I'm Alex Filippenko, astrophysicist and enthusiastic science popularizer at the University of California, Berkeley. AMA!

Hi Alex! I took your Intro to Astronomy class in 2008 and I could not get enough of it! You made complex topics very digestible and interesting, which is no easy task for a college professor. My question: Elon Musk has stated that he has a goal of getting the first manned mission to Mars by 2025. Do you see this as a viable timetable, and why or why not?

That was my favorite class I took at UC Berkeley. I think I took it in 2008 as well! Go Bears!

I took Astro C10 in 2009 and had a similarly wonderful experience. One time I asked Filippenko about rainbows around the moon and whether they're an illusion or what -- I had had a pretty revealing acid experience the week before wherein I'd seen rainbows shooting out of the moon, and we were about to learn about the moon in class anyway -- and he winked at me and said something to the effect of "depending on your perspective, the rainbows are definitely real." Then he played "Dark Side of the Moon" in class later that week. I miss his course!

Cool, I'm glad you liked the class and remember that particular experience!

Rainbows are actually pretty fascinating, if you delve into what's actually going on. I love looking at them -- and knowing how they work makes them even more beautiful to me.

Fun fact: the "ice crystal version" of a rainbow is a solar or lunar halo. These are centered on the Sun or Moon (best to be full or nearly full, so it's bright), with a radius of 22 degrees (that's about 1/4 of the angular distance from the horizon to the zenith) -- as compared with a rainbow, which is centered on the point opposite the Sun and has a radius of 40-42 degrees (nearly halfway from the horizon to the zenith). It's caused by refraction (bending) of light through hexagonal ice crystals (instead of liquid raindrops) high in cirrus clouds.

Thanks, and GO BEARS!

Go Bears! I feel like a vast majority of us have taken his class at Cal. Hems such a great professor!

Thanks! Spread the word to your friends! Fall 2016 will be the next time I teach it (i.e., in about 2 months).

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 watched his online course around that same time. Really got me into Astronomy. His course was easy to follow, and very informative. Thanks Berkeley for making these courses available for free online!

Thanks! We can't make the newer versions available online because of various intellectual property rules. But you can purchase a longer version (and some shorter versions) that I created with The Great Courses (also known as The Teaching Company). Look them up online and wait for the spectacular sale that they have on each course every few months (about 70% off -- I'm not kidding).

What project or idea has your greatest attention right now?

This AMA

Although you're being down voted, I'll have you know I found this funny

Funny reply! Yes, this AMA will be my focal point for the next 2 hours!

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.

Black holes to me are fascinating, but i will freely admit that i don't really understand them at all.

We speak about them as "holes" that nothing can escape from - but that is referring to the effect of gravity right? A black hole at it's core is matter right? It's just so dense that even photons of light can't go fast enough to achieve escape velocity?

In short - for the layperson on reddit - just what is a black hole made up of? Or is just simply impossible for us to know?

BTW - been a huge fan of yours since seeing your passion for astronomy and astrophysics in "The Universe" series. Very inspiring.

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!

What's the least favorite thing about your career, what's a tough day at the office for you?

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

What are your thoughts on the possibility of extraterrestrial life?

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)

Prof. Filippenko,

What's you opinion of the "Great Filter"?

I hope you're still jumping between the tables to demonstrate electrons jumping energy levels? And hope you haven't broken any more bones teaching! I took your Astro 10 class in 2000 - the year the Readers were labeled "Astrology 10" by mistake. In office hours you explained how you applied yourself to becoming an excellent teacher just as you would apply your research in the hard sciences. I believe that was your answer to why there were so many Simpsons references (probably explains how we got to hear They Might Be Giants - Why Does the Sun Shine? in class - they updated with some corrections as “Why Does the Sun Really Shine? The Sun is a Miasma of Incandescent Plasma" )

It turns out we're cousins by marriage for about 10 years now, mazel tov!

See you in Oregon in 2017 - turns out the location is accessed through my mother-in-law's brother's place.

Let me look up the "Great Filter" some other time and get back to you. I'm not sure what you mean.

Great that you took my class long ago and still remember the funny "Astrology 10" Course Reader. I still show a photo of that incorrect Course Reader in my Astro C10 class on the first day, reminding students that the course will be about astronomy instead of astrology but that there's a lot of confusion among the general public.

Cousins by marriage? Yay! Who are you?

Yes, the total solar eclipse that's coming up on August 21, 2017 will be totally amazing! If you haven't seen one, please, please go to the path of totality and see it. Doesn't count to be close to the path of totality: a total eclipse is completely different from a partial eclipse. It's like the difference between being pregnant and not pregnant; you can be partly pregnant. Anyway, total solar eclipses are a breathtaking experience... incredibly moving. I know I sound like a nut ("lunatic"!?) to those of you who have not seen one, but go see one for yourselves and you'll probably understand what I mean. Use your favorite search engine to find the path of totality and then make plans to go there. If you want to go with a group that I've set up, search for "Cal Discoveries Travel total solar eclipse Oregon 2017" and you will find it. I hope you sign up! But regardless of whether you go with my group, just go SEE it! I've seen 15 in my life, throughout the world. (Great excuse to travel to exotic places, by the way.)

Its been 3 hours and he hasn't answered a single question, and his post doesn't say anything about starting the AMA at a later time.

Is this little line the real point of this "AMA"?

The Reddit community can engage and assist with this stellar research, technology development, education, and public outreach by making a donation here.

edit: And it's now removed. Dammit, Alex!

So sorry! I was not in charge of organizing this AMA. I was asked to do it at the Aspen Ideas Festival, and I made it pretty clear that the post should state that I would start at 3 pm Mountain Daylight Time. But my instructions weren't followed... oh well, sorry again.

Hey Alex! I think in one instance, on History's "The Universe" episode on the Speed of Light, someone finally pointed out that we say the universe is 14 billion years old because that's how far we can see; and that from a point 14 billion light years away, one would observe themselves at the center of a universe that also has a radius of 14 billion light years. That was the happiest moment in tv science history for me.

What I would ask is, is the cosmic background radiation boundary just the point at which our ability to distinguish longer radio wavelengths ends? Like trying to see a boob in scrambled 80's Cinemax broadcasts?

Yes, no matter where you are, you would appear to be in the center of the Universe, with the cosmic background radiation coming in from directions all around you after having traveled for 13.8 billion years.

No, it's not that we reach a limit in the length of radio waves that we detect. Instead, it's a limit to how far back in time we can see because before that time (380,000 years after the Big Bang), the Universe was fully ionized and thus opaque to electromagnetic radiation (light). So the CMB comes from an opaque "wall" -- kind of like you can't see the interior of a fog bank if you're outside.

However, neutrinos and gravitational waves do travel through ionized gas, so in principle we can observe the Universe at times <380,000 years by looking at neutrinos and gravitational waves. Both are very hard to detect. (But it's wonderful that gravitational waves were recently finally detected, 100 years after being predicted, from pairs of merging black holes. The gravitational wave signals from merging black holes are much stronger than those from the early Universe, though.)

I just wanted to say I loved all your commentary on The Universe series. Thanks for doing this AMA!

Can you give me an ELI5 or an "Explain like I'm 5" M-theory and how it relates to string theory? That's one concept that's difficult for me to easily conceptualize :)

Thanks! I've enjoyed being on The Universe series and bringing astronomy/science to the general public. However, I'm not in charge of the script or the visuals, so sometimes errors creep in. That's one frustration for me... I'm not the producer and I'm not in charge. On the other hand, if I were in charge, I probably wouldn't have time to do it.

Briefly: string theory says that particles are 1-dimensional vibrating bundles of energy, and the different vibrational modes correspond to different fundamental particles (kind of like different vibrational modes in a violin string correspond to different notes). In M-theory, the bundles of energy are multidimensional -- for example, a 2-dimensional membrane. So, it's an extension of string theory. All of this stuff is difficult to conceptualize, especially since (to make the math work out) the vibrations have to be in a bunch of dimensions that differ from the xyz that we see. Generally, there's a total of 11 dimensions, so 7 of them (all except xyzt) need to be hidden. In most cases, they are probably incredibly small, like 20 orders of magnitude smaller than a proton.

There can also be some big dimensions in the theory: our perceived Universe is a "brane" (membrane) embedded within a higher-dimensional "bulk." Don't confuse our xyz membrane with the tiny membranes in M-theory mentioned above, though.

What is your favorite documentary you have done And why?

I think my favorite was one on gravity as part of The Universe series: I was able to take a trip on the "Vomit Comet" (Zero G) while being filmed talking about physics.

Did you find the television series "Cosmos" to be an effective means of communicating scientific ideals and achievements? If so, do you recommend any others?

Yes, the original Cosmos (featuring Carl Sagan) was a tour de force and helped inspire a lot of kids (and adults!). It's still worth watching now, despite being out of date; parts of it are timeless. The more recent remake was also good, though there are some inaccuracies. But it reached a very wide audience, being on the Fox channel.

I recommend The Universe series, NOVA shows on astronomy, and many others. Though many of these shows greatly simplify what's going on in order to not lose the audience, they help get people interested in science.

Why did you give me a B?

Because you must not have earned enough points in the class to get an "A" -- sorry!

Hello Alex.

It's been a pleasure for me to follow your work with the videos and talks you have done about your research. Recently, I had the pleasure to visit Lick Observatory as well. I would like to listen to one of your talks at Berkeley the next time I visit my fellow university students!

I was wondering if you ever find research in theoretical fields unfulfilling. In other words, you risk squandering months or years of work on something with little physical evidence to back it up that can be disproved when further discoveries are made. I know that's how science works, but do you even feel it's a personal letdown, even for you as someone with many successes?

It would like to take this moment to introduce r/filippenko. It's a... fan page? Or something like that. Founded by two students in 2012, it has been revived by two more four years later. Jolly good astrophysics memes if I do say so myself.

I'm a tad disappointed to see you've lost your password to u/AlexFilippenko. I guess there are more important things like pi digits to remember. But that's a lot of trouble to keep track of your karma. It's very important around here.

Finally, let it be known that I supported Lick Observatory by purchasing a $50 Lick jacket at the gift shop. It's quite chilly up there.

Thanks! I'm glad you've enjoyed my contributions to public education in the sciences.

I'm not a theorist, so I personally have not experienced working on a theory that didn't pan out. However, I have worked on observational projects that didn't lead to anything interesting. Yes, that's a disappointment, but it's also just the way life goes -- in every field of endeavor there are ups and downs.

I don't spend much time online. I didn't know about the r/filippenko and u/AlexFilippenko sites. Maybe I should look into them. Thanks for telling me about them.

And thanks for supporting Lick Observatory! You can also do this through the Friends of Lick Observatory (https://www.ucolick.org/main/support/folo.html) and the UC Berkeley Lick Observatory Operations Fund (https://give.berkeley.edu/#Lick%20Observatory).

Can you do an impression of Robin Williams doing an impression of Carl Sagan?

I wish! A lot of people have told me that I remind them of Robin Williams, both in appearance and behavior. I'm really sad that he's no longer with us. Such a funny guy...

Hello Mr. Filippenko, thank you so much for doing this AMA. I'm a big fan of your work and see you provide insight and commentary on a lot of the documentaries that i watch. You always seem so upbeat and happy and genuinely excited about the topic at hand, my question is: what was your inspiration to work in your field? What made you interested in science and astrophysics in general? Also, whats your favorite piece of tech out of science fiction?

Thanks for the compliment! I am indeed very happy to be a scientist. It's really a privileged position to have.

I've always been interested in science, from my earliest years. I remember playing with magnets in the first grade and wondering about why they are pulled together or pushed apart. I played with electronics kits, microscopes, chemistry sets, etc... indeed, from age 10 through 17, chemistry was my main interest. Then, as a freshman in high school, my parents gave me a small telescope as a gift, and that night a "discovered" Saturn on my own (nobody told me to look at that bright "star" in the sky). This was an incredible thrill, despite millions of people having seen it before me. What a rush it must be to discover something really new!

So astronomy became a growing hobby, and sometime in my freshman year of college it passed up chemistry, so I switched to physics with the intention of becoming an astrophysicist. Part of the reason is that with astrophysics, I could "have it all" -- the very small (like the physical chemistry I was interested in) and the very large. It turns out that the large-scale properties of the Universe (stars, galaxies) are governed by the small-scale properties of matter. Amazing! How could I not study it! (Plus, I had several accidental explosions as a budding young chemist, so switching to astrophysics was partly an act of self-preservation!)

Not sure about sci-fi tech: depends on whether you mean what's possible or impossible. Exceeding the speed of light is cool but not possible. Travel through a wormhole is also cool, but almost certainly impossible. Things like Alcubierre drives are theoretically possible, but not for human travel from one place to another (there are many problems).

It is usually said that nothing can escape from the inside of a black hole. Yet, the recent LIGO experiments claimed that about 3 solar masses of matter from the insides of black holes involved in a collision, were converted into energy that was radiated away into universe. How could now black holes lose mass, if they can't lose mass?

Some scientists claim that new amounts of dark energy are constantly being generated out of nothing, as universe expands. Is a perpetuum mobile then possible, if only we could somehow harness that energy?

The initial LIGO detection was of two black holes (36 and 29 solar masses) merging to form a black hole of mass 62 solar masses. So, 3 solar masses were lost in the process. They were radiated away as gravitational waves, a tiny fraction of whose energy was detected by LIGO! A majority of the energy was radiated in the last 0.2 seconds of the merging process. During these 0.2 seconds, the amount of energy released exceeded that of all visible light from stars in the observable Universe by something like a factor of 30 (if I remember correctly -- I need to recalculate this to be sure).

No matter/energy actually came out of the black holes. All of the warping of spacetime that moved outward as a gravitational wave was already outside of the black holes (i.e., outside the event horizons of the two black holes). And theory simply says that the "surface area" (the area of the event horizon) of the final black hole must be greater than or equal to the combined "surface area" of the two individual black holes. Taking non-spinning black holes for simplicity (although the observed cases were actually spinning... this just changes the numbers somewhat), you can see that this was the case here. The surface area is just 4 pi R^2, where R = 2GM/c^2. By putting in the masses I mention above, you'll see that the theoretical constraint is indeed satisfied.

Regarding dark energy: Yes, as the Universe expands, there's progressively more dark energy in the Universe. (This doesn't violate the law of conservation of energy, by the way... the added dark energy is compensated by an increase in the negative gravitational energy associated with it.) But you can't do anything useful with it because it's spread out uniformly and probably can't be harnessed. (It definitely can't be harnessed if it's simply a property of the vacuum.)

Mr. Filippenko,

What is the possibility that our universe is a false vacuum, and that it may switch to a true vacuum state and wipe out the entire observable universe?

Thanks.

That's a great question -- the quality of questions on this Reddit AMA is very high.

It's conceivable that the "dark energy" that's currently driving the accelerating expansion of the Universe is analogous to the "inflaton" that produced the early inflation (exponential growth) of the Universe. In that case (a false vacuum), it could indeed be the case that someday, the false vacuum will "decay" to zero energy (or at least a lower-energy false vacuum). This could be the end of us, if (for example) the gravitational force decays into two other manifestations of what was once a unified force.

That being said: I wouldn't worry about it too much. Solar flares and coronal mass ejections, killer asteroids/comets, the gradually rising power of the Sun (leading to the evaporation of oceans within 1-2 billion years), etc., are much greater cosmic existential threats.

What is the greatest moment of your career thus far?

When our team (and in particular, my postdoctoral scholar Adam Riess) discovered the accelerating expansion of the Universe. Totally mind-blowing!

Hi Alex, fellow Golden Bear here! What are your thoughts on the Breakthrough Starshot project?

Really an interesting project, though somewhat of a long shot (it will require sustained funding over a long period of time, as well as international agreements about firing powerful lasers, etc.). I'm for it! Very generous of Yuri Milner to provide initial funding for this bold endeavor.

If Dark Matter neither emits nor absorbs light, than can it really be called "matter"? Also is there a way to quantify the effects of light seen 'through' dark matter? Would we even know if the dark matter was in the way between the observer and the light source?

It's matter, but it doesn't interact through electromagnetic forces. It could perhaps more appropriately be called "transparent matter."

Dark matter does affect the curvature (warping) of spacetime, though, so you can definitely see its effects. Look up "gravitational lensing of galaxies" online and you'll find great examples, specifically photos taken with the Hubble Space Telescope. And last year, one of my postdoctoral scholars (Pat Kelly) found the first example of a background supernova gravitationally lensed by a galaxy into 4 distinct images!

I'm an undergraduate who's done a bit of work in Type Ias over the last couple years. From what I've seen there's a lot of debate in the field as to the extent to which these supernovae are caused by the merging of two white dwarfs as opposed to a single white dwarf accumulating mass and then exploding. I know the jury is still out on this so to speak, but do you have an opinion? And if it does turn out that most Ias are caused by white dwarf mergers do you expect this to hurt their ability to be used as standardizable candles?

P.S. Are you taking any grad students next year?

I think many or most of the Type Ia supernovae probably come from merging white dwarfs, but that the single white dwarfs also contribute.

The changing demographics with time (the ratio of one kind to another varying with time) can affect precise measurements of the expansion history of the Universe, but if we are careful and have sufficiently big samples of supernovae (as in upcoming surveys like LSST), we should still be able to use them really well as standardizable candles (we've known for a long time that they're not really "standard").

I could take on another graduate student next year, especially if he/she gets an external fellowship such as from the National Science Foundation. (Funding is often an issue.)

What's the most interesting thing you've learned?

Quite possibly that we are made of star stuff -- the atoms of carbon, oxygen, calcium, iron, and other heavy elements in your body were made through nuclear reactions in stars billions of years ago!

Is looking into or inside a blackhole possible?

Only if you enter inside it -- but then you will never get out.

How come you're the only guy I remember from space documentaries?

Maybe because I'm usually smiling?! (Seriously, I really love the subject, which is why I smile; I can't help it. But sometimes people criticize me for smiling so much, even when I'm discussing end of the world scenarios such as a supernova going off near Earth.)

Hi Alex! There's been some debate in the community about public vs private funding. I personally think private funding (like Making & Science :] ) is terrific, although I can see how private donations can fund iffy science. Do you have any thoughts on the issue? How do you feel about using things like Kickstarter to fund projects (like the infamous WTF star)? Thanks!

Scientists doing pure research (like astrophysics) are very grateful that there's some Federal funding for them. That being said, the amount of funding, at least in the USA, is declining in real dollars. That's a real shame -- we should be investing more in science, and there are certain sectors of govenment spending that I personally think could be trimmed. But, this is the reality right now.

So, we are beginning to rely progressively more on private funding and foundations. A great example, as you pointed out, is the Google Making & Science gift to the University of California's Lick Observatory. We are enormously grateful for that! I think it has been quite mutually beneficial, and I'm hoping that the first gift was just the courtship phase of what will be a long relationship. Moreover, getting the support of Google gave us a stamp of approval, increasing our credibility. This helped us secure partial matching donations from individuals, and we are continuing to seek these through the Friends of Lick Observatory (https://www.ucolick.org/main/support/folo.html) and the UC Berkeley Lick Observatory Operations Fund (https://give.berkeley.edu/#Lick%20Observatory).

Kickstarter can also be used, though I personally have not used it. The WTF star is really interesting, by the way!