What counts as an observation in QM?

• HomogenousCow
In summary, the article discusses the fuzzy and undefined concept of observation in quantum mechanics, and how it is unambiguously "defined" as a macroscopic limit of the properties of a non-isolated system exchanging a large quantity of information (entropy). It also discusses the concept of quantum states and how measurements in QM are equivalent to density matrix operations.
HomogenousCow
Hi I was day dreaming the other day when I realized how incomplete and vague the fundamental postulates of QM are, I mean what counts as an observation?
For example simply being in the presence of a charged particle can be an observation, since (using a point particle for the sake of argument) then I can feel the electric force and deduce how far the particle is from me and which direction it must be in.
If this were to be a proper observation, then by that logic every electron and proton observes every other one.
Another problem I have is that the notion of an observation is not a very physically sound one, I can deduce many things without directly measuring them, for example I can deduce that everybody who will reply to this thread is on earth, however I did not go ahead and measure that. With this I can go ahead and put limitations on how accurately your momentums can be measured.

Why post links to articles that cannot be read online for free ? and the title does not sound like a consensual position.

The concept of observation in QM indeed first appears as fuzzy and undefined. But it is only so as long as you keep applying the equations to a small isolated system. In fact it turns out to be quasi-unambiguously "defined" as a macroscopic limit of the properties of a non-isolated system exchanging a large quantity of information (entropy) with the outside. This is the process called decoherence.

In my introduction to quantum physics I describe the mathematical structure of quantum states and measurements in a simple way I did not find so clearly expressed elsewhere, while staying rigorously equivalent to the established things (density matrix).

What counts as an observation in QM?

In quantum mechanics (QM), an observation is any interaction between a quantum system and a measuring device that results in the collapse of the system's wave function. This can be thought of as the measurement of a physical quantity, such as position or momentum, which forces the system to take on a definite state.

Is an observation in QM the same as a measurement?

In QM, an observation is often used interchangeably with the term measurement. However, it is important to note that an observation does not necessarily have to involve a conscious observer or a physical measuring device. Any interaction that causes the system's wave function to collapse can be considered an observation.

Can an observation in QM be predicted?

In general, the outcome of an observation in QM cannot be predicted with certainty. According to the Copenhagen interpretation of QM, the act of observation inherently introduces uncertainty into the system. This is known as the observer effect, and it is a fundamental aspect of QM that distinguishes it from classical physics.

What role does the observer play in QM observations?

The role of the observer in QM observations is a topic of much debate and interpretation. Some theories, such as the Copenhagen interpretation, suggest that the observer is an essential part of the observation process. Others, such as the Many-Worlds interpretation, propose that the observer is just another part of the system being observed.

Can a single observation in QM determine the state of a system?

In most cases, a single observation in QM is not enough to determine the state of a system. This is due to the inherent uncertainty and probabilistic nature of quantum systems. Multiple observations or measurements are often needed to gain a better understanding of a system's state and behavior.

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