Indulge me with a thought experiment:
I have two identical cups. I fill one with loosely packed sand, and another with loosely packed gravel. I then start pouring water into both cups at the same rate. Which cup will fill first (in other words, does sand or gravel have more space for the water to fit in)?
We’ll get back to this!
In my Honours year, I took an elective called Scientific Methods, in which we had some interesting discussions about the nature and philosophy of science, science publishing and science communication. I had to produce a poster ‘about science’ for the course, and read The Unnatural Nature of Science, by Lewis Wolpert, as inspiration. In the book, Wolpert explains the distinction between ‘common sense’ and science, in that science often leads to conclusions about reality which seem counterintuitive. For example, scientific experimentation and observation tells us that what we see as solid objects are, at an atomic level, mostly empty space.
I keep running across examples of this distinction, so I cringe whenever I hear someone arguing an anti-scientific stance by appealing to ‘common sense’ – it might work most of the time (that’s why we evolved to have it!) but when trying to strip away the subjectivity of human experience and find out what’s really going on, it doesn’t trump a scientific approach.
One such example has recently been drummed into me, because I had to mark 60 first year Earth Sciences reports. The laboratory looked at porosity (the volume of pore spaces in a substance) and permeability (how fast water can flow through the substance) in sand and gravel. The results tended to indicate that fine sand had more pore volume than coarse sand – in other words, it takes more water to fill up a cup containing fine sand than coarse sand. Extrapolating this, it would seem that gravel has less pore space than fine sand.
Hmmm. The idea that sand has more space in it than gravel is a bit odd, right? You can SEE the gaps in gravel, but sand looks more solid, doesn’t it? Many of the students said that, despite their data indicating otherwise, that gravel has a larger pore volume than sand. It’s common sense!
Let’s reverse the situation for a moment. You have a fist-sized lump of rock, and place it in a container that fits it almost perfectly. Then, you take the rock out and smash it with a hammer. You then drop the bits loosely into the container. Are they going to fit as snugly as before? In this case, your common sense is probably telling you the right thing: no, they won’t, because all the bits will fall together in uneven ways, leaving more gaps, so it won’t go back into the original container as easily. Extrapolate this trend, and you’ll find it’s the opposite of the first example! However, this time, we’ve got it right.
If the particles have similar shapes, smaller particles will tend to have more space between them, when loosely packed, than larger ones. That’s the idea we were trying to convey in the Earth Sciences lab, but common sense, or intuition, overwhelmed many student’s confidence in the data they’d just collected, so they went with their gut feeling and got it wrong. The moral of this story? Natural phenomena can be tricky to understand, and common sense and intuition are no substitute for experiment and data!