Reality Q&A: Live from Willoughby Library with Geraint Lewis (489)

“To a photon, it was emitted and absorbed at the same time. There is no time that passes for that photon.” That line, from astrophysicist Geraint Lewis, lands early in this live Q&A recorded at Sydney’s Willoughby Library, and it sets the tone for a wide-ranging session in which Lewis fields audience questions about dark energy, the nature of time, consciousness, and what happens to our atoms after we die. Lewis, author of The Shortest History of Reality, answers with a mix of precision and humility, often admitting what physicists still do not know.

Dark energy: a confident bet, but not a house bet

An audience member asks how confident astronomers are that 97 percent of the universe is made of dark energy and dark matter. Lewis walks through the discovery: in the 1990s, astronomers observed distant supernovae (exploding stars that act as “standard candles” of known brightness) and found they were fainter than expected. Using Einstein’s general theory of relativity, they concluded the universe must be accelerating — and the only way to get acceleration in Einstein’s equations is to introduce a repulsive form of energy, now called dark energy. “Matter causes the expansion of the universe to slow down; dark energy pushes and causes it to speed up,” Lewis explains. He adds a crucial caveat: the alternative is that our theory of gravity is wrong. “The majority of people say