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Quantum Mechanics


Bohr-Einstein debates
 

EPR

non-locality

 

 

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Einstein claimed that all events in the universe have a “local” cause. What does this mean?

Physicist, John Bell:

“Local causality is the idea that what we do ‘here,’ in this region of space, has no immediate effects in a distant region of space. In fact, it has no effects until a lapse of time, which [effects are] limited by the velocity of light. Now, this was an important principle in Einstein’s [special] relativity, one to which he was strongly attached, and it is one which is not present in quantum mechanics, because quantum mechanics doesn’t give you this analysis of the world into different spacetime regions – the whole thing is treated as a unified [whole], and Bohr insisted quite strongly on the indivisible nature of quantum phenomena. If two particles had interacted and had then separated, they were still one system, and it was wrong, from Bohr’s point of view, of how to divide it up with something happening in this spacetime region and something happening in that spacetime region.”

In other words, according to Einstein, if an event occurs in one region of spacetime, it won’t be known in another spacetime region until such information is carried to it, but no faster than the speed of light.

Distant events are "non-local." For example, our Sun is 8 light-minutes away. This means it takes 8 minutes for a photon emitted from the Sun to reach us on Earth. If the Sun exploded, we wouldn’t know about it for 8 minutes.

All this is true, and so what is Bohr’s contention? It has to do with “entangled” particles, which, Bohr asserted, are never separated. But, strictly speaking, these are not yet particles but only particles in superposition, particles in potentia, only a wave function until a measurement is taken.

 

 

Editor's last word:

Locality: things affect other things only at their mutual location.

Non-locality: things affect other things though separated and at a distance; if you want to affect something at a distance, you have to send something to that distant thing.