Questions tagged [measurements]

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1answer
78 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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1answer
56 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
139 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
175 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 ...
2
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3answers
86 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
226 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
40 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
131 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
59 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
173 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
447 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
729 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
104 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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0answers
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
57 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
41 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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0answers
79 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
57 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
110 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
114 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
136 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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0answers
94 views

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) ...
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0answers
23 views

Can we track phonon's path similarly to how we track electron's path in a bubble chamber?

An answer to a recent question argues that Standard Model's particles like electron are "real" because we can see their tracks in e.g. bubble chamber. So now I wonder: can we track quasiparticles ...
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1answer
48 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 ...
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1answer
191 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 ...
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1answer
402 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 ...
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0answers
111 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 ...
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4answers
301 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 ...
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0answers
58 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 ...
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2answers
104 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 ...
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1answer
69 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)^...
2
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1answer
89 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 ...
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1answer
232 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\...
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0answers
85 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 $...
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2answers
305 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 ...
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1answer
96 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
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2answers
89 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
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1answer
50 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 ...
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1answer
98 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 ...
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1answer
112 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 ...
2
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1answer
105 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 ...
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1answer
162 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 ...
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1answer
514 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 ...
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1answer
145 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
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1answer
57 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 ...
2
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1answer
61 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 ...
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1answer
354 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 ...
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84 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 ...
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3answers
252 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 ...

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