Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
| [1] In some sense, particle-spin is a rotation about its own axis, but this classical concept is limited. The electron can spin clockwise or counter-clockwise, and these two values are denoted by spin 'up' and spin 'down'. |
| [2] A particle can be given a desired spin through a Stern-Gerlach device, in which electrons are made to pass through a magnetic field. |
The connectedness is even more perplexing in another experiment. In twin-particle-system of Zero spin [1], spin of each of the particles cancels the other.
If one of the particles has a spin 'up' the other particle has spin 'down'. If the first particle has a spin 'right', the second one has a spin 'left'. No matter how the particles are oriented, their spins are always equal and opposite.
Now, if these particles are made to go off in opposite directions, i.e., if one goes to New York, and the other to Bombay or one on the earth and the other on the moon, their combined spins will still always be Zero. Supposethe particle in New York [2] is given a spin 'up', we do not have to make a measurement on the other particle in Bombay, because we know that its spin is equal and opposite to its twin, i.e. 'down'. Somehow, the particle in Bombay knows that its twin is spinning up. In other words, what was done in New York affected what happened in Bombay.
This strange phenomenon is known as the Einstein-Podolsky-Rosen (EPR) effect. How does the particle in Bombay know which axis has been chosen? There is no time for it to receive the information from the other particle by any conventional signal. EPR's thought experiment is the Pandora's Box of modern physics. It inadvertently illustrated an inexplicable connectedness between particles in two far away different places.
This is the crux of the EPR experiment, and this is where Einstein disagreed with Bohr.
