The Double Slit Experiment

[Astro78]

I presume the 'double slit experiment' is well known amongst most of you guys but want to start a thread anyway :)

http://www.youtube.com/watch?v=DfPeprQ7oGc - well worth the 5 mins.

Just me or mind blowing?

[OneOfOne]

Yes, I have heard of the experiment but must admit have never done it myself, I have to buy some more bullets for my electron gun some day!

It raises some interesting questions and concepts. The effect caused by observing the electron has created some very "unscientific" sounding logical conclusions, ie. because matter changes how it acts if it is observed could lead you to the theory that the universe only exists because we observe it, if it was not observed, the matter would behave differently and would not exist (sort of like saying that if a tree falls over in a forest and no-one is there to hear it, it makes no noise, but of course a tree falling over is not a "quantum" event and so is not subject to the same laws but the concept is similar). The latest issue of Astronomy magazine has a brief article on some of these thoughts.

[janoskiss]

Can add that *everything* is like that! ie. exhibits waves-particle duality. E.g., light waves <-> photons. Trevor you can do the double slit experiment using a laser pointer and a home made double slit (e.g., score an overexposed negative). Well you can produce an interference pattern at least. Observing which slit each photon goes through might be a little tricky. :)

Re the act of observing changing the outcome: it's not as mysterious as it may seem at first. To observe it you need to illuminate it, i.e. fire a photon at it (or some other particle which will also exhibit the same wave-particle duality btw). The more precisely you want to locate it the more energetic a particle you need to fire at it, so the more you'll perturb the motion in an inherently unpredictable way -> Heisenberg uncertainty principle. The mechanics of this process is well understood, ie we have precise mathematical description of what's going on and how, how the observer will muck up the outcome etc, but that takes nothing away from the weirdness of it all.

[avandonk]

This can even occur with C60 or Buckyballs!

See here
http://physicsworld.com/cws/article/news/2952

Or search with buckyballs interference


Bert

[AGarvin]

Hi all,

I think the uncertainty principle is a little more complex. It is often described as the result of particles colliding thereby "altering" the outcome but this is actually wrong. From what I understand it is based around a bottom limit placed on a quantum system.

The uncertainty is based on the product of the position (p) and momentum (m) being equal to or above this bottom limit (p*m>=hbar/2) where hbar is Plancks constant h divided by 2pi.

Lets say the bottom limit is 16 (ie, hbar/2 = 16) and the uncertainty of the position is 4, then the uncertainty of the momentum must be >= 4, as 4*4 (m*p) must be >= 16, which is true in this case.

If we reduce our uncertainty in the position (p) to 2, then the uncertainty in the momentum (m) can no longer be 4 as 2*4 (p*m) = 8, which is under our bottom limit 16. So by reducing the uncertainy in the position to 2 we increase the uncertainty in the momentum to a minimum of 8 (2*8>=16).

Hope I've got that right and I'm not on drugs :screwy:.

Cheers,
Andrew.

[janoskiss]

Anyone interested in more in-depth but fairly non-technical explanation of the intricacies of this experiment and the implications about the quantum weirdness of nature, have a listen or read of the lecture "Quantum behavior" by one of the greatest physicists and ever, Richard Feynman. All the Feynman lectures are very informative and entertaining to listen to btw.

[avandonk]

Steve I was truly frightened by the implications of these experiments. What the take home message is each buckyball or C60 interferes with itself and goes through both slits. It seems to be the classic quantum wave going through all possible paths as a putative wave function and only collapsing back to our reality as it hits the screen or detector. What that means is our mere existance could only be due to our observing it. We are all just wave functions until somone reads our post!


Bert

gone again

[Astro78]

I believe Feynmann had nothing to do with the single 'electron' experiment and through two slits (this 1st occurred in 1989). Until this experiment took place, this phenomenon was not yet discovered.

The phenomenon - being the electrons altering their path to our classical understanding (producing an interference pattern) when being viewed from afar (just the effects on the wall), but not producing the same effect when being viewed at an atomic level.

Almost like there are programmed secrets in the quantum world we are not suppose to know. God?

This in turn opening a debate on the consciousness of all matter.

[xelasnave]

What happens if you fire one electron an there are three slits??
alex

[avandonk]

Alex the number of slits is immaterial. You will always get diffraction. The pattern will change in a mathematically predictable ways. The variables basically being the ratio of the width of the slits and their separation and the 'wavelength' of the incoming 'wave'.

I used to solve the atomic structure of protein molecules using xray diffraction. You could think of the protein crystal as being a three dimensional array of protein molecules and the diffraction pattern that was also in three dimensions had spots whos spacing depended on the spacing of the protein molecules. The intensity of the spots give you information about the electron density in the PM and hence the position of the atoms.

When you have slits or rectangular apertures the resulting diffraction pattern is described by sin to some power functions and in the case of circular apertures Bessel functions of the first kind. I am going on a fading memory here. I would have to look it up as I did this stuff in the late sixties early seventies.


Bert

[AGarvin]

Hi Alex,

If you fire one electron at a time and have one slit, you will get the electrons building up on the screen behind the slit, as one would expect, but even here the first hint of quantum wierdness begins to display itself. If you continue to fire one at a time through the one slit, the pattern begins to show diffraction as Bert said ... how can individual particles, fired one at a time, produce diffraction which is a property of waves ..... aren't our electrons particles ...???

If you have three slits, fire your electrons one at a time, you will get your interference pattern ... once again a wave property .... hang on, not only are we firing particles we're doing it one at a time. Even if they do exhibit wave properties how can we get the interference pattern when we're firing them one at a time, their "wave properties" can't interfere ... but they do :D ..... or should I say the electron goes through all the slits and interferes with itself.

Freaky stuff ....

Andrew.

[xelasnave]

Thanks Bert for sharing your exciting work ..that was really neat.

Andrew the point is finally I take it that one electron behaves as if is were more than one.. it is most perplexing to wonder what is going on.

There must be a simple explanation its just no one knows what it is...is that it?
Maybe what we think is one electron is in fact much more... if it in fact has many parts one could work from there.

mmmm dont think many would buy that one.

I will think how the gravity push Universe explains it... in a sea of particles flowing all over the place a single electron may cause some sort of knock on effect

mmm
probably no sale there either.

I have never believed in waves as others see them but more in the nature of the movement of many particles... well like a wave on a pond ..what we see are many particles moving yet we call what we see a single wave... maybe some sort of knock on effect in the medium we fire the electron into.. air?..I presume however they would do this in a vacuum otherwise you would have to take into account maybe the effect of the particles in air. Still do we ever get a true vacuum.

Interesting to wonder about... but the way I see it many are intrigued by what happens.
alex

[xelasnave]

If we think of it like a pond and that the waves on the pond are really showing the movement of many particles..ones which at the moment "they" dont talk about as being there:D.. could this solve the problem:shrug:..the electron hitting anything in its way..at speed of C I presume or pretty fast out of their gun.... even if small may give them a fair clip of speed..passed on that is to these other ..yet to be discovered pasrticles;)..and these particles scattered would go everywhere ...some thru each hole and out the other side would interefere with each other... but how would they record as an electron on the wall??? when you think of a radio wave is it not in effect on electron sending out a wave.. which in if you think of waves like the pond is not a single thing but a way to describe many little things acting in the same way...


Anyways Just thinking dont worry about me I will think about it for days and I have probably covered the most rational thing I will come up with:lol::lol::lol:.........
alex:):):)

[AGarvin]

Alex,



I guess that's kinda it Alex. I'm certainly no expert, but here's how I understand it from what I've read.

Mathematically the double slit experiment can be perfectly explained in terms of the wavefunction, which explains how the electron evolves over time. As the electron moves on its journey towards the slits, it's wavefunction spreads out. Think of it like this. If you have a big box and realease an electron in one corner, the chances are pretty good that straight after release it will be somewhere in that corner. If you come back in an hour it could be anywhere in the box. The wavefunction descibes this.

If by the time the electron reaches the slits its wavefunction has spread far enough to encompass both slits, it will go through both, with two new wavefunctions emerging from the other side of the slits. As they move towards the back screen they to begin to spread out, and if they start to cross over they interfere just as a "real" wave would and produce our interference pattern.

However in terms of a deeper meaning about what the electron is actually doing, that's the 64million dollar question. From what I've read there's a few interpretations. The most common seems to be the "Copenhagan" interpretation developed by Bohr, which basically says since you can't tell what's actually happening without effecting the outcome, there's no point trying. All you have available is what you can actually see, the result. There's a few other interpretations I'm aware of such as the deBroglie-Bohm and many-worlds versions. There's probably heaps more.

Andrew.

[xelasnave]

Well Andrew I am happy to say I had it more or less right as to the way all sees it afterall...what they see not what I may see that is.

My point with wave function is this ...we talk of wave function as a math thing and thats good because the formuleas show us where it is etc... however I say this... a wave must be more than a math concept in reality ..what we seek to describe is the tip of the ice burg ..for a wave to exist it ..in my view..as a layman not trying to disagree with the math...it must be in a medium..so the way I see it is the electron is the stone in the pond..what makes up the water of my pond I am not sure ... but partcle in nature... if we admit for a moment that something like the old eather is there it will make sence ... still I have not thought about this at all today..what a blessing for me...a family day for me..a too rare thing..still may explain why I think about this stuff inbetween those times..

Anyways it seems one thing is for sure there are not many ideas anyone is real happy with to explain it ..but I feel if they could happily answer this it would possible tell us so much really...



I wonder if you placed slits on a panel behind the gun would you notice something of a wave pattern there as well..I think you may..if not exactly behind in another position where you could calculate ..if my approach is correct..where the wave should be strong and interact with the slits.

Dam I have started thinking about it.

Thanks for taking the time for such a well thought out reply.

alex

[xelasnave]

On topic for those interested...
http://www.lbl.gov/Science-Articles/Archive/CSD-double-slit.html ...........this is rather interesting .. titled smallest double slit experiment

alex

[g__day]

Heinsenberg's rule has stunning implications beyond assigning statistical probabilities when defining situational outcomes - it allows you the bend or break any other rule of physics, in a finite volume of space for a finite time with a finite probability. The deeper implications of Heinsenberg's rules is a far more advanced Uni level study leading into quantum mechanics - it truely is the most impressive law of physics - up there with e = mc^2, hence the saying goes:

Schrodinger rules the waves, but Heinsenberg waives the rules!