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Questions tagged [measurements]

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2
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1answer
42 views

When the barometer drops, my well level seems to rise - how?

I have a deep water well, with a length of 1/4" drip tubing, open at the bottom, taped to the pump 180' down. The other end of the tubing is connected to a gauge pressure sensor in the cabinet of my ...
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0answers
6 views

Calculating Uncertainty with Mean Absolute Deviation

I'm taking an Introductory Physics course and, when learning about approximating uncertainty, I was wondering if a more quantitative method would be to calculate the Mean Absolute Deviation using the ...
2
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1answer
54 views

Heisenberg uncertainty principle meaning and using it to determine approximate sizes confusion

I was always under the impression that the Heisenberg uncertainty principle tells us to what precision we can make a momentum measurement given a position one. However, when doing quantum mechanics ...
0
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1answer
70 views

Reflection Vector (Ray Tracing)

Snell's law of refraction at the interface between 2 isotropic media is given by the equation: \begin{align} \tag{1} n_1 \,\text{sin} \,\theta_1 = n_2 \, \text{sin}\,\theta_2 \end{align} $\qquad$ ...
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0answers
16 views

Attempting to calculate the mobility from SCLC measurement (space charge limited current)

I performed an SCLC measurement on a p-type material (hole transport layer) for a solar cell. Referring to the wikipedia page:https://en.wikipedia.org/wiki/Space_charge The relationship between $J$ ...
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0answers
28 views

Can traps measure which path in an interferometer?

it is possible in an interferometer to block the paths with screens so that the particle is well located in one of the paths. I wonder if it is the same when instead of screen we use traps in which a ...
0
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1answer
84 views

Are there any physical quantities that are loglog?

Most units, e.g. for length, time, mass, etc, are linear in relation to the underlying property. But there are some special quantities, where the units can vary several orders of magnitude within the ...
1
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1answer
60 views

A delicate measurement?

The identity operation means that we are not disturbing the system at all. However, if we consider the following operation $$ \begin{pmatrix} 1 + \epsilon & 0\\ 0 & 1 + \...
1
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1answer
40 views

Experimental measurement of the radial excess

Update: I have updated the the question, see below. The radial excess due to the curvature of spacetime is a well-known prediction of General Relativity that is often mentioned in introductory texts, ...
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0answers
17 views

Porous nanostructure in TGA and DSC

How does the porosity in porous nanostructures, such as mesoporous silica, influence the observed response in Thermal Gravimetry Analysis (TGA) vs Differential scanning calorimetry (DSC) thermograms?
1
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1answer
37 views

Profile of a platinum-iridium meter bar

The former meter standard, platinum-iridium meter bar, had a specific cross section somewhat resembling mixed variant of letters "X" and "H" with serifs (Image source): What was the reasoning behind ...
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3answers
75 views

Shot noise in optics

There are many explanations to be found about shot noise in optics, but the answers I find are incompatible. There are three ways shot noise in optics is explained. (Note that according to ...
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2answers
77 views

Work definition does not make any sense for coherent states? TPM (two projective measurement) scheme

I'm talking about the relation that if we are in a isolated, quantum system that only allows for work exchange with the surrounding system we know from the first law of thermodynamics that $\Delta E = ...
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0answers
10 views

Can radio waves be detected using a coulomb torsion balance?

Radio waves are almost everywhere. shouldn't the coulomb torsion balance be sensitive to any passing through the device?
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1answer
100 views

How can a Lego version of a Kibble balance measure the Planck constant?

As the picture shows below [][1] in a Kibble balance, one can drop out the measurement uncertainty of $B$ (magnetic flux intensity) and $L$ (length of coil) by the use of two modes, force mode and ...
2
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1answer
39 views

Calculation of Thermometer Time Lag?

A couple of days ago, contributor 'Rogan Parks' posed the question below the page break. I was about to fully answer it when the OP for no apparent reason deleted his question. I find it an ...
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1answer
66 views

Why does my thought experiment involving entanglement appear to violate the speed of light?

I have a thought experiment that "seems" to allow messages passed at faster than the speed of light. Since that is impossible I'd like to learn what is wrong with my experiment/reasoning. Start with ...
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2answers
41 views

Relation Between Speed of Light & Reflected Angle (Fizeau–Foucault)

I have a bachelor's in physics & its recently struck me that I do not understand, semantically, what phenomenon allows us to measure the speed of light through air in a small room with a laser and ...
1
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1answer
58 views

Is a quantum channel essentially either a unitary evolution or a measurement?

I'd like to understand exactly what people mean when they speak of quantum channels. As I understand it, we can represent a channel by a set of Kraus operators, $M_i$, which satisfy $\sum_{i}M^{\...
2
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2answers
50 views

Relation between momentum and momentum uncertainty

I have read the following sentence in a section which is explaining why particle accelerators need such high energy: As we require high precision position measurement, the Heisenberg uncertainty ...
0
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1answer
35 views

How to write down the state after a measurement if the result was not recorded?

I measure the spin of an unknown qubit in the computational basis but I do not record what the outcome of the measurement was. How should I now describe the state? If it is described as a maximally ...
1
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1answer
140 views

A new definition of mass using Planck constant

Tonight in a italian television news channel well known internationally I have heard that almost certainly the definition of mass that we currently know will be obtained by means of the constant ...
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0answers
8 views

MOLE - interferometers of PTB and NMIJ

For the definition of the mole, two interferometers were used: the first is a Fizeau spherical interferometer belonging to the PTB (Germany) and the second is the NMIJ interferometer, in order to ...
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1answer
90 views

Is the uncertainty principle a circular argument? [closed]

Uncertainty is due to the measurement techniques humans tend to use requiring photoelectric effect. The Planck constant is due to the photoelectric effect. If the standard deviation of measurement was ...
2
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3answers
61 views

Is it possible to suggest a physical experiment in which measurement errors are dramatically increasing? (due to the accumulation of errors)

Is it possible to suggest a physical experiment in which measurement errors are dramatically increasing? (due to the accumulation of errors) Maybe some magnetic field can be used here? Or any other ...
0
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1answer
49 views

Nielsen and Chuang 2.2.3: state of a qubit after measurement

(Apologies for the poor typesetting of kets. I haven't been able to figure out how to do that on this site). On page 85 of Nielsen and Chuang's textbook, they write that the probability of obtaining ...
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1answer
65 views

Problem in measuring uncertainty

While performing an experiment involving a bar pendulum, we had to measure the time period of one oscillation by measuring the time taken for 30 oscillations. There is some confusion regarding ...
0
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1answer
18 views

Error of capacitnace measurement using a multimeter

physics stack exchange Many multimeters have a setting for measuring capacitance. The multimeter I have has a precision of four significant figures, or in other words, it returns the value in the ...
0
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1answer
34 views

How do multi-meters measure capacitance?

Many multimeters have the ability to measure capacitance along with the ability to measure AC voltage, DC voltage, current, resistance ... The only equations I am aware of that determine capacitance ...
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1answer
25 views

Changing transmittivity of polarizing filters for different wavelengths

I suppose, that a polarizing filter works as described here: The polarizing filter absorbs or reflects some of the photons in the light beam, which is why the energy/power measured after the ...
0
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4answers
99 views

Difference between unpolarized beam and half H+V polarized beam

Let's say that we have got two light beams. Beam A is not polarized at all. Beam B is composed of half H polarized photons + half V polarized photons. Is there any way to measure those two beams in ...
2
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2answers
52 views

Is a quantum gate different from taking a measurement?

I'm reading a book on quantum computing. It is a very non-technical book, and I do not need a very technical explanation. I keep on seeing the words quantum gate pop up, and I'm wondering whether this ...
1
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1answer
123 views

Heisenberg choice of the observables and different outcomes [duplicate]

Somebody could help me to clarify how its possible that different choices of the observable to measure can lead to different outcomes of the observed system state?
3
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1answer
46 views

How to measure width of a very narrow spectral peak without expensive instruments?

Lasers typically have very narrow peaks in their spectrum, much narrower than 1 nm. It would require quite a high-resolution spectrometer to measure such a peak precisely. But if I simply need to find ...
0
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1answer
17 views

Strain gauge vs linear variable displacement transducer

For measuring deformation experiments, it seems that strain gauges and linear variable displacement transducers (LVDT) are commonly used. If the material exhibits geometric linearity, then couldn't we ...
2
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6answers
1k views

Are uncertainties higher than measured values realistic?

Whenever I measure a positive quantity (e.g. a volume) there is some uncertainty related to the measurement. The uncertainty will usually be quite low, e.g. lower than 10%, depending on the equipment. ...
0
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1answer
62 views

Measure temperature of a heater wire

I would like to measure the temperature of a heating wire (tungsten) that's wrapped around my sample. Does anyone have a better idea than using a infrared thermometer, which has relatively low ...
2
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1answer
44 views

Does measurement with unknown outcome transform a pure superposition state into a mixed state?

$\def\+{\!\!\kern0.08333em}$ Say I have a spin-1/2 particle in a general, pure superposition state $$ |\psi\rangle=\alpha|\+\uparrow\rangle+\beta|\+\downarrow\rangle, $$ or equivalently $$ \rho=(\...
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0answers
29 views

Entangled photons

How do you identify correlated events between a pair of entangled photons if these photons arrive at different times at their respective detectors?
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1answer
31 views

Precision of measuring $N$-fold period one time vs single period $N$ times

One quick (I was told, trivial) question. Is it more precise to measure $N$-fold period one time, or to measure a single period, $N$ times? Ex. You could let the circular pendulum do 10 swings and ...
1
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1answer
38 views

The uncertainty of a toy car's measured average speed

Suppose I have a battery-powered toy car, and I am told to design an experiment which would allow me to determine the average speed of that toy car over, say, a meter, and to account for experimental ...
2
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1answer
62 views

Sequential spin measurement misunderstanding

I'm having some trouble understanding something concerning sequential measurements. Suppose we have a wave function consisting of a superposition of spin up and spin down in the z direction (spin=1/...
2
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0answers
24 views

Resource to learn modelisation of quantum measurement

I need to learn how we do model quantum measurement (for experiments for example). If we study a quantum system X, when we measure it we usually consider a projective measurement (so we use the ...
2
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3answers
49 views

Confusion with regards to uncertainty calculations

Let’s say we have a scenario of a ball being released from the top of the building. This can be modeled simply with the kinematics equation $S=ut +\frac{1}{2}at^2$, which reduced to $S=\frac{1}{2}at^...
0
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3answers
62 views

Uncertainty when multiplying two measured values

Until now I have been using this rule: "The result of the multiplication or division must not have more significant figures than the operands." when dealing with uncertainty in multiplification....
0
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1answer
17 views

Measuring change in magnitude of electric current due to change in conducting material characteristics

I am new to this community and hence may not be aware of all the rules. Please do let me know if you think I am violating any. I will try to get it corrected. I am trying to get an answer to the ...
2
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1answer
49 views

Covariance matrix after projection on gaussian state

I cannot understand the proof in Eisert article (https://arxiv.org/abs/quant-ph/0204052) about finding the covariance matrix of a state after projecting some of its modes onto a gaussian state. We ...
1
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1answer
35 views

I couldn't figure out something about mass and weight measurement

Today when I was studying physic for my University enterance exam I have stucked in one topic. I understood the difference between mass and weight. Mass is the amount of material an object contains ...
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1answer
71 views

Non-Hermitian measurement operators

I am familiar with the von Neumann projection postulates but I don't know how one can write a non-Hermitian measurement operator using von Neumann measurement model. Does anyone can help?
13
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2answers
191 views

Numerical Simulation of Stochastic Master Equation using Stochastic Schrödinger Equation (Wave Function Monte Carlo)

Consider a time independent system coupled to a Markovian bath, the equation of motion for the density matrix of the system has to take the form \begin{equation} \dot{\rho} = - i \left[H,\rho\right] -...