Stars as Laboratories for Fundamental Physics - MPP Theory Group

556 Chapter 15
charged particles in the galactic magnetic field would be curved, leading
to an energy-dependent time-delay (Barbiellini and Cocconi 1987).
Because the same argument can be applied to photons, the signals from
radio pulsars also allow one to set a limit on a putative photon electric
charge (Cocconi 1988). However, the resulting dispersion effect scales
with photon frequency in the same way as the effect caused by a photon
mass or by the plasma effect so that this method, again, is limited by
the standard dispersion effect (Raffelt 1994). One finds a bound on the
photon charge of Q γ ∼ < 10 −29 e.
Returning to a hypothetical photon mass, its value can be extracted,
in principle, from the spatial distribution of static magnetic fields of
celestial bodies. The measured fields can be fitted by an appropriate
multipole expansion in which m γ is kept as a free parameter. The
most restricitve limit of this sort was derived from Jupiter’s magnetic
field on the basis of the Pioneer-10 observations; Davis, Goldhaber,
and Nieto (1975) found a limit m γ ∼ < 0.6×10 −15 eV. The same method
applied to the Earth’s magnetic field yields an almost equivalent bound
of m γ ∼ < 0.8×10 −15 eV (Fischbach et al. 1994).
As detailed in a review by Barrows and Burman (1984) more restrictive
limits obtain from detailed considerations of astrophysical objects
in which magnetic fields, and hence the Maxwellian form of electrodynamics,
play a key role in maintaining equilibrium or creating long-lived
stable structures. The most restrictive such limit of m γ ∼ < 10 −27 eV is
based on an argument by Chibisov (1976) concerning the magnetogravitational
equilibrium of the gas in the Small Magellanic Cloud
which requires that the range of the interaction exceeds the characteristic
field scale of about 3 kpc. This limit, if correct, is surprisingly
close to 10 −33 eV where the photon Compton wavelength would exceed
the radius of the observable universe and thus would cease to have any
observable consequences.
15.5 Free Quarks
It is thought that quarks cannot exist as free particles; they always
occur bound in hadrons which are neutral (“white”) with regard to the
“color charge” of the strong interaction. In order to test this hypothesis
of confinement it remains an important task to search for single
quarks. The observation of fractional charges in the experiment of
LaRue, Phillips, and Fairbanks (1981) has never been confirmed. However,
if their observations were caused by unconfined quarks bound to