# Tag Info

16

Albert Einstein rather famously said The only reason for time is so that everything doesn't happen at once. and John Wheeler added Space is what prevents everything from happening to me! Now, those quotes may sound silly and self-referential, but they are meant to draw you attention to something very, very basic. Things do happen at different ...

12

Strictly speaking vacuum is the state of lowest energy. That means no matter or radiation (photons or any other particles). Note that space is not a perfect vacuum. Also note that, technically, a gas of planets and comets etc. has a pressure (there is usually little reason to care about it though). There is also radiation pressure due to the photons. ...

8

our lab has an ultra-high vacuum stm system (10-11 torr), and all parts that go in the vacuum system has to be extremely clean. Here is what we do: first i want to point out that the material you use for UHV is very important too. The commonly accepted material is 316 stainless steel and oxygen free pure copper. For other specialized material, you should ...

6

Gravity can quite easily be repulsive due excessive negative pressure as mentioned by @Stan Liou. Let us work this out. We will need two equations - Einstein's Equation $$R_{\mu\nu} = 8 \pi G \left( T_{\mu\nu} - \frac{1}{2} T g_{\mu\nu} \right)$$ where $T = g^{\mu\nu} T_{\mu\nu}$. This equation describes how matter affects the curvature of space-time. ...

6

Water boils when the pressure is less than it's vapour pressure (there is a table of vapour pressure vs temperature here). At 20ºC the vapour pressure is 2339Pa, so if your balloon exerts a pressure greater than this the water won't boil. If the pressure exerted by the ballon is less than this, the water will start to boil and the steam generated will ...

6

You're right that the vacuum is the state that minimizes the energy. In the classical limit this is easy to do. Let's take $\phi^4$ theory for example. Then the Hamiltonian is $\dot{\phi}^2/2+(\nabla \phi)^2/2+\lambda \phi^4/4!$. The lowest energy configuration is thus the one where $\phi$ is constant sitting at $\phi=0$, the bottom of the potential. ...

6

1) Most materials you use in everyday life contain far more moisture than you might believe. This is a major reason materials meant to be exposed to space are specially designed and tested. In a general vacuum, most fabrics and many plastic will outgas - all of the absorbed moisture and oils will work their way to the surface and boil off - which is a major ...

6

Both the free and interacting vacuum are invariant under translations, assuming that translation invariance isn't spontaneously broken. Usually we expand around spatially homogeneous and time-independent field configurations, so that you don't have to worry about spontaneously breaking translations. There are some cases where translations are broken in the ...

5

Has anyone ever constructed an ultra-high vacuum system with half-assed, or no cleaning of parts? Haven't we all done that at some point? How'd it turn out? Badly! Water and hydrogen are easy to bake off the internal surfaces, but get any hydrocarbons, skin grease, silicone, etc on it and you'll be baking for days.

5

A very small pop - possibly the worlds smallest thunderclap Surprisingly the average thermal velocity of air molecules (or any ideal gas) is around the speed of sound at that temperature and pressure. This is about 330m/s at room conditions so the air would rush to fill the 0.3m gap in 1ms

5

The phrase is a translation of a quote from Democritus, an ancient Greek philosopher. The quote is not intended to refer literally to the details of modern physics. It is simply an example of an early expression of the naturalistic viewpoint. Carroll's general philosophy is that the universe can be understood in terms of natural laws. There is nothing ...

5

The speed of any object is constant if there are no forces acting on the object. This applies to light and all other matter. Without forces (e.g. friction), an object that is moving will never stop moving. By "perpetual motion" people usually mean a machine that can produce more work than the work required to run it. It's hard to think of an everyday ...

5

Assuming you're willing to accept General Relativity as a valid theory, your question has a well defined answer because we can solve the equations of GR for an empty universe. The result (well, the simplest result) is Minkowski spacetime. You might think that nothing much can happen in an empty universe, but even though no matter or energy is present there ...

4

The reason why classical solutions add a "lot" to the path integral is that their action (phase) is stationary i.e. almost the same phase as the action (phase) in their reasonably large vicinity of the configuration space; one gets positive interference as a consequence. More generic paths cancel with the adjacent ones whose phases are different and random. ...

4

There is a video on YouTube of such an experiment. The corresponding paper is as follows: Can a Siphon Work In Vacuo? Adrian L. Boatwright, Simon Puttick and Peter Licence. J. Chem. Educ., 2011, 88 (11), pp 1547–1550. DOI: 10.1021/ed2001818 After watching the video (but not reading the paper), my first thought was that the liquid in question has unusually ...

4

Creating a vacuum above carbonated drinks causes the CO2 to outgas faster--simply because there is no CO2 above the drink to diffuse back into the liquid. In physical terms this means there is no vapor pressure of CO2 above the liquid, so net movement of CO2 is from the drink to the space above it. If you leave a closed carbonated drink bottle long enough, ...

4

There are lots of related questions on this site but I couldn't find one that answered your question exactly. If you're interested try searching the site for boiling vacuum or something similar. The boiling point of a fluid depends on the external pressure. Specifically a fluid will boil when its vapour pressure is greater than or equal to the external ...

4

But as I understand it, the fields in QFT are not operators, I'm not sure where you heard that, but quantum fields are operators. Or more precisely, operator-valued functions of position: a quantum field $\psi$ maps every point in space, $\mathbf{x}$, to an operator, $\psi(\mathbf{x})$. The VEV $\langle 0\rvert\psi\lvert 0\rangle$, or (perhaps more ...

4

I just realized what the problem is. It actually doesn't have anything to do with the detector. When working in vacuum systems you have to worry about the dielectric breakdown of the air as the pressure is reduced. It turns out that the breakdown voltage hits a minimum around $\sim 1$ Torr depending on the species of the gas (see the curves below). This ...

3

In Newtonian mechanics this term is pretty easy to define - vacuum is a region of space free of matter. In quantum mechanics, things get a little bit complex, for more details I recommend you to read the corresponding section of the Wikipedia article you've put the link to, it explains the term well enough for a beginner.

3

It has nothing to do with action at a distance the way you talk about it. There is some sort of instantanious action at a distance to the particles knowing where are holes to go ? Not quite. Consider that case of the barrier with vacuum inside. From one side, the air atoms are constantly colliding with the surface, and exerting a force on the barrier ...

3

I believe the true answer to your question is that our observation of space fits a mathematical model for a 3-dimensional geometry. My wording is backward compared to historical development, which is why it's hard for people to decouple these things. Humans learn about the degrees of freedom in the space around them as an infant, and possibly even sooner. ...

3

is empty space really nothing? The physicist's 'nothing' is an example of something to the philosopher for which 'nothing' is well, let this philosopher explain in a review of "A Universe from Nothing" by Lawrence Krauss: empty space governed by quantum mechanics (or any other laws of physics, or even just the laws of physics by themselves) is not ...

3

My understanding is that as you state, these states do not belong to the same hilbert space but are formally connected by the symmetry transformation. The essence is that the degeneracy of the vaccum is expressed in the notion that there are multiple equally well suited states (from separate hilbert spaces) from which one can build up excited states. Nature ...

3

Quantum field theories usually have a unique ground state – by the ground state, I mean the Hamiltonian eigenstate corresponding to the lowest energy eigenvalue. This may be demonstrated in various ways, depending on the assumptions we're allowed to make. For example, if a quantum field theory is a free field theory, the ground state may be constructed ...

3

We have the functional of the external source $J$, which gives us v.e.v.s of field operators, by functional differentiation: $$e^{-iE[J]} = \int {\cal{D}}\phi\, e^{iS[\phi]+iJ\phi}$$ $$\phi_{cl}=\langle\phi\rangle_J = -\frac{\delta E}{\delta J}$$ Where $\langle\phi\rangle_J$ is the v.e.v of $\phi$ in presence of external source $J$. That could be ...

3

First of all, classically (neglecting loop corrections), we obviously want to expand around the true minima that are found in the vacuum, which means around one of the states $|0_\pm\rangle$ – these two are really equivalent to one another due to the gauge symmetry. The state around $\phi=0$ is a maximum of energy, not a minimum, so Nature doesn't spend much ...

3

Expectation values in QFT mean the same thing as they do in quantum mechanics. It's just that certain of these guys, the so-called vacuum expectation values (VEVs) $\langle 0|\rm{operator}|0\rangle$ turn out to be especially useful and important in QFT. In particular, the correlation functions (aka Green's functions) of the QFT, which for the theory of a ...

3

The vacuum is polarizable. The polarization can be with respect to electric charge or color charge. In the presence of an electric field, virtual electron-positron pairs briefly exist (created from virtual photons of sufficient energy). The virtual pairs act as dipoles and orient with respect to the field. For example, near a proton, the virtual electron ...

2

What causes these constants to have the values they do is simply our choice of a system of units. When you have a unitless constant, it makes sense to ask why it has the value it does. For example, two of the lines in the visible spectrum of hydrogen have wavelengths in the exact integer proportion of 28/25. When this was first discovered, it made sense to ...

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