# Tag Info

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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 ...

8

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 ...

7

The big bang in relativity is not what you are imagining--- it isn't a localized explosion. You don't have stuff rushing out from a point, you have everything getting denser in the past in a homogenous way. This is complicated a little by the fact that a Newtonian big bang has things rushing out from a single point. But even in a Newtonian bang everything ...

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

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

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

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

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

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.

4

In the string theory literature, the term "vacuum" is usually synonymous with "perturbative string background", i.e. a target space of a 2d CFT with the right central charge. This target space comes equipped with a metric satisfying the Einstein equations, as well as a host of other background fields, dilaton, B-field, and whatnot, all satisfying the ...

4

In algebraic QFT, the Hamiltonian is always well-defined, and the vacuum state is an eigenstate of zero energy. Thus no such terms arise - they are eliminated by careful renormalizations before a continuum limit is taken. In the path integral approach, the problem is still present, hidden in the renormalization prescriptions for making perturbative sense of ...

4

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 ...

3

Measurable particle anti-particle pairs are never created out of vacuum, but only out of other particles or  out of externally applied fields. (The cross section for creating anything from the vacuum state but the vacuum itself is exacly zero.) The reference to creation out of the vacuum is for unmeasurable virtual particles only, which are visual ...

3

The sealing effect is caused by pressure differences. Once you put the lid back on, the warm air in the box cools, reducing the gas pressure inside the box below atmospheric pressure. So the atmospheric pressure outside the box tends to seal the box if there is contact all around the lid to prevent air moving between inside and outside. Water vapour ...

3

The usual procedure involves Careful preparation for ultra high vacuum Do you reach a good high vacuum? Can all the equipment withstand high temperatures over a long period of time? If not you might to cool those parts while heating the rest of the chamber. Try to remove all materials that might have a high outgassing rate, otherwise ultra high vacuum is ...

3

It seems like your idea of "perfect vacuum" is something like "nothingness", which you definitely cannot find anywhere in our universe. Firstly, if we travel 10000 light years away, we would still be well within the Milky Way galaxy. Interstellar space, while mostly empty, still contains a good amount of hydrogen and other debris. Even if we travel a ...

3

You say: With perfect vacuum I mean that there are no particles, not even virtual photons but the quantum mechanical vacuum is a complicated place. For example see the Wikipedia article on the QCD vacuum. If you count virtual particles then there is nowhere in the universe that is a perfect vacuum in sense that there are no virtual particles present. ...

3

The reason attractive $\lambda \phi^4$ is unphysical is because a sufficient density of the $\phi$ particles has a self interaction which compensates for their mass-energy, so it is less energy to make a condensate of particles with a large density than to leave the vacuum alone. This means that the vacuum will spontaneously decay by a monstrous explosion ...

3

$\cal H \sim \frac{\lambda}{4!} \phi ^4$ (note that this term goes with the opposite sign in the Lagrangian). $\lambda$ has to be real because of unitarity and has to be positive because of vacuum-stability or, equivalently, since the Hamiltonian must be bounded from bellow. If $\lambda$ were negative, the larger the value of $\phi$, the more negative the ...

3

To answer "Is there an intuitive description of vacuum entanglement?", we would like to point out to define entanglement in a quantum theory (defined by a Hilbert space and a Hamiltonian), we need to assume that the total Hilbert space is a direct-product of local Hilbert spaces: $\cal{H}_{tot}=\otimes_i \cal{H}_i$. (For example, in a lattice model, ...

3

Nothing goes on; the vacuum is completely inert. Virtual particles don't exist in time, except in a (literally) figurative sense. They don't have associated states, hence no expectations, probabilities, uncertainties. See http://physics.stackexchange.com/a/22064/7924 and Chapter A8 ''Virtual particles and vacuum fluctuations'' of my theoretical physics FAQ ...

3

No, because in a vacuum, there is no way for the two tuning forks (I think you meant this, rather than pendulums) to communicate. The reason a second tuning fork with the same resonance frequency will begin resonating is because, physically, sound waves are hitting it at its natural frequency. Sound waves travel in a medium, so in a vacuum, there's nothing ...

3

It comes down to the change in the speed of sound at the interface. The speed of sound in air is approximately 343 m/s. The speed of sound in a solid object is typically much, much higher because the stiffness is much higher. For example, copper is 3901 m/s, brick is 4176 m/s and there are many other materials you can look at for reference. On the other ...

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

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

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 ...

2

The slides by Summers misuse the conventional terminology (though for a formally justified reason explained below), thereby introducing confusion. Entangled states are, by the conventional definition (as given,e.g., by Wikipedia), defined in a tensor product with more than one factor of dimension $>1$. On the other hand, the vacuum state of a free ...

2

In reality the small fraction of the air you hadn't managed to remove. Even if you could make a perfect vacuum pump (you can't) then there would be molecules from the material making up the walls of the container which will boil off into the vacuum. Ignoring engineering details and with a perfect vacuum there would still be a sea of infrared ...

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