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26

Why is space a vacuum ? Because, given enough time, gravity tends to make matter clump together. Events like supernovae that spread it out again are relatively rare. Also space is big. Maybe someone could calculate the density if visible matter were evenly distributed in visible space. I imagine it would be pretty thin. (Later) Space is big. Really ...


20

The energy is borrowed from the Heisenberg Uncertainty Principle to create virtual particles and has to be paid back in a very short time. $\Delta{t} \geq \frac{\hbar}{2\Delta{E}}$ This is why virtual particles live for very short times (i.e pop in and out of existence). We cannot manipulate this energy.


20

You aren't creating a vacuum, but you are reducing the pressure in your lungs when you inhale. In effect your lungs are working as a diaphragm pump. When you pull your diaphragm down, and/or expand your chest, this increases the volume inside your lungs. Boyle's law tells us: $$ P_0V_0 = P_{\rm inhale}V_{\rm inhale} ,$$ where $P_0$ and $V_0$ are ambient ...


14

Whether you can extract energy from this or not (and I strongly suspect not) the Casimir effect is a consequence of vacuum fluctuations. Essentially when two metallic plates are very close to each other, the wavelengths of virtual particles that can be created between the plates is restricted and hence there are fewer particles between the plates and ...


11

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 ...


10

Let's consider the simplest case of a quantum harmonic oscillator, with creation and annihilation operators $a^{\dagger}$ and $a$ respectively. The ground state of our system is, $\lvert 0 \rangle$ which has energy, $$E_0 = \frac{1}{2}\hbar \omega$$ Every time a creation operator acts, the state $\lvert n \rangle \to \lvert n+1 \rangle$, modulo some ...


8

I take your question as Is there any substance with condensed (solid or liquid) equilibrium phase at zero pressure? No, because of statistical physics. Let's consider two things. (1) The potential energy of interaction between molecules. (2) The thermal energy distribution for molecules. The potential energy of interaction can generally be of any ...


8

I think the key conceptual hurdle is that the vacuum state is not nothing. Quantum field theory describes matter as excitations in quantum fields. These quantum fields are very strange things, and I don't know of any easy way to explain to a non-physicist what a quantum field is. The key thing is that the quantum fields fill all of spacetime. So a vacuum is ...


7

This creates a point of extremely focused energy at the middle point where the bubble collapses. In theory, this point focuses enough energy to trigger nuclear fusion. It is not currently accepted mainstream science to say that collapsing bubbles focus energy enough to cause nuclear fusion. Temperatures over 10,000K can be acheived, but are still well ...


7

Let us look at the instantons of an ordinary pure Yang-Mills theory for gauge group $G$ in four Euclidean dimensions: An instanton is a local minimum of the action $$ S_{YM}[A] = \int \mathrm{tr}(F \wedge \star F)$$ which is, on $\mathbb{R}^4$, precisely given by the (anti-)self-dual solutions $F = \pm \star F$. For (anti-)self-dual solutions, ...


7

It means it's "the end of the line". The vacuum state is, as you correctly say, not the zero state. It has energy content, and physical meaning - it's the state with no particles. Annihilating the vacuum leaves...nothing. Trying to take a particle out of it is not possible - it gives you the zero vector, which does not represent a physical state, since it is ...


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 ...


6

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 ...


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

The image of space being bent is just an analogy, it is not meant that anything is actually being deformed. Gravity distorts the notion of distance on spacetime, i.e. the presence of matter somehow causes the metric to change. A way to visualize this is to think of spacetime being bent, as you say, but really, spacetime is not made of anything, the idea of ...


5

Yes, water still has surface tension in a vacuum. Water/vacuum surface tension is 72.8 dyn/cm experimentally according to Zhang et al. J. Chem. Phys. 103, 10252 (1995). Surface tension is caused by the fact that water molecules in the bulk (not at the surface), are surrounded by other water molecules with which they interact through intermolecular ...


5

I assume you are asking why we are not drawing air out of a balloon like container so as to create the lower density that helium or hot air gives us. The answer is that it is hard to maintain a vacuum with a thin enough, so as to be almost weightless, rigid contaning surface. A balloon with gas inside equalizing the atmospheric pressure with the gas ...


5

Virtual particles refer to actual, nonzero features in the quantum fields of real objects, but they are features that are not particles in many ways so you should not expect anything from their being named "particle". Basically, the idea of virtual particles was invented as a device for when you want to hold on to the particle picture while doing quantum ...


4

Space is sometimes described as a vacuum better than mankind could create in any laboratory. But it is not a vacuum, but a tenuous plasma carrying the interplanetary medium (solar wind). It is also structured, forming the Heliospheric current sheet. This means that space has the characteristics of a plasma. It is electrically conductive, carries magnetic ...


4

The questions you ask are really difficult to answer. Mass is not a property of space (or space-time itself), but of physical objects in classical physics. In General relativity, it is difficult to speak about mass clearly, there is no good general definitions. Now, there are two naive metaphysics about space-time. The substantivalists think that space-time ...


4

If (hypothetically) you could find a far off region of space where there is no radiation of any sort, and you place a hot object there, then it would radiate away its heat and gain no heat back from its surroundings. The rate of radiation would gradually decrease but eventually it would lose its last photon and enter a ground state of absolute zero ...


4

Even in a "perfect vacuum", i.e. barring quantum fluctuations, all fields are present. Their field values just are zero, corresponding to no particles or electromagnetic fields present. Still the fields are there! Therefore, just as a matter wave (which is nothing but a particle), light waves, i.e. waves in the electromagnetic field, can propagate through ...


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 ...


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

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

The answer kinda is "You can, but why would you". It is indeed possible to extract energy from the vacuum. It has been studied, both theoretically and experimentally, using a variety of metal plates and other Casimiresque gizmos. The problem is just that it basically acts like a spring. To put the Casimir effect in action, you must first approach together ...


4

No. Just like in Chemistry and Thermodynamics, we never get anything for free. On a mechanistic level, it's important to recognize that zero-point (vacuum) energy represents the lowest energy state waveform. I remember thinking that because the EM fields are everywhere and quantized, that there was some sort of magic taking place. Realistically, ...


4

Why can't fermions have a non-zero vacuum expectation value (VEV)? Lorentz invariance. If anything other than a Lorentz scalar has a non-zero VEV, Lorentz invariance would be spontaneously broken. For example, suppose we have a Lorentz invariant term in a Lagrangian for a vector $$ \mathcal{L} \supset m^2 A_\mu A^\mu. $$ Now suppose the vector obtains 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 ...


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 ...



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