While Traveling the Galaxy in a Spacecraft, You and a Colleague

question 61

Multiple Choice

While traveling the galaxy in a spacecraft, you and a colleague set out to investigate the 10⁶M _$\odot$ black hole at the center of our galaxy. She hops aboard an escape pod and drops into a circular orbit around the black hole, maintaining a distance of 4 *10⁶ km, while you remain much farther away in the spacecraft. After doing some experiments to measure the strength of gravity, your colleague signals her results back to you using a green laser. What would you see? Hint: you will need to calculate the location of the event horizon.

• A) You would see her signals unaltered in wavelength because she is orbiting well outside the event horizon of the black hole.
• B) You would see her signals shifted to a much redder wavelength because she is close to the event horizon.
• C) You would see nothing because your colleague has crossed the event horizon around the black hole.
• D) You would see nothing because no light can escape the gravitational pull of a black hole no matter how far she is from it.
• E) You would see her signals shifted to a much bluer wavelength because black holes can make highly energetic light.

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