I just saw a comment that linked to an article explaining how Nobel prize winners recently discovered the universe is not locally real. My brain isn’t functioning properly today, so can someone please help me understand what this means?
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I wrote up an explanation back when this was making headlines. Linked and quoted below: https://www.reddit.com/r/QuantumPhysics/comments/y1dqgy/comment/irx9x44/
> “Locality” is the principle that things can only affect and be affected by other things in their immediate vicinity.
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>You can push someone right next to you, but you can’t push someone a mile away from you. In order to do that, you have to physically travel to them. Even things which seem to affect distant other things require something else to travel that distance.
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>You can see far away objects because a photon bounced off that object where it was, traveled towards you and hit a sensitive cell in your eyeball. The interactions happened between the object and the photon at the object’s location and between the photon and your eye at the eye’s location.
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>So a “local” universe is one where all interactions happen like this and any interaction between distant object requires that something (another object or signal of some kind) travels between those objects, and that thing is limited in how fast it can travel by the speed of light.
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>“Realism” is the principle that objects have definite properties even when they aren’t interacting with anything.
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>Let’s say you have two particles that are going to collide. If you want to know how the collision will affect each particle, you need to know their speeds and masses, so their momentum.
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>In a universe where realism holds, each particle has a definite momentum and when they collide, they interact with each other based on those values and then fly off each with a new momentum.
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>If realism does not hold, then before they collide, each particle has a range of possible values it could have for its momentum, and interacting with each other forces the momentum of each particle to become a single definite value. The particles then interact using those definite values for their momenta before flying off with a new range of possible momenta until they interact with something else.
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>For a long time, scientists thought that the universe was locally real. That means that particles only interact with particles that are near them with all interactions over distance being restricted by the speed of light, and particles have definite values for all of their properties even when not interacting with other things. We may not know what the value is when they aren’t interacting, but the interaction reveals the pre-existing value to us, it does not cause the object that didn’t have a defined value at all to take one on for the purposes of the interaction.
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>Quantum mechanics, and entanglement in particular, threw a wrinkle into this view.
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>If you prepared a set of particles so that they are entangled, it means that measuring a property of one particle will tell you something about the other particle, because they are correlated.
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>If I take a pair of shoes and stick each shoe in a separate box, opening one box to find a left shoe will tell you that you would find the right shoe in the other box if you were to open it.
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>Similarly, you could prepare a set of particles so that they have opposite spins. If you measure one and find it is spin up, it means that a measurement of the other will have a value of spin down.
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>Curiously, however, the math of quantum mechanics says that these properties are indeterminate until they are measured, and that both particles are in a superposition of spin up and spin down until a measurement or other interaction forces them to take on one or the other state.
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>Furthermore, even if you separate the entangled particles over a great distance and measure them at the same time, the results will still be correlated. This presents a bit of a problem, because if the properties of each particle aren’t determined until they are measured and the measurements happened so far apart that no signal traveling at the speed of light or slower could have been exchanged by the particles, how does particle A “know” that it should be spin up to particle B’s spin down and vice versa?
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>This is what Einstein referred to as “spooky action at a distance” and he and others at the time proposed that our understanding of quantum mechanics must be incomplete and there is some value we have not yet discovered that pre-determines the result of the measurement ahead of time. The result isn’t random, it just looks that way because we have not discovered the thing that causes the result to be what it is, a so-called “hidden variable.” This would neatly solve the problem and take us back to a world with both locality and realism, since the properties of each particle are set from the time they are entangled and no communication would need to take place for the results to be correlated.
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>Much later, in comes John Stewart Bell who is able to demonstrate mathematically that there are certain predictions that quantum mechanics makes that can never be replicated by any theory that incorporates a hidden variable in this way. This means that either quantum mechanics is not just incomplete but wrong or else locality and realism cannot both be true. You could have one or the other (or neither) but not both.
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>The Nobel prize was awarded for devising and conducting experiments for which these two competing theories give different results for the expected outcome, and determining that the actual results in the real world match the predictions of quantum mechanics, which precludes both realism and locality from being true together.
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>Thus one or both of the following must be true:
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>Particles only have defined properties when interacting with other things and not between interactions
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>It is possible for a particle to directly interact with a distant particle without having to send a signal at or below the speed of light.
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>Thus “local realism”, the concept that objects always have defined properties and all interactions are limited by distance and the speed of light, cannot be true of the universe that we live in.
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