# The Theory of Causal Fermion Systems

## Notions of Entropy

In [entropy21] a notion of entropy was introduced for causal fermion systems. This entropy is a measure of the state of disorder of a causal fermion system at a given time compared to the vacuum. It should be regarded as a quantum entropy. It also gives rise to a corresponding entanglement entropy.

The connection of this entropy to other notions of entropy (like the von Neumann entropy of the corresponding quantum state or the entanglement entropy constructed from the kernel of the fermionic projector) is currently under investigation.

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