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I'm studying up on the physics around black holes and got myself confused with the concepts of gravitational redship and Doppler effect.

  • Imagine a spacecraft orbiting a non-spinning black hole. It flashes blue light as signals every second (according to the pilot). As an observer far away at a safe distance observing, what (qualitatively) would you see of the lights? I'm thinking because of gravitational redshift, you'd see red lights instead of blue, and because of Doppler effect, you'd see the lights at longer periods than one second. Is this correct?

  • Will these observations change if the spacecraft's orbit shrinks? Provided it's still outside of the event horizon.

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The blue light would be yellow, orange or red, depending on how close to the event horizon the spaceship was. The longer interval between signals is mainly because of gravitational time dilation, not Doppler shift. There is a doppler blue shift when the spaceship is moving towards you and a red shift when it is moving away. In the case of a receding spaceship, there would be a longer time interval between signals. Is there such a thing as a non-rotating black hole? All stars rotate, and this rotation speeds up as they shrink to become neutron stars or black holes.

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