Stars as Laboratories for Fundamental Physics - MPP Theory Group

554 Chapter 15
Actually, a certain reduction of the true globular-cluster ages relative
to their apparent ones would be a welcome cosmological effect as
they are, at best, marginally compatible with other cosmic age indicators.
In view of the current limits on ĠN/G N summarized in Fig. 15.2
this possibility cannot be excluded at present.
15.3 Test of the Equivalence Principle
The equivalence principle of Einstein’s general theory of relativity implies
that the space-time trajectories of relativistic particles should be
independent of internal degrees of freedom such as spin or flavor, and
independent of the type of particle under consideration (photons, neutrinos).
A number of astronomical observations allow one to test this
prediction. Laboratory tests of various consequences of the equivalence
principle are discussed in Will’s (1993) book.
Nonsymmetric extensions of general relativity (e.g. Moffat 1991)
predict that different polarization components of electromagnetic waves
propagate with different phase velocities in gravitational fields. This
birefringence effect would lead to the depolarization of the Zeeman
components of spectral lines emitted in magnetically active regions of
the Sun. The absence of this depolarization effect leads to significant
constraints on Moffat’s theory and others (Gabriel et al. 1991).
In a similar approach one uses the difference of the Shapiro time
delay between different particles or between different polarization states
of a given particle which propagate through the same gravitational
field. In Sect. 13.3 the absence of an anomalous shift between the
SN 1987A photon and neutrino arrival times gave limits on violations
of the equivalence principle because both pulses moved through the
same galactic gravitational potential.
Also, one may search for differences in the arrival times of leftand
right-handed polarized electromagnetic signals from distant pulsars
(LoSecco et al. 1989). The best bound was obtained from an analysis of
the pulse arrival times from PSR 1937+21 which is about 2.5 kpc away
from Earth. One may write the effective gravitational potential in the
form V (r) = V 0 (r) [1 + A 1 σ ·ˆr + A 2 σ · v + A 3ˆr · (v × σ)] where r, v, and
σ represent the location, velocity, and spin of the particles (photons,
neutrinos). The PSR 1937+21 data then yield a constraint |A 1 | <
4×10 −12 and |A 2 | < 1×10 −12 (Klein and Thorsett 1990), apparently
the most restrictive limits of their kind.