HOW ARE GRAVITATIONAL WAVES MADE?

Wave your hand in the air. You just created gravitational waves. Already, they are rippling outwards through space-time. They have left the Earth. They have passed the Moon. In fact, they are well on their way to Mars. In about four years’ time they will reach the nearest star system. We already know that one of the three stars of Alpha Centauri is circled by a planet. If it hosts a technological civilisation that has built a gravitational wave detector, at the beginning of 2022, it will be able to pick up the gravitational waves you created by waving your hand a moment ago!

Mind you, the detector will have to be super-sensitive. This is because gravitational waves, which are produced whenever mass changes its velocity, or ‘accelerates’, are extremely weak. The reason for this is that gravity itself is extremely weak. An equivalent statement is that space-time is extremely stiff. Imagine banging a drum. Now imagine replacing the drum skin with something a billion billion times stiffer than steel. That’s the stiffness of space-time. This extreme stiffness means that only the most violent movements, such as the merging of super- dense bodies like neutron stars and black holes, can create appreciable gravitational waves.