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Neil deGrass Tyson "Orbital Erosion"

  1. Aug 6, 2015 #1
    I listened to one of NDT's lectures on a podcast this morning and he made a passing reference to two black holes orbiting each other that would eventually "spiral into each other due to orbital erosion" (his words)

    I'm familiar with orbital decay for satellites in orbit around the earth due to friction, emf etc; and I'm familiar with planet and moon orbits expanding from tidal forces, energy lost to gravitational waves etc. However I'm struggling to understand Neil's reference to orbital erosion in relation to Black holes (although from the context of his comment I don't think his comments were specific to Black holes), my understanding until now has been that two bodies orbiting each other would be stable unless other forces act inn them?

    Can anyone enlighten me as to what Neil may have been referring to?

  2. jcsd
  3. Aug 6, 2015 #2


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    Two bodies orbiting each other will be stable forever in classical mechanics, but when you introduce general relativity, the picture changes somewhat. Gravitational waves will carry away energy/angular momentum from the system, resulting in the two bodies spiralling into each other. In the Earth-Sun system, this effect is absolutely negligible (1/300th of a hydrogen atom each year), but in orbiting black holes or neutron stars, this effect can be significant. In fact, observations the orbital decay of a pulsar-neutron star binary is considered some of the strongest evidence for gravitational waves. https://en.wikipedia.org/wiki/PSR_B1913+16

    You can read more about how gravitational waves result in orbital decay here:
  4. Aug 6, 2015 #3
    Fantastic reply. Many thanks indeed.
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