Questions tagged [measurements]

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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?
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1answer
84 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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219 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
82 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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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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169 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 ...
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1answer
72 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
74 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
50 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 ...
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1answer
72 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^{\...
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2answers
58 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 ...
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1answer
40 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 ...
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1answer
267 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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95 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 ...
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3answers
66 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 ...
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1answer
69 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
73 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 ...
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1answer
24 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 ...
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1answer
44 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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47 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 ...
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4answers
126 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 ...
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2answers
53 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 ...
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1answer
133 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?
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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 ...
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1answer
114 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 ...
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6answers
2k 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. ...
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1answer
69 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 ...
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1answer
60 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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33 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 ...
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1answer
54 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 ...
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1answer
81 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/...
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0answers
28 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 ...
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3answers
58 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^...
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3answers
73 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....
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1answer
19 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 ...
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1answer
86 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 ...
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1answer
44 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
106 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?
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2answers
302 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] -...
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1answer
136 views

Why does taking more readings reduce random error? [closed]

So I was tossing a coin And I did two experiments Experiment 1: Tossing same Coin with no fan with different torque each time and did'nt much care about orientation of the coin, 8 times I got 5 ...
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1answer
73 views

Misconception in difference between Accuracy and precision?

I know that The more systematic errors > the less accurate The more random errors > the less precise So lets say we have a stopwatch which has a zero error -1 (every ...
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42 views

Can time be described as a distance?

Can we do away with time in the coordinate system? When I picture the x,y and z planes and plot positions, the idea of adding a time element seems redundant. The time element adds a reference to an ...
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3answers
53 views

Error estimation during measurements with high standard deviation

I want to measure average distance between fixed metal construction and water, as shown in the picture below to predict water flood. Let's call this distance water level h. If the water level starts ...
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1answer
40 views

Measured Earth local gravity

I am looking for measured data of the local gravity on Earth. I know this website (https://www.ptb.de/cartoweb3/SISproject.php). The idea is nice but it looks a bit old and I would like to have an ...
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61 views

Sequential Stern-Gerlach experiment on a budget

The sequential Stern-Gerlach experiment is important for historical and theoretical reasons; it demonstrates in a conceptually simple manner the consequence of having non-commuting observables with ...
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1answer
49 views

Uncertainity calculation [closed]

The length ($l$) and width ($w$) of a rectangle is ($15.6\pm 0.1$) cm and ($10.80\pm 0.01$) cm respectively. Calculate the area and uncertainity. Solving: $$\text{Area}=15.6\times 10.80$$ $$\text{...
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1answer
50 views

Measurement coil for magnetic fields, linearity and frequency of the field

I am trying to measure an oscillating magnetic field, of a solenoid and in the air gap of a transformer core. I have a Hall effect sensor, but its frequency range is limited, so I have put together a ...
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1answer
77 views

Precision in cosmological constant

The observed cosmological constant, what is its precision? How precise do we know is it not a fluke or misinterpretation of cosmological data?
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2answers
90 views

What does “as small as a fraction of an angstrom” mean?

I was reading my school textbook in which the following statement was given: The resolution of such an electron microscope is limited finally by the fact that electrons can also behave as waves! (...
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Special Theory of Relativuty [duplicate]

I'm trying to learn STR on my own. As I read through a few books and saw a few video lectures - I feel to really understand STR you need to really understand and appreciate precisely (and formally) ...