The Light Never Broke the Speed Limit. Space Itself Stretched.

Lex Fridman opens at the largest scale he can find: will the universe expand forever, or collapse on itself? Alex Filippenko, an astrophysicist at the University of California, Berkeley and a member of both teams that found the accelerating expansion, says the matter density already looks too low for a recollapse. On top of that sits a repulsive effect physicists named dark energy for want of a better term, and it appears to be speeding the expansion up. Asked for investment advice, he would bet on expansion forever, because he thinks the culprit is likely the zero-point energy of the vacuum. The ground state of empty space is not a state of zero energy, he says, and that leftover energy has a repulsive effect that does not change with time.

He is quick to add that nobody really knows what dark energy is. Different theories predict different histories for the expansion, and the data so far agree with a constant-density vacuum energy. Agreement, though, is not proof. Match the numbers to eight, ten, even twelve decimal places, and a discrepancy in the thirteenth would mean the dark energy is not the vacuum after all, but some new field filling space. Fields of that kind, grouped under names such as quintessence, could in principle change, even reverse sign, and become gravitationally attractive in the far future.

There is a precedent. When the universe was a trillionth of a trillionth of a trillionth of a second old, a dark-energy-like substance — the inflaton — blew space up exponentially and then decayed into ordinary, attractive matter. Deceleration then ran the show for about nine billion years, until a small amount of dark energy began accelerating the expansion again some five billion years ago. Whether that second acceleration will last is, Filippenko says, a question we would like to answer and may not answer soon. He even floats, without endorsing, the idea that other bubble universes in a multiverse could be tugging outward on ours.

Then he admits to a darker thought. Sometimes he wakes up screaming, he says, because dark energy and dark matter might be twenty-first-century Ptolemaic epicycles. Ptolemy's geocentric scheme could, with enough nested circles, reproduce planetary positions to arbitrary accuracy; it was, in a sense, the beginning of Fourier analysis. The math can fit and the physics can still be wrong. Other data eventually showed that Earth goes around the Sun. As a scientist, Filippenko says, he has to stay open to the possibility that today's patches are just as misplaced.

That talk of epicycles puts Filippenko in Copernicus's shoes. Five hundred years ago the Polish astronomer had a philosophical preference for a Sun-centered system, not a decisive win on the numbers. He saw that the Sun is the source of light and heat, and he knew it is much farther away than the Moon, so the fact that the two disks look the same size meant the Sun is vastly bigger. Something that large and bright, he felt, belonged at the center. The measured positions of the planets in the early sixteenth century were not a significantly better match under heliocentrism than under Ptolemy, whose system had lasted 1,500 years.

Copernicus was still a man of his time. He dedicated the work to the pope and kept circular orbits, with the Sun a little off center — a slightly off-center circle looking a lot like a slightly eccentric ellipse. Kepler needed Tycho Brahe's better data to show that the orbits are ellipses. Actual evidence for the heliocentric hypothesis, Filippenko says, did not arrive until Galileo's work around 1610. Venus goes through a full set of phases, from new to crescent to gibbous to full. In the Ptolemaic setup, with Venus always roughly in the direction of the Sun, you could only get new and crescent. When Venus was gibbous or full it also looked smaller, as it should if it were then on the far side of the Sun. That, he says, was the nail in the coffin.

Galileo's other famous observation was that Jupiter has moons, and that those moons travel with Jupiter as it moves. Earth is not the only body with satellites, and a moving body can carry its moons along. Filippenko finds that less definitive than the phases of Venus. He also notes an irony of scientific history: the Roman Inquisition placed Galileo under house arrest, which he does not condone, and that confinement gave Galileo time to assemble and publish experiments he had done decades earlier. Newton later used those results. Without the arrest, Filippenko says, it is not clear the book would have been written.

Fridman asks how human civilization ends if we dodge the near-term ways we might destroy ourselves. Filippenko puts anthropogenic threats, including nuclear ones, above the celestial list, and he sees no path for biological humans to the heat death of the universe. Machines that he thinks of as our evolutionary descendants might last longer, though even they will have less and less energy to work with as the lowest-mass stars burn out over trillions of years. In our own solar system the Sun becomes a red giant in about five billion years. The tighter deadline is sooner: within one or two billion years the Sun will have brightened enough that, without compensatory atmospheric changes, the oceans evaporate. You would need much less carbon dioxide to hold the temperature down, and plants would not like that at all. It is a bind.

On a nearer timescale he worries about impacts. A civilization-changing asteroid is only a kilometer or so across; a true mass extinction wants something ten kilometers or larger. Ninety-nine percent of near-Earth objects ultimately come from the asteroid belt, and those can be tracked. Find one fifty or a hundred years out, and clever applied physicists and engineers can deflect it. A tiny change in speed is enough: Earth moves through one of its own radii in about four minutes, so arriving a little early or a little late misses the planet. Months of warning are another story. By then you would be trying to shove the thing through a right angle.

Comets are worse. They come from the Kuiper Belt beyond Neptune, from the scattered disk, and from the Oort Cloud, a reservoir of a trillion or ten trillion icy bodies sitting a good fraction of a light-year from the Sun. A passing star can knock one inward. By the time we see it, it may be only a year or two away, and outgassing from sublimating ices acts like a rocket, shoving the trajectory in ways we cannot predict. Filippenko's nightmare is deflecting a comet that would have missed and thereby making it hit — "the ultimate Charlie Brown." With only months to go, the job would be to predict the impact site and try an orderly evacuation, though he is no longer confident, after a year of watching governments in 2020, that extreme competence is what we should expect.

The more realistic cosmic threat, he thinks, is a giant solar flare. A coronal mass ejection can slam Earth's magnetic field, induce currents in long power lines, and burn out transformers that are expensive, hard to replace, and hard to move. Astronomers already watch the Sun around the clock; a day or two of warning is often enough for utilities to shut the grid down and save most of it. "Support your local astronomer," he jokes: they may tell the power companies to cut the juice, or find the asteroid a century early. A supervolcano is less funny. The Toba eruption more than 70,000 years ago threw up enough soot to crash global temperatures, and some estimates put the human population at 5,000 to 10,000. A fifteen-degree drop on a short timescale would, even with today's technology, starve hundreds of millions. Imagine North America without power for a year. The people who would do fine, he says, are the rugged individualists already living in the forest. "We're incredibly lucky. But we've set ourselves up to be very, very fragile."

Fridman puts the beauty of the night sky next to Elon Musk's engineering claim that humans must become a multiplanetary species to survive. Filippenko agrees with the prudence and rejects a common caricature. Going elsewhere is not an excuse to neglect Earth; we should take the best possible care of this planet and still plant seeds somewhere else, because things can go wrong here. Humans are explorers by nature, he says. Climbing the next mountain is in the job description. Mars is a reasonable next apartment. He just does not believe it will be a thriving colony on the timetable Musk likes to quote. Visionaries need optimistic clocks to get people moving. The reusable rocket that lands upright is a genuine game changer — imagine throwing away the airplane after every San Francisco to Los Angeles hop — but the clock for a real city on Mars still looks, to him, far too short.

Mars is a harsh sublet. The atmosphere is less than one percent of Earth's and mostly carbon dioxide, so you would need a dome almost immediately, and a dome does not house many people. Water sits in permafrost or in polar ice, which means melting or canals. Dust storms are violent and the temperatures are generally cold. At a meeting Filippenko attended on the most efficient nutrition for humans on Mars, the answer was insects: high protein, easy to farm. It will not be as simple as growing potatoes in *The Martian*. You would be terraforming, or living under glass.

Outside the solar system the real estate is plentiful. Kepler watched more than 150,000 stars and found planets around about one percent of them; because only about one percent of systems are oriented so that a planet transits from our point of view, a back-of-the-envelope count says something like 50 to 100 percent of stars have planets, and roughly one in five is thought to host a roughly Earth-like world. There are lots of possible homes. The problem, he says, is getting there.

Sirius, the brightest star in the sky, is 8.7 light-years away: the light we see left about nine years ago. At Earth's escape speed of 11 kilometers per second, the trip takes about a quarter of a million years. Call a human generation 25 years, and that is 10,000 generations of people who must grow their food, recycle their water, and process their waste on a vehicle they never leave. Filippenko's verdict is blunt. "I just don't see it happening." Even Mars is not a three-day hop like the Moon; a year or two in deep space means a lot of radiation that living tissue does not handle well.

What he can imagine, if we avoid destroying ourselves first, is machines. Robots can hibernate. They need to do almost nothing for a long time, and if a cosmic ray hits the circuitry they can be told to fix themselves. On arrival they copy themselves from materials they brought or from local resources and set up factories. It sounds futuristic. He thinks it is much more feasible than sending flesh and blood across interstellar distances. Fridman likes the thought that silicon descendants might carry some of what makes humans beautiful — curiosity, compassion, the full mix — rather than becoming paperclip-maximizing zombies. Filippenko is willing to be poetic about that. He likes the idea that thoughts and feelings come from ones and zeros, and that our creations could be evolutionary descendants even if they do not share our DNA.

Special relativity offers a loophole that physics then slams shut. Go close to light speed and your own clock can make the trip short, but E = mc² makes the energy bill unfathomable. The Large Hadron Collider can accelerate a proton to 99.99999 percent of the speed of light. A person is gazillions of protons, plus the rocket. As v/c approaches one, the Lorentz factor 1/√(1 − v²/c²) heads toward infinity, which is why a massive object cannot reach light speed through pre-existing space. Yuri Milner's Starshot idea — a tiny camera, shoved toward Alpha Centauri at perhaps 20 percent of light speed by high-powered lasers, snapping pictures of exoplanets 4.2 light-years away — is a project Filippenko supports. Sending a camera past a star, he notes, is not the same as sending people with mining gear who then go forth and multiply. Suspended animation would collapse 10,000 generations into one sleepy crew, and it still would not make the kinetic energy small.

Fridman asks whether black holes and the fabric of spacetime might someday be transportation. Filippenko does not want to play Lord Kelvin, who announced a century ago that fundamental physics was finished and that the rest was engineering — just before special relativity, general relativity, and quantum physics arrived. He also thinks we now know more about our limits. On present knowledge, most faster-than-light schemes look far-fetched if not impossible.

A non-rotating black hole is a no-go: the singularity is a point, and you get squished reaching it. A rotating black hole has a ring singularity, and in theory there are paths through the event horizon that miss it. Filippenko still would not buy a ticket. You fall in with so much energy that the wormhole pinches shut, or you hit a blast of incoming and outgoing energy that vaporizes you — the mass inflation instability. Even if some scrambled information made it through, traversable wormholes raise a problem physicists take personally: the grandfather paradox. Come back before you left, stop your grandparents from meeting, and cause and effect fall apart. Some people say you return unable to change your journey, or that you land in another universe. To Filippenko those fixes feel contrived, and landing in a different universe is not going home.

The Alcubierre drive, which compresses spacetime in front of a ship and expands it behind so the craft rides a wave, has the same kind of bookkeeping problem. Miguel Alcubierre himself called the idea fanciful. Beyond the energy requirements and the puzzle of how you exit the bubble, Filippenko's objection is simpler. To warp the space in front of you, you first have to get there, at ordinary sub-light speed, and arrange the warp. You have not saved any time. You might as well have flown the original route. You cannot snap your fingers and make the space to Alpha Centauri fold up.

On intelligent life, Filippenko puts himself with the pessimists — not for the observable universe, which may well contain other minds, but for our own galaxy. Light-travel time out to the edge is roughly 14 billion years; accounting for expansion, the radius is more like 46 billion light-years, a volume holding something like 10²² to 10²³ stars and a comparable number of Earth-like planets. The Milky Way, with several hundred billion stars, is another matter. He would side with the school that says we are alone here, or at most a handful, because he does not see human intelligence as a natural evolutionary destination.

He offers four arguments. First, more than ten billion species have lived on Earth, and nothing else has approached our combination of intelligence, mechanical ability, and curiosity; whales and dolphins are bright, but there is no evidence they think abstract thoughts or build machines to study the world. Second, early hominids are only four or five million years old and *Homo sapiens* about a quarter of a million; for most of the history of life, a passing alien would have found nothing particularly clever. Third, it is not obvious that our intelligence is a long-term evolutionary advantage. The last couple of centuries improved hundreds of millions of lives at the risk of destroying us. Fourth is the Fermi paradox: if they are common, where are they? UFO reports and Roswell do not, in his view, clear the bar of scientific evidence. SETI has scanned only a couple of hundred light-years, a tiny patch, but the galaxy's chemistry could have made Earth-like planets billions of years ago. Civilizations that far ahead, growing on the same exponential we have seen in a few hundred years of technology, could have filled the galaxy in 10 million to 100 million years. They would already be here.

One explanation he likes is that the truly intelligent civilizations are the ones that decided not to colonize. Exponential probes — send one, make two copies, repeat — fill a galaxy quickly, and then the next large galaxy, Andromeda, is 2.5 million light-years away, a different scale. They may have survived precisely by getting over the aggression and self-destruction that seem to come with intelligence, which would itself be a great filter. They might live like a monastery, seeds planted in several places so a supernova or a comet is not a single point of failure, and they might follow something like *Star Trek*'s Prime Directive: explore, do not interfere. Humans call it exploration, Filippenko says, and are honest enough to admit that a desire to conquer often sits underneath. Or this planet is television for them. Still, his most likely account of the silence is that such creatures are very rare. Carl Sagan's estimate of 100,000 galactic civilizations would, he thinks, already have shown itself.

Primitive life is a separate question, and it may be the hard step. Filippenko uses the old Sahara-watch argument: you do not trip over a timepiece in the desert and decide sand grains assembled it. Even the simplest prokaryotic cell is an extraordinarily complex structure. The jump to eukaryotic cells, with a nucleus and the energy budget that lets complexity explode, may be another great filter. The jump from single cells to multicellular organisms probably is not, at least on Earth, because the evidence says it happened independently dozens of times.

That is why a discovery that would look like a triumph could be grim. If we found bacteria — or worse, trilobites, or a fossilized civilization — on Europa or elsewhere, and if we still take the Fermi silence to mean that no one else is around at our level, then the filter is ahead of us, not behind. Lots of life, few minds, a bad omen for the long term. Fridman catches him on the word *pessimist*. Being alone can be good news: it means we were the lucky ones who got through. Filippenko agrees he had meant pessimism in the Drake-equation sense, few others out there, and that this may be optimism about our survival. Sitting here talking is itself an observational selection effect. Life's origin, eukaryotic cells, and human-like intelligence could each be filters, and we might have won several lotteries.

He parts company with Stephen Hawking on whether we should hide. He knew Hawking, admires the unification work and Hawking radiation, and thinks that if Hawking were alive he should have shared the 2020 physics Nobel for black holes with Roger Penrose. On broadcasting, he respectfully disagrees. We have been leaking radio and television for a hundred years. Any alien that could come here and squash us already knows, and probably does not care, the way you do not feel obliged to crush every ant on the sidewalk. They only need to watch whether we approach a technology that could threaten them. Advertising our presence, or not, is irrelevant.

Fridman brings up the pile of UFO reports, including the idea that sightings clustered after nuclear weapons appeared, and the 2004 Tic Tac encounter described by Navy commander David Fravor. He believes Fravor saw something weird. What bothers him is the instinct, across the Navy, the government, and much of science, to pretend nothing happened. Filippenko does not roll his eyes as a reflex. We should investigate, he says. The trouble is that nobody has brought a hunk of kryptonite — tangible material you can take into a lab. The evidence is anecdotal, and most of the photographs are ambiguous.

He quotes Carl Sagan: extraordinary claims require extraordinary evidence. Some famous crashes later turned out to be weather balloons. Astronomy departments get phone calls whenever Venus sits next to a crescent moon, a striking pair that people report as a UFO. There are more convincing cases, and he has seen some of the footage. There are also so many decoys that a finite number of scientists with finite time have to choose their problems. As a young man he looked at some of this himself, found it unconvincing, and went another way. That is a career choice, not a categorical denial. Slow travel to other stars violates no law of physics; Voyager and Pioneer will pass near other stars in a few million years, and we can even name which ones. Active aliens altering a craft's trajectory in real time, spying, and zipping home or sending signals across many light-years is a different claim, because it implies they have also beaten the light-speed barrier.

He would welcome a young scientist who finds meaning in doing the work properly. He also notes that astronomers spend their lives looking at the sky and almost never see the unexplained lights that fill the report databases. Fridman, for his part, thinks that if a genuine craft showed up there would be plenty of hard-to-dismiss evidence, and that a modern government is not competent enough to hide all of it. Filippenko's filter is the same one he applies to colliding-universe signatures in the microwave background: the more amazing the claim, the more independent, high-significance evidence you need. A five-percent chance it is a fluke is not how you announce another universe, and it is not how you announce visitors.

Then Fridman asks the question that sounds like a contradiction. The radius of the observable universe is 45.7 billion light-years. The age of the universe is 13.7 billion years. How is that possible if nothing outruns light? Filippenko wishes he had brought the classroom prop he has used for years: ping-pong balls on a rubber band. Space itself between clusters of galaxies is expanding. Light leaves one cluster, traverses some distance, and while it is still traveling the stretch of road it already covered keeps lengthening. Thirteen-point-seven billion years is how long the cosmic microwave background, the afterglow of the Big Bang, has been on the way to us. Multiply that age by a factor of about 3.25 — he jokes that it is roughly π, then corrects to three and a quarter — and you get a radius of some 46 billion light-years.

The light is not speeding. Locally, if you measured the photons zipping past, they would not exceed c. Speed in relativity is a local quantity. Giving the light credit for the extra stretch that happened behind it makes it *look* faster than light, and that is not how speed works. Two ping-pong balls far enough apart on a stretching band can recede from each other faster than light with no trouble: take a 400,000-kilometer gap, double every centimeter in one second, and the gap grows by 400,000 kilometers in that second, which already beats 300,000 kilometers per second. Special relativity's speed limit applies to objects moving through a pre-existing space. Nothing in special or general relativity forbids space itself from expanding faster than light. Einstein, Filippenko says, would not have had a problem with the universe cosmologists now observe.

The Big Bang itself is easy to cartoon as a point. Filippenko calls that glib. If we live in a closed universe, a three-dimensional analog of a sphere, it was always that topological shape; you cannot turn a point into a shell. It started at very high density, and we do not even know whether it was finite or infinite. Inflation, developed by Alan Guth, Andrei Linde, Alexei Starobinsky and others, can take a tiny finite region and expand it so fast that light sent out to see whether space curves back onto itself never finishes the trip. Operationally, that is a working definition of infinity: you cannot measure a finite size. Filippenko sometimes teases theorist friends by asking whether a godlike slice of time, taken from outside, would still look finite. They tell him he cannot stand outside, and that in relativity even what counts as a slice of time tilts when you move. He is an observational astronomer. He knows which end of the telescope to look through. If you cannot measure finiteness, he is willing to call it infinite.

What happened before t = 0, and whether other universes exist, sit on the boundary of science: hypotheses with plausible physics behind them, not theories with direct tests. A quantum fluctuation out of nothing can look as if it violates energy conservation until you notice that an inflating universe can keep a tiny total energy by pairing positive stuff with negative gravitational energy, the way a falling apple trades potential for kinetic. Roger Penrose has suggested that some information might leak through from before the Bang. Filippenko's instinct is that it does not, and he has not seen convincing evidence in the microwave background. A collision with another universe would leave a temperature pattern there; some astrophysicists have claimed one at about the five-percent level, which he does not regard as enough for a claim that extraordinary. The accelerating universe caught on quickly in 1998, he notes, because two supernova teams saw it and then other groups, using independent methods with different systematic errors, came to the same conclusion.

Filippenko never expected, as a realistic young scientist, to stand near a Nobel-level result. You hope to add something to how nature works and be satisfied. He was in the right place at the right time, with luck and a lot of work, on both the Supernova Cosmology Project and the High-Z Supernova Search Team. The 2011 prize was for the observational fact of accelerating expansion, not for an explanation of dark energy. You would expect galaxies to recede more slowly now than billions of years ago, the way an apple tossed upward slows under gravity. Looking at more and more distant supernovae, the teams found that the expansion did decelerate for the first nine billion years and has been speeding up for the last five. Adam Riess, then Filippenko's postdoc, a Miller Fellow at Berkeley after MIT and Harvard, was charged with measuring the brightnesses. The distant supernovae were fainter, hence farther, than expected. Filippenko was shocked when Riess showed him the calculations.

The prize went to the two team leaders, Saul Perlmutter and Brian Schmidt, and to Riess. Filippenko was second author on the High-Z paper, Riess's direct mentor, and a member of both teams. The three-person cap is tradition, not a line in Alfred Nobel's will; a teammate checked the will in a Stockholm museum during Nobel Week 2011 and it was not there. The leaders knew the result would not have existed without the people in the trenches, so they invited the teams. Gerson Goldhaber, whom Filippenko also names as deserving, had died in 2010. The Breakthrough Prize in Fundamental Physics later recognized both teams, then still rolled out the red carpet for the same three and split the other half of the money among roughly fifteen others who did not get into the ceremony. Gravitational waves in 2017 went to three people from a collaboration of a thousand. The Higgs discovery involved some 6,000 physicists and engineers and has never been recognized on the experimental side.

At the Nobel banquet Filippenko sat between two members of the Swedish academy, kept pouring wine through a long ceremony, and in a polite, diplomatic way asked pointed questions. They admitted that if four or more people seem equally deserving, the committee waits for one to die or does not give the prize. The committee is not always right. Jocelyn Bell discovered pulsars with antennas her adviser Anthony Hewish had conceived; his first reaction to her data, Filippenko says, was a condescending "rubbish, my dear," and the prize went to him. She won every other prize and did not let it destroy her. Vera Rubin, a central figure in dark matter, won everything except the Nobel — in part because dark matter itself has never been a Nobel result — and the large survey telescope in Chile now bears her name. Filippenko has known physicists whose lives were ruined by a prize they did not get. His was not. He is in the work for the thrill of the discovery. If the Nobel were perfect, Fridman suggests, it would be less interesting. Humans like heroes, competition, and the unfairness that makes a story land. Filippenko is glad to see teams recognized in the press and glad that Andrea Ghez became the fourth woman to win the physics prize, a signal to girls that they have role models.

A supernova is an exploding star. Most stars die quietly; the Sun will become a red giant and incinerate Earth on a slow schedule. A minority end in a titanic blast, and that blast is not just a spectacle. Heavy elements synthesized in nuclear reactions during a star's life, and during the explosion itself, get ejected into the cosmos as raw material for new stars, planets, and life. They are also beacons. Calibrate how powerful an exploding star really is in galaxies of known distance, then compare that intrinsic luminosity with how bright a distant one appears, and you have the distance to its host galaxy. Filippenko's analogy is headlights at night: you have already calibrated how bright they look a few meters away, so a faint pair means a far car. Your brain even cross-checks the angular gap between the lamps.

The cosmological workhorse is the Type Ia supernova, from a white dwarf. The Sun will become one in about seven billion years: roughly half its present mass squeezed into a volume the size of Earth, so dense that the electrons form degenerate matter, racing because of the Heisenberg uncertainty principle and the Pauli exclusion principle, and supplying extra pressure far above ordinary heat. A lone white dwarf does not explode. In a pair it can steal gas from a companion until it nears an unstable limit of about 1.4 solar masses, the Chandrasekhar limit, figured out by Subrahmanyan Chandrasekhar at about age twenty on a voyage from India to England and rewarded with a Nobel Prize fifty years later, in 1984. His adviser, Arthur Eddington, ridiculed the work at a conference. Most people, Filippenko says, would have quit astrophysics. The lesson he offers students is to keep going no matter what the adviser says — and also not to ignore advisers, who sometimes have sage advice. Past the limit, carbon nuclei fuse, degenerate matter cannot expand and cool the way the Sun can, and you get a thermonuclear runaway, an uncontrolled fusion bomb instead of the Sun's controlled reactor.

In theory each blast should have nearly the same brightness. In practice they do not. Trace heavier elements, the white dwarf's age, and other details change the fusion rate. Part of Filippenko's work was to show that Type Ia supernovae are not all the same. In 1991 he studied two extreme oddballs, a faint one and a luminous one, 1991bg and 1991T, and grew skeptical that the class could be used as cosmological tools. The game changer was a 1993 paper by his colleague Mark Phillips, on Schmidt's team: the intrinsically brighter Type Ia events decline — and rise — more slowly. Calibrate that relation on nearby supernovae and a distance that used to be 100 plus or minus 50 becomes something like 112 plus or minus 15. Filippenko dislikes the phrase *standard candles*. There are none. There are standardizable candles. The High-Z team had 16 well-measured objects; the Supernova Cosmology Project had 40 of lower quality; the statistical uncertainties came out comparable. A supernova would have to go off within about ten light-years to be an existential threat. The galaxy is 100,000 light-years across. What they mostly do is enrich the next generation of clouds. Filippenko tells his Berkeley intro students there are only five or six things he wants them to remember, and one of them is this origin story: the carbon in cells, the oxygen we breathe, the calcium in bones, the iron in blood, the phosphorus in DNA, all cooked in stars and blown out so that, 4.5 billion years ago, a chemically enriched cloud could make a rocky planet and, eventually, us.

As a Caltech graduate student working with Wallace Sargent on active galaxies, Filippenko took four courses from Richard Feynman, including graduate general relativity, and still ranks him as the physicist with the most direct intuitive grip on nature he has met. Feynman diagrams turned pages of quantum electrodynamics into lines and vertices with rules; more important to Filippenko was the demand that if you cannot explain something simply to a non-scientist, you probably do not understand it. In a room of students too scared to look foolish, only he and Jeff Richman, now a physicist at UC Santa Barbara, kept asking, until Feynman asked the others whether they were even curious and later took the two of them to lunch. On the meaning of life he is almost stubbornly modest. Meaning is what you make; it tracks goals; helping a dying person, raising a family, reading, or traveling can be enough. Most of nearly eight billion people will not change the world the way Newton or Michelangelo did, and that does not make their lives empty. Inanimate rocks and black holes cannot wonder about their origins. We can. Tweak a constant as dull as the neutron-to-proton mass ratio and you often get a boring universe of only hydrogen, or only helium, or only iron, without a rich periodic table, let alone bacteria, slugs, and humans. Either we are wildly lucky, or the dice were thrown many times and we live in one of the interesting rooms. Through us, Filippenko says, the universe found a way to know itself.