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This question already has an answer here:

I've come across another article in "list verse" which says that the empty space is not actually empty at least for a while. I've tried to find about this, so I googled it .It also quotes a word "Quantum foam" .But all the explanations are way too hard to understand .Any one who knows about this could help . Thank you in advance....enter image description here

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marked as duplicate by John Rennie, Qmechanic Feb 5 '16 at 11:27

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicate of Are vacuum fluctuations really happening all the time? $\endgroup$ – John Rennie Feb 5 '16 at 8:47
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    $\begingroup$ @JohnRennie You are too fast in declaring duplicates. In this case, when the OP says that he /she cannot understand what he finds on the net, pointing to more of such answers is not helpful. I just managed to put in my answer which I think is elementary enough to be understood by similar readers. $\endgroup$ – anna v Feb 5 '16 at 11:50
  • $\begingroup$ @Qmechanic the above is also for you $\endgroup$ – anna v Feb 5 '16 at 11:50
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    $\begingroup$ This is not a duplicate. Reopen it. $\endgroup$ – John Duffield Feb 5 '16 at 13:33
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    $\begingroup$ @annav (and John): This is a duplicate because the question is the same. If you feel the answers that already exist at the linked dupes do not address the question at the level the OP wants to understand it, and you feel the OP deserves an answer, then you should add an answer to the old questions. $\endgroup$ – Danu Feb 5 '16 at 14:35
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First let us address "emty space".

Empty space is a theoretical concept, a space where there is no matter and no energy.

In our universe, no matter how far away one goes in space, it is not empty. It contains the cosmic microwave background radiation, cool photons, which is at a temperature of 2.7 K .

Within quantum mechanics and elementary paricles, the "empty space" , ignoring the CMB existence and assuming empty space exists, can be mathematically populated. What does this mean? A mathematical model exists where every elementary particle in this table populates "empty space" with the ground state of the solution for the field of that particle. This means that the existence of that particle, an electron on the way to the sun, for example, appears as an excited state on this ground state solution for electrons, and a probability distribution of its trajectory in space accompanies it, as the Heisenberg uncertainty principle , HUP

hup

defines a region around the trajectory where the probability of finding the electron at x, constrains the probability of its momentum being p.

The quantum foam is an extension of this format, taking the HUP and examining a region of space for a tiny time interval delta(t) and saying then there must be a delta(E) for that energy.It then goes further, saying that the positron field also exists at that (x,y,z,t) and a "virtual" electron positron loop could be drawn existing for very small time with the energy allowed by the hup.

This is a misuse of the Feynman diagrams where such loops exist, misuse because in empty space no interactions are happening, and virtual particles are just lines in Feynman diagrams representing mathematical forms to be integrated over. A closed loop cannot be integrated over if the limits of the integral are (0,0). A real particle interaction is needed with real outgoing particles to be able to define intermediate virtual "particles", which have the name but not the mass of its name.

virtual

So the "quantum foam" picture is a handwaving hypothesis, based on the HUP, but really not calculable in any way so as to give anything measurable larger than zero in empty space. It is only within interactions that the effect of virtual loops is observed as measurable, as in the Casimir effect and the Lamb shift.

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Such phenomena as Casimir effect (especially dynamic version of the effect) and Lamb shift are typical manifestations of the quantum fluctuations. An existence of quantum fluctuations is really intuitive fact if you use an uncertainty principle, which tells us that particles can pop out of the vacuum during a very short time interval.

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The universe is not EMPTY. It is governed by dark matter (80%)!.The universe is also filled with neutrino. THE universe wouldn't then be expanding because new matter particles is being created(however they again disappear). Think it like a balloon being blown, as more particle goes in, the balloon expands. In other words, expansion is directly proportional to particles/matter. Therefore, as the universe is expanding, it is ought to be that the UNIVERSE IS NOT EMPTY

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