A rookie question on double slit

[SlimPickens]

First of all, hi guys. My first post here though I've been reading for a bit now. I'm in no way a physicist or even a physics student but I've always considered myself a fan of science the way some people are fans of baseball. I may not be the starting pitcher but I still love to watch the game. I'm sure you guys are sick like swine flu of double slit questions but please help me out here if you can.

I do my best to read everything I can and thus far I was convinced that the profoundness of the double slit experiment was that it demonstrated the duality of particles. I had tried to stay away from things like Qr. Quantum and cartoons that attempt to explain quantum mechanics but alas I could hold out no more.

So my question is this: Do I have a fundamental misunderstanding of the entire dual slit experiment? Is the most important part that the observation device changed the results of the experiment? To me that just seemed secondary I suppose. I mean, isn't implying consciousness in matter, the way Dr. Quantum and the Bleep movie does just multiplying things beyond necessity? I had always thought what the experiment told us was that the observation device just corrupts the experiment somehow. If I bump into a cup of coffee I don't assume the coffee decided to spill.

David Deutsch recently had a TED video posted on their site where he says that the explanation that is hardest to vary is most likely the correct one. Doesn't even the suggestion of something like consciousness in matter bring about near infinite variables that can result in the same explanation for the results in the dual slit? It seems just as unprovable to suggest that invisible aliens force the particles to do their bidding and would have the same result.

Sorry if this is an annoying question you get all the time or if its just 101 stuff that bores the snot out of you. Its always been my understanding that any time you try to fill in blanks in knowledge with speculation or what Deutsch says, "wizards", that all you are doing is creating fiction and not progressing our understanding in the slightest. Misinformation is like knowledge cancer to me. Thanks in advance for any info and thanks for all the knowledge you guys have already imparted even if it isnt reflected in this post =/

[Ken G]

Is the most important part that the observation device changed the results of the experiment?Yes, that's the important part, though I'd nitpick that the apparatus didn't change the experiment, the apparatus was the experiment. Comparing two different experiments is comparing apples to oranges-- when we eat an apple, we don't imagine that it was changed from an orange.


I mean, isn't implying consciousness in matter, the way Dr. Quantum and the Bleep movie does just multiplying things beyond necessity?That movie is a nice introduction, but should not be taken too seriously. It promotes several misconceptions.

[SlimPickens]

Thanks for the reply Ken. It seems I need to go back and read a few things again. I was under the impression that an observation device was added to see how the particles passed through the slit after the experiment had already yielded the duality results and was then run again. Looks like I am a victim of some misinformation myself. Thank you kindly for the clarification.

[Strange]

Yes, that's the important part, though I'd nitpick that the apparatus didn't change the experiment, the apparatus was the experiment.

Does this mean that it is the change to the apparatus (i.e. the experiment) that is significant not the change in observation per se? It is the latter that popular, alternative or "new age" interpretations of quantum theory always seem to pick up on ("we, like, create the universe, like, from our thoughts, man").

[Ken G]

Does this mean that it is the change to the apparatus (i.e. the experiment) that is significant not the change in observation per se? Exactly. Indeed, is there a difference between the apparatus and the experiment? Just because we can imagine we are measuring something doesn't mean we actually are measuring what we imagine.

It is the latter that popular, alternative or "new age" interpretations of quantum theory always seem to pick up on ("we, like, create the universe, like, from our thoughts, man").Yeah, that "new age" view of things is not regarded as terribly informative for practicing scientists! The scientist creates the experiment by building some particular apparatus, that much is clear.

[Amber Robot]

Does this mean that it is the change to the apparatus (i.e. the experiment) that is significant not the change in observation per se? It is the latter that popular, alternative or "new age" interpretations of quantum theory always seem to pick up on ("we, like, create the universe, like, from our thoughts, man").

The word "observation" is often abused in this sense. Anything that interacts with the quantum state of the experiment could be counted as an "observer". Not just the human running the experiment.

[Jens]

I do my best to read everything I can and thus far I was convinced that the profoundness of the double slit experiment was that it demonstrated the duality of particles. I had tried to stay away from things like Qr. Quantum and cartoons that attempt to explain quantum mechanics but alas I could hold out no more.

So my question is this: Do I have a fundamental misunderstanding of the entire dual slit experiment? Is the most important part that the observation device changed the results of the experiment?

Other people seem to be disagreeing, but my own understanding is similar to yours. The double slit experiment was important for demonstrating the duality of particles. Wasn't the observer issue (isn't that called the Copenhagen interpretation?) a way that was formulated to explain that result? I think that at a basic level, the experiment demonstrated simply that particles seem to have a double nature.

[Strange]

There is some risk of confusion here as there isn't really a single double slit experiment. Is it worth a quick summary?

There is the classical version where waves (or things that are assumed to be waves) are used, and generate interference patterns. This can be done with things that really are waves (e.g. water) as well as things that are more ambiguous (photons). Mind you I have known people struggle with this level of the experiment.

Then you can repeat the experiment with a beam of things that are more usually thought of as particles (e.g. electrons) and find a pattern. If there are a large number of particles in your beam, you can hypothesize various ways that each particle passes through one slit or the other but in total they generate a pattern.

Next you can reduce the flow of particles so only one at a time is allowed through the slits. Hey presto: still a pattern. A reasonable interpretation is that things will behave like either particles or waves depending on context (and what they "really" are is neither/both of those).

Finally, if you change the experiment in any way that forces the particle to "choose" one slit (even if this is just testing/observing which slit it goes through) the pattern disappears. It is a bit like variables in a weakly-typed programming language: change the context where you use it and it changes its type and semantics.

The important point, as Ken G says is that by doing this you are changing the experimental setup. It's not just because you "looked" and the electron somehow knows this and does something different.

BTW I gather that this has been attempted with some reasonably macroscopic particles (small atoms?) - anyone got any idea on the limits that it can be done with (simple molecules? virus? amoeba? tardigrades?) - oops, hope I am not guilty of thread hijacking... :whistle:

Oji

[SlimPickens]

Finally, if you change the experiment in any way that forces the particle to "choose" one slit (even if this is just testing/observing which slit it goes through) the pattern disappears. It is a bit like variables in a weakly-typed programming language: change the context where you use it and it changes its type and semantics.

This part seems, to me, where the "magic" explanations seem to pop in from people that may be more spiritually or metaphysically inclined. I mean no offense by "magic" BTW for anyone that believes in this, just lack of a better concise explanation. I suppose this has always happened throughout history though. Whenever something lacks an explanation, or a readily understandable one by someone not considered an expert, its too easy to insert something supernatural. Human nature perhaps.



BTW I gather that this has been attempted with some reasonably macroscopic particles (small atoms?) - anyone got any idea on the limits that it can be done with (simple molecules? virus? amoeba? tardigrades?) - oops, hope I am not guilty of thread hijacking... :whistle:


As far as I know it has been done up to molecules and if I'm not wrong the theory is that, in principal it would be the same with any sized particle. Could be wrong though. I think the bigger the particle the harder it is to control the experiment.

[ShinAce]

I've seen the experiment done with neutrons. In theory, any particle will do, but the bigger the particle, the smaller the wavelength and therefore the tighter the 'banding' of the waves. Eventually the bands get so tight that you get a classical result.

You can always look at X-ray diffraction of DNA for something fun.

[NorthernBoy]

Thanks for the reply Ken. It seems I need to go back and read a few things again. I was under the impression that an observation device was added to see how the particles passed through the slit after the experiment had already yielded the duality results and was then run again. Looks like I am a victim of some misinformation myself. Thank you kindly for the clarification.

Well, the experiment rerun with the observation of which slit the electron went through is to show what happens when you try to have it both ways, and simultaneously have the electron as a wave and a particle.

If you run it without the ability to know which slit it went through, then the scatter pattern of electrons on the screen looks like a diffraction pattern. If, though, you set the experiment up in such a way that you can see which slit the electron went through, then the scatter pattern is just the sum of two single slit patterns, with no interference.

The question of consciousness is not really an issue here. We don't need to really look at which slit the electron went through; it is enough that it woudl have been possible to work it out afterwards. So, for example, if we shine a light on the slit, of have some kind of counter on it that sees the electron transiting, but never bother to check this data afterwards, we still destroy the interference pattern.

[Amber Robot]

... but never bother to check this data afterwards, we still destroy the interference pattern.

But if you never bother to check, how can you be sure the interference pattern was destroyed? ;)

[NorthernBoy]

But if you never bother to check, how can you be sure the interference pattern was destroyed? ;)

Well that's the interesting thing, you can still look at the pattern on the screen, but never look at the counter on the left hand slit, and you'll see that the interference pattern has all gone.

In practice, there are so many ways to screw up the pattern that it's really, really hard to ever get it to work. Basically, the existence of any mechanism that would have allowed someone to later work out which slit the electron went through is enough to lose the fringes.

[Strange]

But if you never bother to check, how can you be sure the interference pattern was destroyed? ;)

For the avoidance of doubt, NB was referring to checking the data about which slit the electron went through. You are "allowed" to look at the interference pattern (at any time).