Light, in all its forms, is such a basic part of life for most of us that it’s sometimes easy to take it for granted. Light from the sun is what powers the Earth, from its geological evolution over vast timescales to its weather, the energy we rely on, and ultimately, life itself. Light has physically and metaphorically guided our planetary and evolutionary history for billions of years. And it continues to illuminate our future.

Walk into a darkened room and flick the light switch and, in most cases, what was invisible immediately becomes visible. And with it, different possible futures are revealed. These may be as mundane as avoiding stubbing a toe on a protruding piece of furniture, or as profound as—quite literally—being enlightened by what you can now see.

Light reveals a pathway between where we are now and where we’re heading. It enables us to develop new knowledge as it illuminates the world around us. It allows us to explore how the past influences the future by observing the relationships between cause and effect. It even provides the illumination for many scholars to capture their ideas in writing, and for their students to read and benefit from these—even if that illumination is sometimes the light of a computer screen. More than this, though, light infuses our thinking about what is coming next, as we talk about “seeing” into the future, or “envisioning” it.

Yet, even before humans were on the map in the cosmic scheme of things, light was playing the role of arbiter between past and future.

As the initial maelstrom of the big bang settled down into something approaching the normality we’re now familiar with, the cosmos was flooded with the fundamental particles that act as the building blocks that make up the universe and the glue that keeps it together. We’re perhaps most familiar with those particles that represent visible light—the photons that are emitted from fires, light bulbs, computer screens, and, of course, the sun. But these represent just a small slice of the spectrum that scientists think of as “light.” This spectrum extends all the way from intense, destructive gamma rays to long, lazy radio waves, with visible light sandwiched into a narrow band somewhere in between.

All of these forms of light form connections between the past and the future. This is perhaps most famously seen in Einstein’s theory of relativity, which depends on the speed of light in a vacuum remaining the same, wherever you are and whatever you’re doing. Because light travels at a finite speed, we’re still, quite remarkably, receiving signals from the very earliest moments of the universe. Incredibly, we can actually detect the afterglow of the big bang in the form of cosmic microwaves that have taken nearly fourteen billion years to reach us. These signals from the universe’s past are deeply revealing of where we come from on a cosmic scale, and they help us better understand where we’re ultimately heading.

But there’s an aspect of light that’s even more fundamental to our understanding of the future.

Light is emitted when charged particles oscillate back and forth. This is how transmitters emit radio waves. It’s also why atoms emit light as the negatively charged electrons in them move between energy states.

This connection between the electrons in atoms and light turns out to be deeply relevant to the passage of time between past and future. For every oscillation, every turn of the atom-electron spinning top, emitted light waves slice like a metaphorical knife between what has just been and what’s to come. Without light, there is no past and no future. And without past and future, there is no light.

Fittingly, we actually measure time using the frequency of light emitted by oscillating electrons. A single second is defined as the time it takes for nine billion, one hundred and ninety-two million, six hundred and thirty-one thousand and seven hundred and seventy oscillations of an electron transitioning between two energy orbits in a cesium atom. It’s a frequency that is, sadly, too slow to be seen as visible light. But it can be picked up by a high-frequency radio receiver. And thus, light becomes the metronome that keeps time as the past transitions to the future.

But as light ticks the seconds away, it reveals yet another important aspect of the transition from past to future: movement.