Quantum entanglement is not wierd or crazy.
Particles can become entangled when they interact with each other or are emitted from a common source. When electrons or particles are paired they become part of a system of which the net energy is zero. Let’s say two photons (particles of light) are emitted from a particle (say a pion) with zero angular momentum. The spins of the two particles must add up to zero. So if physicist A (call her Alice) measures the spin of one of the photons as +1, she knows instantly that if physicist B (for Bob) measures the other photon, its spin will be −1. But Alice’s measurement could be -1. And if Alice comes up with -1 she know that Bob will measure +1. The measurements coincide instantly, breaking the laws of causality which say that one event cannot cause another event by transmitting information faster than the speed of light.
Physicists have been reluctant to accept this and an exhaustive series of experiments have been conducted to show that it is true. They have been especially keen to show that ‘local effects’ – the action of the conventionally ‘real’ – make no difference.
How can this happen?
According to quantum laws particles are in undetermined states until measured, which means that they have to be considered as being in all states for which probability exists. Once measured the probability ‘collapses’ which in simple terms is to say that their state is now determined. Once you know where a particle is, or which way it was spinning, it obviously ceases to be subject to probability. So if one electron is in an undetermined state so is the other. And once the state of one is known so is the state of the other, instantly, because it will be determined by the same measurement – because the net energy must be zero – wherever it is in the universe.
So it is not the case that one particle is reacting to the other. Once measured, the unknown state of both particles is revealed.
The dual quantum state where the two particles are detected only through probability is mathematically real, but it is one which humans would normally consider to be imaginary – probability is like a mathematical way of talking about imagination. So we could say that quantum experiments prove the reality of the imagination.
Einstein did not like it and called it ‘spooky’. However here you have to pause and realise that what he and other scientists have been balking at is the fact that this reality of imaginary states can be physically demonstrated. This is what the scientists have been amazed about, and this is the reason they have repeatedly tested it. This is the significance of the slit experiment.
When passed through two slits electrons and particles behave as if they travel in waves, as water does, and form bars on a detector screen. (There have been dozens of more recent and more robust versions of this experiment but the result is always the same.) However what freaks people out is that if you only fire one particle at the slits it still behaves like it’s in a wave and acts according to the interference pattern. It looks like it has gone through both slits and interfered with itself, and this is kind of what it’s done. This is because what we are observing is a probability. The probability that the particle will always pass through the same slit is clearly zero. We could say that we are watching our imagination observe the particle. The particle passes through both slits with more or less equal probability. This does sound weird. It is like we are watching an act of imagination, or observing ghosts of the particle. And that is why as soon as we try to measure it the wave collapses and the particle ceases to have wave motion. Probability becomes certainty. Why is the wave real? Because we can observe it as long as we don’t determine it.
The first problem here is not science but the way we assume that science and imagination are somehow unrelated. It does not require imagination to understand that our lives are lived in a world governed by probability, but it does require imagination to live in it. The other problem is that human beings tend to be intellectually and philosophically deterministic, even though reality clearly isn’t. As Einstein said, God doesn’t play dice.