Effect Preceding Cause: A Quantum Physics Thought Experiment on Usenet

[Robert Noel]

Ten years ago, I posted a long rambling post concluding with a thought-experiment on Usenet, and got no useful feedback. I'd like to try again here. I'd like to post a link to the original post, as it includes details on how I'd go about measuring the effect that I predict, but I don't seem to be allowed to post urls yet. Instead, I'll include a simple drawing that will preclude any need to even read the original post. It involves a simple interferometer using any long-coherence CW laser. Forgive my primitive drawing skills and terminology. If you find quantum physics at all interesting though, I hope you'll have fun considering my thoughts and correcting them as necessary. In the image below, nothing is to scale, and I pulled my distances out of a hat so to speak, but that should not affect the principle I'm trying to understand. If the text looks blurry, maximizing the page should fix it.
Thanks!

 

[AzonicZeniths]

I read your attachment, and you seem to have a mixed understanding of interference. For the interference pattern to happen you must have some kind of slit, because without that slit, you will not get the wave pattern from the photons. I have a pretty solid understand of wave-quantum-particle mechanics but I might not be understanding your experiment much yet. Explain how your beam splitters work and I might understand a little better. But, what you were saying about sending a signal that goes backwards through time, what would this do? How could it be applied for anything?

 

[ZapperZ]

Ten years ago, I posted a long rambling post concluding with a thought-experiment on Usenet, and got no useful feedback. I'd like to try again here. I'd like to post a link to the original post, as it includes details on how I'd go about measuring the effect that I predict, but I don't seem to be allowed to post urls yet. Instead, I'll include a simple drawing that will preclude any need to even read the original post. It involves a simple interferometer using any long-coherence CW laser. Forgive my primitive drawing skills and terminology. If you find quantum physics at all interesting though, I hope you'll have fun considering my thoughts and correcting them as necessary. In the image below, nothing is to scale, and I pulled my distances out of a hat so to speak, but that should not affect the principle I'm trying to understand. If the text looks blurry, maximizing the page should fix it.
Thanks!

I don't get it.

First of all, what is a "long-coherence CW laser"?

Secondly, why is this any different than the many Mach-Zehnder-type interferometer experiments that have been done? See, for example, T.L. Dimitrova and A. Weis, Am. J. Phys. v.76, p.137 (2008).

Zz.

 

[Robert Noel]

Hi ZapperZ. I thought CW was a pretty standard laser term. It means "continuous-wave", as in "not pulsed", like a standard HeNe. "Long-Coherence" means the waves remain in phase over a long distance, allowing interference to occur with large path-length differences. Perhaps I should have been more specific and said "long coherence length", but I though it was obvious.

And the drawing does not depict the experiment per se (though, I did state my original post on usenet contained an idea as to how to measure when the pattern disappears vs when we actually block path B). It is, as stated, a typical Mach-Zehnder interferometer. My question was, if we block the beam in path B 45ns into the light's journey, when does the pattern disappear at the screens (considering that path A's journey is only 5ns total and that the light in path A cannot interfere with it's path B counterpart because we block path B...40ns after the path A light reaches its target)?

 

[Robert Noel]

I read your attachment, and you seem to have a mixed understanding of interference. For the interference pattern to happen you must have some kind of slit, because without that slit, you will not get the wave pattern from the photons. I have a pretty solid understand of wave-quantum-particle mechanics but I might not be understanding your experiment much yet. Explain how your beam splitters work and I might understand a little better. But, what you were saying about sending a signal that goes backwards through time, what would this do? How could it be applied for anything?

Hi AzonicZeniths. The beamsplitters are standard cubes. You certainly can get wave patterns with cube beamsplitters (and plate beamslitters for that matter) as I've recorded holograms with them. What would this do?! Proving that the pattern disappears before we actually block path B would shatter beliefs about cause necessarily preceding effect on the macroscopic scale for one thing (and that's good enough for me). And if it were possible, creating chain reactions of such signals would change the world in more ways than I can imagine, but I'll wait until I find out if it's possible before I go speculating about that, heh heh.

 

[DrChinese]

If you find quantum physics at all interesting though, I hope you'll have fun considering my thoughts and correcting them as necessary. In the image below, nothing is to scale, and I pulled my distances out of a hat so to speak, but that should not affect the principle I'm trying to understand.

As ZapperZ points out, there are recent experiments that address the basic idea here: that effects can precede a cause. However, there are some major caveats: a) No FTL communication possibilities; b)The actual "effect" is still random and therefore the cause-effect connection is not deterministic; c) The predictions of QM are supported even though it appears to lead to logical contradictions.

There are plenty of head-scratching issues in QM, if you choose to think in "everyday" terms. For example, the issue you identify - effect preceding cause - is not a problem if the laws of physics are time-symmetric (particles can go either direction in time). I am not advocating this position particularly, merely stating that the contradictions are in our minds more than in the physics.

 

[Robert Noel]

As ZapperZ points out, there are recent experiments that address the basic idea here: that effects can precede a cause. However, there are some major caveats: a) No FTL communication possibilities; b)The actual "effect" is still random and therefore the cause-effect connection is not deterministic; c) The predictions of QM are supported even though it appears to lead to logical contradictions.

There are plenty of head-scratching issues in QM, if you choose to think in "everyday" terms. For example, the issue you identify - effect preceding cause - is not a problem if the laws of physics are time-symmetric (particles can go either direction in time). I am not advocating this position particularly, merely stating that the contradictions are in our minds more than in the physics.

Thanks for the reply DrChinese! I'll try to look up those references (assuming I can access them), but if it can be proven that the pattern would disappear a full 40ns (in this case) before we actually block path B, then we should be able to create a system whereby the process repeats itself (a chain reaction backwards through time via a feedback loop), in which case I can think of no argument that would negate the possibility of sending a binary signal (on/off or yes/no) as far back through time as the device was active, wether a few milliseconds or a few years!

 

[Robert Noel]

If anyone is interested, I have to go out for the day, but later tonight, I intend to post pictures depicting how I would go about measuring the timing between the disappearance of the pattern vs the blocking of path B (so someone can tell me how it compares to experiments already performed) as well as exactly how I would go about creating the "feedback loop" I mentioned. It's not as sci-fi as you might think...the signal (or lack thereof) would be present the moment the device was activated, that is, it's not as though we would experience one reality and then do something in the future to change that reality and create a new past or anything like that. The reaction would appear to be occurring in a forward through time direction, even though it's not...you'll see.

Wether my thought processes are faulty or not, I think you'll find it amusing since we all obviously find this stuff fascinating. At the very least, you'll have a good laugh at my expense.

Cheers!

Rob

PS: @AzonicZeniths: I was thinking about what you were saying about slits. To clarify, under ideal circumstances, in a Mach-Zehnder interferometer, light would interfere destructively at one exit port of the recombining beamsplitter and constructively at the other exit port. Just so you know, I consider the total absense of light at one exit port due to interference as a "pattern"...that is, I'm not expecting the classic double-slit pattern, though a slight shift in angular geometry of the setup can bring about such a pattern...but I know what you mean, and I know my terminology leaves much to be desired.

 

[Cthugha]

I'll try to look up those references (assuming I can access them), but if it can be proven that the pattern would disappear a full 40ns (in this case) before we actually block path B, then we should be able to create a system whereby the process repeats itself (a chain reaction backwards through time via a feedback loop), in which case I can think of no argument that would negate the possibility of sending a binary signal (on/off or yes/no) as far back through time as the device was active, wether a few milliseconds or a few years!

Ehm, no. I do not see, why the pattern should disappear before you block the path. You quoted Dirac in the picture, but I suppose you misunderstood his famous "each photon interferes only with itself". This statement aims at a situation with low light intensity down to the level of for example one photon passing a double slit at a time. Each photon interferes only with itself in this case as the interference of several photons would mean, that there are sometimes 2 photons, sometimes none and so one. In terms of conservation of energy this is impossible.

Now consider a situation where there are several photons present at a time. Let's start with 2. There have been experiments showing two photon interference, but they emphasized, that 2-photon interference is not the result of 2 photons interfering, but of the superposition of two indistinguishable two-photon amplitudes. If two photons are indistinguishable you can't even in principle tell, whether interference is a result of just one photon interfering with itself.

Going to even more photons - for example a CW laser - one might ask, whether these photons are distinguishable or not. Mandel once showed, that the degree of coherence corresponds with the degree of indistinguishability, so using a laser with long coherence time should not allow you to see an effect before blocking a a path.

 

[Robert Noel]

Ehm, no. I do not see, why the pattern should disappear before you block the path. You quoted Dirac in the picture, but I suppose you misunderstood his famous "each photon interferes only with itself". This statement aims at a situation with low light intensity down to the level of for example one photon passing a double slit at a time. Each photon interferes only with itself in this case as the interference of several photons would mean, that there are sometimes 2 photons, sometimes none and so one. In terms of conservation of energy this is impossible.

Now consider a situation where there are several photons present at a time. Let's start with 2. There have been experiments showing two photon interference, but they emphasized, that 2-photon interference is not the result of 2 photons interfering, but of the superposition of two indistinguishable two-photon amplitudes. If two photons are indistinguishable you can't even in principle tell, whether interference is a result of just one photon interfering with itself.

Going to even more photons - for example a CW laser - one might ask, whether these photons are distinguishable or not. Mandel once showed, that the degree of coherence corresponds with the degree of indistinguishability, so using a laser with long coherence time should not allow you to see an effect before blocking a a path.

Hmmm...I've obviously got some more research to do before I waste my time with more illustrations, but I'm not quite buying it. If two separate photons could combine their effects to contribute to the interference pattern, then you could combine single beams from two separate lasers with the same single longitudinal mode and obtain an interference pattern...you can't...at least everything I've read about holography,(a hobby of mine) suggests that it is not possible. But I'm taking you seriously enough to concede that I've got some more reading to do. Thanks for the reply.

 

[Cthugha]

If two separate photons could combine their effects to contribute to the interference pattern, then you could combine single beams from two separate lasers with the same single longitudinal mode and obtain an interference pattern...you can't...at least everything I've read about holography,(a hobby of mine) suggests that it is not possible.

That's right. You can't just combine single beams and expect an interference pattern to arise. It is at least not that simple. In most cases different light pulses are easily distinguishable.

I am not good at explaining, especially not in English. Maybe this paper might hint at what I mean:

Quantum interference by two temporally distinguishable pulses (Phys. Rev. A 60, R37 - R40 (1999)) (also available on arXiv)

You might also want to have a look at the Hong-Ou-Mandel experiment, but I must admit I do not know, which paper it was reported in. But I suppose, you might find a brief review of it in:

L. Mandel:Quantum effects in one-photon and two-photon interference; Reviews of
Modern Physics 71; 1999

 

[ZapperZ]

Hmmm...I've obviously got some more research to do before I waste my time with more illustrations, but I'm not quite buying it. If two separate photons could combine their effects to contribute to the interference pattern, then you could combine single beams from two separate lasers with the same single longitudinal mode and obtain an interference pattern...

This is incorrect. The interference pattern that we are all familiar with is the single-photon interference! 2-photon interference almost never, ever occur, and when it does, it isn't the interference pattern that you are familiar with. Refer to the Mendel paper that has been cited.

Zz.

 

[Robert Noel]

This is incorrect. The interference pattern that we are all familiar with is the single-photon interference! 2-photon interference almost never, ever occur, and when it does, it isn't the interference pattern that you are familiar with. Refer to the Mendel paper that has been cited.

Zz.

So...you're agreeing with me then? That's exactly what I was hinting at...that all the photons, even in a CW beam, interfere only with themselves and not each other, which is why I believe the pattern must disappear before beam B is blocked. But I'm going to have to research this concept of indistiguishability Cthugha has mentioned. I just can't imagine what difference it can make wether one photon or a billion photons strike the detector at once where the destruction of the pattern is concerned if each photon can only interfere with itself. I'll look up those references and see if I can convince myself to give this up.

You people are a gold-mine!

 

[Robert Noel]

From a brief search for these references (interference involving pulses/photons from different sources), I still can't understand how they relate to my interpretation, but I will continue to research.

As far as I can intuit, when we block the beam near the end of path B, all the corresponding photons that have reached the target via path A had nothing to interfere with, no matter how many were striking the target, period. If it takes the photons leaving the laser cavity 45ns to reach the point where we decide to obstruct path B, and only 5ns to reach the target via path A, I cannot see how the interference pattern can possibly continue to exist until after we block path B since the photons that took 5ns to reach the target via path A are the same photons that would have also taken over 45ns to reach the target via path B to interfere with themselves.

What are the photons taking path A interfering with if the pattern doesn't disappear 40ns before we block path B? Certainly not themselves, since we have prevented that (or more accurately, WILL have prevented that) by blocking path B!

 

[Cthugha]

As far as I can intuit, when we block the beam near the end of path B, all the corresponding photons that have reached the target via path A had nothing to interfere with, no matter how many were striking the target, period. If it takes the photons leaving the laser cavity 45ns to reach the point where we decide to obstruct path B, and only 5ns to reach the target via path A, I cannot see how the interference pattern can possibly continue to exist until after we block path B since the photons that took 5ns to reach the target via path A are the same photons that would have also taken over 45ns to reach the target via path B to interfere with themselves.

That is exactly, why I think the term single photon interference is badly chosen. A laser beam consists of a lot of photons, which are intrinsically indistinguishable during coherence time or stated more precisely if coming from the same coherence volume. So due to indistinguishability one could say at most that only photons from the same coherence volume can interfere. At first you can't tell, whether they do so with themselves or each other. So if you block the long path, taking your terminology there are still photons taking path B, which have already passed the blocking point and came from the same coherence volume as photons arriving at the same time, which took path A and therefore there should be interference.

However, this sounds pretty odd in particle description. It sounds much better, if you take the underlying field and possible paths into account.

Anyway I have to leave this discussion for a while as I am off to NOEKS 9 now. I hope to read some interesting posts, when I return end of the week.

 

[Robert Noel]

Wow! I just looked up NOEKS 9...Nonlinear optics is what I've been hoping would ultimately make my experiment (not the timing measurement but rather the chain reaction) possible -a two-wave mixing scenario using the right material, if it exists. I look forward to continuing this discussion with you! Have fun over there!

 

[Robert Noel]

This indistinguishability concept has me baffled. A photon leaving the laser cavity can only travel at one speed: "The speed of light". I can't understand how anything can circumvent the fact that after 45ns of leaving the laser cavity, a photon can only be at one specific distance along its path (assuming we force it to choose only one path). Therefore, I cannot see how our inability to distinguish which photon is interfering with which photon can have any bearing on the destruction of the pattern, how it can allow a photon to sneak past our barrier before we block it, wether we are thinking waves or particles, single photons or many photons. The constancy of the speed of light (of a photon) in a given medium is what makes me doubt that the pattern can continue to exist after the path A photons reach the target if we block path B 40ns later, preventing those same photons from reaching the target via path B (I hope I'm making sense here...I'm giving myself a headache).

I guess what I'm saying is that I don't understand why individual photons should care if we cannot distinguish them from other photons. Each travels at exactly the same speed and if we know the ones at our obtruction in path B would have taken 40ns less to reach path A, then the path A photons from 40ns ago could not have had anything to interfere with. I feel like an idiot for not "getting it", but my primitive brain just refuses to digest what it's being told, but I'll keep trying.

 

[muppet]

There's another Dirac quote that may be of relevance here. Someone once asked him what he thought happened when a wavefunction collapses. His reply was: "Nature makes a choice". In this experiment, nature is free to consider her options right up until the barrier is implemented.
It's difficult for me to sharpen this argument up much because I haven't studied QFT; I'm not sure if the idea of a "wavefunction" means anything when particle numbers aren't necessarily conserved. But conceptually (and very roughly speaking), it might be worthwhile remembering that wavefunctions commonly aren't strictly localised? A wavefunction assigns to each point in space the probability that a localised particle will be found there. Even though we talk about a hydrogen atom as having a radius of a couple of angstroms, the radial distribution function isn't zero a metre way from the proton; it's just extremely small. A free particle can only be completely localised to any finite region of space if it's described by a superposition of infinitely many plane waves (in which case all talk of it taking one path or the other is meaningless as the uncertainty in its momentum blows up in your face). The best way I could hope to understand this experiment without a knowledge of QFT would be to say that when the barrier is put in place the contribution to the wavefunction from eigenstates traversing that path is reduced to zero, so the interference pattern of the wavfunction vanishes. The "actual location" of the photon (by which it is meant a small region of high probability density of finding the electron) doesn't actually have a great deal to do with it. People who actually know something about the QM description of light please feel free to shoot me down if that's hideously inapplicable?

 

[Robert Noel]

But when photons reach the end of one path (detector or obstruction), the wavefunction collapses, the photon is localized, and since we know the exact length of each path, is it not determined exactly when they would have reached the end of the other path had they taken it? It's not like the speed of light is going to change in one path in order to preserve the pattern so we can't make effect precede cause...Or are we back to the indistinguishability thing again? It feels like we're supposed to believe nature actually cares what we know, like some program designed to foil our every attempt to tamper with it. I don't want to believe it, heh heh!

 

[muppet]

Well... by that reasoning you could time how long the photon took to be detected and work out which path it took without ever actually measuring the position at an intermediate point.

Would anyone care to explain why you can't do this?

 

[Robert Noel]

Well, because there's no way to tell when the photon was emitted or left the laser cavity unless you could trigger the emission of a single photon and measure the timing between triggering and absorption, which by itself, couldn't really be considered an interference pattern. We could, I suppose, do this many many times and see if a pattern still emerges though. Not quite as dramatic as what I'm proposing (trying to make the pattern disappear before we block path B), but I do wonder if it's possible.

 

[Robert Noel]

Here's an image depicting how I'd try to measure the timing if it helps. Again, maximize the image if the text is blurry. I hope I didn't leave out anything obvious. Again, the laser is a continuous wave laser with a long coherence length, but importantly, is also linearly polarized (pretty standard fare...a typical HeNe could fit the bill).

 

[Phrak]

Well, because there's no way to tell when the photon was emitted or left the laser cavity unless you could trigger the emission of a single photon and measure the timing between triggering and absorption, which by itself, couldn't really be considered an interference pattern. We could, I suppose, do this many many times and see if a pattern still emerges though. Not quite as dramatic as what I'm proposing (trying to make the pattern disappear before we block path B), but I do wonder if it's possible.

There are picosecond pulse lasers. A lot of filtering could bring the average photon density down to 1 per pulse. It's feasible, if not practicle.

 

[Cthugha]

This indistinguishability concept has me baffled. A photon leaving the laser cavity can only travel at one speed: "The speed of light". I can't understand how anything can circumvent the fact that after 45ns of leaving the laser cavity, a photon can only be at one specific distance along its path (assuming we force it to choose only one path). Therefore, I cannot see how our inability to distinguish which photon is interfering with which photon can have any bearing on the destruction of the pattern, how it can allow a photon to sneak past our barrier before we block it, wether we are thinking waves or particles, single photons or many photons. The constancy of the speed of light (of a photon) in a given medium is what makes me doubt that the pattern can continue to exist after the path A photons reach the target if we block path B 40ns later, preventing those same photons from reaching the target via path B (I hope I'm making sense here...I'm giving myself a headache).

I guess what I'm saying is that I don't understand why individual photons should care if we cannot distinguish them from other photons. Each travels at exactly the same speed and if we know the ones at our obtruction in path B would have taken 40ns less to reach path A, then the path A photons from 40ns ago could not have had anything to interfere with. I feel like an idiot for not "getting it", but my primitive brain just refuses to digest what it's being told, but I'll keep trying.

Ok, I see the problem here. Now it gets a bit complicated. I hope I manage to formulate my point of view in an understandable manner.

So what is a photon? It is the quantized analogon to the classical intensity of the light field. So in terms of fields it is second order, at a certain place and time it is something like

I(r,t)=E^*(r,t)E(r,t)

Now one can deconstruct the em-field at a certain point into a superposition of several fields. For example, you could have a superposition of a laser and several usual lamps or generally speaking just several sources. How does this superposition effect the intensity. Each field has an amplitude and a phase. Two different light sources do usually not show a fixed phase relationship, so it is pretty random, whether these fields add up (same phase) or cancel each other (phase shift of pi). So in average there will be no intensity created by the product of different light sources. The intensity is then created by the square of a single field.
Now consider the usual double slit. Here each of the two slits is a light source of its own, but as both are created by a single light beam, they show a fixed phase relationship. As you surely know, you will see an interference pattern at the screen. This is due to the fact, that now not only the squares of the fields from each slit contribute to the intensity, but also the product of the two fields, which does not have a random phase relationship and does therefore not cancel.
Moving on to a laser, the principle stays the same. Each atom (or molecule or quantum dot or whatever you use as the active medium) contributes to the final em-field. Due to the lasing processes all of these single fields show a fixed phase relationship.

Now the definition of coherence time is simply speaking a measure of how long there is a fixed phase relationship of the emission of a light source. So roughly speaking, coherence time determines the timespan inside which there are also contributions of products of different fields to the intensity, whereas outside of the coherence time there are just contributions of the squared single fields.

So inside coherence time, you can't just map each photon to a single source (which would be distinguishability), but have to take the whole superposition of all fields into account. If you introduce some delay (like you do) you even have to take the products of the fields at different times into account (inside coherence time) and all of the products of these fields contribute to the intensity, which is at the heart of indistinguishability. You just can't imagine the photon as a bullet traveling from the emitter to the detector anymore.

However, this was a rather classical explanation. To get to the quantum point of view, you just replace the fields with adequate operators.

 

[scjphysicist]

I pretty much agree with the other replies to this question, but let me give a little different viewpoint.

I think the main difficulty in interpretting your proposed experiment is with the energy-time uncertainty relation, often written as \Delta E \Delta t > \hbar. When you regard the time that a photon is launched into your apparatus as being very well-localized, say, to within a nanosecond, you are giving it a small delta t. But saying that it has a very long coherence length means, among other things, that it has a very small uncertainty in its energy, a small delta E. The energy-time uncertainty relation says that you can't meet both of those conditions in the same experiment. What I think this will mean is that if you have a laser with a coherence length of 50 light-nanoseconds, you won't be able to say when a photon is emittited with better than 50 nanosecond precision. My guess for what would happen if you ran this experiment is that as you changed the time that the shutter is switched, the contrast you would see in your fringes would change, gradually disappearing as you blocked the interference for a greater fraction of the time when the photon is in the apparatus.

 

[f95toli]

There are picosecond pulse lasers. A lot of filtering could bring the average photon density down to 1 per pulse. It's feasible, if not practicle.

There are some pretty good single photon sources around, they are not completely deterministic and the fidelity isn't 100%, but they are getting there. Quantum cryptography actually requires deterministic single photon sources in order to be completely secure, so there is a lot of money being spent on R&D in this field at the moment.

 

[Fredrik]

Thanks for starting this thread Robert. I really like this experiment. It's a fun problem that really shows how weird QM can be.

By carefully adjusting the length of the longer beam, we can choose what fraction of the photons will be detected by the lower detector. Let's say that we set it up so that the lower detector never clicks. By inserting the obstacle (permanently) we destroy the interference, and the result is that the lower detector clicks 50% of the time.

Now let's assume that the intensity of the beam is so low that only one photon at a time is emitted. This doesn't change any part of what I just said. With the obstacle in place, the lower detector clicks 50% of the time. Without the obstacle, the lower detector never clicks. This can be explained using path integral methods. (The simple kind that's described in Feynman's QED book). The amplitude associated with the longer path to the lower detector is -1 times the amplitude associated with the shorter path to the lower detector, so the amplitudes add up to zero.

Now we're getting to the fun part. Let's start with the obstacle in place and wait until the lower detector clicks. Now we quickly remove the obstacle. If the photon "takes both paths", moving at speed c, it isn't going to hit the obstacle, so how could the lower detector click?

The only answer I can think of is that the obstacle blocks some of the superluminal paths through space-time. Apparently that's enough to get the detector to click some percentage of the time. (We're not just supposed to add up amplitudes associated with paths through space, we're supposed to add up amplitudes associated with paths through space-time. We can often ignore paths with the "wrong" velocity, but this isn't one of those times).

Now let's do it a bit differently. Let's say that we instead remove the obstacle exactly 10 ns after each time a photon is emitted. (We'd have to change the setup a little bit, but it can be done). This situation is only slightly different from the first. The obstacle still blocks only some of the superluminal paths, but that's enough to change what percentage of the photons will be detected by the lower detector.

Does it matter how long we leave the obstacle in place before we remove it? I think it makes a big difference. I think that if we remove it almost instantly, the result of the experiment will be close to what it would be with no obstacle (50% detected by the lower detector), and if we wait until the last nanosecond to remove it, the result of the experiment will be close to what it would be with a permanent obstacle (0% detected by the lower detector).

Does this mean that the effect is preceding the cause? In a way, yes, because by removing the obstacle at t=10 ns, we're affecting the probability of the possible events at t=5 ns. Does this mean that we can send messages into the past? No, it doesn't. This should explain why:

Suppose that we choose the time to leave the obstacle in place so that the lower detector will click 25% of the time. By increasing the time a bit, we can get it to click 30% of the time. By decreasing the time a bit, we can get it to click 20% of the time. We can let 30% mean "1" and 20% mean "0", and send a binary code message by sometimes going for 30% and sometimes going for 20%. But to send even a single binary digit, we'd have to let many photons be detected. We are, in a way, "sending a message into the past", but there's no way to see what the message says until after we're done.

This is the reason why causality isn't really violated. The interference "pattern" is only present in the data from a large number of detection events. Nothing useful can be learned from a single detection event.

 

[Cthugha]

Now we're getting to the fun part. Let's start with the obstacle in place and wait until the lower detector clicks. Now we quickly remove the obstacle. If the photon "takes both paths", moving at speed c, it isn't going to hit the obstacle, so how could the lower detector click?

The only answer I can think of is that the obstacle blocks some of the superluminal paths through space-time. Apparently that's enough to get the detector to click some percentage of the time. (We're not just supposed to add up amplitudes associated with paths through space, we're supposed to add up amplitudes associated with paths through space-time. We can often ignore paths with the "wrong" velocity, but this isn't one of those times).

Ehm, no. This is quite some usual problem. Popular statements like "the photon takes both parts and moves at c" are ripped out of context and lead to wrong results. If you read post #24 again, you will notice, that the notion of a photon being a somewhat bulletlike entity moving at c is at most valid outside of coherence time. In fact the changes in the underlying fields are moving at c, which is a huge difference. There already is a field beyond the obstacle. You do not annihilate it instantly by putting the obstacle in.

 

[Robert Noel]

Wow...this is a lot to digest! I guess one of my problems is in terminology. I've gotten used to thinking in terms of "coherence lengths" (all holography books refer to coherence length of lasers) and "discrete packets" etc...

"Fields" and "coherence times" and "indistinguishability" are bending my mind, but I really, really appreciate all the input. I have to learn to think in these terms. I'll probably spend days (if not weeks) thinking about what you folks have been saying.

I really do wish I'd posted the image I uploaded in post #22 instead of the one I uploaded in post #1 though. It might not make any difference, but it might better depict where my thought processes went wrong. I think that where my thinking is flawed is in assuming that once the wave-function collapses for the photons in either path (eg. we divert path B to detector 2 in the second image), the time of emission of the photons in either path becomes irrellevant. Obviously I was wrong, and I have to spend some time trying to understand all of this.

I can't thank you people enough!

Edit: I did make major typo in the second image...where it says "at the same time" I should have said "as a result"...but hopefuly it was obvious.

 

[Fredrik]

If you read post #24 again, you will notice, that the notion of a photon being a somewhat bulletlike entity moving at c is at most valid outside of coherence time. In fact the changes in the underlying fields are moving at c, which is a huge difference. There already is a field beyond the obstacle. You do not annihilate it instantly by putting the obstacle in.

I didn't read #24 before, but I have read it now. I don't see how concerns about photons being indistinguishable are relevant when we can just lower the intensity so that the source emits (for example) an average of one photon per millisecond.

You're implying that there's something wrong with what I said, but you didn't say what. The only hint you gave me seems to be a misunderstanding on your part:

Popular statements like "the photon takes both parts and moves at c" are ripped out of context and lead to wrong results.

That was my conclusion too. I thought I made that clear enough in my post.

I have always felt that it's an extreme oversimplification to say that the photon takes both paths. I would rather say that each path through space-time from the emission event to a possible detection event contributes to the probability that the photon will be detected at that event. To calculate that probability, we would have to add up the probability amplitudes associated with N paths (chosen so we can get all the paths by letting N go to infinity), divide by N, take the square of the magnitude of the result, and then let N go to infinity. (That's just the definition of a path integral). Hm, that's quite a mouthful. Maybe that's why the phrase "the photon takes both paths" is used so often.

 

[Cthugha]

I didn't read #24 before, but I have read it now. I don't see how concerns about photons being indistinguishable are relevant when we can just lower the intensity so that the source emits (for example) an average of one photon per millisecond.

Oh, I just intended to say that going to an average of one photon per millisecond does not change anything as we know from coherence theory. Coherent light keeps its coherent character even at low intensities, which is why single photon sources are so hard to create.
If the source emits on average just one photon per millisecond, the intensity is still created by the superposition of all of the fields during coherence time, which again forces us to take indistinguishability into account.
To summarize, I just wanted to emphasize, that it is more intuitive to think of the field going all possible ways instead of the photon.

 

[Phrak]

Just to clear-up a point from the original post.

Lasers are not monochromatic but contain, ideally, a discrete spectrum of wavelengths. The coherency length of a laser is equal to it's length, ignoring the ratio between the indexes of refraction of air and lasing medium. In any case the spectrum is in phase at the reflective surfaces.

 

[Cthugha]

The coherency length of a laser is equal to it's length, ignoring the ratio between the indexes of refraction of air and lasing medium. In any case the spectrum is in phase at the reflective surfaces.

Maybe I am just to dumb to understand: "The coherency length of a laser is equal to its length, ..."

Which length do you mean by "its length"?

 

[Robert Noel]

Yeah, that's got me scratching my head too. With gas lasers, I always thought the shorter the lasing cavity, the fewer possible longitudinal modes and thus the longer the coherence length. Also, with the addition of an intra-cavity etalon, the laser can be forced to operate in "single longitudinal mode", which is as monochromatic as you can get, and can result in coherence lengths on the order of many, many meters if not kilometers.

 

[Fredrik]

The details about how a laser works shouldn't really matter in the discussion about what this experiment says about causality and the principles of quantum mechanics. We might have to know those details to be able to perform this experiment in the real world, but the thought experiment works with any monochromatic light source. In the thought experiment, we can always just imagine that photons that go off in the wrong direction are hitting some obstacle that absorbs them.

 

[Phrak]

Maybe I am just to dumb to understand: "The coherency length of a laser is equal to its length, ..."

Which length do you mean by "its length"?

It goes something like this, with my limited understanding. There's a narrow, but finite bandwidth of frequencies when the electron 'falls' from it's metastable state to a lower energy state. The laser is a resonant cavity that will only amplify the frequencies withing this bandwidth that that meet the boundry conditions imposed by the size and shape of the cavity. In the case of a lazer, it's just two mirrored surfaces. So the length between the two mirrored surfaces accommodates something like one trillion, one trillion and one, one trillion and two, etc. wavelengths.

Additionally, the phase of the wave at the mirrored surface is fixed for all frequencies. So, at the mirrors, they're in phase. Anywhere else, they will not be in phase. Check me on this.

Addtionally, how and where all the transverse components of the fields add-up has something to do with the TEM(x,x) specification. Maybe someone else, fresher on this material, can clarify.

 

[Phrak]

Robert-

An etalon is something new to me. Care to share?

 

[Robert Noel]

An etalon is a Fabry-Perot interferometer, either solid or air-spaced (but solid in laser cavities), and when it is used in the cavity of a laser, the beam enters it and is reflected back and forth by the reflective surfaces...the end result being that when angled properly, only the desired frequency is allowed to pass through while others interfere destructively, allowing the plasma (or whatever medium) to only amplify that single frequency, resulting in an extremely long coherence length. When I attended a holography course in Toronto, the large 50mw HeNe laser they had was equipped with an etalon, which allowed them to ignore the length differences between reference and object beam paths in the recording setup (I had to take care to keep beam paths as equal as possible on my table with my little 5mw HeNe in order to record 8"x10" holograms with a lot of depth, but I did it).

An etalon-equipped laser is standard in professional holography labs, where extremely monochromatic/temporally-coherent light is required.

From Wikipedia (under applications):

"Etalons are used to construct single-mode lasers. Without an etalon, a laser will generally produce light over a wavelength range corresponding to a number of cavity modes, which are similar to Fabry-Pérot modes. Inserting an etalon into the laser cavity, with well-chosen finesse and free-spectral range, can suppress all cavity modes except for one, thus changing the operation of the laser from multi-mode to single-mode. "

They are talking about longitudinal mode (temporal coherence/monochromacity) and not transverse mode (spatial coherence...ie. TEM OO)

Edit: I never really thought about it before, but it's funny how all holography texts I've read refer to the temporal coherence of a laser as the coherence-length (instead of coherence-time).

 

[dkv]

The interference is realized when and only when the paths are available to interfere.
blocking one of paths does not produce any interference.
In other words the intereference can be seen only after 45ns and not 5ns.
If path B is blocked the interfernce then the experiment is equivalent to the single slit experiment.
Is this correct?

 

[Phrak]

dkv, and all.
Here's an interesting factoid regaurding the experiment in question that will show that the analysis of this is not as simple as expected. A continuous wave sent to a shutter that's open for a short time, say, will modify the spectral components of the wave. (The same occurs with the 'half shutter' under question.)

Now you have a lot of different frequencies that have non-zero amplitudes on the down-stream side of the shutter before the shutter is opened--or did someone already mention this?

 

[Robert Noel]

The interference is realized when and only when the paths are available to interfere.
blocking one of paths does not produce any interference.
In other words the intereference can be seen only after 45ns and not 5ns.
If path B is blocked the interfernce then the experiment is equivalent to the single slit experiment.
Is this correct?

That is correct. I'm talking about the interference pattern disappearing after 5ns. I'm also talking about a single-frequency laser. Somehow I assumed most physics labs performing such an experiment would use an etalon-equipped single-frequency CW laser (hence the extremely long coherence length.

Again, I think the second image I posted should make it much clearer what I'm thinking. The interference is already occurring and we purposely destroy the interference, thereby (I assumed), determining the paths/locations of all photons, making all quantum weirdness suddenly irrellevant (or so I thought...I'm still struggling to understand why this isn't so).

Edit: I'll repost the image here with one gross error in phrasing corrected:

 

[Cthugha]

It goes something like this, with my limited understanding. There's a narrow, but finite bandwidth of frequencies when the electron 'falls' from it's metastable state to a lower energy state. The laser is a resonant cavity that will only amplify the frequencies withing this bandwidth that that meet the boundry conditions imposed by the size and shape of the cavity. In the case of a lazer, it's just two mirrored surfaces. So the length between the two mirrored surfaces accommodates something like one trillion, one trillion and one, one trillion and two, etc. wavelengths.

Additionally, the phase of the wave at the mirrored surface is fixed for all frequencies. So, at the mirrors, they're in phase. Anywhere else, they will not be in phase. Check me on this.

Roughly speaking this is correct, but it depends of course on the kind of laser you are using. For pulsed lasers you also may have designs, where you want to have lots of different modes in phase at a certain point of time (mode locking) or to detune the cavity (Q-switching).

However I am still puzzled on the connection to the coherence length. The longitudinal coherence length is related to the coherence time and is in easy cases usually determined via the decay of the autocorrelation, which is in easy cases the Fourier transform of the power spectrum.

 

[dkv]

if the obstruction of path B is dependent on a random outcome of another experiment then
your experiment suggests that the random outcome is deterministic in time which is not possible unless there is nothing random in the definition of randomness.(i.e the interfernce pattern disappears even before the random experiment is performed!)
therefore ,
I don't think that interference can be seen before 45ns.That is if interference can be seen only after 45ns then there is no question of the interference disappearence after 5ns and before 45ns.

 

[Robert Noel]

if the obstruction of path B is dependent on a random outcome of another experiment then
your experiment suggests that the random outcome is deterministic in time which is not possible unless there is nothing random in the definition of randomness.(i.e the interfernce pattern disappears even before the random experiment is performed!)
therefore ,
I don't think that interference can be seen before 45ns.That is if interference can be seen only after 45ns then there is no question of the interference disappearence after 5ns and before 45ns.

There is no randomness involved. We swith the laser on and see the interference occurring, and we then press a button that causes the shutter (or whatever obstruction we are using) to suddenly block the long path, stopping the already-occuring interference.

In the case of the second image (see post #41 above), the polarization of the linearly-polarized beam is rotated, causing it to be diverted away from the recombining beamsplitter to the long-path detector by the polarizing cube beamsplitter, causing interference (which has been occurring before we even start the experiment by diverting the long-path beam) to stop at the recombining beamsplitter and allowing light to reach the short-path detector. The beam is monochromatic (one single wavelength) and I'm confused as to why the subject of wavelengths has been discussed...I assumed that it is taken for granted that an etalon is used in a laser where such long beam-path length differences exist, and that etalon-equipped lasers can produce photons of only one single wavelength (or frequency, whichever you prefer) if desired, in other words, purely monochromatic light.

If I take the definition of photons as "discrete packets of energy" as I've read, I can't understand how interference can keep happening until we puposely divert the long-path beam. To me, it should logically disappear before we divert the beam, but if what Cthugha says is correct, apparently I've got some studying to do.

And for the record Cthugha, I never imagine photons as "bullets" traveling anywhere, or even waves travelling...I find it hard to imagine photons existing at any point between emission and absorption...but that's another matter entirely and is irrellevant in this experiment. When I say "the photon takes the short path or the long path", I'm just using easy to understand language that is commonly used.

 

[dkv]

the effects can precede cause but not in a deterministic way as you are suggesting.
I read the main diagram and not the second one.
Anyways ,
photons do not interfere with themselves...
We can understand the photon self-interference using different concepts.
"Deterministically" ,Effect is realized only after the causes become available.


(wavelength discussion is irrellevant and also the bullet or wave debate)

 

[Cthugha]

And for the record Cthugha, I never imagine photons as "bullets" traveling anywhere, or even waves travelling...I find it hard to imagine photons existing at any point between emission and absorption...but that's another matter entirely and is irrellevant in this experiment. When I say "the photon takes the short path or the long path", I'm just using easy to understand language that is commonly used.

Ok, I did not mean to sound like I "accused" you of thinking so. I just met a lot of people jumping to wrong conclusions from using commonly used language.

If I take the definition of photons as "discrete packets of energy" as I've read, I can't understand how interference can keep happening until we puposely divert the long-path beam.

Hmm, I am not sure, I get you here, but I would like to understand what you mean. Why does discreteness of the energy prevent interference right before one diverts the long path, from your point of view?

Anyways ,
photons do not interfere with themselves...
We can understand the photon self-interference using different concepts.

Photons do not self-interfere, but we can understand this interference, which is not happening with different concepts? That does not make any sense.
How do you explain single photon interference experiments, if photons do not interfere with themselves?

(wavelength discussion is irrellevant and also the bullet or wave debate)

Did you even bother to read the rest of this thread? As your first two posts already show, obviously not, there even was no "bullet or wave debate". I just commented about what one has to keep in mind, when dealing with photons and beam splitters on timescales shorter than the coherence length in post #24.

 

[dkv]

Anyone who has even a bit of the understanding of physics will not bother to discuss the non-commutativity of cause and effect becuase it will lead to contradiction to its very core as I said like introducing the non-probabilistic physics based on which we can not discuss the question of interference from quantum mechanical point of view.

 

[Cthugha]

Anyone who has even a bit of the understanding of physics will not bother to discuss the non-commutativity of cause and effect becuase it will lead to contradiction to its very core as I said like introducing the non-probabilistic physics based on which we can not discuss the question of interference from quantum mechanical point of view.

I strongly disagree. Finding flaws in thought experiments is usually very educative, especially if they are well put. Bohr and Einstein were very successful in doing so. For example one needs a severe understanding of coherence in order to find the flaw in the experiment presented here. As another example delayed choice quantum erasers also do not violate the principle of cause and effect, but it is very educative to find out, why.

As to the second part of your post, I do not get it. No one introduced non-probabilistic physics here, so I am very puzzled.

And I also still do not get, whether you think, that photons do or do not show self-interference. Your posts are not very clear.

 

[Robert Noel]

Ok, I did not mean to sound like I "accused" you of thinking so. I just met a lot of people jumping to wrong conclusions from using commonly used language.

No! Don't worry about that Cthugha! I did not take offense at all. I fully realize that the language I use makes it sound like that is how I think. I just wanted to let you know I don't think it's that simple. Hmmm...at the risk taking this thread way off-topic, here's an excerpt from something I wrote in another post about photons:

"But relativity states that for anything traveling at the "speed" of light, space would have to contract to a singularity in the direction of travel. I don't pretend to know what it's like to experience the knowable universe as a singularity, as I assume a photon must since it obviously "travels" at the speed of light, but I assume that in a singularity, all possible points along the path of the thing doing the traveling must overlap, that is, in some way, every place a photon can go (its knowable universe) exists in one place to the photon (even the term "place" would be meaningless in a singularity). I also assume that from relativity we can deduce that from the photons perspective, it's "journey" would be instantaneous (time dilation), which makes sense since if all possible points along its path overlap, it shouldn't take any time for it to get where it's going. So does it necessarily have to actually exist between emission and absorption? Is it possible that what we interpret as the wave-function is actually just an interpretation we make because we don't quite get what it's like inside a singularity. Could it be that the concept of existing in spacetime to a photon is as meaningless as the concept of existing in a singularity is to us?

I'm just having fun (I like to bend my mind around crap like this), but when thinking about relativity, I can't help but intuit that the photon's target is actually chosen the instant it is emitted...that an atom absorbing a photon has as much a causal effect on the atom emitting it (no matter how far back in time) as the emitting atom has on the absorber. Putting things like lenses in the path certainly affects the probabilities of where the photon can be absorbed, but who knows how a lens can be said to affect a photon when it exists as a part of a singularity? Does the photon actually have to exist between emission and absorption?"

My ideas are probably totally misguided, but that's how I see it at the moment. The main argument against this view is that particles that travel slower than the speed of light (ie. electrons) also have a wave-function, but then who knows how the bound-energy that constitutes an electron sees it's knowable universe? Does anyone know what an electron really is? We call it a "negative charge" but that's really a description based on what it does, not what it is (ie. it attracts particles that we call "positive charges", and changes direction in a magnetic field etc...). It seems that just about everything in nature is defined by what it does and not what it is. Same goes for gravity...some say it is warps in spacetime (I like that one), and some still try to define it by an exchange of hypothetical particles called "gravitons" (I groan whenever I read that one). Nobody knows what any of these things actually are...at least not that I'm aware of. Sorry to get so off-topic...I'll shut up now.

Hmm, I am not sure, I get you here, but I would like to understand what you mean. Why does discreteness of the energy prevent interference right before one diverts the long path, from your point of view?

Actually, it's more the term "packet" that I'm referring to...a small bundle of energy. It implies to me that it's something that can only be in one place (emitted or absorbed by one atom or particle). This concept of it taking multiple paths or existing as a spreading wave, to me, sounds more like an illusion or interpretation than an actual existing phenomenon. If I recall correctly (and I may not be), in one book by Richard Feynman, I was told to view this wave-function that we see depicted in interference diagrams as an abstraction that we use to calculate probabilities, and not an actual thing (I'm misquoting for sure as that particular book is back at the library..."QED" I think).

The concept that it matters wether or not we can distinguish photons in this thought experiment is incomprehensible to me. I have always considered that when we detect a photon (it is absorbed), we automatically determine exactly when it would have been absorbed at any other point in space (taking into account what we interpret as path changes via lenses, mirrors, gravity etc...). Likewise, I would think that we automatically determine exactly when any photon leaving the laser cavity (and again, "leaving the laser cavity", is just simplified language that I use) at the same time as the detected photon would be absorbed at any point in space. I can't seem to wrap my brain around the concept that, at least in this experiment, it matters if we can't distinguish which photon is which, but I'll keep trying.

Everything you're writing is going on my "try to understand" list...and I do appreciate your taking the time to reply!

Edit: The more I reread this post, the more I wonder if I'm making the slightest bit of sense...can you tell I'm confused? heh heh.

 

[Cthugha]

The concept that it matters wether or not we can distinguish photons in this thought experiment is incomprehensible to me. [...] I can't seem to wrap my brain around the concept that, at least in this experiment, it matters if we can't distinguish which photon is which, but I'll keep trying.

The concept of distinguishability is not only important in this experiment, but in any experiment concerning interference. Niels Bohr thought of the concept of complementarity as being fundamental in quantum mechanics.

Maybe you know the famous delayed choice quanum eraser experiments, in which you can either have which way information or an interference pattern. If you can distinguish photons this means you can track them back to their source in principle, which is the same as having which way information. The Englert-Greenberger duality relation even predicts quantitatively how the amount of distinguishability affects the interference pattern.

I have always considered that when we detect a photon (it is absorbed), we automatically determine exactly when it would have been absorbed at any other point in space (taking into account what we interpret as path changes via lenses, mirrors, gravity etc...). Likewise, I would think that we automatically determine exactly when any photon leaving the laser cavity (and again, "leaving the laser cavity", is just simplified language that I use) at the same time as the detected photon would be absorbed at any point in space.

So let's take a beam of light, send it through a beam splitter and send both beams through another beamsplitter, which "reunites" them. If you detect a photon in one of the two beams, which come out of the reuniting beam splitter, do you determine in which of the two beams between the first and the second beamsplitter the photon would have been detected?