Measurement Definition and 1000 Threads

I'm a PhD student who's taken GR, so I have studied gravity and cosmology, and we have the underlying assumption of homogeneity and isotropy, but since this assumption comes from how we've calculated the distribution of galaxies in the visible universe I want to make sure I really understand it...

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 ##...

I am having trouble figuring out the answer to the following question, I am not sure how to get to my answer.Calculate the area of each wall, including any doors and windows (The room is rectangular in shape. The floor is 18 ft. 5 in long by 9 ft 6 in wide. All of the walls are 8 ft high. You...

What is the difference in temperature with different thermometers for the same object? Thanks !

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...

Why protective measurement is important to understand whether the wave function is the ontic property of a single particle and not of the ensabmle of similarly prepared particles? I need some help with understanding the following topic (which is currently above my level so I may easily miss...

At very high energies Earth absorbs a relevant fraction of neutrinos passing through it. Experiments receive more of these neutrinos from above than from below and the difference depends on the mass of Earth. So why not measure it? Neutrino tomography of Earth The uncertainty is very large, of...

In those experiments demonstrating quantum interference which rely on a beam splitter to create a superposition of a photon taking different paths, what would happen if one made a sufficient "measurement" of the momentum of the beamsplitter itself? Would or should it destroy the interference...

I teach aerodynamics. One horsepower is 550 lbs moved one foot in one second. My question is this. Does that assume that the 550 lbs is being lifted vertically against gravity? As opposed to moving, say, a 550 lb chunk of concrete across a floor. And if so, does the gravity value of...

What is the history of the concept that a measurement process is associated with a linear opeartor? Did it come from something in classical physics? Taking the expected value of a random variable is a linear operator - is that part of the story?

For the experiment, we were to determine the relationship between two quantities through linearization after collecting measurements. To verify that P~L^1/2. and determine proportionality constant, C. P=C√L/g= (Cg^-1/2)(L^1/2) Materials used were a timer, string(The pendulum on length), and a...

Homework Statement What will momentum measurement of a particle whose wave - function is given by ## \psi = e^{i3x} + 2e^{ix} ## yield? Sketch the probability distribution of finding the particle between x = 0 to x = 2π. Homework Equations The Attempt at a Solution The eigenfunctions of...

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...

When talking about measurement in QM it's completely normal to think right away about someone doing "something" to get or "observe" a definite quantum state... ...but how about the screen in a standard DS? ...there we get clumps if we measure a quantum state at the slits or fringes if not...

Suppose we have a pair of spin entangled electrons, measured by resp. Alice and Bob. The basises of Alice and Bob make an angle of α=10°. If Alice and Bob wind up in a joint world where Alice measures ##|u\rangle##, then the probability that, in that world, Bob measures ##|d\rangle## is...

A quantum system goes from an uncertain to a certain state upon measurement.This indicates a decrease of entropy--is there a corresponding increase of entropy elsewhere(environment/observer)?Is there any work done on the system in the act of measurement?

I am still confused about the difference between measurement and interaction. I mean when electrons are traveling 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...

<Moderator's note: 2 threads merged as it is an identical topic.> Given Kim's version of DCQE, let's say Bob in on Earth looking at d0, Alice is on Mars holding the prism that deflects the idler coming from (a) to d3 or (b) to d4. After the experiment run ends Bob should get clump at d0 without...

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

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...

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

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...

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...

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...

> 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: > $$\psi_1=(\phi_1+2\phi_2)/\sqrt{5}$$ >...

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...

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...

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...

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)}##...

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...

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...

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...

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...

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...

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...

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...

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?

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...

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.

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 >=...

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...

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

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...

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...

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...

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...

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...

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...

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...

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...