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

question 17

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. He hops aboard an escape pod and drops into a circular orbit around the black hole, maintaining a distance of 10,000 km, while you remain much farther away in the spacecraft. After doing some experiments to measure the strength of gravity, your colleague signals his 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 his signals unaltered in wavelength because he is orbiting well outside the event horizon of the black hole.
• B) You would see his signals shifted to a much redder wavelength because he 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 he is from it.
• E) You would see his signals shifted to a much bluer wavelength because black holes can make highly energetic light.