# How to define measurement in quantum mechanics

• spaghetti3451
In summary, measurement in the Copenhagen interpretation of quantum mechanics involves collapsing the wavefunction of a system into one of its orthonormal basis vectors, which changes the state of the system. This means that we cannot assume a system possesses properties until they are measured. In practical terms, measurement is the act of using physical tools to determine a real number, such as measuring the position of a photon in a double slit experiment with photomultipliers.
spaghetti3451

## Homework Statement

Define measurement in the Copenhagen interpretation of quantum mechanics.

## The Attempt at a Solution

In the Copenhagen interpretation of quantum mechanics, measurement changes the state of a wavefunction, that is, it collapses the wavefunction into one its orthonormal basis vectors (eigenstates). We cannot assume that the system possesses properties if not measured...

I found the second sentence in a book of mine. Problem is I don't know what it means :-)

What do you think it means?

I thought about it for a bit, read a bit on wikipedia and was going to summarize what I read, but i'll just give you the link.

http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics

I just think about it in the practical sense. A measurement is something you do in a lab with rulers and stopwatches that gives you a real number. For example we can measure the position of a photon at a particular time in a double slit experiment by putting a photomultiplier next to each slit, and it will make a loud noise when a photon goes through one of the slits, so we know the photons position (within a particular uncertainty) at the time when the photomultiplier made the noise.

## 1. What is measurement in quantum mechanics?

In quantum mechanics, measurement refers to the process of obtaining information about the state of a quantum system. This can involve observing the position, momentum, or other properties of a particle or group of particles.

## 2. How is measurement defined in quantum mechanics?

Measurement in quantum mechanics is defined as an interaction between the quantum system and a measuring device that results in the collapse of the quantum state into one of the possible measurement outcomes.

## 3. What is the role of the observer in quantum measurement?

The observer plays a crucial role in quantum measurement as their act of observation causes the quantum system to collapse into a specific state. This is known as the observer effect.

## 4. Can measurement in quantum mechanics be predicted?

No, measurement outcomes in quantum mechanics cannot be predicted with certainty. The outcome of a measurement is determined by the probabilities of the different states of the quantum system, as described by the wave function.

## 5. How is measurement related to the uncertainty principle?

The uncertainty principle states that it is impossible to know both the position and momentum of a particle with absolute certainty. This is because the act of measuring one property affects the uncertainty of the other property. Therefore, measurement in quantum mechanics is closely related to the uncertainty principle.

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