# Why are quantities called operators

1. Feb 5, 2012

### dpa

Is it because measurement of those quantities involves action on the system.
And is the idea that as light is to be used to measure momentum which effects its position fundamental of QM or is it merely like an analog to understand.

2. Feb 5, 2012

### tiny-tim

hi dpa!
the quantity is not called an operator …

its representation in quantum theory (in the equations) is called an operator, because it operates on the wave-function

eg, momentum is represented by a derivative, which is obviousy an operator!
no, the fundamental point is that when we measure both position and speed (in the same direction), there is a limit to the accuracy …

the disturbance caused by light (or anything else) is merely a consequence of that

3. Feb 5, 2012

### dpa

tim,
1. What causes the limit to accuracy.
2. How does momentum operate. I mean momentum is an quantity, clasically internal to body and it's motion.

4. Feb 5, 2012

### tiny-tim

heisenberg's uncertainty principle
momentum is a physical quantity, it doesn't operate

5. Feb 5, 2012

### dpa

sorry, i realize that I had not got something fundamental.
Again,
1. I meant to say how/why uncertainity principle. I mean, why is there uncertainity.
2. So what's an operater.

6. Feb 5, 2012

### juanrga

An operator is a mathematical object that operates in a given function.

Operators are not quantities, but many quantities $a$ have associated an operator $\hat{a}$ and verify the eigenvalue equation ($\Psi$ is a function)
$$\hat{a} \Psi = a \Psi$$
Light is not «used to measure momentum».

7. Feb 5, 2012

### tiny-tim

because that's how the universe is

8. Feb 5, 2012

### dpa

is that like saying einstein's GR is intuitive/imaginative.

9. Feb 5, 2012

### dpa

is that like saying einstein's GR is intuitive/imaginative.