When we gaze up into the night sky, we look out into the past. Adam Frank makes this point eloquently in a recent post. And it is a point redolent with consequence in the field of physics. It is the starting point of Einstein's special theory of relativity.
But is it right to suggest, as Adam does, that when I look into the face of my loved one across the table from me, what I see, really, is how she looked a tiny fraction of a second earlier? Adam writes:
When you look at a mountain peak 30 kilometers away you see it not as it exists now but as it existed a 1/10,000 of a second ago. The light fixture three meters above your head is seen not as it exists now but as it was a hundred millionth of a second ago. Gazing into your child's eyes you see her not for who they are now but for who they were a 10-billionth of a second in the past.
Should we think of seeing here in the sublunary sphere on the model of our visions of the solar system in the night sky?
I think not. Here's why.
Thinkers used to worry a lot about the fact that retinal images are upside down. Why don't we see the world upside down? By far the most common proposal — it was Leonardo's — has been that somewhere inside of us, in the brain, presumably, the image is switched around. We experience the switched around image, not the retinal image. Seeing happens, really, only when we inspect this internal image.
But this raises a puzzle. If we suppose that something inside us, the mind's eye, sees the internal image, then don't we need to explain how this seeing happens? And doesn't that put us right back where we started? Don't we face an infinite regress? (This has been called the homunculus fallacy, because it seems to require the existence of little minds inside us to do our cognitive work for us.)
The way out of the puzzle — this was fully appreciated already by Descartes — comes when we ask: The retinal image is upside down relative to what? It is only if we think of the image as a picture, as something to be looked at, that it makes sense to say that it puts things upside down.
The brain isn't in the business of making pictures for us to inspect, pictures which it then presents to consciousness in some special place in the soul, what Daniel Dennett called "the Cartesian Theater." The brain's job is helping us achieve and preserve contact with the world around us. As Dennett puts it, the brain's job is finding out, not filling in pictures in the mind.
Well, as with inverted retinal images and spatial orientation, so with time. Who's to say that the when of our experience of what's going on around us is the same as the when of neural events? As if someone inside notes the arrival of a signal from outside and says, Now! Human events don't elapse at the time scales of light's passage, or even at the much slower time scale of events in the nervous system. Think of a melody. You hear it, now, but what you hear, now, includes a contour stretching out of the past into the future. A whole melody can be present now.
And the time it takes for light-caused neural events to propagate within us is not the time it takes for us to receive the message. We are not hidden inside us. We are are not homunculi trapped inside the body.
Let us take the idea that the brain's job is helping us stay in touch with the world around us, in contact, seriously. Then we won't think of light as a signal from the object but as something more like the medium in which we, and the world around us, transact. (This was J.J. Gibson's idea. Gibson is the most original scientist to think about seeing at least since Hermann von Helmholtz.)
What I see, here and now, depends as much on what I do — I move around it, I blink, I rotate the object in my hand — as it does on anything the object, thought of merely as a source of reflectance, does to my nervous system. We are coupled with the things in our midst. (It also depends on what I care about, what I know, what matters.)
Seeing is a way — it is one of the ways — we can do this, one of the ways we touch things near us and know them.
We don't have this sort of contact with that which is very far away. That is why, when it comes to the bodies we glimpse in the night sky, we are limited only to twinkles and traces of how they were. We aren't able to transact with them. They are too far away for that. We can't really see them at all. (We are like detectives working with finger prints. To examine a finger print is not to espy the perpetrator.)
Things around us, the things we know, are not merely remote sources of signals producing neural happenings inside us. Seeing is not the reception of a signal. It is more like dancing.
In a way the point — this was Gibson's main idea — is that we can't make sense of vision and the role of light in vision so long as we confine ourselves to light as the physicist thinks of it. We need what Gibson called an "ecological" conception. We need a science not of what goes on inside of us. We need a science of what we do.