Quantum Mechanics may be the ultimate mindfuck. Einstein was no fan -- he famously wrote "God does not throw dice," referring to his distaste for the statistical formulation underlying quantum theory. Despite his misgivings, quantum physics has proven effective at describing all manner of natural phenomena since its formulation in the early 20th century. That said, the formalism does lead to some rather odd or paradoxical conclusions.
Not least among these is the notion of Quantum Entanglement -- that observable qualities of systems may be statistically correlated, even though the systems may be separated by vast spatial distances. An example is a pair of entangled electrons -- one will have spin up, one spin down, but the individual states are indeterminate until a measurement is made. However, once the spin of one electron is measured, the spin of the other will become determinate instantaneously, and more importantly, superluminally. Einstein labeled this theoretical phenomenon "Spukhafte Fernwirkung" -- "spooky action at a distance."
These graffiti, placed on opposite ends of a bridge, embody the notion of entangled states, each with Einstein's appellation and a measured spin wavefunction -- up for Alice and down for Bob, whose wavefunction has collapsed once Alice makes her measurement.
For more on entanglement and Einstein's view, I also suggest reading about the EPR Paradox.