# Questions tagged [measurements]

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775 questions
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### Tools for measuring vibration/sound of shaking

I would like to know how I can measure the vibration coming from my hand shaking something. My experiment is like this. I put a ball made of strong glass into a closed empty box made of strong glass ...
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### Pure state before and after measurement

Before the measurement of an observable, the quantum state is $$|\Phi\rangle = \sum_i c_i |\psi_i \rangle,$$ with $|\psi_i \rangle$ called "pure states". Once the measurement is done, the quantum ...
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### Measuring the entropy of mixing ideal gases

Suppose I have two compartments of volume V separated by a barrier. Each one has an ideal gas, pure, of N molecules, and with equal pressure and temperature. However, I don't know whether both gases ...
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### Uncertainties with GUM for multiple measurements

I work with the GUM method to determine measurement uncertainties. As I understand the method, the model created for a particular measurement setup provides an uncertainty for a single measurement. ...
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### Mean velocity in unsteady flow

I'm starting hydraulic experiments, where I'd have to measure velocity in an unsteady flow with a device called Acoustic Doppler Velocimeter (acquisition rate: 100 Hz) . In DSP terms, I'd have a ...
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### Is there a way to measure this observable in QM?

Let a quantum system be described by Hilbert space $\mathscr{H}$ and let $|\psi\rangle$ be an arbitrary state. Define the operator $$P=|\psi\rangle\langle \psi|$$ This is hermitian. It has two ...
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### Observables in Relativistic QM

I am thinking about observables in relativistic quantum mechanics (RQM). The position operator (X) is considered to be an observable in QM vs time, which seems to be just a parameter. What about RQM? ...
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### Best ways to record data on interference pattern using Tektronix DPO 5204B oscilloscope?

I am looking at an interference pattern signal using a Tektronix DPO 5204B oscilloscope which has windows 7 and TekScope software. This interference pattern grows and shrinks with time (approximately ...
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### How to physically make sense of these generalized measurements?

In Quantum Mechanics one often by postulate has that the system is described by a Hilbert space $\mathscr{H}$ and observables are described by hermitian operators $A\in \mathfrak{A}(\mathscr{H})$ on ...
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### How to define approximate meter using primitive to no tools? [closed]

Imagine you've lost somewhere in the wild due to some catastrophic event, and don't have any measurement tools with you. How do you find approximate meter, millimeter, etc. with materials like sticks ...
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### Simple question about propagation of error/uncertainty

I'm stuck on a seemingly simple question about propagation of error. Say we repeatedly measure the speed of a particle, and we estimate the uncertainty in the measured speed to be 10 percent. What is ...
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### Several thought experiments on the difference between direct and indirect measurements

I have read several books regarding basic concepts in physics but I'm still puzzled by own lack of knowledge. I shall not give definitions, because I presume we are all familiar with them. I do not ...
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### How do I find the instantaneous velocity in real life? [closed]

I am learning about Instantaneous speed and velocity in Khan Academy. I understand the concept and calculation, but how do I know the different instantaneous speed at different points in real life?
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### Propagation of errors

I was going through a problem in finding error in measurement of focal length, if object and image distances are given with appropriate errors. I found two methods to find the error at different ...
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### Measurement of a real apparatus

Do we know the approximate form of measurement operators for a realistic measuring apparatus? For instance, consider a cloud/bubble/spark chamber. What operator $\hat X$ can be considered to measure ...
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### How to measure the Thermal Noise in a conductor?

Johnson Noise in a conductor is very small: between 10-9 to 10^−12 V How to measure it using an oscilloscope and amplifier? If it were a resistor that would be easy, but conductors have much smaller ...
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### Uncertainty in the average of two measurements (with their respective uncertainty)

I know that generally the uncertainty in the mean of a sample should be equal to: $\frac{V_{max} - V_{min}}{2}$ where $V_{max}$ is the maximum value and $V_{min}$ the minimum value of the sample ...
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### Human body as a measuring instrument

One can easily find descriptions of human eyes and ears used for measuring phenomenons. Finding what resolution do eyes have or what frequencies can human hear is no trouble. It is also seen in scales ...
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### How did they measure -20 db sound?

Current world record for quietest room is Microsoft anechoic chamber that was measured at -20 db.How did they measure it? I mean,lowest noise measurement microphone I found is 1 inch special low ...
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### Measurement of decay rates and statistics

During my nuclear physics lab I was measuring decay rates of Fe59. The average was around 20 decays per second. But I did two series - in one I was measuring number of decays per 90 seconds and in the ...
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### How do you physically measure voltage and current?

Short Version: How exactly do (modern) voltage/current meters measure the numbers they do, from first principles? References much appreciated. We measure length with a ruler, time with a clock, and ...
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### Are there places in space completely devoid of light?

I'd be interested to know how far from the sun you'd have to be for it to feel 100% dark, as the human eye sees it. But also, are there places in space so far removed that there is nothing visible ...
### What experimental bounds do we have on big $G$?
I know that there has been a large amount of controversy surrounding the exact value of the gravitational constant $G$, but I know that there is not a substantial difference in the measured value. So ...
Let {$E_{i}$} be a set of 2x2 POVM operators, satisfying $\sum_{i}E_{i}=\mathbb{I_{2x2}}$. We know that a general 2x2 Hermitian matrix (say, $H$) can be represented by  H = \left[{\begin{array}{...