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

Is everything deterministic?

  1. Jul 18, 2008 #1
    Hi, my question is simply: "Is all physical phenomena deterministic?" or said different, "Can all physical systems be predicted exactly given initial conditions?"

    I am not very far into my studies in physics, but I am aware there is such a thing as the "Heisenberg Uncertainty Principal" and it has to do with an uncertainty of the state of very small particles.

    Is it generally accepted by physicists that the uncertainty principal is simply modeling phenomena that we are currently unable to explain analytically or is it suspected that there is an actual randomness or a sort of divine influence in events that actually makes the future indeterminate?
  2. jcsd
  3. Jul 18, 2008 #2


    User Avatar

    vanesch has written a lot of great posts about this.

    I can't find the one I was looking for, but here's one:
    Here's another good thread:
  4. Jul 18, 2008 #3
    From reading his posts I gather that physicists do not currently know or have any suspicions as to whether the randomness inherit in quantum mechanics is "reducible to ignorance" or not.
  5. Jul 19, 2008 #4


    User Avatar
    Gold Member

    I read something once which said once we have a theory of everything, if we knew everything about the universe at any one time we could predict the out come. I think I read that in one of steven hawkings books. Surly thought the uncertanty principal would stop that from ever being true.
  6. Jul 20, 2008 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In the "orthodox" views on quantum theory, the randomness is irreducible. For all we know, there isn't a way to *find out* that information, even in principle, before the actual event happens. In the classical example of the Heisenberg uncertainty of position and momentum, if you know the position of a particle with a certain position, and you are going to perform a momentum measurement on it (with a minimum uncertainty, given by the uncertainty principle), then there is absolutely no way to find out what that measurement is going to give in any more precise way before you actually do it. Unless one day quantum theory is falsified/modified/..., as long as quantum theory is a good description of nature, we're stuck with it (simply because quantum mechanics runs into fundamental problems if we WERE able to find out what was going to happen). So on a quantum mechanical level, the randomness seems to be irreducible.

    However, it is not because *we are not, even in principle, able to predict* that it doesn't mean that "nature doesn't knows better". This is open to discussion. Is it because we are by one or other principle unable to find out all information in nature (but if we were able to have a "gods eye view", we would be able to predict), or is "nature itself" ignorant, and is there nothing in the ontology of nature that "determines" what will actually happen ?

    There are ontological models of quantum mechanics (Bohmian mechanics for instance) which have it that "nature" is perfectly deterministic, but that for some or other reason, we cannot get at that extra knowledge. There are even funnier ontological models of quantum mechanics (many world views - my favorite) which solve the issue in an even more bizarre way: nature is deterministic, and all outcomes actually happen in parallel, only, you observe just one of them (and the randomness is now displaced into which one you are going to observe, of which many world views have no deeper explanation - so at this point there IS indeed something fundamentally random, but it is not in the workings of nature, but in the nature of your subjective observation).

    Nevertheless, all these views agree with the claim that it will be impossible for you to make any better predictions than are allowed by standard quantum theory.
  7. Jul 20, 2008 #6
    Does the "Heisenberg Uncertainty Principal" not in any way pertain to our inability to measure a characteristic of an object without fundamentally changing the characteristic which is being measured?

    I know that if I attempt to use a low impedance meter to measure a voltage across a component an electrical circuit, then I will change the impedance of the component when I attach my meter leads in parallel with said component. Regardless of the accuracy of my meter, I can't possibly obtain an accurate measurement. By using a very high impedance volt meter (which I generally do) I am able to reduce the error in my measurement -- but I can never eliminate it altogether.

    Doesn't the same situation apply at the quantum level? Is that what is meant by the theoretical "Gods Eye View"? Does that refer to the ability to ascertain information about a particle without affecting the particle being measured?
  8. Jul 20, 2008 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No. Unfortunately, many textbooks (especially intro textbooks) explain it that way, but that is making an assumption which is incompatible with the very quantum mechanical description in the first place. Probably this is done to find an analogy with situations the reader might be more acquainted with, such as the one you suggest, but the error is the following:

    In order to CHANGE a characteristic, it would mean that the object HAS the characteristic. However, the quantum-mechanical description of the state does NOT allow for a simultaneous value of say, "position" and "momentum": a "position state" is made up of several momentum states, and a momentum state is made up of several position states. So there is no quantum state which corresponds to some precise position AND momentum value. So quantum-mechanically, a particle cannot be in a state where it HAS both a precise position and momentum value. It is not that the value *changes* arbitrarily, it is that it doesn't EXIST.

    Upon a measurement of, say, the momentum, the quantum state has to change from a superposition of many momentum states into a state which has only one momentum state. It is this process (the famous "projection") which appears to us to be random and which is badly understood. The particle has to flip from a state in which it has "several momenta in parallel" into a single one of them. It is not that it had "a certain momentum" and then "got a random kick".

    The point is that knowing the impedance of your meter, you could CALCULATE what deviation that impedance introduced, and hence correct for the error (and hence obtain a very precise measurement of the impedance).

    Quantum mechanically, there's no "correction" possible.
  9. Sep 11, 2008 #8
    But still in Quantum mechanics you can minimize the uncertainty on account of other non-commutable observable....like "the squeezed states of light "

    so I think by gathering more and more information about the behavior of the system....we can minimize the uncertainty of a particular observable on account of the other ... but yes still the heisenberg uncertainty principle holds true...

    correct me if i am wrong .. also this is my first post on PF :)
  10. Sep 11, 2008 #9
    According to QM, the wavefunction evolves according to a deterministic law, therefore QM is deterministic. The difference with classical physics is that an observation effectively erases information from the perspective of the observer.

    This is perhaps best illustrated by a thought experiment. We place a person in an isolated room and administer a deadly poison gas that kills the person with 100% certainty within 5 minutes. So, the wavefunction of the person describes a dead person after 5 minutes.

    According to classical physics we could, in principle, observe the dead person and collect enough information to be able to compute the hypothetical situation in which the person had not been killed.

    According to quantum mechanics, we can still talk to the person after the 5 minutes have passed, provided we make sure the wavefunction of the contents of the box doesn’t decohere (in practice this is an unrealistic assumption). This works as follows. Asking a question to a person amounts to applying some perturbation to the Hamiltonian. Recording the answer is equivalent to measuring some observable. But in this case we want to undo the effect of time evolution since the poison was adminstered. The wavefunction evolves in time according to:

    |psi(t)> = Exp(-i H/hbar t) |psi(0)>

    Asking a question induces some unitary evolution Q on the wavefunction. We want to apply this to living person
    |psi(0)> when given the dead person |psi(t)>

    Q |psi(0)> = Q Exp(i H/hbar t) |psi(t)>

    Suppose that the person was sure to give some definite answer to the question. Then Q |psi(0)> is an eigenstate of the operator, A, that corresponds to measuring the answer (the eigenvalue lambda):

    A Q Exp(i H/hbar t) |psi(t)> = lambda Q Exp(i H/hbar t) |psi(t) >

    And it follows that:

    B |psi(t)> = lambda |psi(t)>


    B = Exp(-i H/hbar t)Q^(-1) A Q Exp(i H/hbar t)

    So, measuring the observable B when the person is dead gives the same result as asking the question and recording the answer to the person when he was alive. But if you just open the room and see the dead person lying in the ground, then you can't do this anymore. The wavefunction |psi(t)> will (effectively) collapse and information about the past will be lost.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook