What is Measurement: Definition and 1000 Discussions

Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependent on the context and discipline. In natural sciences and engineering, measurements do not apply to nominal properties of objects or events, which is consistent with the guidelines of the International vocabulary of metrology published by the International Bureau of Weights and Measures. However, in other fields such as statistics as well as the social and behavioural sciences, measurements can have multiple levels, which would include nominal, ordinal, interval and ratio scales.Measurement is a cornerstone of trade, science, technology and quantitative research in many disciplines. Historically, many measurement systems existed for the varied fields of human existence to facilitate comparisons in these fields. Often these were achieved by local agreements between trading partners or collaborators. Since the 18th century, developments progressed towards unifying, widely accepted standards that resulted in the modern International System of Units (SI). This system reduces all physical measurements to a mathematical combination of seven base units. The science of measurement is pursued in the field of metrology.

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  1. entropy1

    I Is a measurement a correlation?

    If we make a measurement, does that mean that the outcome of the measurement gets correlated with the measured value?
  2. entropy1

    I Is There Only One True Outcome in the Many-Worlds Interpretation?

    Suppose we have a photon in superposition of reaching detector A or detector B. Then, in Everett-worlds, both outcomes (detection at A/detection at B) are true, but in different worlds (##|U_x\rangle|x\rangle##). But if we observe the law of conservation of energy and the quantisation of the...
  3. Dr. Courtney

    Direct Echo-Based Measurement of the Speed of Sound - Comments

    Greg Bernhardt submitted a new PF Insights post Direct Echo-Based Measurement of the Speed of Sound Continue reading the Original PF Insights Post.
  4. E

    I Energy measurement on superposition of states

    We have a 1 dimensional infinite well (from x=0 to x=L) and the time dependent solution to the wavefunction is the product of the energy eigenstate multiplied by the complex exponential: \Psi_n(x, t) = \sqrt{\frac{2}{L}} \sin(\frac{n\pi x}{L}) e^{-\frac{iE_n}{\hbar}} Now, I want to create a...
  5. G

    What is the unit of measurement in this X-Ray?

    Hi I hope I'm not posting under the wrong forum but I'm pretty sure this is physics related. If you go to have an X-ray, in the film there will be information detailing the dosage you just received. However I encountered a rather unusual one like this: 70kV 100mAs 100mS -- dGycm2 El_s:300 As...
  6. E

    I Wave function collapse and measurement rule

    Hi everyone, I'm kind of new in the QM world and I'm having difficulties understanding the superposition and the measurement principles together with the have function collapse. This is how I understand these principles: Superposition: While not measuring, the particle is in a superpsotion of...
  7. A

    Quantum state of system after measurement

    > Operator $$\hat{A}$$ has two normalized eigenstates $$\psi_1,\psi_2$$ with > eigenvalues $$\alpha_1,\alpha_2$$. Operator $$\hat{B}$$ has also two > normalized eigenstates $$\phi_1,\phi_2$$ with eigenvalues > $$\beta_1,\beta_2$$. Eigenstates satisfy: > $$\psi_1=(\phi_1+2\phi_2)/\sqrt{5}$$ >...
  8. R

    A Null measurement in the which-path experiments

    Say we set up a which-path experiment in which there is a detector at only one of the two slits ("Slit A"). In the case of a null measurement, where an electron arrives at the ultimate screen without triggering the detector at Slit A, is there any physical interaction between that electron and...
  9. Jarvis323

    I Definition and Rules of Quantum State Observation

    I was wondering how the rules work for observation in a quantum system. Particularly, about what happens if two separate entities try measuring at the same time. And also, what kinds of interactions are happening all the time that are considered measurements, for example in quantum...
  10. entropy1

    I History of the Universe and measured outcomes

    This thread is a split-off of this post: https://www.physicsforums.com/threads/do-macro-objects-get-entangled.946927/page-2#post-5997089 So my issue is this: if, for convenience, we use a Copenhagen interpretation, and we measure an observable WF ##\alpha |A \rangle + \beta |B \rangle##, then...
  11. olgerm

    I Calculate measurement uncertainty? I made one measurement....

    How to calculate measurement uncertainty of m. I understand I should use these formulas to calculate it if I had data of many measurements, but when have only measurement then it becomes undefined, because of 0/0 in standard deviation formula. ##u(m)=\sqrt{u_a^2(m)+u_b^2(m)}##...
  12. Johny Boy

    A Decoherence of measurement outcomes

    Suppose we have a quantum system ##Q## with an initial state ##\rho^{(Q)}##. The measurement process will involve two additional quantum systems: an apparatus system ##A## and an environment system ##E##, hence giving the initial state of the system ##\rho^{(AEQ)} = \rho_{0}^{(AE)}\otimes...
  13. L

    I Generalizing the Bell Inequality for Arbitrary Measurement Axes

    EDIT: I realize now that I have fundamentally misunderstood a crucial aspect of deriving the Bell inequality for this case which is the existence of the third axis. The setup of the problem did state that the axes were chosen at random. Therefore I can't just look at the possibility of choosing...
  14. Paul Colby

    Voltage Monitor for an LC circuit

    Hi, I have an air wound 0.736 mH coil in series with a 3.5pF capacitor being driven with a function generator. Ideally the series resonant frequency should be around 3.13 MHz. The internal impedance of the function generator is 50 ohms or so. At resonance the voltage across the cap should be...
  15. Derek P

    I Heisenberg Uncertainty vs Measurement Error

    Spun off from https://www.physicsforums.com/threads/is-quantum-physics-retro-deterministic.945431/#post-5984157. @Gerinski said: if we measure a particle's position at time X (not caring about its momentum) and we measure it again at a later time Y and we find it at some other position (again...
  16. R

    I Measurement Problems in Quantum Mechanics: Wave/Particle Duality

    Can we distinguish two measurement problems — 1) Why measurement appears to cause a discontinuous change in the wave function for a single quantum entity/system? As in Schrodinger, "The abrupt change by measurement … is the most interesting point of the entire theory…in the realism point of...
  17. Pattarasak

    I Measurement of the mean value of the number of photons?

    Hi everyone, I do experiment in field of quantum optics and I want to calculate the mean of the number of photons (MNP). In the most of papers, MNP can be calculated by using average power which is measured by power meter. But my power meter is defective. For now, I have only the avalanche...
  18. microsansfil

    I How the probability amplitude is estimated in practice

    Hi, I would like to know how the amplitude of probability is estimated/determinated in practice, for a given experiment. In this example 1.3.2 Analysis of Experiment 2 it is assumed that the probability for each of the two possible states are equiprobable. Than from the experimental results...
  19. W

    I Accuracy of Relativity vs Quantum Mechanics Measurement

    Just for a bit of fun, which theory , relativity or quantum mechanics has the most accurate ( i.e to how many decimal places confirmation between theory and experiment) measurement ever made?
  20. K

    I Measurement effect ('collapse') question

    I routinely read that in quantum theory, measurement 'collapses' the wave function. I am unlikely (a geneticist, not a physicist) to understand this very well, but I think my central question is not entirely naive. Whatever the measurement 'collapse' effect really means, I don't understand...
  21. adamaero

    System turns on alarm once temp. dips below 10.5°C

    Homework Statement Homework Equations (Above given. I think it's a hint.) The Attempt at a Solution How is delta T = 0.25?? Where is the 10kΩ coming from? We were given a solution for this homework, since it's not collected. It just seems out of order.
  22. P

    Measurement of Lx: Result of Measurement?

    Homework Statement If we have a wave function ##\psi =zf(r)## and we take a measurement of ##L_x## what is the result of the measurement? Homework EquationsThe Attempt at a Solution So i know we can write ##L_x=\frac{1}{2}(L_+ + L_- )## and that ##|\psi > = g(r) |1,0> ## so ##L_x |\psi >=...
  23. E

    Measurement Errors in Kirchhoff's Voltage Law

    Homework Statement For a lab, we explored Kirchhoff's Laws. I made a procedural mistake while measuring my voltage values across my different elements. I know that all of my calculated voltage sums are correct, so I was wondering what I might have done to have loops ACBA and CDBC have almost...
  24. Manoj Sahu

    Why do we use a Contact Resistance Measurement test?

    I was wondering about contact resistance measurment test. Why do we take such test?
  25. J

    B How Do You Properly Calculate Uncertainty in Measurements?

    I've come across it it at least half a dozen classes in my life but I never really learned how to do uncertainty calculations properly. Right now I am torn between what is taught and what makes intuitive sense. In every book or website there is a different explanation of this concept with...
  26. V

    Linear measurement of a curved part

    Below is a part proposed to be manufactured. The inspection of this part poses a problem, because vernier callipers only measure distances between parallel surfaces. I know there is some photographic method of measuring distances, however I would like to know is there no way of physically...
  27. haushofer

    I Some (unrelated) questions about the measurement problem

    Dear all, every now and then I get this itchy feeling and start to think about quantum mechanics. Which raises, of course, some questions. These concern the measurement problem. I decided to post them in 1 single topic, so I enumerate them. If someone has some insights clarifying my confusions...
  28. A

    I Reversible Measurement: Is It Possible?

    Reversible computation is a somewhat well-known topic. (Quantum computers, for instance, must use reversible gates). Apparently, though, quantum measurements can be reversible too. This also means you could recover the original state by “unmeasuring” the system. Imagine being able to “see” a...
  29. P

    I What material is used in weak measurements for Young's experiment?

    hello i try to look for some informations about the weak measurments that have been used in the Young's experiment. The slit was made with absorbent material to detect which slit the photon passes through but the fringe interferences are destroyed because of that. Which material is it? Does the...
  30. Posty McPostface

    Is the measurement of "IQ" outdated, and needs replacement?

    Upon reading about what members of this forum think about "IQ" (they don't think highly of it if you're wondering), and the rather inherent discrimination that measuring IQ comparatively to other individuals/nations/races/genders have/has, is there any attempt in the field of psychology, or...
  31. Danny Boy

    A Probability of obtaining general quantum measurement outcome

    The Fundamental Theorem of Quantum Measurement is stated as follows: Every set of operators ##\{ A_n \}## ##n =1,...,N## that satisfies ##\sum_n A_n^{\dagger}A_n = I## describes a possible measurement on a quantum system, where the measurement has ##n## possible outcomes labeled by ##n##. If...
  32. W

    Weak measurement quantum circuit

    Homework Statement The circuit in the attached photo has a one-qubit operation ##U## which is controlled by the first qubit. The box with the meter and arrow denotes a measurement. If ##U=H## what is the probability of finding 0 or 1 in the final measurement of the second qubit? Homework...
  33. K

    Switching observers in a quantum measurement

    Homework Statement (c is a constant) The attempt at a solution -In the first measure we got a1, so the state of the system would be psi1. -In the second measure, there's no information about what eigenvalue we got. Would the state of the system still be psi1? Psi1 is written in terms of B...
  34. Danny Boy

    A Fundamental Theorem of Quantum Measurements

    The Fundamental Theorem of Quantum Measurements (see page 25 of these PDF notes) is given as follows: Every set of operators ##\{A_n \}_n## where ##n=1,...,N## that satisfies ##\sum_{n}A_{n}A^{\dagger}_{n} = I##, describes a possible measurement on a quantum system, where the measurement has...
  35. dRic2

    About Nm^3 and Flow Measurement

    I'm panicking right know, and I wonder if someone can answer these questions: 1) Nm^3 is a unit of measurement for Volume or Moles? 2) I have a flow at T=500K and, let's say, it is Q = 100 Nm^3/h When I find the value of the flow at T=500K I do ##Q_{real} = Q* \frac T {273.15K}##. The new Q...
  36. J

    Measurement (didn't understand solution)

    Homework Statement Homework Equations For sine wave RMS/Avg = 1.11 For DC, RMS = Avg = Peak Value The Attempt at a Solution I didn't understand the solution. There is a 24V DC supply. I give it to an AV Voltmeter. The Voltmeter responds to average value. So the voltmeter would give 24V DC. So...
  37. Danny Boy

    A Defining Krauss operators with normal distribution

    I am interested in defining Krauss operators which allow you to define quantum measurements peaked at some basis state. To this end I am considering the Normal Distribution. Consider a finite set of basis states ##\{ |x \rangle\}_x## and a set of quantum measurement operators of the form $$A_C =...
  38. Roger Dodger

    B Least / Smallest / Minimum Detectable Difference

    In scientific experiment, we often have a physical property that can change but have no detectable impact on the measurement. For example, suppose I have a mass of (say) 30 grams attached to a string passing over a pulley. I can add up to another 2 grams and the system doesn't budge. In our...
  39. M

    I Understanding Magnetometer & SQUID Property Measurement Systems

    Hello, Please I don't get how a Magnetometer property measurement system (MPMS) and a Superconductor quantum interference device (SQUİD) are working, can someone explain it ? Thank you very much
  40. T

    Understanding current measurement on an irregular waveform

    I hoped to use a Keithley 2400 source meter to generate a ~120Hz AC signal by outputting a 2-point sweep from 1A to -1A on an infinite trigger. The waveform image is attached below. The end goal is a resistance measurement for bulk semiconductor materials, we prefer AC measurement to avoid...
  41. ggraham76

    Justification for no properties before measurement

    Hi. I’m a PhD student at University of Toronto, in philosophy of physics. I have a number of questions which are quite fundamental: just trying to get everything clear in my head in the most succinct way possible, so I’m sorry if the questions seem remedial. First question: what is the...
  42. Paul Colby

    I Measuring Light Speed Anisotropy

    Sorry, me again. In another thread #47 I gave a experimental setup to detect an anisotropy in light speed of the form ##c(\hat{n}) = c_o + \alpha (\hat{n}\cdot\hat{v})##. It was pointed out quite correctly that the error incurred in the long arm to the sensor would cancel the anisotropy making...
  43. S

    Measurement of the Speed of light

    Hi, What is people measuring when they measure the speed of light (c) actually? the photons or the waves? or some kind of manifestation of light? I ask because I was reading about photons and that they "might" have a very small mass and therefore are not traveling at c. If photons have a...
  44. S

    Non-coherent spread spectrum for distance measurement

    Hi,I’ve heard of a technology that enables to measure the distance of a satellite to the ground station by using non-coherent spread spectrum. Can anybody tell me what’s the name of this technology?Senmeis
  45. A

    Pressure and temprature measurement in a tiny cylinder

    Hey everybody! as part of a project i need to solve this problem: i have a small closed (50 mm height, 5 mm radius) tank made of PMMA fiiled mostly by salted water and a bit air. the water get a 200 (C) pulse for a very short time of 20 micro-seconds and start to transform into gas (there is a...
  46. Danny Boy

    A Quantum measurement operators with Poisson distribution

    The following is a somewhat mathematical question, but I am interested in using the idea to define a set of quantum measurement operators defined as described in the answer to this post. Question: The Poisson Distribution ##Pr(M|\lambda)## is given by $$Pr(M|\lambda) =...
  47. Guilherme Franco

    I Error in the average of same-value measures

    My question is simple, and I'm only asking it because most places talk about more advanced problems than this one: I've measured the radius of a sphere (a very regular one) with a micrometer of 0.01 mm resolution. I took 3 measures (rotating it between each measure), and all of them were...
  48. MichPod

    A Reasonable measurement of both coordinate and momentum?

    Can a reasonable observable operator be defined which measures a two-component observable, first component for the approximate coordinate and the second for the approximate momentum (so that the precision of each measurement do not contradict Heisenberg inequality)? I am actually thinking of...
  49. F

    Pressure measurement using a Piezometer

    Hello everyone, I understand the hydrostatic fluid pressure in static fluids: static pressure varies linearly with depth and the infinitesimal fluid parcels located at the same depth in the fluid experience the same isotropic hydrostatic pressure. By Pascal principle, the atmospheric pressure...
  50. T

    B Eratosthenes angle measurement

    I am writing an article on Eratosthenes and am having a little trouble on how he actual found the angle of the stick. Because as far as I can gather there was no series solutions for any of the trig defin no calculators. All he knew was the length of the shadow and the height of the stick. I...
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