Modern Physics vs Classical Physics

Senjai

Just curious, what's the difference?

I was told be someone that modern physics has ways of accurately defining what a force is, while classical physics does not define forces, only acceleration in terms of force and mass.

Again, just curious.. What are the major differences? Is one wrong? (similar to how high school students were taught the bohr model earlier to discover it was wrong later.)

Senjai

dmtr

Correct me if I wrong in my summation. Modern physics deals mostly with the dynamics of the space, classical - dynamics in the space.

Pythagorean

one simple difference is that classical physics deals with slow, big things (from bullets to planets) while modern physics was required for smaller, faster moving things (like electrons).

GR is a counter example, of course.

arunma

There are a few varying definitions of what separates classical physics from modern physics. Some day that everything pre-1905 is classical, and everything afterwards is modern. The defnition I prefer, and have heard most often, is that every branch of physics that does not utilize a quantum treatment is classical, and any physics that is based on quantum mechanics is modern. So classical physics includes mechanics, E&M, the kinetic theory of gases, optics (including the whole wave/particle duality thing), and special and general relativity (yes, I've heard a cosmologist call GR a classical theory). Modern physics then includes non-relativistic quantum mechanics, quantum statistical mechanics, nuclear/particle stuff, and field theory.

Regarding forces, I think you might have heard slightly incorrectly. In quantum mechanics we actually don't talk about forces at all. In non-relativistic quantum mechanics, potentials (potential energy and the magnetic vector potential) play a more direct role than in classical physics. And in relativistic quantum mechanics, or quantum field theory, we get rid of potentials altogether and talk about fields. It's actually somewhat difficult to do a quantum mechanics calculation while talking about "forces," without fudging the formalism somewhat.

Pythagorean

The experimental history of light is another interesting case that betrays my simple assertion above.

And non-linear (chaos) dynamics perhaps too.

bbos

Classical physics is based on Newtonian physics where as modern physics supplements upon that with general relativity and quantum mechanics