S. James Gates Jr., a theoretical physicist at the University of Maryland at College Park, explores the bizarre physical reality that lies behind our everyday experience of the world as a featured guest on the four-part NOVA series “The Fabric of the Cosmos,” airing this month on PBS. The first episode, broadcast last week, probed the nature of space itself, which Gates calls “one of the deepest mysteries in physics.”

Brian Vastag

Why is space such a mystery? Isn’t it just empty nothingness?

As strange as it may seem, theoretical physicists have an enormous problem understanding space. Let me go back to the Greeks. They had this notion that space was a stage. And this picture of an empty stage seemed to work pretty well until Uncle Albert — that’s Einstein — got hold of it. In 1916, Einstein said space is kind of like a mattress. It’s really like a box spring mattress. You can jump up and down on it and create ripples. So it wasn’t just an empty stage.

In “The Fabric of the Cosmos,” you call space squishy. What does that mean?

You can think of space as a flexible sheet that is all around us. Or think of it this way: If you stuck a spoon in a jar of syrup and began to twist the spoon, some of the syrup will move along with spoon. That’s what gravity does to space. That’s why the Earth bends the space around it. We saw this with a satellite called Gravity-B, which launched in 2004. It actually measured the dragging of space around the Earth. That was the first time we actually saw it.

So space can bend and drag; but is it really empty?

Don’t say there’s nothing there to a physicist. Space has a seething quantum structure to it. I like to think of it as a pot of water on the stove with bubbles coming out. Space is like that. It’s always bubbling. We could see it if we had a powerful enough microscope.

What exactly is bubbling up out of space?

It’s matter and energy. It bubbles up in a totally random and spontaneous way.

Theory predicts this, but can we measure it?

Yes, we can, through something called the Casimir effect. If you take two sheets of metal and separate them by the thickness of a sheet of paper, what happens is this bubbling goes on in the space around the plates. You have bits of matter and energy coming into existence and disappearing. But the short distance between the plates restricts this bubbling. It’s like having a pressure differential, so you can measure the force. It’s not just play.

The Nobel Prize in physics this year was awarded for the discovery in 1998 of another weird aspect of space, dark energy. Dark energy pushes space apart at an accelerating pace and accounts for some 73 percent of everything in the universe. Had any physicist predicted this?

No one had predicted it. As a community, physicists view our job as checking out every possibility about how reality can work. But this one was a surprise. And so our understanding of dark energy is absolutely in its infancy. I have yet to see a really good explanation for why dark energy occurs. The real mystery is why is it the size it is. Our theories about how big dark energy should be are wildly off from the observations. They’re off by a factor of multiplying 10 by itself 122 times. I call this the all-time worst error in physics.