No matter what stock analysts, political pollsters and astrologers say, we cannot predict the future. In fact, we cannot even predict the past.
So much for the work of Pierre-Simon Laplace, the French mathematician, philosopher and king of determinism. In 1814, Laplace declared that if it were possible to know the speed and position of each particle within the universe at one particular moment – and all of the forces that act on it – “nothing would be uncertain for such an intellect, and the future, like the past, would be for it the present.”
Laplace’s dream remains unfulfilled because we cannot measure things with infinite precision, so small errors propagate and accumulate over time, leading to even greater uncertainty. As a result, in the 1980s, astronomers, including Jaques Laskar of the Paris Observatory, concluded that computer simulations of planetary movements couldn’t be trusted applied more than 100 million years into the past or future. For comparison, the universe is 14 billion years old and the solar system is about five billion years old.
“It is impossible to create an accurate horoscope for a dinosaur,” Scott Tremaine, an expert in orbital dynamics at the Institute for Advanced Study in Princeton, New Jersey, commented recently in an email.
The ancient astrological chart has now become even more blurry. A new set of computer simulations that take into account the influence of stars moving past our solar system have effectively limited scientists’ ability to look back or into the future another 10 million years. Previous simulations considered the Solar System as an isolated system, a clockwork universe in which the main disruptions to the planets’ orbits were internal and resulted from the presence of asteroids.
“Stars matter,” said Nathan Kaib, senior scientist at the Planetary Science Institute in Tucson, Arizona. He and Sean Raymond of the Laboratoire d’Astrophysique de Bordeaux in France published their results in Astrophysical Journal Letters late February.
Scientists have found that a Sun-like star called HD 7977, which currently lies 247 light-years away in the constellation Cassiopeia, may have passed close enough to the Sun about 2.8 million years ago to shake the largest planets in their orbits.
This additional uncertainty makes it even more difficult for astronomers to forecast beyond 50 million years and to correlate temperature anomalies in the geological record with possible changes in Earth’s orbit. This knowledge would be useful as we try to understand the climate changes currently taking place. Dr. Kaib said that about 56 million years ago, Earth apparently went through the Paleocene-Eocene Thermal Maximum, a period lasting more than 100,000 years during which average global temperatures rose by as much as 8 degrees Celsius.
Was this warm period caused by some change in the Earth’s orbit around the Sun? We may never know.
“So I’m not an expert, but I think this is the warmest period in about 100 million years,” Dr. Kaib said. “And it’s almost certainly not due to Earth’s orbit alone. However, we know that long-term climate fluctuations are linked to fluctuations in the Earth’s orbit. So if you want to understand climate anomalies, it’s worth being sure what’s happening with the Earth’s orbit.”
Dr. Tremaine noted: “The simulations were fastidiously performed and I imagine the conclusions are correct.” He added: “This is a comparatively small change in our understanding of the history of Earth’s orbit, but a vital one from a conceptual standpoint.”
The really interesting story, he said, is how chaos in Earth’s orbit could have left a mark on the paleoclimate record.
The ability to track the movements of stars just outside our solar system has been dramatically improved by the European Space Agency’s Gaia spacecraft, which has been mapping the positions, motions and other properties of two billion stars since its launch in 2013.
“For the first time, we can actually see individual stars,” Dr. Kaib said, “X-ray them in time or forward and find out which stars are close to the Sun and which are not, which is really cool.”
According to his calculations, every million years there are about 20 stars within one parsec (about 3.26 light-years) from the Sun. HD 7977 could have come as close as 400 billion miles to the sun – or about the distance to the Oort Cloud, the vast reservoir of frozen comets at the edge of the solar system – or remained a thousand times farther away. Gravitational effects from the closer encounter could have disrupted the orbits of the outer giant planets, which in turn could have disrupted inner planets like Earth.
“This is potentially powerful enough to change simulation predictions of Earth’s orbit more than about 50 million years ago,” Dr. Kaib said.
As a result, he said, statistically almost anything is feasible should you look far enough into the future. “So we find, for example, that if we go back billions of years forward, not all planets are necessarily stable. The probability of Mercury colliding with the Sun or Venus in the next five billion years is about 1%.
Whatever happens, there’s a chance we won’t be around to see it. Lost in the present, we don’t know for sure where we came from or where we’re going; the future and the past dissolve into myth and hope. Yet we press forward, trying to see beyond our horizons in time and space. As F. Scott Fitzgerald wrote in The Great Gatsby: “So we sail in boats against the current, constantly going back into the past.”
