There was a movie (released 2004) What the #$*! Do We Know!? that has the crucial scene where a kid is holding a basketball and he tosses it and is carrying on about "It depends on how far down the rabbithole you want to go". I call this scene, "crucial" in that it directly communicates the viewer that the discoveries of quantum mechanics apply to basketballs. https://www.imdb.com/title/tt0399877/
In this thread, I will be laying out my personal understanding of this topic , that is : the applicability of quantum mechanics to certain physical contexts , situations, and systems, and under what conditions (temperature, scales) require a descriptions written in the language of QM. (I will then briefly cover aluminum wires and other cantilever experiments here too, to cover all my bases.)
So lets begin.
Domain of Classical Physics
The room you are in now, is under 1 atmosphere of pressure, and the temperature is roughly 290 Kelvin. Your surroundings are bathed in multi-spectrum white visible light and infrared photons are saturating everything. Your body is emitting billions of them haphazardly due to your body heat alone. The room you are in likely is very noisy. TV's blaring. The sound of a ticking clocks on the wall. A fan running is vibrating the air violently. In a quiet enough room, you would even be able to hear the sound of your blood being pumped by your heart.
Classical physics dominates everything in human "everyday life". To any degree of precision you would desire, you could faithfully simulate everything you see, hear, touch, and feel using a computer simulation that only operates by the equations of classical physics. The degree of precision is only bounded by the amount of computing power, and NOT bounded by the "inaccuracy" of classical physics.
Your everyday life is dominated utterly by the mechanical, physical, material phenomenon perfectly captured by and described by Classical Physics, with all its mechanical determinism. If you smash your foot into a concrete wall, it will break your bones and bruise you. A delivery van can still run over you and kill you. If you fall out a 5th story window, you will be injured upon impact below. Basketballs are described by elastic collisions, vibration, kinetic energy, angular momentum, F = ma. Quantum mechanics and Special Relativity do not in way contradict these claims
Bottom line : Descriptions of Physical systems requiring Quantum Mechanics, Relativity, or Quantum Field theory occupy the very extremes of physical law. Extremely cold, extremely small, extremely hot , extremely fast -- these are regimes in which Quantum Mechanics must be applied. Quantum Mechanics does not state that classical physics is "wrong" -- it describes physical contexts at the very extremes, in situations in which classical physics breaks down.
Despite the narrative visuals in the movie, What the #$*! Do We Know!? , Quantum mechanics does not describe basketballs.
Domain of Quantum Mechanics
Classical physics does break down , yes, but only in the extremes of size scales.
Classical physics will describe every physical system that is larger than a molecule. Molecular-scale simulations of physics can adequately capture every phenomenon you would ever want by describing molecules as point-particles. We all know in the back of our minds that molecules are not "actually" points. But any computer simulation of molecular scale can approximate them as if they were. We can faithfully reproduce the phenomenon of heat and pressure inside a chamber simulating carbon dioxide as if every molecule were a single point. We can simulate the dissolving of table salt into water by simulating the water molecules as if they were the points, and simulating the sodium-chloride molecules, as if they were singular points. Such a simulation would reproduce "dissolving" to a precision that would be effectively indifferentiable from reality.
Carbon dioxide is actually shaped like a bar, with two oxygen atoms at the ends. Water molecules are actually shaped as a triangle. These shapes have no bearing on their "large-scale behavior". In a dissolving simulation, we can plug in the properties measured in nature as ad-hoc parameters, and that approximation is both precise and effective. If a smart, combative person wants a simulation to "give rise to" dissolving, then these inter-molecular forces would be required to be simulated ab initio.
That is to say, a combative person who would ask "Okay that's fine but WHY does salt dissolve in water?" Again, you can faithfully simulate dissolving in a computer that only uses the equations of classical physics, with properties of dissolving concentrations plugged in ad-hoc as magical parameters. To answer the WHY-question, does indeed require quantum mechanics. That is, it requires the faithful representation of intermolecular forces, and those forces cannot be described with Classical Physics. The simulations that actually give rise to the triangular shape of bounded oxygen and hydrogen molecules have a name. They are called Ab-Initio Quantum Chemistry simulations. They do indeed contain representations of the Schroedinger Wave, and they consider the orbitals of electrons. Dissolving happens due to the magnetic forces that manifest around the shape of bounded oxygen and hydrogen atoms, and how those electrons create magnetic poles. None of those physical phenomena can be described by Classical Physics.
(Speaking in rough-hewn terms) you can can depict chemical reactions into a simulation in terms of pure concentration curves and heat, and your classical computer simulation will faithfully give rise to chemistry to any level of precision desired. It is only when you start asking question like "Why does flourine burn in air?" do you have to start addressing inter-molecular forces and thus bring in Quantum Mechanics. But a computer simulation can accurately capture flourine burning in air without quantum mechanics -- provided you plug in the realistic parameters "ad hoc", by hand. If you wanted your simulation to give rise to those parameters, naturally in terms of its own dynamics, only then would you need to start adding in Quantum Mechanics to your codebase.
What would be importantly communicated to Deepak Chopra -- is trying to get him to appreciate how extremely tiny molecules really are in comparison to objects we interact with in our daily lives. And that is something Micheal Shermer tried to tell him, with little success.
Cantilevers and Aluminum Wires
This section is for those naysayers and other combative readers who may want me to address the placing of macroscopic objects into superpositions.
Cantilevers and aluminum-nitride wires were indeed placed into superpositions in recent years by researchers in prestigious institutes of science and their adjoining laboratories. This seems to strongly suggest that basketballs, trees, cars, and rocks go into superpositions too --- ergo any hyksos claim about the applicability of QM to certain scale sizes are outwardly contradicted.
Not so fast.
The cantilevers placed into a superposition were under high vacuum; which can only be produced by very expensive lab equipment. The apparatus holding the cantilever was cooled near absolute zero. The researchers then used laboratory-grade lasers to set the cantilever into a resonance, using very dim light that was likely previously entangled. A tiny bump on the table, a stray infrared photon passing through, or any kind of magnetic field induced nearby would destroy the superposition of the cantilever immediately. These experiments, were as it were operating the EXTREMES OF PHYSICAL LAW (cough cough) and are using instruments with atomic-scale precision and sensitivity.
For you Chopra's out there, the room you are in is not under high vacuum, it is saturated with infrared photons, it is about a million times hotter than the cantilever was. The room you are in is likely noisy, and the energy from those sounds is dissipated as heat into the surfaces that absorb them. There is no clean-single-spectrum light sources anywhere near you (lasers), and so photons of all kinds of wavelengths are splashing around.
Researchers in Finland and Australia made a microscopic drumhead go into a superposition. The drumhead was composed of aluminum nitride and contained roughly a trillion atoms. Read up on it, as much as you want, to confirm the following extremal conditions required to do this :
- The temperature of the apparatus is 0.001 K , or one one-thousandth of a Kelvin.
- The aluminum nitride was laboratory-grade purity. (this really matters)
- The lasers were lab-grade microwaves of excruciating precision.
- The slightest noise destroyed the superposition, something the researchers repeated a dozen times in interviews.
- The superposition was maintained for timescales far shorter than intervals of individual frames shown on a TV screen.
You cannot grab a crappy aluminum can off the side of a road and expect it to be in a superposition. The can is warm, its lattice of atoms is riddled with impurities, and the can is being bombarded with the pressures of air molecules at 1 atm. Your fingers alone are emitting a storm of infrared photons that are bombarding the impure lattice, setting off waves of phonon-based heat knocking, distorting and wobbling the atoms in a random dance. Remember, those cantilevers in a lab in a superposition were under high vacuum and cooled to a temperature that is never manifest at the surface of the earth, even in Antarctic winters.
Our human reality does consist of cans on the side of the road. Of rocks, cars, trees , furniture, and basketballs. The delivery truck that is about to run you down at a crosswalk does not "manifest" by you observing it. As Shermer said, Quantum Mechanics applies to very tiny objects. In a more honest work-a-day tone, you could even say that QM describes the behavior of fundamental particles.
I fully agree that classical physics does break down ( I am not a hidden-variable theorist). But it only breaks down at the extremes of physical law. The behavior of matter will not be classical when its temperature is hot enough to split the nucluei of atoms apart (roughly 10 million Kelvin). You need QFT to describe the physics there. Matter cooled near a billionth of a Kelvin will not act classically, and instead QM dominates its behavior. Matter moving at astronomical speeds will not act classically, or even appear to be classical, and in those situations Special Relativity is required.
