The main challenge in understanding QM is the fact that we are all intelligent animals whose brains developed in what is for all practical purposes a classic newtonian universe. When you were one year old, crawling on the floor at your parents' house, your brain was learning to operate within the realm of classic physics.

In such a universe, things are very clear-cut. An object must be either here, or there. Something must be either in this state, or the other. Particles behave like billiard balls, and waves behave like, well, waves. Things either exist, or they do not. Space and time have perfect accuracy, there's no "smearing" of coordinates either in space or in time.

The problem is, that is not at all the case at microscopic levels. You've learned what a marble is as a kid, and you think of electrons as little marbles racing around the nucleus - but that's utterly wrong. You've watched water waves on a pond and you think that's how a beam of light behaves, but that's ridiculously simplistic.

The fundamental reality at the quantum level is the awkwardly named wave function (awkward because there's no wave proper there). That's what electrons, and photons, and neutrons, etc, are. It's a function that describes a probability amplitude as a complex number. That's it. They're not little marbles - although sometimes they kind of behave like that. They're not little waves - although they kinda-sorta look like that. They are bundles of probability amplitudes smeared out over space and time intervals. This is not a metaphor. This is stark reality.

The familiar classic universe with crawling floors and marbles and water waves is just an epiphenomenon. It's a collateral that emerges on top of the quantum world if you zoom out a trillion-fold and ignore all the little details.

The "smearing out" of space, time, momentum and energy parameters at the quantum level simply means these are not the truly fundamental things you think they are on that level. Take energy and time; in a quantum system, you could measure energy with infinite precision, but then you can't say anything about the time intervals. Or you could measure time very precisely, but then you don't know anything about energy. That's not a limitation of your equipment. It simply means neither energy nor time are the precious snowflakes you thought they are - but their combination is.

A particle that does not exist has exactly zero energy and exists for exactly zero seconds. But wait, this is the quantum world. That means a deviation from zero energy is permissible, as long as it does not take place longer than a correspondingly short interval. The higher the energy deviation, the shorter the duration of the blip. That means, in completely empty space, you will be able to measure that particles seem to appear out of nowhere, exist a very short time (the more massive the particle, the shorter the time), then vanish.

But do those particles actually exist? In QM, that's meaningless. Remember, there's no such thing as a particle? Only bundles of probability density? Remember that? And those bundles obey the basic relations about energy/time and momentum/location. As long as you're talking about fluctuations the size of an electron, then yes, pure vacuum would seem to produce electrons continuously, but they only blip in and out of measurement for a very short time.

But are they "real"? If you can measure it, it's real. Full stop. But classic categories such as "it exists" or "it does not exist" get blurred at the quantum level. Anything could potentially, and does actually, exist - provided it does not violate basic QM relations.

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So that "quantum fog" exists in a vacuum all the time. Usually it's completely chaotic. But magnetars have such strong magnetic fields, they impose a structure over those short-lived virtual particles. And then when light passes through, it interacts with that structure and gets polarized.

So do those virtual particles exist? Well, they changed the light, didn't they? But realistically - "exists", "does not exist", that's newtonian bullshit. At the quantum level, which is the basis of reality as far as we can tell nowadays, it's all about probability functions.

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If you feel compelled to visualize this stuff, think of quantum particles as little "clouds" of probability. More dense in the middle regions, getting thinner and thinner towards the periphery - but basically it keeps going on an on, it's just a stupendously small value far from the thickness. Of course, this is just a poor visualization, but for some folks it's better than nothing.