# 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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### Does vacuum of interacting fields contain virtual particles or real particles

People often say that the particles in ground state of interacting fields are virtual (for instance, from youtube video "What is 'Nothing'?" by Sabine Hossenfelder). However, from my ...
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### Is the speed of light $c$ invariant with the vacuum space energy density? [duplicate]

Vacuum space energy density is related to the speed of light in a vacuum $c$ as: $$\Lambda=8 \pi \rho_{v a c} G / c^4=\kappa \rho_{\text {vac }}$$ where $\rho_{v a c}$ is the vacuum energy density ...
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### In the calculation of VEV in QFT, we don't take into account that the vaccuum is a superposition of field configurations

The vaccuum expectation value is given by the path integral: $$\int e^{iS[\phi]} d[\phi]$$ where $S[\phi]= S_{free} [\phi]+\int J\phi d^4x$ And I guess that the path integral is computed for field ...
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### If water instantly boils/vaporizes in space how do comets made of ice exist?

I am studying the enthalpy of fusion, enthalpy of vaporization, and latent heat. I’m not extremely skilled in physics or formulas so would appreciate answers in simple form. To take the question a bit ...
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### If there is a way to create an antigravitational chamber how would it be created? [closed]

First off, physics is not my major rather it is a hobby that I have gained basic knowledge of over the years. Private research has led me to a hypothetical question: If an electromagnetic device was ...
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### Piano on the moon

Imagine a piano on the moon. You strike a chord. Since there is no atmosphere, there is no medium for the sound to travel. so where does the sound energy go? Does it just dissipate as another form of ...
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### In the event of a vacuum phase transition, could the universe be left into a state with no fundamental symmetries?

I was having a discussion with a physicist asking him whether there could be any process, compatible with our current knowledge of physics, where the universe would be left without any fundamental ...
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### Does bringing 1 cubic meter vacuum from space to Earth require more energy than the energy required for creating 1 cubic meter vacuum on Earth?

I imagine both bringing significant amount of vacuum from space to Earth and creating the same vacuum on Earth would take so much energy but I wonder which one would require more energy or would they ...
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### Can electron/positron vacuum bubbles interact with vacuum photons?

The proposed Feynman diagram below represents two electrons exchanging photons with an electron/positron vacuum bubble which itself exchanges a photon with the electromagnetic vacuum. Is this diagram ...
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### Can any pair of operators $(a,a^*)$ defined as such be seen as ladder operators?

The ONLY thing that we know about $p$ and $q$ is their commutation relation $[q,p]=2i$. In other words, can we find some Hilbert basis such that one of them has the form I am asking this because it ...
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### Do all string theory vaccua have the same vacuum energy?

Of the $10^{500}$ or so vaccua in string theory. Do they all have the same energy? i.e. are the vaccua degenerate? Or do they have different energies? (If this is even possible.) It is said that the ...
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### For an action that has multiple vacuums. Are path integrals multi-valued?

Consider some quantity from QFT $\langle 0| \hat{F}|0\rangle$. This can be written in terms of path integrals as: $$\frac{\int F[\phi] e^{iS[\phi]} D\phi}{\int e^{iS[\phi]} D\phi}$$ if the limits are ...
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### Why is the Vacuum state got by a limit to imaginary time?

Given a (non-relativistic) propagator $K_t(A,B)$ giving the 'conditional amplitude' to go from state $B$ to state $A$ in time $t$, it is known that one can find the vacuum wavfunction by (independent ...
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### Why do we "normal-order" instead of just subtracting off vacuum energy?

The Hamiltonian is arbitrary upto a constant anyway. Why don't we just subtract off the vacuum energy? The Hamiltonian was always observable only upto a constant. Instead, we do normal-ordering, in ...
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### The expansion of universe releases vacuum energy

The quote below has been taken from the Wikipedia article on cosmological constant, https://en.wikipedia.org/wiki/Cosmological_constant. Einstein included the cosmological constant as a term in his ...
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### Free Dirac field Vacuum has zero total angular momentum

I have been reading this pdf (it is released on https://www.physicsforums.com/threads/angular-momentum-in-qft.448715/)about the total angular operator $J_{Z}$ of Dirac field \begin{equation} \psi(x)=\...
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### Inconsistency in Mathematics of QFT

$\newcommand\dag\dagger$ I am learning Quantum Field Theory right now and I stumbled a bit of math that buffles me a lot. In David Tong's notes on QFT, page 76, he used the following trick: \begin{...
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### Propagation of light in vacuum [duplicate]

Suppose, you and your friend are present in a pitch black, perfect vacuum side by side. Your friend has a flashlight which he flashes in front of him. Will you be able to see/perceive the light or ...
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### Pressure dropped in vacuüm chamber after vacuuming the chamber and closing it. How?

I was doing a leak test for a low vacuüm application by vacuuming a small chamber to 60 mbar and then measure the pressure for 16 hours at constant temperature. I chose 60 mbar to prevent evaporation ...
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### Do vacuum solutions of Einstein's field equation violate Mach principle?

The Schwarzschild metric is mainly used to define the gravitational field on exterior region of an isolated, spherically symmetric body, but isolated bodies, according to Mach's Principle, are ...
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### An example of the vacuum emitting photons?

Imagine we have two parallel wires with a potential difference of $V$ volts that form the opposite sides of a square of size $\lambda$. Any virtual electron-positron pairs that form between the wires ...
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### Does an electric field cause the vacuum to emit photons?

As I understand it electron-positron pairs pop in and out of the vacuum on a time scale $T$ inversely proportional to the electron mass. Imagine we put a static electric field across the vacuum. ...
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### Why is "a phase for the QCD vacuum" considered a fundamental constant and what does it even mean?

We can read about fundamental physical constants in Wikipedia: There is, however, no single "correct" way of enumerating them, as it is a matter of arbitrary choice which quantities are ...
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### Could a glass cup containing a vacuum rise into the air?

https://what-if.xkcd.com/6/ has been mentioned here before, but I'm questioning whether or not the glass cup with the bottom half as a vacuum would rise at all. To start with, a vacuum exerts no force....
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### Why Superfluid Vacuum Theory has not proved yet that Vacuum space is possible a superfluid?

Our Universe and subatomic world being defects of an omnipresent type of superfluid we call "Vacuum Space" therefore a medium, unknown phase of matter is an intriguing idea expressed by SVT. ...
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### Playing cymbals in vacuum [duplicate]

When two cymbals collide each other in our atmosphere (so with the presence of air), the kinetic energy make their atoms vibrate, then this energy is dissipated as sound waves and you here the loud ...
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### Does the quantum vacuum fluctuate in time?

Let's take a look at the quantum vacuum (which, on closer inspection is not faithful as it shows constant periodic behavior): There surely is something going on. Now I don't want to speak about the ...