# Tag Info

24

There have actually been cases of (accidental!) exposure to near-vacuum conditions. Real life does not conform to what you see in the movies. (Well, it depends on the movie; Dave Bowman's exposure to vacuum in 2001 was pretty accurate.) Long-term exposure, of course, is deadly, but you could recover from an exposure of, say, 15-30 seconds. You don't ...

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. ...

10

As others have said, it's almost empty, but not quite, as there are gas particles and so on floating around. As wikipedia states: Generally free of dust and debris, intergalactic space is very close to a total vacuum. The space between galaxy clusters, called the voids, is probably nearly empty. Some theories put the average density of the ...

10

As I recall from Susskind's course, there is no actual vacuum in string theory. There are some pieces of information, which can be helpful, like terminology developed for 2 decades. Please, note the dates. String theory is believed to have a huge number of vacua — the so-called string theory landscape. Terminology starting from almost nothing: "In ...

9

Dear GJ, "vacuum" and "empty space" is always the same thing, but one must always be careful what these two synonymous terms mean. General relativity implies that the only "information" that the vacuum carries at each point is the so-called "metric tensor" - a set of numbers that allow one to calculate the distance between any two nearby points. This is ...

9

First: Whether the metastable region is acceptable is somewhat debatable. I think that most experts would say No. Even if the tunneling may be very slow, one would have to explain why the Universe started in a configuration whose energy is very far from the minimum, in a metastable valley. There are other papers that already put the observed mass to the ...

8

The running coupling $\lambda(\mu)$, as a function of renormalization scale $\mu$, does run negative for large $\mu$ in the SM if the Higgs is not too heavy. But "renormalization scales" are not particularly physical things to talk about. A more physical quantity is the renormalization-group improved effective Higgs potential, $V(H)$. For large values of ...

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

I might add some further notes to the actual material things existing in intergalactic space. One might wonder but the notion that there is space is already stating that there is more than nothing. It implies that there is at least vacuum which is a pretty interesting thing on its own. Classical harmonic oscillator Maybe you know that the harmonic ...

7

You'd freeze to death faster in the Atlantic ocean. Space has essentially no thermal conductivity. All the heat you lose will be radiated away. According to the Stefan-Boltzman law, $W = \sigma T^4$, you would lose at most 500 watts per square meter of body surface area. By contrast, the convective heat transfer coefficient in water is about 12,500 ...

7

The Einstein field equations actually don't say anything at all about the nature of matter. Their structure is that they relate a certain measure of spacetime curvature G to the stress-energy tensor T: $G_{ab}=8\pi T_{ab}$. The stress-energy tensor describes any matter that is present; it's zero in a vacuum. Trivially, you can write down any equations you ...

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

The concept of vacuum in physics indeed comes from two different theories. The General Relativity Vacuum is a space-time model region without matter. In General Relativity all of space-time has a "curvature" which relates to the metric which can all have measurable effects, such as the bending of light rays (in the vacuum) near a massive object. One may ...

6

Inflation is a rapid stretching which result in cosmic smoothness and uniformity on large scales; as such, inflation is a key component of almost all fundamental cosmological scenarios. Not only does inflation explain the overall uniformity of the universe, but quantum fluctuations during inflation plant the seeds that grow into galaxies and clusters of ...

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 ...

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

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 ...

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the original strategy of Feynman and wheeler was really about the desire to get rid of all self-interactions. In the modern language, it would eliminate most loop diagrams. In particular, consider an electron propagator, in the modern language. One may attach a photon propagator on it. That modifies the electron's self-energy, and this is the kind of a term ...

5

I don't think the particle-anti-particle picture is a very good one to grasp what's going on. Essentially, it's a consequence of zero-point energy. In classical physics, the lowest energy state of a system, it's ground state, is zero. In quantum mechanics, its a non-zero (but very small) value. The easiest way to see how this zero point energy arises is ...

5

Heat is not caused by thermal energy being radiated from particles due to their energy heat is the ramdomized (i.e. neglecting bulk flows) energy of motion in any material (including, for instance, photon gases). Any vacuum that we can make or have access too includes a small amount of matter, and the temperature of that stuff can be measured. Not ...

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

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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.

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