Questions tagged [measurements]

The tag has no usage guidance.

1
vote
1answer
36 views

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 ...
3
votes
1answer
118 views

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 ...
1
vote
1answer
233 views

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 ...
0
votes
0answers
51 views

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. ...
0
votes
0answers
72 views

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 ...
6
votes
4answers
231 views

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 ...
1
vote
0answers
45 views

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? ...
1
vote
0answers
52 views

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 ...
2
votes
2answers
94 views

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 ...
0
votes
1answer
50 views

Adding uncertainties when you cannot make any assumptions about the measurements [duplicate]

From what I read, when adding two $x$ and $y$ measurements with uncertainties $\delta x$ and $\delta y$, the resulting uncertainty is determined by doing: $$\delta z = \sqrt{(\delta x)^2 + (\delta y)^...
1
vote
1answer
83 views

High order (>3) derivatives measurements

Is it possible to directly measure derivatives of higher order than 3? It can be jerk for instance. By "directly" I assume that we can measure a signal proportional to the jerk value without ...
0
votes
1answer
114 views

What's the meaning of the $\sigma$'s of a particle physics measurement?

In particle physics experiments, one often quotes the result of measurement of an observable with $1\sigma$, $2\sigma$, $3\sigma$ ranges. The experiments typically give a best-fit value with a $3\...
2
votes
0answers
71 views

Indicator of the acoustic level in near field

Usually in acoustics, it is assumed that what we hear is the acoustic pressure. In far field - when the distance to the source is larger than the size of the source and than the wavelength - one has $...
5
votes
1answer
159 views

Homodyne detection as quantum measurement

How is homodyne detection a quantum measurement? In quantum mechanics, the way I'm used to think about obtaining the expectation value of an operator $A$ when the state is $| \psi \rangle $ is as ...
1
vote
1answer
49 views

How to average the loudness of a audio file from its amplitude?

I would like to average the loudness of different sounds so that their means could be used as a dependent variable in a statistical regression, but I have some troubles to understand how loudness ...
2
votes
2answers
79 views

Is there any experiment that doesn't have a finite countable number of outcomes?

There are a lot of random variables in physics that are, in principle, continuous and unbounded (canonical examples: time, position along each direction). In practice, it is my understanding that the ...
2
votes
1answer
47 views

Is an atomic clock's measurement principle independent of what a light clock provides

Special Relativity has the light clock which basically counts how often a photon goes forth and back along a fixed distance. Our most accurate clock, defining the second (at least as of today), is an ...
2
votes
1answer
88 views

Which program to analyse and graph laboratory measurements?

I'm interested in which program would you recommend for drawing graphs with x and y errors. Also to be able to analyse data and then graph it. My ideal graph would look like this: Until now I was ...
1
vote
1answer
85 views

How to check if a thing I have is exactly of 1 kg or not?

The definition of a kilogram is as far I know, Kilogram: The mass of a cylinder made of platinum-iridium alloy kept at International Bureau of Weights and Measures is defined as 1 kg. But it is ...
1
vote
1answer
68 views

How is the Length of a Meter Physically Measured

I have two parts to this question. First, I understand that the meter is defined as the distance light travels in 1/299,792,458 seconds. But how is this distance actually measured? The second is ...
1
vote
1answer
132 views

Accurate measurement of total lung capacity / vital capacity

I'm currently working on a practical problem: How to measure your total lung capacity and/or your vital capacity accurately. Any suggestions or experience? One of the methods that can be found with a ...
-1
votes
1answer
183 views

Fractional uncertainty question

Q. A ball falls freely from rest with an acceleration g. The variation with time t of its displacement s is given by s = 1/2 gt^2. The percentage uncertainty in the value of t is ±3% and that in the ...
0
votes
1answer
118 views

Smallest uncertainty ever achieved in position measurement in QM?

The Heisenberg uncertainty principle states that $$\Delta x\Delta y\geq\hbar/2$$ Since the magnitude of $\hbar$ is $10^{-34}$ we could measure both $x$ and $p$ with an uncertainty magnitude of $10^{-...
2
votes
1answer
46 views

Ultra low level light detection: What measurement principles are used?

I was reading up on how Avalanche Photodiodes(APD) are used to detect very low light levels, some are even used in "photon-counting" experiments (in Geiger mode). However, a survey of some commercial ...
1
vote
1answer
48 views

Is it possible to measure the number of spacetime dimensions?

My question is pretty straightforward to be stated but I don't know whether the answer is as easily reported. It is remarkable and very interesting that Physics work in (almost) any number of ...
1
vote
1answer
121 views

What is the relation between a measurement and an observable?

Observables are represented by Hermitian operators. First of all, it's a little strange (to me) that some measurable physical quantity is represented by a transformation (or linear map), given that I ...
0
votes
0answers
78 views

How much of Sun's mass is “regular” matter? How precisely do we know Sun's composition?

This is a question which spun of this Can the Sun / Earth have a dark matter core? I have argued that it is possible to pretty accurately estimate how much mass of Sun is in the regular matter, so ...
0
votes
3answers
180 views

Superposition of position eigenstates after measurement of position?

In a discussion with someone recently, the person made the following statement: Even when we observe a particle, it is still in superposition of position states within the resolution of the ...
-1
votes
3answers
98 views

How come 84000 has 2 Significant Digits and 84000.0 has 6 Significant Digits?

Isn't there a decimal point after every whole number. So shouldn't 84000 also have 6 Significant Digits?
2
votes
0answers
43 views

Question about statement of uncertainty

I have a doubt about uncertainty of a measurement. I have a sample of a specific material which is my reference material and it is calibrated and NIST traceable. This sentence is in its data-sheet: ...
1
vote
2answers
66 views

A doubt related to Significant Digits

Could someone please explain this statement to me "Reporting the result of measurement that includes more digits than significant digits is superfluous and also misleading since it would give a ...
0
votes
2answers
230 views

How the last digit in significant figures is considered doubtful?

If a reading of a length on meter rod is 44.6cm with least count of 1mm And last point of the length is exactly on 44.6 not in between of 44.6 or 44.7 Then how is it doubtful?
1
vote
0answers
99 views

Causality in Quantum mechanics

If we consider a quantum preparation procedure on a system followed by a projection measurement as constituting two sequential events in space-time (which seems plausible ,given that the Copenhagen ...
1
vote
2answers
163 views

Radioactive Dating Accuracy After 60,000 years [duplicate]

Why is radioactive or Carbon 14 dating only accurate for 60,000 years? In theory, there should always be at least SOME Carbon 14 in everything that was living at any given time. This is because no ...
1
vote
0answers
94 views

How is thrust per unit power measured?

Typically, thrust efficiency is measured in specific impulse, as thrust per fuel consumption rate: $$I_\text{sp} = \frac{F_\text{thrust}}{\dot{m}_\text{fuel}} \quad \text{or} \quad I_\text{sp} = \...
-2
votes
3answers
132 views

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 ...
2
votes
2answers
101 views

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 ...
1
vote
1answer
42 views

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 ...
-1
votes
4answers
638 views

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?
0
votes
1answer
226 views

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 ...
2
votes
0answers
32 views

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 ...
0
votes
0answers
82 views

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 ...
0
votes
1answer
2k views

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 ...
3
votes
2answers
97 views

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 ...
2
votes
1answer
67 views

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 ...
1
vote
1answer
41 views

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 ...
5
votes
3answers
266 views

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 ...
3
votes
2answers
225 views

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 ...
2
votes
2answers
81 views

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 ...
1
vote
1answer
63 views

Generalised representation of a 2x2 Positive Operator-Valued Measure

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}{...