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What is an observer?

  1. Nov 27, 2011 #1
    Hello all,

    I was just wondering what 'an observer' might be in quantum physics, and any definition I can come up with just doesn't make sense.

    I strongly oppose the idea that an observer has to have a conscious. Because in that case, one would have to wonder: what is consciousness? Let's say we observer in the form of image: light triggering the sensors in our eyes and sending a signal to the brain. Note that that would not be observing yet, not until the thoughts have been completely processed, because you are not conscious of it yet. Hence history, in this case, would only exist after someone became conscious of it.
    Then I can only wonder if your observations make your reality real until I observe it. If not, that means you guy are all a creation of my imagination.
    More seriously, this would only leave one conclusion: our brain is supernatural. If not, then there is no real conscious. It is only chemicals in our brains, that are based on exactly the same physics that don't exist until it is observed.
    It just doesn't sound right to me one bit.

    Then there's the other thing I can think of, that any interaction with any other particle is an act of observation. For instance, a photon colliding with a particle would be an observer, as would a magnetic field. This sounds a lot more plausible to me and yet it doesn't completely add up to me.
    Consider the double slit experiments with electrons. If one observes the electron (in this case, have another particle interact with it) right at the slit, it loses its wave-like property. However, during the entire experiment the electron is "observed" by the gravitons created by earth.

    Hence my question: what is an observer exactly?


    Thanks in advance
     
  2. jcsd
  3. Nov 28, 2011 #2
    I feel the same way. I don't see the point of considering this point of view until an observer is defined.
     
  4. Nov 28, 2011 #3

    Fredrik

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    Observation doesn't need to involve consciousness, but it always involves storage of information in a form that can be approximately described by classical physics. That's probably the most important thing involved in consciousness too. So the two aren't completely unrelated, but there's certainly no need to think that something supernatural needs to be involved.
     
  5. Nov 28, 2011 #4
    In its simplest form an observer is a coordinate choice under which the system is to be considered. Mathematically where (x,y,z,t) all equal zero in Cartesian coordinates. It is not in itself a physical choice but defines the variations in the measured values relative to an alternative coordinate choice. The observer frame terminology was implemented because each of us view the world in terms of a coordinate choice in which our own coordinate position is (0,0,0,0) at any specific moment.
     
  6. Nov 28, 2011 #5
    Thanks for your answers guys.

    Okay, so this is the second option, where an observer is basically an interaction with another particle, correct? But then again, is there no interaction with earth in the form of gravity during the experiment? And assuming that gravitons exist, would they not in some way reflect the changes?
    Imagine in a distant future where we can actually create and detect individual gravitons, and we'd fire them in a straight line in front of the slits in the double slit experiment for electrons. Then we could detect the gravitons further away and use their locations to determine the choice of slit - which should have destroyed the wave-like property. But if this experiment would be possible, then so would gravitons from earth, right?
    Or is there some reason (uncertainty principle, or some properties of gravitons?) such an experiment would be impossible?

    So it's a choice of coordinate system, you'd say. Now, who choses this? Does it have to be an observer with a supernatural kind of conscious, or does this choice of coordinates happen when the particle interacts with other particles?


    Thanks for your help
     
  7. Nov 28, 2011 #6

    Fredrik

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    There's no such thing as gravitons or even gravity in the theories we should be talking about, i.e. non-relativistic quantum mechanics and special relativistic quantum mechanics. So issues that involve gravitons aren't worth thinking about until you're going to try to develop a quantum theory of gravity.

    The "observer=coordinate system" idea is appropriate in classical physics, but in quantum physics, an observation is an interaction that creates of an almost classical information record. So an "observer" should be a system that makes such an interaction happen.

    Given a curve in spacetime that represents the motion of a particle, and an event on the curve, there's a coordinate system that takes that event to (0,0,0,0), has a time axis that's tangent to the curve, and...satisfies a few more requirements that I'm too lazy to list. What's important to know is that statements about what someone or something "experiences" in SR are really statements about the coordinates assigned by the coordinate system associated with the curve that represents his/her/its motion. This is the reason why statements like
    From A's point of view, B's clock is slow by a factor of 0.6. From B's point of view, A's clock is slow by a factor of 0.6.
    aren't immediate and obvious contradictions. They are statements about coordinates assigned by two different coordinate systems.
     
  8. Nov 28, 2011 #7

    A. Neumaier

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    An observer is a macroscopic piece of matter, large enough for thermodynamics to be valid, and interacting with the observed system.
     
  9. Nov 28, 2011 #8
    Thanks for your answers. That makes a lot more sense, especially lacking gravitons.
     
  10. Nov 28, 2011 #9
  11. Nov 28, 2011 #10
    An observer is someone observing say an electron, or an atom (Uncertainty Principle) this is related to Quantum Mechanics if you want an example and a demonstration search Double Slit Theory on you tube and go to Dr. Quantum

    http://youtu.be/0ElSXo1HWS4

    here is a link to save you some time.
     
  12. Nov 28, 2011 #11
    What I always think of are hydrogen atoms. The electron and proton are observing one another. That is, the electron always has a definite location vis a vis the proton. We can prove this because measurements show that the rotation of the atom is around the center of mass, which would not be possible if the locations were undefined.

    However outsiders do not know what these locations are. So the two particles are observing one another and have a definite relation, but all such possible definite relations are superimposed.

    Now consider a hydrogen molecule, which is two hydrogen atoms. The four particles always have certain relationships that are preserved, but all possible such relationships are superimposed.

    This goes on and on, without limit. The math says that if a human being gets involved then there are certain relationships between the molecule and the human that are preserved, but the total system stays in a state of superposition. For some mysterious reason we are aware only of one such state.
    ---
    Another thing to look into is decoherence. These are not all-or-nothing states, these superpositions. With the two slit experiment it isn't a matter of either/or, of interference or not. Recently there was an experiment where buckyballs were fired through two slits. The buckyballs were heated, and as the temperature went up the amount of coherence gradually decreased and the interference pattern on the screen gradually faded away. Surprisingly for me, they had to heat the buckyballs until they were glowing[2000K], before they got complete decoherence and the interference pattern completely disappeared. I would have thought it would be much less.
     
  13. Nov 28, 2011 #12

    Fredrik

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    The Dr Quantum video is mostly OK, but the stuff it says about "observation" is misleading. The detector wouldn't be an eyeball that just passively records what is happening, it would be a device that interacts with the particle, strongly enough to produce an approximately classical record of the result.
     
  14. Nov 29, 2011 #13
    Maybe the word "measurement" is better than "observer", but, even then, you still have to keep in mind than it's an impersonal sort of measurement that doesn't care if a scientist is around to record the data. After all, measuring devices are nothing other than normal physical process that just happen to be set up by us so that we can record the result. It's sort of incidental that it's an experiment. Nothing special about human involvement or the involvement of any conscious creature.

    If no one is watching the double slit experiment, the result will be the same.

    I guess it's a little like the old "if a tree fell in the woods and no one was there to hear it, would it make a sound?" question.
     
  15. Nov 29, 2011 #14

    A. Neumaier

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    This is not an observation in the sense the word is generally used by physicists.
    An observing device must be macroscopic, so that the counter or pointer acts irreversibly.
     
  16. Nov 29, 2011 #15

    A. Neumaier

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    There is no result unless there is a screen to observe the particles that went through the slit.
    The screen is the true observer of the particles, while _we_ only observe the screen (and infer properties
    of the particles).
     
  17. Nov 29, 2011 #16
    Yes, when I said double slit experiment, that includes the screen. But it doesn't matter if no one is there to see the screen or record data.
     
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