Could aliens in another galaxy see dinosaurs on Earth?

"For example, reader David Erickson had this on his mind: If there were aliens 66 million light-years from Earth, how big a telescope would they need to see dinosaurs? Ha! I love this question. I've thought of it myself but never worked out the mathexcept to think, Probably pretty big, which turns out to dramatically underestimate the actual answer."

"First off, why does it matter that the aliens are 66 million light-years away? It's because light travels a distance of one light-year per year through space, and the Chicxulub asteroid impact that wiped out the nonavian dinosaurs occurred about 66 million years ago. The light from that event would just now be reaching a galaxy 66 million light-years away, more or less. At that distance, observers there could still see (the very last of the) dinosaurs, assuming they felt like building a really big telescope."

"Now the question needs to be split into two parts: How big is a dinosaur from that distance, and how big must a telescope be to see something that size? Because the sky looks like a gigantic sphere surrounding us, astronomers use angles to measure apparent size. The basic unit for that is a degree; for example, the angle from the horizon to the point directly above an observer, called the zenith, is 90 degrees. The moon has an apparent size of about 0.5"

Light from the Chicxulub impact and the final nonavian dinosaurs would be reaching regions 66 million light-years away now. An object’s apparent size depends on its angular size as seen on the celestial sphere, and resolving small angles requires telescope apertures large enough to overcome the diffraction limit. A typical dinosaur at that distance subtends an extraordinarily tiny angle, so the telescope diameter required to resolve individual animals becomes astronomically large and impractical. The calculation illustrates limits set by light-travel time, angular resolution, and diffraction, and it highlights how extreme distances render direct imaging of small, Earth-sized events impossible.

Read at www.scientificamerican.com