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The 'double slit' Diffraction Paradox is rewritten...


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I'm still trying to get my head around this one....301.gif

The author doesn't really understand what's going on. You need to understand that - according to quantum mechanics - a photon is not a particle, it's a probability distribution - just like a wave - once understood everything makes sense. Each individual photon passes through both slits, it can only "arrive" and be registered at points where the waveform recombination is not self cancelling.

The bit about needing a threshold to register is irrelevant. There is no threshold illumination for CCDs, and with film it can be negated by "flashing" the film to create enough fog to get the density above the flat toe of the curve. At this point the film is reasonably efficient, the photon capture efficiency of a fast film can be well above 10%.

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The author doesn't really understand what's going on.
Good point. And most likely. :)

I think it fair to say, something s/he shares with one or two others. These are the sort of things that appear in Chapter One of the average text book on "Advanced Quantum Mechanics". But as someone once remarked, it was rather FEW, even among quite "solid" Ph.Ds who, if pressed, really understood many such things. :o

It was always a pleasure (and humbling too!) to see one or two "true geniuses" in action. I could think of one particularly, who was "hated" by undergrad (PC101) students, but his tutorials were fascinating. He began with a premis of being a sole occupant of a desert island, with nothing but blackboard and chalk for company. For a fleeting moment, he was able to convince me I almost understood too... :)

Aside: Thankfully, "big science" allows room for people willing to write "code", wield a soldering iron, hammer, drill, get covered in concrete dust, freeze to death, drink heavily etc. Wo/men, I salute you! :cool:

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as someone once remarked, it was rather FEW, even among quite "solid" Ph.Ds who, if pressed, really understood many such things. :cool:

Nobody really understands quantum mechanics, it's so counterintuitive. However some of us do understand the mathematics, we can "visualise" the wave equation representation of a photon (or electron, or whatever) and see how the thing works.

Incidentally the "double slit paradox" arises because the diffraction pattern from a double slit remains an interference pattern even when the photon arrival rate is infinitesmally low ... what you would expect is that, in the absence of interference between photons going through two slits at the same time, what you would get is the superposition of two single slit patterns i.e. a smooth intensity curve (with a flat or dipped peak if the distance between the slits is large in relation to the slit width) rather than the multipeaked pattern. If the author's logic relating to "minimum intensity" was correct the pattern would be substantially unaltered, just the intensity lowered with the faint grey shades reduced to black.

As a photon is indivisible, if it is going to create an interference pattern with itself (as it must if the intensity of the light beam is so low that at most one photon at a time is in transit between source and detector) then it must necessarily pass through both slits - counterintuitive, certainly, but it all makes complete sense if you simply apply Heisenberg's Uncertainty Principle - when you detect the photon, you are in essence "accurately" measuring its position, therefore you cannot be certain of its momentum, so it could have come from either of the slits.... or both!

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However some of us do understand the mathematics...
Oh, lest there be any doubt, it was never my intent to suggest otherwise - IF indeed that was ever thought. :)

General Aside: In Alphabetic terms, my Maths A level would probably not have even secured an interview at a modern seat of learning! I think it GREAT that people ask questions - Great when others provide answers. I do lament the... somewhat uneasy relationship that seems to exist among "lay" public, amateur scientists and their professional counterparts. It is actually rather rare that I would "come out" and admit to being onetime the latter. On a brighter note: Perhaps testament to the present company? :)

But if someone just once stops saying: "I couldn't possibly understand", there is HOPE for us all? :cool:

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The Double Slit Paradox is as old as - well, as quantum mechanics itself - we certainly had it covered it in my physics course (40 years ago). I think the general explanation and consensus was: understand Schroedinger properly, and you'll understand the Double-Slit...

Did anyone actually try the experiment in real life - with a dim light source and photographic emulsion? Or was it merely a gedanken-experiment - like Einstein's Twin Paradox - or Schroedinger's cat? I would imagine that in the conditions set down, noise would completely swamp any meaningful data. And many of us are astroimagers, we know all about noise!

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The Double Slit Paradox is as old as - well, as quantum mechanics itself - we certainly had it covered it in my physics course (40 years ago). I think the general explanation and consensus was: understand Schroedinger properly, and you'll understand the Double-Slit...

Did anyone actually try the experiment in real life - with a dim light source and photographic emulsion? Or was it merely a gedanken-experiment - like Einstein's Twin Paradox - or Schroedinger's cat? I would imagine that in the conditions set down, noise would completely swamp any meaningful data. And many of us are astroimagers, we know all about noise!

I think it's been done with a set up that fired individual electrons???

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Did anyone actually try the experiment in real life - with a dim light source and photographic emulsion? Or was it merely a gedanken-experiment - like Einstein's Twin Paradox - or Schroedinger's cat? I would imagine that in the conditions set down, noise would completely swamp any meaningful data. And many of us are astroimagers, we know all about noise!

It's actually easy - if you arrange a very dim point source - about equivalent to a 13th magnitude star as seen with a 4" scope - the photon arrival rate is ~10 per second. Sure you get a noisy image but the longer you let the experiment run the less noise there is.

Detection at that level is well within the reach of CCD cameras, or hypered and pre-flashed Kodak Technical Pan 2415 Recording Film when it was still available.

The "Twins Paradox" can be experimentally tested by flying a very accurate clock round the world even on a subsonic jet transport, and comparing the time elapsed with a "stay at home" duplicate experiment. You need to make allowances for the reduction in gravitational field caused by the altitude variations (a general relativity effect which actually swamps the small time dilation predicted by special relativity) but the experiment has been done.

The Schrodinger's Cat experiment is performable but it is not possible to deduce anything from the outcome of the experiment ... it's simply pointing out that you cannot deduce anything about the quantum state of a system until you collapse it to make the readout. Anyway, if you did try to perform, it you'd probably be lynched by rampaging animal rights protesters, and (since the actual performance is totally meaningless) I'd agree that the rights of the experimental subject are more valuable than the rights of the experimenter.

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  • 3 weeks later...

1) The author says that a continous wave can produce discrete dots.

2) Thereby you cannot infer that light is formed of discrete packets (photons), but rather a continous wave.

Statement 1 is true. But the inference in statement 2 is not necessarily true. Reason?

Just think about the "Photo Electric Effect" for which Einstein won the Nobel prize. Light is shone on a photoelectric material and a current of electrons is observed to flow. Without changing the frquency of the light, its intensity is increased gradually. The result observed is that more electrons of the SAME energy start flowing. This implies that light is packaged in discrete units (called photons), each photon knocking out one electron of the same energy. Increasing the intensity of light (more photons) merely increases the number of electrons released. If light was a continous wave, the energy of electrons would increase, not just their number.

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Thanks for that. The "Modern View" section on Photoelectric effect was an eye-opener. Quoting - "It has now been shown that it is not necessary for light to be quantized to explain the photoelectric effect".

Sad that Einstien won his first Nobel prise in error.

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  • 3 weeks later...

The double slit experiment is indeed deeply disturbing. Like a lot of counter intuitive descriptions in quantum theory it is particularly disturbing if you adhere absolutely to the tensed theory of time (past, moving present, future.) Maybe what it is really trying to tell us is that we should junk this view of time once we leave our local environment, as we do when we try to peer inside an atom. Many of the shocking aspects of quantum theory dissolve if you forget your routine notions of time. The notion of 'one photon at once' goes out of the window and with it the attendant problems. What replaces the tensed theory of time is anybody's guess but it is useful to start looking for something more appropriate.

Forgive me, I'm not a physicist and am only a rather half baked philosopher but I would be interested to hear what others think about this.

Olly.

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Forgive me, I'm not a physicist and am only a rather half baked philosopher but I would be interested to hear what others think about this.

You're quite right, we're programmed to think in 3D space plus "the arrow of time" and it ain't the way the universe works. Our flawed perception is, however, a convenient shortcut for largish objects (roughly between dust particles and galaxies in size) moving at relatively low speeds (up to about a tenth the speed of light).

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Indeed. What I love abut this outer edge of physics is that it makes you confront all sorts of preconceptions and so realize that that's what they are. Einstein said something like, 'Common sense is the name given to the set of prejudices you have built up by age eighteen.'

Olly

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