If you love old-fashioned pendulum clocks, you’ve probably noticed that ghostly phenomenon where two pendulums will eventually synchronize with one another. A study reveals that it may be that the pendulums are simply transferring energy to one another through sound pulses, The Guardian reports. So, not a ghost. Researchers created a complex mathematical model, and it turns out that the phenomenon closely matches their theory. This model could help scientists understand other systems demonstrating the same phenomenon such as the synchronized flashing of fireflies.