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Why do two people see the same experimental results?

  1. Mar 11, 2014 #1
    Before quantum mechanics hit the scene, I would expect most physicists (and people) would have answered the above question with the basic assumption:

    Two observers see the same result in an experiment because we live in the same universe.

    Today, most people (and some physicists) would probably still agree with that answer. However, with all the interpretations of quantum mechanics, I would guess that there would be additional responses from physicists such as but not limited to:

    1. The question is outside the scope of legitimate questions that physics can answer.
    2. Observers don't live in the same universe (for example, each possible outcome creates a new universe).
    3. Observers don't observe the same thing (for example, each observation is an imperfect approximation of a mathematical universe).

    So my questions are:

    • Am I correct that this assumption is now "up for grabs" and being questioned?
    • If so, are the above responses an accurate summary of current responses?
    • Are there any other responses besides the ones above?
    • Has anyone attempted to experimentally show a difference in observations (response 3 above)?

    I have no agenda with these questions. I am just trying to wrap my head around quantum mechanics and its corresponding interpretations.
  2. jcsd
  3. Mar 11, 2014 #2
    There is another answer, but I'd rather phrase it in the form of a question.

    What would it be like to live in a universe where others have completely different experiences? If you were to ask them what their experiences were would they even hear your question? Would you see them? How would you know they were there?
    Last edited: Mar 11, 2014
  4. Mar 11, 2014 #3
    Thank you for the question.

    I don't think a single universe where two observers see different things and those measurements cause different results is possible. I would guess that there is some fundamental law or mechanism that prevents it. However, I don't think the assumption can stand on its own without explanation or exploration, especially with the proposition of multiverses and parallel universes.

    I may be wrong in seeing that a current direction of quantum mechanics is to push beyond a "single universe" to explain quantum mechanical features. If so, let me know. Or if you have another interpretation, I would be interested in reading about it.
  5. Mar 11, 2014 #4


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    There is no problem if they do not interact. they may see what they want. When they meet and interact they always agree.
    Maybe something occurs during this interaction.
    It is the point of view of Rovelli.
  6. Mar 11, 2014 #5
    I wouldn't say that there's a current push beyond a single universe coming from a desire to interpret QM. Everett proposed the Many Worlds Interpretation over 50 years ago. The current interest in the idea of a multiverse seems to be coming from cosmology.
  7. Mar 11, 2014 #6
    Interesting. Thanks.
  8. Mar 11, 2014 #7


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    The "interpretation" of QM makes no difference: everybody calculates the same set of probabilities for the results of a well-defined experiment.

    Thus if you make enough trials your results should should differ by statistically insignificant amounts from each other. If this is not the case, somebody has a problem with their setup/instrumentation/etc.
  9. Mar 11, 2014 #8
    I guess some are still extending a theoretical multiverse theory (Tagmark's Our Mathematical Universe) based on extra dimensions instead of positional distance. I have not read it yet, but that was the basis for response 3 in my original post (if I understood a synopsis of the book correctly).
  10. Mar 11, 2014 #9
    I may have misused the word "interpretation". I was using it in the sense of "Copenhagen interpretation", which is one explanation of what happens when we take a measurement. I was not referring to the actual experimental results or the mathematics that predict the experimental results.
  11. Mar 11, 2014 #10


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    Popularizations can give the impression that conciousness causes collapse is a mainstream interpretation that physicists worry about. It isn't. Most interpretations such as Copenhagen, Ensemble, DBB etc assume the existence of a world out there independent of observation. This conciousness causes collapse view never held much of a sway and now is very much a backwater view since one of the high priests of it, Wigner, abandoned it when he heard about some early work of Zureck on Decoherence; without going into the detail of how Decoherence throws a spanner in the works of the reason for conciousness being involved in the first place.

    Basically what you have nutted out is one reason why that interpretation is downright crazy anyway, and part of the reason its very backwater.

    The modern view is this. Decocerence leads to what is known as effective collapse which occurs independent of human conciousness etc. If you consider the collapse actual, and their is simply no way to tell the difference between actual and effective (apparent is another word used), then all these issues simply disappear.

    If you want to pursue the issue further check out (see section 3.2):

    Last edited: Mar 11, 2014
  12. Mar 11, 2014 #11


    Staff: Mentor

    Actually Copenhagen doesn't explain what happens when a measurement occurs. It simply assumes the existence of a classical common-sense everyday world and we know about quantum systems by measurements, observations etc etc that appear in that world. It also has a particular view of a quantum state along the lines of the Baysean view of probabilities that distinguishes it from similar interpretations like the ensemble interpretation:

    Within and of itself its a perfectly valid interpretation that pretty much solves all issues. It's 'blemish' however (note the careful use of the word blemish - it does not disprove Copenhagen) is how does a theory about things like measurement etc that appear in an assumed classical world explain that classical world?

    Much progress has been made in solving that but some issues remain and research is ongoing.

    Last edited: Mar 11, 2014
  13. Mar 11, 2014 #12
    Thanks for the reference. I am wading in the shallows, and this looks to be over my head, but not in the deep end. So I will get as much as I can from it.
  14. Mar 11, 2014 #13
    Again thanks for the reference. This is much more accessible. Wikipedia is all over the map in terms of its articles and the level of expertise each requires.

    I hope the blemish does not turn into a sore that turns into an infection.
  15. Mar 11, 2014 #14
    I'm not sure if you noticed this, but you actually answered your own question here (which was the purpose of my question). If the only possible universe, (for an observer to be in, at least), is one where observers see the same results then it should be no suprise to find yourself in such a universe. As for a fundamental law, it doesn't get more fundamental than this. It's just a simple piece of logic. It might, at first, seem like a circular argument but it's not.

    You don't even need physics for it. It's firmly rooted in metaphysics and no physical discovery or model can change it. I'm sure if you trawl the history of metaphysics, you'll find a very similar argument, attributed to philiosophers from long ago, including theologians.

    If you're interested in this line of reasoning, then you should read about the Anthropic Principle:

    There is a degree of controvesy associated with it. It's accepted amongst many high profile physicists, including Susskind and Tegmark, for example, though I've seen Turok and Smolin become quite agitated when asked to discuss it. They seem to share the view that it has nothing to contribute to the field of physics, though I haven't yet fully understood why they hold this view.

    In my opinion, many questions that we currently consider unanswerable, will have accepted answers following this principle.
    Last edited: Mar 12, 2014
  16. Mar 12, 2014 #15


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    I found no "effective collapse" and no "apparent collapse" in this link.
    What is it?
  17. Mar 12, 2014 #16


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    I found no "effective collapse" and no "apparent collapse" in this link.
    What is it?
  18. Mar 12, 2014 #17


    Staff: Mentor

    Decoherence transforms a superposition into an improper mixed state. If it was a proper mixed state collapse would have occurred. But it isn't. There is no way to tell the difference, and that is what is meant by effective or apparent.

    See section 1.2.3 for a discussion.

  19. Mar 12, 2014 #18
    I haven't read it either, but Tegmark's ideas are exactly the kind of thinking I'm alluding to. I really should read it, but I fear it would kill the fun.

    I suspect he argues that in an infinite universe all the laws of nature need to do is facilitate correlation and all that we observe can in principle, be explained with statistics and probability theory.
    Last edited: Mar 12, 2014
  20. Mar 12, 2014 #19


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    I can undestand that collapse can give a mixed state.
    Are there measurements that give as an outcome a mixed state (POVM?) if yes can you give examples?
  21. Mar 12, 2014 #20
    Thanks for not force feeding me dogma.

    If you open a can of worms, best be prepared to eat them. I guess that is why I asked my original questions. I was wondering if a can of worms was truly opened and how the mess was being addressed. It sounds like there is not really that much of a mess.

    I would agree that certain assumptions have to be made and accepted (either implicitly or explicitly) in order to even do physics (or any science). If not, nothing would get done.

    However, I would probably fall into the "renegade" camp that maintains that things can get interesting when assumptions are questioned for a good reason. Just look at the assumption that "time is constant". Where would we be if that was not questioned.

    Even though questioning assumptions can be productive, I am also not ready to discard a single universe that we share and that is independent of observers. I would think that the following points at the very least would have to addressed (not that I am an expert) before taking such a huge step.

    1. There are rules to define a universe, its boundaries, and how it behaves.
    2. There exists well defined exceptions to a single universe
    3. There is causality between universes
    4. There is an explanation for why two observers see the same thing
    5. There is an explanation for how causality is maintained when two observers do not see the same thing (if possible)
    6. There is a well defined physical explanation for consciousness

    That seems like a tall order. I guess the current assumptions will be kept for the foreseeable future.
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