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Questions tagged [vacuum]

A region of space devoid of any fluid molecules resulting in a zero pressure in ideal cases. In practice, even space has a small but finite number of molecules. In QFT, it signifies the ground state devoid of excitations/particles.

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How fast will air travel to fill a vacuum?

Suppose I have a sealed glass cylindrical vessel of radius r and length l that is evacuated. I smash one end. How long will it take for air to (a) reach the end of the cylinder (b) fill the vacuum ...
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Strong empirical falsification of quantum mechanics based on vacuum energy density

It is well known that the observed energy density of the vacuum is many orders of magnitude less than the value calculated by quantum field theory. Published values range between 60 and 120 orders of ...
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What is the difference beetween vacuum tube and x ray tube? [on hold]

I ask for their structure, because if they are the same, shouldn't vacuum tube generate x rays?
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Why do people think that rockets can thrust in the vacuum of space? [duplicate]

A force is required to change the momentum of the rocket exhaust which consists mainly gas moving out of the rocket chamber. People think that it is the rocket that pushes the gas out. If this were ...
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Can our universe be a true vacuum bubble?

The paper "Spontaneous creation of the universe from nothing" by Dongshan He, Dongfeng Gao and Qing-yu Cai claims that our universe was created by the quantum fluctuations in the metastable false ...
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Are wave equations equivalent to Maxwell's equations in free space?

In free space, do Maxwell's equations contain the same amount of information regarding electric and magnetic fields as is contained in the wave equations derived from them? If so, how?
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Divergent integral problem

When expanding the scalar field vacuum energy $$\sum_k \frac{1}{2} \omega_k = \frac{1}{2} (L/2\pi)^{n-1} \int \omega(k) d^{n-1}k = \frac{(L^2/4\pi)^{(n-1)/2}}{\Gamma(\frac{n-1}{2})} \int_0^\infty (k^...
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Could the cosmological constant be due to vacuum fluctuations in a box, i.e., in a finite universe?

Assumption: If the universe were a finite box whose boundary is the cosmological horizon, then there would be a zero-point energy inside that box. Consequence 1: This zero-point energy would be given ...
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Why can't we superpose two quantum vacuum states?

i read in this paper (Spontaneous Symmetry Breaking as the Mechanism of Quantum Measurement by Michael Grady) that we are not allowed to consider the superposition of two vacuum states. i do not ...
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Is there only one vacuum solution of the Einstein equations?

I am thinking about this: A vacuum solution means vanishing Ricci tensor. The Ricci tensor is a contraction of the Riemann, which itself involves contains second derivatives of the metric. Thus they ...
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Expressing the vacuum projection operator in terms of number operator

I've been reading this book, in which the author expresses the vacuum projection operator $\vert 0\rangle\langle 0\vert$ in terms of the number operator $\hat{N}=\hat{a}^{\dagger}\hat{a}$, where $\hat{...
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Vacuum energy in continuous phase transitions

In a first-order cosmological phase transition, the tunneling from the false to the true vacuum (see left picture below) releases false vacuum energy corresponding to the potential difference $\Delta ...
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Layperson wondering about The power within one protium atom (1H) or H

Within 1 hydrogen atom (1H) protium we have 1 electron the half life of this stable isotope is 10 to the 34th power years. It can fuse into Helium if 4 atoms fuse when the Coulomb force is overcome ...
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How to calculate vacuum energy density? Do we need to sum up the zero-point energy of all kinds of fields in the vacuum?

If we are not sure whether the Standard Model has already included all the fields in the nature, then how can we calculate vacuum energy?
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Could pressure differentials be used to produce power?

There is potential energy between something like a vacuum in a container and a gas on the outside of it, or vice-versa. Could this energy be harvested in a vacuum, or close to vacuum situation like on ...
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Harnessing molecule vibration energy

Disclaimer: I am learning physics for fun, please dont kill me but explain where I am wrong. As I understand all molecules have a lowest energy state in which they posses some amount of momentum even ...
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Einstein-Hilbert action and Lagrangian density for vacuum Ricci scalar

From the action, $$\int L\,\mathrm dt=\int R \sqrt{|g|}\,\mathrm d^4x,$$ why is the Lagrangian density for the gravitational field replaced by the Ricci scalar, which yield field equations in vacuum $$...
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Is it possible to slow down the speed of light in a vaccum? [duplicate]

Is it possible to slow down the speed of light in a vaccum?
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Speed of waves in vacuum

All electromagnetic waves travel at speed of light in vacuum. Gravitional waves also travel at speed of light. I am having this vague notion that all waves in vacuum must travel at speed of light. Is ...
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Is the quantum harmonic oscillator energy $E = n\hbar\omega$ or $E = (n + 1/2)\hbar\omega$? (Feynman Lectures)

Please, read the whole question. I've discussed a few contradictions and so far have not found an explanation for them. I was reading The Feynman Lectures on Physics (vol. 1), the part where he talks ...
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3answers
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Can there be charges in free space without it being on some particle?

We see leptons and quarks possessing charges. Can there be charge in free space devoid of matter(neither leptons nor quarks or anything else)?
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Questioning whether gravity is strictly attractive. Is there a possibility for repulsion?

My teacher wanted to test our understanding before starting a new topic. The question he gave goes something like this “There are 2 vacuums present here about which there is an abundance of mass ...
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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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Is it possible to drain all oceans' water to space? [duplicate]

Assuming: That I have an ideal straw (a very long pipe) from the ocean to space; That the straw (pipe) is ideal (will not break); That i can pump all the air from the pipe creating vacuum inside the ...
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How does entanglement create empty space?

It is claimed that empty space is full of quantum fields which their waves perfectly cancel out each other, but this can only happens when all the waves are entangled. Is this claim valid because I ...
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What does the 'cosmological constant' represent?

Newtons theory of gravity involves a gravitational constant $G$, however one does not refer to it directly, we speak instead of gravity or the force of gravity. Now Einsteins introduced a ...
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3's a crowd. Particle me but may i have this dance? [closed]

In a particle accelerator (built for 3 particles), could we smash them into a vacuum with only enough space for 2 particles?(electrons, protons, neutrons) what would be the result?
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Scattering of light in Quantum Vacuum

The quantum mechanical vacuum (i.e. the vacuum of a typical QFT) viewed as the zero-point state of the system has an energy (of course, here enters the Vacuum Catastrophe and all the mess that follows)...
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SUSY vacuum has 0 energy?

This is related to Modern Supersymmetry: Dynamics and Duality by Terning. Consider $N=1$ SUSY. $\{Q_a,\bar{Q}_{\dot{a}}\}=2\sigma_{a\dot{a}}^\mu P_\mu$. Sum over $a=\dot{a}=1,2$ and this yields $4P^...
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Stable bubbles in a vacuum

I want to make space bubbles that hold some gaseous stuff - at very low pressure - for a long time and through nothing but surface tension. I can't use soap water as the water will evaporate quickly. ...
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Trying to make some sense of the cosmological constant

The question is simple: Does the naive reasoning below have some physical sense, or is it pure gibberish junk? (I use $c = 1$ and metric signature $\eta = (1, -1, -1, -1)$) I suppose that the ...
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What kind of average could give a Lorentz invariant energy-momentum tensor?

The electromagnetic (EM) radiation energy-momentum tensor is of the following shape, in the case of incoherent superposition of EM plane waves (I'm using $c = 1$ to simplify things, and metric ...
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Natural classification of fundamental constants of Nature

I was wondering about the fundamental constants of Nature since several years, and still pondering on them. Of course, I have read a lot of papers on them, but never found any satisfying ...
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Could false quantum vacuum decay change fundamental laws of physics? [closed]

It is often said that low energy/effective physical laws (i.e fundamental physical constants) could possibly change if a quantum false vacuum decays into a true (stable) level of quantum vacuum. But, ...
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Virtual particles

I use Wikipedia to learn physics. In a recent search I found an ample description of virtual particles in terms of a discontinuous (in the present) and asymptotic distribution of these particles from -...
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Why should vacuum energy be zero for LSZ formalism?

Can anyone explain why vacuum energy must be zero if we are to use LSZ formalism?
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How can zero point energy vacuum be Lorentz invariant?

What distribution of electromagnetic radiation is Lorentz invariant? How can radiation look the same regardless of inertial frame? According to Marshall and Boyer a cubic distribution like $ \rho(\...
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Gauge transformation on vacuum

Suppose we have two electromagnetic vector potential operator that differ by a gauge transformation in free field theory. $A'_{\mu}= A_{\mu}-\partial_{\mu}\alpha$ is the state $A_{\mu}|0\rangle$ ...
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Normalization for the overlap $\langle \phi_a | 0 \rangle$

This question is related to my previous post here. According to Wienberg's Volume I (9.2.9), we have the result $$ \langle \phi_a | 0 \rangle = \mathcal{N} \mathrm{exp}\left( - \frac{1}{2} \int d^{3}\...
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Will Vacuum Energy persist at the Heat Death of the Universe?

By "vacuum energy" I mean the underlying background energy that exists in space throughout the entire Universe, codified in Heisenberg's energy–time uncertainty principle, a special case of zero-point ...
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Question regarding the degeneracy of vacuum state in various spacetimes

In a talk, it was mentioned that if one attempts to do quantum gravity in the following spacetimes (of any dimension), then the vacuum state has the following degeneracy. In $AdS$ - There is a ...
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Is this how vacuum works?

Vacuum exerts a net zero force, or i guess pressure? on what's inside it, right? So if the universe is expanding at an accelerating rate, is the vacuum of outer space is growing? Like space is ...
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Einstein equation outside of the source

When solving Einstein equation outside the source it is assumed that we have $R=0$ where $R$ is Ricci tensor. But if we have as a source Earth and for example, a black hole, how can this equation ...
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What are the “basic things” we need in physics to define any kind of energy, including “mass”?

After having commented an answer there: Relative potential energy vs Absolute potential energy, I realised that the energy concept may be much more subtle than what we usually believe, even if we ...
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Electron interaction with vacuum fluctuations

The cause of Lamb shift is interaction between vacuum fluctuations and electron in the atom. Will electron (or photon) in free space interact with vacuum fluctuations? How momentum will transfer ...
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101 views

Absorbing the Cosmological “Constant” in the standard Energy-Stress Tensor

Recently I found some publications on Cosmologies with variable cosmological constant. The Bianchi Identity then implies that the divergence of the modified Energy Stress: Tensor $$\hat{T}_{ab}=T_{ab}...
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Energy/mass of Quantum Vacuum

So far as we have measured the volume of the universe, has anyone calculated the virtual energy/mass of the Quantum Vacuum? If so, what? Or if not, a local volume such as 1 sq. meter? This relates to ...
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Inner product in QFT

When we write inner product in QM for example $\langle\psi\rvert x \psi\rangle$it means (in position space) $\int\psi^*(x,t)x\psi(x,t)dx$. But when we write, in QFT, $\langle0\rvert0\rangle=1$ what ...
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What is in space?

I was recently on a chat server having a random discussion about science stuff and someone I was talking to then made the comment that "space is not a complete vacuum and it's full of plasma / matter"....
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Is the term “quantum fluctuation” an aide to understanding? [closed]

I would like to ask if anyone has found a tight enough way to define the term "quantum fluctuation" so that it can become a useful rather than a misleading piece of physics terminology. Terminology ...