Friday, February 20, 2009

The branch tapping at the windowpane

David Z.Albert is never less than interesting. He first came to my attention with a provocative paper, which suggested that the global state of the universe might be the vacuum of quantum field theory. His 1992 book, Quantum Mechanics and Experience, is something of a unique work, successfully expounding a many-worlds interpretation of quantum theory without the use of difficult mathematics, and extending the notions of quantum superposition and branching worlds to include subjective experience.

Albert, however, was hookwinked by the makers of the disreputable 2004 film, What the Bleep Do We Know!?:

He spent four hours patiently explaining to the filmmakers why quantum mechanics has nothing to do with consciousness or spirituality, only to see his statements edited and cut to the point where it appears as though he and the spirit warrior are speaking with one voice. "I was taken," Albert admits. "I was really gullible, but I learned my lesson." Yet the real shame with this film is that it plays on people’s fascination with science while distorting and misrepresenting that science.

Albert has now co-written an excellent article in Scientific American on quantum nonlocality, (the branch tapping at the windowpane of modern physics). In this phenomenon, two particles can be prepared into a so-called 'entangled' state, a superposition of correlated states. In such a state, a particular property of both particles is indefinite, but it is guaranteed that when a measurement of that property is performed on one of the particles, the state of the entire system will collapse into one of the correlated states in the superposition, and the value of that property will become definite for both particles. Although the preparation of the entangled state requires the particles to be in close proximity to each other, they can then be separated to a great distance without breaking the entangled state. A measurement can be performed upon one of the particles, instantaneously selecting one of the correlated states in the superposition, thereby instantaneously selecting a definite value for the relevant property on both the measured system and the remote system. This appears at first sight to be 'spooky' action-at-a-distance, which violates the notion of locality associated with relativity.

Abner Shimony, however, proposes an interesting resolution to the apparent discrepancy between quantum theory and relativity. Shimony proposes that the quantum state describes the evolution of objective potentialities, as well as objective actualities, and suggests that while actualities satisfy relativistic locality, potentialities need not:

Relativistic locality is the domain of actuality, while potentialities have careers in space-time (if that word is appropriate) which modify and even violate the restrictions that space-time structure imposes upon actual events. The peculiar kind of causality exhibited when measurements at stations with space-like separation are correlated is a symptom of the slipperiness of the space-time behavior of potentialities.

Whilst one actuality cannot instantaneously cause another spatially distant actuality, the transformation of potentiality to actuality (otherwise known as the 'collapse of the wave-function'), is instantaneous. In the case of a spatially separated entangled system, the transformation of potentiality to actuality in one place causes the instantaneous transformation of potentiality to actuality in a spatially distant place.

Note, however, that unless some notion of absolute simultaneity can be re-injected into modern physics, the instantaneous transformation of potentiality to actuality in two spatially distant places, will presumably only be simultaneous in the reference frame of the system performing the measurement on one of the particles.

Shimony's proposal also requires a significant extension of our physical ontology to embrace the existence of objective potentialities, a notion which would require significant fleshing out...


Bob said...

I like to think that separated entangled particles are 'not really' separated. That would be the spooky connection, we just call it spooky because we don't know what it is yet.

Of course, opening the doors for such assumptions is very unsientific.

Or I like to think that it is just logical that two 'entangled' particles behave in the same way. The thing is, as I understand it, we cannot force one particle to be in a certain state and thereby forcing the second particle to be in a certain state; we can just measure the first one and and then assert that the twin particle did the same thing.

Could you please explain the phenomenon ? :-)

Gordon McCabe said...

Correct, Bob. You basically establish a superposition:

v(1) x w(1) + v(2) x w(2)

where v(1) and v(2) are possible states of one particle, and w(1) and w(2) are possible states of the other particle.

You know that when you measure the entangled system, the state will collapse into either v(1) x w(1) or v(2) x w(2), so you know that the states of the systems are correlated, but you don't know which correlated state you will find.