The past causes the future. The future is undetermined, and cannot reach back to have an effect on the past. Call this the “principle of causality”. It is so ingrained in our thinking that we don’t know how to make sense of the world without it.
And yet, quantum mechanics subtly defies the principle. A young Einstein built the causal structure of reality into his relativity theory (1905), and a mature Einstein (1935) struggled with quantum mechanics precisely because of the way it was inconsistent with causal logic.
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