More on Gravity Waves

In Relativity by Brian Koberlein5 Comments

Yesterday I wrote about how general relativity predicts gravitational waves. While we haven’t yet observed gravity waves directly, we know they exist. That’s because gravity waves carry energy away from their source, just as light waves carry light energy.

When two stars orbit each other, they produce gravitational waves. The gravity waves in turn take away some of the energy from the binary system. This means that the two stars gradually move closer together, an effect known as inspiralling. As the two stars inspiral, their orbital period gets shorter (because their orbits are getting smaller). Eventually they become so close that they merge, usually creating a supernova or black hole.


Period decay of a binary pulsar

For regular binary stars this effect is so small that we can’t observe it. However in 1974 two astronomers (Hulse and Taylor) discovered an interesting pulsar. You may remember pulsars are rapidly rotating neutron stars that happen to radiate radio pulses in our direction. The pulse rates of pulsars are typically very, very regular. Hulse and Taylor noticed that this particular pulsar’s rate would speed up slightly then slow down slightly at a regular rate. They showed that this variation was due to the motion of the pulsar as it orbited a star. They were able to determine the orbital motion of the pulsar very precisely, calculating its orbital period to within a fraction of a second. As they observed their pulsar over the years, they noticed its orbital period was gradually getting shorter. The pulsar was inspiralling, and would eventually merge with its companion star in about 300 million years.

In the figure above, I’ve plotted the measured orbital periods of pulsar with the theoretical period shortening due to gravity waves. As you can see, the data lines up almost exactly. The system is losing energy just as general relativity predicts. Hulse and Taylor had demonstrated that gravity waves exist. Which is why they were awarded the Nobel prize in physics in 1993.


  1. How can that be explained:
    “1936 Einstein wrote to his friend Max Born: “Together with a young collaborator, I arrived at the interesting result that gravitational waves do not exist, though they had been assumed a certainty to the first approximation. This shows that the non-linear general relativistic field equations can tell us more or, rather, limit us more than we have believed up to now”

  2. When you allow time to vary depending on the specifics of the observer, you shouldn’t be surprised when 1 + 1 doesn’t equal two (now). For what is “now” anyway? BTW, I like gravity waves — when mass moves (in a classical sense) it creates gravity waves. I guess Weber is out of vogue?

  3. I don’t think general relativity is necessary to predict gravity waves. Also, we can see the result of gravity waves as the result of the ocean tides on Earth. The moon is a massive body close enough to Earth that we can see the result of several motions, such that the tides happen. If the moon wasn’t there and wasn’t orbiting the Earth we wouldn’t have this happen. How would this not be a gravity wave?

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