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Q&A with Berkeley astrophysicist Alex Filippenko

June 8, 2022
by Pat Joseph
Filippenko and a crowd looking at the sky

Ask an astronomer and they’ll tell you we’re living in a kind of golden age.   

In the last several years, a string of breakthroughs have radically changed our conception of the universe, and many of the most important discoveries have come out of Berkeley. Just look at the Nobels. In 2020, Berkeley’s Reinhard Genzel shared the physics prize for the discovery of the supermassive black hole in the center of the Milky Way galaxy. In 2017, Berkeley alumnus and Caltech professor Barry Barish ’57, Ph.D. ’62, won laurels for leading the LIGO project that directly detected gravitational waves, the ripples in the fabric of space-time predicted by Einstein. In 2011, Berkeley cosmologist Saul Perlmutter, Ph.D. ’86, shared the prize with Adam Riess and Brian Schmidt for the stunning discovery that the expansion of the universe is not slowing down, but accelerating. That gave rise to the notion of dark energy, a mysterious repulsive force that is believed to comprise roughly 70 percent of the universe. 

Berkeley astrophysicist Alex Filippenko, the subject of the interview that follows, was central to that discovery; in fact, he was the only scientist who was, for a time, on both of the teams—Perlmutter’s and Schmidt’s—that were racing to publish their results. Riess was his postdoc at Berkeley, as a Miller Research Fellow. 

While Filippenko didn’t share in the Nobel himself, he says he wasn’t disappointed. Sitting in his office in Campbell Hall, wearing a Berkeley Astronomy T-shirt (the graphic depicts the Campanile à la Van Gogh’s Starry Night), Filippenko said, “Most scientists make small contributions to the overall state of knowledge, and we’re happy with those contributions. Never in my wildest dreams did I think that I would be an important part of such a fundamental discovery. I have to pinch myself that I was at the right place at the right time.”

In addition to being one of the most widely cited astronomers in the world, Filippenko is also a celebrated educator—voted Best Professor at Berkeley a record nine times, named National Professor of the Year in 2006, and, most recently (2022), winner of the American Astronomical Society’s Education Prize. Who better to go to with my dumb questions? 

Filippenko holds up a ball
In totality: Alex Filippenko explains the mechanics of a total solar eclipse in Sunriver, Oregon, 2017.

I should add that Filippenko is also a devoted umbraphile, or eclipse chaser.
I first became acquainted with him on a Cal Discoveries Travel trip he led to Oregon to see the Great American Eclipse of 2017. It was one of the most memorable moments in my life. The discussion that follows, which has been edited for length and clarity, begins there. 

You and I first met on the eclipse trip, which was fantastic. I understand there’s another Great American Eclipse coming in 2024 and that this one is even bigger. 

That’s right. This one’s bigger in the sense that, at maximum, it’s around four and a half minutes long. That’s twice as long as my 18-eclipse average. Americans had to wait 38 years for the 2017 eclipse. This one is only seven years later. Many people who missed it last time, thinking it was no big deal, have heard from friends how great it was. So this one could be much more observed than the 2017 U.S. eclipse. That’s great, to show more people the grandeur of the heavens.

I can’t think about total eclipses without tripping on the fact that the sun and moon appear the same size in the sky. 

Yeah, that is a real trip. It’s a complete coincidence. Unlike, for example, the fact that we see the same side of the moon at all times. That’s not at all coincidental. The moon used to be rotating quickly, but Earth produces tides on the moon—not in the form of water, but in the form of rock. And so as it rotated quickly, the moon got squished back and forth. That caused friction, releasing energy, thus slowing down the rotation, until it got locked, rotating at the same rate that it revolves around Earth. It’s now a stable configuration. 

But the moon being the same apparent size as the sun, that is coincidental. And indeed, the moon is gradually moving away from Earth, again as a result of tides, at a rate of about four centimeters a year on average—about the rate at which fingernails grow. So in about half a billion to a billion years, the moon will always be too far from Earth for total solar eclipses to be possible. So, people should act now!

How did the moon form in the first place? 

It’s thought that within the first few tens of millions of years of Earth’s formation, a Mars-sized object slammed into Earth. And that blew off part of Earth’s crust and upper mantle that formed a debris disk that circled Earth. Within a century or two, this coalesced to form the moon. 

Anyway, that’s the theory—or, rather, the best hypothesis we have. I would say it’s a hypothesis in the sense that I don’t know that there’s enough evidence to make it rise to something like Newton’s theory of gravitation, or Einstein’s theory of relativity. These have so many facts supporting them that they rise above the hypothesis level. People who don’t understand this often say, “Oh, it’s just a theory.” But, no, to scientists, theory is a pretty high level of evidence. 

We now have evidence for a black hole at the center of the Milky Way. Do we expect to find black holes at the center of every galaxy?

Yeah, pretty much, every big galaxy, especially those that have a big bulge. In spiral galaxies, there’s a flattened pancake disk in which spiral arms occur and where new stars form. And then there’s an older, more spherical or elliptical distribution of stars called the bulge. Even going back to my own thesis work in the mid-1980s, and when I was a postdoc here, I found indirect evidence for black holes in many, if not most, nearby galaxies. I can’t say I was the only one who was doing this work, and it’s conceivable that something else produced those observations, but the most natural and consistent explanation was that there’s a black hole in those galaxies as well. The reason the evidence is best in the middle of our own galaxy is that the center is only 26,000 light years away, rather than 26 million light years away. And so we can see much greater detail. 

Are black holes really holes? 

They are holes in the sense that, according to general relativity, they are a warping of space and time. The space is easier to imagine. It warps off into a different dimension, a fourth spatial dimension that we can’t see. Can’t go there, can’t see light from there. The analogy we like to use is a trampoline. When there’s nothing on it, you can think of it as just a two-dimensional thing, you can go forward or backward, or left and right, but the up and down dimension to you does not exist, because hypothetically, the laws of physics constrain you to be on the surface of this trampoline. But mathematically, you can imagine that there’s an up and down dimension, and you can call it z. Then you put a bowling ball on this thing, and it curves in the z direction. You’re still stuck in the x and y directions, but it curves into the z direction. And if you put more and more mass there, and you make it denser and denser, the warping becomes bigger and bigger. And finally, it becomes so deep and so steep that the sides become vertical. So it’s a hole, but it’s not like you took x and y—this rubber sheet—and took some scissors and cut a hole out of it. 

In the parlance of general relativity, it’s such a severe warping of space-time that not even a photon, not even a light ray, can get out. Anything trying to escape loses energy as it’s climbing out. And by the time it gets out, it has no energy. So in a sense, it hasn’t gotten out, the light becomes trapped.

The comedian Steven Wright used to pose this question in one of his bits. He’d say, “You’re traveling in your spaceship at the speed of light. You turn on your headlights. Does anything happen?”

So that’s the question, in slightly different wording, that Einstein asked when he was coming up with relativity. He asked, basically, if I look in a mirror that’s traveling away from me at the speed of light, will I see myself? 

So, here’s the deal (and this sounds like a cop-out, but it’s not): Your spaceship can’t get to the speed of light because it would take an infinite amount of energy to do so. And you could say, well, what if it could? But then you would have to abandon the laws of physics as we know them. And then anything could happen. It becomes kind of nonsense to talk about it. 

The Hubble Space Telescope recently recorded the oldest star ever observed: close to 13 billion years. The universe is around 13.8 billion years old. I have to ask: Will we ever be able to see back to the beginning? 

The answer is no. Because, for the first 380,000 years, the universe was a plasma, it was hot enough to be an ionized gas. And electromagnetic radiation doesn’t travel through ionized gas. 

The microwave background radiation that my colleagues are measuring comes from an age of 380,000 years, when the universe became cool enough that the electrons combined with protons and formed hydrogen atoms. At that point, the universe became transparent, like the molecules in this room are transparent. With electromagnetic waves, we cannot see farther back than that. But there are also primordial gravitational waves. Those are really hard to find, but they would take us back to the tiniest blink of an eye of the universe’s existence—not quite Time=0, but almost. With light, 380,000 years is the best you can do.

What comes before the Big Bang? 

That’s something physics cannot entirely answer. You can have all kinds of hypotheses, but you can’t test them in the lab. Nevertheless, we can come up with a plausibility argument, based on physics that we think we know. But it’s at the boundary of what we can call physics. That’s the caveat. All right? So here’s the idea: The universe may have been a quantum fluctuation out of nothing at all. 

What do I mean by nothing? Do I mean a pre-existing hyperspace in which there were laws that allowed a quantum fluctuation out of nothing—a little bit of energy that came from zero energy? If you accept that, you’ve kicked the can further down the road. What created the hyperspace and the laws of physics that allowed this to occur? 

It’s like the question “Who created God?” Which makes me wonder: If we surveyed astronomers, would we find many who believe in a divine creator?

Certainly some, but probably fewer than in the general population. I think many probably have something resembling my own personal feeling. And that is that we can never really use science to prove or disprove the existence of a greater being that made this all happen and may even be watching over us. 

I actually object to those scientists who try to use science to disprove the existence of God. My own feeling is that they are fanning the flames of discord between science and religion that most of us have been trying to quench for hundreds of years. 

Let people believe what they want to believe. I mean, if it gives them hope and faith, fine. But my own god is closer to what I think Einstein meant when he said that his god was like that of Spinoza, the philosopher who basically said that God is nature. Maybe carrying it a step further, God is the laws of physics. I don’t know how they came about, and maybe that’s God. That’s one of the things that’s beautiful about the universe: It achieves this amazing complexity through this relatively simple set of laws. 

And isn’t it amazing that I’m this thinking, feeling, emotional being, not just a rock or a black hole or a star? Yeah, that’s amazing. But I don’t personally think that there was any higher purpose dictated by an omniscient, omnipotent creator. It’s just a wonderful, wonderful coincidence that we exist. And that is how I find meaning: that we are the only creatures on Earth, possibly a very rare form of creature in the entire universe, that can come to understand the origins of their own existence, the origins of the chemical elements, things like that. The Big Bang doesn’t answer why we are here, and what our purpose is, and what your moral code should be. But it at least answers how we are here, right? 

But why are we here? I have no idea. It’s something that I’m really frustrated by, honestly. I’m an intensely inquisitive and curious person. I want to know things. But I will never know that for sure. And I won’t know what, for example, happens to humanity. What will we know, what will we learn in 100 or 10,000 or a million years? Will humanity even be around? Or will we destroy ourselves, intentionally or unintentionally, or through neglect, etc., etc.? 

Did you watch Don’t Look Up?

Yeah, I did. I liked it. It has a number of messages, actually. For one, climate change is clearly occurring. And a lot of people just don’t want to believe it, and don’t want to look at the evidence. 

I do have a few little complaints. One is when the Leonardo DiCaprio character says, “Look, there it is, you can see it.” We’ve seen lots of comets, and just because you see something in the sky doesn’t mean it’s going to hit. And so the people on the street would still have to trust the scientists who are saying, “Based on our observations, in our calculations, this thing is going to hit us.” 

I also didn’t like the very ending. But I did like the part near the end where the DiCaprio character and his family are having dinner as all this destruction is going on around them. And they’re just having this conversation. “Well, you know, we had really great lives. It was really amazing that we were here, and let’s just calmly have this last dinner.”

That was an interesting touch. 

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