Is Quantum Mechanics (QM) universal?

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Is Quantum Mechanics (QM) universal?

Post by Inquisitive on Sun Aug 28, 2016 1:52 pm

Sir, in his lecture2 notes has asked Is there any physical change which does not involve QM?
My plausible answer is :
QM is the science of the small. Since everything is composed of molecules it can be said that there is no physical change in which QM is not involved. But since the matter around us is very large, classical mechanics is thought to be a good approximation of QM.
Is it correct?

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Re: Is Quantum Mechanics (QM) universal?

Post by Keep Guessing! on Sun Aug 28, 2016 11:38 pm

Though I agree with you, its a bit confusing as to how the approximation happens. An electron has some uncertainty dx associated with it, but a macroscopic object, which comprises of many electrons, has a much lesser uncertainty. So instead of adding up, the uncertainties gets cancelled out. Or is it that they act as vectors?

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Re: Is Quantum Mechanics (QM) universal?

Post by Inquisitive on Mon Aug 29, 2016 7:04 pm

I think the value of plank constant (h) is the reason why uncertainity is less in macroscopic world as when u divide h by mass of electron, the value is still considerable. but when u divide h by 1 kg.. its too small to consider. So its not adding up of uncertainties, rather the value of h.

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Re: Is Quantum Mechanics (QM) universal?

Post by Sabiya on Mon Aug 29, 2016 9:03 pm

Actually, Keep Guessing!, the electrons in atoms and molecules are present in specific orbitals around the nucleus which are of many different shapes. The uncertainty in position of electrons is still there, but here there is the nucleus, which attracts the electron towards itself. Hence the electron is VERY likely to be found near the nucleus in these orbitals. Therefore the all the electrons of the atom are mostly present in a certain volume, and this volume is very small in comparison to the things that our eyes can see. Hence we see mostly 'solid' things and not hazy clouds.

Also, Inquisitive, I agree with what you are saying, but I think that uncertainties of the microscopic constituent particles do affect the macroscopic objects they make up. This can be understood by trying to answer the question, "What is your de Broglie wavelength if your body is at rest?"
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