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

Confused about time on the quantum scale

  1. Jul 9, 2006 #1
    In the macro, non-relativistic world, lets say that the present is the interval that separates the future from the past. And that this is an analogy for the interval within which the probability waves in a region in space collapse into particulate matter resulting in a measurable event which in turn embodies reality (whatever that is).

    Now lets scale down to the micro world where a single electron is fired from an emitter towards the double slit wall. On its way there, lets say that the electrons wavefunction does not collapse. While in this state, the second hand on the experimenters watch ticks forward as usual. But in the microscopic world, this interval lacks the same meaning as in the macro world because no measurable events occurred with respect to the electron while in this state.

    I am confused here. Does time not act as a non-spatial continuum within which probability waves can propogate, co-mingle or whatever else it is that they do? Or have I just missed the boat on this one...?
  2. jcsd
  3. Jul 9, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    I think that this is a very sensible way of looking at the act of measurement. Unfortunately, this view is not well accepted in the physics community.

    Here, I disagree with you. Let me put it into an analogy with the manner in which a stressed crystalline material cracks. When the experiment was in the "future", the electron was described by a wave function. You might think of this wave function as being a description of how much space-time is stressed at each point by the electron. When the experiment is in the "past", the spacetime has snapped and relieved the stress. The path that the electron took is usually the path that had the highest stress (and therefore probability) but spacetime has fluctuations that can cause a path with a lower stress (and therefore lower probability) to snap as well. In physics, we understand very well the way that stresses effect spacetime. That is, we have equations that allow us to compute the stress at a point in spacetime given knowledge of the stress at other points in spacetime. These are called "Schroedinger's Equation", or propagators, etc. But we do not have a description of how it is that spacetime actually snaps, and chooses which path for the electron.

    So when you write "On its way there, lets say that the electrons wavefunction does not collapse," you are assuming that you can choose to look at the electron's passage as it is in the future, that is, as a wave, but when you write "While in this state, the second hand on the experimenters watch ticks forward as usual." you are possibly supposing that time, for the experimenter, is moving forwards, so that for the experimenter, the experiment becomes a part of the past. This is not possible because both the experiment and the experimenter are part of the same universe. Time must move forward for both at the same time. That is why it is so difficult for the experimenter to describe the action of measurement.

    I think that you can eliminate your confusion by splitting your understanding of "time" into two portions. Let the time of "spacetime" be the usual time of quantum mechanics. In that spacetime, there is never any measurement, and one can write down simple wave equations that tell how wave functions change with time (or more accurately, change with spacetime). The time of spacetime is bizarre in that there is no past, present or future in it. There is just, at best, a coordinate, t, whose actual value, plus or minus, is of no more importance than the coordinate x. That is, the values of t depend entirely on an arbitrary choice on the part of the physicist.

    In the time of spacetime, there is no past, no future, and no present. This is what Einstein referred to near the end of his life when he said that according to physics, there is no particular point in time that is the "present". This sort of time is completely contrary to our intuitive understanding of time, but it is very useful in physics.

    The time of "past, present and future" can only be defined with respect to an observer. It is the passage of this time that defines the act of measurement. Measurement time cannot be described inside the equations of quantum mechanics. Instead, we use the rules of quantum mechanics to determine how wave functions spread in spacetime, and we use Born's probability rule to determine the probability of various possible measurements, mesurements that must occur over the interval of past, present and future.

    My opinion is that physics will not be complete until we have a good theory that explains how to get past present and future into physics. Right now, it's done with a probability postulate that is rather difficult to justify.

  4. Jul 9, 2006 #3
    As far as I know, spacetime itself does offer a notion of past, present and future. For any event (say event A) in spacetime, there is its future and past lightcones. Any spacetime points in those regions are the future and past of the event, no?
  5. Jul 12, 2006 #4
    Thanks very much for clearning that up Carl. I think that much of my confusion, as you managed to pinpoint, comes from my lack of physics background, which amounts to a couple of university classes 10 years ago...

    Your thoughts actually reminded me of something that a prof told me which I had forgotten. He said that the quantum level intuition can be a real hinderance and one really needs to trust the physics and math to shed any light down there.

    One thing that I didn't know was that fluctuations in spacetime were a factor in causing a path with a lower stress to snap as well! As far as probability waves go I always considereded them as a mathematical representation of the electron's movement and not really as a description of the "stress" at any given point in space-time. Your analogy was very effective though.

    Thank's again for taking the time that you did!

    Last edited: Jul 12, 2006
  6. Jul 12, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, RELATIVE TO AN EVENT, there is a clear notion of future and past of course, but the event itself already contains "a time tag". If you take another event (for instance in the future lightcone of the first event), then the notion of "future and past" are different.

    I think that what was said here, is that there is, contrary to the Newtonian notion, no absolute concept of "present", because, exactly, you have to refer to an event in order to classify other events.
    For instance, saying that Julius Ceasar lived in the past is, relativistically speaking, a meaningless statement, while it makes entirely sense in Newtonian (and commonsense) views. Julius Ceasar "is still there" on the same level as you are, reading this post on PF right "now", because it's just a big bag of events. You can say that Julius Ceasar is in your past lightcone, but you can just as well say that you reading PF is an event in Ceasar's future light cone. So, past, present and future are now concepts that are "observer event dependent".
    This was not the case in Newtonian physics: there was supposed to be a physical status to "present", and "past" and "future" were ontologically non-existent concepts: the past "was gone", and the future "still had to come", objectively.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook