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Classical vs Quantum Mechanics

  1. Sep 25, 2005 #1
    I'll get straight to the point - my knowledge is based on Newtonian physics, I have not been introduced to quantum stuff in a classroom setting, only in casual conversation. I am in physics 111 at a university (calc-based), so keep in mind that I have not been introduced to most of the topics I’m delving in to. Still, I am very interested (and intrigued) by this sort of physics, and have a few elementary questions that are way above my head.

    I have two questions that relate to each other;

    Q1: According to Newton's laws, perpetual motion is impossible. However, what I do not understand is the force that drives subatomic particles. What force is exerted on electrons to make them move about the nucleus? I understand things work differently at this level (where quantum mechanics comes in), however to me it just doesn't make sense.

    Q2: I remember always being told that light has mass - this is why light cannot escape a black hole. Einstein said that light accelerates instantaneously to 299,792,458 m/s. He also said that if you were to hop on a bike and petal to 9/10ths the speed of light, the light will still travel away from you at the speed of light. This makes sense, but what I don't understand is what determines that speed, and also what force is expended on light to propel it to that speed in any situation?

    Obviously, Newton’s laws no longer apply at this level. I would like to at least understand the vague concept behind my questions. But then again, I ask a complicated question, I should get a complicated answer….so me asking to put this in layman’s terms is like telling you to convert apples to oranges. Replies are greatly appreciated

  2. jcsd
  3. Sep 26, 2005 #2


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    the answer isn't complicated, but there is some insight required. i might suggest that you look at the Wikipedia article on Planck units: http://en.wikipedia.org/wiki/Planck_units . that speed of propagation (of either E&M or gravity) of 299,792,458 m/s is a number that is totally dependent on the units (meter and second) that it is expressed it. but when we measure anything, we ultimately only measure dimensionless numbers. (When one commonly measures a length with a ruler or tape-measure, that person is actually counting tick marks on a given standard or is measuring the length relative to that given standard, which is a dimensionless value. It is no different for physical experiments, as all physical quantities are measured relative to some other like dimensioned values.) that numerical value of 299,792,458 is only a consequence of the units of length and time we humans have decided use.

    In Planck units the speed of light is always 1 Planck length per Planck time. if some God somehow changes [tex] c [/tex], it's still 1 Planck length per Planck time. so the question is: "why is there 5.39121 x 10-44 seconds in a Planck Times and why is there 1.61624 x 10-35 meters in a Planck length? in other words, why did we choose our unit time and unit length to have the reciprocals of those two numbers -- to have that many Planck times in a second and to have that many Planck lengths in a meter?

    now, i don't know why an atom's size is approximately [tex] 10^{25} l_P \ [/tex], but it is, or why biological cells are about [tex] 10^{5} \ [/tex] bigger than an atom, but they are, or why we are about [tex] 10^{5} \ [/tex] bigger than the cells, but we are and if any of those dimensionless ratios changed, life would be different. but if none of those ratios changed, nor any other ratio of like dimensioned physical quantity, we would still be about as big as [tex] 10^{35} l_P \ [/tex], our clocks would tick about once every [tex] 10^{44} t_P \ [/tex], and, by definition, we would always perceive the speed of light to be [tex] c = \frac{1 l_P}{1 t_P} \ [/tex] which is the same as how we do now, no matter if it could conceptually be changed to another speed.
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