Talk by Adam Caulton (Cambridge)

[Karim Thébault]

Speaker: Adam Caulton (Cambridge)
Wednesday 21st May 2014
Location: Ludwigstr. 31 room 021
Time: 16:15 - 17:45

Title: Adventures off the mass-shell

What is the correct possibility space for a given particle of a quantum
field? The firm orthodoxy, initiated by Wigner in 1939, is that we should
look to the irreducible representations of the Poincaré group. But the
justifications for this are murky, so I offer two rational reconstructions.
According to one, the justification is that the Poincaré group comprises
all the actions that can be performed on a simple system; but this is
false, made particularly relevant in the case of interactions.
According to another, conflicting reconstruction, Poincaré invariance is a
reality condition; but this applies only to the entire system of fields,
not to individual particles.  Therefore it ought not constrain the
possibility spaces of individual particles.

Consequently, I argue, we have reason to consider "larger" state spaces
than are normally considered.  On one such approach, which I will advocate,
the elements of these state spaces represent entire four-dimensional
histories rather than instantaneous states---most of which are "off
mass-shell", in that the relativistic mass condition is violated.  I
conclude by arguing that off-mass-shell particles, far from being mere
formal inventions for the sake of S-matrix calculations, are just as "real"
as their free counterparts---indeed, they are to be found as much in
classical relativistic theories as in quantum relativistic theories.
_______________________________________________
philphysmunich mailing list
philphysmunich at lists.lrz.de
https://lists.lrz.de/mailman/listinfo/philphysmunich
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.lrz.de/pipermail/philphysmunich/attachments/20140515/24dd99ca/attachment.html>