Friday, June 15, 2018

Are Space And Time Quantized?


Forbes |  Throughout the history of science, one of the prime goals of making sense of the Universe has been to discover what's fundamental. Many of the things we observe and interact with in the modern, macroscopic world are composed of, and can be derived from, smaller particles and the underlying laws that govern them. The idea that everything is made of elements dates back thousands of years, and has taken us from alchemy to chemistry to atoms to subatomic particles to the Standard Model, including the radical concept of a quantum Universe.

But even though there's very good evidence that all of the fundamental entities in the Universe are quantum at some level, that doesn't mean that everything is both discrete and quantized. So long as we still don't fully understand gravity at a quantum level, space and time might still be continuous at a fundamental level. Here's what we know so far.

Quantum mechanics is the idea that, if you go down to a small enough scale, everything that contains energy, whether it's massive (like an electron) or massless (like a photon), can be broken down into individual quanta. You can think of these quanta as energy packets, which sometimes behave as particles and other times behave as waves, depending on what they interact with.

Everything in nature obeys the laws of quantum physics, and our "classical" laws that apply to larger, more macroscopic systems can always (at least in theory) be derived, or emerge, from the more fundamental quantum rules. But not everything is necessarily discrete, or capable of being divided into a localized region space.


The energy level differences in Lutetium-177. Note how there are only specific, discrete energy levels that are acceptable. While the energy levels are discrete, the positions of the electrons are not.

If you have a conducting band of metal, for example, and ask "where is this electron that occupies the band," there's no discreteness there. The electron can be anywhere, continuously, within the band. A free photon can have any wavelength and energy; no discreteness there. Just because something is quantized, or fundamentally quantum in nature, doesn't mean everything about it must be discrete.

The idea that space (or space and time, since they're inextricably linked by Einstein's theories of relativity) could be quantized goes way back to Heisenberg himself. Famous for the Uncertainty Principle, which fundamentally limits how precisely we can measure certain pairs of quantities (like position and momentum), Heisenberg realized that certain quantities diverged, or went to infinity, when you tried to calculate them in quantum field theory.