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The CRT and Electron Superposition

  1. May 31, 2009 #1
    If according to quantum theory electrons are in superposition until their waveforms collapse by the observer observing them, does this mean that there are more than one image on a CRT such as a TV or PC screen when nobody is looking at it?
     
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  3. May 31, 2009 #2

    mathman

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    The interaction with the CRT screen is enough to collapse them. No one has to be looking at it.
     
  4. May 31, 2009 #3
    This is an ill-posed question, therefore it can't be answered. Whatever the answer you get (for example, I may say that there are angels dancing on the screen while nobody is watching) you have no means to verify it. In order to verify you need to look at the screen, but this is forbidden in the original question.
     
  5. May 31, 2009 #4
    But we can set up a video cam and view the video later to see if there are angels dancing on the screen or not
     
  6. May 31, 2009 #5
    What about the CRT's anode - would its pull on the electrons collapse them?
     
  7. May 31, 2009 #6
    Yes, but if this were done to electrons it would be an act of observing and would therefore collapse the waveform.
     
  8. May 31, 2009 #7
    It is irrelevant how exactly you (or somebody else) get the information about the screen appearance. The important moment is when you obtain and digest the information. That's when the "wavefunction collapse" occurs and probability changes to actuality.

    Possibly this quote from Niels Bohr would explain this point better than I ever could: "It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

    The distinction between what nature is and what we can say about it was not apparent in classical physics, but in quantum mechanics it is absolutely essential.
     
  9. Jun 1, 2009 #8
    Will the video camera collapse the wavefunction if it can see only half of the TV screen?
    How do you know what would collapse the wavefunction, what would not?
     
    Last edited: Jun 1, 2009
  10. Jun 1, 2009 #9

    ZapperZ

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    There is a certain amount of misunderstanding here that needs to be corrected.

    Let's say you have a wavefunction [itex]\Psi[/itex] that describes a particular system. When you make a measurement (and this should be emphasized very clearly), it corresponds to operating some operator (call it operator A) we named as an observable to this wavefunction. Now, assuming that this is an eigen operator of that wavefunction, you can get one of the many possible values (called the eigenvalue) as the outcome of this measurement. So when you do this and get a value, we call this as "collapsing" the wavefunction because it now has a particular value for this observable.

    But you need to remember that this observable can possibly be one of the many observables that can be measured. You have possibly only collapsed the superposition of this observable. If there is another observable, called it B, and it does NOT commute with observable A, the act of measuring A doesn't change the superposition state of observable B!

    So what you are "collapsing" isn't the wavefunction, but rather the representation for that particular observable. That's it. This means that asking if something collapses if it is "observed" is rather vague. WHAT exactly is being "observed"? It's position? What was the wavefunction before? Was there a superposition in position? Is this what is being "collapsed"?

    There is also another issue that should be mentioned. When we deal with "free" electrons in vacuum, especially for a CRT, you will note that practically ALL of the descriptions for such a system rely on purely classical physics, not QM description. In fact, go to a particle accelerator and look at the codes they use to study the beam dynamics of the particles that are zooming around in such accelerators. You'll see that inevitably, they are using classical particle description to describe such a system, not quantum mechanical. Why? Compare to conduction electrons in metals, these free particle are so far away from one another, each of their "wave" nature does not overlap, and they are essentially classical particles. We track them the way we normally would any other classical particles. That's why we can design the CRT on your old TV without ever using QM, and the electrons can be predictably controlled without any "weird" quantum properties popping up.

    So the question of collapsing such "wavefunction" on something like the position of such particles is a bit puzzling in light of what we already know and can do. Now, if you pass such a beam through a Stern-Gerlach type setup whereby you attempt to measure the spin of the electrons, that's a different matter. In this case, the observable will be the spin states, or more accurately, the spin projection operator. That observable is certainly in a superposition and will only be determined once it is measured.

    Zz.
     
  11. Jun 6, 2009 #10
    So if you are saying that if a specific observation is made, then there is still the potential for other observations and hence outcomes to be made. This reminds of Paul Davis conclusion that quantum theory implied the existence of parallel universes.

    So if certain instrumentation is designed in such a way to expect particles to behave in a 'classical manner', then I don't see how this will cause the electrons to lose their "weird quantum properties".
     
  12. Jun 6, 2009 #11

    ZapperZ

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    Er.. no, this has nothing to do with those "parallel universes". The latter is an attempt at describing superposition. That isn't what I was getting at.

    Look at your parents. Do you see any weird quantum effects? No?

    When a system can be adequately described via classical mechanics (see: The Building of your house), then you are not detecting any quantum effects. It would be nuts to insist on still using QM to describe such a system, especially when we clearly have not quite yet figured out how the quantum-classical transition looks like.

    Look at particle tracking code such as PAMELA that is used for tracking particles in an accelerator. There are no quantum components in there at all. And these are some of the codes that we use to design particle accelerators. I rest my case there.

    Zz.
     
  13. Jun 6, 2009 #12
    The Stern-Gerlach experiment is a very important measurement, worthy of a Nobel Prize. But the discovery was an accident. There was no image on the screen when the experimenters removed the detector plate from the vacuum system. As Otto Stern recalled, after both he and Gerlach were examining the screen, a dark image slowly began to appear. It was an accidental discovery, because Otto Stern smoked cheap cigars, which caused the wave function to collapse and image to appear. See
    http://www.fhi-berlin.mpg.de/mp/friedrich/PDFs/ptsg.pdf
     
  14. Jun 6, 2009 #13

    ZapperZ

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    Not sure what this has anything to do with what I wrote. I did say that if one were to consider spin state superpostion, that is still a valid scenario for even free electrons.

    A lot of important discoveries were done by accident. The discovery of superconductivity by a student of Kamerlingh Onnes was one. So I'm not ignorant of this fact.

    Zz.
     
  15. Jun 6, 2009 #14
    From zz
    In the [Stern-Gerlach case], the observable will be the spin states, or more accurately, the spin projection operator. That observable is certainly in a superposition and will only be determined once it is measured.
    From Bob S
    [The Stern-Gerlach experiment was accidentally discovered] because Otto Stern smoked cheap cigars, which caused the wave function to collapse and image to appear. See
    There was no image on the plate when it was removed from the vacuum system. When did the wave function "collapse" onto the plate? the image could not be measured until it was observed.
    It was the sulfur in Stern's cheap cigars that developed the silver atoms on the plate, so as they watched, they saw the wave function collapsing and the measurable image forming on the plate.
     
  16. Jun 6, 2009 #15

    ZapperZ

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    I still have no clue in what your beef here with what I wrote.

    You shoot electrons. The spin state is undetermined until you measure it.

    Which part of that do you dispute? This is also a SIDE item in my post that you are focusing on, and not relevant to the thread. Why I'm being taught about the history of Stern-Gerlach expt, I have no idea.

    Zz.
     
  17. Jun 7, 2009 #16
    When the electrons are moving in macroscopic trajectories it's enough to know their positions with precision of ~1mm. Therefore the deviation in their speeds will be ~10cm/s (in accordance with the principle of Heisenberg) which is negligible amount of their real speed. So, like Zz said, the descriptions for such a system rely on purely classical physics.

    Ps. sorry for my bad english...
     
  18. Jun 7, 2009 #17
    My concern is the semantics of the word measure. After Stern and Gerlach turned off the silver beam, they opend up the vacuum system, and removed the flange plate. The could not see any image. Was something measurable at this time, even though they could see no image? Only after Stern's cigar smoke, with sulfur in it, developed the silver deposited on the plate, could they see a measurable image. Was the spin state measurable immediately after the silver atoms left the high gradient magnet in their experiment, or when there was a measurable image?
     
  19. Jun 8, 2009 #18

    ZapperZ

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    Then you are nitpicking on something completely off-topic. Please start another thread, preferably in the History forum.

    Zz.
     
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