Table 1.

Current limits on the PPN parameters

Parameter	Limit	Remarks
γ - 1	2.3 × 10-5	Cassini spacecraft tracking
	4 × 10-4	VLBI radio deflection
β - 1	3 × 10-3	perihelion of Mercury
	2.3 × 10-4	no Nordtvedt effect
ξ	10-3	no anomalous Earth tides
α1	10-4	no anomalies in lunar, binary-pulsar orbits
α2	4 × 10-7	alignment of sun and ecliptic
α3	2 × 10-20	no pulsar “self” accelerations
ζ1	2 × 10-2	combined PPN bounds
ζ2	4 × 10-5	no binary “self”-accelerations
ζ3	10-8	no Lunar “self”-acceleration
ζ4	-	not independent