Friday, September 18

Quantum mechanics means some black hole orbits are impossible to predict

Even if you could measure three black
holes’ locations as precisely as physically possible, you still might not know
where the black holes would go. Such a trio’s complex dance can be so chaotic
that the motions are fundamentally unpredictable, new computer simulations

The paths of three black holes orbiting each other can be calculated based on their positions and velocities at one point in time. But in some cases, the orbits depend so sensitively on the black holes’ exact positions that the uncertainty of quantum physics comes into play. Tiny quantum uncertainties in specifying the locations of objects can explode as the black holes’ gyrations continue over tens of millions of years, astrophysicist Tjarda Boekholt and colleagues report in the April Monthly Notices of the Royal Astronomical Society. So the distant future of the black holes’ orbits is impossible to foresee.

Such extreme sensitivity to initial conditions is known as chaos. The new study suggests, in the case of three black holes, “quantum mechanics imprints into the universe chaos at a fundamental level,” says astrophysicist Nathan Leigh of Universidad de Concepción in Chile, who was not involved with the research.

In chaotic systems, tiny changes can generate
wildly different outcomes. The classic example is a butterfly flapping its wings, thereby altering weather patterns, possibly
producing a distant tornado that otherwise wouldn’t have formed (SN: 9/16/13). This chaos also shows up
in the orbits of three black holes and other collections of three or more objects,
making such orbits difficult to calculate, a conundrum known as the three-body

To test whether the black holes’ motions
were predictable, Boekholt, of the University of Coimbra in Portugal, and
colleagues checked if they could run computer simulations of the orbits both
forward and backward and achieve the same result. Starting with a given set of
locations for three initially stationary black holes, the researchers evolved
those orbits forward in time to an end point tens of millions of years in the
future. Then, they rewound the simulation, reversing the motions to see if the
black holes ended up where they started from.

Computer simulations have a limited level
of accuracy. In this case, for example, the locations of black holes were known
only to a certain number of decimal places. That tiny imprecision can balloon
over millions of years of the simulation.

According to quantum mechanics, it is
impossible to determine the position of any object better than an utterly tiny
distance called the Planck length, about 1.6
times 10-35 meters, or 16 billionths of a trillionth of a trillionth
of a millimeter (SN: 4/8/11). Yet even
with accuracy the size of the Planck length, the researchers found that about 5
percent of the time the three black holes wouldn’t return to the same spots
when the simulation was reversed. That means, even if you measured where the black
holes were to the quantum mechanical limit, you couldn’t rewind to find out
where they had come from.

“These systems are fundamentally
irreversible,” says Boekholt. “You can’t go forwards and backwards for these 5 percent
of systems in nature. And that was quite a surprising result.”

The result is theoretical and can’t be applied to real black holes, says astrophysicist Nicholas Stone of the Hebrew University of Jerusalem. For example, measurement errors would swamp the importance of quantum physics. But that doesn’t detract from the study’s importance, he says: “It is still quite interesting from a conceptual perspective.”