# The concept of measurement

#### nughret

Measurement is probably the most important act when we attempt to understand our universe; without measurement we would have no information at all. The problem is including measurements in our theory leads to complications which our difficult to describe. I will outline a couple of the problems I have and would be happy for any input into the conclusions of these:

1) Every measurement is an interaction:
To gain information about a system, i.e. measure it, we must interact with it. For example we could perform a measurement by touch. Classically two objects interact by applying a force to each other. This then leads us to the conclusion that any reference frame which is measuring is non-inertial, and in classical mechanics (as well as in relativity) where we are performing continuous measurements, this will have a serious effect on our theory; obviously depending on the strength of the "force of measurement".

2) A measuring device measures itself:
For this point consider only the measurement of position. Space is relative; this means our measuring device can only measure the position of an object relative to itself. For this to be the case our measuring device will “know” that its position relative to itself is zero (we are here assuming that our idealized measuring device has zero extent). You may consider this point trivial, but you are fortunate enough to be endowed with a consciousness; surely we cannot assume every measuring device has a consciousness, but if our measuring device measures its own position this problem is resolved.
Now though we have to deal with complications arising from this self measurement. I cannot think of any classical problems (I would be glad if someone can come up with some) but they can arise in the case of QM, especially with continuous self measurement. Basically if our quantum mechanical continuous measuring devices self-measures then its wave function will always be (in its reference frame) proportional to δ(x), and this will lead to problems if we have other types of measuring devices, i.e. momentum measuring device.

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Gold Member
Well alot of things are relative when measured. Simple things such as voltage, what is 0v? Thats why its PD... the difference, just depends on what you take as 0, as long as its always the same then it should work.

#### russ_watters

Mentor
1. As you say the magnitude of the effect depends on the force. For most measurements, it is trivially small, but it is important to know how accurate your measuring device is. All in all, this is something scientists pay attention to, but is not typically a huge issue except in specific cases.

2. You don't say anything about why this is a problem macroscopically, but in any case, what you are saying is just a fundamental principle of physics called "relativity".

Gold Member
relativity does make things more confusing, mostly because you never think about it in every day life. Its always there though we just ignore it most of the time.

#### Andy Resnick

<snip>

1) Every measurement is an interaction:
To gain information about a system, i.e. measure it, we must interact with it. For example we could perform a measurement by touch. Classically two objects interact by applying a force to each other. This then leads us to the conclusion that any reference frame which is measuring is non-inertial, and in classical mechanics (as well as in relativity) where we are performing continuous measurements, this will have a serious effect on our theory; obviously depending on the strength of the "force of measurement".

<snip>
My understanding is that the is precisely the model used for decoherence: any system in thermal contact with the environment (a thermal reservoir) undergoes 'measurements' by the environment as it approaches thermal equilibrium. These 'measurements' are what leads to the emergence of classical behavior.

#### pallidin

Measurement(or observation) is dualistic. It can be passive or interactive.

For example, If I were a DEA, FBI or CIA agent observing your movements from afar using optical observation techniques, your movements are in no way affected by my observations as long as you are not aware(or suspect) of them. This is "passive"

On the other hand, if you were aware or suspect of external observation then your movements would be modified if you desire non-disclosure. Thus, in this case, my observations, real or perceived, have an interactive affect on your movements.

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