
At which point do Newtonâ€™s physics laws differ
from Quantum Physics? 
Question Date: 20170609   Answer 1:
You may have a better understanding of Quantum
Physics after you learn about such concept in
college. It is probably not easy to explain in
just a few words. Basically you can view the
classical mechanics as a derived theory from
Quantum Physics when the quantum effects
only play a small role. There is a good diagram
showing the domains in wiki:
classical mechanics .
Basically in the macroscopic world with
relative low velocity, the classical mechanics
dominate. But in the microscopic scale, it is
where the quantum effects are not negligible.
One key point to differentiate the Classical
Mechanics and Quantum Physics is the difference
between determinism and probability. In the
classical world, you know the past then you know
the future. While in the Quantum world, the
knowledge of the present will only determine the
probability of the state in the future.
There is a simple rule to tell that if you need
a plank constant "h" in the formula, then
you are in the quantum world; otherwise the
classical theory would be fine for description.
As for the plank constant h, you may learn it
in the future.
Best,
  Answer 2:
Reality deviates from Newtonian physics at all
scales. Because the effects of quantum
mechanics are most important at small scales, the
deviation due to quantum mechanics gets larger the
smaller you get, however. Generally speaking,
for most physical principles, you need to get
on the scale of a nanometer or below for quantum
effects to become important. This said, there
are visible effects of quantum mechanics on large
scales: for example, ice is less dense than water,
which is unusual for solids to be less dense than
their molten form. The reason why water is
weird is because of quantum mechanics, and you
can see that when you drop an ice cube into a
glass of water  or, for that matter, an iceberg
that a ship might crash into.
You didn't mention relativity, but it's
also a place where Newtonian physics breaks down.
Like quantum mechanics, relativity exists at
all scales, but the effects only become
significant at very high velocitie (special
relativity), or over great distances or with
enormous masses (general relativity).
  Answer 3:
There's a nice diagram in this wikipedia
article about Classical Newtonian Mechanics vs
Quantum Mechanics:
classical mechanics
The diagram shows that:
1. Classical Newtonian mechanics deals with
things that are larger  generally large
enough to see, and quantum mechanics deals with
things that are tiny  a nanometer or less, which
is the size of atoms.
and:
2. Classical Mechanics deals with things that
move somewhat slowly, while quantum mechanics
deals with things that move at something like the
speed of light.
  Answer 4:
Quantum mechanics starts to dominate over
classical, Newtonian, mechanics, when the mass of
a particle gets down to small scales, such as the
range of 10^{9} meters.
We know that light acts both like a wave and
a particle. This is true for massive
particles, too. However, these waves are not quite
like sine and cosine waves, but more like
probability distributions, looking like a
bell curve. A wave for a massive particle would be
the probability of finding that particle in a
certain region. For very very massive particles,
this probability distribution becomes so small,
that we say the particle is most definitely in one
place, and not distributed over some region.
When the mass of a particle gets to smaller
ranges, we start to see probability
distributions. These depend on the potential
and kinetic energy of the particle. as a low
energy particle will want to be in areas of lower
energy.
I hope this answers your question on quantum
mechanics. Visualizing this property of mass is
very counterintuitive, however it is key to
understanding the basics of atoms, electron
structures, and optics!
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