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Questions tagged [heisenberg-uncertainty-principle]

This tag is for Heisenberg's quantum mechanical uncertainty principle. DO NOT USE THIS TAG for uncertainty in a non-quantum measurement.

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Measuring the Momentum of a Quantum System using Position Measurements

A basic way to measure the momentum of a charged particle is to know that (classically) it follows a circular trajectory in a uniform magnetic field perpendicular to the plane of its trajectory, with ...
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Heisenberg uncertainty principle - Can a particle be a wave and particle at the same time?

I understand the traditional definition of Heisenberg uncertainty principle and how it is direct consequence of wave particle duality. I am wondering if it is a consequence of the fact that a particle ...
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Can't we see electrons without affecting their movements? [closed]

Can't we see electrons without affecting their movements? I was thinking about the double slit experiment and something is writing on Feynman's book like photons can affect their movements too. But I ...
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Diffraction and HUP [closed]

Please help, I cannot find explanation anywhere. Using Heisenberg’s uncertainty principle, give an account of how light diffraction and interference may be treated as a phenomenon based on photon-...
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38 views

Energy conservation when exchanging meson

In Prob 1.2 of Griffith's "Introduction to Elementary Particles" 2nd ed., it says that: However, I don't understand why it states that "they must temporarily violate the conservation of energy by ...
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1answer
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Length and velocity of a pulse of particles

We are given a pulse of protons of duration $10^{-7}s$ and energy $2KeV$. I know I am supposed to use the uncertainty principle to solve this. I need to get the length and the indetermination of the ...
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Quantum mechanics paradox

Let us consider a collision problem of two particles 1 and 2. After collision we measure the position of the 1st particle and momentum of the 2nd particle simultaneously. Now from momentum ...
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1answer
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Question about point particle vs. wave equation location

Another uncertainty question, this came up in another forum. As I understand it an electron, for example, is a point-like particle. I take this to mean it exhibits dimensionless properties, but ...
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1answer
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A basic question on Heisenberg uncertainty and experiment

I’m not a physicist or anything even close. But what I understand from the reason for the Heisenberg uncertainty is that the photon is changing the volecity/momentum when it hits the particle. ...
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Uncertainty Principle for a locally constrained particle

According to the Uncertainty Principle we cannot calculate the precise position or momentum of a quantum particle at the same time, trying to calculate one of them would make the other uncertain. One ...
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0answers
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How fundamental is the uncertainty relation? [duplicate]

I was having a discussion over the uncertainty relation with some with of my friends with people splitting over the nature of uncertainty relation. So there were two groups formed (a) One of the ...
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3answers
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Heisenberg Uncertainty Principle derivation question [closed]

So I'm rading Shankaar's book and got stuck in this place. $ (\Delta \Omega)^{2}(\Delta \Lambda)^{2} \geq \frac{1}{4}\left\langle\psi\left|[\hat{\Omega}, \widehat{\Lambda}]_{+}\right| \psi\right\...
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How in experimental practice does a momentum measurement reduce a state to a momentum eigenfunction?

It's easy to think of ways to reduce the state of a particle to a position eigenfunction (or at least a narrow spread in position space), whether by trapping the particle in a potential well or by ...
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Maximum wavelength of electromagnetic radiation; is it limited through uncertainty principle?

I have already asked one related question; Maximum wavelength of a photon/electromagnetic radiation? And I am now developing this thought further by a question. First, I want to point out that; "the ...
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2answers
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Interpretation of the energy-time uncertainty [duplicate]

From the uncertainty relation it follows that: $$\Delta E \ \Delta t = \hbar$$ $\Delta E$ is the energy uncertainty of a state, $\Delta t$ should be the uncertainty of the lifetime $\tau_b$ of the ...
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1answer
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What's the intuitive interpretation of quantum uncertainty $\Delta \hat{A}=\sqrt{\langle\hat{A}^2\rangle-\langle\hat{A}\rangle^2}$?

As per this video, if $\hat{A}$ is a quantum operator, the uncertainty is given by $$\Delta \hat{A}=\sqrt{\langle\hat{A}^2\rangle-\langle\hat{A}\rangle^2}$$ I understand what this expression means ...
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Plotting quadrature uncertainties in phase space

In most books like in the picture given below, the uncertainties regarding quantum states like coherent and squeezed states are represented in phase space plot by some area enclosed within a circle or ...
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Confused about using uncertainty principle to determining momentum

A question asks to estimate the energy of a neutron, if the neutron is composed of a proton and electron by using the uncertainty principle. The basic idea is to let x = 1 fm and calculate p using: $...
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2answers
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Are superposition and uncertainty principles logically dependent?

If we assume superposition and define an Hilbert space with canonical commutation relations we can derive uncertainty relations. So it seems the uncertainty principle isn't required, or should be ...
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2answers
55 views

What happens to the uncertainty principle when I have a particle contained within an inelastic box?

Say I have a box made of inelastic material such that when a particle hits the box, energy is lost through heat. I then put a particle inside of this box and squeeze the box down. How does this not ...
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3answers
210 views

Understanding the statement of the bandwidth theorem

I know that the Bandwidth Theorem (BT) and the Heisenberg Uncertainty Principle (HUP) are basically the same thing, and stem from the fact that for operators $A,B$, we have: $$\Delta A \Delta B \geq \...
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3answers
102 views

Why do neutron stars with more mass have smaller volume?

I know about Heisenberg uncertainty which makes more localized neutrons have a wider range of undefined momentum, and Pauli exclusion principle which prohibit neutrons from getting too close or "...
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1answer
79 views

Accuracy of Heisenberg Uncertainty Principle for estimating ground state energy of particle in potential well

I've understood the assumptions and logic behind the 'proof' that the ground state of a particle in an infinite potential well has a non-zero energy using the Heisenberg Uncertainty Principle. ...
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Momentum uncertainty for a free particle [closed]

Free particle is in state psi k, where k is the wavenumber. Now i am trying to findout uncertainty in its momentum. I know that for free particle position is uncertain (delta x approaches to infinity) ...
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Where this interpretation for the field modes comes from?

I'm reading the book "Modeling Black Hole Evaporation" by Alessandro Fabbri and Jose Navarro-Salas, and in section 3.3.2 they talk about wavepackets at $\mathscr{I}^+$. It all starts like this: one ...
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1answer
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What does Hawking mean by “in empty space the field can’t be fixed at 0 because then it’d have both a precise value and a precise rate of change of 0”

Slightly modified the language in the title to make it fit in 150 characters. The above comes up in the part leading up to the explanation of how black holes emit particles.. My impression was that ...
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How to get the Number Phase Uncertainty relation from the Energy time relation?

I can arrive at $\Delta H\Delta t \geq \frac{\hbar}{2}$, but how do I get from there to $\Delta N\Delta \phi \geq 1$ for the number states of light? I know we write $H = \hbar \omega (N + \frac{1}{2})...
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momentum in calculation of Bohr's radius

Some usual calculations of the Bohr's radius (see 2.5 at Feynman's here, or this text here) starts by defining a radius $a$ ( most probable radius ? average radius ? ). Next, from Heisenberg's ...
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Why accelerating observer can see Unruh radiation?

I read that uncertainty principle allows a pair of virtual particles to spawn in a vacuum as long as time is $h/E$ and nature forbid negative energy to exist, a accelerating person will see the photon ...
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Prove a version of the uncertainty principle

Hello this question is about the wave nature of the electron and the uncertainty principle. The question goes as follows: A finite wave train has a constant velocity $v$ and its profile as a ...
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2answers
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If the system is in an eigenstate of $B$, then $\Delta B = 0$, but $\frac{|[A,B]|^2}{ 4} \not = 0$, so how can uncertainty relation hold?

Let $A,B$ be two non commuting observables, then we do know that $\langle (\Delta A)^2 \rangle \cdot \langle (\Delta B)^2 \rangle \geq \frac{|[A,B]|^2}{ 4}.$ Also, we do know that, when the system ...
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1answer
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Heisenberg uncertainty principle meaning and using it to determine approximate sizes confusion

I was always under the impression that the Heisenberg uncertainty principle tells us to what precision we can make a momentum measurement given a position one. However, when doing quantum mechanics ...
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De Broglie Wavelength of a particle at the turning point of a fountain

If you think about an atom which is launched in some kind of straight fountain, upward in the gravitational field. At the turning point the velocity and therefore the momentum of the particle seems to ...
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Explanation of Different factor of $2$ in Formulas for the Uncertainty Principle [duplicate]

I am readind about Heisenberg's Uncertainty Principle and although the main formula most texts use is $$\Delta x\Delta p_{x}\geq \frac{\hbar}{2}$$ or equivalently $$\Delta x\Delta p_{x}\geq \frac{h}...
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1answer
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Clarification on Heisenberg thought experiment

Heisenberg imagined a high energy photon was absorbed by an electron, but in order to make a measurement on that electron it will have to absorb another photon which means it velocity gets higher but ...
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A question on Heisenberg's uncertainty principle [closed]

According to Heisenberg's uncertainty principle it is impossible to determine accurately momentum and position of an electron simultaneously. Whereas it's not much of a problem in case of a football ...
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1answer
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minimum uncertainty in measurement of position

Suppose, I made a measurement of the position of an electron using a photon. How the minimum uncertainty in the measurement of the position be one wave length? Also how do we use waves to measure the ...
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How does Heisenberg's uncertainty work with more than one quantum field?

How does Heisenberg's uncertainty principle work with more than one quantum field? I am specifically asking about the time-energy uncertainty: $$\Delta E \Delta t \ge \frac {\hbar}{2}\tag{1}$$ Imagine ...
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If perfect blackness violates the Uncertainty Principle, how isn't dark matter a violation?

In a recent tweet, Dr. Michio Kaku stated that perfect blackness would violate the Heisenberg Uncertainty Principle, i.e. every object must emit some radiation. I have two questions regarding this ...
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How to get the radius of Bohr from the uncertainty principle?

I would like to know how to get the following result: \begin{eqnarray} a_0=\frac{4\pi\varepsilon_0 \hbar^2}{\mu e^2} \end{eqnarray} from the beginning of the uncertainty of heisenberg. If possible, I ...
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Are phase and particle (photon) number in QED conjugated variables?

I found in A. Zee's book "QFT in a nutshell" (1.edition) the interesting relation (8) respectively (9) in chapter III section 5 (p.173) which states that in a collective of non-relativistic bosons the ...
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1answer
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What is 'definite' variable in QM?

I have gone through a few of the questions on the website regarding this particular query, but I have not understood what they meant. When a question says that a particle has definite momentum, are ...
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1answer
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I have a thought experiment that defies the uncertainty principle - what's wrong with it?

There's a particle somewhere on the x-axis (this whole thing is 1-dimensional) at x0, which we do not yet know. I fire a photon at it from the origin with a known speed (c) and a known wavelength λ1 ...
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How can electron have known momentum?

In many exercises it is given that electron has a know speed, e.g. 5*10^3 m/s. But how is this possible when we deal with Heisenberg principle?
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Bond breaking stretch length with QM

In QM and assuming you could repeat the same exact bond-breaking experiment arbitrary number of times, do bonds always break at the same stretch length, or does the uncertainty principle require some ...
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2answers
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Notation in Heisenberg Uncertainty Relation: greater than with tilde: $\gtrsim$

In "The Road to Reality" (pg. 523), Roger Penrose writes: Heisenberg's uncertainty relation tells us that the product of these spreads cannot be smaller than the order of Planck's constant, and ...
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1answer
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Would a neutron star-like celestial body made of bosons collapse because of missing Pauli repulsion?

Imagine the neutron star-like celestial body would be built up of bosons (assume that neutrons would be bosons). In that case there would be no Pauli repulsion (the exchange force of the exchange ...
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1answer
65 views

Is there an Heisenberg uncertainty relation for light polarisation?

While reading some questions about the polarisation of a photon, I was wondering about its quantum mechanical representation as a classical vector in the electromagnetic field operator : \begin{...
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1answer
43 views

Finding the uncertainty from a probability distribution?

If you have two properties, $A$ and $B$, that do not commute, and thus have a commutator $C$, and the uncertainties $\Delta A$ and $\Delta B$ obey the relation $$(\Delta A)(\Delta B)\geq \frac{1}{2}|C|...
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How to determine the approximate order of the strength of nuclear force from Heisenberg's Uncertainty Principle

Today I came across a question , where I was asked to find the Strength of nuclear force from Heisenberg's Uncertainty Principle... Please can anyone help me out... I have come across a solution ...