Final answer:
Kai should explain that black holes are invisible and cannot be seen directly; astronomers detect them by their effects on surrounding matter. He should mention that it's impossible to observe a black hole with a backyard telescope and clarify the nature of an event horizon, beyond which no escape or communication is possible.
Step-by-step explanation:
Kai should respond to his friend with skepticism and explain basic black hole physics. Black holes are regions of spacetime exhibiting gravitational acceleration so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from them. Since black holes themselves are invisible, because they do not emit light, astronomers look for the effects black holes have on their surroundings. For example, they might observe how a black hole affects a nearby companion star or the outpouring energy from quasars, which are luminous due to material falling into the black holes at their centers.
Additionally, Kai could point out that while one can observe the effects of a black hole, seeing one directly is currently not possible. A person viewing from Earth, or even using a standard backyard telescope, would not be able to see a black hole. If someone were to hypothetically travel into a black hole, they would not be able to escape or send information back due to the extreme gravitational pull within the event horizon.