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Is the Moon there when nobody looks at it?

  1. Sep 11, 2005 #1
    Please help me with this. Can anyone direct me to the specific mathematics that is interpreted as saying that "the Moon does not exist when nobody looks at it."
    Is there any book or discussion of the actual math and physics of this?
    What specifically was Einstein reacting to when he said:
    "I cannot believe that the Moon exists only because a mouse looks at it." ?
    Please feel free to write or send me anything you wish about this subject. I have no axe to grind, and I only would like to understand this better. Thank you.-Ron
    Last edited: Sep 11, 2005
  2. jcsd
  3. Sep 11, 2005 #2
    To add to the question, what would constitute "looking". Would a video camera that displays the image on a screen in an empty sealed basement that no one can enter be considered "looking?

    What if the images were captured digitally and were later destroyed without anyone looking at it?

    or perhaps someone does look at it later.

    The issue of looking, taping and viewing presents an interesting discussion topic.

  4. Sep 11, 2005 #3


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    Welcome to PhysicsForums, Ron!

    I keep a page on my site which explains this in more detail:

    Bell's Theorem with Easy Math

    This is partially based on a paper by N. David Mermin called: "Is the moon there when nobody looks? Reality and the quantum theory" (1985). As you might guess, my page is probably a bit easier to follow for most people, especially as to the math.

    The fact is: Einstein (sadly) died before Bell's Theorem appeared. He likely would have changed his opinion had he known about it. Experiments support the ideas of Quantum Mechanics in this area quite solidly.
    Last edited: Sep 11, 2005
  5. Sep 11, 2005 #4
    There are some recent:http://www.physics.uq.edu.au/people/andrew/publications/2005/weakvalue.pdf [Broken]

    workings on the experimental setup?

    I like your pages on the setup principle, though simplistic it has the essence of answering some questions, whilst I must admit it has lead to me asking "more-than" the original question!
    Last edited by a moderator: May 2, 2017
  6. Sep 11, 2005 #5
    The moon is there when nobody is looking at it. You don't have to look at
    something for it to exist. QM never said you did.
  7. Sep 11, 2005 #6


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    No one is actually talking about the moon. At least, I'm not. But if you are saying there are values for particle attributes independent of actual measurement, then I am going to disagree with you.

    I believe this position is as consistent with experiment and theory as it is possible to state: the act of observation changes reality, even after the fact.
  8. Sep 11, 2005 #7
    Hello Tardis, Dr.Chinese, and Spin Network, Thank you for your prompt replies. It is my pleasure to meet you. I downloaded David Mermin's paper, but could not open the file. How is the question of whether the Moon exists when nobody looks at it tied into EPR and Bell's Theorem?
    Can you show me the equations that can be interpreted as showing that the Moon(or anything else) does not exist when nobody looks at it. I am familiar with issues about observer-created reality. Is this issue about the Moon a result of scaling up a quantum physics fact to macroscopic sizes, sort of like how this is done with Schrodinger's Cat? If so, what is it specifically that is being scaled up? Thanks, and Best Regards,-Ron
    Last edited: Sep 11, 2005
  9. Sep 11, 2005 #8
    Why doesn't this statement naturally lead to application with macroscopic bodies, such as the moon. The moon is composed of fundamental particles, is it not?

    I have read a little about a particular Copenhagen stance considering macroscopic objects as different entities than fundamental particles. Something about a macroscopic measuring device being able to collapse the wave function, so there is some kind of interplay between the macroscopic world and the microscopic world and both are needed to create the human experience. Is this sort of what you're saying? That since the moon is macroscopic it is capable of having a definite position even when not being measured? It just seems to me that if you take the statement that fundamental particles don't have definite positions unless they're being measured and follow it to its logical conclusion, the moon also can't have a definite position unless it's being observed. ???
  10. Sep 11, 2005 #9
    Actually, I was very careful to say that the thing [moon] "exists" and not
    that it "is there" implying a particular position. As you know Dr. C, to exist
    in QM is to have a unit probability of being somewhere in the universe
    without actually being any one place most of the time.

    It's a freaky business and (quoting myself from another thread) the person
    who figures out the physical basis of quantum superposition will be a rock
    star on par with Einstein.

    Edit: As far as the macroscopic-composed-of-microscopic goes, Ellipse, to my
    way of thinking the best answers so far are given by the folks who say that
    a large object perturbs (or observes if you will) itself and this is what gives
    rise to the emergence of more classical persistence of position.
    Last edited: Sep 11, 2005
  11. Sep 11, 2005 #10
    So, if I observe the Moon, does this "effect" the motion of tides here on Earth?

    What would such consequences be if everyone that can observe the Moon at the same instance, did so. Would there be a verifyable difference in Tidal action?

    What I am asking is the number of observers influential on imparting "effects" to the observed ?
    Last edited: Sep 11, 2005
  12. Sep 11, 2005 #11

    No, beacuse the gravity of the moon is there (and is the same) even when you don't look at
    it- more evidence that you don't need a human observer to make a thing
    manifest in a particular position.
  13. Sep 11, 2005 #12


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    That is a good way of describing it, yes.

    What is being scaled up is the idea that particles have definite attributes independent of observation. The Heisenberg Uncertainty Principle (HUP) restricts the "simultaneous reality" of various particle attributes according to its relations. Observation of position affects momentum, for example. QM treats HUP as fundamental; therefore the existence of attributes varies according to the nature of the observation. All known experiments satisfy the HUP.

    EPR (Einstein) tried to use measurements on entangled particle pairs as a way to outsmart the HUP. However, the HUP came through as QM would predict, after Bell came along and showed the way. You see, many thought the HUP was simply an experimental limitation; Bell concluded it was fundamental to the nature of reality. Most people do not look at Bell in terms of the HUP, but that is in fact at the base of the debate.

    I do not mean to imply that anyone really knows what an observer is - i.e. must it be a conscious mind? I only mean to say that the act of observation is somehow fundamental to the outcome as to what reality becomes.
  14. Sep 11, 2005 #13


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    Talking about the moon confuses the issue greatly, because it is a macroscopic object. The combined uncertainty of the particles composing the moon is not a factor in macroscopic observations. The question is not whether the particles composing the moon exist. It is whether those particles have distinct values independent of individual observation. The answer is that they don't. That does not mean that the particles don't obey gravity - because they do. They also bond into atoms. Etc.

    But no individual atom escapes the rules of the HUP. This is fundamental to QM. And this is the debate. People simply talk about the moon as a metaphor. It is not intended to be literal.
  15. Sep 12, 2005 #14


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    Referring to my paper:

    Einstein said: "I think that a particle must have a separate reality independent of the measurements. That is: an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it."

    If you look at my Table 1, you will see 8 cases ([1]...[8]) listed with all permutations for the hypothetically real polarization attributes at 3 angle settings - 0, 120 and 240 degrees. The existence of these 8 permutations is what is being debated. If there is reality independent of observation, then all 8 exist. But QM/HUP denies this explicitly! In fact, the mere assumption that there are 8 permutations leads to a burdensome requirement that was not previously suspected, and this requirement is called Bell's Inequality. Bell's Inequality cannot be supported experimentally, although the predictions of QM/HUP can. This is Bell's Theorem.
  16. Sep 12, 2005 #15
    Hi Dr. Chinese, Please help me understand if this Moon-existence issue results from the whole take on the Copenhagen Interpretation, or not. I think I see now how it is related to EPR and Bell's, because the moon-existence thing is related to the question of whether there is or is not a "deep reality" independent of observations. Let me be straight with you: After spending the last 4 months trying to learn The Quantum Theory, (including the Bra-Ket notation), I came across a startling argument for the existence of God, (I am an agnostic.), which is that "If the Moon continues to exist when nobody looks at it, that this is because there is a conscious observer who is continuously observing the moon 24 hours a day." Naturally, I would like to see exactly where this follows logically from the science that yields the notion that "the moon does not exist if it is not observed by anyone."
    What do you think about this? If the moon-existence paradox, (MEP), is actually a kind of "non-issue," so that MEP follows from a metaphor that conveys the essence of the Copenhagen Interpretation, then that is one thing. However, if, as you have pointed out, it leads straight back to The Heisenberg Uncertainty Principle, which has never failed an experimental test, that seems a lot stronger. Best regards, Ron
    Last edited: Sep 12, 2005
  17. Sep 12, 2005 #16
    Have finally gotten Mermin's paper to open, and am studying it and Dr. Chinese's discussion on his site. This is deep stuff. Fascinating and amazing stuff. Thanks. If you guys think I'm mixed up or confused, please tell me. I have no axe to grind or point I need to make. I just want to learn it and understand it.
  18. Sep 12, 2005 #17


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    :smile: If you *really* want to be politically correct, you should say that in quantum theory, the act of observation changes PERCEIVED reality, even after the fact.
    The whole discussion (amongst those who accept of course current predictions of QM) resides in what is the underlying reality, and what part of it is perceived. It is only in the case you identify the underlying reality with the perception you have, that you can make such a statement.
    In mathematical terms, this comes down to saying that the quantum state represents "reality", and that observation (projection) affects "reality" or that observation only comes down in picking one of the terms as the *perceived* reality.
  19. Sep 12, 2005 #18
    How can something not exist if it does exist? There are somethings that maths simply fails to explain.

    If no one looks at the sun for about 8 minutes 20seconds and when we look back would sun be there or would it not be there?????

    If it happenes to moon it will surly happen to sun!
  20. Sep 12, 2005 #19
    Why are you so certain that the moon must have a definite position when it's not being observed? It *does* exist, as someone pointed out earlier, fundamental particles/waves always exist, they just don't always have a definite position. So why wouldn't quantum theory naturally lead to the fact that if there are no moon observers, there is no definite position for the moon.

    However, I have come accross a similar type of argument, that perhaps the one thing that seperates an observer from the quantum world is a soul. I really don't understand why wave functions should ever collapse, but they obviously do when humans are involved, so should that lead to the conclusion that there is something humans possess which isn't built up from the quantum world? Oh well, :tongue:.
  21. Sep 12, 2005 #20


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    I think the whole point is that QM tells us that percieved reality is the only reality. This also goes right to the question regarding Conscious-mind observer vs. video recorder. Perhaps the statement in the OP could better be expressed as, "YOUR Moon only exists when you are looking at it". If the video camera is looking at the Moon, then the Moon exists for it, but you cannot prove that it exists for you. Of course, if you are not looking at the camera, then I suppose you can't even say that "the Moon exists for the camera", because the question then becomes "what camera?", since the camera also does not exist for you when you're not looking at it, right?

    But for the Moon to exist as "moon", many articles/waves must be in definite positions. Those that make up the material in the loose dust on the surface must be above those that make up the carbonaceous rocket and magma underneath, with perhaps some others making a tiny metal core at the center. If all of these bits have only "existence" without a particular location, then they cannot be assembled together in their proper places to form a Moon.
    I think that it only looks that way to a human.
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