There have been various attempts to find modifications to general relativity. The most famous of these are the Brans–Dicke theory (also known as scalar–tensor theory), and Rosen's bimetric theory. Both of these theories proposed changes to the field equations of general relativity, and both suffer from these changes permitting the presence of bipolar gravitational radiation. As a result, Rosen's original theory has been refuted by observations of binary pulsars. As for Brans–Dicke (which has a tunable parameter ω such that ω = ∞ is the same as general relativity), the amount by which it can differ from general relativity has been severely constrained by these observations. Many other alternatives to general relativity have also been ruled out by analyses of the neutron-star merger GW170817.