What prior experiments justified the notion of entanglement?

[ontodva]

In 1935 Albert Einstein, Boris Podolsky, Nathan Rosen, and Erwin Schrödinger introduced the notion of quantum entanglement. What prompted that notion and what justified their belief it was a real phenomenon?

 

[DrChinese]

There were no prior experiments at that time. In fact the first experiments with entangled particles was in the 1970s, I believe.

The idea of entanglement was a deduction from the newly formed Quantum Theory. Much like the idea of anti-particles was a deduction from theory.

 

[ontodva]

The idea of entanglement was a deduction from the newly formed Quantum Theory.

That is what I suspected. So much for the topic header. Next...

Which parts of quantum theory specifically led to entanglement and how? What specifically convinced these physicists that it was a real phenomenon?

 

[.Scott]

What specifically convinced these physicists that it was a real phenomenon?

The term was coined by Schroedinger. Both the person who coined the term and the context of its first use give clues to its manner of deduction:
From https://www3.nd.edu/~dhoward1/Early History of Entanglement/sld003.html:

It would appear that the entire body of then-known QM knowledge was being considered in an overall examination of particle interactions. At the time the term "entanglement" was coined, Schroedinger was highlighting this type of interaction as a particular issue - without predicting specific characteristics of the entanglement.

 

[DrChinese]

Entanglement as a quantum state of 2 or more particles took a while to catch on. By the time of Bohm's classic Quantum Theory (1951), electron spin entanglement had been studied a bit and written about. Bohm himself adapted the EPR paradox to electron spin (EPR-B).

There were some early experiments by Bleuler and Bradt (1948) and Wu and Shaknov (1950). Yet it remained something of a curiosity until Bell's Theorem came along (1964). After that, the pace of new work in the area took a big leap. The idea of ruling out local hidden variable theories was a powerful motivator, and research has accelerated since. There are now well over 1000 papers a year published on the subject.

 

[bhobba]

Which parts of quantum theory specifically led to entanglement and how? What specifically convinced these physicists that it was a real phenomenon?

Dr Chinese is correct. It took a little while to catch on but is a simple deduction from the principles of QM which took its final form at the end of 1926 when Dirac published his transformation theory or about 1932 when Von Neumann published his mathematical Foundations of QM depending on your attitude to mathematical rigour in physical theories.

Simply its an extension of the principle of superposition to different systems. Suppose two systems can be in state |a> and |b>. If system 1 is in state |a> and system 2 is in state |b> that is written as |a>|b>. If system 1 is in state |b> and system 2 is in state |a> that is written as |b>|a>. But we now apply the principle of superposition so that c1*|a>|b> + c2*|b>|a> is a possible state. The systems are entangled - neither system 1 or system 2 are in a definite state - its in a peculiar non-classical state the combined systems are in.

If you observe system 1 and get state |a> then you know system 2 is in state |b>, and similarly if you observe system 1 and get |b> you know system 2 is in state |a>. That's all this entanglement weirdness is - a correlation.

It's now known to be absolutely fundamental to QM:
http://arxiv.org/abs/0911.0695

Thanks
Bill

 

[Weddgyr]

Which parts of quantum theory specifically led to entanglement and how? What specifically convinced these physicists that it was a real phenomenon?

I suppose at the time of the EPR paper the conclusion was probably based around the uncertainty principle. Two paired particles and conservation laws would otherwise allow an experimenter to get around the principle by measuring the properties of one particle to determine those of the other. But not if the particles are "entangled" however that works.

 

[bhobba]

I suppose at the time of the EPR paper the conclusion was probably based around the uncertainty principle.

The original EPR paper gave the correct QM analysis:
http://www.drchinese.com/David/EPR.pdf

Thanks
Bill