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Thread: Quantum mechanics

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    Senior Member Tim's Avatar
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    Quantum mechanics

    Does anyone here have much of a grasp on Quantum and Newtonian mechanics and some of the math behind them?

    I have a question.

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    Subscriber and Workshop Member MGrayson's Avatar
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    Re: Quantum mechanics

    Fire away....

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    Senior Member Tim's Avatar
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    Re: Quantum mechanics

    I've searched google but never found an answer or the question.

    If Quantum mechanics and hence its math is only applicable at sub-atomic sizes and Newtonian maths used in "the real world" at what size point does one set of maths stop being applicable and the other apply?

    The obvious answer is at the atom level but that is not a clear answer, even if it it correct.
    I have always been under the impression that Newtonian math is not 100% accurate or true any way.

    Can you educate a novice like me about this as brief as possible?

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    Subscriber and Workshop Member MGrayson's Avatar
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    Re: Quantum mechanics

    What makes a model applicable? It has to 1) make a prediction that is measurably different than a simpler model. And 2) you have to be able to compute what the model predicts. Quantum mechanics, at least the QM theories that are presently useful, are true as far as we know everywhere except in very strong gravitational fields. But as the system gets larger, QM fails on both 1) and 2). How large, and which one fails depends on the experiment. Model a baseball with QM and you'll get answers indistinguishable from Newtonian mechanics. But even single atoms are too complex to analyze if you want to predict their magnetic properties (I may be out of date on that one. It was too hard 20 years ago).

    But there are glaring macroscopic effects that need QM to explain. Here's a photography related one - How does light partially reflect from glass? Say 94% of the light gets through and 6% reflect. How does a photon decide? Newton knew that he didn't understand what was going on. What does "understand" mean in this context? I could say that partial reflection is a property of glass and that would be a perfectly good model. But quantum electrodynamics is a simple (to set up, not to calculate with) model of light and electron interactions that predicts perfectly well all optical properties of glass.

    As for 100% true, no model is 100% true, although not every model has actually been observed to fail. OK, General Relativity on a cosmological scale is confusing. Dark matter and dark energy may just be error terms. We don't know. The Standard Model (the name for the quantum field theory which accounts for all forces except gravity) has no observed problems. Yet GR and the SM are not compatible. They can't even be defined simultaneously. And if they could be, bad things would happen on extraordinarily tiny scales.

    I hope I haven't just confused you more,

    Matt

    BTW: Feynman's short book "QED: The Strange Theory of Light and Matter" is possibly the best physics book ever written. The glass reflection example is from there. It requires very little math, but cuts no corners on the physics. If you have ANY interest in quantum mechanics, get a copy!
    Last edited by MGrayson; 11th March 2016 at 03:42.
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    Re: Quantum mechanics

    Qm is basically an approach to understand and calculate the behavior of matter and energy using mathematics based on wave propagation concepts, probability and statistics.
    It arose from the inability of classical physics to properly explain the behavior of light and eventually atomic particles.
    Some classic outcomes were the Heisenberg uncertainty priciple, which demonstrates a connection between the speed of light and the ability to observe both position and velocity: if you start to zero in on exactly where a particle is, it gets fuzzier, the act of observing interferes with the measurement. Another was the dilemma over whether light was particles or waves. How do particles pass through glass? How does a wave pass through a vacuum...what is waving?
    Great stuff, completely turning the world of physics around fro the late 1800's on.
    My grad school advisor was Bernt Craseman and one of my mentors was john Powell, who together wrote the "introduction to quantum mechanics", an interesting pairing of a theoretical and experimental physicist and a couple of great guys with a landmark textbook.
    Feynman however is legendary, wrote a great series of so-called introductory textbooks that I still read today, and a series of video lectures at cal tech. Truly a visionary who could explain the most complicated concepts in a very entertaining way
    Last edited by jlm; 11th March 2016 at 15:11.
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    Re: Quantum mechanics

    Thanks. There are also a couple of youtube videos in which Richard Feynman explains the light reflection observations very lucidly.
    With best regards, K-H.
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