I’m fascinated by how the humblest observations can lead you to the profoundest conclusions, and here’s one I learned from Thanu Padmanabhan, an eminent theoretical physicist at the Inter-University Centre for Astronomy and Astrophysics in Pune, India. Suppose you cup your hands around a glass of cold water to warm it up. From this simple act, you can deduce that the water consists of molecules.
The heat energy you apply has to go somewhere and, as 19th-century physicists such as Ludwig Boltzmann showed, it goes into the motion of water molecules. You can even count those molecules. If you dump in a lot of energy and the temperature barely budges, that energy must be spread out over a lot of molecules; if the temperature shoot up, that energy must be spread out over fewer molecules. If water were a true continuum—having an infinite number of molecules—the temperature would stay stuck at absolute zero. Such a system could never come into equilibrium with its surroundings; it would suck in energy without limit. “The fact that such a fluid can store and exchange heat energy cannot be understood within the continuum theory,” Padmanabhan has written.
As he explains, you can apply the same thermal reasoning to spacetime. It, too, has a temperature. It can heat up and cool down. After all, what makes heat heat, rather than motion, is ignorance. You can’t follow the details of molecules and see only their aggregate effect. Much the same happens when you become ignorant of processes in space. Suppose you’re stationary or moving at a constant velocity. The vacuum of space looks empty, meaning that vibrations in fields, such as the electromagnetic field, cancel one another out. But if you accelerate, you recede from certain regions of space (those way in back of you) too fast for light ever to catch up. You see a horizon around you, marking the farthest you can see, and that leaves you unavoidably ignorant about those field vibrations. To you, the supposed vacuum looks like a hot gas, throbbing with particles at a certain temperature.
These thermal phenomena imply that spacetime, like the glass of water, consists of some sort of “molecules” or “atoms.” Padmanabhan runs with this idea to develop a quantum theory of gravity. Einstein attributed gravity to the structure of spacetime, so, if spacetime is atomic, it stands to reason that gravity must reflect the behavior of those atoms. Following up pioneering work in the mid-1990s by Ted Jacobson (whom I featured in an earlier video), Padmanabhan has sought to derive Einstein’s equations of gravity from spacetime’s thermal properties. In fact, he puts forward an even more general theory of gravitation that accounts for the accelerating expansion of the universe; predicts that the atoms of spacetime, like molecules of water, can rearrange themselves into multiple phases; and might even explain some of the mysteries of quantum mechanics.