1

Entanglement refers to one particle needing no time to react to change of another particle, not even the time for speed of light to travel. (that even gravitational force requires speed of light to travel so that two particles attract).

Is it by experiment that entanglement was found, or is it by some other theory that we can "deduce" entanglement, or both?

nonopolarity
  • 332
  • Putting aside the silliness in the first paragraph in order to answer the question in the second: Once you posit that the state space of a composite system is the tensor product of the state spaces of the components, it's a nearly trivial exercise to deduce that almost all states are entangled. – WillO Aug 02 '19 at 03:16
  • 1
    "Entanglement refers to one particle needing no time to react to change of another particle" No, that's a (fairly common) misunderstanding of what entanglement means. The 2nd particle needs no time to react because there isn't any reaction happening. – PM 2Ring Aug 02 '19 at 06:14
  • @PM2Ring so do you just view it as "an integral part of the universe" that one is not separated from the other"? That they are just one thing and doing something to part of it always make it happen to the other part of it? – nonopolarity Aug 02 '19 at 07:46
  • I think this answer may be helpful. – PM 2Ring Aug 02 '19 at 08:04

2 Answers2

1

It is called quantum mechanics with its postulates and its equations of state. Quantum mechanics is a complete mathematical model of the micro world of particles atoms molecules and nano states.And it has been discovered to be the theory for all experimental data up to now, i.e. it has not been falsified.

A quantum mechanical solution of a system of particles by definition means that all the particles are "entangled", i.e. not independent of each other not only in kinematics ( as is the case with classical mechanics) but also with quantum numbers. It means that once there is a wavefunction $Ψ$ solution for the system each "event" building up the probability distribution for the system which is given by $Ψ^*Ψ$ will have the particles under consideration in specific quantum states.It is these quantum states that are used in the populist examples of "entanglement" .

Since mainstream physics states and can prove that all classical physics emerges from the quantum underlying framework,in principle everything is "entangled". BUT the complexity is such that it can be shown that macroscopic systems lose their coherence. This is mathematically modeled with the density matrix formalism.

anna v
  • 233,453
  • It is perhaps a bit of a stretch to say that all particles are entangled. There are entangled states, but that are also separable states. – flippiefanus Aug 02 '19 at 04:27
  • @annav so entanglement is not proven or "shown to be true" by any experiment yet? (meaning, was there any experiment that showed, one particle perhaps spinning one way, and you change the other particle's spinning and the first particle is affected with 0 time?) – nonopolarity Aug 02 '19 at 04:37
  • On the contrary, all elementary particle and generally atomic molecular nuclear etc measurements validate it. Validate a theoretical model means that all measurements follow the theoretical model,and all the predictions of the model come true. – anna v Aug 02 '19 at 04:40
  • @annav so you mean... it actually can be shown or proven by experiments already? – nonopolarity Aug 02 '19 at 04:41
  • yes , it is the result of fitting experiments with quantum mechanical models – anna v Aug 02 '19 at 04:44
  • @flippiefanus that is why there exist the density matrix https://ocw.mit.edu/courses/chemistry/5-74-introductory-quantum-mechanics-ii-spring-2009/lecture-notes/MIT5_74s09_lec12.pdf, to describe the "separable" states within the theory. – anna v Aug 02 '19 at 04:51
  • Are you saying that density matrices are only valid for separable states? (Why do you put separable in inverted commas?) Any state can be represented by a density matrix, regardless of whether they are entanglement or not. – flippiefanus Aug 02 '19 at 07:32
  • @flippiefanus The universe' s density matrix is composed of matrices about the diagonal, within the accuracy of measuring the $ρ_{i,j}$ as different from zero ; these are the "separable" states – anna v Aug 02 '19 at 11:17
1

It may be misleading to say that entanglement refers to a particle reacting to the observation of another particle's state with which it is entangled. Instead, the entanglement refers to a quantum correlation that already exists between these particles even before the observation. The observation merely reveals the specific state of the other particle that is correlated with that of the observed particle.

The concept of entanglement comes from the EPR paper and was later developed further by Schroedinger. The term entanglement was coined by him.

The EPR paradox was later formalised in terms of Bell's inequality. Then Alain Aspect (among others) showed that nature violates Bell's inequality. By implication nature allows states to be quantum entangled.

flippiefanus
  • 14,614