The dual slit experiment seems like...well...BS!

Discussion in 'Physics & Math' started by rustyw, Mar 15, 2016.

  1. rustyw Writer Registered Senior Member

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    Hello again,

    This should, I guess, be posted in quantum physics but I don't see that topic...so it's here.

    I have to say that the two slit experiment as mysterious as it seems, also seems (at least parts of it) like… well, bullshit! I read that if you make ‘observation’ at the two slits (to see which slit the photon is passing through) that the wave pattern vanishes. Some say that this is because when you ‘observe’ the particle (or anything for that matter) you hit it with a photon. Well I do not know how people have ‘observed’ which slit the particle is going through but if you have to ‘hit’ it with anything to observe it…then there’s the answer!

    By the same token, when I ‘observe’ something…I am not hitting it with a photon! My eye is not shooting out any photons…I’m simply receiving photons that are already hitting it and are already bouncing off it—photons that already exist.

    Furthermore, if you ‘observe’ which slit the particle is going through (however this is done) and the wave pattern on the screen behind it changes (is lost), are you telling me that if you block the observation (put a playing card in front of detector) that the wave pattern reappears? What about if the human watching this looks away?!

    Sorry but I just don’t get it. I’ve read a lot on it over the years and perhaps I just need to dig deeper. Nothing ever seems to detail how this observation is made. My reading seems to say that if you look at something you change it. That implies that the action of intercepting a photon which has already left the ‘observed’ thing somehow has a backlash that impacts that ‘observed’ thing before the photon left it therefore traveling backwards in time. BS on so many levels...it seems.

    Please forgive my ignorance on this subject. Also, try not to use formulas or talk over my head.

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    Rusty
     
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  3. rpenner Fully Wired Valued Senior Member

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    Quantum mechanics has the boundary condition that what has been measured happening, happened.

    That sounds reasonable, because we wouldn't want Alice and Bob getting into a fight over whether their cat is alive or dead.

    But as a consequence, there is either something non-local going on when all physical law appears to be local, or something non-real going on when everything we can in principle measure is real.

    Here's a mathless summary from Chad Orzel's How to Teach Physics to Your Dog:
    Central Principles of Quantum Mechanics
    1. Wavefunctions: Every object in the universe is described by a quantum wave function.
    2. Allowed states: A quantum object can only be observed in one of a limited number of allowed states.
    3. Probability: The wave function of an object determines the probability of being found in each of the allowed states.
    4. Measurement: Measuring the state of an object absolutely determines the state of the object.
    But there is not much you can do without math.
    (Augmented with cats)
    1. Wave functions: The state of the cat is described by a complex quantum wave function. \(\left| \psi \right> = \cos \theta \left| B_{\textrm{Alive}} \right> \; + \; \sin \theta e^{-i\phi} \left| B_{\textrm{Dead}} \right> \)
    2. Allowed states: Alive or Dead the cat was, is, and shall be. \( \left< B_i | B_j \right> = 0 ; \quad \textrm{if} \, i \neq j\)
    3. Probability: We need square of absolute value because probability is a real number between 0 and 1. All probabilities sum to 1.\( P_i(\psi) = \left< \psi | B_i \right>\left< B_i | \psi \right> = \left| \left< B_i | \psi \right> \right| ^2, \; \sum P_i(\psi) = 1\)
    4. Measurement: Assuming cats change state slowly, measuring the cat prevents the cat from changing state. \(P_i( \left| B_i \left> \right< B_i | \psi \right> ) = 1\)
     
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  5. Confused2 Registered Senior Member

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  7. exchemist Valued Senior Member

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    Nobody says the double slit experiment is easy to rationalise pictorially. (see for example rpenner's unimpeachable but possibly somewhat inaccessible reply.) But I think you are right to dismiss the notion that a conscious observer has anything to do with it. Some popularisations make a song and dance about this, as if to argue human consciousness has (gosh-wow!) a role in determining reality, but I think that is obscurantist bullshit, and does a disservice to the understanding of science. (If that were so, then the pattern would change when the experimenter went away to get a cup of coffee! That sort of idea might have appealed to Bishop Berkeley, but not to most scientists today.)

    A better way to think of it, in my understanding, is that the properties of a quantum object become determined when it interacts with something, be it a detector or something else.
     
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  8. Layman Totally Internally Reflected Valued Senior Member

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    They have used many kinds of different particles to detect this effect, even a lot of particles that do not even react with the electron. Basically, particles physicist already know that it seems like BS, and no one really understands why this type of behavior occurs. It is also one of the reasons why no one really understands quantum mechanics. Then it is what has been found to occur in every experiment, and in science experimentation is king. You just simply have to accept that is what occurs in the experiment, and that makes it a fact of life. I am still trying to figure it out myself, and being able to explain why this effect occurs would be a big breakthrough in particle physics.

    I believe it has something to do with the relativity of the hidden variables in quantum theory, because relativity deals with observers a lot. Then those variables are hidden from us, so we cannot explain why it occurs mathematically or logically. Then that is just my best guess at best.
     
  9. exchemist Valued Senior Member

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    My understanding is that support for the idea of "hidden variables" is something of a minority pursuit, these days. Or is that just local hidden variables.......??
     
    Last edited: Mar 15, 2016
  10. Confused2 Registered Senior Member

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    Just for fun, take as many variables as you like to 'explain' the double slit result ... what ones would you use and what would you do with them?
     
  11. Layman Totally Internally Reflected Valued Senior Member

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    You're right. Most physicist really don't support trying to discover hidden variables, although some still do. I admit that I try to use relativity a lot to put my head around a lot of the stranger things in quantum mechanics in place of the hidden variables. For instance, the act of observation can also make it more difficult to determine the exact speed and location of a particle. The more scientist determine the accuracy of the speed, the harder it becomes to determine its location, and vice versa. The more they determine about the location of the particle, the harder it is to determine the speed of the particle. Although, this effect is reduced for particles that have more mass and bodies that are made up of many particles. Well, massive bodies travel more slowly, so I remember this fact with the "mnemonic device", if you will; that massive bodies travel more slowly, and they are not affected as much by relativity...

    Then I believe a unified theory of quantum mechanics and relativity could possibly work out, even though the two theories are incompatible so far. I also believe that the hidden variables is just the math needed to be discovered to unify these two theories. Then I have no idea really how this could be accomplished, but it is really my desperate attempt to really try to make logical sense out of all it. Sometimes I have thought that it would be impossible, and hidden variables may always have to remain hidden. So, we may end up having to face the fact that quantum mechanics will always have to just be done by using probabilities, since relativity can become undefined at the speed of light.

    Quantum mechanics is like trying to determine the speed and location of a baseball in a baseball game using only the teams statistics and the batters averages, who is up at the plate at the moment. Then taking into account the pitchers averages, you can only guess how the play ends up. Knowing the hidden variables would be like if you cheated and measured the velocity of the ball at any given moment and determined its next location from there. Then you could just do normal physics, instead of just using the players statistics to determine the outcome of the game at any given moment.

    In a way, string theory is an attempt to discover a more fundamental underlying theory. That is how most physicist approach the problem, but some particle physicist try to even describe more fundamental particles that they believe even fundamental particles are made up of like the preon. Then strings in string theory are what particles are made of. Then a vibrating string can be a particle or in it's simplest form, a photon, which I believe is most fundamental. We discovered that mass is made up of mostly energy after all with E=mc^2. Then matter and anti-matter collisions just produce photons.
     
  12. Layman Totally Internally Reflected Valued Senior Member

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    Here is a paper that suggest that particle action at a distance cannot be described by relativity, because relativity is a local theory. Then it really doesn't prove anything beyond the writers ability to not be able to do it or resolve the problem. Its funny that I have come across other sources of confusion about this concept, since I had mentioned it on these forums. Then I am not the only one that has these beliefs or assumptions...

    http://arxiv.org/abs/0809.2904
     
  13. exchemist Valued Senior Member

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    That strikes me as rather a silly suggestion. You would have to read, in depth, one or more of the various hidden variable theories before attempting to say what these hidden variables would represent, and how they might appear in a theory involving them. Wouldn't you? Eh?
     
  14. Bruinthor Registered Member

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    The double slit experiment can be reproduced in a basin of water. You need an agitator to create the waves, a divider with opening to allow some of the waves through and some means of recording the wave pattern at the far end. For some combinations of agitation and slit geometry you will record a regular interference pattern. If you place some kind of obstruction near one of the openings the recorded wave pattern will be modified or destroyed.
    If you have a fancy lab you can recreate the experiment with electrons. The obstruction could be a beam of light or electrons. The math can look much different but the results the same.
    The point of the double slit experiment is that things that classical physics calls particles can and do behave as waves.
    I really wish NOVA and other such producer would stop using humans observers to demonstrate the effects of the extra object/phenomena. I guess it must sell too well.
     
  15. Layman Totally Internally Reflected Valued Senior Member

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    I think you got it completely backwards. It really doesn't sell to people at all, and that is why we have this thread here right now. Modern education teaches people to think more scientifically, and more and more people are becoming more atheistic every day. I wouldn't be surprised if the OP shied away from physics, because it seems to be based on pixie dust and fairy tales. From every book I have read about it, it is described to be an actual effect of the observer, which is simply a detector to measure the location of a particle in this case. You could go to the physics isle of your local library and close your eyes and then spin around 3 times pointing a finger, and that book will tell you the same thing. Most of them all include a detailed description of this experiment.
     
  16. Bruinthor Registered Member

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    Layman: I guess you have more optimistic opinion of the average TV viewers capacity for critical thinking than I.
    Bring religion into the discussion can only serve to alienate those with conventional theological convictions.
     
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  17. zgmc Registered Senior Member

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  18. The God Valued Senior Member

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    Is there any alternative explanation to this observation. You see experimental observations cannot be denied, one way is explained by QM, Dual nature etc..Is there any other possibility, which at least can explain this experiment's observations ?

    PS : To OP, Experiment is certainly not BS, may be you are talking about explanation of the same under QM.
     
    Last edited: Mar 16, 2016
  19. exchemist Valued Senior Member

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    If by "observer" you mean the (inanimate) detector then I agree. But this is a poor use of English, because in normal parlance an "observer" is a conscious human perceiver of the event observed. It is the interaction of the quantum object with a detector (which is inevitably involved in the process of measurement of its properties) that leads to what is called "collapse of the wavefunction", a.k.a. determination of the property.
     
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  20. zgmc Registered Senior Member

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  21. exchemist Valued Senior Member

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    Thanks for this. As usual from Feynman, a very nice description.

    Note he makes the point that it is the inevitable interaction, occurring in the course of detection, that collapses the wavefunction, not some woo about conscious observers

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  22. Layman Totally Internally Reflected Valued Senior Member

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    I don't believe that it is poor use of English at all. There are many words in the English language which have different meanings in different fields or context. I do not know of one word in the English dictionary that only has one definition and is only used in one context. I use the word "observer", because this is the word which was used many times over by different authors in physics where I learned about it. Then I would not think that the word observer in this context would be used as slang.

    Like I stated before, this experiment has been tested by other particles that the electron doesn't interact with somehow (something I learned from watching a Richard Feynman Youtube video). Then your definition would be inaccurate, because it has been tested when there is no interaction. As far as I know, the wave function doesn't even have to collapse in order for there to be an effect from the act of observation (be it any act done known in experiment to make a measurement). Take the example of just quantum uncertainty itself. If you measure the speed very accurately, then no known experiment can then pin down the position accurately. Then you do not actually get a situation where you detect a particle and it only has discrete properties.
     
  23. exchemist Valued Senior Member

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    I stand by my previous statement.
     

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