Vacuum should contain something in it. Because nothing is perfectly empty that's what I feel, but what is there left in it? Is there any matter or its just enegry. Can energy be pulled out of some space?
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Modern physics doesn't make a fundamental difference between the vacuum and matter and radiation. The physical vacuum, when it is not excited, is what we used to call "the vacuum" and is considered empty, and in its excited state it contains radiation and matter. In a sense, if you want to be metaphysical about it, the physical vacuum is all there is... it's just not a simple object but it has rather complicated properties. – CuriousOne Feb 16 '16 at 11:28
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2Tip: Check the right margin for duplicates. – Qmechanic Feb 16 '16 at 11:39
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In practice, no vacuum is perfect. I've done a lot of ultrahigh vacuum work, and no matter how fast you pump, there is always residual gas, usually hydrogen, coming out of solution in the chamber walls. Outer space also has a very dilute gas, thinner as you move to interstellar and intergalactic regions. Then there is the neutrino flux, and remnants of the Big Bang. Its basically very untidy out there! – Peter Diehr Feb 16 '16 at 11:49
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In quantum field theory, the vacuum is the state containing exactly zero particles anywhere in space and at all times. Since it is an eigenstate of the number operator, there is no uncertainty at all about this.
On the other hand, empty space between matter (i.e., what is informally called a vacuum) is never completely empty; it is still filled with the quantum fields emanating from the matter. Just like the space between the sun and the planets is not empty but filled with the gravitational field.
If this field is strong enough one can extract energy from it. For example, a ball falling in a conventional vacuum gains kinetic energy from the gravitational field.
Arnold Neumaier
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Is there any limit to how much energy we can extract from vaccum/ field? – Anubhav Goel Apr 21 '16 at 10:12
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@AnubhavGoel: The maximum energy one can extract by free fall is the difference between the potential energy at the top and at the bottom. But you need to be somewhat clever to convert the kinitic energy at the bottom to useful energy. Water power is of this kind. - On the other hand, no energy can be extracted from a vacuum not containing fields. – Arnold Neumaier Apr 21 '16 at 17:13
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Sorry! I was not clear. Consider you have a positive charge of 1C in space. Clearly, electric field is now well defined around it. Now , I bring a test charge, energy it will acquire is q∆V. From, same field we can extract different energies based on q. Isn't it peculiar? Is their any upper hand limit to it? – Anubhav Goel Apr 21 '16 at 17:23
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@AnubhavGoel: Nothing peculiar there: From the same gravitational field you can acquire more kinetic energy if you let a heavy ball fall than if you let a tiny ball fall. The point is that to turn this into a continuous source of energy you need to find a place such as a waterfall where there is a continuous supply of high potential objects to fall down. The same problem is in your situation, with the Coulomb potential in case of the gravitational potential. – Arnold Neumaier Apr 21 '16 at 18:15
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Polite cough: Arnold, a ball doesn't actually gain kinetic energy from the gravitational field. The gravitational field converts internal kinetic energy aka potential energy into external kinetic energy, that's all. When you catch the ball and dissipate that kinetic energy, you're left with a mass deficit. The mass of the ball is now less than it was (we ignore the effect on the Earth because it's so slight). It's similar for the electron and the 13.6eV hydrogen binding energy. – John Duffield May 01 '16 at 11:54
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@JohnDuffield: You speak a different language than I. I was not talking about quantum gravity (where there is no vacuum state at all). – Arnold Neumaier May 01 '16 at 18:18
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Vacuum is in fact not empty. According to our current understanding all of space is permeated by fields which due to quantum mechanical effects only tend around a zero energy value. This means that the vacuum is subject to fluctuations in the fields permeating it.
In essence particles pop into existence more or less randomly as a result of excitations in these fields making a vacuum a boiling sea.
The fluctuations are related to the Heisenberg uncertainty principle.
These fluctuations have been experimentally observed and are quite significant to modern physics. The Casimir effect describes the fluctuation in electromagnetic fields and has been observed in a lab environment.
An interesting article on quantum vacuum fluctuations can be found here: http://www.hep.caltech.edu/~phys199/lectures/lect5_6_cas.pdf
Jaywalker
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1-1: This is the popular science view. It has no sound basis in quantum theory. – Arnold Neumaier Apr 20 '16 at 09:53
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I do not know to what extent this could be considered a popular science view as there are plenty of respectable physicists that have conducted work around the casimir effect – Jaywalker Apr 21 '16 at 09:42
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Is this casimir effect due to comment of Peter Diehr or it indeed require no matter? – Anubhav Goel Apr 21 '16 at 10:15
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In the Casimir effect, particles do not pop into existence. See, e.g., https://www.physicsforums.com/posts/5448495, the references there, and the subsequent discussion there. See also https://www.physicsforums.com/posts/5449166/ and the subsequent discussion. – Arnold Neumaier Apr 21 '16 at 18:20
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The paper you quote makes on p.2 under the heading ''Vacuum and vacuum fluctuation'' a lot of unsubstantiated claims taken from the popular literature. The derivation of the Casimir effect given on p.3-5 makes no use at all of any of these wrongly claimed properties. The observed Casimir effect cannot be taken as proof of properties not used in its derivation! – Arnold Neumaier Apr 21 '16 at 18:33