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Predicting the Future

  1. Jul 19, 2012 #1
    I read somewhere that if you detect which slit a particle stream goes through in the double slit experiment that you will no longer get the fringe paterns associated with the double slit experiment. I'm told that as soon as you record the detection of which slit the particles enter you loose the patern. But if you don't record which slit was entered you do get your double slit patern. You can even keep the detecting device on that sends a signal to the recording device, but as long as you don't actually record which slit, you get your fringe patern.

    So... what if you send your detection signal over a very long distance and back again, perhaps many times, to create a delay between detection and possible recording? For example what if you bounce your detection signal off of a satilite many times so that it takes a whole day to reach the recording device. (assuming no temporary data storage in the satilite, etc). Then you could decide a day later whether to actually record the patern or not. Could this be used to send information back in time so that you could read the future? Perhaps you could send coded messages back in time about the Stock Market... in an automated fashion to avoid any paradoxes you might otherwise encounter about your free will.

    Or would the detection be considered stored in the data stream? Then the question is how brief of a detection signal is required before it is not considered storage of data?
    Last edited: Jul 19, 2012
  2. jcsd
  3. Jul 19, 2012 #2
    I believe that it's the actual detection that is important there: whether it is recorded or not is irrelevant. For more wackiness about the timing in the double-slit experiment, read about the delayed choice variation on the experiment.
  4. Jul 19, 2012 #3


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    As a rule: the "something" you do to detect which way (or slit) the particle goes, always precedes the actual detection. So there is no opportunity here to send a signal from the future to the past. As others have commented, the decision to record the results is not relevant.
  5. Jul 19, 2012 #4


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    It sounds like you are describing the
    The result is that it doesn't matter when you decide to measure which way the particle goes. The decision to make the measurement can be space-like separated from the slits, and the appearance of the interference pattern still "depends" on whether the measurement of which way is made or not.

    I put depends in quotes because this is not a causal relationship, but rather a universe consistency requirement. We can't say if the measurement of which way caused the interference pattern to disappear, or if the disappearance of the interference pattern caused the measurement of which way to occur. The law of cause and effect is a classical way of thinking and simply doesn't cover all scenarios in quantum mechanics, which is a non-local theory.
  6. Jul 19, 2012 #5
    Right! The question is what constitutes a "measurement" or what constitutes a "detection". If the experimenter will never have the ability to gain knowledge of the detection (it's never recorded), then is it "measured"? Can a detecting device remain in place but not be activated? And at what point is it activated, where do you put the switch that activates the detector? Where do you cut off the signal to the recorder to disable the detector? This is like asking whether the microphone is working if it's not connected to a speaker or a recorder or an oscilloscope or ear or other measuring device. If the sound is off, how do you know the microphone is sending a signal (or is detecting sound)?
    Last edited: Jul 19, 2012
  7. Jul 22, 2012 #6
    I'm still not clear as to what is possible. So let me suggest a thought experiment with various switches to make it easier to discuss the options. I make no claims here. Every assumption is open to debate.

    Suppose we have a double slit experiment using electrons. And we have detectors on each slit that can be switched on with Switch A. And we can operate Switch B to record the "which slit" detections if desired. We also have a screen beyond the slits, and the screen is divided into many pixels so we can detect where on the screen the electrons hit and with what intensity. There is a Switch C to enable the screen detections. And there is a Switch D to store the screen data. We shoot electrons at the double-slits one at a time so that it is possible to detect which slit it goes through, if desired, and where on the screen the electrons hit. There is also a Switch E which send both the which slit data and the screen data to an integrated circuit which calculates the correlation between slit and screen. Switch F determines whether that calculation shows up on an monitor for human observation. And of course a human can decide whether to look at the monitor or not.

    At what point do we loose the fringe pattern associated with having which slit data? Is it Switch A that activates the slit detectors, Switch B that store it in memory, Switch C that enables the screen detectors, Switch D that stores screen data, Switch E that sends all data to the computer for any necessary calculations, Switch F that sends the calculations to the monitor, or when a human observer looks at the data? Any help with this would be appreciated.
  8. Jul 22, 2012 #7
    Switch A. It's nothing to do with a conscious observer knowing or recording the which path information. The very fact that you've "asked nature a question" is enough.

    I've posted this before but I think it answers these sorts of questions brilliantly

    Hope this helps.
  9. Jul 22, 2012 #8
    This doesn't satisfy me because if the beam consists of electrons and the slits are small enough, then wouldn't the charge of an electron going through a slit disturb the molecules on the surface of the slit it goes through? And wouldn't this disturbance on the surface propagate through the rest of the material that makes up the slit? Then it would seem in principle that it is unavoidable that the electron detection would always be permanently recorded in the slight but permanent displacement of the atoms in the material of the slit due to the charge of the electron passing through it. The only difference seems to be that no one accesses this data (it would be very difficult) to correlate it with where the electron hit on the fringe detector screen.

    I see no difference then if solid state amplification is used to amplify the signal of this disturbance down some wire to a memory circuit where it is more accessible. For example the two slits might be incorporated on a silicon chip made of different semiconductor regions with different doping to create transistors, wires, and memory cirucits. These circuits are getting smaller and smaller all the time. We are still talking in that case about disturbances in the surface atoms of the slit propagating through material and effecting arangements of atoms, only this time not so slightly so that the information is more accessible. So I think this leaves the question open as to at what point does the data (or knowledge) of the which slit information effects the fringe pattern on the screen. For it seems the information is being store (in the slight rearrangement of atom in the slit) in any event. Would the fringe pattern disappear just because someone applied a voltage to some material near the slits? What if it's very low voltage?

    Or perhaps the electrons going through the slit must not have enough energy to disturb the atoms on the surface of the slit before a fringe patterned can be detected. Is that even possible in principle, for example, for the force of the passing electron to be less than some zero point vibrational energy of the atoms on the surface of the slit?
    Last edited: Jul 22, 2012
  10. Jul 22, 2012 #9
    But then the interference pattern wouldn't show up, because which-way information is available.
  11. Jul 22, 2012 #10
    Are the atoms that make up the surface of the slit disturbed or not when an electron goes through that slit? If they are disturbed, can that be considered a record of which slit information?
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