Often when reading about cutting edge physics and the amazing feats of the Large Hadron Collider, we are treated to crazy scenarios involving “virtual particles”, also variously referred to as “ghost particles” or worse. These labels clearly distinguish the involved concepts from "real particles" like atoms. Not being bound by restrictions of reality, virtual particles “borrow” energy from nothing, go faster than light, travel back in time, do an infinite amount of loops creating an infinity of other virtual particles during every single infinitesimal moment.
For some examples of this clearly going too far, read “Extreme Laser to tear Space Time Apart So Ghost Particles Enter From Other Dimensions?”, which refutes the hype around the Extreme Light Infrastructure ELI, part of Europe’s Ultra-High Field Facility.
ELI Hungary – one of four locations in the total ELI project
Virtual particles are an important ingredient of modern physics. Many now insist on that virtual particles are “real”, although the whole point of “virtual” was to distinguish them from real particles. What makes the issue confusing is the "Feynman interpretation", which is the following definition via the "mass shell":
Feynman lecturing on his favorite diagrams.
A real particle is “on mass shell”, which basically means that it has the mass it is supposed to have, no less, no more. A virtual particle is “off mass shell”. In a Feynman diagram, the virtual particles are those that connect vertices; they are the internal lines like the wavy one in the diagram below. Actually, they represent internal propagators, but you may think of them as the tracks of the particles. The real particles are those that enter and those that leave the diagram. In fact, the longer the line, the more the represented particle is “on mass shell”, and if it is infinitely long (never again interacts), it is perfectly “real”.
Anybody in their right mind calls those objects that are observed and interact with us "real". “Virtual” meant that something is not so "real", especially if something can on principle not be observed. Yet, the above “Feynman interpretation” is easily misinterpreted as claiming the opposite, namely calling the particle that interacts the virtual one, while the one that is never interacting again becomes the more "real" the less it is interacting (perfectly real if it is never observed). Is the meaning of “real” here put on its head?
The funny Feynman interpretation is easily explained: You, the observer, may be represented by the left track and the real particle by the right track. The particle is “real” because the interaction that goes between the tracks is an observation of the particle. It is the whole track that is thus real relative to the other.
The particles leaving are indeed the real ones. Calling the short connecting lines "virtual" makes especially sense if the whole diagram is just a part of a complicated particle interaction which happens in its entirety long before any human experimenter can observe the results (see the so called “Penguin diagrams” below).
One more gratuitous example to stress the fact of that these diagrams can get infinitely complex:
Especially the virtual particles internal to calculations that sum up an infinity of them, are not "real" in a common interpretation of that word.
Virtual particles are theoretical entities; they are tentative interpretations of mathematical tools inside the theoretical apparatus with its shortcomings. They are useful in order to calculate for example the Hawking radiation around a black hole. It may turn out that we cannot find better ways of calculating these interactions. Should we thus call them "real" and what would that add? As I wrote at the end of “Extreme Laser to tear Space Time” which goes into the Hawking radiation example in more detail:
“The theory that calculates Hawking radiation does indeed have virtual particles in its mathematical description, which is a semi-classical calculation, so we know it is getting to the correct result via an in a sense wrong approach (of which we happen to know however when results are still reliable). These are intuitive interpretations which ease dealing with theoretical tools. Virtual particle pairs are ripped apart by a gravitational field which does not exist in the better (not semi-classical) description??? Well, be glad about them being virtual, which basically means that they do not exist either.”
There are different kinds of realisms, all the way from direct realism over modal to structural realism. So you may object and say “being real” does not equal “existing”, but surely such sophistication cannot be an excuse for writing “virtual particles are real” when aiming at a wide audience that is not already at home with such intricacies.