First collisions at the LHC with unprecedented Energy! (Ask a particle physisicist)

[Alex K]

Well, I don't know whether I was really taking time out. I was sitting between some trees in a shady garden, birds chirping, a piece of cake and a glass of wine in front of me. Writing a treatise on forces was just the right dose of physics to enrich such an idle afternoon...

That sounds lovely.  The only way it could be much better would be to have an entire bottle of wine, and someone to share it with and...

From The Rubáiyát of Omar Khayyám, as translated by Edward FitzGerald, fifth edition:

XII.


 A Book of Verses underneath the Bough,
 A Jug of Wine, a Loaf of Bread--and Thou
   Beside me singing in the Wilderness--
 Oh, Wilderness were Paradise enow!

https://en.wikisource.org/wiki/The_Rubai...h_edition)

Lovely - I'd only heard of the Rubaiyat in connection with the mysterious dead spy chiffre case, but never read any of it...

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First of all...

OK, I'm lost already.
But I'll take a shot at demonstrating my ignorance.
Particles trade particles to other particles to make particles or to change particles or to hold particles together against other trades of particles.
But we don't know what a particle is, just how it acts, what it does.
Its sort of particle economics with trades of derivative products which are composite things that don't really exist except for the purpose of trading things to other things on the NYSE to eventually make real things like cash or land ownership.
Sorry, I can't do the math so I have to talk in metaphors.

So in these exchanges, do the 'real' particles know about each other in order to trade 'unreal' particles?
Do they find out about each other by 'fields' (other things whose only evidence of existence is what they do to other things.)
If all these interactions are by exchange of particles, how do the 'real' particles pick which virtual particles to exchange?
And why do you need so many of them to get any rock crushing done?

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you have to know that all forces can be seen as the exchange of virtual particles. the electrostatic and magnetic forces for example are communicated by the exchange of virtual photons, the same guys that make up light if you give them some more energy.
What is a virtual particle? Well, that's kind of a technical issue. Think of it as a particle for which there really isn't enough energy present to make one, but which for a short amount of time can defy this hindrance thanks to some sort of uncertainty principle, which allows particles to appear for a short time.
Now, there are many more types of particles in nature which can also cause forces when they are exchanged, but most of them have a much much shorter range than electromagnetism - for different reasons. There are the W and Z bosons, and when they are exchanged one talks about the "weak force". It is actually not weak, but of very short range, because the W and Z bosons are so heavy. This makes it more unlikely that they propagate spontaneously as virtual particles over longer distances, and hence the range is short. The most obvious effect of the weak force can be observed when W bosons are exchanged, because they change the type of particle. They make electrons out of neutrinos and vice versa, and protons out of neutrons, as an example. Therefore, the exchange of a virtual W boson can turn one type of atomic nucleus into another lighter one,  and produce an electron and a neutrino out of the spare energy. That's beta decay.

There are also the Gluons, and they have a somewhat longer range because they are actually massless like the photons, but since they bind to each other, they basically keep themselves from travelling long distances. These guys lead to a very strong binding force between particles which can interact with them - themselves and the Quarks, and the exchange of Gluons can therefore bind Quarks together to form Protons and Neutrons. This is often called the strong force.

But nit just elemdntary particles can cause forces. Pions which are actually pairs of a quark and an anti quark, themselves can be exchanged between protons and neutrons. This isthe force that holds the nuclei of atoms together - even overpowering theelectric  repulsion between between protons. This effect is also calledstrong force.

Any clearer?

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So again, you wonder whether or how we can talk about particles without knowing what particles actually are. As I have alluded to earlier, I don't think this is necessarily in the realm of science - but - I also believe that we regularly leave the realm of science when we talk about nature. This is a deep philosophy of science question and I am no philosopher of science, so I can only offer you a half baked cake of my own making. I think the question what particles really are, might possibly not be a valid question. My impression is that we can't help but talk about the objects and goings on in the world in terms of our theories about these objects, and that only in the framework of a theory a multitude of phenomena can be unified into a single object. What is a cup of coffee really? Is it still the same cup if you rotate it 180° and why? In order to talk about *the cup*, we have to relate a multitude of phenomena in different channels, touch, sight, and its reaction to touch, and a theory of rigid rotations in space and how they form equivalence classes between objects, all have to relate these things in a theoretical construct you call *the cup*. I don't think particle physics is all that different, except that quantum weirdness makes it harder to express the state of an object in everyday language.

Concerning the second question, I have wondered that myself. The interpretation of forces as particle exchange is an intuition that comes out of the mathematical construction of Feynman diagrams. It is real work to try and map this question (how do the particles that are exchanged decide to get flying, do they see their goal?) to the maths of the derivation of feynman diagrams. My preliminary answer is thusly: Particles send off virtual particles all the time in this picture. If another is in sight, it gets caught. But this does not explain why particles don't "lose " energy via virtual particles all the time.