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Questions on the test that led to Many Worlds

  1. Nov 3, 2009 #1

    This concerns the test where you have a barrier with two slits cut out of it, set in front of a wall or blackboard (or something that will show hits upon it) and devices that will shoot sub-atomic particles.

    My understanding is this: when particles are shot into the barrier with two slits, you get a wave-like pattern on the wall behind it. However, if you watch the barrier (or take measurements?) to see where the particles are going, the pattern on the wall behind it changes and you get two lines with no wave-like interference pattern.

    There is a goofy little video that shows this test here:
    The test is shown about half way through the video.

    I hope that I can find someone who knows the details of this test. I of course find it very hard to believe and want to find out just how deep the rabbit hole goes. I’d heard of this test and the implications long ago but somehow I missed the part about the sub-atomic particles changing their patterns from wave-like to particle-like when ‘watched’.

    My first questions are the obvious ones:

    Do I understand correctly that completely non-evasive and passive acts of ‘watching’ or ‘measuring’ really cause a change in how the particles acted?

    Exactly how was the barrier ‘watched’ or ‘measured’?

    Searches using ‘Many Worlds’ has not shed light on these questions… does the test have a name or, what search would get me to more information?

    Any help will be greatly appreciated!

    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Nov 3, 2009 #2


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    It's called the double slit experiment, and you should find lots of threads about it here if you search for it. The Dr Quantum video is essentially correct, but it gives you the wrong idea about what an "observation" is. There's no such thing as "just looking" (the term they actually used in the movie they took that video clip from). An observation is a special kind of interaction between the particle and its environment, and not a one-way transfer of information from the particle to an observer like the clip is suggesting. Since the particle is participating in an interaction with another physical system (the observer's measuring device) before it reaches the slits, it isn't completely absurd that it behaves differently those times when it's "observed".

    Antoher thing I didn't like about the youtube video is that they claimed that the particle "splits in two". You shouldn't take that seriously. Unfortunately quantum mechanics doesn't tell us what "actually happens". It just tells us how to compute that interference pattern. There are several "interpretations" of QM that attempt to tell us what actually happens, but there's no way of knowing if any of them is correct. It's not all clear that QM can be interpreted as a description of what actually happens. It's possible that it's just a way to calculate proabilities of possibilities.

    Most of the interpretations are pretty poorly stated, so it's not even clear what they're saying. The many-worlds interpretation is one of them. It also seems to me that the double slit experiment is a very bad choice of an experiment to illustrate the features of the MWI. (The stuff about decoherence and "worlds" will be pretty much the same regardless of whether there's one slit or two). The Schrödinger's cat experiment is better, but it would take a long time to try to explain the MWI description of that. I'm sure someone will try, but you should not expect to receive any answers that really clear things up for you (about that particular topic).
  4. Nov 3, 2009 #3


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    Welcome to PhysicsForums, rrw4rusty!

    I agree with Fredrik's comments. Thought I might add a few more.

    There are a variety of ways to determine which path the particle goes though, but I would not use the term "passive" or "non-evasive" to describe all of them. As mentioned, some of these are somewhat interpretation dependent. First, the standard technique is to simply block one side. Then you know eveything went through the other. Then reverse the covered slits. There is no interference pattern. The rule is: if you make the particle go through only one slit, there is no interference. More properly, if you CAN know which slit the particle goes through, there is no interference.

    Second, using entangled particles (usually called Alice and Bob - not sure if you know how these work - that's another story), you can attempt to do one test on Alice while looking at Bob to see which slit. (Their paths are symmetrical, so observing one should tell you something about the other.) Turns out that technique only works if you "erase" the results of one of the tests; a so-called double slit quantum eraser. The point being that even in a case in which you think you are being "passive", you aren't.

    In sum: quantum objects display either particle-like or wave-like behavior in accordance with the Heisenberg Uncertainty Principle. If it is particle-like, you don't see interference. If it is wave-like, you do. Considering you are using the same source at all times in the double slit experiment, clearly what you see is a result of the context of the setup since that is all you change.
  5. Nov 5, 2009 #4

    Thank you both for replying! It cleared up a lot for me. DrChinese you don't look Chinese in your pic and I was a programmer for 20 years (retired now).

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