Everyone knows that time moves forward, but answering the reason why remains a fundamental challenge of physics, a CalTech professor told a capacity crowd at the Reuben H. Fleet Science Center on Monday.
“Time has a direction. The problem is that’s not how the fundamental laws of physics work,” said Sean M. Carroll, a research professor of physics and author. He spoke at the Fleet’s first event of 2015, addressing crowd of 250 that packed the IMAX dome theater.
Carroll explained that even though atomic processes are reversible, time never moves backward. This is because entropy — a scientific measure of disorder — is always increasing. “The arrow of time is because entropy is increasing,” he explained.
What isn’t understood is why the universe began in a “hot, dense and smooth” state of low entropy. “Understanding why is a profound challenge for modern cosmology,” he said.
He said the low entropy at the beginning could be because the Big Bang 13.8 billion years ago truly was the beginning of the universe, but the latest data only goes back to one second after that event. “The truth is the Big bang isn’t the beginning of the universe but the end of our understanding,” Carroll said.
The Big Bang wasn’t an explosion, as popularly perceived, but the coming into being of the universe: “the coming into being of space itself.”
Entropy is increasing because the universe is expanding at an accelerating rate. Eventually the stars will burn out and even black holes will dissipate, leaving “a thin gruel of elementary particles.” Carroll showed a completely black slide to illustrate the future universe.
Turning to human civilization, he noted that life converts low entropy to high entropy. “The reason why we can have life is that the sun is a hot spot in an otherwise cold sky. We get low entropy energy from the sun…and we radiate away high entropy radiation to the universe,” Carroll said. “We are still living off the afterglow of the high level of organization we had at the Big Bang.”
During the question-and-answer period, he dispelled the idea of time travel. “Time is a one-way street. We will always just move into the future,” he said.
Prior to his talk, the American Institute of Physics awarded Carroll with the 2014 Andrew Gemant Award, an annual prize recognizing significant contributions to the cultural, artistic or humanistic dimension of physics.