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Natural Science Forum / Physics / Particle Physics / May 2005Electron and Positron Question
"For each kind of particle, there is an associated antiparticle with the same mass but opposite electromagnetic, weak, and strong charges, as well as spin." from wiki. Electrons and positrons can have a spin of +1/2 or -1/2, so if an electron and positron both have the same spin, they are not anti-particles of each other. Is my statement correct? Is the above definition wrong? I am I missing something? > "For each kind of particle, there is an associated antiparticle with > the same mass but opposite electromagnetic, weak, and strong charges, [quoted text clipped - 7 lines] > Is the above definition wrong? > I am I missing something? Your statement is not quite right, and the definition is not quite right. The electron has spin 1/2 (only). It can have spin *projections* of +1/2 or -1/2 with respect to an external axis. In addition, antiparticles have the same spin quantum number as their particles. Thus, anti-electrons also have spin 1/2. PD | "For each kind of particle, there is an associated antiparticle with | the same mass but opposite electromagnetic, weak, and strong charges, [quoted text clipped - 7 lines] | Is the above definition wrong? | I am I missing something? Your statement is wrong. Yes, you are missing something--a good particle physics book. ;-) FrediFizzx > "For each kind of particle, there is an associated antiparticle with > the same mass but opposite electromagnetic, weak, and strong > charges, as well as spin." from wiki. The part about spin is wrong. With respect to spin projection, the positron's magnetic moment may be of opposite sign to that of the electron's, which to zeroeth order, is given by the Bohr magneton: mu_e ~ mu_B = e * hbar / 2 * m_e > Electrons and positrons can have a spin of +1/2 or -1/2, That's not spin. That's spin projection. > so if an > electron and positron both have the same spin, they are not > anti-particles of each other. Wrong > Is my statement correct? No. > Is the above definition wrong? Parrtly. > I am I missing something? Yes: Lepton Spin ( S = 1 / 2 > 0 ) Lepton spin projection ( S_z = - S or + S ) [Old Man] > Lepton Spin ( S = 1 / 2 > 0 ) > Lepton spin projection ( S_z = - S or + S ) Hi Old Man. May I ask a question deeply mired in ignorance? Ok. How can the "projection" of spin have the same magnitude as spin itself? Unless of course the damn thing is alligned with the projection axis: but somehow I never pictured that as being the case. >> Lepton Spin ( S = 1 / 2 > 0 ) >> Lepton spin projection ( S_z = - S or + S ) [quoted text clipped - 4 lines] > itself? Unless of course the damn thing is alligned with the > projection axis: but somehow I never pictured that as being the case. Been away and back a couple of times since hearing from Ed Green. Nice to hear from one who enjoys physics, the lack of which impels one to go-away, while the want of such tempts one to return. As for spin projection .... at times, Old Man's knowledge base is on par with that of a parrot .... spin and spin projection come in increments of hbar / 2. So, if spin projection were less than spin, lepton spin projection would have to be zero. Stern- Gerlach says that's not so. Is that a cop-out ? [Old Man] On 21-May-2005, "Edward Green" <spamspamspam3@netzero.com> wrote in message <1116686924.363278.120220@o13g2000cwo.googlegroups.com>: > > Lepton Spin ( S = 1 / 2 > 0 ) > > Lepton spin projection ( S_z = - S or + S ) [quoted text clipped - 4 lines] > itself? Unless of course the damn thing is alligned with the > projection axis: but somehow I never pictured that as being the case. Angular momentum states labeled "j" actually have total angular momentum j*(j+1) (in units of h/2pi, of course) and possible projections of -j, -j+1, ..., j-1, +j. So a spin-1/2 particle actually has total spin 3/4.  SignatureJim Heckman On 21-May-2005, "Jim Heckman" <wnzrfeurpxzna@lnubb.pbz.invalid> wrote in message <118vmha3dm1b131@corp.supernews.com>: > On 21-May-2005, "Edward Green" <spamspamspam3@netzero.com> > wrote in message <1116686924.363278.120220@o13g2000cwo.googlegroups.com>: [quoted text clipped - 10 lines] > Angular momentum states labeled "j" actually have total angular > momentum j*(j+1) (in units of h/2pi, of course) D'oh! Make that sqrt[j*(j+1)]. I was thinking of the eigenvalues of the J^2 operator, not J itself. > and possible > projections of -j, -j+1, ..., j-1, +j. So a spin-1/2 particle actually > has total spin 3/4. And make that sqrt(3/4). SignatureJim Heckman >> Lepton Spin ( S = 1 / 2 > 0 ) >> Lepton spin projection ( S_z = - S or + S ) [quoted text clipped - 4 lines] > itself? Unless of course the damn thing is alligned with the > projection axis: but somehow I never pictured that as being the case. Because S = + or - h_bar/2 And since h_bar is the unit angular momentum plus or minus half of it ofsets you exactly 90 degrees in either direction. S_z is also related to magnetism in the following manner: S_z = mu_B m_e / e Where mu_B is the Bohr magneton m_e is the mass of the electron e is unit electron charge in Coulombs in this perspective, plus and minus refer to the relative magnetic orientation of the electron with respect to the magnetic aspect of other electrons, which can be either parallel (both are plus or both are minus) or to antiparallel orientation (one is plus the other is minus) the latter is how the covalent bound is established between two atoms. André Michaud if he electron is a 'point particle' how can it have a 'parallel' or antiparallel' orientation?? parallel and antiparrallel are geometric properties that is againt the HUP that tell sus that we cannot know anything geometric about the electron. right?? ..... and 'ofsets of 9o degrees in either direction' all the best Y.Porat ----------------------------- > if he electron is a 'point particle' > how can it have a 'parallel' or antiparallel' orientation?? 'Point particle' is only a handy mathematical representation, just like in Newtonian gravitation, planets and sun are dealt with as if they were points at their own center of gravity to calculates the orbits. Obviously, if they can have orientation, and they do, then they cannot physically be dimensionless points. > parallel and antiparrallel are geometric properties > that is againt the HUP I don't think it is. The hup only states that we cannot know at the same time its localisation and velocity. > that tell sus that we cannot know anything geometric about the > electron. > right?? ..... Well, we certainly do, in this case. Electrons are electromagnetic particles, and as such, they have to conform to electromagnetic orthogonality, which is definitely "geometric". > and > 'ofsets of 9o degrees in either direction' Generally speaking, the spin can only be at the same time normal to the velocity vector which, whether expressed or not, is at all times normal to the electrostatic vector, which is directed towards the nucleus. André Michaud if so can we say that in the Deutron nuc the proton is located next to the Neutron? is it geometric knowledge?? all the best Y.Porat ------------------------- No answer??? --------------------- Y.Porat ----------------------------- > > "For each kind of particle, there is an associated antiparticle with > > the same mass but opposite electromagnetic, weak, and strong [quoted text clipped - 32 lines] > > [Old Man] my name says it all... sounds like is related to why we cannot transport matter target re-assembling has 50/50 chance of being correct position. > "For each kind of particle, there is an associated antiparticle with > the same mass but opposite electromagnetic, weak, and strong charges, [quoted text clipped - 5 lines] > > Is my statement correct? I guess, if we accept the Wikipedia definition http://en.wikipedia.org/wiki/Antiparticle, then your statement is correct. > Is the above definition wrong? How does one determnine that? Poll the experts in that area and see whether or not a majority agree with the definition? > I am I missing something? I think so. It's hard to get an understanding of physics from an encyclopedia. They are nevertheless useful in that they summarize and may tell a fellow which way to go to learn more. Take the electron-positron pair as an example. Look up pair production and see what the conservation laws tell you about the charge and spin of each member of the pair. |
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