As we explain in the video lesson, we know that "anything that far away to appear from the Earth the way quasars do, it would have to burn steadily at a rate that produces more light than 90 billion suns would produce. But nothing that burns at a rate that produces that much light could exist for more than about 100 million years."
So a quasar can't exist for more than about 100 million years but they are so far away that their light takes at least 500 million years to reach Earth.
Therefore, (E) must be true, i.e. "No quasar that has ever been seen from Earth exists any longer."
Hope that helps! Please let us know if you have any other questions.
marisawoodleyon January 6, 2020
I too am confused with this. If a quaser cannot exists for more than 100 mil years but take 500 mil years to reach earth, how could they ever have been seen from earth at all?
dmedeiroson September 24, 2020
Exactly my question!!!!
alanylhuangon November 8, 2020
The quasars generate light while it exists. The light does not reach the earth instantly. It needs to travel through space. The quasars cease to exist after 100 mil years while the light it generated is still traveling through space to reach earth.
We are used to the light reaching us instantly on earth so we equate the existence of light equals the existence of the origin of the light. But the light here, which is in space, keeps on traveling even when the original source of the light ceases to exist.
I am also a student. This is just my understanding. Hope it makes sense. Please do correct me if I'm wrong.
Isaiah-Rubioon January 7, 2021
piggy backing off of a previous student comment to verify their explanation.
The quasars existence and the quasars light are two different things, the passage states that the quasar can't exist for more than about 100 million years, but a quasars light can still travel even though the quasar no longer exists. Due to the distance of the quasar from earth, it took the light from this extinct quasar at least 500 million years to reach Earth, thus verifying (E) "No quasar that has ever been seen from Earth exists any longer."
hopefully this helps!
SOULCAGES71on July 27 at 07:56PM
Exactly as stated above. It's because the light can still be seen even after the quasar has burned out.
