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Regarding human influence and Heisenberg

  1. Feb 2, 2006 #1
    Ok...I'm new to these forums and am just beginning to understand quantum physics, so please be kind. I've been reading those Brian Greene books and I have a question that I don't think he addressed.

    If I understand correctly, some believe that IF we could measure the position, velocity and direction of all particles we could then predict with absolute certainty where they will be at any time in the future or would have been at any time in the past. Heisenberg says we can't do this, not even for one single particle because the more accurate our measurement of the location of a particle becomes, the less accurate our measurement of its speed and direction becomes. So far I get it.

    But lets assume we could get both the postion and velocity and direction of all the particles in a system (say...me and a cup of coffee with cream). We make our calculations and predict that in 10 minutes the system will look like X. During those 10 minutes I stick my finger in the system and stir it around.

    My question is this...Will knowing the exact position and velocity and direction of all the particles that make up ME somehow predict that I am going to stir the coffee? Do I have any freewill? Or is my every move somehow predictable based the current state of my matter?

  2. jcsd
  3. Feb 2, 2006 #2


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    In the classical conception as presented by the great physicist Laplace, that's exactly what would happen. This assumes of course that you don't have free will, but in this Laplacian deterministic universe, you don't.
  4. Feb 2, 2006 #3


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    There are different "interpretations" about what the universe is really doing when we aren't observing it. All these interpretations make the same physical predictions, which is why they don't really qualify as scientific theories. The interpretation known as Bohmian mechanics would indeed say that particles have a definite position and speed at all times, and that if we could know these details exactly then everything would be exactly deterministic. In the context of this interpretation, the answer to your question is that in order to make perfect predictions about the coffeecup we'd need to know the exact state of every particle in your body and brain (and in the rest of the universe too), not just every particle in the coffeecup itself.

    But other interpretations would say that the problem is that particles don't have an exact position and momentum at every moment, not that they have them but we just can't measure them. For example, some interpretations would say that the ultimate reality is the "wavefunction" which is ordinarily used to make probabilistic predictions about the outcome of different measurements (such as measurements of position and momentum). So in such an interpretation, total knowledge of reality would be knowledge of the state of the "wavefunction of the universe", not knowledge of the position, momentum, etc. of every particle.
  5. Feb 3, 2006 #4
    Is there any difference ? Would not "total knowledge of reality...of wavefunction of the universe" be the same as "total knowledge of the reality...of the position, momentum, etc of every particle" in the universe ?
    That is, what do we find in the mathematics of the first that is absent from the second ? Just asking, I have no answer.
  6. Feb 3, 2006 #5


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    The other way around: there is no wavefunction that gives "position, momentum etc" of every particle. It just doesn't make sense. It is like asking what is the exact frequency and arrival time of a signal. A signal is a function f(t), and if it has an exact arrival time, then it will have several frequency components, and as such, no exact frequency.
  7. Feb 3, 2006 #6


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    The wavefunction of a system only assigns "amplitudes", which are complex numbers (with both a real and an imaginary component), to every possible outcome of a measurement of a particular variable. When you make a measurement of a particular variable like position, you can use the wavefunction to compare the probability of getting one outcome (particle found at x=3 nm) vs. another (particle found at x=2 nm), by taking the square of the absolute value of the amplitude that the wavefunction assigns to each outcome (the square of the absolute value of the complex amplitude is always a real number between 0 and 1 which can be treated as a probability).

    In between measurements, the amplitude that the wavefunction assigns to each possible outcome evolves in a continuous and deterministic way. But in terms of the quantum formalism, when you make a measurement of a particular value this causes a discontinous and random "collapse" of the wavefunction, so that immediately after your measurement all the amplitude is concentrated on the outcome you actually got, with the amplitude now being zero at every other possible outcome for the same variable. This is part of what is meant by the "measurement problem" in quantum mechanics, the fact that the rules for the dynamics of a system seem to be fundamentally different in between measurements than when measurements are made, which seems illogical since the measuring device is just another physical system whose dynamics could be treated in the same way as the system being measured. So most interpretations of QM would try to explain things in a way that says there is really no fundamental distinction between how the laws of physics work when something is being measured vs. how they work between measurements, and that the "collapse of the wavefunction" is just a matter of appearances, not reality.
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