In quantum mechanics, you can put an object into a superposition of states for "here" and "there", but we never seem to see those. And we say that there is decoherence, but what causes it? This has been a topic of much debate since the beginning of quantum mechanics. Now, Igor Pikovksi of the University of Vienna and colleagues offer an idea based on time dilation due to general relativity. The amplitudes for "here" and "there" will, in general, evolve in time at different rates, due the local variations in the gravitational field. Put in the numbers close to Earth, and perhaps surprisingly, the effect is not small, and varies with the square root of the number of particles in an object and inversely with both the temperature and the distance between "here" and "there". For a gram-scale object, and a distance of 1 μm vertically, one gets about a millisecond. The idea could be tested in the near-zero-gravity environment of space.