A Measurement (Problem) of Reality – Additional Information

Copenhagen interpretation

 

The quintessential interpretation of quantum mechanics. The Copenhagen interpretation asserts that prior to the measurement it makes no sense to ask about the state of the system.The act of measurement itself requires both a quantum system to be measured and a classical observer; there is a strong dichotomy imposed between the pair. Upon measurement by the observer, the state of the system collapses to a fixed, observable state.

 

According to the Copenhagen interpretation, asking about the state of the photon whilst in superposition is meaningless; the photon only takes on properties once measured. A clear distinction is drawn between the photon being measured, and the apparatus performing the measurement; the apparatus acts solely as an “outside” observer. Only once measured does the state of the photon collapse to one of two possible outcomes.

 

Many-worlds interpretation

 

In the many-worlds interpretation, the measurement apparatus is an intrinsic part of the quantum system. A measurement results in the outcome displayed by the apparatus becoming intrinsically entangled with the state of the system being measured. Upon measurement, the universe is hewn into multiple, objectively different realities, each representing one of the possible outcomes of the measurement.

 

For the many-worlds interpretation, a less stark line is drawn between the photon and the device for measuring it. Instead, upon measurement, the state of the device becomes intrinsically entwined with the state of the photon. Once the act of measurement occurs, the universe is hewn into two, objectively different realities, corresponding to one outcome given by the measuring device, or the other.

 

Quantum Bayesianism

 

In quantum Bayesianism (QBism), the state of a system, and hence the outcomes of measurements, are subjective relative to the state of the observer (they have no objective reality). A measurement apparatus is an extension of an agent – quantum measurement outcomes make no sense without the presence of a conscious agent, even with a measurement apparatus present.

 

According to quantum Bayesianism (QBism), the state of the photon, and hence the outcomes of any measurements performed on it, are subjective relative to the state of an observer (they have no objective reality). The measurement apparatus is thus an extension of you, the observer, and quantum measurement makes no sense without the presence of a conscious agent, even with the measurement apparatus present.

 

 

 

 

Many Thanks to Sam Pallister and Henry Semenenko from the Bristol Laboratory for Quantum Physics.