to be or to be virtual?

A substantial chunk of Honours has passed now, quite quickly. I can tell you that it is the 58th day of Honours - yep, I've been keeping count! One of the tasks we are recommended to do (although thankfully we are not assessed on it) is to keep a logbook of our progress throughout the project, which serves as a useful reminder of where you've come from as time goes on. I'm usually pretty bad at these kind of things, but it's going okay so far!

Getting back into physics has been relatively easy. I wondered for a while, after spending 18 months focusing on Japanese, whether physics would make any sense at all. But it's fun, and it's definitely nice to be back doing physics things.

One such 'physics thing' that never ceases to be a source of joy (not…) is the "Physics Assignment". We used to get two assignments per subject per semester, but now we get three per subject (which only last six weeks or so). When you are doing 3 or 4 subjects, it's a bit of a domino effect really.

Physics assignments are usually not terribly hard but quite horribly ambiguous, and you end up spending a fair bit of time working out what the question is. Not necessarily because they are worded badly, but because we are dealing with fairly specific subjects at this stage, and as such fairly meaningful terminology. Which brings me to my first Physbit: the notion of virtual particles.

One of the questions on our first Standard Model assignment asked us to decide whether the particles in a certain interaction (electron-positron -> Z0 or photon -> tauon-antitauon, if you're interested) were virtual or not. In some ways, it's a philosophical question, and is really not a simple question because this area of particle physics is not yet complete. There are many definitions out there of what a virtual particle is (search it!) and a lot of confusion surrounding the subject of what is virtual, and what exists.

Our understanding of it, or the conclusions we wrote down in the assignment, is as follows. A particle can only be said to exist if the total energy put in is greater than or equal to the amount of energy needed to equal its rest mass (think Einstein). So, for example, our particular example involved an amount of energy that was not enough to form the Z0, although we know it happened because the tauon pair came out the other side.

So what's going on there?

Very, very simply (because I certainly don't understand it entirely!), a virtual particle is able to "borrow" energy from somewhere. Where? I'm not sure anyone really knows. Dirac once referred to a "sea of negative energy", and this conception of a "sea" is perhaps something akin to what particles are. Quantum physics first messed with our minds in the early 20th century, and the notion of a particle as a probability distribution (a disturbance in a medium we don't quite understand) is maybe the best way we can understand how things work.

It perhaps means that all particles everywhere exist within this "medium" - our lecturer joked about this being the new "ether" and he might be right - and this medium is able to lend energy to facilitate reactions. In this sense, the Z0 was "virtual" because we hadn't put enough energy in the electron-positron reaction for a real Z0 to form. But, luckily, the unknown field lent a hand and we still had the result we wanted.

Had we put more energy in, enough that the Z0's mass could be formed, then this "virtual" particle would no longer be virtual, and we could say that yes, it exists.

Kind of bizarre, right? I hope you think so! There are so many questions in physics that are unanswered, because with time physics is moving away from answering the obvious questions (what happens to a ball rolling down a slope? how does ice become water?) to big and confusing questions like what exactly matter is and, more importantly, why it is. It is clear that probability plays a key role in things. Unfortunately, quantum physics also seems to be quite important (it's a big headache for me). Why exactly this mysterious background field decides to lend energy for reactions to happens is a strange, strange question that I don't know the answer to.

They might find some clues in the LHC, once it starts working. They might find the Higgs boson! I think however what is for certain is that they'll find more questions. Even their results will be conclusions drawn from a statistical analysis of millions, probably billions, of interactions, searching for the ones that contain signals that the Higgs boson, to date only a theoretical and hence virtual particle, has been made real, through a heckload of power.

So here, and in general, to be or to be virtual is really a matter of energy.

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License