# Tag Info

14

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 ...

7

As zonk said, there is no perfect vacuum. Even the 'vacuum' of space contains a few atoms per cubic meter on average. In the lab, the lack of a high vacuum usually results from not having a pump that can effectively extract enough of the particles inside the chamber you're trying to evacuate. There are several different kinds of pumps used, depending on ...

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

The space between atoms depends very much on the medium you are talking about. In solids the typical distance between atoms is about the same as the size of the atoms themselves. In everyday gases at room temperature and pressure the distance between molecules is many times their size, and in deep space you can get densities as low as one proton per cubic ...

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 ...

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

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. ...

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 ...

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, ...

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

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. ...

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

If the container full of air is spinning around you, the drag will eventually set you spinning as well, regardless of the rotational speed or the air density. Low air density just means that it will take much longer. Eventually the air and you will share the same rotation, so that as you speed up, the air and the container will slow down. Only in (complete) ...

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

I've always been curious how long the Pioneer plaque would last, so I tried doing a very rough order of magnitude estimate - which could be applied similarly to entire spacecraft, or objects on the moon, etc. Note that I'm trying to make the estimate conservative, so I use pessimistic estimates. Some Numbers: $v\sim500 \textrm{ km s}^{-1} = 5\times10^{7} ... 3 (Adding another answer as my response to Hurricane's question is too long for comments) Glad it helped. Richard Terrett is correct, (charged) anti-matter is confined in a magnetic trap in as high of a vacuum as we can get. Uncharged anti-matter must be contained using laser traps ('optical tweezers' is something to look into if you're curious). There will ... 3 There are many ways to carry heat. The first is conduction, which is about the "vibration" of atoms on one material passing to another by simple physical contact. (Example: you touch something hot and get hurt). The second is convection: hot molecules simply move from one place to another (Example, water starts to boil in the bottom of a pan, but moves on ... 3 In Newtonian mechanics this term is pretty easy to define - vacuum is a region of space free of matter. In quantum mechanics, things get a little bit complex, for more details I recommend you to read the corresponding section of the Wikipedia article you've put the link to, it explains the term well enough for a beginner. 3 It has nothing to do with action at a distance the way you talk about it. There is some sort of instantanious action at a distance to the particles knowing where are holes to go ? Not quite. Consider that case of the barrier with vacuum inside. From one side, the air atoms are constantly colliding with the surface, and exerting a force on the barrier ... 3 We have the functional of the external source$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 ... 2 The current record vacuum, made at CERN, is around 1000atoms/cubic cm For comparison intersteller space (well away from the solar system) is around 1 atom/cc and deep intergalactic, really empty, space is around 0.001 atom/cc The bits of space we can reach, in Earth orbit, are only about the sort of vacuum you can get in an ordinary lab vac pump. 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 ... 2 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 ... 2 I find in this link of the Beams department at CERN, how to clean parts of a quadrupole because it will be in a vacuum. Cleaning of the parts Since a quadrupole is working under vacuum and high currents are used it is essential that they are absolutely clean from any oxides, grease or other contaminants. It is also of high importance that the ... 2 Yes, this is another diagram that contributes to "vacuum polarization", also called self-energy. If you want to make a consistent calculation of the self-energy in perturbation theory, you need to include the two diagrams that you mention, plus the diagram where the gluon splits into$\bar{q}q\$, plus, depending on your gauge choice any ghost diagrams.

2

In KKLT, a stable AdS vacuum is uplifted to a metastable dS vacuum by the addition of antibranes. And one way to get a Higgs mechanism in string theory is to have a single stack of branes separated into two, parallel, very close stacks. So let's suppose we have a KKLT-like construction in which there are two such adjacent stacks of antibranes. Since the ...

Only top voted, non community-wiki answers of a minimum length are eligible