Atom interferometry has become a powerful tool for measuring gravity or rotation (see Focus: A Better Quantum Gyroscope). Atoms in a small cloud are cooled to near absolute zero, so that their quantum wave nature becomes observable. Lasers send the atoms along two paths that are like the arms of a light interferometer. When the paths merge, the interference between the atoms’ wave functions depends on the gravitational acceleration they experienced. Current techniques can measure gravity at a particular location with an uncertainty of a few parts in 109 .
One way to improve on atom-based gravimeters is to squeeze the atoms together until their wave functions begin to overlap, resulting in a so-called Bose-Einstein condensate (BEC). The main advantage of using a BEC is that its diameter in the typical experiment is between 10 and 100 micrometers, which is about 100 times smaller than the usual non-BEC cloud of cold atoms. The smaller size means that the BEC is less affected by the nonuniform cross section of laser pulses used to manipulate the atoms.
What is to be done? The next president will face a quandary often called the “Thucydides Trap.” This concept was popularized by the Harvard political scientist Graham Allison. Its premise is that through the 2,500 years since the Peloponnesian warfare that Thucydides chronicled, rising powers (like Athens then, or China now) and incumbent powers (like Sparta, or the United States) have usually ended up in a fight to the death, mainly because each cannot help playing on the worst fears of the other. “When a rising power is threatening to displace a ruling power, standard crises that would otherwise be contained, like the assassination of an archduke in 1914, can initiate a cascade of reactions that, in turn, produce outcomes none of the parties would otherwise have chosen,” Allison wrote in an essay for TheAtlantic.com last year.
No sane American leader would choose confrontation with China. The next president has no rational choice but to keep trying to make the best of this relationship. The two countries’ cooperation on climate and energy is the main thing that gives the rest of the world even faint hope of progress. U.S.–Chinese collaboration and compromise were essential to reaching the Paris accord on greenhouse gases last year, and the equally important Kigali agreement to ban the very damaging HFC (hydrofluorocarbon) refrigerant chemicals in October. Without China’s support (and Russia’s), the deal to control Iran’s nuclear program would not have been struck.