Electromagnetism and the strong and weak nuclear forces are now understood using quantum field theories. Quantum mechanics shows that at high energies, electromagnetism and the weak force combine to form a single force, known as the electroweak force. A theory that shows that the electroweak and strong nuclear force combine into a single force at high energies is known as a grand unified theory GUT. This is because we have not yet combined gravity with quantum mechanics, as we have done with the other forces.

The probability of any given result can be determined using the Born rule. We could put this problem aside and hope that these dynamics will be discovered in time, but then we still have to solve the problem of how quantum states can interact with ordinary matter at all.

This is known as the measurement problem. The measurement problem is similar to the problem faced by mind-body dualists like French natural philosopher Rene Descartes, who argued that there are two fundamentally different substances in the universe discussed in Chapter 26which interact, despite exhibiting different properties and obeying different physical laws.

Antirealists argue that all previous scientific theories have been proven false and so we should not take the invisible entities postulated by our current theories literally either.

This argument is countered by the fact that instruments have been built that rely on these invisible entities to work. Both the Bohm interpretation and the collapse approach predict that an observer will record only one result in accordance with its probability.

Neither branch is more real than the other. Everett argued that we do not appear to experience every possible result because we too behave like a quantum object, and the individual elements of a superposition do not affect each other.

This is because the theory applies to the universe as a whole. When we become aware of the result of a quantum experiment, the world does not literally split, we simply realise which world we are already in.

This superpositional universe is known as the multiverse. Two entangled particles may be separated and then observed by two people. Probability and free will Probability and classical mechanics Classically, we understand probabilities in terms of decision theory.

A rational person should try to bet on the number that has the highest objective probability. If we actually throw dice and count how many times each outcome occurs, then a set of objective probabilities is expected to emerge, but no matter how many frequency trails are run, we can never know for certain if the dice are weighted.

This raises the question of why we should be rationally compelled to use our objective knowledge of probabilities when placing bets. This property will be defined independently of the decision-theoretic link but will come to the same conclusions. Primitivism is the view that we should accept the decision-theoretic link as a fundamental law of nature and not look for a deeper explanation.

Cautious functionalism is the view that we will one day find a functional definition and, in the meantime, we can use the decision-theoretic link as such. This allows scientists to continue to use decision theory when considering objective probabilities.

Free will and classical mechanics At first glance, we do not appear to have free will. Classical mechanics shows that if we knew all of the natural laws, then we should be able to predict the future.

Ayer suggested that free will should be contrasted with constraint instead of causality.

This idea leads to the question of whether we would be able to choose to act differently if we were made aware of what our future actions will be. American physicist Henry Stapp described how: The idea that they be not alternatives but all really happen simultaneously seems lunatic to [them], just impossible.

This theory was extended by American physicist Wojciech Zurek in Mathematically, these branches are said to decay exponentially, but they do not disappear completely and so decoherence can only give us an approximate appearance of definiteness.

It is still unclear whether the approximate nature of decoherence shows that the many worlds interpretation is incorrect, or whether it can be accounted for.

It may be that our minds have evolved to only comprehend definite objects, or because decoherence is precise enough to explain our observations when combined with a material theory of the mind discussed in Chapter There is no objective uncertainty because every physical possibility actually happens.

The problem of how we can ascribe probabilities to events at all, given that every physical possibility is certain to happen, is known as the incoherence problem.The preferred basis problem. The many worlds interpretation solves the measurement problem by stating that macroscopic objects also obey the laws of quantum mechanics, but it then faces a similar problem known as the preferred basis problem.

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Quantum Gravity