Laser diffraction through a single slit

[swansont]

Hehe... that ois probably why my physics teacher hates quantum mechanics. He cannot understand it.

 

I think I just have to accept that if a photon has been given many paths to go through' date=' it will go through all these paths, no energy is lost. It just clones itself!

 

Is that a correct way of thinking about it?[/quote']

 

Any "correct" model predicts the outcome properly. So, whatever works for you is correct. But I think that visualizing it as a photon that splits up might eventually lead to trouble - you're still thinking of the photon as a classical "thing."

 

BhavinB's comment about the shortest path ties in with Fermat's principle of least time (least action), but that's also classical - it explains the overall behavior but not some of the details. Or at least that's how I was interpreting it. The path integrals, while related to least action (and reduce to it for classical systems) involve all possible paths, but most of them iterfere with each other and end up not contributing anything to the final answer. (as I understand it - this is not something with which I do much work)

 

add: Feynman path integrals

[aommaster]

Thanks alot for that explanation Swansont! It really cleared things up. I guess I'll need to take a look at the Feynaman path integrals (by the way, is he the same scientist that created the Feynman diagrams, used for decay of subatomic particles?)

 

Thanks alot for your time Swansont!

[5614]

Yep, it was Richard Feynman who did Feynman path integrals AND Feynman diagrams, and a hell load of other stuff, clever bastard!

[aommaster]

Haha.. don't you just wish you were like him???

 

Thanks alot guys!

[swansont]

Haha.. don't you just wish you were like him???

 

 

You mean...dead?

[BhavinB]

you're right, I was talking about Fermats Principle. My bad...I always assumed all other paths cancelled out.

[Bettina]

What a great thread:-)

I too had trouble, but believe it or not Klaynos's fish tank really got me to understand it, then topped off by 5614. I have done this experiment several times but always failed to "get it"....I got it now.

 

Bettina

[swansont]

you're right, I was talking about Fermats Principle. My bad...I always assumed all other paths cancelled out.

 

They do usually cancel, unless there are interference effects that persist.

[hoagy]

err' date=' yeah, back to what I was going to say when I said "on top of"... if you try firing a particle and then while it's moving then shutting one of the 2 holes the particle will know that there is only 1 hole and will make the interference pattern as if there is 1 hole.

 

[/quote']

 

Let's say the particle you're speaking of (called particle 1) has an entangled twin (called particle 2) that was also in motion approaching a double-slit somewhere else. If shutting one of the two holes in the first double-slit yields a single-slit interference pattern from particle 1, what interference pattern would result from particle 2 if both holes in the second double-slit are left open?

[Klaynos]

I think (and my knowledge of this is pretty sketchy so please someone confirm or call me a newb for this) that you only entangle one quantum state, so the interfearence pattern from particle 2 would be the same as if both particle 1's slits where left open.

[5614]

Klaynos is correct.

 

Entanglement does not effect the interference pattern.

 

A particle will show the right interference pattern for the experiment setup at the point when it passes through the slit(s).

ie. at the instant it passes through the slit(s) if there is 1 slit open it will show a standard single slit interference, whereas if at the instance the particle passes through the slit(s) there are 2 slits open it will produce standard double slit interference.

 

The answer to hoagy's question is that it will produce standard double slit interference because after all, it is going through 2 slits.

[Royston]

Apologies if this seems slightly off topic, but there's something I need to clear up. I've been trying to get my head round the discrepancy between relativity and quantum mechanics with regards to determinism, I understand both are correct...and quantum mechanics only threw out indeterminism because it deals with very small scales, this all makes sense so far.

 

What I keep on slipping up on is when people refer to the double slit experiment and say that an electron only appears to be in more than one place at a time, is when it's observed...surely if you set up the experiment and there was no-one there to observe or measure it, the electron will still behave the same.

 

Sorry if this seems like a dumb question.

[5614]

This is not a dumb question. This is a question which many people (incl. physicists at the time this was discovered) do/did not like the answer to.

 

The answer is that the electron quite literally does go through both holes at once.

 

When the first books and textbooks came out they all started with 2 things. Uncertainty and the double slit experiment. Why? Because if you truly understand both of those you know a lot about QM and because soo much of QM relates to one of those 2 things. Indeed "how can a particle go through 2 holes at once?" relates to many things. The main one would be wavefunctions, superpositions & observation.

 

The simple reason why we say an electron goes through both holes is because as we are not watching it so we cannot know. This is an important point of QM, what you do not observe you do not know anything about, you can't assume or guess... you don't know!

 

So why do we say that it goes through both? Why don't we just say it goes through one or the other and we do not know? That is a complicated question I don't think I could fully answer. At the end of the day it comes down to the fact that superposition says that when something is in a superposition of states it is in both states (not 1 or the other).

[Royston]

Thanks a lot for making that clear 5614. So if it wasn't for relativity, the implications of quantum mechanics would be a lot stronger with regards to uncertainty and determinism, which is reassuring. Having said that, it's a kind of a catch 22 situation for both theories to gel.

[omerta]

I think the trouble some have is thinking of a single particle floating around doing loodyloops through the slits. This probably isnt a perfect picture but feynman diagrams describe it best for me. I think of a pool of plastic balls. Say somewhere in the middle a ball moves in a circle or whatever. Now you have this motion dispersing out where it could go through any number of slits only to arrive at the wall as a packet of energy. This isnt perfect but I just substitute quanta of energy for the plastic balls and the motion for the wave.