Gravity

Elliot has a beautiful set of brightly colored blocks, given to him on his birthday by Emily and Gerald.  We were playing with these recently, and he picked up a small cardboard box that had previously held a pair of earrings.  Together we discovered that one of the colored blocks fit perfectly inside the small cardboard box.  The fit was so tight that it was impossible to pull the block out with your fingers. 

So I showed Elliot how to tip over the box and slide the block out.  He was fascinated.  I repeated the motion: put the block inside the box, rotatate your wrist until it’s upside down and the block comes out.  Then I gave it to him.  He did just as I had, turning it upside down to dump the block out. 

Yay!  I thought.  He understands gravity.   

I reset the device: took the box from him, put the block in it, and gave it back.  He tipped it again, but not quite far enough to let the block slide out.  He waited there, watching the opening of the box intently.  Then he tipped the box upright and stared at the opening, mystified.  Again he turned the box to the side, again not far enough to slide the block out.  Then he got bored and crawled over to play his mini-piano. 

I realized I had misinterpreted his actions.  I thought when he flipped the box upside down, he understood the principles at work.  But to Elliot, there was no larger principle, just a simple rule:  flip box and block will emerge.  As far as he knew, the rule applied only to this box and this block.  A larger theory of gravity was not implicated here. 

Which got me thinking about this notion of gravity.  It seems so fundamental, so deeply woven into our understanding of the universe, that there could never have been a time when we didn’t understand it.  Like saying that there was a time we didn’t know how to breathe, or poop. 

But watch a baby in a high chair, throwing objects over the edge and then peering down after them.  What are they doing but experimenting with gravity?  They're saying, Does it work every time I throw it?  Does it work with every object?  Does anything ever go up?

I tried to do some research into when babies understand gravity.  The academic literature is in flux:  conventional understanding is around a year.  But some researchers argue that humans are born with an understanding of physics, and they have some studies suggesting we understand gravity at 4 months. 

To me, there seems to be a fairly obvious way to tell.  A baby understands gravity when he stops chucking spoons off his highchair and watching them fall.