What exactly is nothing?

Last month I got to visit three middle schools in Baltimore, New York and Pittsburg. I talked to almost 2,000 students, and they asked dozens of questions. One of the most interesting questions came from two girls in Pittsburgh who wanted to know, "What is nothing?"

It's a great question, because we can answer it on three different levels.

Let's say you walk into a room and there is "nothing in it" -- no objects of any kind, no furniture and no people. It's just four walls, a ceiling and a floor.

Even though we think of the room as empty, what this room contains is air. Floating around the room are an unbelievable number of atoms and molecules. The air in the room contains nitrogen, oxygen, carbon dioxide, water vapor and all sorts of other chemicals. But we cannot see all these atoms because they are transparent. So we think of the room as full of nothing, even though it is full of atoms.

To get to a real form of "nothing", we need to go into outer space. Imagine that you go to the farthest, emptiest corner of the universe. This is as close to nothing as we are ever going to get. What we are looking for is a section of space that contains zero atoms. No atoms at all -- it is a perfect vacuum. That is the best approximation of "nothing" that we have in our universe today.

The girls asked another question: "What color is nothing?"

What we think of as "color" comes from light that hits our eyes. Small units of light, called photons, have to leave the object we are looking at in order for our eyes to see a color. Photons can either be produced by something, like a light bulb, or they can bounce off of something and get reflected into our eyes. Those photons are what our eyes "see."

Since "nothing" contains zero atoms, there is nothing in "nothing" that can produce photons, or reflect them - so there are zero photons. Our eyes see zero photons as black. So the color of "nothing" is black.

But here's a deeper question: Is a section of space that contains zero atoms really "nothing"?

Not really. Space, even if there are no atoms in it, is "something." For example, photons can move through space even if the space contains zero atoms. So can gravity. So can radio waves. So can a magnet's field. And we can measure space -- a chunk of space has a length, a width and a height. And time elapses. In other words, empty space is a measurable framework that has the ability to transmit certain types of energy.

"True nothing" would be truly nothing -- no space. This is hard to get a grasp on, because we cannot imagine this kind of nothing. We have never seen it. It is, presumably, what existed before the universe existed. Apparently, at the creation of the universe, there was truly nothing. Space, with its ability to transmit different types of energy, was created when the universe was created. Then energy in this space condensed into matter -- the atoms that we find all around us today.

"True nothing" is that immeasurable, zero-energy, non-existent thing that did not exist before the universe, and all the space in it, came into existence.

Who knows what that was like?