why there is more matter than antimatter...

Mike Hewson
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RE: For the Young's Slits

Message 86909 in response to message 86906

Quote:
For the Young's Slits experiment, are there any interesting effects on the interference pattern that are dependant on the material used for the physical obstruction?


I'd say there ought be some dependence on the material, I don't know if it would be interesting though. From the point of view of one of our particles ( photon or electron ) traversing the apparatus, the 'hard' walls etc represent an area of altered potential ( electromagnetic ) to be overcome/transited, or not. Note that the reason that we use x-rays, say, is their ability to cause dense materials to be transparent. Essentially atomic/lattice level variations in energy are not felt by those more energetic particles. So if the walls between the slits effectively disappear as we ramp up the photon energy then the interference pattern ought progressively fade. We'd be left with some ray-tracing/straight-line model similiar to imaging in free space.

This is also without considering tunneling where suitably thin layers can allow a significantly non-zero portion of the wave function to appear on the other side. Hence some finite probability of slipping across a volume that would otherwise trap a particle.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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RE: not sure what it means

Message 86910 in response to message 86905

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not sure what it means that there are four components to the wave function yet only two choices for the form of the metric - I had recalled the conventions of using (-,+,+,+) or (+,-,-,-) for (t,x,y,z), and I'm getting to understand it all a little better


Symmetry. That is, why should a particular spatial dimension/direction be special? So x,y,z have to go with each other. There aren't multiple time dimensions to choose from, so no issue there.

However what symmetry holds between the spatial and time dimensions? The requirements of Special Relativity have the physical laws remaining constant b/w frames, all frames are equal etc. That's a symmetry. What achieves this democracy is the constancy of the speed of light across those frames. For light to propagate outwards from a source as time proceeds then you need the time variable to be of opposite sign to the spatials. If it weren't then our light metric/measuring-stick would alter in length from frame to frame.

Here's an example:

Spaceship blasting past you at some notable speed, say 0.5c. The captain of the ship is typing on his keyboard, perhaps an email to you. Say he hits the 'a' key and then the 's' key one second apart in his time, and one centimetre apart by his ruler. For him the spacetime distance between those two events is:

d(ship, 'a', 's') = SQRT[(1cm)^2 - (c^2)*(1^2)]

From your point of view he hits the keys at vastly different spatial positions - he's really shooting through your district - and his clocks are slow too.

d(you, 'a', 's') = SQRT[(big_distance)^2 - (c^2)*(a_bit_more_than_1_second)^2]

These distances must be the same under SR or else light propagation will not be equal in all frames. If you look at the spatial term in the metric it really increases from the captain's frame to yours, so how do we get the entire metric to come down in size to remain equal? By subtraction in the time component, or whatever arithmetic sign offsets the spatial term. Note, by the by, that only a small change in the time term from the captain's value to yours is needed to achieve this - because the time term has the 'advantage' of c^2 as a multiplier !! :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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Two beers and a dash of

Two beers and a dash of quantum and my head can't help but hurt!

Mmmmmm...

So... Getting physical and experimenting with the Young's Slits experiment but using light that is incoherent but monochromatic from an LED and...

For some very narrow closely spaced slits in Al foil there's a hint of distorted fringes but nothing definite. A retest with coherent laser diode light must await some batteries for a laser pointer...

So it's back to the question of what is the maximum size/spacing for the slits that will still produce interference. A quick search suggests that the slits must be within a wavelength or few. So is this all an effect of evanescence after all?

[edit]
This adds an interesting additional light on the experiment:

Do the "Double Slit" Experiment: The Way it Was Originally Done

So... The significant part is the central barrier and its thickness.
[/edit]

And then to stir a little more intrigue, there is:

Double-slit quantum eraser (pdf)

Mmmm?

I'm still wondering that we're just in a magical Alice-in-Wonderland dream lost in the mathematical assumptions made for describing a reality that in reality works to assumptions that are more deterministic.

Or is our future truly random for individual events even if our aggregate future can be probabilistically predicted?

Dice anyone?

Cheers,
Martin

(I'll come back to the various other points over the next few days!)

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Mike Hewson
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RE: Do the "Double Slit"

Message 86912 in response to message 86911


Outstanding! I never knew that! Two alternative pathways, one either side of the card, diffracting back around distally to interfere. Certainly not monochromatic either, but you'd want a darkened room.


Whoa! That's a deep rabbit hole, or warren. It is probably easier to grasp if you're under the influence! :-)

I'd interpret that as a 'pre-recording' of the entangled pair state at it's birth, one half of which is then later engaged in interference with itself. You don't actually get a non-interfered pattern when the which-path detectors are active, but one can deduce/assume what would have been the pattern had the path detectors not been operating. The exact time this reconstruction occurs after the creation of the entangled pair ie. before or after path detection/revelation is thus not relevant.

So it's not so much the erasure of effects that would have bumped an interference pattern, but the other half ( that wasn't put through the double slit ) of the pair retaining timing information for later accounting. So you deduct those from the counts and are left with those that ran through the apparatus ( missing the which-path detectors ) to give the ripple pattern. The recovery of the interference pattern is NOT the loss of any disturbing influence ( path detectors ) after the fact but the ability to see, by deduction, the pattern formed by those that were never detected near the slits. We still cannot know which slit was taken by those photons that did interfere. Nothing new in that regard then.

Now because it was determination of polarization angle that was employed ( one direction for one slit and the orthogonal direction for the other ) then the usual Heisenberg momentum/position uncertainty was said not to be responsible for interference loss. They have said that the detector did-not/could-not disturb the path - this is an assumption. [ but this is tricky, and I see alternative views here ].

The fact is the interference pattern was none-the-less disrupted when the which-path detectors were active [ hence in 'particle mode' you have the traditional explanation of deflection ]. And if you pay no attention what-so-ever to the other entangled particle then we have the traditional two slit scenario. So the entanglement accounting becomes merely a form of co-incidence ( timing ) detection, regardless of exact polarization ( you place a polariser on the non-slit arm at 45 degrees orientation relative to either path detector, and thus use entanglement to exclude the paired photons from the slit path counts ).

In any case the traditional position/momentum duality of Heisenberg uncertainty was originally meant to be pre-dictive not post-dictive - developed in the period when the rules of evolution of wave functions was being constructed. It is important to appreciate this as misunderstandings/inconsistencies will occur. So if you detect a particle one CAN make a statement about it's position and what the momentum must have been to get it there - but NOT from then onwards. Similiarly with this apparatus you can say what photons got intercepted at what detectors, and thus what their characteristics were to achieve that, BUT you can't say at one of the path detectors where a photon will hit the screen. You can only say that in retrospect. All the photons getting path detection still sprayed, and you can't predict/postdict which one went where from data at the slit plane alone.

Hence there is still a subsequent lateral momentum uncertainty ( which over the slit-to-screen distance will determine where photons the screen ) that is induced by path-detection. Being able to draw that subset out of the data, using entanglement or other means is not exciting as it might sound! Cause and effect is still as safe a bet as it ever was, and alteration of the past ( even slightly ) is not on....

Quote:

Mmmm?


Indeed!! :-)

Quote:

I'm still wondering that we're just in a magical Alice-in-Wonderland dream lost in the mathematical assumptions made for describing a reality that in reality works to assumptions that are more deterministic.

Or is our future truly random for individual events even if our aggregate future can be probabilistically predicted?

Dice anyone?

Well, it depends. Does unpredictability equate to randomness? Or just ignorance?

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Chipper Q
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Wow, sure is easy to recreate

Wow, sure is easy to recreate the effect. Here's a picture of my apparatus:

Here are pictures of the results – the light areas were a bit too bright for the camera (and get progressively brighter towards the central region) and so the dark areas are a bit washed out by the bright areas, but the light/dark areas were sharp and clear to the unaided eye (click on the thumbnails to see the full size image) :

I stood there watching, taking pictures. I observed the edge-on view and could see light interacting with both sides of the strip at the same time, and the effect didn't go away. Clearly when doing it with two slits, the material between the slits functions the same as a single solid strip of material. It's interesting to note also that there are some circular interference patterns present (I think caused by the camera's optics) – point being that the mysterious 'wave function' didn't collapse despite all these additional concurrent observations and interactions ...

Presumably however, if I were to perform the experiment in a more elaborate fashion where only a single photon is emitted at a time, and were to use a suitable detector to record where each photon hits the ceiling, then I would still (after collecting many samples) end up with the same dark / light / dark / light / dark interference type pattern. I'm also betting that if I were to use electrons (or even C_60 molecules) that the results would would be the same – point being that the condition of being wave-like instead of particle-like is still present and mysterious as ever, but it's good to know that this condition is not so fragile when it comes to observation, or at least more robust than I had thought ...

Thanks for posting that link, Martin! (edit: What you asked earlier about what would happen using the opposite of the slits was spot on! Good one! It would be like using two closely spaced strips...)

Mike, I'm still working on that spacetime interval example – but regarding my question about four components to the wave function and two choices for form of the metric, I was wondering if this meant that when matter is transformed from energy into particle and antiparticle, are there four possible states constrained by two forms of manifestation? For example, (left-handed / right-handed) (matter / antimatter) --> (+time / -time). The slashes connote 'or' conditions between choices of state, so you end up with combinations like left-handed matter forward and right-handed antimatter reverse. From the total possible combinations, matter and antimatter can coexist, or pair-up as in the case with two electrons sharing the same orbital (because one has opposite spin) – one of these electrons could be a positron running with a signature where time is negative. I recall the article on the Dirac equation mentioned more about the mixing of the four components ... this thread's a tough one to keep up with, but I think that makes it all the more worthwhile :)

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RE: Wow, sure is easy to

Message 86914 in response to message 86913

Quote:
Wow, sure is easy to recreate the effect. Here's a picture of my apparatus:


You really do need to improve your diet! I hope you don't eat those things!!

Quote:
Here are pictures of the results...


Very good experimenting there, and you beat me to the batteries!

Quote:
... (or even C_60 molecules) ...


Now, would that indeed be the case? And with or without requiring a BEC and silly low temperatures??

Quote:
Thanks for posting that link, Martin! (edit: What you asked earlier about what would happen using the opposite of the slits was spot on! Good one! It would be like using two closely spaced strips...)


I'm still wondering what are significant aspects of the "slits".

Do you get a different interference pattern for a square edge termination vs a rounded edge termination?

With all of this, we are assuming that there are additive and subtractive wave effects going on. Except, that a single wave will have nothing to add or subtract with. Is what we are seeing an interference-like pattern in the probability of where a photon/electron will deflect (refract?) ?

When experimenting with singular photons/electrons, would a detector in a dark region of the target screen fringes detect an equally balanced population of additive and subtractive photons/electrons?

Quote:
Mike, I'm still working on that spacetime interval ... this thread's a tough one to keep up with, but I think that makes it all the more worthwhile :)


It makes a refreshing change to more earthly trivia! Certainly gives rise to rather more obscure pub talk. Was once nearly floored because some very down to earth types nearby thought that we were "taking the piss" out of them...

A good question is still what is mathematical contrivance vs reality...

But the best part is... Galileo style, we've blundered into to doing some real experimenting. Cold fusion next? :-)

Keep searchin',
Martin

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Mike Hewson
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Ah Chipper, you are the man!

Ah Chipper, you are the man! It is a beautiful pattern of quantum mechanics writ large. You have the skills - and the paper clip, pine board, 'third hand' claw, and quality biscuits. All true signs of the earnest investigator! :-)

Is that copper foil, as the card, being held across the laser?

Think of 'slit = alternative'. In fact two slits is about as simple as one can get to demonstrate QM pathways. It is very hard not to think, as a photon, going down one side of the card 'what do I care what is on the other side?' But, as ever, experiment rules scientific discourse, so yes it is like that ..... and all this pathway stuff is happening all day, every day to each of us. It can be hard to encompass that we are made/operating of the same stuff, not casual observers at all. In our smallest detail, we are like this too. Some particular scenarios like Young's highlight it. Feynman talked about us demonstrating some phenomena in one little corner of a huge chessboard, but with us still having little idea ( except in generalities ) of the real play. So we deal in models and tinker to refine.

Now right handed matter goes like left handed antimatter I think is the relation you are after. So if I flip the charge and run backwards in time, while swapping left for right then the universe should run the same - the laws ought to be CPT ( Charge/Parity/Time ) invariant. Or almost, which is the original question in this thread. With three characteristics you have eight combo's to play with. Perhaps a cubic view is helpful - it has eight corners - and going from one to an adjacent corner via one edge will flip a character. CPT does three and so goes to the diagonally opposite corner right through the cube's center. Hence all pairs of opposite corners ( there are four such pairs ) give universal duos that should be indistinguishable within each.

Two electrons occupying the same orbital is a separate issue again. They are Fermions meaning that they combine amplitudes with opposite sign ( each electron is as 'identical' as the next ) when going into the same state. So whatever is the amplitude for one to do that, the other will subtract precisely that. So zero amplitude ( for the combined state ) yields zero probability ie. never. Hence there must be some difference in state between the two. For an atomic orbital all numbers are decided bar spin - so one up, one down as you say is the only way to go.

Martin I think it might be easier to not try to see QM as waves AND/OR particles. It really is neither. Wave and particle are approximations grown from historical sequence, and alas we have yet to get a comfortable, yet accurate, macroscopic analogy to the reality of the microcosm. As for pub chat, I do worry that the Theory Of Everything will be born in an ale house and yet be lost forever due to concussive/amnestic effects!! :-)

Cheers, Mike.

( edit ) Yup, the rings are from the camera optics. No fixed relation to the pattern but viewpoint/angle dependent.

( edit ) With the appropriate formula you could deduce the wavelength and compare with the laser's specs! Since each peak is formed from path lengths differing by an integral multiple of wavelengths then :

n * wavelength = d * sin(theta)

applies, where :

n = order of peak/fringe [ call the central peak zero ]
d = slit separation
theta = angle between a given peak and the midline

and you could ( to good approximation for large slit/screen separations and small angles ie. near-midline fringes ) have

sin(theta) = x / L

where

x = distance to a given fringe from midline
L = distance from slit to screen

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Chipper Q
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RE: Do you get a different

Quote:
Do you get a different interference pattern for a square edge termination vs a rounded edge termination?


Quote:
Is that copper foil, as the card, being held across the laser?


Good questions, the material I used for the strip was cut from a regular (orange colored) note card, so the edge was as square as the scissors could cut. I just this morning tried using an old x-acto knife blade, with one side a fair bit more oxidized than the other side (see pictures this post). I first positioned it with the sharp edge down and just the tip crossing over the laser, and the result looked like a mix of the normal round spot of light and the interference – so then I adjusted it so the tip was well past the beam with the wider (or taller/higher) portion more centered in the beam. It worked, and I did note a dim circular pattern similar to what's in the previous pictures near the central region – it didn't seem to move as I tilted my head one way and the other ... Next I tried adjusting the blade from straight up and down to giving it a slight angle – recall that the sharp side is down, so this gives the leading edge a triangular geometry – and the result was a bit shocking: The usual interference pattern formed directly above the apparatus, but to one side of it the light regions didn't get weaker in luminosity, but continued on much farther until eventually getting brighter (though not as bright as the central region directly above the apparatus), and that's not all – after the brighter region of the long continuation of the interference pattern, the pattern still continued on past that but the light regions were very dim. It's hard to see in the pictures:

For the pictures on the left I was facing north (image was on ceiling, slightly less than 7 ft. floor-to-ceiling). After fiddling around with the position of the blade (from angled to straight up/down), I found it was pretty easy to get this pattern out of it. For the picture on the right I was facing south. I had done the fiddling between these pictures, so technically not images for the exact same position of the blade. For both pictures, the bright round spot is directly over the apparatus. The top image is the whole pattern, and the middle and lower images are close-ups of both sides of the extended pattern ... not quite sure what to make of it – some reflection? The surface of the blade was hardly mirror-like!

Note the remains of the orange note card in the upper right.

(Yes, Martin, as Mike said, quality biscuits! :) )

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RE: RE: ... (or even C_60

Quote:
Quote:
... (or even C_60 molecules) ...
Now, would that indeed be the case? And with or without requiring a BEC and silly low temperatures??

Hmm, these are probably very naïve ideas, but off the top of my head I can think of trying using pressure (inert gas, maybe N2) to force the particles out a nozzle with a strip of material in front of it, thereby providing the quantum mechanical option of two possible paths. The detector screen would be a card covered with adhesive. I'd have to check, but I heard that C_60 can be found in the soot particles caked to the sides of an exhaust pipe of a combustion engine. Alternatively, there are probably many fine-grained powders (paint pigments, etc.) that might also be tried. Anyway, after spraying the particles out of the nozzle, see what type of pattern eventually forms as a sufficient number of samples collects on the detector. Speaking of paint and spraying, what about trying a can of spray paint with a strip of material in front of the nozzle (or card with two slits)? Isn't this fluid mechanics then? That is, would it be surprising to see wave-like qualities in fluids or would it be more surprising not to? ... Possibly another way would be to extrude the particles and let gravity do the work where they would simply drop over the strip to eventually land and collect on some type of detector (like a glass microscope slide) that wouldn't necessarily have to be coated with adhesive (although without the adhesive, particles landing where others have already collected may displace some of those into adjacent 'dark', or otherwise lower-probability regions, possibly smearing the pattern).

As for an update on the extended interference pattern, some better imaging equipment makes quite a difference, but now it's evident that I need a tripod – I think you were right again, Martin, about the optical bench! (Note: camera used was Kodak C913 digital, 9.2 Mpxl)
This image shows some jitter holding the camera, but more closely matches my earlier description of an extended pattern, which gets brighter and then and then still continues after the light regions get abruptly dimmer:

Note: for this one and the one below, I had the blunt side of the blade incident in the beam.

Strange thing about this one is that the extended part seems to lack the interference pattern – it looks more like whole photons were being reflected. By the way, at a high angle of incidence, the x-acto blade does indeed look shiny. The abrupt change from bright to dim as distance from central region increases is still present.

For these next two, I tried to get the extended pattern using the strip of note card, adjusting the incidence angle with the beam. I was unable to reproduce the way it happens for the x-acto blade, but here are two bizarre looking attempts:

This one has a slight off-axis interference pattern and the normal spot of light (seen without any material present) is very enlarged and still spherical, has no discernible circular interference pattern but is spread out in puffy patches that decrease in size as distance from the zenith (location directly above apparatus) increases:

This next one is actually slightly extended, has definite circular patterns, and the the gaps in the interference pattern seem more staggered with the bright regions proportionally longer than the dark regions:

Still not quite sure what to make of it - I think the paper and metal have different optical characteristics for one thing, and the extended pattern seems to be a combination of the expected interference pattern mixed with reflection off the side of the material (for metal, but not so much with paper) ... ?
edit:
It's easy to see how performing more meaningful experiments gets a lot harder and takes a lot longer – the next step here would be to test all permutations in 1) material composition, 2) material geometry, and 3) material orientation in the beam. That's an awful lot of materials, shapes, and positions!
edit: and 4) frequency of laser light, and don't forget that category 1 above, these days, includes materials with a negative index of refraction ... somehow I don't think QED would fail predict the pattern with the experiment properly defined ...

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I think some further laser

I think some further laser and LED and sunlight experimenting is required! Now where's those spare bits laying around?...

There's a few ideas there to try yet...

Meanwhile, here's an interesting aside on the Casimir effect:

Casimir effect goes negative

Happy New Year!
Martin

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