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Classical Physcics VS Quantum Physics

  1. Mar 10, 2010 #1
    Hello people, its been a while since i used physics forums. My assignment is basically a comparison between classical physics and the Quantum physics and to explain why we don't use classical physics. While surfing on the web, i came across a text implying that the definitions of light, energy and matter for classical physics are outdated. When i searched for why they were outdated, I either found nothing or something that made no sense to me. For example;
    Energy- what i dont get about the classical definition of energy is that it states "In classical physics energy is considered a scalar quantity, the canonical conjugate to time." I get that it is considered to be a scalar quantity, but what the heck is a canonical conjugate? I searched everywhere and can't find the definition. I know that it shares a special relationship to time, but dont know what sort of relationship that is. Any help would be very much appreciated. (also, if anyone can give me more info as to why Quantum physics is better than Classical Physics that would be much appreciated also)
  2. jcsd
  3. Mar 11, 2010 #2


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    Whoa, hold on. People do use classical physics all the time. :tongue2: Well, I'm using the definition of classical physics being pre-quantum and pre-relativity physics. But by that definition, one can successfully argue that most engineering, science, and technological endeavors to this day are accomplished using nothing but classical physics.

    Quantum physics, special relativity and general relativity are pretty much only used if necessary. Generally speaking, quantum physics is used if you are dealing with something really, really small. Special relativity is used if you are dealing with something light, but moving very, very fast. General relativity can deal with things that are extremely, extremely dense, very massing, and/or traveling very, very fast. But for everything else -- which is most everything else -- classical physics is commonly used. (There are a couple of exceptions that I can think of. One is modern chemistry, which is based upon quantum mechanics. Another is the manufacture of semiconductor devices [e.g. computer chips, etc.] which is also based on quantum mechanics. Global positioning systems [GPS and GLONASS] account for both special and general relativity, although that's not necessarily true with other satellites.)

    In general, unless you are working with the extremely tiny, extremely fast, or extremely dense, classical physics models do just fine, and might be the best choice for the task at hand.

    Hmm. I'm not sure what to make of that quote. From my knowledge, energy is a scalar in all branches of physics; classical and quantum alike.

    Canonical conjugates are Fourier transform pairs. It means that if you have a function representing one of the conjugate pairs, you can take its Fourier transform to get the other conjugate pair. But the canonical conjugate relationship between energy and time is a quantum mechanics property -- not classical. Classical mechanics doesn't have this Fourier pairing between energy and time. Canonical conjugate pairs are things which are subject to the Heisenberg uncertainty principle.

    This quantum mechanical pairing between time and energy can be interpreted multiple ways. But it often means that it is difficult to determine something's energy over a short period of time. But given a longer amount of time, its energy value is more certain.

    Other canonical conjugates exist in quantum theory. Position and momentum are popular ones.

    I wouldn't necessarily say that it is better, just different.

    Quantum mechanics is by far more accurate, particularly when it comes to the world of the tiny. Quantum mechanics has succeeded every time its predictions have been put to the test in the laboratory.

    But the standard model of quantum physics, at least as we know it now, doesn't involve gravity. Gravity is completely ignored and unexplained (there are many physicists exploring new, potential theories linking quantum thoery and general relativity. But no such complete theory exists just yet. Some are promising, but none yet complete). So obviously, since we experience gravity every day, and the [quantum] standard model can't yet explain this, quantum theory is obviously less than perfect.

    Quantum physics is also very complicated mathematically, compared to classical physics. So if you are looking for a predictive model of some aspect of the world, and that aspect is not tiny enough to show noticeable quantum effects, classical physics might be your best choice to get the job done.
  4. Mar 11, 2010 #3
    Wow, thanks collinsmark:cool:. Looks like I'll be heading for an A. Just one more question, who contributed the most towards classical physics besides Newton and what did he/she do?
  5. Mar 11, 2010 #4
    also, does classical physics work on a molecular level or a macroscopic level? my classmate and I have been debating on this issue for quite a while.
  6. Mar 12, 2010 #5


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    Oh, my. That's quite a loaded question. :uhh: I don't feel qualified to answer that. Perhaps an Internet search on classical physics history might get you started in the right direction.

    But without putting much thought into it, certain names pop into mind such as Leonard Euler, Joseph Louis Lagrange, William Rowan Hamilton, and Johann Carl Friedrich Gauss. Although technically, most of these people are considered mathematicians, they did change the face of classical physics. But there are so many others too.

    Oh, and there's Benjamin Franklin to thank (or to blame) for the convention of positive vs negative charge (Oh, not the glass, please give the silk the "positive" sign! Oh, darn. Too late. :cry:)
  7. Mar 12, 2010 #6


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    You'd probably want to include Michael Faraday and James Clerk Maxwell, since classical electrostatics and electrodynamics are part of classical physics. And then there's classical thermodynamics ... so you'd want (at minimum), Lord Kelvin (William Thompson), Rudolf Clausius, Ludwig Boltzmann and of course Josiah Willard Gibbs. I'm sure I'm forgetting a bunch, but at least your list is longer now :wink:
  8. Mar 12, 2010 #7


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    The makeup of atoms themselves, and anything smaller than that (i.e. subatomic particles) are inherently quantum. Classical physics totally falls apart when modeling electrons and nuclei, and anything at that scale and smaller.

    Molecular bonds between atoms can be considered to be somewhere on the dividing line between quantum and classical physics. it depends on the situation. Quantum effects are certainly present, but depending on the situation, classical physics might make good approximations, sometimes.

    Interactions between molecules that do not involve breaking chemical bonds, and anything at a larger scale that that, are often done using classical models. At that scale, molecules are sometimes modeled as just a whole bunch of polarized balls bouncing around. But there exceptions at this scale where the consideration of quantum effects are necessary, such as estimating the leakage current through the gate of a very small transistor.

    Of course this is all very general. There are other notable exceptions too (superconductors, super-fluidity, lasers, etc.). The above is just a rule-of-thumb, so to speak.
  9. Mar 13, 2010 #8
    thanks guy, :smile:
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