How did it come about that  2  particles  were considered to be entangled?

What were the preconditions for this to occur?

I imagine (I am just guessing) that there was a theory and that this theory was confirmed when its predictions  were observed.

How was the theory arrived at?

As far as I know, entanglement is a fundamental aspect of Quantum Mechanics, where the quantum state of each particle affected cannot be described independently of the state of the others in the grouping.
IOW, the states ( such as position, momentum, spin, polarization ) of theaffected particles in the 'entangled' grouping are represented by a single mathematical wave function, until 'decoherence' occurs due to an interaction.

It was 'accepted' theory, until A Einstein, B Podolsky and N Rosen proposed the EPR paradox which violated local realism and/or causality; the original "spooky action at a distance", and Einstein's ( and Shroedinger's ) belief that unknown 'hidden' variables were involved.
J Bell put the 'nail in the coffin' to hidden variable theories, and this year's Nobel Prize winners, J Clauser, A Aspect and A Zeilinger, have been recognized for estabilishing that there is no local reality.

See here      Quantum entanglement - Wikipedia

Edited October 9, 20223 yr by MigL

6 minutes ago, MigL said:

As far as I know, entanglement is a fundamental aspect of Quantum Mechanics, where the quantum state of each particle affected cannot be described independently of the state of the others in the grouping.
IOW, the states ( such as position, momentum, spin, polarization ) of theaffected particles in the 'entangled' grouping are repItresented by a single mathematical wave function, until 'decoherence' occurs due to an interaction.

It was 'accepted' theory, until A Einstein, B Podolsky and N Rosen proposed the EPR paradox which violated local realism and/or causality; the original "spooky action at a distance", and Einstein's ( and Shroedinger's ) belief that unknown 'hidden' variables were involved.
J Bell put the 'nail in the coffin' to hidden variable theories, and this year's Nobel Prize winners, J Clauser, A Aspect and A Zeilinger, have been recognized for estabilishing that there is no local reality.

See here      Quantum entanglement - Wikipedia

Thanks,I will take a look tomorrow. 

19 hours ago, geordief said:

Thanks,I will take a look tomorrow. 

 

19 hours ago, geordief said:

Thanks,I will take a look tomorrow. 

OK I can see that the maths involved in Bell's theorem is too hard for me  to follow for now (even though I have come across the notation** used  in the past)

I see there are a few ways that entanglement can arise and learn now that it was considered  by Evin Schodinger (sp?) to be absolutely  central to QM.

 

I will just keep at it and maybe it will become clearer eventually. 

 

** I think it looked like a tensor or a cross product they were  using  before I gave up trying to follow.

 

It was almost certainly the tensor product. The statement "these two particles are entangled" is, more formally, "the combined state of these two particles is not a pure tensor element in the tensor product of the state spaces of each element separately".

While I don't know the history itself, entanglement itself is a rather natural aspect of the fact that quantum physics is fundamentally linear, and that the tensor product is the "natural" way of dealing with bilinearity.

Try this

Bell's Theorem (utoronto.ca)

also includes some history and insights into 'reality separate from observation', and non-locality.