Differentiate physics from other branches of science?

Answer 1

Classical mechanics and quantum mechanics are subfields of the branch of physics called mechanics, that deal with two realms of size, the big and the small, respectively. The border between big and small has not be scientifically defined yet, but almost every object we deal with can be assigned to a respective group (i.e. galaxies, stars, planets, people, ants, and dust particles are all big. Atoms, quarks, photons and electrons are all small).

Classical mechanics is a set of physical laws and their corresponding equations that describe/govern the motion and interaction of big bodies within the universe. These equations are Galilean invariant which means they do not apply to non-inertial reference frames. Classical mechanics is sometimes still called Newtonian mechanics because it's basis is on the work of Isaac Newton. Classical mechanics is an approximation of General Relativity in a weak gravitational field.

Quantum Mechanics is a set of physical laws and their corresponding equations that describe/govern the motion and interaction of small bodies within the universe. Quantum mechanics as we know it is the Copenhagen Interpretation which has a set of several main principles . There are two widely taught formulations of QM, the wave formulation (Schrodinger), and the matrix formulation (Heisenberg).

In the most general sense, the equations that describe a Baseball being thrown cannot describe an electron in an accelerator. Likewise, the equations for the electron cannot describe the baseball. There is a small caveat here though, the quantum statistical expectation value of the position and momentum obey Newton's laws on average.

Answer 2

Classical: Optics, Mechanics, Wave Physics, Electrodynamics/Magnetism, Thermal, Electricity. Special and General Relativity can fall into both depending on the depth you go into... this can be said for many of the different branches though However many of these have cross-overs, i.e. Quantum Electrodynamics - See work by Feynman, Quantum Optics etc Modern Physics is more hazy... Particle, nuclear, atomic, Quantum. The definition of modern physics could be generalized to anything that involves the quantization of energy (lumps of energy, rather than continuous). Many problems in modern physics are more accurate (not perfect) descriptions of classical things. For example, Newtons laws of motion work in everyday use, but when you get to about 5% of the speed of light the relativistic effects come into play. So although the Newtonian stuff is an approximation it works very well as long as the velocity is much less than c. Good examples of the segregation of modern and classical physics are things like the photoelectric effect (Einstein's nobel prize winning work) which shows how light must be a particle. Also, Young's' Double Slit experiment shows how light is a wave.. Hence, Wave-Particle Duality purposed in De Broglies (ridiculously short) PhD thesis.

Answer 3

In QM, you can't know for certain both the momentum and position of an object. However, in CM you can. For instance, take a chair sitting still in a room. It is still, so it's momentum is zero. You are looking at it, you can go over and measure all its dimensions with a ruler and relative to other points, so you know its position. You know the momentum and the position of the chair completely. The chair won't suddenly become larger, for example. However, this level of certainty would not be possible in QM.

Here is a general rule of thumb: What separates a QM problem from a CM problem is the applicability of what is called Planck's constant, denoted by h. If the values of the problem (mass, etc) are on a similar size scale, then you have a QM problem. On the other hand, if the values of the problem make h look ridiculously small when compared to them, then you have a CM problem. As you could probably guess, h has a small value.

Answer 4

Modern physics deals with photoelectric process, quantum physics, electromagnetism, ext. whereas classical physics deals with kinematics, rotational dynamics, simple motion, gravitation, ext.. In other words: "modern" physics deals with relatively new concepts whereas "classical" physics deals with older concepts.

Answer 5

Physics is the science of everything in the universe. And then by default all the other sciences are categories of physics.

Answer 6

One of the important concepts of modern physics which Aristotle didn't use is experimentation. While modern physics relies on experiments, Aristotle mostly philosophised.