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.