measurement

  1. entropy1

    I Conservation of energy and measurement problem

    If we have a two dimensional measurementbasis, then we have two possible outcomes of the measurement. Now I figured: considering the law of conservation of energy, if one particle goes in, one and only one can come out. So outcome "both results simultaneously" cannot happen, for that would...
  2. entropy1

    I Ontology of spin measurement

    Summary: If a measurement outcome depends on the measurement setup, is de measured not real or the measurement? If the factual outcome of an electron-spin measurement depends on the orientation of the SG magnet, for instance up or down in one orientation and left or right in the other, does...
  3. entropy1

    I Conservation of possibility?

    Suppose we have a quantum object in superposition to some measurement basis, given by: ##\frac{\sqrt{2}}{\sqrt{3}}|a \rangle + \frac{1}{\sqrt{3}}|b \rangle##. (1) Suppose the measurement is made, and the system evolves, according to MWI, into ##\frac{\sqrt{2}}{\sqrt{3}}|a \rangle|W_a \rangle +...
  4. Peter Morgan

    A Schrödinger's cat: what's the state?

    A physicist prepares a box and tells us that in the box there is a cat that is in a superposition of being alive and being dead. How can we be sure whether they're telling the truth? Is the state a superposition or a mixture? If we open the box and measure only whether the cat is alive, using...
  5. entropy2information

    B Doesn't the choice of measurement prove free will

    I was reading the free will theorem and it basically says that subatomic particles and observers have to have free will because there's nothing prior to measurement that predetermines the outcome. Here's more: The free will theorem states: Given the axioms, if the two experimenters in question...
  6. G

    I Send the measurement device through a double slit

    Hi. Double slit experiments are being performed successfully with increasingly large molecules. Some physicists (e.g. Anton Zeilinger) believe it might work with viruses as well. Assuming it works with a system that qualifies as a measurement device (be it a virus or something else complex...
  7. D

    I Can I steer myself into one of the Many Worlds like this?

    Given an ideal "box" as used by Schrodinger; - have a quantum event occur inside it, e.g. sudden cat death with 50% probability. - have a machine in it that sends out a qubit, fully entangled with the box' internal state, at regular intervals. - the qubit is a polarised photon - outside, use a...
  8. C

    Energy Measurement in QM

    Let's say I have a system whose time evolution looks something like this: This equation tells me that if I measure energy on it, I will get either energy reading ## E_0 ## or energy reading ## E_1 ## , when I do that, the system will "collapse" into one of the energy eigenstates, ## \psi_0 ##...
  9. A

    How do I use full-bridge strain gauge circuit to measure F?

    Hey all, I am using a Wheatstone bridge with 4 strain gauges as resistors. I have a formula for the output voltage (Vout). My questions is how do I make it so instead of voltage I measure force? Do I simply apply a set max. force (let's say 140N), see what voltage I get (Vout,max) and then use...
  10. Nevada City Bob

    Bob from Nevada City, CA

    Hi. I'm a retired software engineer who has always been fascinated in science. I was a math major in college, before the time that computer science majors were a thing. My key areas of interest these days are particle physics and quantum mechanics. I'll be asking some questions that have...
  11. D

    B Measurement vs. Interaction

    I am still confused about the difference between measurement and interaction. I mean when electrons are travelling from source to the screen through the slits, there are air molecules in their way. And even if the electron double slit experiment is carried out in total vacuum in a completely...
  12. 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?
  13. entropy1

    I Quantisation of measurement

    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...
  14. 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...
  15. 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...
  16. 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...
  17. 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...
  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. adamaero

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

    1. Homework Statement 2. Homework Equations (Above given. I think it's a hint.) 3. 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.
  20. E

    Measurement Errors in Kirchhoff's Voltage Law

    1. 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...
  21. 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...
  22. 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...
  23. 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 =...
  24. 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...
  25. 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...
  26. A

    QM: Purity, two-outcome measurements

    1. Homework Statement 2. Homework Equations in addition to those provided in the questions, I used the following: Tr(B) = sigma<x_j|B|x_j> purity = Tr(rho^2) 3. The Attempt at a Solution I find calculating trace and purity very confusing. Am I on the right track with question 1? With...
  27. sergiokapone

    I Velocity measurement by a stationary observer in GR

    In almost general case, the space-time metrics looks like: \begin{equation} ds^2 = g_{00}(dx^0)^2 + 2g_{0i}dx^0dx^i + g_{ik}dx^idx^k, \end{equation} where ##i,k = 1 \ldots 3## - are spatial indeces. The spatial distance between points (as determined, for example, by the stationary observer)...
  28. Auto-Didact

    A Falsifications and constraints due to GW measurements

    This thread is to serve as - a collection of theories that have been falsified by and/or have had new constrained placed on them by the ongoing gravitational wave measurements. - a place to discuss the further constraining/falsifying of still existing models using GW data. I'll start by posting...
  29. 1977ub

    B Understanding Measurements / Collapse

    I'm trying to understand the impact of past measurements, and when measurements occur. As I understand it, in the simplest case, you've got a particle emitter in the center of a circle, and a measuring plate around the circle. Here in the ideal case the particle is emitted and has equal...
  30. C

    B Is mass a measurement of energy?

    That's pretty much it. Are physical objects essentially clumps of energy, and that energy is measured as mass? And do objects become more massive as they receive more energy through push?
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