But if both copies were in an undefined state prior to their collapses, how do those copies “know” which one to collapse to so they agree with the other? In other words, what the paper says isn’t a definite value, until either Bob or Alice (or some other subscriber) looks at theirs, but as soon as either does, the other’s copy instantly becomes definite too, with the same values. This is true even if Alice and Bob are separated by light years. When Alice looks at her copy, she knows what Bob will see, even though Bob hasn’t looked at his yet. Under standard interpretations of quantum mechanics, it is meaningless to talk about what the paper actually says until someone looks at it.īut, as soon as Bob or Alice actually look at their paper, the wave function of the quantum copy collapses into a definite value. One branch of the superposition says the stock market went up yesterday, the other says it crashed. So what’s the big deal with entanglement? Well, let’s say that a very special issue of the Times comes out, a quantum version of the paper, one that is in a superposition of possible states until a reader actually looks at it. (Technically since information is always conserved, it spreads the entanglement around, but let’s not get sidetracked.) All it really does is break the entanglement between them, that is, erase his ability to use his paper to know what’s in Alice’s copy.
If Bob alters his copy of the Times, it doesn’t effect Alice’s. Alice and Bob each know what the other is seeing, but can’t use that information in any way to communicate with each other. You could say that each copy of the same issue of the Times is entangled with every other copy, including Alice’s and Bob’s, which is to say, they share a causal history that enables information about one to provide information on the other. When Alice gets a particular issue of the Times and looks at it, she knows that Bob is getting the same issue with the same information. Let’s further suppose they both have very reliable and timely delivery of their paper. Imagine both Alice and Bob, living far away from each other, each have a subscription to the New York Times, and each of them knows about the other’s subscription. But that description, for most people, isn’t very enlightening. When two quantum objects interact, they become entangled with each, meaning that they’re described by an overall common wave function. That understanding is that entanglement is inherently about information. I first got it when reading Adam Becker’s What Is Real?, but wanted to wait to discuss it until I’d gotten some confirmation from Sean Carroll’s Something Deeply Hidden, which I’m currently reading. This doesn’t come from a pessimistic view of the possibilities, but from an understanding of what entanglement actually is, an understanding I have to admit I’ve only recently fully come to appreciate. Doesn’t this, as a fair amount of science fiction implies, mean that there might be some effect we could use in the future for faster than light communication? Entanglement, we are told, is a non-local effect. Often when these facts come up in discussions, someone raises the possibility of using quantum entanglement for communication. But that’s the fastest they enable it at. As the unit of all electromagnetic radiation, they enable communication at the speed of light. They always travel at the speed of light.
Things are marginally more hopeful for photons, which have no mass. To actually reach the speed of light, it would need to acquire infinite mass, zero passage of time, and zero length, which would require infinite energy. The closer to the speed of light it gets, the higher its mass climbs, the slower its passage of time, and the shorter its length. A rocket ship attempting to accelerate to the speed of light encounters some well known effects: time dilation, mass increase, and length contraction. Albert Einstein, with his theory of special relativity, established that the speed of light is the absolute speed limit of the universe.
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