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Name: John
Status: student
Grade: other
Country: USA
Date: Winter 2012-2013

Can light at opposite sides of the Universe have interaction? Can it be instantaneous?

Hi John, Thanks for the question. Yes, in principle, light at opposite sides of the Universe can interact and the interaction will be instantaneous. However, in practice, the interaction depends very strongly on the distance between the photons. When I say "in principle", I am referring to a theoretical or mathematical treatment. There may be quantum entanglement, which goes beyond the scope of this service.

I hope this helps. Please let me know if you have any more questions. Thanks Jeff Grell

Hi John,

Yes, if they are entangled.

Quantum mechanics allows particles to be entangled, one object cannot be described without considering the other(s). They remain in a quantum superposition and share a single quantum state until a measurement is made. The Schrodinger's Cat problem is loosely tied to this.

Einstein, Boris Podolsky and Nathan Rosen first considered entanglement as particles having position and momentum. This thought experiment, EPR paradox, hinges on the principle of locality. One presentation of the paradox is: two particles interact, perhaps having a two-state spin, one up-one down. They fly off in opposite directions as far as possible. Measurement of one particle determines the corresponding measurement of the other. They remain in a quantum superposition and share a single quantum state until a measurement is made. The superposition means the interaction must be instantaneous.

Hope this helps! Peter E. Hughes, Ph.D. Milford, NH

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