• Question: What are gravitational waves and how do they work?

    Asked by 326grte48 to Emma, Bose, Alice, Christian, Steve on 13 Mar 2016. This question was also asked by Lara A, 685grte48, 938grte48, Diya.
    • Photo: Steve Marsden

      Steve Marsden answered on 13 Mar 2016:

      A gravitational wave is created whenever an object repeatedly moves from side to side. These are usually incredibly tiny, and impossible to measure. For us to have any chance of measuring them, they have to be created by an object with a huge mass, like a black holes, and accelerate very quickly, like an instance that two black holes orbit around each other and ultimately collide. In this case a huge amount of energy is put into gravitational waves, at which point we can just about measure them.

      A very hand wavy metaphor for gravitational waves is to compare them to the Doppler effect you can hear when an siren drives past you at high speed. As it drives towards you the frequency is shifted up, as it drives away the frequency is shifted down. As the two black holes orbit each other they alternately move towards you, and away from you, in quick succession at speeds close to the speed of light. This essentially results in the gravitational force to be Doppler shifted, increased as the black hole comes towards us, and decreased as the black hole moves away. This quick succession of the gravity increasing and decreasing repeatedly travels through space at the speed of light, and are referred to as gravitational waves.

      This is important as relativity states that gravity must travel at the speed of light, and cannot be instantaneous. So if say the sun were to vanish, we would continue orbiting where it was for several minutes before flying off in a tangent. If gravity were instantaneous, and not limited by the speed of light, this Doppler shift would not be present. So by seeing these gravitational waves, we know that gravity is limited to the universal speed limit, and that once again general relativity is correct.

    • Photo: Emma Dean

      Emma Dean answered on 15 Mar 2016:

      Massive objects like planets, stars and black holes all stretch spacetime. This works in a very similar way to how a bowling ball on a trampoline stretches the fabric. The curved shape of the fabric created by the ball is what we call gravity. So heavier object stretch space more and create a steeper curve, so we say they have stronger gravity. Lighter objects don’t stretch space as much (like a marble on the trampoline compared to the bowling ball), so their gravity is weaker.

      If you put two bowling balls on a trampoline and they move around each other, lots of ripples on the trampoline would appear. This is where the elastic fabric is being stretched and squashed in different places. Gravitational waves do exactly this. They stretch and squash the spacetime in our universe as they move through it.

      Here is a really good video from the European Space Agency on what gravitational waves look like http://www.esa.int/spaceinvideos/Videos/2015/09/Gravitational_waves.
      See how the hoops change shape – this is the stretching and squashing motion of the gravitational wave that effects space.

Comments