# Definition of Pointer in Measurement

• SheikYerbouti
In summary, a pointer is a readout on a measuring apparatus that tells you the outcome of an observation. It is subjective, and requires common sense, since there doesn't seem to be an obvious limit as to what can be treated as part of the quantum realm.
SheikYerbouti
Definition of "Pointer"

I am in the process of writing a review-type paper for my intermediate quantum mechanics course. I have chosen to do my paper on the topic of measurement, with a focus on weak measurement. In all of the papers that I am reading, the term "pointer" is thrown around, but I have not seen it clearly defined as all of the papers assume familiarity with the topic. My (vague) understanding of the pointer is that it is coupled with the measured system and, upon measurement, its value will shift by the measured eigenvalue, at least for a projective measurement. However, I am not entirely sure if this is correct and would greatly appreciate elaboration on what the pointer is in the context of measurement.

A 'pointer' is simply some kind of readout that tells you what the outcome of an observation is. The states associated with those outcomes are sometimes called pointer states.

Actually rigorously defining what an observation is in QM is no easy task. It requires decoherence and my suggestion for you is to consult THE textbook on the subject - Decoherence and the Quantum-to-Classical Transition by Schlosshauer
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

I have a copy and recommend it VERY highly.

There you will find definitions of all the usual terms, the various parts to the measurement problem, and what decoherence does and does not do in relation to the issue.

A cut down exposition can be found here:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

But basically it is along these lines. Decoherence converts a superposition to an improper mixed state. The states it is composed of are the possible outcomes of an observation so these days the idea is to say an observation has occurred just after decoherence. This is a totally quantum process and avoids issues of exactly what counts as a measuring apparatus etc.

What you have are so called pointer observables whose eigenstates are stable with respect to the decoherening effect of the environment. These eigenstates are often called pointer eigenstates and is part of solving the preferred basis issue of the measurement problem.

Thanks
Bill

Last edited:
In the "orthodox" or "shut-up-and-calculate" Copenhagenish view of quantum mechanics, one divides the universe into macroscopic and quantum realms. The measuring apparatus is on the macroscopic side, and the pointer is simply the thing on the measuring apparatus that tells you the result of the measurement. It is subjective, and requires common sense, since there doesn't seem to be an obvious limit as to what can be treated as part of the quantum realm. Within these interpretations, this is the measurement problem. http://www.tau.ac.il/~quantum/Vaidman/IQM/BellAM.pdf

There is a process called decoherence, which although it does not solve the measurement problem, does pick out the "pointer basis", ie. the possible eigenstates into which a system may collapse when a measurement is made on it. http://arxiv.org/abs/quant-ph/0306072

## What is a pointer in measurement?

A pointer in measurement is a device used to indicate the value of a measurement on a scale or dial. It is typically a thin, pointed object that moves across a calibrated scale to show the measurement.

## How does a pointer work in measurement?

A pointer in measurement works by being connected to the measuring instrument, such as a scale or gauge. As the instrument measures a quantity, the pointer moves along the scale to indicate the value. This allows for a quick and easy way to read the measurement.

## Are there different types of pointers used in measurement?

Yes, there are different types of pointers used in measurement, including mechanical pointers, magnetic pointers, and digital pointers. Each type has its own unique way of indicating the measurement and can be used for different purposes.

## What are the advantages of using a pointer in measurement?

Using a pointer in measurement allows for a visual representation of the measurement, making it easier to read and interpret. It also allows for quick and accurate measurements, as the pointer moves in real-time with the measurement.

## Are there any limitations to using a pointer in measurement?

One limitation of using a pointer in measurement is that it may not be precise enough for very small measurements. Additionally, the pointer may experience parallax error, where the viewing angle affects the reading. This can be minimized by ensuring the pointer is viewed straight-on.

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