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There are not really different laws. QM holds in the macro world as well as in the micro-world. On the other hand, Avagadro's number (roughly the number of protons in a gram or protons) is so big (about 6*10**23) that statistical descriptions are useful (and necessary.) Even simple things like the central limit theorem mean that the fluctuations in things go like 1/Sqrt(6*10*23) which is about 1/(8*10**11). Thus measurements of for example a couple of grams of hydrogen gas may fluctuate by about .000000000008 or in practice, the gas acts classically.

Quantum effects usually only take place at small lengths too. One can amplify quantum effects (transistors, lasers, etc.) to the macroscopic level. Normally though, ordinary Newtonian mechanics is sufficient. In the limit of a very large number of particles (and the limit if Planck's constant were to go to zero), matter acts classically. 0

Similarly for relativity (special and general). Only for large distances or times of masses does relativity differ in practice from classical mechanics.

In the limit of a very large number of particles (and the limit if Planck's constant were to go to zero), matter acts classically

I'd add the condition that T >>0. Even with large numbers of particles, matter acts quantum mechanically (see Bose-Einstein condensates, superfluidity, the temperature depedence of the heat capacity) if you get close enough to 0 K.

...and the limit if Planck's constant were to go to zero....

Doc, is that "if" or "of" Planck's constant? (Are there two "if" statements here, or just one?)

Just trying to follow you here. I'd thought that Planck's constant was a defined value; yet maybe it's something one can "fiddle with" (i.e., trying different values in one's equations) to see what happens? I gather if different values are assigned for the cosmological constant, extraordinarily, drastically different results would obtain, theoretically speaking.

These connections between the micro and macroworlds truly are fascinating. I have a lot to learn!

Thank you for writing, Doc.

Er, if I may interrupt. You said:

QM holds in the macro world as well as in the micro-world.

What are you saying then about Schrodinger's cat? Is he both alive and dead in your macro world?