double slit experiment.

can anybody explain in a quantum mechanicvs for dummies kinda way (me being the dummy) why the matter behaves like a particle and a wave and why their behavior changes when observed. i saw the experiment on a documentary a while back and ive been thinking there must be a lot more to it than was explained.

Originally posted by stellspalfie
can anybody explain in a quantum mechanicvs for dummies kinda way

Even clever people can't understand quantum mechanics. Its so different from the Newtonian mechanics of every day life that it is practically impossible to give analogies or simple explanations.
why the matter behaves like a particle and a wave and why their behavior changes when observed.
I would say that:
1. waves look like particles when zoomed out, and particles look like waves when we aren't sure where they are.
2. Its not so much 'observed' as 'interacted'. Whenever there is an interaction, it reduces the possible states of the universe.

Originally posted by twhitehead
Even clever people can't understand quantum mechanics. Its so different from the Newtonian mechanics of every day life that it is practically impossible to give analogies or simple explanations.
[b]why the matter behaves like a particle and a wave and why their behavior changes when observed.
I would say that:
1. waves look like particles when zoom ...[text shortened]... racted'. Whenever there is an interaction, it reduces the possible states of the universe.[/b]

its the "particles look like waves when they arent sure where they are" bit that i find hard to "get".
in the documentary the "interaction" was a video camera filming the results. would this really have an effect or was a camera used just to illustrate a point.
would i be anywhere near the mark if i said that its similar to schrodinger's cat and something can be in 2 or more states at the same time depending, like you said "on the possible states of the universe"?

Originally posted by stellspalfie
can anybody explain in a quantum mechanicvs for dummies kinda way (me being the dummy) why the matter behaves like a particle and a wave and why their behavior changes when observed. i saw the experiment on a documentary a while back and ive been thinking there must be a lot more to it than was explained.

Nobody knows why a quantum entity behaves like a quantum entity, any more than anyone knows why a field behaves like a field.
The best way to think of it, I find, is not to insist that a quantum entity behaves either like a particle or a wave, but that macro particles and macro waves behave like quantum entities at the extreme ends of their behaviour. For example, an electron does not behave "as a particle" when it hits another electron and knocks it out off its orbit, nor does it behave "as a wave" when it passes a double slit. In both cases, it merely behaves as a quantum entity should, under those circumstances.
Rather, a macro particle behaves as it does because (due to its large size) it is always at the extreme "particle-like" end of the behaviour spectrum, and a macro wave always behaves as a wave because it is always at the extreme "wave-like" end. In theory, even large objects, like ping-pong balls or even humans, could exhibit the other parts of the spectrum; it's just that, because of our size, it is more than astronomically unlikely that we ever will.

Richard

Originally posted by stellspalfie
its the "particles look like waves when they arent sure where they are" bit that i find hard to "get".

I may even have got that wrong. Its a little bit more than simply not knowing where a particle is, it is as if the particle is everywhere that it could possibly be. In the two slit experiment, its as if the particle goes through both slits at once.

in the documentary the "interaction" was a video camera filming the results.
It depends on your experimental set up, but I don't think the video camera would have been the detector, so no, I don't think it was the "interaction". Typically, there is a light source (or source of whatever particles you are testing on) then two slits, then a 'screen' which is what the photons 'strike' or interact with and where the wavelike pattern emerges. That we can record the wavelike pattern by watching the screen with a video camera is irrelevant. What matters, is that as far as the rest of the universe is concerned, there is no way of ever knowing which slit the photons go through, so it is as if they go through both.

would i be anywhere near the mark if i said that its similar to schrodinger's cat and something can be in 2 or more states at the same time depending, like you said "on the possible states of the universe"?
The ultimate question is whether or not schrodinger's cat truly is in two or more states at once. The fact is that if it is in a particular state it is impossible to tell. So, if you cannot know something, is it real?
I am not sure if it is possible to design an experiment that determines whether the cat is in both states at once or one unknown state. Maybe someone who knows more than I do can tell us.

Originally posted by twhitehead
I may even have got that wrong. Its a little bit more than simply not knowing where a particle is, it is as if the particle is everywhere that it could possibly be. In the two slit experiment, its as if the particle goes through both slits at once.

[b]in the documentary the "interaction" was a video camera filming the results.
It depends on your ...[text shortened]... at once or one unknown state. Maybe someone who knows more than I do can tell us.[/b]

sorry i didnt explain myself very well, i meant that in the experiment the camera was used as an observer, when the experiment was observed it behaved differently, as if it was aware it was being watched. does this imply that the universe only acts how we think it acts when we are watching???? ahhh my tiny brain is being blown!!!!

Originally posted by stellspalfie
sorry i didnt explain myself very well, i meant that in the experiment the camera was used as an observer, when the experiment was observed it behaved differently, as if it was aware it was being watched. does this imply that the universe only acts how we think it acts when we are watching???? ahhh my tiny brain is being blown!!!!

Please give more details as to where the camera is placed in the experiment.
It is a popular misunderstanding of quantum mechanics that some form of concious observer is required. This is not the case at all. All that is required is some form of interaction with the rest of the universe.
In the two slit experiment, if there is any way at all of knowing which slit, each individual photon passes through, then the wave pattern disappears. But this is not about whether or not a camera 'observes' or a concious being 'observes' but whether the universe as a whole 'observes', ie if there is any effect whatsoever on the universe, be it a change in trajectory of some neutrino on the far side of the galaxy, or an image on a camera, the wave pattern will disappear.

Originally posted by twhitehead
Please give more details as to where the camera is placed in the experiment.
It is a popular misunderstanding of quantum mechanics that some form of concious observer is required. This is not the case at all. All that is required is some form of interaction with the rest of the universe.
In the two slit experiment, if there is any way at all of knowing ...[text shortened]... o on the far side of the galaxy, or an image on a camera, the wave pattern will disappear.

ill have to track down the documentary, it might still be on bbc iplayer.

I tend to think of the wave particle duality to be more along the lines of ..

Particle nature tells you what happens when your 'particle' hits something.
Wave nature tells you where the particle is likely to hit something (or be found).


So in the dual split experiment,
The wave nature of the particles gives you the
probability distribution of where the particles are likely to hit the detector.

The particle nature tells you what happens when it hits the detector (ie it hits
a single point as a complete packet of energy and doesn't hit the entire detector
as a smear of energy)



However this is just trying to understand what our model of what is going on is saying.
It isn't necessarily a true representation of what is actually happening.

It might be, but there is no way of telling if our model is true or if there is some deeper
level of reality that underpins it that we haven't yet discovered.
(or alternatively that there is an alternate explanation that gives the same answers
mathematically but with a different explanation)

i found the documentary - http://topdocumentaryfilms.com/what-is-reality/

the bit about the double slit is 19mins in.

Originally posted by stellspalfie
i found the documentary - http://topdocumentaryfilms.com/what-is-reality/

the bit about the double slit is 19mins in.

Nice documentary. I have read quite a few accounts of the double pslit but for me the preferred answer is indirect.

What you seem to want is to reduce the experiment to common sense. That is like saying you want to observe the moons around Jupiter without using a telescope.

We can use a telescope to see Jupiter in detail. We can use mathematics to study the behaviour of atoms.

Originally posted by stellspalfie
can anybody explain in a quantum mechanicvs for dummies kinda way (me being the dummy) why the matter behaves like a particle and a wave and why their behavior changes when observed. i saw the experiment on a documentary a while back and ive been thinking there must be a lot more to it than was explained.

I found the book How to Teach Physics to Your Dog very helpful. http://dogphysics.com/

Originally posted by epiphinehas
I found the book How to Teach Physics to Your Dog very helpful. http://dogphysics.com/

Like the sound of that. For the time poor among us, here's a quote from the site which to my mind demonstrates why we can only "see" this stuff with mathematics. Common sense (human variety) does not come into it. The point is that the maths does work and we make things with it. Talk your way out of that one if you can!

"Can you give an example?

Well, take the dog's bowl, for example. Every now and then, we put scraps from dinner in the bowl when she's not looking, and she's become convinced that her bowl is magic-- that tasty food just appears in it out of nowhere. She'll wander over a couple of times a day, and look just to see if anything good has turned up, even when we haven't been anywhere near the bowl in hours.

This puts her in a better position to understand quantum electrodynamics than many humans.

It does?

Sure. One of the most surprising features of QED, in Feynman's formulation, is the idea of "virtual particles." You have an electron that's moving along, minding its own business, and every now and then, particle-antiparticle pairs just pop into existence for a very short time. They don't stick around very long, but they have a real and measurable influence on the way electrons interact with each other, and with other particles.

You're making this up, right?

No, not at all. One set of these interactions is described by a number called the "g-factor" of the electron, and this has been measured to something like fifteen decimal places, and the experimental measurement agrees perfectly with the theoretical prediction. If there weren't electrons and positrons popping out of nowhere, there's no way you could get that sort of agreement.

So, what's this have to do with the dog?

Well, like I said, the dog is perfectly comfortable with the idea of stuff popping into existence out of nowhere. If a great big steak were to suddenly appear on your dining room table, you'd probably be a little perturbed. The dog, on the other hand, would feel it was nothing more than her due.

So she's perfectly ok with the idea of virtual particles, unlike most humans, who tend to say things like "You're making this up, right?" She was already convinced that there were bunnies made of cheese popping in and out of the backyard, and just regards QED as a solid theoretical justification for her beliefs.

And this helps humans, how, exactly?

Physics has a reputation as a difficult and unapproachable subject, especially in fields like quantum mechanics, where the predictions of the theory confound our human preconceptions. If you can put aside a few of your usual notions of how the world works, and think about how things look to a dog, some aspects of physics that seem absolutely impossible to accept become a lot more approachable."

If you care to follow this thought further, I am impressed by the way so much modern science is done by mathematicians - literally, people with a pencil and paper doing sums. Not people with telescopes, microscopes, laboratories, great experiments and falsifiable hypotheses. Those are the guys who take the mathematician's work and do the reality check afterwards. Roger Penrose quotes examples of mathematicians first coming up with new ideas in pure maths which later are disovered to have an actual existence in the material universe, but would never have been seen otherwise.

My daughter's partner is a typical mathematician except that when he finished his degree he played for some years in a band. But he still did mathematics in his leisure time and then someone offered him a job doing it. Now he is publishing a paper that answers an interesting question about epidemiology - the question being how to establish when an outbreak of a diseaase will become an epidemic. Turns out that he worked out a way to do the calculations which others have found impenetrable. But the nearest he has been to anything resembling an epidemic is the common cold. The next scientific problem he tackles might have nothing whatever to do with medicine - all he cares about is the type of maths it requires.

Originally posted by finnegan
If you care to follow this thought further, I am impressed by the way so much modern science is done by mathematicians

Mathematics is all about doing things in abstract. We ask 'for the general case, what are the rules, and what are the implications'. All the other sciences are what is called 'applied maths' which means you take what you are researching, find out which general mathematics rule applies then see what the implications are for your specific case ie you apply the rules. So, when it comes to discovering rules - the mathematicians are king. The rest of the scientists can at best, only discover whether or not a rule applies to a real world scenario.