Light travels fast, but not infinitely so. This simple truth has profound implications discovered only a century ago, which rocked the foundation of our understanding of ourselves and the universe from which we emerged.
Just as we hear a thunderclap a few seconds after it happens, so also do we see the lightning flash after the fact. Everything we see happened in a past whose remove depends on the distance spanned. We see one another a handful of nanoseconds ago. The Moon appears to us as it was a second and a half ago, but we see the Sun eight and a half minutes in our past. Jupiter is 43 light minutes away, and it took four and a half hours for the signals from the New Horizons probe to reach us from Pluto.
Click here to watch a simulated pulse of light travel from the Earth to Mars in real time (you will need to be patient)!
Vega, a bright star in the Summer Triangle, lies 25 light years distant and is one of our the closest bright stars. When we look at it on a June evening, we look back to the mid-1990s. Deneb, the brightest star of the Northern Cross, is almost as bright as Vega but lies 2600 light years further away. Our view of Deneb shows us the time of the Buddha. A tiny blur of light visible to the naked eye in the chest of Hercules is the globular star cluster M13, a ball of half a million stars swarming around their center of gravity. Our eyes leap 22,180 years into the past when we see it, to the time of the last Great Ice Age when mammoths walked the valleys of Wyoming. If we stay up late enough or wait till fall, we can lift our eyes to the east and survey the Great Galaxy in Andromeda, more than 2.5 million years ago. With our naked eyes we can peer into a past before humans walked the Earth and before the Yellowstone Caldera came into being.
As Einstein surmised more than a century ago, traveling faster and faster makes clocks run slower and slower. This has been borne out to the last decimal place by astronauts and GPS satellites. The only thing that can travel at the speed of light is light itself, which can travel at no other speed and can never stand still.
The time dilation effect of Special Relativity means that time stands still for light itself! The time elapsed for a photon reflected from your eyes into mine is precisely zero. Similarly, from the point of view of the photon it take light exactly zero time to traverse the distance from the Moon to Earth, or from the Andromeda Galaxy to my camera. Light connects distant objects and observers simultaneously! Only matter requires time to traverse distance.
Light and time and space itself is bent by matter. Gravity is nothing more nor less than the curvature of space. More than a century ago, in a time of kerosene lamps and candles, Einstein showed that “the curvature of space shows matter how to move, and matter shows space how to curve.”
The universe began 13.7 billion years ago as an infinitely dense and infinitely curved point containing all of matter and energy and space and time. It flashed into existence and inflated, stretching further and further and colder and colder and darker and darker. It stretches still, expanding the distance between all pairs of points as the galaxies and atoms rush apart.
At the speed of light, it took no time at all for this to happen, and the flash of light at the beginning of time is all around us, permeating our bodies and filling the empty spaces between radio and TV stations as static. The entire universe is a single, simultaneous event, connected right now at the speed of light.