Photons do not travel in straight lines

[tickle_monste]

I've been thinking about this for some time now and I wanted to get some feedback on it. If photons are supposed to travel along straight lines (for the sake of simplicity let's neglect the curvature of space-time), then how could one photon possibly be used in a double slit experiment? More specifically, how could one photon traversing a straight line possibly create an interference pattern with itself? I have never heard of even pseudo-logic that would allow for something this absurd.

I understand that upon absorption (or some other event) the possible paths of the photon will "reduce" to a single path that takes it, in essence, in a straight line from emission to absorption; I'm not concerned with the reduction part of it, I'm concerned with the "all possible paths" that we assume the photon to be traversing simultaneously. What possible logic could then say that the photon travels along a straight line, and why do we still teach that this is the case? If we're saying that the photon travels along an infinite number of straight lines, simultaneously, during the time between absorption and emission, that would make sense, but every single day I have to struggle with the fact that different sources explain the propagation of light in different ways. If I'm reading about optics, I'm going to get the "photons travel in straight lines" interpretation. If I'm reading about pure quantum physics, I'm going to get a completely different story, where photons are evolving over multiple paths simultaneously, until they reduce to having traveled a single path.

If one is true, the other is not. Which one is it?

 

[negitron]

Both are (or at least appear to be) true. Light sometimes seems to act as a pointlike particle that travels in straight lines but at others it seems to act as a wavelike entity. Chances are it is actually neither of these things, but some other construct altogether haivng some properties of both.

 

[Demystifier]

Even classical light does not travel in straight lines. (Otherwise, mirrors and lens couldn't exist.) Hence, there is no reason to believe that photons travel in straight lines.

 

[jnorman]

photons travel by all possible paths. read QED by Feynman - excellent presentation of this topic by a true master.

 

[pellman]

They only instantaneously travel in straight lines.

 

[negitron]

Even classical light does not travel in straight lines. (Otherwise, mirrors and lens couldn't exist.)

This makes no sense.

 

[Dale]

Yes, in general photons do not travel in straight lines, they travel as waves. However, it is often a reasonable simplification to treat a wave as a ray or otherwise to say that waves travel in a straight line. They are not mutually contradictory explanations, one is an approximation of the other and both are correct in situations where the simplifying assumptions are valid.

 

[Red_CCF]

Photons are EMR waves which travel in straight lines, so in essence it travels both in a straight line and in a wave motion. One photon cannot create an inference pattern with itself, it needs another photon which may arrive in phase or out of phase to create an interference pattern.

 

[kote]

It's not helpful to think of photons having permanent existence as either particles or waves. The underlying "real" structure is unknown and may be unknowable, but in certain interactions the underlying structure will manifest itself as a wave, and in certain interactions it will manifest the properties of a particle. Neither "waves" nor "particles" are persistent objects, they are just useful theoretical tools for visualizing something, depending on the situation, that we don't have complete knowledge of.

Take the property of being red. We say that being red is NOT an intrinsic property of an apple. It is a property that manifests itself in specific situations. When a person is looking at an apple, to that person, the apple manifests the property of being red.

The particle-property of momentum works the same way. Momentum is a property of the underlying photon structure that manifests itself during collisions. When the photon is not actively colliding, it has no intrinsic property of momentum. When it is colliding, then, to whatever it is colliding with, it manifests the property of having a certain momentum. The same can be said of the other particle-properties of location, mass, etc.

Similarly, a photon does not have a persistent wave property of polarization. A photon will manifest the property of polarization while passing through polarized glasses.

The property of being red, the property of being 1 gram, and the property of being linearly polarized at 0 degrees, are all fleeting, extrinsic properties that manifest themselves in certain (experimental / observational) situations.

Realize that any framework you use to visualize basic objective reality will be inconsistent and illogical. We do not have a consistent framework for ascribing persistent properties to real objects. It is very likely that as humans we are necessarily limited to visualizing reality in the falsified, macro, classical mechanical sense of billiard-ball style particle determinism.

So which is real, particles or waves? Well... neither is objectively basic, as the properties of each can only exist in certain situations, but particles and waves are both just as real as the color red and the meaning of the words I'm typing. And that may be as "real" as we can get.

FYI that (above) is Bohr in a nutshell, with a little Kant for flavor.

 

[negitron]

One photon cannot create an inference pattern with itself, it needs another photon which may arrive in phase or out of phase to create an interference pattern.

Not true.

http://www.physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm

 

[Tac-Tics]

[...] how could one photon traversing a straight line possibly create an interference pattern with itself? I have never heard of even pseudo-logic that would allow for something this absurd.

Science gets things a little backwards. We believe that photons can interfere with each other because they DO. We know from experiment. After we can accept that, we come up with a theory about it retroactively.

Science has many moving parts, all stitched together. That light travels in straight lines is a consequence of classical electromagnetism and general relativity. That light is discrete and can interfere with itself comes from quantum mechanics. Classical theories don't always agree with quantum theories because massive objects (beams of light) act very differently than small objects (individual photons).

 

[kote]

Classical theories don't always agree with quantum theories because massive objects (beams of light) act very differently than small objects (individual photons).

In general I agree with your post, but I have to disagree on this. Individual photons can act exactly the same way beams of light do, wave properties and all. The differences aren't a question of scale. We are also seeing entanglement experiments of larger and larger systems, bringing small object phenomena to a larger scale.

 

[Manojg]

Just curious, How do you know that only one photon is hitting the slit or how to make only one photon to hit the slit? It can't be done just by pinhole.

 

[kote]

Just curious, How do you know that only one photon is hitting the slit or how to make only one photon to hit the slit? It can't be done just by pinhole.

You only record one photon at a time on the other side of the slit. Notice the experiment shows individual dots showing up on the screen on the other side. They show up one at a time. I've seen this done with electrons as well in radioactive situations where you can only be emitting one electron at a time from each atom.

The nature of particles as discrete objects necessarily means that, at arbitrary precision, they must be passing through the slit one at a time, in a discrete order. Only when considered as continuous waves is it even possible to talk about photons simultaneously passing through a slit... although "passing through" isn't exactly the right terminology for talking about waves either.

 

[ZapperZ]

I've been thinking about this for some time now and I wanted to get some feedback on it. If photons are supposed to travel along straight lines (for the sake of simplicity let's neglect the curvature of space-time), then how could one photon possibly be used in a double slit experiment? More specifically, how could one photon traversing a straight line possibly create an interference pattern with itself? I have never heard of even pseudo-logic that would allow for something this absurd.

I understand that upon absorption (or some other event) the possible paths of the photon will "reduce" to a single path that takes it, in essence, in a straight line from emission to absorption; I'm not concerned with the reduction part of it, I'm concerned with the "all possible paths" that we assume the photon to be traversing simultaneously. What possible logic could then say that the photon travels along a straight line, and why do we still teach that this is the case? If we're saying that the photon travels along an infinite number of straight lines, simultaneously, during the time between absorption and emission, that would make sense, but every single day I have to struggle with the fact that different sources explain the propagation of light in different ways. If I'm reading about optics, I'm going to get the "photons travel in straight lines" interpretation. If I'm reading about pure quantum physics, I'm going to get a completely different story, where photons are evolving over multiple paths simultaneously, until they reduce to having traveled a single path.

If one is true, the other is not. Which one is it?

Look at the SIZE of the light source!

For example, if you have a beam spot that is 1 mm in diameter, and your slit is more than 1 mm apart, do you think a photon will then bend its path to pass through both slit? Try it!

The spot size of 1 mm requires that the slits be separated at LESS than 1 mm. This means that AT THE SOURCE, a photon can be emitted anywhere within that 1 mm cross section! Given that scenario and the ability for each path to interfere with each other, only then will you get the interference pattern.

It is why, when many of these experiments are done using thin laser beam, one often use a beam expander - to increase the spot size!

Zz.

 

[Manojg]

You only record one photon at a time on the other side of the slit. Notice the experiment shows individual dots showing up on the screen on the other side. They show up one at a time. I've seen this done with electrons as well in radioactive situations where you can only be emitting one electron at a time from each atom.

The nature of particles as discrete objects necessarily means that, at arbitrary precision, they must be passing through the slit one at a time, in a discrete order. Only when considered as continuous waves is it even possible to talk about photons simultaneously passing through a slit... although "passing through" isn't exactly the right terminology for talking about waves either.

I understand. But the source is flashlight bulb which emits many photons at a time, then how one can isolate one photon to hit the slit? Is it possible by simple pinhole arrangement?

 

[jtbell]

We believe that photons can interfere with each other because they DO.

Photons do not interfere with each other except via a rare higher-order process, Delbrück scattering.

 

[Red_CCF]

Photons do not interfere with each other except via a rare higher-order process, Delbrück scattering.

then what's creating the interference patterns on Young's Double Slits?

 

[Red_CCF]

Not true.

http://www.physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm

wow I've never heard of this in my life. I always thought that interference pattersn can only exist from multiple photons/waves.

Can you explain how this works?

 

[ZapperZ]

then what's creating the interference patterns on Young's Double Slits?

It is the "single photon interference".

2-photon interference almost never happen - it is a higher order interaction.

Zz.

 

[Red_CCF]

It is the "single photon interference".

2-photon interference almost never happen - it is a higher order interaction.

Zz.

Can you explain single photon interference? My textbook states that interference patterns are created, in Young's Double Slit for instance, from 2 light sources. I've always assumed that the photons from these 2 light sources interfere with each other.

 

[kote]

Can you explain single photon interference? My textbook states that interference patterns are created, in Young's Double Slit for instance, from 2 light sources. I've always assumed that the photons from these 2 light sources interfere with each other.

When light is a wave, it's not a particle. There is no single photon particle interfering with itself. There are continuous waves interfering with themselves. When it's time for the waves to act as a particle, they can manifest as a single particle. It's either waves or a particle, not both.

I tried to explain the duality earlier in the thread.

 

[Dale]

It's either waves or a particle, not both.

That is not correct. A quantum mechanical entity like a photon has a single unified description, i.e. it is always both. Sometimes a "particle" feature like momentum will be more prominent, and sometimes a "wave" feature like interference will be more prominent, but they are always both present.

 

[kote]

That is not correct. A quantum mechanical entity like a photon has a single unified description, i.e. it is always both. Sometimes a "particle" feature like momentum will be more prominent, and sometimes a "wave" feature like interference will be more prominent, but they are always both present.

You can put it that way if you want, but a photon cannot have a persistent momentum or location while having a persistent polarization for example. It simply cannot have the properties of a wave and a particle persistently and simultaneously. The EPRB type experiments prove this. Results of experiments are inconsistent with any conception of persistent particle properties or else the wave function probabilities would match classical probability distributions, and they don't.

Saying that it is a particle at all times while acknowledging that it does not at all times have a location or a momentum is misleading in my opinion and doesn't fit with the common definition of a particle.

 

[Red_CCF]

When light is a wave, it's not a particle. There is no single photon particle interfering with itself. There are continuous waves interfering with themselves. When it's time for the waves to act as a particle, they can manifest as a single particle. It's either waves or a particle, not both.

I tried to explain the duality earlier in the thread.

But doesn't that refute what jtbell and ZapperZ said or did I misunderstand your statement?

 

[kote]

But doesn't that refute what jtbell and ZapperZ said or did I misunderstand your statement?

Nope! Single-photon interference is actually the result of multiple waves interacting. Just because you have an amount of energy etc enough to produce only a single photon in particle form, that doesn't mean that there is only one discrete wave associated with it.

What will manifest as a single particle during the eventual collision and measurement can, in the meantime, be represented by multiple interfering waves.

It has been mentioned before that nearly all interference is a result of single photons interfering with themselves in this manner.

 

[Red_CCF]

Nope! Single-photon interference is actually the result of multiple waves interacting. Just because you have an amount of energy etc enough to produce only a single photon in particle form, that doesn't mean that there is only one discrete wave associated with it.

What will manifest as a single particle during the eventual collision and measurement can, in the meantime, be represented by multiple interfering waves.

It has been mentioned before that nearly all interference is a result of single photons interfering with themselves in this manner.

Oh so in the double slit experiment, the interference pattern produced by the two waves is called single photon interference? So the waves that interfere are separate but part of the same photon? That is quite confusing. How can one photon be split into two and then reform?

 

[tickle_monste]

Oh so in the double slit experiment, the interference pattern produced by the two waves is called single photon interference? So the waves that interfere are separate but part of the same photon? That is quite confusing. How can one photon be split into two and then reform?

Moreover, if it even has the possibility of splitting into two photons, than ALREADY it's probability amplitude is greater than 0 outside of the straight line it's supposed to be traveling. This is enough for me to say "Ok, I guess they don't actually travel in straight lines." I know that once they reach the absorption plate and become absorbed, these probabilities will reduce so that it's 1 only along the "straight line" (disregarding space-time curvature, mirrors, lenses, etc as special scenarios). But saying that it's probability is 1 along the straight line and 0 elsewhere AFTER the photon has been recorded is the same as saying the probability of me picking black socks over white socks is 1 AFTER I pick the black socks, even though that had nothing to do with the actual probability of it BEFORE I picked the socks. Before the measurement, the photon has must exist as a possibility along all possible paths; there is only one way that it could interfere with itself, and that's if it's in 2 places it at once, hence: PHOTONS DON'T TRAVEL IN STRAIGHT LINES! It may be useful to think of photons as traveling in straight lines, in some scenarios, but wouldn't this "Always a straight line photon" simply be a limiting case of the photon going down all possible paths? Perhaps the photon that "acts like a particle" and "only travels a straight line" is just a photon where all possible paths lie along the same line, but this wouldn't be the general case, because if it were, the double slit experiment would NEVER EVER EVER EVER EVER work, EVER.

If anyone is still in doubt about this, please take a piece of paper, draw a square to represent a photon emitter and out of this photon emitter draw a straight line to represent a single photon. Now, perpendicular to this straight line, draw your double slit. Try as hard as you possibly can to make that single straight line go through both slits, and remember, no refraction due to lenses, no reflection due to mirrors, and no bending due to the curvature of space, and don't try any other silly things to get around this. It's just simply impossible.

 

[Cthugha]

So the waves that interfere are separate but part of the same photon? That is quite confusing. How can one photon be split into two and then reform?

No, the photon does not split up.

Most of the confusion, which came up in this topic and which generally comes up in discussions of the double slit is a consequence of some misunderstandings and common misconcepts of what some famous physicists said.

Starting with Dirac's famous statement about the double slit experiment

Each photon then interferes only with itself. Interference
between two different photons never occurs.

one sees this statement misinterpreted very often. Roy Glauber comments on Dirac's quote in his Nobel lecture "100 years of light quanta"

It is worth recalling at this point that interference simply
means that the probability amplitudes for alternative
and indistinguishable histories must be added together
algebraically. It is not the photons that interfere physically,
it is their probability amplitudes that interfere—
and probability amplitudes can be defined equally well
for arbitrary numbers of photons.

In a nutshell, this shows us that one must interpret Dirac's quote from a modern point of view: Interference between several photons happens, but interference between several DIFFERENT photons does not happen. As soon as photons are indistinguishable the naive ballistic photon picture breaks down and collective behaviour starts to dominate. In this case it becomes clear, that the underlying fields or probability amplitudes are more fundamental than photons (which are second order products of the underlying fields). In everyday life however, we almost never encounter light with a high degree of coherence and therefore the "straight line picture" is ok, but not completely correct.

By considering interfering fields or probability amplitudes instead of photons the situation becomes much clearer and lots of the ambiguities are avoided.

 

[Dale]

You can put it that way if you want, but a photon cannot have a persistent momentum or location while having a persistent polarization for example.

To the best of my knowledge the polarization commutes with the momentum, so you can in fact have an eigenstate of both. In any case, even with non-commuting operators you can still prepare states where a "wave" operator and a "particle" operator have minimally uncertain values.

Saying that it is a particle at all times while acknowledging that it does not at all times have a location or a momentum is misleading in my opinion and doesn't fit with the common definition of a particle.

No, but it fits the quantum mechanical definition, which is what I stipulated earlier. Have you studied any quantum mechanics at all, if not, I would recommend starting with the Leonard Susskind video lectures.

 

[tickle_monste]

By considering interfering fields or probability amplitudes instead of photons the situation becomes much clearer and lots of the ambiguities are avoided.

That makes sense. So before the photon is absorbed by, say, an atom in your eye, it can be considered as "delocalized" across an area? Is this area considered to be a sphere emanating from whatever the photon originated from, expanding at the speed of light, sort of like a bubble? Information must be present everywhere on this (supposed) bubble, does that information suddenly "collapse" when the photon is finally absorbed by that atom in your eye? I would guess that once it is absorbed, you would be able to trace back the (a single) path to the source, that would be the equivalent of a straight line (when you account for lensing, reflection, the curvature of space-time, etc.), is this correct?

 

[Cthugha]

That makes sense. So before the photon is absorbed by, say, an atom in your eye, it can be considered as "delocalized" across an area? Is this area considered to be a sphere emanating from whatever the photon originated from, expanding at the speed of light, sort of like a bubble? Information must be present everywhere on this (supposed) bubble, does that information suddenly "collapse" when the photon is finally absorbed by that atom in your eye?

Well, what the value of some variable is, before it is measured, is one of the great questions in qm. Different interpretations of qm will give different answers to this questions ranging from a simple "the value is not well defined" to a more complicated answer a Bohmian would favor. What one can indeed say, is that the field, which originated from some light source, spreads out as a sphere and so does the probability density to detect a photon. Whether there is some real and localized entity connected with it before the detection of a photon, is again difficult to answer.

I would guess that once it is absorbed, you would be able to trace back the (a single) path to the source, that would be the equivalent of a straight line (when you account for lensing, reflection, the curvature of space-time, etc.), is this correct?

Not necessarily. If you try to find the path to the source in the double slit experiment for example, you will only find out, that one of the slits was the source, but not which one. If you knew, you would not find an interference pattern.

1) Photons, being Bosons, do not and cannot interfer with themselves or each other.

Do photons know that, too?

2) It is only the effects of photons on matter that can show interference. Interference phenomena therefore require intervening media such as slits and biprisms etc. That is why you will never see an interference experiment with only light sources and detectors.

Strange that interference experiments with synchronized and phase locked lasers work completely without a slit or other matter. Michelson interferometers also do not use slits and show interference. 2-photon-interference experiments using consecutive photons from single photon sources also work without a slit, using just a beam splitter. Or is even a beam splitter too much matter for your taste?

3) Photons travel in straight lines until they interact with matter. It is the only way momentum is conserved.

Prior to a measurement the best thing you can do is to consider the coupled system of photon and photon source, which has some well defined momentum. If you measure the measure the momentum of the photon or the emitter after an emission process, you have a photon with well defined momentum. Before that measurement, the momentum is simply not well defined. As said before, attributing real values to unmeasured variables can go pretty wrong. What at last completely kills your argumentation is the problem, that a description using a well defined and localized photon density does not give sensible results for polychromatic fields. If you define a localized photon probability density, its maximum will not be locally connected to the energy density and it will also not be locally connected to the detection probability density (see Mandel and Wolf, Optical Coherence and Quantum Optics, chapter 12.11, around page 637 in my edition). So these 3 distributions will peak at different positions, which renders the concept pointless.

4) Interfernce experiments have been done with photons of different wavelength, therefore it is not true that interference requires the same type of photons.

Sure, if they come from the same source or have some other fixed phase relationship this can work. However, even in this situation the probability amplitudes will still be indistinguishable. After some detection you are not able to tell, which photon is which, even if you could do that beforehand (see for example "Quantum interference with distinguishable photons through indistinguishable pathways" by Kim and Grice, JOSA B, Vol. 22, Issue 2, pp. 493-498 or on arxiv), so it is again not meaningful to talk about different photons, because one has no means to tell them apart. Again, there is no problem, if you use probability amplitudes as the basis of your description in these cases.

 

[bjacoby]

Oh so in the double slit experiment, the interference pattern produced by the two waves is called single photon interference? So the waves that interfere are separate but part of the same photon? That is quite confusing. How can one photon be split into two and then reform?

Well it is confusing. And of course QM standard model is even worse. So instead of reading a freshman textbook describing all the magic, and fairies that go measure the second slit for the photon, and the religious stories of things that don't exist until you look at them, let's look carefully with logic and reason instead.

What happens in a double slit diffraction experiment? It IS known.

You fire a single photon at the slits. It goes through one of them. It's straight path is deflected at the slit in some unknown manner. It lands on the detector and you count it as a "flash" at a certain position. Well and good.

But it doesn't start to get interesting until you do this over and over a great many times. Much to your wonder and amazement a pattern starts to emerge on the screen which is not a simple mechanical scattering of hard particles. It is easily recognized as an interference pattern such as that produced by a wave going through two slits. But note carefully that this is NOT a wave interference pattern! It is a statistical function of the counting of photon landing sites which for some odd reason TAKES THE FORM of a double slit diffraction pattern!

This is the mystery! Block one of the slits and even though each photon particle clearly does not have the "reach" to interact with the second slit, it clearly "knows" it's no longer there! And that is proved because the statistical landing pattern changes!

The Standard Model "explains" this by saying that photon particles are actual dual entities that are both waves and particles at the same time and switch back and forth depending on what you are doing to them etc. Well that has to be nonsense. Since there are two things happening apparently, then let's just MAKE them two separate things! We might say therefore that there are photon-particles as well as some kind of waves that seem to interact with them and matter by changing their trajectories. This could be true and is the basis of some theorizing, but we should remember that we have little proof that the statistical landing patterns that are observed to be similar to diffraction functions are actually DUE to any kind of waves. There remains the possibility that somehow the mathematical functions produced are identical with those observed with waves for some other unknown action having nothing to do with waves. The assumption that mathematical wave solutions are always produced ONLY by waves is, in a phrase, a leap of faith.

So if one assumes that photons are logically traditional particles, it is clear that any "splitting apart" explanation is nonsense. The photon clearly must go through only one slit. The fact that it's deflection statistics produce wave function solutions is the mystery. Clearly it must be that SOMETHING ELSE is "associated" with each photon that does the "measuring and deflecting". What that is seems to be pretty much open for speculation at this time. There are theories that in addition to a photon there are actual waves of some type that may actually be the "probability waves" of QM traveling from the source to the target or in the other direction that somehow "deflect" photons as they traverse the experiments. But it should be clear from the wild illogical "explanations" of QM that there is MUCH that is little understood here.

 

[Cthugha]

You fire a single photon at the slits. It goes through one of them. It's straight path is deflected at the slit in some unknown manner. It lands on the detector and you count it as a "flash" at a certain position. Well and good.

First of all: In almost all experiments you do not fire single photons at the slits. In most cases you do not even fire a well defined number of photons at the slits. Coherent light as well as thermal light does not have a fixed photon number, but some intrinsic photon number uncertainty.

The Standard Model "explains" this by saying that photon particles are actual dual entities that are both waves and particles at the same time and switch back and forth depending on what you are doing to them etc. Well that has to be nonsense.
[...]
So if one assumes that photons are logically traditional particles, it is clear that any "splitting apart" explanation is nonsense. The photon clearly must go through only one slit.

The idea, that photons are logically traditional particles is wrong. The idea that photons are traditional particles going through just one of the slits is simply not compatible with experiments. However, contrary to what you say, qm does not predict photon particles to be dual entities switching back and forth between wave and particle behaviour. Probability amplitudes have wave character. This is true for any probability amplitude, whether it is about electrons, photons or buckyballs. Photons are quantized excitations of the em-field. If you think of photons as traditional (classical) particles, you deny the role of the underlying fields, which must go wrong as any basic text on QFT or quantum optics will show you.

 

[DrChinese]

The Standard Model "explains" this by saying that photon particles are actual dual entities that are both waves and particles at the same time and switch back and forth depending on what you are doing to them etc. Well that has to be nonsense. Since there are two things happening apparently, then let's just MAKE them two separate things! We might say therefore that there are photon-particles as well as some kind of waves that seem to interact with them and matter by changing their trajectories. This could be true and is the basis of some theorizing, but we should remember that we have little proof that the statistical landing patterns that are observed to be similar to diffraction functions are actually DUE to any kind of waves. There remains the possibility that somehow the mathematical functions produced are identical with those observed with waves for some other unknown action having nothing to do with waves. The assumption that mathematical wave solutions are always produced ONLY by waves is, in a phrase, a leap of faith.

So if one assumes that photons are logically traditional particles, it is clear that any "splitting apart" explanation is nonsense. The photon clearly must go through only one slit. The fact that it's deflection statistics produce wave function solutions is the mystery. Clearly it must be that SOMETHING ELSE is "associated" with each photon that does the "measuring and deflecting". What that is seems to be pretty much open for speculation at this time. There are theories that in addition to a photon there are actual waves of some type that may actually be the "probability waves" of QM traveling from the source to the target or in the other direction that somehow "deflect" photons as they traverse the experiments. But it should be clear from the wild illogical "explanations" of QM that there is MUCH that is little understood here.

QM is a formalism, and it is a lot better than you give it credit for. For example, where is it wrong?

The issue is when people try to give explanations beyond the scope of the formalism. Whether it is a wave or a particle matters not to the formalism. The Heisenberg Uncertainty Principle alone can explain double slit interference, see for example: http://arxiv.org/abs/quant-ph/0703126

So if you have an issue with the QM explanation, it seems to me to be with the semantics and not the formalism or its application (of which you have provided no specific criticism).

If you are not aware of it, the Bohmian model operates more closely to your description of particles/waves. It is equivalent to QM. It states that a photon goes through a specific slit (the closer one to where detected) while a related pilot wave goes through both slits.

 

[bjacoby]

QM is a formalism, and it is a lot better than you give it credit for. For example, where is it wrong?

So if you have an issue with the QM explanation, it seems to me to be with the semantics and not the formalism or its application (of which you have provided no specific criticism).

The problem is not with a formalism which has obviously been tweaked to a high degree, but with the concept of "wrong". You say I take issue with mere "symantics". I say that words actually have meanings. I say logic and definitions have a role in science and elastic versions of these "explain" nothing.

No question that the formalism "works" to a degree, meaning that mathematical predictions can be obtained that approximate what is observed. But the fundamental questions of philosophy remain. What is a "photon"? If a particle has a certain meaning then one must reconcile that meaning with the particle nature of photons. A particle does not have a wave nature. And to say so is nonsense. A wave has a wave nature and to call that wave a particle is likewise nonsense. Words really do have meanings.

So the question here is not to disprove the results of the formalism, but to establish a logical philosophy that does not require magic, does not violate causality, or has a lot of mysterious undefined operations and terms. Obviously when one attempts to describe the formalism in words it doesn't work. There is a great wandering in the wilderness. The formalism works. The description of it does not!

Why is that? Well one reason in my opinion is that everyone wants to pretend that there are no mysteries here. Nobody is willing to say "We really don't know why this works as it does." And the end result is a myriad of "explanations" that include bogus ideas like "action at a distance", things that don't exist until you look at them, and "particles" that split and go through two holes at once and magically recombine in some fashion to give correct answers.

No. The problem is not in our stars or our semantics, but in your philosophy!

 

[bjacoby]

First of all: In almost all experiments you do not fire single photons at the slits. In most cases you do not even fire a well defined number of photons at the slits. Coherent light as well as thermal light does not have a fixed photon number, but some intrinsic photon number uncertainty.

The idea, that photons are logically traditional particles is wrong. The idea that photons are traditional particles going through just one of the slits is simply not compatible with experiments. However, contrary to what you say, qm does not predict photon particles to be dual entities switching back and forth between wave and particle behaviour. Probability amplitudes have wave character. This is true for any probability amplitude, whether it is about electrons, photons or buckyballs. Photons are quantized excitations of the em-field. If you think of photons as traditional (classical) particles, you deny the role of the underlying fields, which must go wrong as any basic text on QFT or quantum optics will show you.

First of all, I don't care about all experiments, only the one I'm talking about. Shine a laser at a dual slit. Put up a channel plate viewer to see single photons. Insert ND filters until photons are coming one at a time. How long do you want me to wait between photons?

Obviously you can't say the idea that photons are traditional particles is "wrong" since you really can't tell us what photons are, can you? It is clear, however, that photon propagation involves much more that simple traditional particles. What exactly do the words "quantized excitations of the EM-field" actually mean? Can you explain that for us in detail?

So what is wrong with the idea of saying that LIGHT is a combination of particles and waves of some type? Why is there a requirement that both must be somehow merged into a mythical wave-article?

We DO know that light is NOT waves. Energy transfer is too fast. And energy transfers are limited to certain values. Plus we know that the physical dimensions of these light "particles" are too small to in any way sense the presence of the other slit. The bottom line is exactly as I have stated it. Particles in the classical sense appear to be shooting along but have their trajectories modified by matter in some way so as to produce trajectory statistics that are wave solutions. (please note, however that these are NOT actual wave solutions which are mathematically continuous and differentiable, while our "diffraction" results are statistical distributions of points which are NOT continuous nor differentiable)

Maxwell noted long ago that energy (information) can only be transferred in two mechanisms: by particles or by waves. There are no other mechanisms. Light energy has long ago been determined not to be transmitted by waves for the reasons I stated.

So as you say, the logical assumption is that SOMEHOW a "wave" must also be involved in this operation. Of course you can't possibly tell me what exactly a "probability wave" is. But we all know that such formalism actually does give more or less correct predictions even if we can't explain why.

So why won't you admit you can't explain why? Then perhaps we can begin to speculate as what might be involved...

 

[Cthugha]

First of all, I don't care about all experiments, only the one I'm talking about. Shine a laser at a dual slit. Put up a channel plate viewer to see single photons. Insert ND filters until photons are coming one at a time. How long do you want me to wait between photons?

I am sorry to hear that. There are lots of other experiments, which tell us a lot more about the properties of light, but I will see how far I can get without citing them. However I would like to stress that using a ND filter in laser beam path does not give you single photons. You just damp the amplitude of the coherent state. The photon number uncertainty of a single photon (or other Fock) state is 0, while the photon number uncertainty of a coherent state is always on the order of the mean photon number. If you really use single photons, for example one out of two photons of an entangled beam you will not see the double slit pattern because the light you use is too incoherent. However, this is not central to your concern, I suppose, but one should keep in mind that having single detector clicks does not assure that you have single photons. By the way, if you are really just interested in the double slit, you might find this paper interesting:
http://arxiv.org/abs/quant-ph/0703126
It is a preprint of T.V. Marcella Eur. J. Phys. v.23, p.615 (2002) and

Obviously you can't say the idea that photons are traditional particles is "wrong" since you really can't tell us what photons are, can you? It is clear, however, that photon propagation involves much more that simple traditional particles. What exactly do the words "quantized excitations of the EM-field" actually mean? Can you explain that for us in detail?

Well, how deep is your understanding of QM? I could tell you stuff about QM formulations of harmonic oscillators and stuff, but that would not help much, if you do not have some basic knowledge in QM, QFT and quantum optics. If you are a layman, let me boil down all of that to a simple "a photon is the sum of its properties". Besides that, this forum has a great FAQ article (in the general physics subforum) on wave-particle-duality and why there is not really one in QM. You might want to read it.

So what is wrong with the idea of saying that LIGHT is a combination of particles and waves of some type? Why is there a requirement that both must be somehow merged into a mythical wave-article?

Well, QM usually does not really do that. QM has one single description from which both particle and wave behaviour can be explained. QM always deals with probability amplitudes. This is where the wavelike aspect comes from. However it does not only apply to electrons, photons and such stuff, but theoretically also to cars, trees and such stuff. The reason why you do not see tree interference in double slit experiments is a matter of decoherence. This is also where the intuitive classical description goes wrong. The classical particle point of view is usually what you have after decoherence: Some discrete and definite position. However in QM the particle aspect boils down to discrete and quantized photon numbers. This is not the traditional meaning of a particle.

We DO know that light is NOT waves. Energy transfer is too fast. And energy transfers are limited to certain values. Plus we know that the physical dimensions of these light "particles" are too small to in any way sense the presence of the other slit. The bottom line is exactly as I have stated it. Particles in the classical sense appear to be shooting along but have their trajectories modified by matter in some way so as to produce trajectory statistics that are wave solutions. (please note, however that these are NOT actual wave solutions which are mathematically continuous and differentiable, while our "diffraction" results are statistical distributions of points which are NOT continuous nor differentiable)

Energy transfer is too fast? This is nonsense. That the energy transfer is limited to quantized values is of course true. This is what constitutes particles in QM. Your statement about the physical extents is however wrong. If you wanted to define some spatial extent for light, the closest thing you get is the coherence volume, which is the volume inside which photons are indistinguishable. By the way this is also the quantity, which defines whether there is a double slit pattern. If both slits are inside of one coherence volume, you will see interference. Otherwise you will not.
By the way, note that I never said that photons are waves. You just do not get around seeing wavelike behaviour because everything shows wavelike behaviour in QM.

Maxwell noted long ago that energy (information) can only be transferred in two mechanisms: by particles or by waves. There are no other mechanisms. Light energy has long ago been determined not to be transmitted by waves for the reasons I stated.

Well, you should start with the very basics of QM. As I said before, the wavelike aspect, which is often mentioned is a consequence of QM itself, not of the photons.

So why won't you admit you can't explain why? Then perhaps we can begin to speculate as what might be involved...

Going back to the deepest reasons of course I do not know why. However, this is not physics and just not interesting. I also do not know, why gravity works as it works on the deepest level, why there is electromagnetic force and why there is magnetism. You can always ask "why" on a deeper level and get to the final speculative answers

a) because god made it that way.

b) because the natural constants accidentally have exactly such values that our world turns out to be the way it is.

c) because the flying spaghetti monster arranged it.

So - to me - a photon is the sum of its properties. And to me the simplest model explaining all of its properties is the model I adopt. So to me it is a quantized excitation of the em field.

 

[bjacoby]

1. I just knew I was going to get into trouble for using the words ND filter! Let's just say or equivalent. While I'm using the double slit (or single slit for that matter!) as an example there are a lot of interesting subjects here having to do with coherence, self-confined beams and the like. I am purposely trying to not turn this into a discussion of the details of high-powered mathematics so that lurkers are not lost. On the other hand, I am trying to keep such fundamentals in mind so that I am not saying things that are demonstrably wrong.

2. You are obviously dying to tell me to go read a freshman textbook. However, suffice it to say that I studied graduate level QM under R. Mills (yes THAT Mills) and Yang. I know you find it difficult to understand how someone so properly indoctrinated would go bad and start questioning the gospel, but it happens. The only difference between then and now is that a few more practical problems have been calculated using QM. Back then the hydrogen atom was about it. Harmonic oscillators and energy wells are just mathematical exercises. They are not practical problems, even though everyone pretends that they are. I guess my problem is I need to upgrade to the latest version of QM...

3. I can see we actually ARE having problems with semantics here. I am assuming that the "on" in photon implies it's a particle. You are saying that the word "photon" is just a word like "turtle" that implies all the properties of the object. Thus your word is a name for the entire object and mine implies certain properties of an object which while associated with additional properties may or may not have those additional properties be part of it's own.

4. So I say there is no requirement that a "light particle" be a merger of both particle and wave properties and you say, yes it does because QM derives both these properties from some vague probabilistic formalism. But the fact is that QM is NOTHING but formalism. It does not and probably cannot describe what a light particle "is". It only predicts how it is likely to behave to some degree.

5. So where we stand is you are defending a vast and complex formalism of epicycles because it nicely explains the motion of the planets. And I am suggesting that perhaps one needs a new way to look at things that simplifies everything by a change of point of view.

6. But while you note that wave-like behavior is a property of QM, light is nevertheless a fooler because in the classical macroscopic sense it looks all the world like a high-frequency version of Maxwellian EM radiation. And in fact "electromagnetic spectrum charts" have always portrayed it as such. But if one looks close enough it's obvious that this idea is wrong. As I said before, energy transfers are too fast for light to be merely high frequency waves in the luminiferous aether. And there are other problems as well as we both note. But wave characteristics do persist. And they transfer to statistics. And we find they take the form of 'probability waves" whatever that might be. Just what is the medium that "imaginary probability waves" propagate in? Obviously the standard answer is that these waves need no medium just as no EM wave needs a medium. All these waves propagate quite nicely in "nothing at all". But that is a philosophical paradox like talking about the properties of "nothing at all". One cannot logically separate properties from the object that possesses those properties. It's nonsense. Particles on the other hand transmit energy and information from one place to another with no philosophical conundrums. Which is why I suggest the need for a higher point of view.

7. Let me say I am not asking "why" as you suggest. I'm asking "what logical model?". You on the other hand are suggesting that QM formalism is some kind of model. In spite of it's ability to predict to a degree, I suggest that there are too many unexplained definitions (such as probability waves) and illogical constructs (waves without media) to qualify as a model. You can accept it as useful, but don't call it a model.

As for the original question if photons travel in straight lines, I suggest that they do unless they interact with matter or fields and are deflected. Which oddly enough is a property of classic particles.

As for "why" I kind of like your spaghetti monster theory!

 

[DrChinese]

4. So I say there is no requirement that a "light particle" be a merger of both particle and wave properties and you say, yes it does because QM derives both these properties from some vague probabilistic formalism. But the fact is that QM is NOTHING but formalism. It does not and probably cannot describe what a light particle "is". It only predicts how it is likely to behave to some degree.

5. So where we stand is you are defending a vast and complex formalism of epicycles because it nicely explains the motion of the planets. And I am suggesting that perhaps one needs a new way to look at things that simplifies everything by a change of point of view.

So where is the criticism? How is QM a "vast and complex" formalism? If simplification can give us something, where is the simpler model? You imply that everyone else is in the "Earth is the center of the Solar System" camp and you are in the "Sun is the center" camp. Great. As far as I know, everyone here is open to a better theory and everybody here is interested in seeing scientific progress. But it takes a lot more than words, and a lot more than desire. And certainly a lot more than empty assertions of "logic".

The only person trying to play games with the definition of a photon seems to be you. Your criticism basically is: it must be either a particle or a wave and not both. A strange assertion indeed for someone with your background.

 

[Cthugha]

1. I just knew I was going to get into trouble for using the words ND filter! Let's just say or equivalent. While I'm using the double slit (or single slit for that matter!) as an example there are a lot of interesting subjects here having to do with coherence, self-confined beams and the like. I am purposely trying to not turn this into a discussion of the details of high-powered mathematics so that lurkers are not lost. On the other hand, I am trying to keep such fundamentals in mind so that I am not saying things that are demonstrably wrong.

Ok, I work in a field, where the small difference between a damped coherent beam and a real single photon might matter, so I always put special emphasis on this point. However, as you know, what you are doing, let's move on.

2. You are obviously dying to tell me to go read a freshman textbook. However, suffice it to say that I studied graduate level QM under R. Mills (yes THAT Mills) and Yang. I know you find it difficult to understand how someone so properly indoctrinated would go bad and start questioning the gospel, but it happens.

No, I just wanted to know, on which level we can discuss.

The only difference between then and now is that a few more practical problems have been calculated using QM. Back then the hydrogen atom was about it. Harmonic oscillators and energy wells are just mathematical exercises. They are not practical problems, even though everyone pretends that they are. I guess my problem is I need to upgrade to the latest version of QM...

Well, with the arrival of lowdimensional semiconductor structures like quantum wells, wires and dots, energy wells have indeed become practical problems.

3. I can see we actually ARE having problems with semantics here. I am assuming that the "on" in photon implies it's a particle. You are saying that the word "photon" is just a word like "turtle" that implies all the properties of the object. Thus your word is a name for the entire object and mine implies certain properties of an object which while associated with additional properties may or may not have those additional properties be part of it's own.

Ok, there are a lot of other "on"s out there and usually this indeed means that the things mentioned are particles or quasiparticles. No problem with the assumption that "on" means a particle. However we might not agree on what a particle is...

But the fact is that QM is NOTHING but formalism. It does not and probably cannot describe what a light particle "is". It only predicts how it is likely to behave to some degree.

Sure, that is what models do. But what "is" a photon to you besides all the properties one can measure? Any model can only predict behaviour and does not tell us more.

5. So where we stand is you are defending a vast and complex formalism of epicycles because it nicely explains the motion of the planets. And I am suggesting that perhaps one needs a new way to look at things that simplifies everything by a change of point of view.

Go ahead. If you find another model that predicts the outcomes of QM and is easier or needs less assumptions, you are very welcome to publish it.

As I said before, energy transfers are too fast for light to be merely high frequency waves in the luminiferous aether.

I don't get your point. The maximum energy transfer rate happens at c. I do not see, where this is too fast for waves traveling at c.

And there are other problems as well as we both note. But wave characteristics do persist. And they transfer to statistics. And we find they take the form of 'probability waves" whatever that might be. Just what is the medium that "imaginary probability waves" propagate in? Obviously the standard answer is that these waves need no medium just as no EM wave needs a medium. All these waves propagate quite nicely in "nothing at all".

Well, empty space is different from nothing at all from a philosophical and physical point of view. Why do you need another medium?

But that is a philosophical paradox like talking about the properties of "nothing at all". One cannot logically separate properties from the object that possesses those properties.

To quote Glauber's Nobel lecture again:
"It is worth recalling at this point that interference simply means that the probability amplitudes for alternative and indistinguishable histories must be added together
algebraically."

If you like to, you can consider this as a property of our empty space (and time).

As for "why" I kind of like your spaghetti monster theory!

Hehe, everybody does. ;)

7. Let me say I am not asking "why" as you suggest. I'm asking "what logical model?". You on the other hand are suggesting that QM formalism is some kind of model. In spite of it's ability to predict to a degree, I suggest that there are too many unexplained definitions (such as probability waves) and illogical constructs (waves without media) to qualify as a model. You can accept it as useful, but don't call it a model.

Well, I do not see the problem with probability amplitudes and do not see the logical fallacies you see. The more philosophical approach to which picture one should have on what probability amplitudes are, is a matter of the interpretation of QM, which I suppose is what you would consider as a model. However, there are plenty of interpretations, from Kopenhagen over Bohm to multiple worlds. Therefore to me, QM is definitely a model. Models explain and predict stuff you can measure. What you call logical model is in my eyes a question of the interpretation of QM. And there are indeed still plenty of people out there trying to make heads or tails out of the philosophical implications of the fundaments of QM. You will find plenty of discussions on the pros and cons of Kopenhagen/Decoherence/Bohm/MWI around here. However my favorite interpretation comes from N.D. Mermin. His "in a nutshell" version of his interpretation always makes me laugh:

"My complete answer to the late 19th century question "what is electrodynamics trying to tell us?" would simply be this: Fields in empty space have physical reality; the medium that supports them does not.

Having thus removed the mystery from electrodynamics, let me immediately do the same for quantum mechanics: Correlations have physical reality; that which they correlate, does not."