Monday, May 07, 2012

more mental physics

the other cool thing that makes no sense to me whatsoever is so called 'action at a distance' which sort of combines relativity with quantum ideas in an experiment. nothing can travel faster than the speed of light (more accurately nothing will have a speed greater than the speed of light in a vacuum). this limits how quickly you can pass information around. ie if something is 1 light year away (the distance light travels in a year) it will take one year to send or receive a message from them. the key concept is things don't happen instantly.

what some clever people did, is they created two particles (electrons or photons), a particle where one of the properties can only have one of two forms. so a photon would be polarised either one way or another, or electrons would be created with either spin 'up' or 'down'. when they created these particles, the property is taken completely at random. conservation laws means that one particle takes one form, the second particle takes the other (eg one is spin up, t'other is spin down.)
when you create them though, you don't know them straight away which is which until you measure it, which you don't do straight away. you send the first in one direction, and the other in the opposite direction. you set up measurement equipment very far away, and arrange to measure one first, and shortly after, the second. the second will always be opposite to the first. you measure them so close together in time, that there is no way for information to be passed from the first to the second. so how does the second know which property it's supposed to have?

it takes time for the news of the other measurement to reach you, but how did the particles already know?

some clever people thought that perhaps they have another 'hidden' property when they're made, so that they both know which to take? that was disproven by an Englishman called Bell, googleable by searching Bell Theorem.

this is all quite weird. more weird stuff about light can be found in a 4 part series of lectures by feynman given in new zea land. there's a great bit in the first lecture where he tells people that "don't understand" what he's about to say, not confuse not understanding what he says, with not wanting to believe what he says. lots of things are difficult to believe but are explained perfectly by theory, such as the probabilistic nature in reflection of light. eg if you fire 100 photons at a surface, how does the surface know what % to reflect back?!

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