On hadron colliders, dark matter and black holes

[RPMiller]

I forgot to ask my question... So what is it about the anti-matter that makes it get destroyed by matter? That has always been a big question of mine.

[torstan]

Hmmm, it's all a bit quantum. The first time that you get the creation and annihilation of matter is in quantum field theory. In QFT you have to understand particles as the excitations of a field. The interaction of particles is then the interaction of modes of a field. Now you cannot have a continuous spectrum of excitations of the field - they are quantised. We identify these quanta as particles.

Right, now we've got fields with states that we interpret as particles. When those states interact we are interested in the interaction of fields. The details of this are seriously messy - but the quantising of fields throws up things called creation and annihilation operators in your maths that correspond in reality to the creation and annihilation of particles. When you construct consistent theories with this framework you find that some things must be conserved. These are called quantum numbers. Now an electron carries a few different quantum numbers - one of which is called lepton number. A positron carries the opposite quantum numbers. Therefore you can have a term in your equations in which an electron and a positron annihilate because their quantum number cancel out. Before the interaction you had a total lepton number of 0 (+1 for the electron, -1 for the positron) and after the annihilation you have a photon, and a lepton number of 0 (photons don't carry any lepton number).

Unfortunately that's about as far as I think I can go without actually writing equations. I know that it's a bit like saying 'because they do' (spot the sentence above that could be substituted for that ). If you've got any specific questions then ask away and I'll do what I can to answer them.

[RPMiller]

So a photon is a light particle right? Is it a field that produces light... er something? I guess the related question would be what exactly is light?

[torstan]

Light is an excitation of the electromagnetic field.

There's a useful distinction to bring in here. There are bosons and fermions in the world. These are two separate types of particle that are differentiated by something called spin. Spin is another of those quantum numbers. It's not that these things actually spin round, but rather that the maths of spin is similar to the maths of angular momentum. Anyway, a fermion has a spin of 1/2 (or more generally half integer) and a boson has a spin of 0 or 1 (or more generally integer values).

All very good, but what use is this? Well it turns out that bosons are force carriers whereas fermions make up the matter in a theory. So quarks and leptons (that include electrons) are fermions whereas photons - the particles of light - are bosons.

Forces appear in a theory when you impose something called gauge invariance. Now this is the equivalent of saying that if you do certain things you don't expect there to be any physical change. Say for example that you have a perfectly circular cake. You rotate it any amount and it's still observably identical to how it was before you rotated it. That's called rotational invariance. Now say you draw a line straight through the middle. It's now only rotationally invariant for rotations of 180 degrees, 360 degrees, 540 degrees and so on. You have broken the continuous symmetry down to a discrete one.

Right, this is going somewhere, trust me.

Now if you construct a theory in which there are only electrons an you require it to obey a symmetry - in this case the symmetry of quantum electro-dynamics - then you find that you can't. If you apply a transformation to your electrons you get an observable result. To make sure it obeys the symmetries you have to introduce a new particle that interacts with the electrons in a very specific way. The new particle must be a boson. You then find that this particle is massless and only interacts with charged objects. We call it the photon. It is the particle that carries all electromagnetic force.

Now it turns out that our eyes also interact with photons that have certain energies - so we can see them. Hence light is a subset of electromagnetic radiation. Hope that makes a bit of sense.

[RPMiller]

Wow... it actually does... for the most part. So if photons are massless, how do they interact with a reflective surface and "bounce".

[torstan]

The short answer is that they interact with the electrons that surround atoms - they get absorbed and then re-emitted. The direction they are re-emmitted in is (I believe) governed by the conservation of momentum. I'm not 100% on the details of optics and there's definitely a whole load of complicated physics that gets you from individual particles back up to large scale optics with diffraction, refraction and so on but the basic process is the interaction of photons with the electrons in the atoms.

[RPMiller]

That seems simple enough to understand. Thanks for answering all the questions.

BTW, this all gives me ideas for possible superhero powers at some later date.

[torstan]

Not a problem at all. I'd be intrigued to hear about the ideas for powers!

[RPMiller]

That's the cool thing about the Hero System, you build powers based on special effects... Quantum Man can destroy matter by firing his Quantum beam of anti-protons... that sort of thing.

In the system that would be simply built as Ranged Killing Attack, Armor Piercing, Penetrating. So not even armor would completely protect you unless it was Hardened x2.

[torstan]

That's quite neat. I may have to look at Hero when I get a chance to actually play some games again. Moving country seems to have eaten my free time, and trying to organise games over a 5 hour time difference is a bit of a killer.

If quantum is the defining feature of your person the best property would be something like blinking (to use an easy D&D analogy). Quantum behaviour is best embodied by the probabilistic behaviour of objects that only become deterministic when you look at them. So a quantum person would be in a number of places at once until someone looked at them - at which point they would be in a specific place. Anyway, enough physics for tonight.