Our intuitive understanding of the universe is that it is locally real. For the universe to be local means that things are only affected by their immediate surroundings, and to be “real” means that things have a definite state at all times.

Weirdly this is not true. A particle can be in a superposition where it simultaneously is in multiple states at once. Also entangled particles can affect their counterparts at any distance, faster than light.

“Real” = an object and its properties continue to exist even when nothing is interacting with it. A basket of 5 apples will still have 5 apples even when no one is looking.

“Local” = in order to change an object’s properties, something needs to physically interact with it. If you throw another apple into the basket of apples, the basket will not contain 6 apples until the apple you threw reaches it. It is assumed there is a maximum speed at which that apple can travel.

“Not locally real” = it has been observed that the basket registers that it contains 6 apples the moment you throw the 6th apple rather than when the 6th apple reaches the basket. The properties of the object have changed without direct interaction.

Remember Shrödinger’s cat? As long as you don’t look in the box, the cat is both alive and dead and only when you open the box the cat “collapses” into either a live or dead cat.

Now imagine the cat has a twin, in another box, also both alive and dead until observed. BUT! Should you look into the first box and the first cat collapses and lives, the other cat **instantly** dies.

That’s what they did in the experiment: they opened the two boxes at exactly the same time, and saw that both cats collapsed into opposite states with seemingly no connection.

Under our previous understanding of a “locally real” universe, there should be some information transfer between them: how else could the cats know each others fate?

This information transfer could only happen at the speed of light, but now this experiment has closed all loopholes in that possibility. The collapse is instant, faster than the speed of light.

It could also mean that we think of as independent particles are actually just *one* really spread out object, not that the particles communicate.

Like separating the front and back side of a coin (say, with a laser) and then discovering that one is heads (or tails) and that the other is always the opposite. No, they were always like this, but it’s our discovery of that which was confused.

Another misconception is that we can operate on one side of the entanglement and have that operation be reflected at the other side. That’s not what it means, I’ve read. There is no connection between the objects… it’s just that they were created as opposites.

Hi /u/kabir9966!

Quantum entanglement is a phenomenon, in which the measurement results of two entangled particles are correlated. I.e. if I measure the spin of 100 pairwise entangled particles along the same axis, the results of the entangled pairs will always correlate. In other words, when one measurement gives spin up, measuring the other will always give spin down. This holds true, no matter how far the two particles are apart, or how short the time between the two measurements is.

One possible explanation of this phenomenon goes as follows: The measurement results follow a secret plan that is created together with the entangled pair. That is, the measurement results are deterministic. You can imagine this like hiding a small item in one of two identical boxes. Then you take one of the boxes to the moon and open it. If you find the item, you instantly know that the other box is empty. This would be a very neat solution, as no signal would have to be exchanged for you to gain this information, thereby side-stepping the problem of relativity. Furthermore, this theory is [realist](https://en.wikipedia.org/wiki/Realism_in_physics#Local_realism), in the sense that the state of each object is well-defined at all times.

This is called a [local hidden-variable theory](https://en.wikipedia.org/wiki/Local_hidden_variable). Here, the term “local” signifies, that this theory holds on to the constraints of relativity, any object can only influence its immediate surroundings. This constraint is also called “locality”. The idea of this theory is, that the measurement result of all quantum mechanical particles is pre-determined from the moment of their creation in such a way, that conservation-laws are respected. When we measure one particle of an entangled pair, we get the secretly pre-determined measurement result, and thereby instantly know the state of the other particle, without the need for any signal to be exchanged between them.

As it turns out, we can test whether or not such local hidden variables exist using the [Bell inequalities](https://en.wikipedia.org/wiki/Bell_inequalities): Veritasium has made a [pretty good explainer how this test works](https://www.youtube.com/watch?v=ZuvK-od647c).

The bottom line is, that such a hidden-variable theory would lead to different outcomes that what we measure.

Consequently, the local realist theory described above cannot be true. We have to let go of at least one of these constraints: The universe can respect realism, but not locality; or it could respect locality, but not realism; or it could respect neither.

A theory that respects locality but gives up local realism would mean quantum states really remain in an undetermined state of superposition until they are measured, and in the moment of the measurement, the wave function of both particles instantaneously collapses (according to the Copenhagen Interpretation anyway). There are no hidden variables pre-determining the outcome of these measurements, and no signal is exchanged faster-than-light.

The Nobel price was given for experimental evidence that realism does not hold locally.

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