Undergrad What exactly is a measurement device and how does it work?

[Quantum Alchemy]

What exactly is a measurement device and how does it carry out a measurement?

For instance, in the double slit experiment, you always hear about particle/wave duality. When it's not being measured it behaves like a wave but when a measuring device is placed by the slits, it behaves like a particle.

What makes up this measuring device and exactly how does it interact with the system to carry out a measurement?

 

[A. Neumaier]

What exactly is a measurement device and how does it carry out a measurement?

For instance, in the double slit experiment, you always hear about particle/wave duality. When it's not being measured it behaves like a wave but when a measuring device is placed by the slits, it behaves like a particle.

What makes up this measuring device and exactly how does it interact with the system to carry out a measurement?

There is no precise definition. A measurement device is anything that interacts with the system measured and allows one to read some measurement result that is sensitive to the property probed.

 

[Demystifier]

There is no precise definition. A measurement device is anything that interacts with the system measured and allows one to read some measurement result that is sensitive to the property probed.

In unitary QM, somewhat more precise definition is possible. A measurement device is anything that gets entangled with the measured system, such that the full state splits into a superposition of macroscopically distinguishable states.

 

[A. Neumaier]

In unitary QM, somewhat more precise definition is possible. A measurement device is anything that gets entangled with the measured system, such that the full state splits into a superposition of macroscopically distinguishable states.

That's not enough; this only gives a distribution of possible values.

To meaningfully talk of a measurement, the device must produce a definite, observable measurement value - i.e., move into exactly one of the macroscopically distinguishable states.

How this is possible is the content of the famous measurement problem.

 

[Demystifier]

That's not enough;

I agree, but I think that's what is shared by all interpretations of QM.

To meaningfully talk of a measurement, the device must produce a definite, observable measurement value - i.e., move into exactly one of the macroscopically distinguishable states.

How this is possible is the content of the famous measurement problem.

Yes, but that part depends on the interpretation.

 

[A. Neumaier]

I agree, but I think that's what is shared by all interpretations of QM.Yes, but that part depends on the interpretation.

This is the reason why there cannot be a precise specification of what a measurement device is.

 

[EPR]

Anything that leaves a mark in the macroscopic world?

 

[PeterDonis]

Anything that leaves a mark in the macroscopic world?

That just pushes the vagueness from "measurement" to "macroscopic" (and "leaves a mark", too, since that's ambiguous).

 

[Quantum Alchemy]

There is no precise definition. A measurement device is anything that interacts with the system measured and allows one to read some measurement result that is sensitive to the property probed.

Thanks for the response.

What type of measuring device is used in the double slit experiment and how does it interact with the system that's being measured, i.e. the electrons or photons being fired at the slits?

Also, why doesn't air molecules interact with the electrons or photons and cause a measurement to occur before the particles arrive at the slits? Why doesn't the barrier between the slits interact with the particles and cause a measurement to occur?

 

[Demystifier]

Also, why doesn't air molecules interact with the electrons or photons and cause a measurement to occur before the particles arrive at the slits? Why doesn't the barrier between the slits interact with the particles and cause a measurement to occur?

They interact, but the question is whether they interact enough to produce significant decoherence. Decoherence is "significant" when the full state has the form
$$\sum_a c_a|a\rangle|\phi_a\rangle$$
such that $\langle\phi_a|\phi_{a'}\rangle\approx \delta_{aa'}$. Here $|a\rangle$ are states of the measured system (e.g. electron or photon), while $|\phi_a\rangle$ are states of the environment (e.g. air or measuring apparatus).

 

[Quantum Alchemy]

They interact, but the question is whether they interact enough to produce significant decoherence. Decoherence is "significant" when the full state has the form
$$\sum_a c_a|a\rangle|\phi_a\rangle$$
such that $\langle\phi_a|\phi_{a'}\rangle\approx \delta_{aa'}$. Here $|a\rangle$ are states of the measured system (e.g. electron or photon), while $|\phi_a\rangle$ are states of the environment (e.g. air or measuring apparatus).

That makes sense.

What does the first term in the equation Ca signify? Is that just for whatever system is being measured?

 

[Demystifier]

That makes sense.

What does the first term in the equation Ca signify? Is that just for whatever system is being measured?

$c_a$ are just complex numbers, the coefficients of the linear superposition. They obey $\sum_a |c_a|^2=1$.

For more on decoherence and relation to quantum measurements see my 3rd lecture in https://www.physicsforums.com/threads/reading-materials-on-quantum-foundations.963543/post-6299524

 

[A. Neumaier]

What type of measuring device is used in the double slit experiment and how does it interact with the system that's being measured, i.e. the electrons or photons being fired at the slits?

A barrier with two slits and a screen. The barrier approximately bilocalizes the particles at the two slits and the screen at the point where it leaves a mark.

Also, why doesn't air molecules interact with the electrons or photons and cause a measurement to occur before the particles arrive at the slits? Why doesn't the barrier between the slits interact with the particles and cause a measurement to occur?

Photons in air are effective particles that account for the air by moving at slightly lower speed than in vacuum. For double slit experiments this can be ignored.

 

[Morbert]

Since nobody has given the consistent histories account of a measurement device, and since CH is clearly the only correct interpretation of QM, I'll briefly present the account here.

A device is capable of measuring whether or not a quantum system $s$ possesses a property $P_\epsilon$ if the following is satisfied:

i) If $s$ is prepared with the property $P_\epsilon$, it is highly likely that the measurement device, upon interaction with $s$, will produce a record $R_\epsilon$ of the property.
ii) If $s$ is prepared without the property, ie with $I_s-P_\epsilon$, it is highly unlikely that the measurement device, upon interaction with $s$, will produce a record $R_\epsilon$ of the property.
iii) The record $R_\epsilon$ is a datum that can be registered by an observer, like a dial position or an audible click or a healthy cat.

 

[PeterDonis]

since CH is clearly the only correct interpretation of QM

Claims like this are out of bounds in this forum. Please review the guidelines:

https://www.physicsforums.com/threads/quantum-interpretations-and-foundations-guidelines.978330/

 

[Demystifier]

since CH is clearly the only correct interpretation of QM

Clearly to who?

 

[Morbert]

Clearly to who?

A confession: I made that comment with tongue in cheek. I was being a delightful jester.

 

[PeterDonis]

Clearly to who?

Please do not respond to off topic comments. You can report them if you feel the need to flag them somehow, but when a Mentor has already responded to them in the same thread, that's clearly not necessary.

 

[PeterDonis]

I made that comment with tongue in cheek. I was being a delightful jester.

First, it's a lot harder for people to realize you are jesting online, since we don't have all the nonverbal cues that normally signal a jest.

Second, unless you already have enough of a reputation as an expert in the subject that people will realize you can't possibly be serious, you should be very, very careful about making a jest that could be mistaken for a rules violation.

 

[bland]

That just pushes the vagueness from "measurement" to "macroscopic" (and "leaves a mark", too, since that's ambiguous).

Wouldn't 'leaving a mark', mean something that could reveal the forbidden knowledge, even if only in principle, like the way information is never lost, but it often is lost in every way albeit 'in principle'.

 

[PeterDonis]

Wouldn't 'leaving a mark', mean something that could reveal the forbidden knowledge, even if only in principle, like the way information is never lost, but it often is lost in every way albeit 'in principle'.

I have no idea. That's why I said the phrase was vague.

 

[PeterDonis]

This thread is now closed.