Quantum mechanics and the macroscopic

  • B
  • Thread starter jaydnul
  • Start date
  • #1
516
11

Main Question or Discussion Point

I know the key tenants of quantum mechanics, and am not interested in the measurement problem. What we do know is that a particle has a wave function that describes the likelihoods of it having certain EXACT values when we measure it.

That's all good, but I am still confused how the classical world emerges from an underlying quantum framework. As I understand it, decoherence is different from "wave function collapse". For example, when I press my finger on my desk, are the desk atoms acting like they are being measured and taking exact values to repel my finger,? Or does my finger become part of the same quantum system and nothing has "collapsed"?
 

Answers and Replies

  • #2
PeterDonis
Mentor
Insights Author
2019 Award
27,856
7,733
What we do know is that a particle has a wave function that describes the likelihoods of it having certain EXACT values when we measure it.
The likelihood of what having certain exact values?
 
  • #3
jfizzix
Science Advisor
Insights Author
Gold Member
753
346
One way to see how the classical world emerges from the large-scale limit of the quantum world is to understand that quantum effects are only really observable when the quantum object is isolated from interacting with the environment.

If you try to consider the system-plus-environment as a larger scale quantum system, and calculate the quantum state of the system as part of the larger whole, that state will almost never just be a pure superposition unless it is completely independent of the environment. Since everything we see/hear/touch is due to second/third hand information about particles that interacted with the objects in question (e.g., a table), it is understandable that we don't see quantum effects in everyday life.


If someone were to describe the state of you-plus-desk, you can imagine that before you touched the desk, the joint state of you-plus-desk factors as the state of you, times the state of the desk. Because of your interaction, the physical systems "you" and "the desk" become entangled, and the quantum state describing "you" is a mixture of relative states depending on the information you obtained from making contact with the desk. Indeed, this loss of coherence due both to your system no longer being in a pure superposition and to this interaction is how measurement is described in "decoherence".

There's still philosophical issues to resolve, but this perspective of examining your interaction with an object as though you and the object are part of a larger quantum system can be quite illuminating as far as measurement goes.
 
Last edited:
  • #4
A. Neumaier
Science Advisor
Insights Author
2019 Award
7,210
3,092
quantum effects are only really observable when the quantum object is isolated from interacting with the environment.
No. In this case nothing is observable. Observation requires an interaction with the environment. Measurement results are visible in the environment only if it has interacted with the system.
 
  • #5
jfizzix
Science Advisor
Insights Author
Gold Member
753
346
To be technically correct (the best kind of correct), quantum phenomena are most easily observed when the quantum object is isolated from everything else, except the measurement device.
 
Last edited:

Related Threads for: Quantum mechanics and the macroscopic

Replies
43
Views
8K
  • Last Post
Replies
16
Views
6K
Replies
5
Views
6K
Replies
7
Views
3K
  • Last Post
Replies
7
Views
3K
Top