Until the End of Time: Mind, Matter, and Our Search for Meaning in an Evolving Universe

by Brian Greene

You’ve never clutched hold of your morning coffee and found it hotter than when you poured it. That’s because heat flows only from higher temperature to lower temperature, and so your hot coffee transfers some of its heat to the cooler environment, causing the coffee’s temperature to decrease.12 For our large cloud of gas, heat also flows from the hot central core to the cooler surrounding shell. Now, I can’t fault you for thinking that this flow of heat will cool the core and warm the shell, bringing their temperatures closer together, much as heat transferred from your coffee to the air brings your hot mug closer to room temperature. But—and this is remarkable and remarkably important—when gravity is directing the show, the conclusion is reversed. As heat flows out from the core, the core gets hotter and the shell gets cooler. This is surely counterintuitive, but understanding it is just a matter of connecting dots we’ve already marked. As the surrounding shell absorbs heat from the core, the additional energy drives the cloud to swell even further. The outward moving molecules, once again, strain against the inward pull of gravity, and hence are slowed even further.13 The net effect is that the temperature of the expanding shell goes down, not up. Conversely, as the core relinquishes heat, the decrease in energy causes it to contract even further. The inward-moving molecules, flowing in the same direction as the inward pull of gravity, pick up speed as they fall, and so...