Have you ever gazed into the frothy remains of a bottle of beer and pondered the fundamental nature of spacetime? (Of course you have. Who hasn't?)
It turns out, that frothy cap on your brew might actually offer a fair analogy for what reality looks like on the tiniest of scales, if it was possible to magnify spacetime down as far as possible. Spacetime, according to some of our best theories, is not smooth. It's foamy. And if you had a microscope that was powerful enough to see down to the most basic quantum levels, what you'd see is quantum foam.
The idea of quantum foam arises out of Einstein's idea that gravity is caused by the warping and curving of spacetime. This conception implies that spacetime is a real, physical entity that is dynamic, and if so, then it ought to also be subject to quantum physics. In other words, the idea of quantum foam is what we get when we apply quantum physics to the fabric of spacetime itself.
Think of it like flying over the ocean. Looking out through the window of a plane from above cloud level, the ocean will probably look like a smooth, structureless blue surface. However, if the plane begins to descend, eventually you'll be able to see that the ocean is actually wavy. As you get even lower, it might start to look choppy with whitecaps. And at lower levels still, you might even make out the foamy bubbles that are generated by the swashing of the ocean waves.
To see the foam of spacetime, however, you need to magnify it to impossible levels, all the way down to the Planck length, a measurement that equals 1.616229(38)×10−35 meters. Just how small is that? Well, humans are closer in relative size to the size of the observable universe than they are to the size of the Planck length. In other words, when compared to the scale of a human body, the Planck length is smaller than the observable universe is large.
Something this small will probably never be possible to observe, so quantum foam exists only in the minds of theorists for now. But some experiments have been performed that seem to corroborate the idea. For instance, scientists have measured that photons arriving at Earth from distant stellar explosions seem to arrive at different times depending on their energy level. Since the speed of light is supposed to be constant, something must have interrupted the path of these particles. Could it have been quantum foam?
These experiments need to be replicated before any conclusions can be drawn, but they at least demonstrate that the idea of quantum foam can potentially be testable, even if we can't directly observe it.
So perhaps we're all awash in a wavy, rippling, undulating, foamy spacetime sea. Like ocean lather, like spit from the mouth of God. Or maybe, not. Either way, it's certainly something worth pondering over a sudsy pint.