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

Implications of Quantum physics

  1. Jan 8, 2010 #1
    I just wanna make sure I understand some of the greater implications of discoveries made by quantum physics experiments and theories like the double slit experiment and Schroedinger's cat.

    Because observation effects results, it becomes clear that an observer cannot extract themselves from the system, and are indeed a part of it.
    True, yes?

    And I'm curious what people think about the implications of this for science in general. I mean sure, these things more or less disappear when you reach macroscopic levels, right? But then, what about more complex systems?
    Is it possible to observe biological systems without effecting it? Do effects actually matter?

    Really I'm interested in this because a recent criticism I've heard of science is that it has objectivity, which really doesn't exist, built in as an assumption. Do you think that's true, and does it matter? If not why not? Is thinking that we can be separate from things an unhealthy habit to get into?
    I'm certainly of the opinion that even tiny bits do add up and make a difference, and that even subtle differences are compounded over time/space to eventually have large effects.
  2. jcsd
  3. Jan 8, 2010 #2


    User Avatar
    Gold Member

    I would like to note first that similar thing you have with relativity and so you can not strictly say that observation problem disappears at macroscopic level.
    But I don't think there are much of implications for science in general because if you step outside physics this is not something really new.
    However even if objectivity is always conditional it does not mean that we can not acquire useful knowledge.
  4. Jan 8, 2010 #3


    User Avatar
    Science Advisor
    Gold Member

    Sometimes the "objectivity" component is not that critical to the outcome, and other times it might be. In those cases, it may indeed be necessary to consider the observer as part of the system and make allowances for that. I know that many medical experiments must consider the role of the observer carefully, for example, and that is why there are double blind studies. I certainly don't think that invalidates all science, so it depends on the context.
  5. Jan 8, 2010 #4
    The OP gets into EPR concepts, does he not?
    Also, wasn't it Dirac (or Heisenberg) who called the arbitrary limit between the quantum and the classical, the "blade", or something like that? In any case, I forget the exact term he used...
  6. Jan 8, 2010 #5
    "Observer" implies that we need a human observer. This is not true! But a quantum experiment does require a measuring device, as well as a measured result. In fact, all parts of the experiment are non-separable and must be considered as a single entity.

    In classical physics, which, in general, describes the behavior of macroscopic systems, the value of the measured quantity is not affected by the measurement process. Here, the different parts are separable and can be discussed independently.

    Classical physics is objective! We believe there is a real universe outside of our measuring devices. This mechanical universe is well described by the laws of Newton, Maxwell, and Einstein, among others.

    Quantum mechanics, on the other hand, is a purely statistical theory. It does not describe physical processes. Rather, it only gives the possible results of a measurement and the probability distribution of those results.

    This is a classical concept - that the macroscopic properties are determined by microscopic events. Not so in quantum mechanics! As far as we know, there is no underlying system that gives rise to the statistical nature of quantum phenomena.
  7. Jan 9, 2010 #6
    I meant both really.

    So in Classical physics, your measuring deviced doesn't exist. And Quantum mechanics the whole thing is not even connected to reality at all? I don't get what your saying, can you be a little more clear?

    I mean... isn't there some thing, like what Anderson was talking about, where you kinda have to switch rules... like it's not unified, so neither is true cuz people can't figure out how to make what quantum physics says apply to bigger stuff (and the bigger stuff is 'close enough')?
    Anderson, I dunno what those crazy acronyms you're talking about are.

    Dr.Chinese, you said you don't think it invalidates science: What is the goal you're talking about? Invalidates it for what?
    I guess the sort of thinking I'm talking about is a bit unusual in it's goal. The goal perhaps being to experience reality to it's maximum and come to a true understanding, perhaps visceral (not simply conceptual, but complete) understanding, and act on that understanding.

    I hear you guys saying sometimes it matters and sometimes it doesn't... is that to say that the world isn't really connected? They are kinda like... two worlds - one where the observer matters, and one where it doesn't? I tend to see the world as connected...
    Am I misunderstanding something?

    Is it inaccurate to say that all other sciences essentially rest on the bedrock that physics provides? Sure, they might not thing about it but their paradigms assume all the things that physics tells them about the universe?
    Last edited: Jan 9, 2010
  8. Jan 9, 2010 #7


    User Avatar
    Science Advisor
    Gold Member

    Here is the problem. Some folks want to know WHY the universe is the way it is. That may not be a truly scientific question in the sense that any answer is necessarily speculative. On the other hand, a theory can be a great theory without answering the WHY questions. Instead, it is judged based on its utility.

    In quantum mechanics, the observer apparently has a role in determining reality. This is the essense of the EPR paradox, which has been decided in favor of QM. And the observer is considered as a fundamental to the outcome.

    QM is a useful theory, perhaps the greatest of all time in that respect (utility). So if you seek a scientific answer that meets criteria that the theory does even attempt to cover (answers to the WHY questions, or answers that are observer-independent), you may go away unsatisfied at this time. Science is measured by relative progress rather than in absolute terms. Do we know more today than yesterday? The answer is YES, and that is why the science remains valid. Ultimately, the critique is not valid unless someone can show a better way - in which case even more progress is made!
  9. Jan 9, 2010 #8


    User Avatar
    Science Advisor
    Gold Member

    Classical physics is limited in scope to those areas in which quantum-level results can be ignored. Obviously, the universe is in fact dependent on the observer at the quantum level, so the classical view of an objective universe is problematic in this regard. See the EPR paradox.

    QM does not postulate a physical model to accompany its mathematical formalism. You could say the same about General Relativity and it would be equally meaningful (although I guess you could say there is "something" of a physical model for both QM and GR). But so what? Are you implying that classical theory is somehow superior to QM? I hope not.
  10. Jan 9, 2010 #9
    The 'why' / 'what makes this do this or that' are the things that I have the most interest. To me, without understanding those things, the rest of it is just getting things to be utilized and used, to some extent.

    Teachers have to teach what works and those things that work best.

    I think that some theories have more problems than others; and, some have several 'accepted' things that may not be right, but are 'accepted' to the point that those things are taken with more truth than maybe they should be and the rest of that theory is then skewed to some extent.
  11. Jan 9, 2010 #10
    Measuring devices are necessary components in both classical and quantum experiments. But quantum events are fundamentally different from classical ones. We observe both kinds of events and it is important that we understand the difference between them. We must especially accept the fact that quantum objects do not move through space-time according to the laws of Newton and Einstein.

    In classical physics the measuring device is separate from the classical object. Assume we intend to measure a particle’s momentum. The momentum has a value whether or not we measure it; It doesn’t matter whether the measuring device is on or off. The object has a value for momentum even if we don’t measure it.

    In quantum physics, on the other hand, the measuring device and quantum object are not separate entities! The momentum has no value until it is measured. In the original EPR paper, it was assumed that a particle behaves classically, i.e. it had momentum prior to being measured - and the results were erroneous. This seems strange to most of us, but all it really means is that quantum particles do not behave in classical ways.

    The reality of the quantum world is not the reality of the mechanical (classical) universe. You are correct that quantum mechanics makes no connection with (classical) reality. For example, a quantum experiment does not tell us how a quantum particle gets from its source to the detector. But, by repeating the experiment many times we get all possible results for that experimental configuration and we get the statistical distribution of those results. That’s it! That’s all there is! And that is exactly what quantum mechanics gives us; for a given experiment we can predict the possible results and the statistical distribution of those results.
  12. Jan 10, 2010 #11
    How does utility come in without a why? Doesn't utility preclude the why? So, what kinda utilities are you talking about here?

    Progress towards what?
    I guess the goal is not defined,but remains to be set by the user?

    Eaglelake/Dr.Chinese... I'm a bit lost as to the usefullness of such quantum physics. I guess quantum computers and the eventual bridging of that gap is it's utility, yea?
  13. Jan 12, 2010 #12
    Just because quantum mechanics is purely statistical, and does not describe "physical reality" as classical mechanics does, does not mean it is not useful. Although we can only predict probabilities, much of our modern technology is based on quantum mechanics. For example, you can argue, and I would agree, that we have no idea of what is going on inside a nucleus. But quantum mechanics provides a statistical model that "tells" us how to build nuclear reactors as well as bombs, among other things. Powerful stuff, indeed!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook