'Photons only exist at the moment they are emitted or absorbed' ()

[Mentz114]

There are strong sounding arguments that suggest that the EM field retains its discreteness when its not interacting with matter. I still think this is an undecidable ( by experiment ) question.

arguments against discreteness
------------------------------
I'm not an expert but in quantum optics it's hard to 'pin down' the photon. The photon number operator can have vacuum fluctuations which are large compared to the average photon number. There's no conservation of photon number, obviously.

The first quantization wave equation of the photon lacks localizability, which may be interpreted as saying that the photons are everywhere at the same time.

In quantum field theory ( second quantisation ) photons are just notches on a stick, ie Fock space states.

How can one define the 'size' ( spatial boundaries) of a photon ?

[end of arguments]

Whether one believes that EM fields are a lot of photons flying about, or a non-local energy field, makes little difference in practice. The fact is that in current mainstream theories, there are no photons per se, although the word is used a lot.

If we define a photon as 'a quantum of the EM field' and leave it there, we should get along fine.

I would like to amend my original claim to "we may not be able to determine if the EM field is discrete (quantised) other than when interacting with matter".

 

[CaptainQuasar]

So it needs to be localized now. Why?

"Needs to be"? What do you mean by "needs to be"? It seems like you're doing something other than trying to understand the question under discussion. As if you're engaging in some sort of legalism, though for what purpose I can't guess.

So anyways, if you look at what the people in the thread have said who seem to have been able to grasp the question immediately without any semantic hangups, they speak of the photon being "in flight" - transitioning between points in space. If a photon was completely diffuse and omnipresent, if it was everywhere at once, that would be inconsistent with the impression of it moving through space between points - so clearly the idea of a photon which they've somehow arrived at together is not an omnipresent entity but a localized one.

It's obvious to me that being localized is a characteristic that people are associating with photons from the way they're talking about it. And I think the only reason you wouldn't see that is if you were trying on purpose to ignore it. It's as though you're being didactic or something, like you think it's your job to teach the rest of us a lesson about articulating questions.

You don't need to pretend as though I am not describing various aspects of an intelligible question that is at least tangentially related to what people have been talking about here, or act as though the points I'm making are bizarre and incoherent or something. It's fine if you think that there's some additional question that can be formulated out of the discussion here that's radically different from what I'm describing. Sure, go ahead and go off and play in semantic-land. But seriously, cut the crap, you don't need to deny my formulation of the question to make your own.

I have another question for those of you who support the view that photons don't exist between emission and detection:

Would you say that quantum states exist between measurements?

See, now, there you go. You don't think we have explored the meanings of the terms "exist" and "physical" deeply enough, but you're going to toss off in a single sentence an equivalent to a question that DrGreg used six paragraphs, a bulleted list, and a request to read most of a separate thread to ask - which you're claiming was inadequately articulated?

I think that, as I said above, you, I, and everyone else understand the question under discussion just fine. At this point I'm beginning to think that your quibbles about semantics are actually some sort of underhanded, circumspect way of making indirect points about what you think photons are.

I wanted to find out if the view expressed by Mentz114 had some theoretical justification behind it.

My impression is not that Mentz114 was saying that he's working off of any particular comprehensive theory that definitely says that photons do not exist between absorption and emission, but making the point that there are models like his "cup of water and the ocean" analogy that would not counterfactualize the experimental results we have pertaining to the emission and absorption of electromagnetic energy.

He's simply pointing out that, although thinking of photons as distinct and individual localized entities that transit between emission and absorption locations is a handy model to work on and certainly a natural way to think of it, to date we don't have evidence to definitively say that it's accurate. As has often happened in the history of science a model that has served us well up to a point may be pulled back to reveal something that is much more complex or exotically different from what we'd thought.

Some simple models seem to work and persist just fine - like (I think, correct me if I'm wrong) conservation of energy and momentum or symmetry, but elsewhere you get... well, you guys undoubtedly know better than I how weird things can get.

To take something more my speed, consider the 19th century concept of aether as a medium for light - of course light's waves have a medium, there has to be something for light to be a wave in! But Michelson-Morely showed that no, light behaves as a wave without a medium. (That we know of, so far.)

If we are talking about the emission and reception of a single photon, what is the entity responsible for the transfer of energy-momentum from the emission event to the reception event? To me, it seems entirely reasonable to describe that entity as "a photon" even if it doesn't much resemble a classical particle. I side with Fredrik here in that I don't really make any distinction between the mathematical theory that models the propagation and the propagation itself.

Like I said, it's reasonable and handy to think of an individual, localized packet of energy transitioning through space between the locations of the emission and absorption events - but do we really know incontrovertibly the case? Couldn't some sort of cosmic accounting mechanism, that "knows" there must be a future absorption event in a location determined by a bunch of geometric rules, also explain everything we have observed?

The mathematical theory doesn't require individual localized energy packets transitioning through space, does it? It specifies methods of calculating the amounts of energy and momentum involved, et cetera, and the geometry and all that - but is there anything that would make localized moving energy packets necessary for things to work properly?

⚛​

 

[lightarrow]

I have another question for those of you who support the view that photons don't exist between emission and detection:

Would you say that quantum states exist between measurements?

It depends on the quantum state and on how we define "exist".

 

[lightarrow]

In my opinion, that's already included in the definition. I required that a theory must be able to make predictions about the results of experiments. A mathematical model alone can't do that. For example, in special relativity we define "proper time" as the integral of a certain quantity along a curve. Then we postulate that the numbers displayed by a clock is the proper time of the curve in Minkowski space that represents its motion. That postulate must be a part of the theory. Otherwise it wouldn't be a theory according to my definition. Minkowski space is the mathematical model of spacetime used by SR, but the theory of physics that we call SR consists of a set of postulates that identify things in the real world with things in the model. The "things in the real world" must be defined operationally.

Which is the operational definition of "photon"? That is, what stays for the clock of your example?

 

[Dale]

I think we need to pin down the term "photon" more than the term "exists".

 

[Cthugha]

So let me test my understanding here: photons are trapped in one of these Fabry-Pérot cavity things confined in some way so that they can only coexist in a finite number of states, the way that electromagnetic force traps electrons around an atomic nucleus where electrons can only exist in a finite number of states. There are also these rubidium Rydberg atom thingies in the cavity and each atom is coupled in some way to exactly one photon. (Or are they coupled to each other en masse? I wasn't clear on that.)

In a nutshell, there is a cavity with a certain number of photons inside. Now you prepare one atom in a specific and well defined spin state and let it evolve and fall through the cavity with the photons. The spin will precede and will have some specific value after it fell through the cavity. Now loose speaking the precession of the spin depends on the em field around, in particular on the photon number. So the final spin state will depend on the photon number inside the cavity. Of course - due to uncertainty restrictions - in most cases it is not possible to have an unambiguos relation between photon number and spin state, so you just get conditional probabilities, that there are n photons present. But to resolve this problem you just take another atom and let it fall through the cavity and another one and so one. This method is nondestructive and after a (rather short) while, the conditional probability to have one certain photon number inside the cavity converges to 1.

arguments against discreteness
------------------------------
I'm not an expert but in quantum optics it's hard to 'pin down' the photon. The photon number operator can have vacuum fluctuations which are large compared to the average photon number. There's no conservation of photon number, obviously.

You are right here, although mostly the fluctuations are due to the emission process. In a laser for example the photon number can be treated as the emission of an ensemble of statistically independent emitters, which gives a Poisson distribution for the photon number statistics, where the fluctuations of the mean photon number are always on the order of the mean photon number itself. The fluctuations due to vacuum fluctuations are most prominent in the emission process as well, mostly as the reason for spontaneous emission. Once the emission is on its way to the detector, there are usually no additional vacuum fluctuations altering the emission.

How can one define the 'size' ( spatial boundaries) of a photon ?

Well, the best guess is the coherence volume, I suppose, which in turn relates to coherence time and therefore to the uncertainty of the exact point of time, when a photon was emitted.

If we define a photon as 'a quantum of the EM field' and leave it there, we should get along fine.

Definitely.

 

[CaptainQuasar]

In a nutshell, there is a cavity with a certain number of photons inside. Now you prepare one atom in a specific and well defined spin state and let it evolve and fall through the cavity with the photons. The spin will precede and will have some specific value after it fell through the cavity. Now loose speaking the precession of the spin depends on the em field around, in particular on the photon number. So the final spin state will depend on the photon number inside the cavity. Of course - due to uncertainty restrictions - in most cases it is not possible to have an unambiguos relation between photon number and spin state, so you just get conditional probabilities, that there are n photons present. But to resolve this problem you just take another atom and let it fall through the cavity and another one and so one. This method is nondestructive and after a (rather short) while, the conditional probability to have one certain photon number inside the cavity converges to 1.

Okay, I think I understand that... yeah, I'm understanding the graphs on page 3 better. So the y-axis of the graphs at the bottom of page 4 of that report, which is labeled "n" but is clearly a continuous value instead of an integer value because it's all spikey - is that something like an averaged or summed measurement of something related to the spin states of the atoms after they've fallen out of the cavity?

Also - another thing I wasn't clear on earlier - it looks like they were running this experiment many, many times and the data in the charts page 3, for example, is grouping the data for all the 1st atoms in every run, 2nd atoms in every run, 3rd atoms in every run, on up to the 50th or 110th atoms in every run, right?

Thank you for patiently explaining this to me.

⚛​

 

[Cthugha]

Okay, I think I understand that... yeah, I'm understanding the graphs on page 3 better. So the y-axis of the graphs at the bottom of page 4 of that report, which is labeled "n" but is clearly a continuous value instead of an integer value because it's all spikey - is that something like an averaged or summed measurement of something related to the spin states of the atoms after they've fallen out of the cavity?

Do you mean the graphs showing plateaus with some spikes? The y-axis labeled <n> gives the mean photon number. If you have a look at graph 2c for example, you will notice, that there are situations, where just one photon number has maximal probabilities and all others have almost 0 probability and there are situations, where several photon numbers have not too small probability amplitudes simultaneously. These second situations lead to non integer values of <n>.

Also - another thing I wasn't clear on earlier - it looks like they were running this experiment many, many times and the data in the charts page 3, for example, is grouping the data for all the 1st atoms in every run, 2nd atoms in every run, 3rd atoms in every run, on up to the 50th or 110th atoms in every run, right?

The charts on page 3 are just showing one run of 110 atoms each. The field starts in a state, where each possible photon number has the same probability and then progressively collapses towards one of them. So it is just the probability distribution after 1, 2, n atoms have fallen through the cavity. Summing over whole runs would not give any good results because each run is independent and the final photon number inside the cavity will be different every time.

 

[CaptainQuasar]

Okay, along with another re-read of the paper that clarifies things even more. Understanding how the probability distributions are constructed might be beyond me, but I'm still uncertain as to what measurements are being taken as inputs to the process of constructing the probability distributions.

Measuring the spin along O–u is performed by submitting it, after cavity exit, to a pulse R₂ whose phase is set to map O–u onto O–z (Fig. 1b).This rotation is followed by the measurement of the atom’s energy, equivalent to a spin detection along O–z. The combination of R₁ and R₂ is a Ramsey interferometer.

So, is the action of the Ramsey interferometer both preparing the atoms before they enter the cavity and measuring them subsequent to their exit from the cavity? And is a Ramsey interferometer a kind of http://en.wikipedia.org/wiki/Atom_interferometer" ? What are these pulses composed of - light / EM radiation or something else?

⚛​

 

[ThomasT]

Interesting thread. I've read quite a few, but not all yet, of the posts. Just thought I'd offer my two cents on this.

... is the original quote in this post actually meaningful?

I think so. To say that photons exist independent of the means used to detect them (ie., independent of detection) in some level of physical reality underlying, and inaccessible to, our sensory perception is physically meaningless.

The word 'photon' refers to experimental materials and instruments, the preparations and observable behaviors of those materials and instruments, and the mathematical shorthands, the models and theories (as quanta of the electromagnetic field), which represent, and quantitatively relate and communicate all experimental productions of photons.

Other than this, the word 'photon' has no physical meaning.

Quantum theory is about the behavior of emitters and detectors. What happens apart from that behavior is a matter of metaphysical speculation. The behavior of an underlying quantum reality might in some ways correspond to the evolution of a wave equation model or operations in an imaginary space, but there's no way to know. It's for just this reason that Heisenberg developed and offered his matrix mechanics representation which makes no pretense to having anything to do with any sort of hidden, underlying quantum reality.

The only unambiguous, current definition of 'photon' is, via QED representation, as a quantum of the electromagnetic field. Again, what aspects of QED might or might not correspond to an underlying quantum reality is a matter of speculation.

My own metaphysical view is that there is an underlying quantum reality and that it is essentially wavelike. Particulate structures and media, and hence macroscopic particulate detection phenomena, arise via the interactions of a hierarchy of bounded wave complexes which is not comprehensibly represented in any theory.

So, of course, whatever it is precisely that's happening to produce photon detection events has something to do with what's happening between emission and detection in an underlying quantum reality. Unfortunately, due to the existence of a fundamental quantum of action, that quantum scale reality is untrackable, and there's no way of talking about it precisely enough to make any difference regarding what's experimentally observed.

 

[wawenspop]

My own metaphysical view is that there is an underlying quantum reality and that it is essentially wavelike.

If you were a photon, so to speak, then no time would pass from emission to absorption for you (as a photon). Indeed, you could travel to the other side of the Universe (eg as a snapshot image of you falling off a chair - for illustrative example) and after 90 billion years of travel, exactly no time would have passed for you at all (remember, you are a photon now).
Conversely, the distance between emission and absorbtion is zero (for you as a photon - because the journey took no time).

So as a photon its not surprising that you only exist at the points of observation - and even that is only a realignment of states. So from the photon's point of view it really does not exist between observations - how could it?

 

[ThomasT]

If you were a photon, so to speak, then no time would pass from emission to absorption for you (as a photon). Indeed, you could travel to the other side of the Universe (eg as a snapshot image of you falling off a chair - for illustrative example) and after 90 billion years of travel, exactly no time would have passed for you at all (remember, you are a photon now).
Conversely, the distance between emission and absorbtion is zero (for you as a photon - because the journey took no time).

So as a photon its not surprising that you only exist at the points of observation - and even that is only a realignment of states. So from the photon's point of view it really does not exist between observations - how could it?

Unfortunately, (or maybe fortunately?) that point of view isn't available to us.

 

[wawenspop]

Also, an electron 'does not exist' going from point A to Point B unless something interacts with it to reveal a state (eg position) at some time, t.
By existence what do we mean? It only ever has a probability of being at some place (existing?) at some time - so in that sense there is not a point particle at all, only the probability of observing one - (ie its 'position state').

Even then, its only a realignment of states - its not as if it suddenly *pops into existence* - it is observed at an instant in time - the 'collapse of the wave function' is misleading as it kind of gives the impression of probabilities ending and particles becoming *real*. No, its a realignment of states that allows observation. Same for the photon. You could also say it must entangle itself with something else to be observed - but that is another bag of philosophical worms - if there is such a thing as a philosophical worm.






I had a philosophy girlfriend once, but she kept trying to prove I did not exist.

 

[lightarrow]

I had a philosophy girlfriend once, but she kept trying to prove I did not exist.

My last girlfriend hadn't studied philosophy, but was trying to prove the same. I've left her

 

[Mentz114]

Posted by atakor in this thread - it's a great read.

https://www.physicsforums.com/showthread.php?t=269121&goto=newpost

Hi,
I recommand the reading of What is a Photon?

(http://stochastix.files.wordpress.com/2008/05/what-is-a-photon.pdf) (PDF - 1.30 MB) by the Optical Society of America (OSA).

 

[WaveJumper]

Just thought of quantum tunneling and the nature of movement of photons. It seems my classical way of thinking is overriding the little possibility there is that i could ever picture how elementary particles move.

When in flight, photons exist as waves. Or more like a probability wave with different amplitudes for places where that wave might be. Now, quantum tunneling says that partciles are described by their wave-function which represents the probability amplitude of finding that particle in a certain location. And sometimes that location happens to be on the other side of the wall.

Now, it seems that according to our classical idea of movement the particle would have to have gone through the wall. But we know it didn't happen, so it leaves us with only one possibility - that movement at the quantum level has nothing in common with movement at the macro level, and hence the photon in question in the OP exists only at emission and absorbtion.

But then, photons are said to move at the speed of light and quantum tunneling happens FTL as in:

"There have been various reports in the popular press of experiments on faster-than-light transmission in optics — most often in the context of a kind of quantum tunneling phenomenon. Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light. But, recall from above, that a superluminal phase velocity cannot be used for faster-than-light transmission of information. There has sometimes been confusion concerning the latter point."

http://en.wikipedia.org/wiki/Faster_than_light



How do you explain this "movement" through barriers in a FTL fashion with our human classical way of thinking?
What is the nature of movement at the quantum level - skipping from one decohered state of superpositions to the next at Planck time units(thus skipping to a position that is on the other side of the wall)? What if the particle is not observed/measured? How does it "move"? Do all eigenstates skip/hop with the emitted photon to the absorbtion event and then only the right trajectory photon is absorbed(the one that arrived first through the shortest route - whether that's in straight line or via quantum tunneling over a hill)?

 

[ZapperZ]

There is nothing exotic here if you also understand classical optics. It is well-known that the phase velocity in classical optics/classical E&M that the phase velocity can be larger than c under certain circumstances. This isn't new and one does not need "tunneling" to observe something like this.

Zz.

 

[bassplayer142]

Just thinking. Doesn't it coincide with our views that if a photon were to disappear that it would have a certain probability to reappear in another stop. This would be inversely proportional to distance so that the farther away the less the probability. If you were to compare this to a source of light like the sun and how at a distance X number of photons strike yet the farther away you get the number declines.

I don't see how this theory would differ in appearance to the currently accepted theory.

 

[wawenspop]

Just thought of quantum tunneling and the nature of movement of photons. It seems my classical way of thinking is overriding the little possibility there is that i could ever picture how elementary particles move.

When in flight, photons exist as waves. Or more like a probability wave with different amplitudes for places where that wave might be. Now, quantum tunneling says that partciles are described by their wave-function which represents the probability amplitude of finding that particle in a certain location. And sometimes that location happens to be on the other side of the wall.

Now, it seems that according to our classical idea of movement the particle would have to have gone through the wall. But we know it didn't happen, so it leaves us with only one possibility - that movement at the quantum level has nothing in common with movement at the macro level, and hence the photon in question in the OP exists only at emission and absorbtion.

But then, photons are said to move at the speed of light and quantum tunneling happens FTL as in:

"There have been various reports in the popular press of experiments on faster-than-light transmission in optics — most often in the context of a kind of quantum tunneling phenomenon. Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light. But, recall from above, that a superluminal phase velocity cannot be used for faster-than-light transmission of information. There has sometimes been confusion concerning the latter point."

http://en.wikipedia.org/wiki/Faster_than_light



How do you explain this "movement" through barriers in a FTL fashion with our human classical way of thinking?
What is the nature of movement at the quantum level - skipping from one decohered state of superpositions to the next at Planck time units(thus skipping to a position that is on the other side of the wall)? What if the particle is not observed/measured? How does it "move"? Do all eigenstates skip/hop with the emitted photon to the absorbtion event and then only the right trajectory photon is absorbed(the one that arrived first through the shortest route - whether that's in straight line or via quantum tunneling over a hill)?

The quantum tunnel can be considered as a local region of distorted space-time:
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

Consider a photon that is passing along multiple paths (using beam splitters, for example),
the wave is passing down, say, 100 optical fibres to interfere at a screen to produce the observable. This shows that the 'wave' *was* in 100 optical fibres. Now if we decide to observe a photon particle, then that particle is found to exist in only one on the optical fibres. i.e. it is not in the other 99.
This is a sort of distributed wave packet - distributed into 100 optical fibres.

So, at the instant a particle photon is observed in one fibre - the wave function in the 99 other fibres must instantly vanish. That 'vanishing' would be instant. Then we ask which way did the photon go, to 'describe' the motion of the particle between observations? We get two answers depending whether we think of a particle or a wave.

Did the photon 'exist' between observations? Well, a wave packet is not a photon, only the probability of one (likewise a wave). So, the question can be rephrased as 'does the wave function' exist between observations. A wave function only 'reveals' a photon if observed (as a wave or particle). So, in this sense the photon, does not exist because it has nothing to define what exactly is exisiting between observations. A wave packet is neither a photon or a wave - its a probability of them having observable states only.
So we cannot really ask the question does a photon exist between observations because its a wave packet that exists - not a real particle or a real wave.

 

[WaveJumper]

Did the photon 'exist' between observations? Well, a wave packet is not a photon, only the probability of one (likewise a wave). So, the question can be rephrased as 'does the wave function' exist between observations. A wave function only 'reveals' a photon if observed (as a wave or particle). So, in this sense the photon, does not exist because it has nothing to define what exactly is exisiting between observations. A wave packet is neither a photon or a wave - its a probability of them having observable states only.
So we cannot really ask the question does a photon exist between observations because its a wave packet that exists - not a real particle or a real wave.

Even if we introduce a wave-packet we are still stuck with "exist". Most people here agree that to exist is to move, so the wave packet has to be moving through space. But how would a probability wave move? I think this question can be answered by a theory of everything. Perhaps a string theorist can tell us how a wave packet is supposed to move through space from A to B.

 

[Primordial]

I think photons only exist in the dimension of time(both proper and relativistic) while in transit.