# Tag Info

1

It all depends on what kind of sensitivity you want in pressure measurement. If you just want to distinguished a vaccum state from atmospheric pressure, then there are many non-destructuve ways: index of refraction of air at atmospheric pressure (and a few orders of magnitude down) is different from 1 - optical interferometry. if you apply a temperature ...

0

Why don't you just put a balloon inside? It will enlarge because the proportion of the pressure inside and outside the balloon will change.

12

Freeze it in liquid helium. Any gas inside will condense out. Spin it quickly then stop it. The internal turbulence of the spinning gas will be visible with a sensitive detector. Apply a short sharp impact to one side. If there is gas inside, the sound energy peak from the sound transiting the gas will be temporally distinct from the spectrum of the sound ...

6

For 1. In principle, the refractive index of a true vacuum is identically 1. For air at atmospheric pressure, the index is 1.000293 for visible light. Therefore, you should be able to determine the deviations between in refractive angles for a jar filled with air and one under vacuum. Since we're talking deviations on the order of one in ten thousandth, it's ...

4

You can use electric discharge of appropriate frequency, as its threshold in gas depends on pressure (and frequency).

0

You have to distinguish between spacetime and proper time: The vacuum would still continue to be part of the Minkowski spacetime continuum of the observers, in particular with respect to simultaneity (e.g. of events happening at this place before or after your experiment). If your experiment is lasting one hour, during this hour the clocks of the observers ...

2

You can certainly create a pressure gradient. Depending on the acceleration, that gradient could be as large as you like and could lead to a very low pressure at the front, which might approach a vacuum. The equation is very simple: $$\Delta P = - \rho g \Delta h$$ So for a $1m$ tube, filled with ambient air ($\rho = 1.2754\ kg/m^3$) and a $1g$ ...

0

Your question is: why is said that the space is expanding ? The answer that everyone are aware, by consensus : We see a reddening of the light received from distant stars and galaxies for the same stellar processes we see in the sun and nearby stars. One way to interpret it, the official way, the only one that your teacher and scientists do know, is: the ...

4

"The way I see it, space, true "space" is literally nothing" Most physicists and I believe many philosophers would disagree. The notion of "Nothing" is impenetrable logically: "nothing" has neither any properties nor relationships with anything else that can be reasoned about. The best you could do would be to assert that "nothing" is somewhat like the ...

2

According to inflation, strictly speaking, space is still being created. I find this idea very interesting. Also, the idea of space as being an empty void to be filled is out of date. Space is full of fields, even when there are no particles present.

4

If you had empty space and then matter expanded from a point in it, then some of the matter would be in the center, seeing everything moving away from it. Some of it would be near the edge and see darkness filling half their world. We see matter moving away, and it seems unlikely that we just happen to be so close to a center of the universe. So we look ...

1

The best experimental evidence comes from photon-photon collisions. These occur during collisions of virtual particles (e.g. quarks) surrounding bremsstrahlung photons in an $e^{+}e^{-}$ collider. From the tone of your question, I get the feeling you're looking for a simple, common experience example. However, the phenomenon is a high-energy quantum effect ...

31

$$\sin(x) = x-\frac{x^3}{3!} + trigonometric\;fluctuations$$ Above you can see why I don't like the language of "quantum fluctuations" -- what people mean by them is just "terms in perturbation series that we can make classical sense of". Similarly the phrase ... particles pop in and out of existence... Is a yet another naive attempt of describing ...

5

My current understanding is that the physical reality of vacuum fluctuations, particle-antiparticle pairs being created and then annihilating, is disputed. The Casimir effect is often cited as physical evidence but there's a few authors which have come to dispute that the Casimir effect is convincing evidence for the reality of vacuum fluctuations, as they ...

3

This phenomenon is called Quantum Fluctuations or vacuum energy and it could be described theoretically by Heisenberg uncertainty relation with the energy term. One of the physical evidences of such phenomenon is ''Casimir effect'' . when two uncharged plates are put close to each other they exhibit a repulsive force, this force is explained by quantum ...

13

Meson Production A significant contribution to forward, production of pions and other mesons is the knock-on of quark-pairs from the nucleon sea. Reactions like $$e^- + p \to e^- + \pi^+ + \text{undetected hadronic junk} \,.$$ For one of many more technical set of discussions, see the $f_\pi$ collaboration's papers:1 http://inspirehep.net/record/535171 ...

23

Just want to bring up that most answers seem to be taking "space" to be a nice uniform medium. However, even within our own galaxy, conditions vary wildly. Here are the most common environments in the Milky Way: Molecular Clouds, $\rho\sim 10^4\,{\rm atom}/{\rm cm}^3$, $T\sim 10\,{\rm K}$ Cold Neutral Medium, $\rho\sim 20\,{\rm atom}/{\rm cm}^3$, $T\sim ... 46 By popular demand (considering two to be popular — thanks @Rod Vance and @Love Learning), I'll expand a bit on my comment to @Kieran Hunt's answer: Thermal equilibrium As I said in the comment, the notion of sound in space plays a very significant role in cosmology: When the Universe was very young, dark matter, normal ("baryonic") matter, and light ... 1 0 Pressure Assuming you fill a balloon with water here on earth, and has some air trapped inside, the balloon will have a volume V1. When you take that balloon into space it will expand and its new volume V2 will be greater than V1. This is because the atmospheric pressure is no longer there. Only the surface tension of the balloon remains, and it ... 6 You need to consider that space is filled with a tenuous plasma, which behaves slightly differently to an ideal gas. First, the electrons will carry sound at a different rate to the heavier protons, but also, the electrons and protons are coupled via the electric field. See: Speed (of sound) in plasma The speed of sound in the solar wind is estimated at ... 35 From the ideal gas law, we know: $$v_\textrm{sound} = \sqrt{\frac{\gamma k_\textrm{B} T}{m}}$$ Assuming that interstellar space is heated uniformly by the CMB, it will have a temperature of$2.73\textrm{K}$. We know that most of this medium comprises protons and neutral hydrogen atoms at a density of about 1 atom/cc. This means that$\gamma = 5/3\$, and ...

4

Given the low density of gas, the speed of sound would be a direct function of the temperature of the gas ie the speed of the molecules/atoms. Since this varies from about 2.7K to millions of degrees near some stars, the speed of sound can change quite a bit.

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