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?
[Physics] Vacuum is not really empty
vacuum
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In Physics "nothing" is generally taken to be the lowest energy state of a theory. We wouldn't normally use the word "nothing" but instead describe the lowest energy state as the "vacuum". I can't think of an intuitive way to describe the QM vacuum because all the obvious analogies have "something" instead of nothing "nothing", so I'll do my best but you may still find the idea hard to grasp. That's not just you - everybody finds it hard to grasp.
Start with the classical description of an electric field (Maxwell's equations). It's not too hard to image an electric field as a field filling space. You can even feel the field: for example if you put your hand near an old style TV screen you can feel the static electricity. You can imagine turning down the electric field until it disappears completely, in which case you are left with the vacuum i.e. nothing.
Now imagine the same field, but this time we're using the quantum description of the field (Quantum Electrodynamics instead of Maxell's equations). At the classical level the field is approximately the same as the description Maxwell's equations give, but now we have fluctuations in the field due to the energy-time uncertainty principle. Just as before, imagine turning down the electric field until it disappears. Unlike the classical description, the (average) electric field may disappear but the fluctuations do not. This means the quantum vacuum is different from the classical vacuum because it contains the fluctuations even after you've turned the field down to zero.
The key point is that when I say "turn the field down" I mean reduce the energy to the lowest it will go i.e. you can't make the energy of the electric field any lower. By definition this is what we call the "vacuum" even though it isn't empty (i.e. it contains the fluctuations). It isn't possible to make the vacuum any emptier because the fluctuations are always present and you can't remove them.
is empty space really nothing?
The physicist's 'nothing' is an example of something to the philosopher for which 'nothing' is well, let this philosopher explain in a review of "A Universe from Nothing" by Lawrence Krauss:
empty space governed by quantum mechanics (or any other laws of physics, or even just the laws of physics by themselves) is not nothing, and not even an “example” of nothing (whatever an “example of nothing” means), but something. And it remains something rather than nothing even if it is a “good first approximation” to nothing (which is what Krauss presumably meant by “good first example”). When people ask how something could arise from nothing, they don’t mean “How could something arise from almost nothing?” They mean “How could something arise from nothing?” That is to say, from the absence of anything whatsoever -- including the absence of space (empty or otherwise), laws of physics, or anything else.
Best Answer
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.