# Tagged Questions

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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### Uncertainty principle in quantum field theory

Can the uncertainty principle be derived in quantum field theory? If yes, does is have a different interpretation than quantum mechanics because the coordinates $x_i$ are now parameters and not ...
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### Heisenberg uncertainty principle clarification

I found these two examples in a books which demonstrate Heiseberg's uncertainty relation: 1) It shows that when we try to locate a moving electron,we transferred momentum via the photon that we ...
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### Heisenberg uncertainty principle applied to large bodies?

Suppose I have a ball of a certain radius inside a box (with the length bigger than the radius) such that the ball fits in the box. The ball has a large mass (1 Kg). Heisenberg uncertainty principle ...
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### Why does the uncertainty principle relate position and momentum when mass is always certain?

Why do physicists relate $x$ and $p$, instead of $x$ and $v$ since mass is well known and is not uncertain?
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### Is entanglement a consequence of the uncertainty principle?

I am an aspiring physicist and once, I asked my professor on what triggers quantum entanglement and he graciously remarked "The great uncertainty principle!" - I was slightly confused and didn't say ...
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### reflection of light and the uncertainty principle

I've heard that light reflects the same angle that it falls and so if we know the position of photon (the spot where it hits the surface) and the direction of where it shall go - that's violation of ...
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### Why do momentum and position have the same direction in space in the Heisenberg Uncertainty Principle?

It is known that $\Delta p \Delta x \geq \frac{h}{4\pi}$. I read that the two uncertainties must be along same axes. Why is that so?
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### How to derive Uncertainty Principle relation?

How to derive Heisenberg Uncertainty Principle relation?
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### Does the electron confinement energy vary with temperature?

I was introduced the electron confinement energy. At room temperature thermal energy of a particle is about $k_B T$, where $T=298K$, giving about $25meV$. I was told that $E_\text{confinement}=50emV$, ...
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### How do tracking detectors in particle accelerators create the pretty pictures we see?

I have read several sources about tracking detectors used in particle accelerators like LHC, but still have not found a more detailed source that can still be understood by a layperson like myself. I ...
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### Why is what Heisenberg's uncertainty principle describes true?

As I understand it, Heisenberg's uncertainty principle (HUP) states that the more precisely we know the location of an electron, the less precisely we know its momentum, and vice versa. However, I've ...
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So the explanation in my text book is this: we have a phase space volume $\Delta x\Delta y\Delta z\Delta p_x\Delta p_y\Delta p_z$. Now they state that Heisenberg's uncertainty principle states $\Delta ... 1answer 63 views ### Uncertainty relation and instrumental errors I was puzzled recently by what I've read in the internets about Heisenberg's uncertainty principle (one probably should never do this). It claimed that in the usual relation $$\overline{(x-\... 3answers 1k views ### Using the uncertainty principle to estimate the ground state energy of hydrogen I have been reading through this estimate of the ground state energy of hydrogen and others like it. In this one it says it is using the uncertainty principal but then proceeded to use the following: ... 1answer 44 views ### Luminal motion and the uncertainty principle I had a thought the other day about a connection between special relativity and the uncertainty principle. According to special relativity, you need an infinite amount of energy to accelerate a ... 0answers 47 views ### Question on Heisenberg's uncertainty principle [duplicate] Can we measure limit of precision of measuring wavelength using uncertainty principle? If yes how can we do that? 1answer 109 views ### Entanglement and the uncertainty principle Suppose you have two maximally entangled particles. You measure the spin about the x axis of the first and the spin about the y axis of the second. But both spins cannot be known simultaneously, so ... 2answers 133 views ### How can the uncertainty principle be used to deduce range of a force from properties of the force carrier? Question: How can the uncertainty principle be used to deduce range of a force from properties of the force carrier? My thoughts? Not too sure how this can be done and would like some advice. ... 5answers 320 views ### Which way will the pencil fall? Let's say you had a perfect pencil, with a point which was just that one point (see this question). The pencil's mass was perfectly distributed, and there are no flaws in the craftsmanship. Let's say ... 6answers 1k views ### What is the uncertainty principle? I looked on Wikpedia for information on the uncertainty principle, but after reading it I still had no idea. I know it has something to do with how many things you can hold at some spot for some ... 1answer 146 views ### Do all quantum systems have zero point energy ? I understand that it is possible to write an uncertainty relation between the Hamiltonian of a system and time, where the time uncertainity is defined by the amount of time it takes an arbitrary ... 2answers 188 views ### Classical notion of trajectory [closed] Why the classical notion of trajectory is meaningless in quantum mechanics? I am asking here about notion of trajectory from classical mechanics and why in quantum mechanics we cannot use it or is ... 9answers 12k views ### Can we theoretically balance a perfectly symmetrical pencil on its one-atom tip? I was asked by an undergrad student about this question. I think if we were to take away air molecules around the pencil and cool it to absolute zero, that pencil would theoretically balance. Am I ... 0answers 57 views ### Why position and momenta are fluctuating quantities? In a coordinate basis we have$$\langle \Psi \mid \Psi \rangle = \int \prod_{i=1}^N d^3q_i |\Psi(\textbf{q}_1,\dots,\textbf{q}_N)|^2=1$$This means that for any quantum state \mid \Psi \... 2answers 122 views ### Could quantum particle have a specific velocity without position at all? I understand that electron spin on different axis is an example of complementary properties, which cannot be measured accurately at the same time. I also understand that if we have an electron is ... 0answers 118 views ### Quantum Gravity Singularity Question? I am not a expert in QG (Quantum Gravity) or GR (General Relativity) for that matter so please forgive if I make any small mistakes, its just a curious question but I know that a singularity is a ... 0answers 64 views ### Minimum Uncertainty Wavepackets I'm reading over some lecture notes on minimum uncertainty wavepackets in quantum mechanics, and I've come across a statement that I'm not entirely convinced by. My take on minimum uncertainty states ... 1answer 73 views ### Energy Conservation in Quantum System? Let us assume, 2 same mass atoms are moving towards each other at equal velocity and they are in course to crash into each other - so what would happen when they do crash into each other? If we use ... 0answers 75 views ### Group velocity and calculating wavelength [closed] A stone tossed into a body of water creates a disturbance at the point of impact that lasts for Dt = 4.0s. Measurements indicates that the wave speed is v = 25 cm/s. (a) Over what distance on the ... 2answers 161 views ### Why uncertainty principle on a large scale doesn't impose limitations on precision? The question is By choosing reasonable numerical values for mass and velocity, show that \Delta x \Delta p >=\frac{ \hbar}{2} doesn't impose any limitations on the precision with which the ... 3answers 310 views ### Are the authors saying that the observer effect plays no role in Bohr's thought experiment of the Heisenberg uncertainty principle? Here is an excerpt from Eisberg & Resnick's Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. Here is introducing Bohr's though experiment to establish a physical origin for the ... 3answers 865 views ### Uncertainty principle and measurement I would like to really understand how the uncertainty principle in QM works, from a practical point of view. So this is my narrative of how an experiment goes, and I'm quickly in trouble: we prepare ... 2answers 60 views ### wormhole time machine and violation of uncertainty? [duplicate] Let's asssume we have a wormhole that allows sending info to the past and receiving information from the future. Next, we have a particle moving in two dimensions, for which we can evaluate p_{y} ... 2answers 115 views ### Heisenberg uncertainty and probabilistic nature of QM I am trying to understand whether the HUP and the probabilistic nature of QM are orthogonal or not. By that I mean that the HUP fundamentally derives from operators not commuting, which is the ... 2answers 56 views ### RH side of the Uncertainty principle: when is it a number and when an expectation value? The uncertainty principle between the position x and the momentum p is given by:$$ \sigma_x \sigma_p \geq \hbar/2,$$whereas for the x and y components of the angular momentum is given by:$$... 3answers 407 views ### Is there a prediction of quantum mechanics that could be construed as representing an “energy-time uncertainty relation?” [duplicate] As the title suggests. Is there a prediction of quantum mechanics that could be construed as representing an "energy-time uncertainty relation?" Does there exist any reference to such a prediction, or ... 0answers 22 views ### Diffraction from a single slit experiment and Heisenberg's uncertainty principle [duplicate] Practically how to separated between Fraunhofer and Fresnel diffraction? Do all diffraction fringes from single slit have same peaks? Why? 3answers 206 views ### Energy of system in eigenstate of Hamiltonian I know how to find the spectrum of the Hamiltonian to get the allowed energies for a system. If the spectrum is quantized, I can get definite values for each energy level. But when the system is in ... 1answer 150 views ### Why can't we define a unique wavelength for a short wave train? [duplicate] Here we encounter a strange thing about waves; a very simple thing . . .namely, we cannot define a unique wavelength for a short wave train. Such a wave train does not have a definite wavelength; ... 1answer 224 views ### electrons inside nucleus and uncertainty principle [closed] State the assumptions of the proton-electron model of the nucleus. Show that if one uses the Heisenberg uncertainty relation to estimate the speed of an electron inside a nucleus with a diameter of ... 2answers 603 views ### The uncertainty principle and spin I realize that this may be a very basic question, but I've been unable to find the answer elsewhere so thanks in advance for the help. Suppose an electron's spin is measured about an axis, and then ... 2answers 106 views ### Can mass be uncertain? If you can have uncertainty in momentum, then wouldn't you have uncertainty in mass and velocity? Why can't mass be uncertain? 6answers 639 views ### Explanation for the EPR-like paradox I am trying to understand the process of Quantum Entanglement for use in Quantum computers. The problem I have is this: Suppose some nuclear process emits an electron-positron pair. Now after ... 1answer 122 views ### Can we put$\Delta p, \Delta x, \Delta E, \Delta t$into one big uncertainty equation? According to my calculations$x$*$p$has unit$[kg m^2/s]$which is what we would get if we multiply$E$by$t$So $$Et = xp$$ Starting from here, can we some how derive a formula that puts all ... 2answers 180 views ### Why only 1 component of angular momentum? Griffiths says that you can have only 1 well defined component of the angular momentum because of the uncertainty principle. From the uncertainty principle, we get that$$\sigma_{L_x}\sigma_{L_y} \... 1answer 84 views ### On quantum randomness, the longest weather predictions and perfect macroscopic caos Which is the maximum number of days we can predict future weather conditions with a reasonable degree of accuracy if we knew all of the initial conditions of everything that effects the weather down ... 0answers 53 views ### Gedankenexperiment to derive the Robertson uncertainty relation without confusing it with the obersver effect In the last years there seemed to be much activity on the meaning Heisenberg's uncertainty relation. The main point of the discussion was Heisenbergs noise-disturbance-relation (see: http://arxiv.org/... 2answers 173 views ### Why isn't the Heisenberg uncertainty principle stated in terms of spacetime? As I understand it, there are two "versions" of the Heisenberg uncertainty principle: Position-Momentum uncertainty $$\sigma_x \sigma_p \geq \frac{\hbar}{2}$$ where$[...
In Heisenberg uncertainty principle why do we only talk about uncertainty in position along $x$ axis, why not along other dimensions as well?
Can someone confirm that the uncertainties in Heisenberg's uncertainty relation are really just standard deviations based on the expectation values? For example, the $\Delta x$ can be computed by \$\...