marlon said:
I think you would better post your assumtions in the Theory Development forum.
Well, we could go on and start insulting each other but as that is not a very creative activity, I will also try to EXPLAIN what I was doing.
Pat posted an interesting question, which was: "why do we want to quantize classical fields we've never even heard of?" and after some missings myself, I think I understood finally what he was aiming at. Because I saw others (including you) post answers next to the point I tried to help convey the point Pat was making, and in doing so, I was thinking loud: when walking around in the standard model, and other quantum field theories, what quantum fields would have a hope of showing their classical behaviour. It is true that my "arguments" which was more a thought process, was handwaving, but I do not think they were fundamentally wrong.
I pointed out that a first obstacle in going from a quantum field to a classical field would be the mass of the particle. You attacked that idea, but I think it is right. On the other hand, you contributed an important point, which I bluntly forgot, that is the fermionic nature of fermions, which will also make it difficult to go to a classical field. So if mass is out, and fermions are out, there's not much that remains: there is EM of course, there are the gluons of QCD and that's it. With QCD, I know there is confinement, so that will be a major obstacle to show classical field behaviour. And then I asked the question if confinement was still there if we took away the quarks (meaning: are colored fermions essential to confinement). You waved that one away with a "QCD without quarks is science-fiction". I then tried to explain why I thought that quarks _could_ play a role in confinement, and you told me that I was wrong, again. The ONLY reason I wanted to know that is that if somehow QCD without quarks was not confined, it might lead to a classical field. There's nothing "speculative" in all the above, it was just a written-down way of a process of reasoning with the idea of stimulating readable answers from knowledgeable people.
Nevertheless, the conclusion seems clear (no matter how "wrong" and "speculative" I've been according to you) that apart from EM, no quantum field will ever give rise to a classical field, in any approximation. This is news to me, honestly! I only realized that during this thread. This makes Pat's point much stronger than it was before: why work with classical fields which we will quantize in the first place ?
Now from what I read here, I take it that Weinberg shows that if you postulate a multiparticle theory and you somehow want to incorporate special relativity, that you can always construct a quantum field that is the quantized version of a corresponding classical field as a bookkeeping device. I assume that he does because I only started reading it, but he says something of the kind in his introduction. But that doesn't take away the interpretative issue: are we finally talking about quantized classical fields, or about a multiparticle theory ? And I think it is an interesting issue, which should be made clear (and which isn't made clear at all) in many QFT texts.
So I'm quite happy because I learned something in this thread.
Ah, and i final remark. I get mails from the QFT-forum from you. I must say that some of your solutions to certain exercises are eeuuuhh of speculative nature ??...
This is the kind of useless aggressive behaviour I was advising you against. The thing that triggered my remark (which was not meant to be nasty, btw) was nothing of what you said to me, but the fact that you asked someone here who has a PhD, a postdoc and years of experience in the field if he was a student in QFT. The problem is that if each time someone advances something he doesn't know completely or is worded in a way you do not understand immediately, you start by questioning if he knows how to count to 10, the discussion stops, or turns into a showing off what one knows and how smart one is, which is not productive, because no-one will dare to truly ask questions and as such expose him/herself to your inquisition.
I don't mind anybody pointing out where I make errors. On the contrary in fact, that's the best way to learn. However, the discussion must remain constructive and respectful, and that's what you apparently have difficulties with. Look at your remark above: don't you think it would have been more helpful to explain me nicely where I went wrong in my solutions than to say something of the kind ?
BTW, as the exercises are still conceptually very basic (although instructive and inducing modesty !) I would really be surprised that there is something "speculative" (except for a sign error or so) in my solutions, which means that if you can point that out to me I would learn a lot :-)
If you are not talking about the few times I posted about the exercises, but about the other posts, if you read them well (which is also not an evident skill) you will notice that they are invitations to discussion, with exactly the aim that I stand corrected if what I write is wrong.
Now that I'm preaching, I can just as well continue
I can give you more or less exactly the level at which I know QFT: it is Peskin and Schroeder, except for the last few chapters which I studied less thoroughly. This is however a few years ago so I don't have everything ready to be exploited and may have forgotten things. I also know some superficial phenomenology (style monte-carlo generators used in experimental particle physics). This means that I lack a lot of deep knowledge, and it is my pleasure to try to fill in these holes at a leasurely pace. But it also means that I should know enough of it so that it is possible to explain me certain aspects of QFT. If I cannot make sense at all of an explanation I take it therefore that the explanation has a problem, not me.
cheers,
Patrick.
)
, my fault
