## Pros and cons of Consistent Histories in a nut shell

### December 28th, 2006

**PROS**

The issues that the Consistent Histories formalism addresses are:

1) The “strangeness” of the **measurement** process which requires the counter intuitive and problematic **division between a Classical subset of the universe and a Quantum one**.

2) The problematic definition of an **external observer**.

3) How does classicality emerge?

The answers it gives are the following.

1) In the History formalism there is no distinction between a Classical subspace of the universe and a Quantum one. In fact the role of measurement as implemented by the Copenhagen interpretation is no more essential in this formalism. In stead what replaces the notion of measurement is the notion of Decoherence. Roughly speaking in the Consistent History formalism the basic elements are histories identified as a sequence of events at different times. Given two or more histories, it is possible to assign, in a consistent way, probabilities regarding properties of these histories iff the members of each history do not interfere between them. This requirement shifts the process of “assigning” properties of histories, from the measuring process to the initial conditions, therefore the concept of measurement as proposed in the Copenhagen interpretation becomes unnecessary.

2) Regarding the second issue the History approach proposes the view that any quantum system is constantly “measured” by the environment, therefore there is no need for the existence of an external observer. Consequently the universe can be rightly viewed as a closed system.

From the above we can deduce that the achievements of this approach are the following:

- It is exactly the same as orthodox Quantum Mechanics with the difference that it rests on a smaller number of assumptions.

- It does not presuppose the notion of measurement as the “moment” in which a Quantum system “becomes” classical.

- No division between Classical and Quantum World.

- No external observer.

- The logic it proposes is distributive and additive,since the only set of histories to which probabilities can be assigned are those which do not present any “Quantum interference”.

**CONS**

Given the present data and some initial states these two can be “joined” by different set of Histories, the union of which does not form a consistent set. This implies that the past is not unique in the sense that present data allow for incompatible sets of histories.

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