Thread: Double Slit Experiment

I just watched a video about the Double Slit Experiment which I found amazing. There are a few things I'm not sure about though, so was hoping someone could help.

First of all... is what the video says true? I've read mixed comments saying it's crap and some say it's right. And do we know why an electron acts differently while it's being observed? I think I read once that you can never tell the exact position of one so that's kinda confused me.

The video is here: http://uk.youtube.com/watch?v=FQBYTU4nQuE


Thanks

First of all... is what the video says true?

Yes, it's true. The experiments have been performed thousands of times in numerous laboratories over the last century. The results are confirmed and accepted.

I've read mixed comments saying it's crap and some say it's right.

They say that about everything, don't they?

And do we know why an electron acts differently while it's being observed?

All we can really say is that this is one of the fundamental laws of the universe. Our job is to discover the laws; we can't always say why some things are the way they are.

Originally Posted by Development

And do we know why an electron acts differently while it's being observed?

I think SteveF's answered you brilliantly. On a philosophical point, it is the Copenhagen Interpretation of quantum physics that makes much of the idea of an observer, or observation. There are at least two other theories (not necessarily as popular among the scientific community) as to what might be happening. One is the famous Many-Worlds idea which simply suggests that any Uncertainty (in the strict Heisenberg sense) is not resolved by observation, but rather by the probability wave function (I've probably got the terminology slightly mixed up) collapsing into different universes for each possibility.

The second alternative, and my favourite, centres around the notion of decoherence (here I'm seriously out of my depth but I read it all in a wonderful book called Where does the weirdness go?) which requires neither an infinity (or thereabouts) of universes nor a 'privileged observer'.

Each of these theories would explain the behaviour of the electron in different terms. Unfortunately, the experiment itself does not validate or disconfirm any of these ideas. The Copenhagen Interpretation is the one that speaks of the electron being 'observed'. In the Many-Worlds interpretation, it is not the observation that collapses the wave function - the wave function is collapsed in every possible way, each generating a new universe - what you 'observe' is simply a consequence of whichever universe you happen to be in. The decoherence notion suggests that the wave function cannot remain coherent when extended to 'macro' contexts and will collapse as a result of the statistical probability of collapse being vastly greater than that of continued coherence for all the individual particles in the large-sized system - ie that Schrodinger's Cat is not an actual paradox. (FWIW, super-cooled super-conducting tori have been put into and maintained in quantum mechanically coherent states despite being definitely macro-sized, visible bodies - but this is a very special circumstance that is not replicated in everyday life.)

I love the counter-intuitive nature of quantum physics, but am always wary of the unfortunate anthropomorphic tendencies that arise when 'observers' are talked about as being crucial to a result: too much bad thinking about science has been generated by post-modernist 'thinkers' who gab on about 'privileging the observer', or speculating about 'the ultimate observer' and so on.

cheer

shanks

You made me think... People can slow down light these days. What would you see if you conducted the experiment while shining the light through the "light slowing stuff" Could you look at it and slowly map out the differances in the point where the radiation was seeming to come from? would that work at all? sorry for busting into the discussion.

Originally Posted by sunshinewarrio

I think SteveF's answered you brilliantly.

I agree.

It might just be worth pointing out that the question "why" is rarely a Good Idea in science; it leads to what is called an infinite regression. Rather like an annoying 4-year old that says "Yes, but why? {answer}....yes but why?" ad irritatum (is that Latin?).

It just effing is!! Get used to it.

as the old saying goes, if quantum mechanics did not shock you the first time you learn't about it then you obviously did not understand it

I thought 'observer' meant any interaction with the outside universe.

Thanks for the replies everyone.

Originally Posted by Guitarist

It just effing is!! Get used to it.

Haha. I think I'll have to. But not knowing why something this strange happens is frustrating, so hopefully someone figures out why.

And sunshinewarrio, I'll check out that book.

Can anyone recommend any other books similar to 'Where does the weirdness go?'?

Originally Posted by MagiMaster

I thought 'observer' meant any interaction with the outside universe.

Hmm... what's an "outside universe" on Thursdays?

I meant outside universe in the sense of anything outside of the finite experiment in question. If that experiment is a single particle, then the outside universe is everything else and being observed would be, for example, when a photon collides with it. Of course, this is just my understanding of things, which is pretty limited.

Ha! So is the fact that the Universe (as it's usually understood) doesn't display quantum indeterminacy due the fact that there is an Outside Observer? Does He collapse our wave function?

Just kidding!

Originally Posted by Guitarist

Ha! So is the fact that the Universe (as it's usually understood) doesn't display quantum indeterminacy due the fact that there is an Outside Observer? Does He collapse our wave function?

Just kidding!

Unfortunately, while you were kidding, many people take this for a philosophical truth. That's one of the reasons why I posted on the alternatives to the Copenhagen Interpretation of observer privilege - all three models currently produce the same observable results, but you probably pick your favourite based upon your philosophical bias. :P

Originally Posted by sunshinewarrio

or speculating about 'the ultimate observer' and so on.

Yes, I missed this bit. Well put.