轩轩
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自由电子的自由度是多少？
为什么他们选H，动量P，和螺旋度h这三个来做力学量完全集，是3个自由度吗？为什么不考虑电子的自旋自由度
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(2004-06-01 13:58:27) 轩轩
super star
| 发表时间：2004-11-24, 09:03:45 |
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权权
发表文章数: 92
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Re: 自由电子的自由度是多少？
螺旋度helicity已经表示了自旋自由度了．
reference: 站长的场论自由度注释讲到了自旋１／２场的自由度．
不忧不惧
| 发表时间：2004-11-24, 18:46:45 |
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sage
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Re: 自由电子的自由度是多少？
well. it actually depends a little bit on the problem at hand.
if we only think about free fermion, the symmetry of the dirac equation is lorentz symmetry. its representation is classified by H and spin. Therefore, we choose those variables. Dirac quation is also translational invartiant, therefore, it is also good to choose momentum to represent its solution.
When there is interaction, one might want to have something else, such as angular momentum in the central force problem.
When couple to electric-magnatic field, there is actually one more quantum number, the charge.
Further more, if we think of weak interactions for electron, there is also the weak isospin charge...
| 发表时间：2004-11-25, 14:03:52 |
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轩轩
发表文章数: 1352
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Re: 自由电子的自由度是多少？
lorentz symmetry. its representation is classified by H and spin.
何以知道,lorentz对称性,H和spin来分类 ,已经足够?
已经足够?
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(2004-06-01 13:58:27) 轩轩
super star
| 发表时间：2004-11-25, 23:48:02 |
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权权
发表文章数: 92
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Re: 自由电子的自由度是多少？
What sage said is quite interesting. Actually, the degree of freedom essentially depends on the energy level of the process.
The energy level of electrical-magnetic process is relatively low compared to strong interaction. In common sense, we regard the "spin" as a degree of freedom of electron because the spin direction of eletron can be easily changed during the electrical-magnetic interaction process, i.e. it changes, so it is usually regarded as some kinds of degree of freedom.
On the other hand, the charge of the electron is regarded as intrinsic property of the electron. Since the charge is hard to change in low energy process. Some other examples are the isospin, strangeness...which cannot be easily changed, so that they are usually not regarded as some kinds of intrinsic properties, while position and spin can.usually be regarded as some kinds of degree of freedom.
In the past, we use mass, charge, spin, etc...to define elementary particles, but we rarely use position to define a particle, since position is to easy to change.
Anyway, we can look at these physical quantities in a unified frame. The so-called particle are just defined by physical quantities, or in other words, by quantum states.
不忧不惧
| 发表时间：2004-11-26, 12:58:17 |
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