I use Wikipedia to learn physics. In a recent search I found an ample description of virtual particles in terms of a discontinuous (in the present) and asymptotic distribution of these particles from -infinity (past) to +infinity(future) BUT on revisiting the site THIS DESCRIPTION WAS NO LONGER THERE. It was replaced with "Since these particles do not have a permanent existence,[clarification needed]".Also missing was a statement to the effect "These particles are extremely short lived. The longer a virtual particle acts, the higher the probability of it reversing in time to its assymptotes." Is the conservation of propability (happening +not-happening = 1) broken and in any case, what does "happening" mean for virtual particles? Can anyone clarify this gap for me?
The term particle together with virtual means we are in the realm of particle physics.
Particle physics has been successfully, in the sense that it has not been falsified, modeled by the standard model of particle physics. You see that I give reference to the wikipedia article, because it has broad lines and a lot of references. This is the use of it, since it is a site where any registered user can always modify a content he/she disagrees with, it cannot be taken as a fixed reference statement by statement. The content you quote is not within mainstream physics about quantum mechanics and the standard model so somebody edited it. Due to these random wikipedia editings , it is not recommended to study physics from it. For seriously studying physics one needs a serious mathematical background, calculus +, anyway, it cannot be done with hand waving and words, at the present age.
Virtual particles belong to the framework of particle physics and quantum mechanics. The concept of virtual particles appears in using the standard model and calculating the integrals implied in the interactions of particles, scatterings and decays, which are the measurable quantities in the laboratory.
Here is an example of such an integral, a Feynman diagram, which is a pictorial representation of the quite complicated integral one has to calculate, in order to predict the probability of electron electron scattering.
Only lines entering or leaving the diagram represent observable particles. Here two electrons enter, exchange a photon, and then exit. The time and space axes are usually not indicated. The vertical direction indicates the progress of time upward, but the horizontal spacing does not give the distance between the particles.
The concept of "virtual" particle comes because in the pictorial representation, the input quantum numbers of the two particles have to be represented and evaluated for the output particles and the line in the diagram represents this transfer, together with the energy momentum transfer by the name of the appropriate particle with those quantum numbers: a virtual photon for this diagram. It is called virtual because it does not have the mass of its name. Under the integral it balances the energy and momentum at the input and output, and as a result it does not have a fixed energy-momentum four vector, and thus not a fixed mass. It is an imaginary particle in this sense, because real particles have a fixed unchangeable mass, the one seen in the table of particle physics in the link above.
That is why the article in the wikipedia was changed because it does not fit within the above mainstream description.The interaction is short lived, and they had attributed real particle status to a mathematical construct.
The statement "virtual particles exist" is talking about mathematics, not physics. Of course the mathematical model has an effect in the probability distributions for the interaction, otherwise it would be useless!! , but in physics one defines as real, quantities that can be measured in the laboratory, and mathematics is used as a tool with no reality except to give the correct input and output four vectors.
Physics uses mathematics as a tool, not as a generator of reality. When in classical physics one calculates the orbit of the moon, the orbit is an imaginary, virtual, line in space, it is not a measurable quantity, except by measuring the position of the moon and checking. The same is true with virtual particles, their effect is not measurable except by checking the computed probability distributions. The virtual particles are hidden within the calculation, and their mathematical existence is necessary to get the correct probability distribution for the interactions, but the virtual particle's four vectors are not measurable, they are a continuum of values under the limits of integration.
You could also read this answer of mine here.
What exactly do you mean conservation of probability ( happening +not happening )? That seems to be a contradiction in itself without reference to science. One thing has to be kept in mind, the fact that we do not "know" is irrelevant. Once we introduce that , you get a series of qualifications. Known to whom? How much effort was made to know ? And so on. So that part has nothing to do with virtual particles in any case.
As to virtual particles, it is a common misconception to think of these as completely hypothetical. Casimir Force, has been experimentally confirmed. So they are as real as any other quantum objects.
So happening has the same meaning for a "virtual" photon as for a "real" photon created in the central region of the sun and absorbed by a Sodium atom in the outer layer of the sun and detected by us as a dark line when sunlight passes through a prism
Obviously a virtual particle has a high probability of disappearing in time. That is after all what makes them. "virtual" . I suspect that the rest of the description you had earlier come across is not based on published and accepted research. So it was eventually removed by other editors.