# Free will theorem

The free will theorem of John H. Conway and Simon B. Kochen states that, if we have a certain amount of "free will", then, subject to certain assumptions, so must some elementary particles. Conway and Kochen's paper was published in Foundations of Physics in 2006.[1] A stronger version of the theorem appeared later.[2]

## Axioms

The proof of the theorem relies on three axioms, which Conway and Kochen call "fin", "spin", and "twin". The spin and twin axioms can be verified experimentally.

1. Fin: There is a maximum speed for propagation of information (not necessarily the speed of light). This assumption rests upon causality.
2. Spin: The squared spin component of certain elementary particles of spin one, taken in three orthogonal directions, will be a permutation of (1,1,0).
3. Twin: It is possible to "entangle" two elementary particles, and separate them by a significant distance, so that they have the same squared spin results if measured in parallel directions. This is a consequence of (but more limited than) quantum entanglement.

In their later paper, "The Strong Free Will Theorem",[2] Conway and Kochen weaken the Fin axiom (thereby strengthening the theorem) to a new axiom called Min, which asserts only that two experimenters separated in a space-like way can make choices of measurements independently of each other. In particular, they are not asserting that all information must travel finitely fast; only the particular information about choices of measurements.

## The theorem

The theorem states that, given the axioms, if the two experimenters in question are free to make choices about what measurements to take, then the results of the measurements cannot be determined by anything previous to the experiments. Since the theorem applies to any arbitrary physical theory consistent with the axioms, it would not even be possible to place the information into the universe's past in an ad hoc way. The argument proceeds from the Kochen-Specker theorem, which shows that the result of any individual measurement of spin was not fixed independently of the choice of measurements. As stated by Cator and Landsman regarding hidden variable theories:[3] "There has been a similar tension between the idea that the hidden variables (in the pertinent causal past) should on the one hand include all ontological information relevant to the experiment, but on the other hand should leave Alice and Bob free to choose any settings they like."

## Reception

According to Cator and Landsman, Conway and Kochen prove that "determinism is incompatible with a number of a priori desirable assumptions." They demonstrate that in certain experiments, "the choice an experimenter makes [about what to observe] is not a function of the past."[2] The philosopher David Hodgson supports this theorem as showing determinism is unscientific, that quantum mechanics allows observers (at least in some instances) the freedom to make observations of their choosing, and so leaves the door open for free will.[4] Some critics argue that the theorem applies only to deterministic models.[5] Others have argued that the indeterminism that Conway and Kochen claim to have established was already assumed in the premises of their proof.[6]