I run an astronomy course for homeschooled children of elementary and middle school ages.Last night’s session wasmy lecture entitled “Distance and Time”, dealing with the rather fascinating fact that in astronomy we are always observing events in the past. Although most educated adults “understand” this fact superficially, spending a couple hours talking about the ramifications of this fact as I expand the range that we’re discussing from the Moon (at about 1.25 light seconds from Earth) to the most distant objects observed by the Hubble Space Telescope (at about 13,000,000,000 light years from Earth) creates a lot of discussion in the class.

Last night, near the very end of the lecture, the youngest member of this class – age 8 I think – asked a very, very profound question. I had driven home the point repeatedly that everything we see happened in the past – even watching me across the room, the light they were seeing had left me a few nanoseconds before they “saw” it. His question (slightly paraphrased):

If everything we see and experience happened in the past, does the Present exist?

This question amazed me on several levels. In his actual phrasing of the question, it was clear to me that this was not an accidental stumbling upon a deep question – he really did have an inkling of what he was asking. The amount of experience he attempted to integrate in that instant was a huge surprise. I am still trying to follow how his mind could have created that question at such an early age.

I had no real hope of answering him in a manner he could understand, but that didn’t stop me from trying. Here is an expansion of what I told the class, in which I tried to explore the issue of observing very short time intervals. Although this was a somewhat technical answer (that no one in the room understood, perhaps including myself at the time), it has lead me further to consider just what the concept “Present” may actually represent.

Similar to how a computer functions, the human mind has a “clock rate”. Unlike a computer, which can perform billions of elementary calculations per second, the human brain’s neurons can fire at most 500 times a second.To receive and recognize visual information may require the firing of dozens of neurons, which brings our visual “frame rate” to maybe 10-50 frames per second. (This makes some sense, since a movie shot at 15 frames a second will appear visually “jerky”, while one shot at 30 or 60 frames per second generally looks smooth). For concreteness, let’s say the mind can receive one frame in 1/50th of a second. That means that anything happening in less than 1/50th of a second will be experienced as simultaneous.

Light travels at 186,000 miles per second. So, using our 1/50th second frame rate, anything that we observe within a range of about 3720 miles at a given instant in “global time” will appear to happen simultaneously. In some psychological sense then, our experience of “Now” has a range of 3720 miles.

As I said, this argument confused us all. And what I’ve written here is much clearer than what I said in the class last night – yet I’m still musing over what it means epistemologically, and what it means for our concept of Time.