# If QM uses 'interaction with macroscopic objects' as the foundation, then how it is a 'fundamental' from which we can built up macroscopic physics?

Suryabarta Saha
1.How is that possible for us to say that a non-interacting microscopic particle can have numerous values of a physical parameter in a given instant? the mixed state of a microscopic particle is simply the probability distribution of the particle having those values of a given physical parameter if a measurement/interaction with a macroscopic object takes place. When a particle doesn't interact how we can know what it really is?

2.From the single photon/electron double slit experiment i.e the interference pattern of separate,discrete particles and the apparent break of casualty, we arrived at the conclusion that it can't really have a trajectory (i.e a definite position at any given instant, so it can't have a definite velocity, lenear momentum, angular momentum, kinetic energy etc. at any given instant in a classical sense) and we have to treat it as a complex probability wave function in between source and screen. Photons/electrons are always registered as discrete particles, it originates as a discrete particle at the source and vanishes as a discrete particle at the screen/detector, to give a reasonable explanation of the interference pattern, from source to screen we have to consider it as a wave of probability which fills up the space between source and screen. Moreover we declared the particle isn't going through one particular slit to protect the casualty.

a)But can we really know what electron/photon is between the emitter and the screen(detector) ? I know we can't separate a particle from its 'associated' probability wave or think it as a discrete classical particle but don't we have a very vague idea about what it really is when it does't interract i.e measured?

b) what makes it to take a perticular/definite value of a physical parameter when measured?

c)What is the meaning of 'measurement' we are talking about? it isn't any kind of interaction, an interaction with another microscopic particle doesn't count as a 'measurement'. An electron in a hydrogen atom interacts with the proton, it isn't a 'measurement'.

d)Does 'measurement' really mean interaction only with macroscopic objects like the screen/detector? but the detector is also made of microscopic particles. If Quantum mechanics use 'interaction with macroscopic objects' as the physical foundation then how it can be a fundamental theory from which we will derive the macroscopic physics?

3.Lastly in our textbooks it seems there are extrapolation of classical ideas/equations in qm, as an example how can we take a term which 'represents the repulsive coulomb interaction between the i-th and j-th electron'? Even in classical physics when a charge is moving it's electric field is not given by coulomb's law. So how can we use potential energy term derived from coulomb's law in Schrodinger equation of many-eletron atom. When the position of an electron is not defined classically how are we even using |ri−rj|? When we are talking about electric field created by an electron in an atom it's obviously not an classical electrostatic situation, electrons are accelerating charged particles (Though I don't clearly understand what 'acceleration' of a microscopic particle means, and whether it's right to use the term 'moving' for something whose physical parameters aren't defined uniquely)

Mentor
When a particle doesn't interact how we can know what it really is?
We can’t. Quantum mechanics doesn’t even try - it is a theory about the results of interactions and refuses to answer questions about what is happening in between.

dextercioby, Dale and Suryabarta Saha
Suryabarta Saha
But then the question remains: if QM uses 'interaction with macroscopic objects' as the foundation, then how it is a 'fundamental' from which we can built up macroscopic physics?

Mentor
what makes it to take a perticular/definite value of a physical parameter when measured?….What is the meaning of 'measurement' we are talking about? it isn't any kind of interaction, an interaction with another microscopic particle doesn't count as a 'measurement'. An electron in a hydrogen atom interacts with the proton, it isn't a 'measurement'.
A measurement is an interaction that is thermodynamically irreversible. A Google search for “quantum decoherence” will bring up some good references, and David Lindley has written a good layman-friendly book on the subject: “Where does the weirdness go?”

dextercioby, vanhees71 and Suryabarta Saha
Mentor
When a particle doesn't interact how we can know what it really is?
We cannot, so we don't make any such claims.

From the single photon/electron double slit experiment i.e the interference pattern of separate,discrete particles and the apparent break of casualty
The double slit experiment doesn't break causality.

how can we take a term which 'represents the repulsive coulomb interaction between the i-th and j-th electron'?
Because it works.

What is the meaning of 'measurement' we are talking about?
This is really the only reasonable question you have in your long post. All of the rest of them are based on misunderstandings.

Your post is really all over the place. This is a fallacious style of argument called the "Gish Gallop", and is made famous by politicians. This is not an appropriate form of discussion in politics and certainly not appropriate for physics forums. I am closing this thread.

Please choose ONE topic, and expand on that ONE topic in sufficient detail that the reader can understand your key concerns and reasoning. We can see the obvious Gish Gallop and do not appreciate it being used here. Focus on your strongest concern only.