Hi. This is a question about Logic. I've read in a book by Michio Kaku, _The Physics of the Impossible_, that it may be possible to receive a signal before it was sent. This to my way of thinking would violate the logic behind causality. And yet on a social level we are effected by what happens in the future. An example would be Christmas shopping. My question is can an effect precede a cause, and if so what does that mean in relation to actuality and reality? Cheers, Pasquale

We normally assume that causes can't precede their effects, but this isn't a logical truth, and in fact it's possible to tell coherent stories where the principle fails. By "tell coherent stories," I don't just mean tell science fiction. As your author may point out (I haven't read the book), it's possible to say how causal loops and backward causation might fit into physical theory, even though there's no strong case for saying that such things actually happen. So there's no issue about the "logic" of causality being violated.

As for your Christmas shopping, this isn't really an example of the future affecting the present. Your present intentions do the causing. You want to make sure that come Christmas day, Granny gets that gorgeous pair of Manolo Blahniks, and it's that present desire and intention that gets you to head off to the shoe store. Granny's beaming grin isn't reaching back from the future to get you to the mall.

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