Can somebody please explain quantum entanglement and retro causality?

Question

This is for me and my housemate so basic terms for me and more complex stuff for my house mate please.....

Answer 1

Take one quantum particle, say an electron. It has momentum (m*v) and for this example , we will isolate one quantum property called spin. Spin can be up or down left or right or in between. Now take another quantum particle - a Photon of energy. It has velocity (c) but photons have no spin. So if we have a VERY enegetic photon (spin zero) and it splits into two electrons (with e=mc etc.) then each electron is entangled through spin. In effect if one spins up, the other has to be spin down - as 1 up +1 down = spin zero - the original spin of the photon. Or left to right etc. However the spin can be in ANY direction and the spin of each electron effectively does not yet exist.. Now if you capture one electron and measure its spin whatever it is, it means the other electron suddenly has to be spinning in the opposite direction no matter how far apart they are. So one quantum measurement forces the instantaneous quantum state of the second at a distance that defies causality. ie One event cannot cause another event if the time between the two events is zero and the distance is greater than C. However this can be explained if you use the full version of the wave function in which probability is time invariant - but that will take a much longer answer..

Answer 2

You're best bet is to read-up on the EPR paradox. But, here's an attempt to summarize.

Quantum Mechanics (QM) as we know it today is called the Copenhagen Interpretation. This very specific model of QM incorporates some very interesting ideas that depart from Classical deterministic mechanics. The following are 2 important ideas relevant to entanglement.

1. A QM system is in an indeterminate state, or superposition of all possible states, before a measurement is made. The act of making a measurement forces the system into a specific state, with a certain probability for the specific outcome. This behavior is directly observable in what's called "The Stern-Gerlach Experiment."

2. Pauli's exclusion principal. Certain objects can't all occupy the same states, but must distribute themselves across available states.

Let's say 2 objects from 2 above interact at some time, when they are co-located, and then travel a great distance apart. At some future time we measure the state of both objects after they've moved far apart. In fact, let's do it in a way that a "signal" traveling the speed of light couldn't pass between them as the measurements are made. And, such that we measure the state of one quickly before the other.

We will find, after making the first measurement, the second will always agree with Pauli's exclusion principle. BUT, keep in mind, we also know the state of the first was only determined when we made the measurement. The collective system was in a superposition of states until at least one measurement is made. So the objects are said to be “entangled” at the very start of this process.

This behavior has been experimentally verified, and is real! It's sometimes called "Spooky Action at a Distance."

Answer 3

Be carefull about asking for wordy explinations of quantum phenomena. It can only really be described in mathematics and by asking for a wordy description you are asking for something to help you visualise it and that is just asking for trouble. Quantum mechanics is well tested and proven but trying to picture it always leads you down the wrong route.

Answer 4

i detect it unusual that each and every individual of the above commentators have received a thumbs down for his or her alternatives. Given the position your interest lies, examining up on the Copenhagen interpretation of quantum mechanics and Stephen Hawkings paintings on the files Paradox couldn't be extra suitable. chuffed examining :-)