AFilippenko

This is my first reply -- it's now 3 pm and I'm on Reddit AMA. I'm not sure why it was made "live" this morning; it was supposed to start at 3 pm because I was occupied with other things at the Aspen Ideas Festival until now.

Anyway, thanks for the compliment about Astronomy C10! I'm glad you loved it so much. I really enjoy teaching it once per year.

Regarding Elon Musk and Mars in 2025: I just don't know enough about the subject to comment as an expert, but that timescale seems optimistic to me (just 9 years from now!). To my knowledge, there's no NASA plan to launch humans to Mars by 2025. Maybe in the private sector, but I just don't think they are sufficiently advanced at this stage (again, my non-expert opinion). There are a LOT of technological obstacles to overcome, and it will be very expensive. But I admire Elon Musk's futuristic thinking and his zeal, and he certainly has accomplished some amazing things in his life, so who knows...

I think primitive life (bacteria, microbes) might be pretty common, but I'm not sure... it all depends on how easily molecules can come together to form something that replicates and evolves. Could be very rare, despite the many billions of Earth-like planets in our Milky Way Galaxy.

I think intelligence and mechanical ability at our level is very rare -- perhaps only a handful in our Galaxy, and perhaps we are the only ones. (But there's 100 billion galaxies in the observable Universe, and I'm not claiming we're the only ones among all these galaxies.) My reasons include the following. (1) We developed only a short time ago on Earth. An intelligent alien looking at Earth over most of its history (until about 200,000 years ago) would have said there's nothing particularly intelligent on Earth. (2) We are the only species out of >10 billion in Earth's history to reach this level of intelligence, along a path that was quite complicated and not inevitable. (3) Our level of intelligence does not have a clear evolutionary advantage. Yes, we've improved the lives of billions of people the past century, but at a cost... and we are the first species that has the ability to destroy ourselves (in several ways). (4) If intelligent life is common in our Galaxy, how come they aren't here yet? (Fermi's paradox)

My research team is studying the expansion of the Universe in more detail, trying to set constraints on the physical nature of the "dark energy" that appears to be accelerating the expansion. We are also trying to better understand which types of stars explode and how they actually explode.

When I spend a huge amount of time writing proposals for funding and then they get rejected, often for crummy reasons.

Okay, so I'm responding to this question without first reading the other responses below... maybe I'll get back to them later, but I want to first cover a lot of ground (a broad range of topics from many people).

Simply put, a black hole is a region of space where matter has been compressed to such a high density that the local gravity doesn't allow anything to escape, not even light. Yes, you can think of it (in a simple, incomplete way) as an escape velocity that equals or exceeds the speed of light, as you said. So, since light (and other things) can't escape, these regions are black. Their boundary (the "event horizon") is the "surface" of no return: go beyond it, and you will never again emerge to the outside world. In Einstein's general theory of relativity (which is needed to properly understand black holes), the warping of spacetime becomes so extreme that light simply cannot get out.

What happens to matter that crosses the event horizon? Well, it continues to gravitationally collapse until it reaches a "singularity" -- according to classical general relativity, a point of zero volume and hence infinite density. But that wouldn't agree with quantum physics: we know there's no such thing as point-like masses. So presumably the region is very small, but not of zero volume, and we just don't understand the properties of matter at such extreme densities. This is at the limit of what modern physics tells us because we don't yet have a unique, fully self-consistent quantum theory of gravity.

I hope this helps!