By Adam J. Pearson
Image: Photons from a ring of lights reaching the eye and being seen.
Physics often reveals very surprising truths. One of these runs counter to our fundamental experience of seeing the world. We usually think that if we are looking at something, then we are seeing it as it is now. But physics tells us that this familiar intuition is false; in fact, you have never seen anything exactly as it is now in your entire life. Every time you look at something, you are seeing it as it looked in its recent past, not in its present state.
What accounts for this staggering fact? It all comes down to the speed at which light travels. When you see something, what you actually see is light reflecting off of it, which travels to your eye. The photons in the light smack into your retina and give up their energy, which activates your optical nerves and sends signals to your brain that get interpreted as visual pictures. This is the physical basis for how we see things.
But here’s the important point: light reflecting off of objects takes time to reach your eyes because it travels at a finite speed, which, in a vaccuum, works out to be 2.98 x 10 to the 8 meters per second. As a result, even when you are looking something that is only a few inches from you, the light from it takes a certain fraction of a second to reach your eye. As a result, when you see, say, a cup in front of you “now,” you are really seeing it as it was a very short while ago in the past. What you are literally seeing in “the present” is actually the very recent past of the objects you are seeing. As a result, physics tells us that we have never physically seen anything in the present moment in our entire lives.
Renowned physicist and popularizer of science Brian Greene illustrates this surprising point on page 133 of his book The Fabric of the Cosmos. He writes that
“Anything you see right now has already happened. You are not seeing the words on this page as they are now; instead, if you are looking the book a foot from your face, you are seeing them as they were a billionth of a second ago. If you look out across an average room, you are seeing things as they were some 10 billionths to 20 billionths of a second ago; if you look across the Grand Canyon, you are seeing the other side as it was about one-thousandth of a second ago; if you look at the moon, you are seeing it as it was a second and a half ago; for the sun, you see it as it was about eight minutes ago; for stars visible to the naked eye, you see them as they were from roughly a few years to 10,000 years ago” (Greene 133).
Thus, even when we look at something ‘in the present,’ we are really only seeing its past, what it looked like at the time the light left it on its superfast journey to our eyes.