No matter what stock market analysts, political pollsters, or astrologers say, we can't predict the future. In fact, we cannot even predict the past.
So much for the work of Pierre-Simon Laplace, French mathematician, philosopher, and king of determinism. In 1814, LaPlace wrote that if it were possible to know the speed and position of every particle in the universe at a particular moment, and all the forces acting on them, “to such an intelligence nothing would be uncertain, and the future would be as present as the past.”
Laplace's dream still remains unfulfilled because we cannot measure things with infinite precision, and small errors propagate and accumulate over time, causing more and more uncertainty. As a result, in the 1980s, astronomers including Jaques Laskar of the Paris Observatory concluded that computer simulations of planetary motion could not be trusted when applied more than 100 million years in the past or future. For comparison, the universe is 14 billion years old and the solar system is about 5 billion years old.
“We can’t give an accurate horoscope for dinosaurs,” Scott Tremaine, an orbital mechanics expert at the Institute for Advanced Study in Princeton, New Jersey, said in a recent email.
Ancient astrological charts are now even more blurred. A new set of computer simulations that take into account the effects of stars passing through our solar system have effectively reduced scientists' ability to look further into the past or future by another 10 million years. Previous simulations had considered the solar system as an isolated system, a clockwork universe with major perturbations to planetary orbits occurring internally due to asteroids.
“Stars are important,” said Nathan Kaib, senior scientist at the Planetary Science Institute in Tucson, Arizona. He and Sean Raymond of the University of Oklahoma published their results in the Asphysical Journal Letters in late February.
Researchers have discovered that a sun-like star called HD 7977, currently lurking 247 light-years away in the constellation Cassiopeia, may have passed close enough to the sun about 2.8 million years ago to shake up the largest planets in its orbit.
This added uncertainty makes it more difficult for astronomers to predict more than 50 million years into the past and to relate temperature anomalies in the geological record to possible changes in Earth's orbit. Such knowledge will be useful as we try to understand climate change underway today. Dr. Kaib said that about 56 million years ago, Earth apparently experienced its greatest tropical period, the Paleocene-Eocene. This period lasted more than 100,000 years, during which average global temperatures rose by up to 8 degrees Celsius.
Were these warm spells triggered by changes in Earth's orbit around the Sun? We may never know.
“I’m not an expert, but I think this is the warmest period in the last 100 million years,” said Dr. Kaib. “And it’s almost certainly not caused by the Earth’s orbit itself. But 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 helpful to be sure about what role the Earth’s orbit plays.”
“The simulations were performed carefully and we believe the conclusions are accurate,” said Dr. Tremaine. “This is a relatively minor but conceptually important change in our understanding of the history of Earth’s orbit,” he added.
The really interesting story, he said, is how disturbances in Earth's orbit could leave traces in the paleoclimate record.
The ability to track the movements of stars just beyond our solar system has been dramatically improved by the European Space Agency's Gaia spacecraft, which has mapped the positions, movements and other properties of two billion stars since its launch in 2013.
Dr Kaib said: “For the first time we can actually see individual stars. It's really cool to project time backwards or forwards to find out which stars are closer to the Sun and which are not. .”
According to his calculations, about 20 stars appear within 1 parsec (about 3.26 light-years) of the Sun every million years. HD 7977 could be as close as 4 billion miles from the Sun (the distance to the Oort Cloud, a huge reservoir of frozen comets at the edge of the solar system), or it could be as much as 1,000 times farther away. The gravitational effects of the closer encounter may have shaken the orbit of the outer giant planet, which in turn could have shaken the inner planet, such as Earth.
“This is potentially powerful enough to change simulations’ predictions of what Earth’s orbit was like about 50 million years ago,” Dr. Kaib said.
As a result, he said, if you look far enough into the future, almost anything is statistically possible. “So, for example, we find that over billions of years, not all planets are necessarily stable. In fact, there is about a 1% chance that Mercury will collide with the Sun or Venus in the next 5 billion years.”
Whatever happens, there's a good chance we won't see it. Stuck in the present, we are not sure where we came from or where we are going. The future and the past disappear into myth and hope. But we move forward to look beyond our horizon in time and space. F. Scott Fitzgerald wrote in “The Great Gatsby”: “So we got into the boat and went against the current, constantly moving past.”