# Quantum Mechanics or Classical Mechanics? [closed]

I'm just a student of grade 11 but, I was interested in knowing about Physics much deeper. In order to start my interest in Physics, I watched this video of Quantum Physics NOVA : Quantum Physics(NOVA)

I have some questions in mind :

$\bullet$ Is Quantum Mechanics also applicable for massive objects like Human Beings, Animals etc. (in comparison with the small atoms) ?

$\bullet$ How much does Quantum Mechanics affects the rules that we have studied yet, like in Newtonian Mechanics and Classical Mechanics ?

$\bullet$ Has Quantum Mechanics been successful in defining the movement of atoms and other negligibly small objects like sub-atomic particles etc. ? Or is there something yet to come...?

$\bullet$ While, I know that any theory is simply NOT based on calculations or only on experiments. Has Quantum Mechanics been able to define all the rules with the help of Experiments etc.?

$\bullet$ As a student, I'm basically confused that which theory is the most accurate and applicable in real life. So, which theory is the most accurate and applicable to the reality?

$\bullet$ After watching the video (linked at the start), I was not able to conclude whether Quantum Mechanics is based on accuracy or on probability ?

$\star$ I'm sorry, I understand that these questions are much broad and may be concluded as foolish or meaningless questions, but, as a student these were the questions which came in my mind. I will appreciate a lot if anyone can help me in clearing my doubts!

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## closed as too broad by Chris White, Kyle Kanos, Brandon Enright, BMS, robJun 27 '14 at 17:32

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

Keep in mind that this site wants single questions at a time. I will answer with simple answers.

∙ Is Quantum Mechanics also applicable for massive objects like Human Beings, Animals etc. (in comparison with the small atoms) ?

You will learn if you continue in physics that the underlying framework of nature as far as we have explored and validated is quantum mechanical. Are bricks applicable to buildings?

∙ How much does Quantum Mechanics affects the rules that we have studied yet, like in Newtonian Mechanics and Classical Mechanics ?

Classical theories are self contained and their overlap with quantum mechanics comes where classical theories fail, as in the photoelectric effect, and the black body radiation. Otherwise classical theories emerge smoothly from quantum mechanics when dimensions get larger than the nanometer scales.

∙ Has Quantum Mechanics been successful in defining the movement of atoms and other negligibly small objects like sub-atomic particles etc. ? Or is there something yet to come...?

Yes it has, as you will find if you continue your studies. It has never been falsified by any experiment to date.

∙ While, I know that any theory is simply NOT based on calculations or only on experiments. Has Quantum Mechanics been able to define all the rules with the help of Experiments etc.?

Theories describe nature if they are validated by experiments. Even one false prediction requires a redrawing of the theory

∙ As a student, I'm basically confused that which theory is the most accurate and applicable in real life. So, which theory is the most accurate and applicable to the reality?

In real life classical mechanics, electrodynamics, thermodynamics suffices unless you want to understand transistors, quantum computation etc. You have to learn the math to deal with the underlying quantum mechanical framework

∙ After watching the video (linked at the start), I was not able to conclude whether Quantum Mechanics is based on accuracy or on probability ?

The theoretical predictions of quantum mechanics are probabilistic, they give the probability of getting a specific measurement. Classical theories predict for example trajectories, whose indeterminacy depends on experimental and calculational errors. In quantum mechanics there is a basic indeterminacy for any measurement coming from the basics of the theory but that does not mean that there are no predictions that can be falsified.

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Is quantum mechanics for massive objects, like human beings?

In principle, we can write down the Hamiltonian for virtually any system, even if it is composed of atoms on the order of $10^{20}$, and solve the Schrödinger equation,

$$H\lvert \psi \rangle = E \lvert \psi \rangle$$

However, in practice, it would be extremely tedious, and currently modern computers are simply not capable of solving the equation numerically. In addition, it is not illuminating to even know $\psi$ for a system as complex as described.

How much does quantum mechanics affect the rules we have studied, e.g. Newtonian mechanics?

As we incorporate further realism into our physical models, e.g. by including relativistic effects, quantum mechanical effects, we potentially increase the accuracy of our predictions. However, one should always pick the most appropriate for a problem - e.g. don't apply QM to a football trajectory. One formulation does not 'affect' the other, but rather reveals its short shortcomings.

Has quantum mechanics been successful in defining the movement of atoms and other particles?

The quantum mechanical model of the hydrogen-like ion is arguably the most famous, and offers quite accurate predictions of, for example, the energy levels of the electron. The approach is vastly different from classical physics; the electron does not follow a fixed orbit. Instead, we assign probabilities for the electron to be within a particular volume; specifically,

$$P(V)=\int_V \mathrm{d}^3x \, |\psi|^2$$

for a normalized wave function, $\psi$, which is the solution to the equation,

$$E\psi = \left( -\frac{\hbar^2}{2m}\nabla^2 - \frac{Ze^2}{4\pi \epsilon_0 r}\right)\psi$$

For more complex atoms, or even molecules, numerical approximation is required. A popular approach is perturbation theory, whereby we model the current system as being a soluble system, i.e.

$$H=H_0+\lambda H_I$$

where $H_0$ is the known system, $H_I$ is the perturbation, and $\lambda$ determines the strength of the perturbation. (If dimensionless, $\lambda < 1$ suggests a weakly perturbed system).

Regarding accuracy and probability. One could argue quantum mechanics is a more accurate model of Nature. Often, it may be the case that the predictions are also more precise. However, if we attempt to model a complex system with quantum mechanics, and have to resort to poor numerical approximations, the predictions may be worse than had we applied a simpler model - possibly.

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Hello JamalS ! Thanks for answering my doubts, but, I'm just a 16 year old beginner... so, may be I will need time to understand this all. Just wanted to let you know, that I have looked at it. :) – Kushashwa Ravi Shrimali Jun 27 '14 at 14:31
Also, in the video there was something mentioned like.. connection between two objects.. I don't know what does it spell actually, can you help in this? – Kushashwa Ravi Shrimali Jun 27 '14 at 14:32
@KushashwaRaviShrimali: The video is 50+ minutes long; please tell me what part you need clarified. – JamalS Jun 27 '14 at 14:33
intangle particles ... something like this :( Something about 35:00 minutes. – Kushashwa Ravi Shrimali Jun 27 '14 at 14:34
@KushashwaRaviShrimali: Quantum entanglement :) That's really for a separate question; it would not be appropriate to update the answer. – JamalS Jun 27 '14 at 14:35

Yeah, Quantum mechanic is applicable to massive objects also but the result can be neglected in daily life. However, it should not be forgotten that quantum mechanics is a key to explore extremely massive universe. Black hole thermodynamics, Prediction of galaxies, exoplanets and so on include quantum mechanics well. Newtonian mechanics and classical mechanics are completely different from Quantum mechanics. Newtonian mechanics is based on fine prediction of experiments while Quantum mechanics doesn't accept it. I mean, the results from Newtonian mechanics and Quantum mechanics are different. And of course, quantum mechanics is able to define the inner world of atom. Today, quantum mechanics is one of the best theory that has survived any experimental results while Newtonian mechanics fails. So, quantum mechanics is correct.

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Thanks a lot Newera for your help. – Kushashwa Ravi Shrimali Jun 27 '14 at 14:43
You can't really say 'quantum mechanics is correct,' because it neglects relativistic effects - that's why we have quantum field theory. So, I'd suggest you use a different phrasing, because the correctness of a theory is not universally defined, and every theory has its limitations. – JamalS Jun 27 '14 at 14:49
Since when is QM used to study galaxies or exoplanets? – Chris White Jun 27 '14 at 14:50
I'm not sure that anything in this answer is correct. – Kyle Kanos Jun 27 '14 at 16:36