Tim Anderson - How the illusion of an objective reality creates time travel paradoxes

Quantum information theory resolves the grandfather paradox but questions our belief in an objective reality.

When down the pub with friends years ago someone might ask how did he know if any of us were actually here, he might be generating the whole reality? It was a popular thought years ago, that we might be living in a computer simulation. I always dismissed all this as nonsense, but perhaps we are all here, with each one of us creating our own reality. Physics keeps getting weirder. 

The alternative explanation is that there are no objective realities but only your subjective reality. Frank Ramsey, Oxford mathematician, philosopher, genius by all accounts and contemporary with Bohr, was one of the early proponents of the idea that probability is a subjective game where our beliefs, the knowledge we have available to us, determines what is real. The very idea of things existing and ascribing logical values such as true, false, or probabilities to such things as coin flips is entirely a mental exercise. Instead, what you observe in a coin flip is a cacophony of sense impressions that your mind orders into logical statements such as heads and tails. The probability of either one is likewise all in your mind. Although he was no quantum physicist, his ideas were at odds with the predominant, Bohr/Einstein viewpoint of a single objective universe. His work has led to a major modification of the Bohr hypothesis to create Quantum Bayesianism (QBism), a subjective interpretation of quantum mechanics that does not invoke additional universes but also does not indicate we have a single objective reality either.

In a QBist or Ramseyan quantum universe, reality is composed of a set of beliefs and information that you hold in your mind. Out there, outside the mind there is no reality, no true, no false. Thus, having realities that are inconsistent between different people, even paradoxical, is entirely possible, even the norm. All probable outcomes are the results of bets that each individual makes with the universe. God doesn’t play dice with the universe. You play dice. In a Ramseyan interpretation, reality is more like a virtual reality game that your mind generates for you from sense impressions. You make bets on what will happen and then interpret the results based on what you bet. The idea that you might have different objective realities based on different probable outcomes is nonsense because you created those outcomes in your head in the first place. Going back to the highway analogy, if you get off your highway and get onto another in your game and change things, even if it looks like you are changing your own past, it is still all just your own personal experience. There is no possibility of paradox because your own life is a linear progression of bets, one after another, and likewise for everyone else. The only reality is your reality and any observation of reality you make is a quantum measurement (according to the universality of quantum physics) but is subject to your interpretation. To quote Christopher Fuchs of Quantum Bayesianism:

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How the illusion of an objective reality creates time travel paradoxes