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Questions tagged [measurements]

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Is this analysis contradictory with the $2$'nd measurment?

Summary and Motivation "The below idea is about making a mathematical statement on system $2$ which induces a measurement on system $1$ while $1+2$ obeys unitary evolution." Introduction There are $...
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3answers
261 views

Scalable quantum computation vs Uncertainty Principle

tl;dr. : If we want the result of a quantum computation to have scalable more bits, we will need inner measurements to have scalable precision, if we already know the result of the computation (from a ...
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0answers
31 views

Is there a way to argue that the appearance of the dependence of past states is stochastic behavior?

Suppose in a particular set of observations that you observe what appears to be a correlation of future results based on past results. Is there a way to be certain that such a phenomena isn't just the ...
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148 views

Energy cost of the measurement without perturbing the system?

Background Let's say I have a Hamiltonian $\hat H$ (assume the Schrodinger equation) and it be in an arbitrary eigen-energy state: $$ \hat H_{\text{system}} | m \rangle = E_m |m \rangle$$ And I ...
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1answer
32 views

Special Relativity - events and measurements

I'm trying to re-ask the Q based on the feedback I got on this forum. Here is the premise for the Q. Consider an IRF, S. Assume only 2-D i.e (x,t). And the observer, O, is as assumed typically "...
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1answer
29 views

If the only two empirical measurements we can make are change in length and passage of time, how are dimensions and units of quantities formulated?

So this is something really confusing to me. Recently a professor of mine gave a talk and said something that I thought was very interesting and kind of crazy. "The only two quantitative measurements ...
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0answers
22 views

Why doesn't a lock-in measure the full voltage from a function generator? (3$omega$ setup)

I'm trying to build a setup for the 3$\omega$ technique, and the equipment is making my brain hurt. When I send a signal directly from the function generator to the lock-in, it only measures about 71% ...
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1answer
179 views

Decoherence Free Subspaces and how they stay this way, using the Zeno Effect

I am currently reading papers discussing the Zeno Effect, which discuss how measuring a system at high frequencies can almost freeze the state of a system, or keep the system in a specific subspace of ...
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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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1answer
44 views

How do you measure voltage from first principles? [closed]

I've always wondered if you were thrown back in time with just my cell phone, how could I charge it... I know how to make electricity, but how do I check the voltage if all I have access to is, shall ...
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1answer
24 views

Uncertainty Decomposition in velocity signal

I have a timeseries with repeated measured velocity signal (almost periodic signal). To find the mean time-varying velocity, I have decomposed the signal and computed the ensemble-average and the RMSE ...
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20 views

SI redefinition of the kilogram - what is one measuring? [duplicate]

I have been reading about the new SI units and specifically, want to get a better understanding of the definition of a kilogram. It was written that the kilogram will be defined in terms of Planck's ...
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1answer
247 views

What are the longest half-lives we can detect experimentally? What stops us going further? Are we trying to?

Xenon 136, apparently, has a half-life of 2.11×1021 years. This strikes me as a humongously long time to run an experiment, clocking in at about 11 orders of magnitude longer than the age of the ...
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0answers
235 views

What is actually happening when a magnetoresistive magnetometer degausses itself?

I'm trying to understand if anisotropic magnetoresistive devices themselves (not other passive ferromagnetic components) can become magnetized, and if so, how the application of a DC 1.1 gauss field ...
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2answers
2k views

Total and static pressure: which one is measured?

Which pressure is measure using measuring device in a pipe flow. My first intuition that it is the static pressure. This is confirmed by this Wikipedia article (link) The concepts of total ...
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0answers
55 views

How does weak measurement affect quantum state?

I'm trying to understand how to describe the quantum state after weak measurement using these two toy examples. Hopefully, these simple examples and your answer will help others who want to learn ...
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0answers
33 views

How to measure a Bell inequality violation in IBM Q? (Or analytically)

I made some circuit to prepare a 2 qubit state, but I am having trouble understanding how to measure Bell's inequality. I know the inequality is of the form $$|E(a,b)-E(a,b')+E(a',b)+E(a',b')| \leq 2$...
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1answer
40 views

Question on finding the relative error

The question is as under: Find the relative error in Z if Z= A/B Z= A(B^-1) Relative error in A is ΔA/A and in B is ΔB/B My solutions are : As A and B are being ...
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1answer
36 views

What is the meaning of true value and uncertainty from a measurement?

I am taking the following example: Lengths of an object are measured as : 5cm , 5.2cm , 5.1cm , 5.5 cm. Calculate the true value and the error . The answer is True value = 5.2 cm error = 0.15 ...
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2answers
56 views

Measuring the Thickness of a Paper

I'm supposed to come up with a way to measure the thickness of paper. I've come up with the following approach. Let us suppose we have a paper of thickness $?$, length $a$, breadth $b$, density $\rho$...
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2answers
56 views

Question on Time Interval [closed]

The question states The mean life of an elementary particle pion is $2×10^{-7}$ nanoseconds. The age of the universe is about $4×10{^9}$ years. Identify the time interval that is approximately half ...
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1answer
20 views

Calculating percentage error with significant figures

I have a problem with significant figures. In an assignment, the paper gives us data to $3$ significant figures to calculate some values but afterwards requires us to calculate the percentage ...
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1answer
6k views

How to calculate uncertainties of a natural exponential function?

(I apologize if this should be posted in mathematics, however I chose to post it here as it's technically about physics) I conducted an experiment in which position of items were shifted on an object,...
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0answers
16 views

Effect of variation in day length on timekeeping

I was reading about atomic clocks when I came to know that precise measurements of time have shown that the time for earth's rotation is slightly more that 24 hrs (on the order of 10^-3). I was ...
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1answer
406 views

How to estimate the frequency of the sample rate?

I'm studying for my class of physics laboratory and I need help with something: Let's say I need to deduce the constant of elasticity of a spring and I will do it using a dual-range force sensor, ...
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0answers
10 views

How to Measure Electric-Field Probe Sensitivity?

I am doing some reading on current methods for detecting electric fields. A figure of merit for the systems is, naturally, the minimum strength of field that can be measured, in units of Volts per ...
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1answer
43 views

What is the difference between resolution and least count of a measurement? [closed]

1.What is the difference between resolution and least count of a measurement? In my physics book the statement of a question is as under : " The time taken by a pendulum for 100 oscillations is ...
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0answers
23 views

Uncertanty in Chi-squared measurements, spectra-like

Let's say I have some observational data. For example: in a certain wavelength interval I measure the flux, 10 000 points, and I have the uncertainty for each one of them. For example: ...
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0answers
39 views

Can we describe the process of measurement in functional form?

The question was can we describe the process of measurement in functional form? We know hermitian operator can be used to represent measurement. In Heisenberg's picture, it even described the "motion"...
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1answer
45 views

Finding the range of time scales by dividing length scales by speed of light

My book states : The scope of physics is truly vast. At one end, it studies phenomena at the very small scale of length (10^-14 m or even less) involving electrons, protons, etc.; at the other ...
5
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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 ...
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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 ...
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0answers
31 views

Subtractive cancellation errors in the lab

I am quite familiar with the problem associated with loss of significance in numerical analysis. This problem seems to be an even bigger problem in the science lab, where subtracting one physical ...
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1answer
79 views

Is it true that in practice quantum fields are *always* measured by means of coupling to particle detectors?

I'm watching some lectures on Relativistic Quantum Information and in one of them - available here - the instructor is talking about measuring quantum fields. He then says basically that in real ...
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1answer
54 views

Why do many phones have very different specific absorption rate (SAR) values for head and body? [closed]

I'm deciding on which mobile phone to buy, and since we don't really know if electromagnetic radiation does or does not have adverse health effects (I'm considering cancer and fertility), I'd rather ...
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3answers
214 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
135 views

A theoretical limit of accuracy?

Is there an agreed upon/conjectured theoretical limit to our ability to resolve the accuracy of time?
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1answer
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
176 views

True white collimated light / nobody uses this?

Based on a previous question is this the difraction limit of our microscope? , I got the idea to use a true white parallel back light to illuminate an object for measurements (target accuracy is 1 ...
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1answer
43 views

Least count of Vernier

Is least count not always 1 main scale divsion-1vernier scale divsion or is it always least reading possible on main scale/ no of vernier divsions. question: 1 main scale reading be 1 mm Lets say 5 ...
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0answers
27 views

Would the voltmeters give different readings in a circuit with induced current?

$\def\vE{{\vec{E}}}$ $\def\vD{{\vec{D}}}$ $\def\vB{{\vec{B}}}$ $\def\vJ{{\vec{J}}}$ $\def\vr{{\vec{r}}}$ $\def\vA{{\vec{A}}}$ $\def\vH{{\vec{H}}}$ $\def\ddt{\frac{d}{dt}}$ $\def\rot{\operatorname{rot}}...
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2answers
3k views

What are the problems with using spirit level experiment to prove the flat-earth theory?

Full disclosure: I do not believe the Earth is flat. However, a guy recently posted a YouTube video that purports to prove that the Earth is flat due to there having been no changes in the position ...
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0answers
80 views

How do we quantify and measure the net entropy of today's universe?

We know that the Universe evolved from a very low entropy state in the early universe to very high entropy state of today's universe. What is the quantitative definition of the net entropy of the ...
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1answer
26 views

The difference in using a predetermined error or standard deviation for calculations

Please edit the thread's tags and content if there are mistakes. I am not sure where If I'm posting my question with the right tags. This is a simple question regarding the study of Linear Motion by ...
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1answer
225 views

Does a precise definition of the meter not involving light exist, so that variation of $c$ can be tested?

The second is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. The metre ...
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0answers
63 views

Spacetime curvature and measurements

From a programming perspective, I've always thought of gravitational influence as a kind of vector field, (crudely drawn) which seems to attribute to the motions of bodies through the field ...
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1answer
73 views

Are there ways to detect/measure photons that don't involve electrons?

As per question, are there ways to detect photons, and/or to measure their energy, that don't involve any interaction with electrons? And if yes, are there detectors which use photon interactions with ...
2
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1answer
234 views

How to measure a static electric field?

I looked up google but didn't find any design for measuring electric field that doesn't vary with time. My own idea is to use two parallel plates (like a capacitor but without the dielectric). In an ...
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1answer
26 views

Convert negative parameters of Altitude Height into possitive using BMP180 Sensor

I am using BMP180 I2C sensor interface with Arduino and with the help of mentioned below code ...
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1answer
232 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 ...