to explain the phenomenon you ask about.

What experiment establishes that an individual photon has a definite classical trajectory between emission and detection?

I am also puzzled by the discussion of the "size" of a photon. A photon is not a classical corpuscle in spacetime. Roughly speaking, in QED a photon is a quantum excitation of the EM field and it is not described by classical concepts such as a definite size and trajectory. Therefore, asking "how small an individual photon is" simply does not make sense.

And just as a joke: if we still insist on asking "how small is a photon?", the answer might as well be "infinitely"  

I'm not even sure what "small" would mean here, since there is no obvious classical size to compare it with. 

The probability density for location of light passing through a hole peaks quite strongly for holes that are large with respect to the wavelength of the light.  And spreads extremely wide as the dimension of the hole becomes smaller than the wavelength.   While this might not explicitly define a size, it certainly has many of the properties of a size. 

[studiot]

The probability density for location of light passing through a hole peaks quite strongly for holes that are large with respect to the wavelength of the light.  And spreads extremely wide as the dimension of the hole becomes smaller than the wavelength.   While this might not explicitly define a size, it certainly has many of the properties of a size. 

Thank you for thinking rather than trying to stick to a fixed idea.

Size is a general term that can be measured in many ways.

How big is a bag of cement ?

What size shoe do you take ?

What size woodscrew do you need ?

When cooking and the recipe calls for a spoonful of ingredient what size spoon do you use ?

How big is a barn ( it is a unit of size in atomic physics) ?


In point of fact a measurable notion of size is used in quantum spectroscopy by way of the uncertainty principle as it defines line width or transition time for a spectrum.
But that is irrelevant to this thread as is quantum theory.

[radiolistener]

The probability density for location of light passing through a hole peaks quite strongly for holes that are large with respect to the wavelength of the light.  And spreads extremely wide as the dimension of the hole becomes smaller than the wavelength.   While this might not explicitly define a size, it certainly has many of the properties of a size.

This shows a relationship between the size of the hole and the resulting diffraction pattern, which is perfectly explained by the wave properties of light, but it does not establish that light consists of a stream of corpuscles with a definite physical size.

The detector tells us about the interaction between the EM field and matter, not necessarily about what the propagating entity fundamentally is. The same applies to the aperture: the diffraction pattern tells us about the interaction between the EM field and the matter forming the aperture, not necessarily about what the propagating entity fundamentally is.

[radiolistener]

If we have one quantum object behind the wall I think you would agree that it is in a superposition of states.

No, I don't agree with that.

If you throw a coin behind a wall and I cannot see whether it landed heads or tails, I don't think this means that the coin is in a quantum superposition. My knowledge of the coin's state is uncertain, and I can even construct a perfectly valid mathematical model with a probability distribution describing that uncertainty. But the fact that this model accurately describes my uncertainty does not mean that the coin itself physically exists in a superposition of states.

To claim that the coin is physically in a quantum superposition would require a physical mechanism and experimental evidence for such a state, not simply the fact that its state is unknown to me.


Someone mentioned above that perhaps the photon is in some sense spacetime itself. I can see why one might be tempted to think along those lines. After all, our operational notion of distance is closely related to the time it takes for an interaction to propagate between physical systems. So it is conceivable that what we perceive as spacetime could itself be an emergent description of some more fundamental process that we do not yet understand.

[TimFox]

Schrödinger’s cat has led to endless confusion about what Quantum Mechanics says about what can be known, and what we choose not to know by putting things in opaque boxes instead of glass boxes.

[MrAl]

If we have one quantum object behind the wall I think you would agree that it is in a superposition of states.

No, I don't agree with that.

If you throw a coin behind a wall and I cannot see whether it landed heads or tails, I don't think this means that the coin is in a quantum superposition. My knowledge of the coin's state is uncertain, and I can even construct a perfectly valid mathematical model with a probability distribution describing that uncertainty. But the fact that this model accurately describes my uncertainty does not mean that the coin itself physically exists in a superposition of states.

To claim that the coin is physically in a quantum superposition would require a physical mechanism and experimental evidence for such a state, not simply the fact that its state is unknown to me.


Someone mentioned above that perhaps the photon is in some sense spacetime itself. I can see why one might be tempted to think along those lines. After all, our operational notion of distance is closely related to the time it takes for an interaction to propagate between physical systems. So it is conceivable that what we perceive as spacetime could itself be an emergent description of some more fundamental process that we do not yet understand.

Hello again,

I had said:
"If we have one quantum object behind the wall I think you would agree that it is in a superposition of states."

Then you said:
"No, I don't agree with that.  If you throw a coin behind a wall and I cannot see whether it landed heads or tails, I don't think this means that the coin is in a quantum superposition."

Your reply this time does not make any sense after I provided a lot more detail, and in the context I was talking about, it MUST be in a superposition of states.
Also after the more detail, we can't talk about throwing a coin behind the wall anymore, we have to describe what it is we are 'throwing' in more detail, which I tried to do.

What surprises me the most though is that you did not recognize this 'experiment' that we originally started talking about.  This is a well-known property of superposition.  In other words, if it is not in superposition then we have nothing to talk about because then it's just like other common objects that we are all very familiar with.

You must agree with the following...
Superposition means we not only do not know what state it is in, even *it* does not have a definite state to be known, at least until we measure it.
Non-superposition means it can have a definite state even if we don't yet know what it is.

[studiot]

If we have one quantum object behind the wall I think you would agree that it is in a superposition of states.

No, I don't agree with that.

If you throw a coin behind a wall and I cannot see whether it landed heads or tails, I don't think this means that the coin is in a quantum superposition. My knowledge of the coin's state is uncertain, and I can even construct a perfectly valid mathematical model with a probability distribution describing that uncertainty. But the fact that this model accurately describes my uncertainty does not mean that the coin itself physically exists in a superposition of states.

To claim that the coin is physically in a quantum superposition would require a physical mechanism and experimental evidence for such a state, not simply the fact that its state is unknown to me.


Someone mentioned above that perhaps the photon is in some sense spacetime itself. I can see why one might be tempted to think along those lines. After all, our operational notion of distance is closely related to the time it takes for an interaction to propagate between physical systems. So it is conceivable that what we perceive as spacetime could itself be an emergent description of some more fundamental process that we do not yet understand.

Hello again,

I had said:
"If we have one quantum object behind the wall I think you would agree that it is in a superposition of states."

Then you said:
"No, I don't agree with that.  If you throw a coin behind a wall and I cannot see whether it landed heads or tails, I don't think this means that the coin is in a quantum superposition."

Your reply this time does not make any sense after I provided a lot more detail, and in the context I was talking about, it MUST be in a superposition of states.
Also after the more detail, we can't talk about throwing a coin behind the wall anymore, we have to describe what it is we are 'throwing' in more detail, which I tried to do.

What surprises me the most though is that you did not recognize this 'experiment' that we originally started talking about.  This is a well-known property of superposition.  In other words, if it is not in superposition then we have nothing to talk about because then it's just like other common objects that we are all very familiar with.

You must agree with the following...
Superposition means we not only do not know what state it is in, even *it* does not have a definite state to be known, at least until we measure it.
Non-superposition means it can have a definite state even if we don't yet know what it is.

I think you misunderstand superposition and also the difference between the coin and Schroedinger's cat.

You may not be able to see the coin but this guy can.

[radiolistener]

You must agree with the following...
Superposition means we not only do not know what state it is in, even *it* does not have a definite state to be known, at least until we measure it.
Non-superposition means it can have a definite state even if we don't yet know what it is.

No, I don't agree with that, and I don't think I should.

This does not follow from the mathematical concept of superposition. In the mathematical model, a superposition is a state that can be expressed as a combination of multiple possible states, with the corresponding amplitudes determining measurement probabilities. Such a description can be used to predict the outcomes of measurements even when we cannot directly check the state of the system.

A mathematical description involving multiple possible outcomes can therefore also describe a perfectly deterministic coin whose actual state is simply unknown to you.

But this mathematical state is abstract and does not by itself tell us that the physical object has no definite state, or even that the object represented by the model is a fundamental physical entity rather than an emergent manifestation of something more fundamental.