# 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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### Negative temperature of the de-Sitter horizon?

I'm considering the $4D$ de-Sitter spacetime, in static coordinates (I'm using $c = 1$ and $k_{\text{B}} = 1$): \tag{1} ds^2 = (1 - \frac{\Lambda}{3} \, r^2) \, dt^2 - \frac{1}{1 - \...
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### Entropy of de-Sitter spacetime and the $10^{120}$ vacuum discrepency

While doing some lazy calculations, I came across a curiosity that I'm unable to interpret. It is well known that the cosmological constant $\Lambda \sim 10^{-52}~\mathrm{m^{-2}}$ is usually ...
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### What is the meaning of 'sputtering' in Ion pumps

So after reading a few articles this is what I understand: Air enters the pump through diffusion In the pump there is a cloud of electrons between two (grounded) cathodes As soon as an electron hits ...
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### The mean free path of electrons in high vacuum is 26 billion kilometres?

I used this formula to calculate the mean free path length of an electron in high vacuum. $$\lambda = \frac{k T}{\sqrt{2}\cdot4\pi r^2 \cdot p}$$ where k is the Boltzmann konstant, T the temperature ...
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If you could produce a very thin tube, say less than the thickness of a human hair, designed to carry a hot liquid inside, the diameter is small so that the liquid would have a high surface area to ...
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### An interesting Casimir configuration [duplicate]

A problem related to an interesting Casimir configuration.  I have the following configuration , related to a modified Casimir experiment.  We have two parallel metal plates and an orthogonal ...
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### Does vacuum energy have an average temperature of 0° Kelvin?

Vacuum energy has zero-point energy. Does this imply that vacuum energy [has] an average temperature of 0° Kelvin?
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### Solving a Casimir configuration, the U - bracket

A problem related to an interesting Casimir configuration.  I have the following configuration, related to a modified Casimir experiment.  We have two parallel metal plates and an orthogonal metal ...
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### QFT: the origin of vacuum correlation

My question is about the origin (or mechanism?) of vacuum correlation. In quantum field theory in flat spacetime, a free scalar field has the following two-point correlation function at the same ...
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### Why aren't spaceships (usually) torn apart

This may have already be ask forgive me if it has but why aren't spaceships torn apart when in the vacuum of space? Is it just because they are strong enough not to be torn apart or because they are ...
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### What is the total mass of orbiting interplanetary unbound atoms in the solar system?

Excuse the poor phrasing of the title, but it became apparent to me that the solar system has some degree of density (i.e it is not a perfect vacuum). So my question is if any of these particles (not ...
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### Poor man's Alcubierre drive?

In general relativity,  the fundamental notions are the curvature (Ricci tensor) and the stress energy tensor. Energy density and curvature are connected .  The Casimir vacuum between parallel plates ...
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### Should particle-wave duality be understood as a description of light's dual nature or as a description of two observable states of light?

In double slit experiments, light is observed in two distinct conditions (no measurement of trajectory / measurement of trajectory) that bring two different results (no interferences / interferences). ...
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### Is it possible to use sound waves to move asteroids?

Does it seem feasible to use sound waves to deflect an inbound asteroid? Would it matter what frequency the sound waves are? Would it matter what the size or mass of the asteroid is?
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### Will a can filled with vacuum move when we let in air? [duplicate]

This picture is from L.C.Epstein's book Thinking Physics. The upper can is filled with compressed air, and, when an opening is made on the right, the air comes out and the can shoots left. The ...
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### What is an vacuum in QFT in curved spacetime?

Sometimes in some public lectures about General Relativity (GR) and Quantum Mechanics, in college, the professors dealt with the vacuum concept, precisely in the context of Quantum Field Theory (QFT), ...
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### Can you separate a 'flattened balloon' in a vacuum?

If a flat, fully sealed balloon is in space, could you pull the walls apart quite easily apart from the elastic force? In other words, will a balloon go from flat to "filled with a vacuum" if it's in ...
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### Vacuum balloon / airship with balloons as shell segments

What are the problems with the following idea for building a vacuum balloon / airship? The question is targeted at the "making it float" part, ignoring other problems with super-light airships (...
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### Would a planet with mirror surface emit radiation due to dynamical Casimir effect? If so, where the energy would come from?

The dynamical casimir effect tells that an accelerating mirror should produce photons. Due to equivalence principle a mirror in uniform gravitational field also should produce photons. So, should a ...
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### Do the apparent Black Hole dimensions contract as the observer falls into Black Hole?

According the Special Relativity, all objects contract their length if moving fast. As the observer falls into black hole, his relative speed to it increases up to the speed of light. But from his ...
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### “Cosmological constant problem” and dark energy

Simply stated: is the "cosmological constant problem" (the discrepancy of about 120 orders of magnitude between measured values of vacuum energy and the ones predicted by Quantum Field Theory) in any ...
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### Vacuum collapse — why do strong metals implode but glass doesn't?

This question has been puzzling me lately. I'm sure you've seen demonstrations of metal containers imploding when evacuated. Here, for example, are two videos of vacuum collapse: experiment 1, ...
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### Has the speed of light ever been measured in vacuum?

According to: https://en.wikipedia.org/wiki/Cosmic_microwave_background the CMB (Cosmic Microwave Background) "is faint cosmic background radiation filling all space" Also, https://en.wikipedia.org/...
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### Why does the lowering operator applied to lowest state have to be 0? [closed]

When solving a problem in QM with raising and lowering operators ($\hat{a},\hat{a}^\dagger,L_{\pm},..$) it is often assumed that: $$L_-|\Omega>=0$$ Why is this assumed? Couldn't the result ...
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### What happens if a traveling sound wave encounters vacuum?

Suppose a sound wave is emitted by an object in a medium like a gas so it travels in some direction. If the wave meets a rigid object, for example a wall, it reflects back as one should expect; and if ...
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### How short is the time for the manifestation of small amounts of energy to justify the existence of quantum fluctuations?

Uncertainty Principle allows the appearance of small amounts of energy from nothing, always on the condition that they disappear in a very short time. Speaking in seconds, how brief should these ...
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### Is vacuum energy transient or permanent?

Vacuum energy is based on Heisenberg's energy–time uncertainty principle which says that $\Delta E$ can exist for $\Delta t$. This $\Delta E$ can produce electron positron pair for $\Delta t$ by ...
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### Confused with pressure and condensation

I'm confused with pressure and condensation of a gas inside a sealed container. Say I boil a liquid to it's gaseous state inside a sealed container and exert more pressure on that container then ...
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### Likelihood of vacuum decay event

My understanding of the matter is currently limited to the wikipedia article about false vacuum. One of my take aways of the article is, that we currently don‘t know if we live in an unstable, ...
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### Virtual particles, conservation of energy & the fabric of space [duplicate]

Do virtual particles take energy from the fabric space and when they annihilate in the split second they distribute that energy back to space, therefore abiding by the law for the conservation of ...
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### $CP$ Invariance of Yang-Mills Vacua in Electroweak Theory

It is well know that quantum Yang-Mills theory has a periodic vacuum structure. Consider electroweak theory. For a single generation of fermions, the theory is CP invariant. I would like to know if ...
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### Energy is in vacuum or not?

Is energy present in vaccum is zero, infinity or particular value depending on volume, length, height etc of container containing vaccum or some other parameter.
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### What is meant by the vacuum structure of ABJM theory?

I was reading the paper Large $N$ behavior of mass deformed ABJM theory. It talks about the vacuum structure of the (mass deformed) ABJM thoery. What does vacuum structure mean in general or in ...
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### How come the universe is considered flat if zero point energy is infinite?

If quantum field theory calculates that the vacuum energy is infinite and Einstein's theory of gravity implies this energy should produce a curvature of space-time then why shouldn't the universe be ...
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### What will happen if a vacuum chamber gets moved with a person inside?

Imagine that a person would be put inside a vacuum sphere, just like the spheres that NASA uses for simulating space's zero gravity. That person would be hovering right in the center of the sphere. ...
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### Is the energy density of vacuum uniform in the whole universe?

This is not a duplicate. I am not asking why the energy density of quantum vacuum stays constant while the universe expands. My question is about how we know/measure that this vacuum energy density is ...
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### Vacuum test in the laboratory

If I put some water in a plastic container and apply a vacuum pressure of -0.20bar the water comes out of plastic container which should not as per design and material of container. This is called ...
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### If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?

A friend of mine told me that if you were to stand beside plate of metal that is millions of degrees hot, inside a 100% vacuum, you would not feel its heat. Is this true? I understand the reasoning ...
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### Is the speed of light in a vacuum constant?

The news site is titled, “Speed of Light May Not Be Constant, Physicists Say.” It talks about how the speed of light in a vacuum is not constant and that estimates of the size of the universe might be ...
239 views

### How would a nuclear explosion really operate in outer space?

(Repost from RPG.SE, per the community's suggestion.) A friend recommended I post this question here - said I'd be more likely to get a useful response than, say, from Reddit. My question is: How ...
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### Definition of medium?

Why do we define vacuum as space devoid of matter? we say that electromagnetic waves can travel through vacuum while longitudinal waves can't. We also say that the velocity of sound waves (a ...
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### What's the difference between “dark energy” and “vacuum energy”?

Let's see if I understood well. "Dark energy" is a term that encapsulates everything that may be accelerating the expansion of Universe, and "vacuum energy" is a kind of energy that permeates the ...
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### Justification for “running” cosmological constant

I am looking for papers which seek out to justify that the cosmological "constant" could be time-dependent. I am aware that within the framework of GR the cosmological constants really has to be a ...
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### If air is an insulator, how does an ungrounded electric generator move electrons?

Firstly, this is NOT a duplicate question. There have been similar questions asked, but no one has adequately explained this particular aspect of electric generators. Yes, they act as an electron "...
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### How to calculate force exerted by vacuum from size and rpm of propeller used to create it?

If a propeller of particular size pushes some volume of air out per unit time. How can the suction force be determined? How big of a particle can it suck?
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### Why is zero-point energy idolized as a good source of energy? [closed]

I've really only seen it in like liquid helium and I don't understand how it can hold the potential portrayed in the incredible movie by the main villian
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### Preparing vacuum state of the EM field

I have some heuristic idea of how to think about state preparation in quantum mechanics. It may be revolving around the idea of using filters, cooling/heating, Stern-Gerlach type setup, etc. However, ...
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### How does vacuum affect an object's temperature?

I have an objected suspended in a vacuum by a very thin point. Thus, it can be heated only at the point of contact? Am I correct in assuming that the object cannot dissipate heat into the vacuum- ...