Selecting which atom we use with careful attention to its excitation states can create entangled particles. Some atoms emit two photons at a time or very closely together, one in one direction, the other in the opposite direction. These photons also have a property that one spins or is polarized in one direction and the other always spins or is polarized at right angles to the first. They come in pairs such that if we conduct an experiment on one to determine its orientation, the other’s orientation becomes known at once. They are “entangled”.
Figure 10 – Entangled Particles
All of this was involved in a famous dispute between Einstein and Bohr where Einstein devised a series of thought experiments to prove quantum measurement theory defective and Bohr devised answers.
The weirdness, if you want to call it that, is the premise that the act of measurement of one actually defines both of them and so one might be thousands of miles away when you measure the first and the other instantly is converted, regardless of the distance between them, to the complement of the first. Action-at-a-distance that occurs faster than the speed of light?
Some would argue (me for instance) that this is more of a hat trick, not unlike where a machine randomly puts a quarter under one hat or the other, and always a nickel under a second one. You don’t know in advance which contains which. Does the discovery that one hat has a quarter actually change the other into a nickel or was it always that way? Some would say that since it is impossible to know what is under each hat, the discovery of the quarter was determined by the act of measuring (lifting the hat) and the other coin only became a nickel at that instant. Is this action at a distance?
It is easy to say that the measurement of the first particle only uncovers the true nature of the first particle and the deduction of the nature of the second particle is not a case of weirdness at all. They were that way at the start.
However, this is a hotly debated subject and many consider this a real effect and a real problem. That is, they consider the particles (which are called Einstein‑‑ Podolsky‑Rosen (EPR) pairs) to have a happy-go-lucky existence in which the properties are undetermined until measured. Measure the polarization of one – and the second instantly takes the other polarization.
A useful feature of entangled particles is the notion that you could encrypt data using these particles such that if anyone attempted to intercept and read them somewhere in their path, the act of reading would destroy the message.
So there you have it – Weird behavior at a distance, maybe across the universe.
Next: Some Random Thoughts About Relativity