Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Double Slit Experiment - Blind Observer

  1. Sep 18, 2009 #1

    I have a question about the famous double slit experiment.

    They say that if an observer like a camera or a person is watching the experiment then it collapses the wave function and two marks appear instead of an interference pattern.

    Does anyone know what what happen if a totally blind person were to witness the experiment?

    If the wave form still collapses would this suggest that we are interlinked with the universe at a much more intricate level, also maybe suggesting that our consciousness plays a role in the collapse of the wave function.

    Julian Mummery
  2. jcsd
  3. Sep 18, 2009 #2
    This is an oxymoron. A blind person cannot witness anything.

    Quantum mechanics (and science in general) should be concerned only with results of observations and refuse to answer questions about non-observable "things". This is the most valuable philosophical lesson of quantum mechanics. If you stick to this idea you'll easily avoid many "paradoxes" of quantum mechanics.
  4. Sep 18, 2009 #3
    If a camera that is not a conscious entity can collapse the waveform then how can you say that a blind person being present is an oxymoron?

    Until the experiment is conducted I am not convinced, especially as it would have other implications.

    By saying it is an oxymoron just means that you are dismissing it as a viable experiment. Surely all experiments no matter how strange should be conducted, especially when we need more clues in this field.

    So if a blind person collapsed the wave function, surely it would have a much more deeper implication? Maybe something like consciousness being entangled with super strings?

    Julian Mummery
    Last edited: Sep 18, 2009
  5. Sep 18, 2009 #4
    Human consciousness/vision has nothing to do with it. You shoot a particle at a pair of slits. If any effect is produced whereby one could tell which slit the particle passed (e.g. you put a device by one slit to detect if the particle passes through it or not), there is no interference pattern. However, if there is no way to tell through which slit the particle passed (because you placed no such device), there is an interference pattern.
  6. Sep 18, 2009 #5
    What exactly you mean by these words? Which experiment is going to tell you that this statement is true or false?
  7. Sep 19, 2009 #6
    Is the interference pattern observable or just mathematical?
  8. Sep 19, 2009 #7
    For a blind person it is only mathematical.
  9. Sep 19, 2009 #8
    It depends on what flavor of Copenhagen Interpretation you use (I dont use CI at all). But if you insist on "collapse" and usage of CI, then "Human consciousness/vision has nothing to do with it" is wrong because in classic CI wavefunctions is just our knowledge about the system and it is subjective
  10. Sep 19, 2009 #9


    User Avatar
    Gold Member

    You have misunderstood something. When we say that a person is watching the experiment, we DO NOT mean that he stands by the two slits looking with his eyes to see the electron coming throught the slits!
    You must keep in mind that an electron is invisible if you dont lit it up with light. A usual atom emits photons with frequencies in the optical electromagnetic spectrum and that`s the reason you see it with your own eyes. But an electron doesnt emit photons by itself!
    So the only way to see an electron is to "lit it up" with a "special torch", and when you see a flash at some position is space, then thats where the electron was!
    In other words: Sitting by the two slits just looking with your own eyes the electron IS NOT observation. So it makes no sense to say what happens if the person is blind!
  11. Sep 19, 2009 #10
    It is just mathematical. The problem is that there is an implicit assumption that when stripes are seen that look like a mathematical interference pattern that some how it IS an interference pattern.

    An interference pattern implies the classical situation of waves in a medium. The waves propagate toward a pair of slits, the waves go through both slits and coming out the other side they tend to interfere with each other according to the angle they make progressing toward the observer screen. So far so good. IF (and only if) your experiment is 1. dealing with waves in a medium, and 2. the medium exists. In such a case the mathematical formula you calculate based upon the wave interference pretty much matches the patterns you observe on your screens.

    So classically the original idea was that light was an electromagnetic vibration that traveled though space as a wave. Hence all the optical phenomena made sense. But if light (and electrons) are particles which are simply painting a TV image on the face of a CRT that is in fact CONTROLLED by some kind of wave, we get closer to the truth. So there are particles and there are probably waves too. And the waves seem to control the particle motion because the statistics of the trajectories form the solutions to wave equations. [Yes, it might not be absolutely necessary to have waves to form such solutions but it may be probable.] So how does it all work? Nobody is quite sure.

    Now here's where it gets tricky. You do this same experiment with light or electrons and it appears at first glance to give the same results. You shine the beam at the double slit and the "screen" (or detector) shows a mathematical double slit diffraction pattern. So, "problem solved" said physics. But not so fast. If you REALLY start to look closely, you find out that what is traveling to the screen are particles. You can see that they are only going through one slit at a time, and further you can lower the intensity until they are traveling to the screen one at a time. So where are the "waves"? Where is the medium for the waves to wave in? It's not there.

    But wait. What about the diffraction pattern we observed so easily? Well careful examination shows that the pattern is made up of a statistical average of great numbers of particle impacts. There is no "wave" making it up. So now we have the problem of how single particles traveling to a double slit somehow ON AVERAGE are guided into a direction such that the statistical average of impacts is IDENTICAL TO a wave interference pattern?

    Well, there is no model to explain this so a lot of "hand waving" is used instead. The theory that "works" involves probability waves. What are probability waves and what do they use for a medium? Who knows? The bottom line is that it's all mathematical, but nonetheless does work to predict results. So what exactly provides the "nudges" that move the stream of particles into mimicking on average the solutions to wave interactions? Nobody knows yet.

    Now the theory that just looking at something causes "waveform collapse" so the thing suddenly exists is really just so much word salad. And it's arrogant as well. I don't think the universe needs me to look at it in order to exist. However, to really provide a cogent explanation of what is going on, would require a true understanding of just where the "probability waves" come from and how they operate. And that understanding does not exist. In fact it's a wonder that such an oddball theory was invented in the first place and actually fits observed phenomena to the degree it does.
  12. Sep 19, 2009 #11
  13. Sep 20, 2009 #12
    But Human consciousness will be involved as we would be reviewing the data of the experiment that has marked one of the slits.
  14. Sep 20, 2009 #13


    User Avatar
    Science Advisor

    Guess you didn't see that Audrey Hepburn movie, then.
    (Or that Richard Pryor/Gene Wilder one.. or.. come to think of it, there are a lot of those films!)

    That doesn't make human consciousness involved in the process. Physics worked the same way before humans existed and before 1926. The universe has no particular concern for what's going on in our heads.
    Last edited: Sep 20, 2009
  15. Sep 20, 2009 #14


    User Avatar

    Staff: Mentor

  16. Sep 20, 2009 #15


    User Avatar
    Science Advisor

    Even if a machine records which slit each electron went through, but then before any human can examine its results the machine is totally obliterated and all its data destroyed, there will still be no interference pattern observed in the total pattern of electrons on the screen according to QM (this is basically a variation on the delayed choice quantum eraser, where the total pattern of photons shows no interference although there can be interference patterns in certain subsets of the data).
  17. Sep 21, 2009 #16
    I think "information is destryed" requires some clarification. When information is destroyed in a practical sense it is not enough to restore the interference. Because theoretically, there are some tracks left - some minor temperature variations etc. So to observe an interference, you must undo the measurement completely, which means, that it must be thermodynamically reversible measurement.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook