Almost nothing is more obvious than the fact that time flows from the past, which we remember, toward the future, which we don’t. Scientists and philosophers call this the psychological arrow of time.
Hot coffee left on your desk cools down, and never heats up on its own, which reflects the thermodynamic arrow of time.
The principles of thermodynamics show that large collections of particles, like the trillions upon trillions of liquid molecules in a coffee cup, always move toward more disorganized arrangements. For instance, hot water molecules clumped together in a cold room need a lot of organization, so warm drinks eventually cool to the surrounding temperature. Physicists say such disorganized arrangements have high entropy, whereas ordered arrangements have low entropy.
Yet the equations physicists use to describe the simultaneous motions of large numbers of particles are equally valid whether time runs forward or backward. Therefore, almost any complex arrangement of matter will gain entropy no matter which direction time flows.
In the past century, however, physicists and philosophers have begun trying to unite the thermodynamic and psychological arrows. Many researchers note that real-world objects that store memories -- such as human brains and computer hard drives -- often heat up as they operate. Heat generation increases entropy and is an irreversible process, so the laws of thermodynamics require that such objects can only run in one direction: from past to future.
But memories don’t have to generate heat, point out Brun and physicist Leonard Mlodinow of the California Institute of Technology. For example, ripples on a pond record a rock falling into the water, and yet could, in principle, travel in reverse. Could such a memory remember the future instead of the past, the researchers wondered?