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Observer effect

  1. May 8, 2007 #1
    Alright, I've been doing alot of reading and talking with my professors at college, and I just can't seem to wrap my head around the observer effect. Basically, Schrödinger's cat.

    I've heard a lot of analogies to compare it to, but I'm still baffled by it.

    I've heard "If a tree falls in a forest and no one is around to hear it, it falls everywhere and it doesn't fall".

    I've heard of course Schrödinger's cat being half alive and half dead.

    I've also heard that if you flip a coin and don't see which way it lands, it's both heads and tails.

    Basically, if you don't know the result of the coin flipping, there is no heads or tails when you think of it. You just think "coin" and have no answer. Same goes for the other analogies.

    From what I understand, if we shoot a stream of electrons (either a bunch at in a stream, or individually to ensure that they aren't interfering with one another) through two slits, we get an interference pattern on a phosphorus plate which is behind the slits. If we try to observe what slit the electron really went through, it collapses the interference pattern (or their superpositioned wave function) and leaves only two single bands of a pattern, instead of an interference pattern.

    Here is what kills me: I asked my professor "Does it matter how we observe the electron?" He said no. I then said "So, if I have a camera sitting next to the slits to observe the electrons that's off, there's an interference pattern. If I turn it on, no interference patter". He said yes.

    I just don't understand the connection of how my reality effects the outcome of something that I didn't interfere with, only observed. There are a billion analogies, but I'm missing something.

    The only thing I read that makes any sense why an observer would have an effect on this, is that observing creates data. Data creation requires energy. There is a connection between that data creation and the energy is uses, and the path of the electron. What that connection is, I have no idea, but it's the only thing I can even semi-grasp.

    Are there any ways for me to better understand the observer effect? My physics prof. said that to understand this is basically philosophy, so I'm hoping for some sort of understandable analogy.


  2. jcsd
  3. May 8, 2007 #2
    The idea that we can observe something without interacting with it is a complete farce. Observers are part of the universe, they don't watch over it. Their constituency is made up from the same material as the material they wish to observe.

    I am always stunned to hear people who seem knowledgeable and rational about the laws of nature to completely choke on the idea that there is no such thing as an independent measurement.
    Last edited: May 8, 2007
  4. May 8, 2007 #3
    I do understand your very abrupt point. Sad that I didn't see it like that before.

    I am to understand that I was understanding correctly that observing creates data, data creation takes energy, the used energy interferes with the behavior of the wave functions, collapsing one. Since it is probability that defines which slit the electron will move through, is there also, albeit a small, probability that the waves won't collapse?

    Another question: I flip a coin and me and my friend don't see the result. It is both heads and tails. If he peeks and he sees heads, is it 3/4 heads and 1/4 tails because I still don't know the result? Are the wave functions relative to observers? Or by him viewing the outcome determine the result on my end? If not, is it possible that when I look, I may see tails?

    Maybe I am asking the wrong questions. The fact that observing changes results...is that even explainable? Or is it just something that is? The same way gravity just is. It's part of the universe, and that's the way it is.

    Last edited: May 8, 2007
  5. May 8, 2007 #4
    This is where the confusion starts. First, coins are not atomic particles, observation does not cause them to change in measurable ways. Second, the laws of QM do not apply to the coin but to the light reflected by its surface.

    Now if you have a coin that is made of a single molecule of nickel, certainly throwing light on it (in order to see it) will change it. Depending on how much light you use, by the time your friend looks it may no longer be nickel. It may not even be there at all.

    The question that creates so much confusion about QM could be put as "how do things look like when they are not reflecting light?". Put that way, it becomes clear it is a meaningless question.

    It's actually a rather mundane fact when you consider that you can't observe anything without taking a certain amount of matter or energy out of it. We tend to think it could be different because the amount of energy we take to see things around us is very, very small, because our eyes are extremely sensitive. Our tongues not so, and you can clearly see how sampling food changes its taste. It's the same thing for all our senses, only in different proportions.
    Last edited by a moderator: May 8, 2007
  6. May 8, 2007 #5
    Still a little hazy, but I'm getting there.

    Basically, the observer effect is a by-product of the uncertainty principle. The only way we can measure the electron is by hitting it with a photon. When the two collide, somehow only one wave is left standing. That wave is the reality of the path the particle should take through one of the slits. Of course, if no photon hits it, nothing collapses and it takes all routes through the slit.

    If this is the case and the electron will take all routes if unobserved, what determines where it will hit the screen, since it could theoretically hit anywhere?
  7. May 9, 2007 #6
    If you go that far you may confuse yourself. On one hand there is the fact that you cannot measure something without interacting with it. That should be simple enough, since the alternative would amount to some magical way of extracting information from a system. If you stil have doubts about this it can be further explained.

    On the other hand there is the conundrum about what we can tell about particles/waves without looking at them, since we must assume the universe doesn't cease to exist when we cease to look. It's, as your teacher pointed out, a purely philosophical question, or I should rather say, a purely intellectual question. ("philosophical" makes one think of metaphysics or mysticism, which really is not the case here)

    As I pointed out in https://www.physicsforums.com/showthread.php?t=167965", it seems the intellectual problems of quantum mechanics essentially nail down to understanding what we mean by negative probabilities. We use the concept to solve real problems, but we can't understand what the concept means. Everything else, from the double-slit experiment to entanglement, apparently follows from there.
    Last edited by a moderator: Apr 22, 2017
  8. May 9, 2007 #7
    Here is another point to consider: observation and measurement are both misleading you; the only thing that counts is that an *event* happens, and it doesn't matter whether you observe it or measure it. Once a quantum thingy (photon, electron, boogertron, etc.) hits, wiggles, copulates, or whatever, the quantum wave function collapses, and it becomes a fact instead of a possibility/probability. Nobody has to be there, nothing has to watch it happen. How else do you think the tree actually falls in the forest, and the universe keeps on truckin'???

    Stop thinking in terms of observation, it just isn't the point at all. You have been duped by the popularized explanation given by bozos who want to write books that sell by introducing metaphysics into quantum physics. :-)
  9. May 12, 2007 #8
    This stuff is actually amazing. I'm curious where you heard all of this. If you're still watching this thread, please tell! The actual story of QM is much more interesting than what they've been feeding your poor brain.

    This rates an OMFG! It's quantum physics for pre-schoolers.

    There is a chance that what you are hearing are gross analogies which [very] poorly attempt to illustrate QM probability ideas.

    Either you misquoted the situation, or your prof needs a good spanking. I'd say "firing", but that might be too adult a solution.

    Honestly, I've had to unlearn so much crap I was taught in school, it's depressing to hear that it still goes on, and at the college{?) level.

    If there's one thing you should take away from this, it's: no matter what somebody is telling you, there's a good chance they aren't getting it right, and it doesn't matter what fancy credentials they have. It's up to you to dig up the facts from as many sources as you can. Kudos to you for looking for independent replies, i.e. on this forum.
  10. May 12, 2007 #9
    I, unfortunately, am very sure that I did not misquote him. The tree falling in the forest analogy I read in some book or magazine. Probably both because it seems the most familiar of an analogy, like I've read it before. The Schrodinger's cat analogy can be found (and it's where I read it in the 5th grade) in the book "In Search of Schroedinger's Cat". I'm sure you knew that though. The coin flipping analogy was given to me by my professor. And yes, at college level. He's a physics prof just to be clear. I don't need you thinking that maybe I asked my English prof about physics.

    Yes, he does need to be fired anyhow. He's the WORST professor. I'm a senior. I have had around 20-30 professors in all fields. He is the worst.

    Cane_Toad, maybe you could find it possible to describe to me how I should understand the Observer Effect. Could you tell me about it and how it works? I don't even understand why probability is in wave form at all. So many popular analogies and such that I need a fresh explanation from one source, because they have all jumbled together in my brain. I will push everything else out of my brain. If you describe it in detail sufficiently, I will be super happy man.

    If you don't find yourself capable of explaining it properly (or don't feel like it), could you at least link me to a credible site?

    Thanks, really man.

  11. May 12, 2007 #10
    I just re-read my above post, and my grammar and sentence structure sucks arse. Just to be clear: Instead of me going and fixing my grammar and sentence structures, I decided to make a post to myself telling myself that I suck at English. I suck.
  12. May 12, 2007 #11
    Heh, good luck with that. Better to compartmentalize it and file it as "suspect".:rolleyes:

    First of all, the disclaimer: I'm not a quantum physicist. I'm trying to understand this stuff at the conceptual level, so I won't be any help with the math. All I can do is try to convey my understanding, which you should also file under "suspect", for good measure. I'm sure somebody here will jump in where I screw up.

    First, the definition of a "Truth" as used below: "as far as I can tell" :-)

    The first big Truth of QM is that really small things appear to behave as mathematical entities. My current favorite way to think about it is as if photons, electrons, etc. are really just objects in a computer simulation. I think this idea was pioneered by Edward Fredkin. The idea of the Universe as cellular automata is often taken as a Truth by its proponents, but I think they're getting ahead of themselves. Nevertheless, if you switch your view of quantum thingies from little balls zapping around, to little data entites getting processed, I find it's easier to get past preconceptions from our macro existence.

    A corollary of the above is that quantum entities get to do whatever the hell they want, and it doesn't have to make sense 'cause it works in the laboratory, and in your computer chips. This isn't directly related to the observer problem, but it will be handy when you start trying to conceptualize all the weirdness presented to you by the experiments where the "Observer Effect" supposedly lurks.

    I'm not entirely sure about this, but I think it must be a convention, since the "wave" nature of quantum entities was first discovered by seeing the interference patterns, which are classically created by waves in a medium, i.e. water.

    Quantum wave probability functions (now *my* terminology is probably breaking down) are different in that the crests and troughs in the function's definitions represent the likelihood that the entity will be in that place at that time. I think of them as analogous waves, since the math used for them resembles that for classical waves.

    The Truth about the "Observer Effect" is that it just doesn't exist, at least in the way commonly tauted. The deal is that in every experiment there is some guy fiddling around with his apparatus. He is considered the "observer" since he decides the positions of stuff. However, he is no more special with regard to affecting wave functions than the wall of his lab if he mis-aligns his equipment. The wall is just as good at deciding quantum states as he is. Think of the term, anthropocentric, and you'll understand why the human is so commonly given a special status.

    Actually, I don't think I've come across one. My understanding of things only came when I started reading exactly how the experiments were done, and what the results were. I then began to be able to distinguish what were facts versus the interpretations of the experiments. It's like that old whisper game where there are a bunch of people in a circle, and one person whispers a story into the ear of the person to the right, who passes on the story by whispering, and so on around the circle. The last person must recite the story out loud, and it usually is hilariously warped beyond recognition.

    One thing that helped me was to get a firm grasp of the Young's double slit experiment. It's the root of all quantum evils. Feynman said it better, but I forget. There have been several versions, but my favorite is Mandel's which demonstrates the effect without even "observing" the primary photon (meaning the one that finally shows itself).

    Here is a page with some cool stuff (I haven't vetted the opinions, but the experiment's description seemed OK):

    http://www.fortunecity.com/emachines/e11/86/seedark.html#Converter" [Broken]
    which should be the same as http://www.fortunecity.com/emachines/e11/86/qphil.html" [Broken]

    For me, this put the role of the observer, measurement, etc. in a pretty solid perspective. The nutshell is that you split a photon twice (partial mirror, then 2 down-converters) down four paths, and then recombine them so you get the interference pattern. Now all you have to do is affect/measure/block *one* of the four child photons (virtual, wave, or whatever they are), and the path is chosen, and the whole four branch tree wave function maddness collapses down to a single path indicated by the photon blocked/measured (i.e. its closest sibling in the sub-branch). Hopefully, the diagrams will be clear.

    Here is a description of the classic Schrödinger's Cat thought experiment, but it's conclusion, the popular one, seems plain wrong to me:

    In reality/practicality the cat *IS* alive or dead, because the decayed particle/photon/whatever hit *something* if only the wall of the box, which is good enough to collapse the wave function. It doesn't take prying eyes to affect the situation.

    However, there was some time interval (nanosec?) when the particle state *was* indeterminate, and everybody has an opinion about what's up with the cat then.

    Contrast the above with the Wikipedia introduction:

    Now, ask yourself: how did this description, where the cat experiment was proposed to more or less *debunk* the link between the cat and the particle, transformed into the previous quote?

    As I read it, it says that even during that small interval where the particle is indeterminate, the cat's macroscopic interaction with the world means that it's state is never indeterminate regardless of how it's fate relies on the particle. The many worlds people will say that the universe splits so that one universe has a live cat and the other a dead cat, but that's a whole other kettle of cats.

    So, finally, the Truth about the "Observer Effect" is that *any* event which determines a quantum entity's position (or momentum) collapses the probability function. This event can be any interaction, whether it bumps into a wall, bumps into a photomultiplier, bumps into somebody's retina, etc. These are all equally valid "measurements". There might be others I don't know about, like field interactions, etc.

    Our existence consists of quantum entities flitting in and out of indeterministic states all around us, all the time. It's only when we catch them at it with our gadgets do we make a fuss about it. They could care less.

    Those who say, "it only happened because I observed it", are patting themselves on the back for a feat of prowess equivalent to getting hit by bird droppings. Yeh, marvelous, now we have evidence of bird droppings because you were looking up at that bird. If it weren't for you, those droppings would never have existed, right?

    Well, I hope I've done more help than harm.

    Last edited by a moderator: May 2, 2017
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