I guess what I am trying to address is: Micro, laws of atoms and smaller.
Macro: laws such as motion of planets and gravity and so forth.
The laws of physics are universal. Hence the ‘macroscopic’ laws are what we call the “classical limit”.
This ‘limit’ is the description for a great number of “particles” (usually intended as ‘atoms’) where you can start to make certain assumptions.
For example for a great enough number of particles (might be as low as a few dozens of atoms) the difference between the ‘energy levels’ of the electrons and vibrations is so small that the discreteness of the levels can be approximated as a continuum.
For example if we had a whole golf ball in a ‘potential well’ also has ‘discrete levels’ but these are so small (not even measurable and also they are especially small compared to the energies usually possessed by a golf ball moving) that the ‘discreteness’ can be ignored (or not even detected).
The same goes for the uncertainty principle.
The ‘uncertainty’ is so small compared to the size of a macroscopic object (a grain of sand is already huge) and the value of its momentum, that such uncertainty becomes irrelevant.
Also the photons we use to detect position and momentum (e.g. by photographing an objct in movement where we basically capture reflected photons) of a macroscopic object perturb the object in an insignificant way (like throwing a tennis ball against a aircraft carrier!).
So the same quantum “microscopic” laws are always valid, but due to the size of an object, certain aspects of nature become irrelevant in our “observations” and can be approximated with the ‘classical’ Newtonian Laws.
The same is valid from going from Newtonian Laws to systems that have high speed (close to light speed) or high gravitational fields.
In this case the approximations we make for law speed and low gravitational fields also are not valid anymore and we must take special and general relativistic effects in account.
Regarding the Big Bang: there are different theories.
It is assumed that the big bang was a singularity, or a system in any case very small to need quantum mechanical description.
But of course cosmology needs also to unify Gravity and the other forces of nature.
The trouble is that there is no ‘quantum theory of gravitation’. There are a few candidates (loop quantum gravity, Kaluza–Klein theory, string theory and other candidates).
We know there was a Big Bang because from observations we know the universe is inflating (through the Hubble Law and red shift of the galaxies) and has been inflating for a while and that requires some ‘starting point of inflation’, since it could have not been expanding forever.
We also know that the universe has probably a ‘flat’ (Euclidean) space on average (by measuring the cosmological constant and the fact that space seems to look anisotropic all around), i.e. an open space, so it will not ‘contract’ again, which leaves out the ‘big bounce model’.
Hence from this we deduce that the universe had a starting point and will (probably) have a so called ‘heat death’ when all it remains is a loose ‘soup’ of particles and photons (although other theories have been proposed).
How the big bang started depends a lot on the ‘quantum gravity theory’ one “likes”. Hawking prefers M-theory and came out with his ‘no boundary theorem’… but in the end it’s only a mathematical model based on an unproven and untested theory.
So basically he is saying: “IF A, B and C are true, then my theory X explains how the universe started”… the problem is that A, B and C themselves are mere untested and unproven theories…
IN the end even if some scientists today claim to “have discovered the secrets of the universe and the Big Bang” the REALITY is that they are still walking in the dark really, since most of their theories have no experimental proof to back them up.
So when they claim ‘we know how the Big Bang started’ (or something similar) they are basically
lying to you.
