particle location

[DParlevliet]

I am not familiar with these type of detectors, but with every explanation of the double slit they are used at the slits input, so I suppose they exist. It could also be done as the measurements with coincidence detection, like in topic Shape of the photon, #224. Here a BBO crystal is used which produces two coherent photon. One is counted directly, the other after the slits.

About the interference, that is normal wave properties where the wave determines the probability of absorption by the detector.

[Delbert]

I am not familiar with these type of detectors, but with every explanation of the double slit they are used at the slits input, so I suppose they exist.

Yes, and apparently when they try to detect which slit it passed with a detector immediately adjacent to a slit, the interference pattern breaks down - that is, it disappears. Presumably because it then becomes two sets of events (as I tried to outline previously). The first pair of events being from the source to the detector immediately adjacent to a slit, followed by the second pair of events with emission from the point of detection to the destination.

 

Now you might suggest that the slit detector is not a photon emitter, and you'd be quite right. Presumably detection disturbs or collapses the process or wave distribution (gauge field or whatever) in some way, such that this event appears as a detected photon to us mortals. The process restarts effectively rendering it as a emitter to the final destination. The situation is thus completely different resulting in no interference pattern. Anyway, that's how I would express the phenomenon.

 

Indeed, I seem to understand there is at least one video on You Tube illustrating this very paradox. It being a cartoon presentation, which is not only amusing, but presents the paradox quite well, I think ( https://www.youtube.com/watch?v=DfPeprQ7oGc ). Although the 'knows it's being watched' suggestion is clearly delving into a non scientific area. But of course, it's only a paradox for us humans when we try to visualise photons as little billiard balls.

[DParlevliet]

The modern definition of a quantum particle is given by Quantum Field Theory.

It is not a classical particle or a wave, or even a combination of the two.

It is an excitation of a quantized gauge field.

 

Does this theory explains the particle and wave properties? Or this topic, the "particle" location?

[Delbert]

 

Does this theory explains the particle and wave properties? Or this topic, the "particle" location?

It seems you're stuck on this particle or wave or both business. The plain fact is it can't be both, and since it can 'appear' as either depending on what method we use to detect it, it can't be neither either.

 

As I tried to suggest previously, what we call a photon is possibly nothing more than an event. An event at the source followed by an event at the destination. What happens in between is open to discussion, but probably some form of energy or wave distribution - gauge field is suggested. Now some might say, I can detect it at one slit (in the case of the two slit experiment), but as we know that changes the result of the experiment completely. The conclusion being that no matter how subtly we peek, watch or detect the photon, the field distribution will collapse as an event that we call detecting a photon. The field or whatever presumably then spreads back out again until another event at the detector. And because the field or whatever didn't go though both slits as a consequence of our detection, no interference pattern.

 

Prof R. Feynman I believe described it as 'sum over histories'. Whereby the transit following the source event consists of a myriad of interactions. The thing we call a photon immediately reacts and changes into and with all sorts of other things. Presumably the conclusion is that the energy of the photon is thus spread out. I suppose one could argue that the photon, apparently being nothing more than an event at the source and destination, doesn't actually exist!

 

Perhaps I'm choosing a not too good example or analogy, but maybe it could be viewed as hitting a steel rod with a hammer. The moment the hammer hits one end of the rod we call a photon. The energy of the hit is then spread out into the atoms of the rod and transferred to the other end, which then feels the force of the hit. The force of the hit is what we call a photon at the destination. In other words, the photon never existed as anything that could be described as an object, it was just the moment of the hits.

 

I'm standing by to be shot down!!

Edited December 14, 2013 by Delbert

[DParlevliet]

The plain fact is it can't be both, and since it can 'appear' as either depending on what method we use to detect it, it can't be neither either.

 

That is right, it is not both, it has properties of both, although that is classical not possible. That is what you read in all books (and Wikipedia). Nobody goes further, so me neither. The rest you write is an opinion, perhaps right, perhaps wrong, but there is no data, no proof or math to discus about.

[Delbert]

 

The rest you write is an opinion, perhaps right, perhaps wrong, but there is no data, no proof or math to discus about.

I was just trying to contribute to the subject matter about particle location. And suggesting, or offering an opinion, that not only location, as we humans understand an identifiable location, is probably doubtful and certainly up for discussion, but also the possibility that what we call a photon as an identifiable object may also be doubtful and up for discussion. Because quite frankly, and certainly because of the two slit experiment if not anything else, clearly demonstrates it's not a little ball of whatever moving from place to place - and possibly not even an object at all.

 

And if the same applies to all subatomic particles... Perhaps I'm going too far!

[DParlevliet]

... because of the two slit experiment if not anything else, clearly demonstrates it's not a little ball of whatever moving from place to place - and possibly not even an object at all.

 

Alright, then let me try further:

 

 

The photons are generated by a BBO crystal which, when radiated by laser, now and then emit two coherent photons with half the intensity of the laser. One photon is detected. the other goes through a 50% mirror and can follow the red path (suppose 1 m) or reflected the blue path (suppose 10 m). When you measure the time between the two detectors you will see that there are only times of 3.3 ns and 33 ns, showing with path the particle went.

[Delbert]

Sorry for not quoting you DParlevliet, but for some reason the quote option isn't working for me (and I haven't got time to investigate as I'm about to depart for the pub!).

 

Anyway, as the two slit experiment clearly and absolutely reveals, when we attempt to monitor, intercept or merely peek to locate these things we call photons, it changes the result of the experiment completely. For example and quoting the two slit experiment yet again, if we observe a photon at a slit before appears to land at the detector a while later, we could say we've monitored its progress. But we clearly know that's not what happens when we don't monitor, intercept or merely peek - as manifestly evident by the resultant interference pattern.

 

As far as I can see that clearly demonstrates your example is not how it happens. That may sound counter intuitive, but yet again, the two slit experiment shows that is the case. The two slit experiment undoubtedly shows that a photon doesn't and isn't a little ball bouncing about not unlike a billiard ball. The photon (call it such an object for convenience only) doesn't go from place to place as in your diagram. That is a human interpretation after we've changed the experiment by observing - as in the two slit experiment.

 

The mirror is effectively a detector and re-radiator. The photon reacts with the atom or atoms of the mirror just as in a reaction when we monitor, intercept or merely peek during our attempts to discover what's going on in the two slit experiment.

Edited December 18, 2013 by Delbert

[DParlevliet]

When you measure the time between the counts in both detectors, what do you think will be the result?

[Delbert]

Sorry, still haven't investigated the 'unable to quote' problem - might be something to do with a MS browser update I receive the other day!

 

Anyway, returning to DParlevliet's query. It seems you are doing nothing more than the equivalent to monitoring a photon at the slit of the two slit experiment. In other words your example is a sequence of events or journeys comprising whatever one likes to call how the energy travels: a gauge field, wave function or sum over histories. When the photon (calling it such for convenience), leaves the source it travels exactly like the two slit experiment without the two slits. An event then occurs at the mirror and the energy field collapses - just like a it does at the destination of the two slit job, or when we try to catch it out by observing at one slit. From there the energy is either re-radiated as what we call a photon either to another atom in the mirror (and probably umpteen atoms until we say the energy passes through), or back outside and deemed to have been reflected.

 

It is nothing more than a sequence of events and certainly not a photon either bouncing of a mirror or passing through it. What really goes on 'in flight' is not being monitored in any way in your example.

 

Out of the two, it's the two slit experiment that's revealing or hinting to us what really goes on 'in flight' with what we call a photon. And clearly what really goes on is not a little billiard ball bouncing around in a classical way - the interference pattern is manifest poof of that. And as said, whatever we do to try and detect, monitor peek or cause it to interact (like with a mirror) destroys the journey which collapses, completely changes the experiment, and gives us the illusion of a classical process. And as said, your example is not one transit of what we might call a photon either going one way or another as you appear to make out, but a whole sequence of events - which means it ain't even the same photon at either destination as the one that left (again calling the sequence of events a photon for the sake of convenience).

[DParlevliet]

Yes it is a kind of double slit.

But my question was: if you measure the time between detector 1 and 2, what is the result?

[Delbert]

Yes it is a kind of double slit.

What part of your example is a kind of double slit? The only place or places to insert a double slit in your example is between the source and the mirror, or between mirror and detector..

 

Your example is a concatenation of pairs of events. The first being from the source to the mirror... and so on. Just like what happens when we peek at the two slit job, it destroys the result. And the situation appears to collapse into a classical path. No interference pattern for the simple reason we've concatenated two pairs of events - one from the source to were we are peeking, and the other from peeking point to detector rendering one of the two slits superfluous. Your example is similar to peeking in the two slit job, and therefore is irrelevant as an example for trying to identify what we call a photon in flight.

 

I think what's called a photon in flight in your example is in between the source and mirror and the various other 'gaps'. Your example makes no statement or whatever about a photon in flight. As said, it is just a collection of pairs of events joined together masquerading as a single photon going from source, through or off a mirror, to one of two detectors.

 

The two slit experiment shows us that what we call a photon isn't a little billiard ball we can plot a path for. If it were then the two slit job would be an unsolvable magic trick. A particle (as in identifiable object) cannot go through two slits at once, or for that matter go through one of two slits and yet form an interference pattern after many firings.

 

The two slit shows as what we call a photon in flight is in fact some sort of distributed field.

Edited December 20, 2013 by Delbert

[DParlevliet]

Then no double slit.

But my question was: if you measure the time between detector 1 and 2, what is the result?

[sb635]

With regard to your asymmetrical double-slit experiment, a QM physicist would agree classically, there exists a nonzero time difference because of the different path lengths between going through the two different slits. But in QM, the math of the double-slit experiment shows how classical thinking is apparently flawed, which is also what experimentation has shown up to now. If timing is observed, the observation of the time consumed along a pathway must be obtained at the same instant (by a clock) at which the position of the “focused” photon impacts (and is determined) on the observatory screen. The position on the observatory screen can only be physically measured within a delta-x (within a pixel, say). The time of photon impact can only be measured within a delta-t, the measurement error of the clock. (How fast we can “take pictures” is extremely important at the LHC.) If the “slop” (measurement error) in either (position or time) is bigger than what the QM uncertainty principle (UP) relations dictate, the diffraction pattern is destroyed. All experiments that have adhered to these extremely “tight” measurement error constraints have always shown, when and what degree of measurement error in any “observable” of QM destroys “entanglement,” which is really what’s going on between photon generation and (later) observation, given absolutely no information is gathered about “the path way” in between. The “information” about the path way can be either the time spent (as you see) or energy induced on the plate. Your “measure the delta-t” setup can be recast as a “measure the plate momentum” experiment which is described in Cohen-Tannoudji’s extensive textbook: “Quantum Mechanics, Vol 1,” p. 50. The QM UP math on their p. 252 shows how one would interpret your time measuring experiment as one that puts the plate on springs (in your diagram, on the left and right) so the plate can move when a photon goes through. The differing classically- induced momenta should allow for path way determination, just as does your time measurements. But measuring the plate’s momentum (or the time spent during “flight”) destroys the diffraction pattern. Time and energy, and position and momentum, are all wrapped up and correlated in the UP of QM. The UP itself serves as one the bases of QFT/QED, where time and energy are actually allowed to go “completely non-classical,” and for short delta-t’s (that can even be “backwards in time”), deterministic conservation laws themselves are violated. And it’s not that they are “allowed” to be violated. Apparently, they are violated, given the equations that follow from the allowance of the violation, and the agreement with experimentation. The agreement implies these mind-blowing violations do occur.

 

An interesting aspect of “timing of measurements” can be used to try and refute the UP. Put the plate on springs, and classically it moves “to the left” or “to the right” given the different photon path ways. If the actual determination of this plate motion could be obtained substantially after the photon impacts on the observatory screen, then according to QM, on face value, the diffraction pattern should appear. No information about path way or plate movement is determined during the course of the experiment, up until photon impact on the observatory screen, and the diffraction pattern should appear. How could it not, if the plate motion measurements are taken after the diffraction pattern theoretically should appear, given the finite distances of photon travel to the observatory screen? The plate motion observations conceptually could be delayed for any amount of “time later,” seconds, months, years, millennia, etc.

 

How would such a “delayed plate motion” observation be obtained? The answer is rather simple, using gravitational radiation. Especially given your asymmetrical plate slit arrangement design, “straight in” and “off to the side.” Say the plate is on springs, and moves “to the left” at photon passage. Since the plate has mass, this sets up a specific gravitational wave of nonzero strength and phase emanating from the plate. If the photon passage is through the other slit, the gravitational wave emitted is different. A gravitational wave detector could be placed light-years (gravitational-wave-years) distant, and these “post diffraction pattern present” data could be used to determine photon path way.

 

There is an interesting “gigantic delta-t” aspect to this “post plate motion observed” thought experiment. The delta-t can be gigantic (time to gravitational wave detection, long ways away) , requiring a very small delta-E, according to the UP. This means the gravitational signal is faint far away, but technology is here assumed to “raise the signal above the noise.” If you want to wait long enough, using gravity waves, you can theoretically determine to a high degree of “accuracy” which slit the photon went through. To determine it “absolutely” would require the gravitational wave detector to be infinitely far away, and infinitely “sensitive” and would require an infinite amount of time for the gravitational wave to “reach” the gravity wave detector infinitely far away. In practicality, nothing can be determined “absolutely” in our discretely quantized and finite universe, but the very fact (if true) that it could be determined in limit, might be of significance.

Edited December 21, 2013 by sb635

[DParlevliet]

Still my question remains open: if you measure the time between detector 1 and 2 (paths are 1 m and 10 m), what is the result?

 

I am not talking (yet) about interference pattern, only if you know which path the photon took.

[Sensei]

I am not talking (yet) about interference pattern, only if you know which path the photon took.

 

Are you making experiments in real, or just theoretic?

Edited December 22, 2013 by Sensei

[DParlevliet]

It is a proposed measurement. I have not done it nor seen elsewhere.

Edited December 22, 2013 by DParlevliet

[Sensei]

Entirely theoretic or you have some lasers, polarization filters, diffraction slits,Young's slits etc. etc. ?

 

I can show you some quick results.

[sb635]

Considering time "absolute," a single "run" of your above device always produces the first photon impact at detector 1, which is theoretically exactly the same delta-time from t = 0 (photon pair generation event and no "clock error" anywhere) from run to run. For a single run, a second impact of either double that time or longer occurs at detector 2. If this second impact time is twice the first impact time, the shorter pathway was taken for that photon. If it was longer, the longer pathway was taken. Yes, you can tell which path one of the photons took.

[Sensei]

I have couple LED monitors.

Let's see what are their photons polarizations.

 

50% photons pass through:

 

Rotate filter -45 degree, image from monitor disappeared, ~0% pass through:

 

Rotate fllter +45 degree. Image is fully visible, ~100% photons pass through filter.

 

Other monitor/tv. different result:

 

By using polarization filters we learned what was source photons polarization.

 

When two polarization filters are one by another with 90 degree rotation, ~0% photons pass through:

What is filtered out by polarization filter is reflected.

In above two filters setup we can see our own reflection in black area.

 

[DParlevliet]

Entirely theoretic or you have some lasers, polarization filters, diffraction slits,Young's slits etc. etc. ?

 

I can show you some quick results.

 

Theoretic, but with above experiment where I ask the time, not interference or polarization.

 

So like sb635 argued. Do you agree with him?

 

Or more exact: you will find 3.3 ns (1 m path) or 33 ns (10 m path). Inaccuray of the time measurement is much smaller. Agree?

[Sensei]

For me it's obvious that photons going longer path will arrive later.

You can see this using fiber wires (which are used by telecommunication companies, especially between different continents).

Data sent at the same time, will arrive with different delay (f.e. longer pinging Australia from Europe/USA).

[DParlevliet]

In that way you know which path the Photon took and that it acted as a particle (which does not mean it is a classical particle). That was the discussion with Delbert about.

 

...... Yes, you can tell which path one of the photons took.

 

Now, according the rules of Feynman, will there be interference at detector 2?

Edited December 23, 2013 by DParlevliet

[sb635]

In that way you know which path the Photon took and that it acted as a particle (which does not mean it is a classical particle). That was the discussion with Delbert about.

 

 

Now, according the rules of Feynman, will there be interference at detector 2?

 

Since no "apertures" (slits) are involved, yes an interference pattern will (eventually) be observed at detector 2. Your above device is an interferometer. Usually, a gazillion laser photons are generated, and 50% bounced "to the left" along some distance that has been otherwise very accurately determined, and bounced back by your "left" flat mirror, and "interferes with itself" at detector 2. I think this is the basis of LIGO, the gravity wave detector. Any slight shift in the accurately-known distance "signals" a potential gravity wave passage (hypothetically changing the distance the laser light travels), as signaled by the slight shift in the interference pattern. You can set the "last wrap" of the "10 m" distance to a very high degree of accuracy by looking at the interference pattern at detector 2. Sit there and look at it after it is "calibrated" and see if a gravity wave has passed through (or a passing train, giggling the detector).

Edited December 24, 2013 by sb635

[Delbert]

In that way you know which path the Photon took and that it acted as a particle (which does not mean it is a classical particle). That was the discussion with Delbert about.

Sorry, if there's an interference pattern it won't be a classical particle. As I think sb635 explained, and as I tried clumsily so to a while back, any detection or reaction will collapse the distributed probability field or gauge field (or whatever one like to call it), totally changing the outcome (no interference pattern) thus rendering the situation a classical construct. And your example has at least one interaction mid journey resulting in some sort of a change of direction.

 

The bizarre fact is if you consider that your example reveals that a photon can and does travel through a particular slit, and then, upon being followed by umpteen others one at a time, produces an interference pattern, then the only conclusion is said photon upon arriving at a slit somehow knows there's another slit close by, works out what sort of pattern might be produced should other photons follow in its footsteps and so land at an appropriate place to be part of a forthcoming formation of an interference pattern.

 

Now I think it's safe to say that Quantum Mechanics and the behaviour of subatomic particles can behave in ways that are contrary to millions of years of human experience, but for a individual photon to be able to act in such a way seems to me to elevate credulity to the stratosphere.

 

I'm sorry, but I think you're missing the point in your example. As said, it involves a least one interaction mid journey, and as I understand it that'll be with an atom of the slit material - you know, effectively the reverse of it's creation when being emitted from an atom in the first place. Presumably the atom then re-emits this energy in the form of another (what we call) photon which we view as continuing toward the final target. Similar if not identical to peeking and destruction of the interference pattern in the two slit experiment.

 

There's also the question of measurement. As I think sb635 also mentioned, the possibility of being able to measure a single photon in such a way may not be within the bounds of the physics of QM and Uncertainty. In other words, it can't be done because it can't happen.

Edited December 24, 2013 by Delbert