Single photon interference

Ahhhhhh the wave / particle duality of light!!

This topic has been argued and discussed for centuries, I think its now been settled that a photon is a packet of a wave!!

At least that's what I think the physicists have agreed to disagree on...

Anything to allow light to behave both as a wave and a particle...

John

It (the wave/particle duality explaination) still means that one photon/wavepacket must split up, so that some goes through each of the slits to produce 2 wavefronts which interfere with each other - then they/it recombines to form a photon/wavepacket of the same energy as the incident one! What about the bit of wave that hit the plate between the slits?

All the explainations still leave my brain hurting!

I would try to explain, but I dont really understand it myself!

All I can do is to give you the name of a book I read back in the late 60's, early 70's that gives a really interesting account of the war between the particle scientists and the wave theory scientists at the beginning of the last century...

Its a fascinating read...

'The Strange Story of the Quantum' by Banesh Hoffmann

published by Penguin Books in 1947 and revised in 1965 at least that's how old my copy is!!!

Its marked as costing me 4/6d so its a little yellow!

But it is a fascinating read, the author makes it easy reading as well.

John.

I should add that in the experiment you mention using slits and a monochromatic light source...

Only the wave like nature of light is agreed upon...

I think some books refer to how the photon can appear to be squashed or in several places at once... depending on where you read it...

If you want a copy of this book, which I highly recommend, and you can't find it in the shops / library contact me and I will lend you my copy...

John.

Er, rationalising the units, that would be £0.225 Sterling...

Excellent Question.

First , it's important to realize in quantum mechanics, a particle, be it an electron, photon, proton, etc., is never somewhere specific. A particle's location is a probability distribution. So, basically, the photon has a finite probability that it travels through one slit, and a finite possibilty it travels through the other slit, and it interferes with itself according to these probabilities, as if it's in two places at once (but actually is in an infinite number of places at once). Sound bizarre? Welcome to Quantum Mechanics.

For what it's worth, I don't think we should ever stop asking the question you just asked. There's definitely something going on that science can't explain well.

I studied applied physics at university. In my first year, I had to take three subjects –physics, applied maths and a.n.other. I chose statistics. I passed the exam for the statistics course, but was told more-or-less never to darken the doorsteps of the Stats dept again. Probability density functions went over my head.

I carried on with the physics & maths, and I was fine 'til it came to quantum statistics – canonical ensembles (grand or otherwise) looked fine for explaining the gas laws & stuff. When probability density functions and their friends started popping up here, I kept my head down, & stuck to the stuff I could understand.

I came out with a 2.II (or II.2, I can never remember which). I just thank god (or whoever) that nobody persuaded me to take theoretical physics.

In my 30-odd years as an engineer, I have often met with experimental scientists, wanting help with their kit. Just to have a feel for stuff like the Mossbauer Effect, gamma ray focussing and phoswich detectors (to give some real examples) stood me in great stead.

The thing that I really got from my time studying, which I guess goes for most full-on study courses, is summed up in my signature …

The problem with quantum mechanics, is no one really knows why it works, but boy does it work. It works so unbelievably well that there is absolutely no doubt that it's correct. It's an extremely awkward position for science to be in, to a have a framework that works extremely well but is unintuitive and impossible to explain in any real world sense.

This bothered the scientists who invented it to no end,

Niels Bohr said:

"If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet."

Erwin Schrodinger said:

"The verbal interpretation, on the other hand, i.e. the metaphysics of quantum physics, is on far less solid ground. In fact, in more than forty years physicists have not been able to provide a clear metaphysical model."

Richard Feynman said:

"I think it is safe to say that no one understands quantum mechanics"

Werner Heisenberg said:

"I myself only came to believe in the uncertainty relations after many pangs of conscience… "

So there you have it, from the guys who made it, except for Feynman, who extended it for fields. Everyone believes it (when the math Predicts the positron before the positron is discovered, its impressive), but no one understands what it means. So you're in very good company.

It's an exciting time to be in physics or engineering.

As you might also remember from your college days, the 'interference' pattern of light and dark bands is observed not only in a two-slit interferometer under the single-photon conditions which you describe, but also in an 'interferometer' having a single slit, or, more commonly, a circular aperture. In the latter case, the pattern seen consists of a central disk surrounded by a series of concentric rings, each slightly thinner and fainter than its predecessor, as one moves away from the central disk. This pattern is called an Airy Disk after George Biddel Airy, a prominent 19th-century British astronomer.

In both the two-slit and one-'slit' (or circular aperture) cases, the same pattern is seen whether or not the photons are arriving singly or en masse. While it is perhaps conceptually simpler to think of the pattern in the latter case as being the result of mutual 'interference' from the wavefronts of multiple photons, this explanation doesn't even come close to the reality of what is actually happening. What makes the single-photon case so puzzling in contrast to its multiple-photon-interference-pattern cousin, is that the 'interference' pattern seen in each case is not the result of interference at all!

Did I just hear someone shriek Heresy!?

Stop and consider for a moment how this phenomenon was mathematically modelled in your college physics class 30 years ago - and probably by nearly every single physics textbook you've subsequently laid eyes on. Wasn't it modelled as if the pattern seen is the result of interference!

As if?

Seldom will you see the author of a physics textbook go on to describe what is actually happening in this case. If the author does Boldly Go and dip his pedantic toes into the actual (quantum) mechanics underlying this mechanism, he or she might subsequently explain the pattern seen as being due to the interference of 'probability' waves! Huh?! Probability Waves?! What the hell is a probability wave? What is 'waving' here, Mr. Textbook Author? Isn't what is waving really just a mathematical artifice to which you have lent an unjustifiable element of Reality - as you seem to have no other rabbits in your hat? And is this your explanation, or the one that other guy parroted from that other guy who parroted from that other guy who...?

Just pick up any intro physics text at any university and you'll see pretty much the same tripe: it's an 'interference' pattern caused by photons or probability waves (ugh!) interfering with each other. If this weren't the case, then you and a gozillion other folks wouldn't be posting questions like these about single-photon 'interference' and such to online forums everywhere. You'd actually have the answers to your questions because somebody cared enough to distinquish the mathematical model from the actual reality. Your query is proof enough that this wasn't the case for you, is it not?

The good news is: it ain't you. It's them.

Sorry for waxing pissy here, but this herd mentality evident among so many textbook authors really turns my crank (can you tell?). Students by the droves are gleefully dispatched every year into the Real World (unless, of course, they go on to graduate school) believing photons - or worse, 'probability' waves - are actually 'interfering' in an 'interferometer' to produce this 'interference' pattern when, in fact, the only thing that is doing the 'interfering' is an artificial mathematical construct used to expedite the analysis. But nobody - not nobody - goes on to explain that it is only a construct and not a representation of reality. It's kind of important to make this distinction, wouldn't one say? Otherwise wouldn't one mistakenly come to believe the Model as the Reality?

And so, thirty years later, we have Walking Wounded speaking of an 'interference pattern of light and darkness,' and of puzzling over single-photon 'interference' in two-slit 'interferometers,' because some prof somewhere - aided and abetted by a number of complicit, plagiarizing parrots - didn't bother to get off his dead collective ass and make this distinction clear. It ain't your fault you're here asking these questions, Mr. Poster, and they're good questions, too. You're not the perp. You're the victim. Having vented, I feel much better now. (Sorry if I offended anyone. Truly!)

It isn't 'interference' that is going on here, Mr. Poster, but something far stranger...

Stay tuned! (It's very late and have to go to bed now. Sorry! I'll try to finish this tomorrow.)

--Europium

I only brought the subject up to rattle a few old brain cells (mine included). I hope I haven't raised anyone's blood pressure too much. (_sorry).

Oh heck no, JohnDG! I like the stimulation!

You oughta hear me rant in class!

Take care,

--Europium

BTW, JohnDG, your 'Shakspearean Avatar' reminds me of the time I read about an American lady who, while visiting London, recognized the person standing across the street as none other than the great George Bernard Shaw himself (probably giving one of his famous street-corner lectures on socialism). Looking both ways, she quickly crossed the street and ran up to him, "Are you Shaw?!" she asked?

"Positive!!!"

--Europium

Gosh! It's been my trading logo for the past 8 years or so, and I hadn't spotted the Shakspearean connection. Thanks!

I guess there are more things in Austin and Earth, Europium, than are dreamt of in our philosophies .

You're welcome!

(pssst: should the box hold an exclusive-or symbol, or am I thinking of something else? )

My idea when I designed the logo was that one input to the and gate represented a problem, the other your's truly. Put 'em together with a bit of hardware and/or software design, and I'll give you a solution !

As it stands, it's something of a mis-quotation (something I've never been known to do ).

Okay. I can see that!

One might also have a bit fun with this by leaving the box as it is (not replacing the & with an XOR as Shakespeare demands), but adding a 2B and a ~2B (which might placate the Bard somewhat), and then leaving the output alone as remaining a Yes, suggests to me that Schrodinger's Cat is clearly still alive - albeit by now quite old.

Ain't that right, Heisenberg? (Meow!)

I love sundays.

--Europium

...not to mention being totally friggin' loony from his being trapped all these years in a little box with his simultaneously-dead self, ~2B.

muiporuE--

Letting the experiment run this long is a dangerous idea. You've got to assume that the poor pussy's shaken off its mortal coil (starvation, asphyxiation, old age etc.), but suppose you open the box to take a peek - and POW! the first disintegration event for a looong time busts the vial - and you're a goner, whether the cat's already snuffed it or not.

Not sure I want my logo to give those kinds of messages!

Good point. But I'm also having this strange deja-vu feeling...

Ah. I know what it is. I was married for 24 years.

--Europium

There is an excellent book called "QED" which covers this rather well. some guy called Feynman or something... :)

I can go for that!

Yes, folks, please do read QED, as Richard Feynman has an uncanny way of explaining this rather difficult subject with amazing clarity.

And besides, reading Feynman's book would certainly save me a bit of typing, so how's about I do all the ranting whilst leaving Dick to do all the dirty work for me?

(thanks!)

--Europium

A "Thought Experiment"

The creation of the photon is like a stone into a pond of water, the resultant single wave front propagating from the stone/water interaction is what we perceive as light (EM Wave). When this wave front interacts with the slits, the interference pattern results. It is not the photon, but the photons EM signature that we perceive.

This "experiment" takes a planar section of the wave front, because unlike a stone in water the resultant wave front from the creation of the photon is spherical.

This is just an opinion, so...

That's the Model, not the Reality. But instead of talking about photons, let's just for a moment consider electrons and pretend - just for a moment - that they are simply overweight photons having a much smaller (DeBroglie) wavelength. The nice thing about using electrons here is that, unlike photons, electrons have an electric charge. This charge lets our Phat Photon interact with other stuff in such a way that it betrays its own trajectory.

Any mass in motion - whether it be an electron or a baseball - has a wavelength. In the case of a baseball moving at National League velocities, it's wavelength is about 10^-50 meters, plus or minus 3 dB (). In the case of an electron, and depending on its velocity, it's wavelength might be on the order of 10 Angstroms or so. The nice thing about electrons, as I said, is the fact that it has an electric charge. That means, for one thing, that we can see its trajectory through a Wilson Cloud Chamber.

As with both photons and electrons, QM says they both can be considered in purely wavelike terms. Unlike with the so-called Copenhagen Interpretation of QM, the wave function of each particle doesn't "collapse" upon measurement, as an electron's path through the cloud chamber - where it is being continuously detected - clearly demonstrates. Otherwise, the wave function would have to recollapse over and over (and over and over...) every time it tickled some nearby liquid hydrogen molecule. So, apparently the electron's wavefront is not spread all over a region of space as large as your typical desktop interferometer. It's pretty localized - damn localized - and so is the wavefront of a photon.

The concept of the photon being some big 'ol wave is, as I said in an earlier post, an artificial mathematical construct used to expedite the analysis. Period. It just so happens that its treatment in this manner - as a wave - gives the right results and, besides, if it didn't, then Dr. Professor would have been obligated to use some other treatment that would itself, and instead, agree with experiment. But in both cases, I strongly suspect that folks would wander off believing it, too, was a representation of Reality. But it's only a treatment - not a representation of the Real Thing - and that is all. Regrettably, most people leave the classroom blissfully ignorant that what they have just witnessed is only a Model. Just a Model.

Please read Richard Feynman's QED for a thoroughly delightful debunking of this popular myth.

--Europium

I just had a wonderful idea for an experiment!

Combine an electron 'interferometer' with a Wilson Cloud Chamber and see what happens! In this setup, the interferometer might be a two-slit, one-slit, or circular aperture immersed in the the liquid hydrogen environment of the cloud chamber.

Not only should this setup let us do electron 'interferometry' in the usual sense, but it will let us see, simultaneously, the trajectory of the electrons through the slit(s) on their way to producing an 'interference' pattern - if such a pattern might actually be produced under these conditions. (And if not, understanding the why behind the not might prove to be an even more fruitful source of insight) After all, if we can see the trajectory of the electron through the cloud chamber - where it is being continuously 'detected' by tickling the surrounding hydrogen molecules - why then should the electron's passage through the interferometer's slit(s) and its subsequent 'detection' be considered a special case deserving a separate treatment? If there's no interference pattern simply because we can see the trajectory, then something much more sublime is going on.

Has anybody ever done such an experiment before?

--Europium

excellent idea!

I haven't heard of it before. Maybe instead of using matter to form the slits, you can create an extremely dense magnetic field (force field) in the exact place of the barriers, and also test out the effect that the atoms of the barrier have on the various trajectories. If all other parameters are identical to the slits, it would be interesting to see that interaction as well. What are the differences of various materials being used to create the slits. (Gold, Glass, Ceramic, Wood....Conducting Insulating, Transparent dielectric, dimagnetic, capacative, semiconductor, crystals, superconducting, radioactive...) How does the cloud chamber magnets affect the slits? etc.

I think there is lots of room for experimentation here.

I would tend here not to apply any magnetic or electric fields whatsoever - at least initially. To get a baseline, I would use an electron interferometer already known to produce interference patterns under normal conditions, thereby eliminating one variable.

I would also not apply the cloud chamber's usual magnetic field for no other reason than to simplify the experimental setup. The trajectory of charged particles in a magnetic field is curved. The degree of curvature depends on the strength of the field, the mass of the particle, the velocity of the particle, and the degree to which the particle is charged (elementary particles are not the only type of particle seen in a cloud chamber: alpha particles are common and are actually helium nuclei, which have a charge of +2). The direction of curvature depends both on the sign of the charge and on the polarity of the magnetic field. If I can conduct my experiment without these complications, so much the better.

Only after the initial experiments would I introduce the additonal variables.

--Europium

I think that is an excellent strategy, and is a good sign of your skills. I have always wondered in 'slit' experiments, what the edges look like at the atomic level. I view them as kind of a ragged edge, and have wondered how to produce a straighter edge.

I suggest a slit that is basically two small radius cylinders, spinning around a vertical axes, tested at varying rpm's. They can be micromachined with a Laser maybe, to produce the smoothest possible surfaces. I wonder what differences might be observed in the scattering if the direction of rotation of these is reversed?

just curious