Why against Stern-Gerlach experiment photons are said to be spin 1?
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From: Jarek Duda on 18 Nov 2009 10:55 Spin corresponds to magnetic dipole moment of particle, so in inhomogeneous magnetic field their trajectories are modified accordingly to alignment of its spin axis (Stern-Gerlach experiment). But as I know, photon's trajectories aren't changed in strong magnetic fields, what suggests that they should be spin 0... So I wanted to ask why in 'common knowledge' photons are said to be spin 1 particles? I've asked this question a few times in different discussions, like to Uncle Al http://groups.google.com/group/sci.physics/browse_thread/thread/4c9918f700d3f207/d06e44263601ee9a?lnk=raot and I didn't get any concrete argument. Attraction/repellence can be transferred even by waves in water. The only reasonable argument I know is that while for example electron deexcitation, spin is changed by one, so spin conservation says that photon has to carry spin 1... But let's look at it - electron e.g changes from spin up to spin down. Giving photon spin 1 makes that this change is kind of transformation happening in one line (its spin axis). But there is also much simpler explanation: that this electron is just rotated 180 deg - in this case photon doesn't have to carry spin, but angular momentum, what can be done much simpler - even waves in water can do it on short range... ???
From: eric gisse on 18 Nov 2009 11:53 Jarek Duda wrote: > Spin corresponds to magnetic dipole moment of particle, so in > inhomogeneous magnetic field their trajectories are modified > accordingly to alignment of its spin axis (Stern-Gerlach experiment). > But as I know, photon's trajectories aren't changed in strong magnetic > fields, what suggests that they should be spin 0... The photons aren't the things that are being changed. Go read your modern physics textbook again. > > So I wanted to ask why in 'common knowledge' photons are said to be > spin 1 particles? It is explained in particle and quantum physics textbooks. [snip rest, unread]
From: Jarek Duda on 18 Nov 2009 12:10 Uncle Al, aluminum-27 and scandium-45 would be split in Stern-Gerlach - I have no problem with their nonzero spin. As You have said on the linked thread "Photons are inert to electric and magnetic fields in vacuum to at lest 10^9 gauss, lab on a small scale (particle accelerators' magnetic and electric detectors) and by observation of pulsars. " How to cope it with nonzero spin? eric gisse - if it's so obvious, please give me one good argument.
From: Jarek Duda on 18 Nov 2009 12:48 Another argument that nonzero spin particles should have internal magnetic structure: Look at quantum rotation operator - while using it to make full rotation around spin axis, the phase rotates 'spin' times - that means around this axis the phase makes rotation. Quantum mechanical formulation of electromagnetism says that phase change along path is proportional to integral of electromagnetic four- potential along this path - so while making loop around this axis - we see that there is magnetic flux through it. Nonzero spin particle's trajectory should be influenced by inhomogeneous magnetic field accordingly to alignment of this magnetic flux going from trough the particle.
From: Jarek Duda on 18 Nov 2009 12:51
Another argument that nonzero spin particles should have internal magnetic structure: Look at quantum rotation operator for particle with spin - while using it to make full rotation around spin axis, the phase rotates 'spin' times - that means on loops around this axis the phase makes rotation. Quantum mechanical formulation of electromagnetism says that phase change along path is proportional to integral of electromagnetic four- potential along this path - so while making loop around this axis - we see that there is magnetic flux going through it. Nonzero spin particle's trajectory should be influenced by inhomogeneous magnetic field accordingly to alignment of this magnetic flux going trough the particle. |