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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About the steps in a book about Quantum Field Theory [duplicate]

I have a question about the steps in the book "An Introduction To Quantum Field Theory" By Peskin & Schroeder. In page 213, i don't understand why the $e^{iPx}$ before the $<\Omega|$ ...
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Zero-point energy? (D-branes C.Johnson's book)

In Johnson's Book about D-branes (11.2) gives the zero point energy. This is given using (2.80) \begin{equation} +\frac12\omega: \text{for boson and}\;\; -\frac12\omega: \text{for fermion}. \end{...
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Regarding vacuum pumps [closed]

A pump has a volume of $20 cm^3$ and is connected to a jar of volume $V = 10 L$. The pump performs $N = 1000$ rotations (strokes) as a compressor and after that it is connected reversely as a vacuum ...
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Vacuum space has mass/energy?

It is said that vacuum spaces contain particles and antiparticles, my question is if so whether empty space has mass/energy?
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Question about vacuum state of fields in Quantum Field Theory

Are the fields in their empty state a single indivisible and static entity? I would also like to know if the gravitational field also has a vacuum state and if the other fields are permanently linked ...
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Could some kind of vacuum decay modify the most fundamental laws of physics?

A false vacuum is a hypothetical vacuum that is not actively decaying, but somewhat yet not entirely stable ("metastable"). It may last for a very long time in that state, and might ...
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Can we have electric current in vacuum?

Suppose we a have an anode and cathode and between them there is vacuum. If we apply a sufficiently high voltage between them, can we have a small electric current?
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Quantum Field Theory, questions about fields and their vacuum state [closed]

Are the fields in their empty state a single indivisible and static entity? I would also like to know if the gravitational field also has a vacuum state and if the other fields are permanently linked ...
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How do we move forward in space without air or atmosphere? [duplicate]

This might be a stupid question, but I am trying to understand the fundamentals. The closest I've come to is Newton's Third law of Motion which states ...
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Dual Bra of the ground state of interacting theory

I'm currently reading Peskin's "An introduction to Quantum Field Theory", but I'm stuck on page 87; I don't understand why he gets such a Bra for the vaccum state of the interacting theory, ...
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Einstein field equation specific solution

Do Einstein's field equations admit a solution such that spacetime was empty in the past of a hypersurface of constant time say $t =0$, but in the future there exists a non-vanishing energy momentum-...
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Can spacetime be curved even in absence of any source? [duplicate]

Einstein's equation in absense of any source (i.e., $T_{ab}=0$) $$R_{ab}-\frac{1}{2}g_{ab}R=0$$ has the solution $$R_{ab}=0.$$ But I think $R_{ab}=0$ does not imply that all components of the Riemann-...
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Casimir energy vs Helmholtz free energy

In what way are the Casimir energy and Helmholtz free energy related / do they differ? In QFT one can compute the Helmholtz free energy as the vacuum expectation value of the zero point function. This ...
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Fermionic vacuum energy from Hamiltonian

I have some questions about some commentsthat Zee makes treating this problem in Sec II.2 of his QFT book. The Hamiltonian density of a spin-1/2 field is $$ \mathcal{H}=\bar\psi(i\vec\gamma\cdot\vec\...
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Why field expansion into positive and negative frequency Fourier components becomes ambiguous in curved spacetime?

Why is the expansion of a relativistic field into positive and negative frequency components $$\phi({\vec x},t)=\int\frac{d^3{k}}{~~(2\pi)^{3/2}}\left[A\left({\vec k}\right)e^{-i\omega_{\vec k}t}+A^*\...
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What is “nothing”? [duplicate]

In some cosmological models, the possibility of the emergence of the Universe "out of nothing" is considered. What do scientists mean when they say "nothing"?
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Propagator from a spacetime point to itself and physical interpretation of vacuum

I am following a lecture note on the QFT. But am a little confused about some parts related to the vacuum bubbles. We define the Feynman propagator, $D_{F}(x-y)$, as giving the amplitude for a ...
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Is free space a 'dielectric'?

The physical constant $\epsilon_0$ is usually called "permittivity of free space" or "vacuum permittivity". Griffiths says: I dislike the term, for it suggests that the vacuum is ...
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Komar mass for vacuum solutions

I'm a little confused about the integral definition of the Komar mass, and how it gives the correct result for vacuum solutions. As given by Poisson, the Komar mass is $$ M_{Komar} = 2\int \left(T_{\...
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QFT: Vacuum invariant, but vacuum correlations aren't

Consider a free scalar field theory. My struggle is that vacuum correlation functions of fields are only Lorentz invariant under a subgroup of Lorentz transformations, despite the invariance of the ...
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Does spacetime itself fall towards gravitational bodies? [closed]

John Wheeler said “Matter tells spacetime how to curve and spacetime tells matter how to move.” So, when objects fall to Earth they travel along the spacetime curves that they and all the surrounding ...
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Normal ordering of hamiltonian

I came across this in the lecture notes of quantum field theory by David Tong. Inside time ordering interactions aren’t taken to be normal ordered. Interaction hamiltonian should be normal ordered ...
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How did Torricelli know that he created a vacuum?

I have read about the experiment of Torricelli, he filled a tube with mercury and placed it standing in a basin filled with mercury, then mercury poured out into the basin from the tube. He concluded ...
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Why product of creation operator with ground state enegy of vacuum is zero?

The product of annihilation operator with zero ground state of a harmonic oscillator is zero since energy cannot be negative or less than zero. I understand this explanation but what is the intuitive ...
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Would everyone die? [duplicate]

When you open the door of your spaceship, the oxygen disappears from the ship. So if I put a very very long tube from the Earth to space, would all the oxygen from the Earth automatically aspire into ...
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1-loop Vacuum energy of a fermion field

Following the method by Peskin and Shroesder 11.4 Trying to calculate the vacuum energy of a fermion. If my method is correct so far the next step is to find gamma function , the formula I have for ...
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Vacuum energy of a free scalar field from path integral

My question has been asked two other times: Spinor vacuum energy (misleading title) and Vacuum Energy Calculation using Path Integral. I am not completely satisfied with the answers and it looks like ...
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Is liquified gas rate of vaporization dependent on the volume of the container?

I have a vessel 50% full of ammonia at outdoor temperature of 20F - 100F depending on time of year. This vessel will be evacuated with a compressor and the vapor will be discharged into an ammonia ...
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Is the value of the scalar components of the Higgs field known?

As I understand the relevant popular sources (example), since the end of the Electroweak epoch, the Higgs field became fixed in the whole known Universe. Thus, we are somewhere on the deep blue part (...
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Question regarding different mode functions in FRW spacetime

I am currently reading Mukhanov's and Winitzki's book with title: "Introduction to Quantum Fields and Classical Backgrounds". At chapter 6, they argue that in a FRW (Freedman - Robertson - ...
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Can we make a drone fly on the Moon by using cylinders with compressed air (or cold helium)? [closed]

Suppose we want to make a drone fly on the Moon (the gravity on the Moon is 1/6 of that on Earth), only by making use of its rotors and air. The drone is as light as possible ($m_{min}(kg)$, has ...
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Using Casimir force as a thruster

In quantum mechanics, there is a phenomenon called the 'Casimir effect'. As two metal plates have a very small distance, the plates work as a potential well, causing limited wave function between the ...
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Why does sending $T\rightarrow \infty(1-i\epsilon)$ in the slightly imaginary direction cause the $n=0$ term to decay slower?

This is in reference to equation 4.27 in Peskin and Schroeder. To derive a formula for the interacting vacuum in terms of the free vacuum we evolve the free vacuum in time with the full Hamiltonian ...
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The nature of the space in which we exist

I'm looking for pointers to give me a better understanding as to what 'space' is. I do not mean the descriptions of what's in 'space' i.e. not interested in descriptions of outer space filled with ...
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Quadruple Mass Spectrometer - Transition metal detection

We use a QMS in our vacuum chamber, mainly for residual gas analysis. In a recent project I tried using it during the an argon sputtering of carbon infested Ni(111). I got a lot of carbon- hydrate ...
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Why is the impedance of an electromagnetic wave written as a ratio between the electric and magnetic fields, $E/H$?

The 'impedance of free space is written like this, too, as the absolute value of the $E$-field over the absolute value of the $H$-field. What does the ratio of the strengths of the two fields of an ...
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Why particles only make sense in flat spacetime?

I don't understand why exactly the notion of particles only makes sense in flat spacetime. From explanations I have read, it says that in general curved spacetime a unique vacuum couldn't be talked ...
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Is there any model or theory which predicts the existence of multiple vacua with different fundamental symmetries?

In certain models of string theory it is predicted that many vacua may exist. These vacua could be governed by different effective laws of low-energy physics resulting from different symmetry breaking ...
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Black Body in empty space

Let's say we have a body of spherical shape with a temperature T > 0 Kelvin for our purposes. It's supposedly in empty space. Completely empty space, no other radiation, no far away stars nothing. ...
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Replacing the $M$ coefficient in Schwarzschild metric with a distribution $M(r)$

Consider the Schwarzschild metric $$ds^2 = -f(r)dt^2 + \frac{1}{f(r)}dr^2+r^2 d\Omega^2,$$ where $f(r)=1-\frac{2M}{r}$. I take it that $M$, although not really the mass of a black hole, is the ...
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To what extent is lambda-CDM a quantum model?

I understand that the $\Lambda$CDM cosmological model is mainly built over general relativity, however many of it's features invoke quantum field theory (such as inflation). I find this confusing ...
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Normal ordering in string theory

So, in QFT where the calculation involves noncommuting operators, we define normal ordering to remove the ambiguity. This is usually done by moving annihilation operators to the right of the creation ...
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Turn over a full bottle, why does it pour?

Why does a full bottle leak when you turn it over? Idea: Let's assume there is a full bottle which is not open, yet. You turn it over such that the opening points towards center of the earth. Now you ...
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Prove that the value of the cosmological constant equals the energy density of the vacuum

I know that Einstein introduced his cosmological constant assuming it as an independent parameter, something characteristic of the Universe, in itself, but the term of it in the field equations can be ...
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Path Integral formulation with a spontaneously broken symmetry?

The path integral evaluates ground state correlation functions, and when a symmetry is spontaneously broken, there are many ground states. Defining the path integral of a QFT in the naive way, I don't ...
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Vacuum with Helium Gas

I am doing some experiments with high vacuums. In the case of air, this is relatively easy, as one has two valves which are open to air. One then turns on a vacuum pump and adjusts the air flow from ...
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The physics behind Ear Candling

So ear candling is basically putting one end of the hollow candle in the ear and the other end is lit with fire. Now apparently this works because the burning creates a 'vacuum' which causes the ...
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Does weight have any function in vacuum? [closed]

As we know, that in a vacuum, bodies with different masses fall with same acceleration, which is due to acceleration due to gravity. But Weight of a body is dependent on mass as well. So, is it so, ...
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Improved stress-energy tensor derivation from Green's function

In chapter 4 of the book Quantum fields in curved space by Birrell and Davies, the authors provide a derivation of the Casimir energy for a massless scalar field (in 4-D) using the method of images ...
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How would an Atom behave in vacuum?

Let's say I put, for example, a Hydrogen atom in vacuum where no external energies have impact on it. Would it just "sit" and exist? Or would it move around? If so, by what force? If some ...

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