and is about 1800 days. In general relativity and other metric theories of gravity, though, the Shapiro delay for gravitational waves is expected to be the same as that for light and neutrinos. However, in theories such as tensor–vector–scalar gravity and other modified GR theories, which reproduce Milgrom's law and avoid the need for dark matter, the Shapiro delay for gravitational waves is much smaller than that for neutrinos or photons. The observed 1.7-second difference in arrival times seen between gravitational wave and gamma ray arrivals from neutron star merger GW170817 was far less than the estimated Shapiro delay of about 1000 days. This rules out a class of modified models of gravity that dispense with the need for dark matter. See also Gravitational redshift and blueshift Gravitational lens Proper time VSOP (planets) Gravitomagnetic time delay References Further reading See Section 15.6 for an excellent advanced undergraduate level introduction to the Shapiro effect. A graduate level survey of the solar system tests, and more. Effects of gravitation Tests of general relativity