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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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Can Poincaré recurrences happen eventually in a universe with zero vacuum energy?

I am interested in the topic of possible vacuum up-tunneling and down-tunneling events in cosmology. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "...
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Why does the imaginary time Euler-Lagrange equation imply the potential goes to zero at infinite imaginary time?

On reading up about the bounce solution for false vacuum decay in S. Coleman's The Fate of the False Vacuum I it was stated that the equation $$0 = \frac{1}{2} \frac{\partial q}{\partial \tau} . \frac{...
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How can a non-derivative interaction involve the derivative of a scalar field?

I was reading up on the paper "The Fate of the False Vacuum" by Sidney Coleman and the claim is made that a scalar field with standard Lagrangian density: $$ L = \frac{1}{2} \delta_{\mu} \...
Adam P's user avatar
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Could a single gamma ray photon break the Schwinger limit? If so, at what energy?

Is it possible for a ray of light, or even a single photon, to 'break' the vacuum of space? If so, at what energies (in eV, e.g.) would this happen?
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Calculate number of air particles going through a hole in high vacuum at molecular flow

I have a vacuum chamber, connected to a vacuum pump with a pipe of diameter 50mm and need to create a high (0.0001mBar) vacuum. I need to calculate the total suction time, and since I was not in the ...
norbre's user avatar
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Are vacuum "up-tunneling" phase transition in AdS and Minkowski spaces impossible?

I am interested in the topic of vacuum phase transitions in models of the universe. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "true" one (in ...
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Vacuum flucutuations = local entanglement between quantum fields?

I'm puzzled by this statement by Dieter Zeh: "Various types of quantum fluctuations (in particular vacuum fluctuations, often visualized in terms of 'virtual particles') are used to describe ...
Husserliana's user avatar
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Static Patch Decomposition of Bunch Davies Vacuum

In the Jerusalem Lectures on Black Holes section 3.3 the author considers a QFT in Minkowski space. He then picks out a space coordiante, say $x$, and divides the Hilbert space $H$ of the QFT in two ...
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Is it possible to lower the energy of the vacuum?

The energy of the vacuum is given by $$\sum_k \frac{1}{2}\hbar\omega_k.$$ However the frequency $\omega_k$ depends on the wavevector $k$ and some constants like the speed of light $c$, which in turn ...
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Could cosmic rays induce a vacuum decay in the future?

I've been told that very energetic cosmic rays could cause a vacuum phase transition or vacuum decay (and even could cause a true vacuum level to go "uphill" to a false vacuum) due to their ...
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Bogoliubov transformation of Bunch-Davies vacuum

Let $\left|0\right>$ be the Bunch-Davies vacuum state of a QFT, for example a free scalar field, in de Sitter space. The creation and annihilation operators w.r.t. this state is a vacuum, i.e. $a^...
Aralian's user avatar
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How does gravity overpower a vacuum?

While watching experiments with vacuum chambers, I had a thought. If you put a sealed box at normal atmospheric pressure inside a vacuum chamber, pumped out the air and pierced the pressurized box I'd ...
Walt Spring's user avatar
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2 answers
916 views

Theoretically, can perfectly flat space exist in the universe?

According to general relativity, mass and energy cause the curvature of space. To have perfectly flat space, there must be a completely empty vacuum state with no mass or energy. Theoretically, is it ...
NOH WHIREA's user avatar
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Do all quantum fields fluctuate in interstellar space?

For a while now I have heard that the explanation behind the non-zero energy of vacuum space is due to quantum fluctuations. Sometimes this phenomenon has been described with virtual particles, where ...
Quantum Wonder's user avatar
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Radial reparametrization ansatz in Schwarzschild metric derivation

The standard derivation of Schwarzschild solution (and Birkhoff's theorem) seem to begin with the most general spherically symmetric static metric $$ds^2 = -U(\rho) dt^2 + V(\rho) d\rho^2 + W(\rho) \...
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What does it mean for particles to be created?

I am right now studying scalar (semi) quantum electrodynamics, i.e. a charged Klein Gordon field coupled to a background classical electric field in 1+1 spacetime dimensions. The setup is as follows: ...
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Is there a formula for calculating the strength of a suction? [closed]

Let’s say I have a plunger and I know the area of where it suctions, the volume of the vacuum, and assuming it had been completely sealed and has 100% vacuum (not likely!) Could I figure out how much ...
security_paranoid's user avatar
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Can absolute silence exist and how are thermal and quantum noise a limit? [closed]

For my exam, I must answer the question: can absolute silence exist? While doing research, I obviously heard about noises internal to our body and noises external to our environment but I also heard ...
Leane Meireles's user avatar
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5 answers
212 views

Vacuum solutions in presence of mass?

Here is the page I will be referencing: Vacuum solution (general relativity) - Wikipedia My point is: if $T_{\mu\nu}=0$ implies that there is no mass, how can Schwarzschild vacuum be a solution, if ...
Elvis's user avatar
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How fast does a gas expand in vacuum?

A gas is initially confined to a small portion of an empty container. At time $t=0$ gas is allowed to expand to homogeneously occupy the container after some time $t$. What is the time $t$? This ...
YoussefMabrouk's user avatar
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Why vacuum does not have absolute zero temperature? [duplicate]

Extrapolation of P vs. T graph gives absolute zero or 0K. Does that mean that if we achieve perfect vacuum, we can have absolute zero? If not why? Let us assume a perfect vacuumn space where there are ...
Vinay5101's user avatar
2 votes
1 answer
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Why does a normal ordered product of operators (in CFT) have 0 expectation value?

Why does a normal ordered product of operators (in CFT) have 0 expectation value? The definition (Francesco - Conformal field theory pg. 174) of the normal ordered product of two operators $A(z)$ $B(z)...
Jens Wagemaker's user avatar
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What kind of general purpose vacuum pump gives me an oil-less high level vacuum?

Vacuuming non dry product made a mayonnaise from my 2 stages vacuum pump oil. How comes this mayonnaise is so stable and turned up to be very hard to break ? What kind of common vacuum pump could ...
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Electrons keeping dynamical quantum fluctuations?

I was thinking about this paper (https://arxiv.org/abs/1405.0298) where the authors argue that there wouldn't be dynamical quantum fluctuations in a De Sitter space as fluctuations would be static ...
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Why is the speed limit of special relativity $C$ equal to the speed of light $c$ and why must there be only one?

DISCLAIMER: this question is different from other questions posted here previously despite accidentally similar titles. This question is not a duplicate of the one linked bc the linked question is ...
ConformalSymmetry's user avatar
9 votes
1 answer
464 views

Physical meaning of Zero-Point Energy

I know that a quantum system can never have 0 energy due the Uncertainty Principle, and its lowest energy is called the Zero point Energy. However, Energy is a relative quantity (atleast in classical ...
Keshav's user avatar
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2 answers
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What would an inter-universal medium be like?

I don't really believe in the Multiverse Theory, but hypothetically, if universes were like soap bubbles in a foam, what might that foam be like? I know this seems like an opinion-based question but ...
Max's user avatar
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The definition of the path integral

I still have big conceptual questions about the path integral. According to (24.6) of the book "QFT for the gifted amateur" from Lancaster & Blundell the path integral is equal to $$Z =\...
Frederic Thomas's user avatar
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Angular momentum, black holes and up-tunneling events in the vacuum

To give some context on the matter, I found these interesting articles (https://arxiv.org/abs/2007.11428 & https://arxiv.org/abs/2003.04927) where the authors analyzed, among other things, the ...
vengaq's user avatar
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What would happen if a human completely disappeared?

How strong effect would roughly human sized and shaped vacuum spontaneously appearing at sea level have? This event is completely fictional. This is related to: What would happen if a piece of matter, ...
ingotangjingle's user avatar
1 vote
1 answer
63 views

Does vacuum decay create/destroy energy?

From what I understand, vacuum decay involves the release of potential energy, but where does this energy come from? Is it created in the moment and how does it affect the energy already present in ...
KleinMoretti's user avatar
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Up-tunneling of vacuum with high-energy events?

I was reading these papers by Sean Carroll (https://arxiv.org/abs/1405.0298 ; https://arxiv.org/abs/1505.02780) in which, among other things, he argues against vacuum up-tunneling occurring in the ...
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Are voids in the Universe more ideal places for Special Relativity and Newton's Law of Universal Gravitation?

If voids are areas with weaker gravitational forces, then wouldn't they be ideal places for the postulates of Special Relativity and the propositions of Newton's Law of Universal Gravitation? I'm ...
William Solomon's user avatar
4 votes
1 answer
121 views

How do we know at the operator-level that the tadpole $\langle\Omega|\phi(x)|\Omega\rangle=0$ vanishes in scalar $\phi^4$ theory?

I'm a mathematician slowly trying to teach myself quantum field theory. To test my understanding, I'm trying to tell myself the whole story from a Lagrangian to scattering amplitudes for scalar $\phi^...
Nicolas Ford's user avatar
2 votes
0 answers
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On vacuum energy in a de Sitter universe?

I have a couple of questions about de Sitter cosmological horizons. Initially I made a single post containing the two questions, but after some suggestions, I asked them in two separate posts. This ...
vengaq's user avatar
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Quark condensate and VEV of $\pi^0$

In David Tong's lectures on the Standard Model I saw that there is a quark condensate, which is just a Vacuum Expectation Value (VEV) of the $\bar{q}_{Li}\, q_{Ri}$ operator, $$ \left< \bar{q}_{Li}...
Gabriel Ybarra Marcaida's user avatar
2 votes
1 answer
107 views

Motivation for the shape of the theta vacua

I understand that the reason why we construct the theta vacua is because instantons allow tunnelling between different vacuum states, $\left|n\right>$. This means that we have to consider a real ...
Gabriel Ybarra Marcaida's user avatar
1 vote
2 answers
206 views

Is there an spherical symmetric Einstein vacuum solution which has circular orbits with flat velocities? Or a proof that it cannot exist?

The Schwarzschild solution shows decreasing velocities with larger orbits - and needs help from dark matter or MOND to explain galaxies. Apparantly no Einstein vacuum solution with flat velocities is ...
Tantal181's user avatar
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What does it even mean?: "Perturbation theory developed around minima of potential (vacuum) is stable."

(Context: QFT, spontaneous symmetry breaking): What I have understood from reading the path integral version of the story is that this "vacuum" is actually the classical solution [i.e. which ...
nmnphy's user avatar
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2 answers
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What, specifically is meant by "particles are popping in and out of existence all the time?"

When I see that wording utilized, I have some confusion about what that means. Are the particle/anti-particle pairs each themselves popping in and out of existence, meaning the exact same particle ...
blacktopshaman's user avatar
1 vote
2 answers
74 views

On the (variable?) nature of $\epsilon_0$ and $\mu_0$

In electromagnetism, the electric displacement field D represents the distribution of electric charges in a given medium resulting from the presence of an electric field E. Its relation to ...
Juan Moreno's user avatar
2 votes
2 answers
118 views

Are vacuum energy, zero point energy and vacuum fluctuations the same thing?

im confused about the relationship between these terms, my intuition tells me that vacuum energy and zero point energy are synonymous and that they are a consequence of vacuum fluctuations. But I ...
KleinMoretti's user avatar
2 votes
1 answer
92 views

Why is the Ramond vacuum a Majorana fermion in type II string theory?

I understand that in order to have a supersymmetric spectrum in string theory, the vacuum has to be a MW (Majorana-Weyl) spinor under $SO(1,9)$. But I don't see where the Majorana condition on the R ...
Ballanzor's user avatar
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3 answers
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What does the state $a_k a_l^\dagger|0\rangle$ represent?

Consider the action of the operator $a_k a_l^\dagger$ on the vacuum state $$|{\rm vac}\rangle\equiv |0,0,\ldots,0\rangle,$$ the action of $a_l^\dagger$ surely creates one particle in the $l$th state. ...
Solidification's user avatar
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2 answers
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What happens to Capacitors at extreme temperatures and pressures?

In my University physics class [first year engineering student] I learned that "for a capacitor in a vacuum, capacitance $C$ depends only on the shapes, dimensions, and separations of the ...
William Banquier's user avatar
1 vote
0 answers
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Bunch-Davies initial condition

I have the Mukhanov-Sasaki equation in terms of $Q(t)$ \begin{align*} Q''(t)+3HQ'(t)+k^2Q/a^2+\left(3\phi'(t)^2-\phi'(t)^4/2H^2+2\phi'(t)V_\phi/H+V_{\phi\phi}\right)Q=0,\ u=aQ \end{align*} and also ...
Jules Alvarez's user avatar
2 votes
2 answers
116 views

Notation for spatially superposed particle using vacuum modes - Why the state of the particle and vacuum is entangled, not separable?

We have two places, call them Here and There where a particle could be. When the particle isn’t spatially superposed we write its state as $ |1\rangle_{Here}\otimes|0\rangle_{There}$ zero being the ...
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Witten's "Semiclassical Stability of Minkowski Space" versus Christodoulou and Klainerman's "Global Nonlinear Stability of the Minkowski Space"

At least naively, there seems to be a tension between two seminal works on the stability of Minkowski space. In Witten's A New Proof of the Positive Energy Theorem, in the section "Semiclassical ...
user196574's user avatar
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Green function and probability amplitude

Consider the following Green function: $$G_{2}(x,t,x',t') = \langle \Omega_{0}, e^{itH_{0}}a_{x}e^{-itH_{0}}e^{it' H_{0}}a_{x'}^{*}e^{-it' H_{0}}\Omega_{0}\rangle$$ for $t' > t$. Here, $a_{x}^{*}$ ...
MathMath's user avatar
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Does the vacuum have gravity? [closed]

Asuuming the vacuum has zero mass (negligible ZPE of the vacuum) and since Black holes generate gravity without the presence of mass (i.e. singularity at the center of a Black Hole) does this mean ...
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