Entanglement Spin Direction

[QuantumT]

I have two very simple questions:

When two particles get entangled, do they settle on a spin direction instantly, or are they still in a superposition when they are separated?

And if they are in a superposition after separation, how can they mirror each other when measured later?

[studiot]

Superposition and entaglement are entirely different things.

Superposition is a general mathematical process used in many branches of the physical sciences and engineering and thus has a classical as well as a quantum manifestaion.
In quantum theory you will find it used in what is known as the LCAO or linear combination of atomic orbitals in the derivation of the molecular orbitals that form when atoms bond together.

On the other hand, Entanglement is an entirely quantum effect which has no direct counterpart in classical physics.

Fermions obey the Pauli exclusion principle so if two entangled particles (electrons) are separated and subsequently one is incoporated in an atomic or molecular orbital where its spin is set by the prevailing conditions of the orbital, then this is reflected in the spin of the other.

Edited February 12, 2019 by studiot

[QuantumT]

1 minute ago, studiot said:

Superposition and entaglement are entirely different things.

Superposition is a general mathematical process used in many branches of the physical sciences and engineering and thus has a classical as well as a quantum manifestaion.
In quantum theory you will find it used in what is known as the LCAO or linear combination of atomic orbitals in the derivation of the molecular orbitals that form when atoms bond together.

On the other hand, Entanglement is an entirely quantum effect which has no direct counterpart in classical physics.

Thank you for clearing that up.

How would you ask the questions then?

[studiot]

Did you spot my edit, whilst you were replying?

[QuantumT]

6 minutes ago, studiot said:

Did you spot my edit, whilst you were replying?

So we can just ascertain that it happens, but not why?

[studiot]

11 minutes ago, QuantumT said:

So we can just ascertain that it happens, but not why?

Here is the nearest classical analogy we can get to entanglement.

Suppose I cut a coin in half so I have two thinner coins, one with a head and a blank face, the other with a tail and a blank face.

Now I send one half-coin to my brother in Australia and keep the other at home.

As soon as my brother receives the half-coin he can determine whether my half is the head or tail half, whether or not I have looked to see myself.

If in fact I did not look he could not communicate that information back to me any faster than the famous Cutty Sark tea clipper. (or perhaps by modern steam radio).

Of course electrons do not have 'blank sides' and can adjust of the distance, but in answer to your question,

I would say how, not why and we do not know the answer to this yet.

Edited February 12, 2019 by studiot

[QuantumT]

2 minutes ago, studiot said:

Here is the nearest classical analogy we can get to entanglement.

Suppose I cut a coin in half so I have two thinner coins, one with a head and a blank face, the other with a tail and a blank face.

Now I send one half-coin to my brother in Australia and keep the other at home.

As soon as my brother receives the half-coin he can determine whether my half is the head or tail half, whether or not I have looked to see myself.

If in fact I did not look he could not communicate that information back to me any faster than the famous Cutty Sark tea clipper. (or perhaps by modern steam radio).

Of course electrons do not have 'blank sides' and can adjust of the distance, but in answer to your question,

I would say how, not why and we do not know the answer to this yet.

I appreciate that, but is it wrong to change the analogy to two coins, who both have two sides, and when your brother opens his box, and sees a tail, you open yours and see a head?

Sorry, I don't want to bug you, but I need to know the basic principle.

[Strange]

25 minutes ago, QuantumT said:

I appreciate that, but is it wrong to change the analogy to two coins, who both have two sides, and when your brother opens his box, and sees a tail, you open yours and see a head?

Sorry, I don't want to bug you, but I need to know the basic principle.

You can modify the analogy that way. Or use left and right socks. Or any sort of matched pairs. 

The difference is that in that classical analogy (either studiot's or your modified version) one of the coins is a head and the other is a tail, we just don't know which until someone looks.

In the quantum world, the "coin" is not defined as being either head or tail until the measurement is made. 

[QuantumT]

Thank you! So they are considered to be A or B, up or down, heads or tail, right or left, when they separate?

Edited February 12, 2019 by QuantumT

[Mordred]

Lets say you have some event that results in two entangled electrons. Now with electrons you only have two possible polarization states. Spin up and spin down. Furthermore you must also comply with conservation of spin. Prior to measurement the superposition which as Studiot mentioned is also in classical theory however isn't restricted to physics either. It is a statistics term so can be used in any statistical analysis. 

The superposition in the above case is that prior to measurement of a given particle it can be either spin up or down. However once you measure the particle the other must be the opposite for the reasons above. 

Now what causes entanglement? Well this is far trickier to answer. However I will take a reasonable stab at it. Entanglement can occur whenever you have some viable means to establish some degree of correlation function between any two objects. Once you have a correlation you gain a means to make a reasonable prediction of the other object via a measurement of the first object. The processes to establish a correlation function can vary and do not necessarily involve an interaction between the two objects

Edited February 12, 2019 by Mordred

[swansont]

2 hours ago, QuantumT said:

Thank you! So they are considered to be A or B, up or down, heads or tail, right or left, when they separate?

No. They are undetermined when they separate. They do not have a state until one is measured.

1 hour ago, Mordred said:

 Now what causes entanglement? Well this is far trickier to answer. However I will take a reasonable stab at it. Entanglement can occur whenever you have some viable means to establish some degree of correlation function between any two objects. Once you have a correlation you gain a means to make a reasonable prediction of the other object via a measurement of the first object. The processes to establish a correlation function can vary and do not necessarily involve an interaction between the two objects

You must also not be able to tell what state the particles are in. 

One way of entangling photons is parametric downconversion. Two photons are emitted from an atom, with a definite polarization correlation, but the only ones that are entangled are emitted in the same direction. If they aren’t, then you can tell which photon has which polarization. 

[Mordred]

Good point and example thanks.