A collision of two neutron stars, detected as gravitational waves GW170817 and a gamma-ray burst GRB 170817A, gave scientists a rare chance to compare the speed of gravity with that of light. The gravitational wave and gamma-ray burst were detected 1.74 seconds apart after traveling 130 million light-years, allowing scientists to measure the difference between the two speeds with a precision of 10⁻¹⁵. This confirmed that gravity and light travel at the same speed, as predicted by Einstein's theory of general relativity. The event, which occurred in a distant galaxy, was observed simultaneously by gravitational wave detectors LIGO and Virgo, as well as by satellites Fermi and INTEGRAL. The probability that this double detection was a coincidence was calculated to be about 5 out of 100 million, validating the link between gravitational waves and electromagnetic signals. The measurement constrained the difference between the speed of gravity and that of light to a range of -3×10⁻¹⁵ to +7×10⁻¹⁶ times the speed of light. This precision is equivalent to measuring a difference in the duration of a marathon across the observable universe with a difference of less than a thousandth of a second. This result excluded several alternative theories to general relativity, including Horndeski gravity and Hořava-Lifshitz gravity, which had been proposed to explain the acceleration of the universe's expansion without dark energy. The findings also confirmed the predictions of the Einstein-Maxwell theory regarding the Shapiro delay, the phenomenon where light and gravity are slightly slowed by the presence of large masses. GW170817 is the only gravitational wave event observed in conjunction with a confirmed electromagnetic counterpart, marking a significant milestone in multi-messenger astronomy. It also confirmed that neutron star mergers are the origin of some short gamma-ray bursts, a hypothesis that had been difficult to demonstrate directly. The event provided more information about the nature of gravity in a few seconds than decades of theoretical calculations. As gravitational observatories continue to refine their instruments, the question remains whether future neutron star mergers will reveal any deviations from Einstein's predictions.