We human beings are curious by nature. Since the time we first began gathering around campfires to ward off the terrors of the night, some questions have haunted us like stubborn ghosts.
Many of these great unknowns have fallen under the weight of passing millennia and the advance of technology. We moderns now know why the ground shakes in an earthquake and why the sky rumbles in a thunderstorm.
But there are other questions that penetrate to the core of not just what we know, but what we can know. When confronting these great questions we are forced to the limits of our ingenuity, where we must rethink both the nature of question and the nature of its possible answers.
"Does the fridge light turn off when the door closes?" is, however, not one of those questions.
Sheesh. The light goes out. OK? It does.
Unless, of course your fridge is broken. But how can you tell if your fridge is broken? Hmmm ...
OK, so even if the fridge thing isn't one of life's great questions, maybe it does point to one: What is the nature of the unseen? And that question, as we will see, inevitably takes us straight to the dark side of reality — i.e., dark matter.
From ghosts to gods, human beings have invested enormous effort trying to understand the invisible. When Roman priests pored over the entrails of their sacrificed goats, they were looking for traces of the unseen. In particular, their divinations were designed to find messages from their gods' invisible world in the very visible (and smelly) world of goat gore. The results, we can only suppose, were not much better than just guessing.
Remarkably, science has learned to play this same game, too, but with much greater success. When was the last time you bumped into a radio wave? No one had "seen" radio waves until 1887, when Heinrich Hertz discovered that an oscillating electric current driven through a wire on one end of the lab could excite another current in another wire set up on the other side of the lab. The unseen world of radio waves was, on that day in 1887, suddenly apprehended.
Science is often faced with questions about domains of reality cut off from perception. In some cases, the unseen exists for the simple reason that we've not been around to see it. We've built a fairly a high resolution account of Earth's 4.5 billion-year history, even though we've only been here to watch about 1 10-millionth of it. In other cases, the unseen world exists owing to limits inherent in our five senses. In all cases, however, we've learned to use what we can measure in this world to infer what exists in the invisible world.
Which brings us back to your refrigerator.
How could you tell if the switchy-light-thing in your fridge was broken without climbing into it? Since you should never ever ever climb into a fridge and close the door, for all intents and purposes the inside of your closed icebox is cut off to your direct inspection. Luckily, science gives you ways to infer what's going inside. Using the laws of nature creatively you can, in fact, get inside that dark box and hang out with the cheese slices, the mayo and Sunday's roast potatoes.
You could, for example, check your electric meter to see if the current usage drops when the door closes and the light bulb, supposedly, switches off. You could measure the outside temperature of the fridge, which is a good, but not a perfect, thermal insulator. A sensitive-enough thermometer should pick up the heat from the light bulb if it is on. You could even weigh your fridge. Einstein showed us that energy and mass are the same thing, so the open fridge with light on should weigh a tiny bit more than the closed fridge with the light off.
Any one of these measurements could tell you if the light was really switching off when the door closed. If you did all three and got the "lights off" result each time, then you'd really be convinced. And if a calculation beforehand gave you an exact prediction for the current decrease, the temperature rise or weight loss that would occur when the light clicked off — and your measurements gave exactly those values — then you'd really feel like you'd nailed it.
So, while a direct view of the closed-door world inside your fridge might be forever out of bounds, from all this work you'd begin to feel pretty intimate with what goes on in there. It would be a world of darkness that was, in your mind at least, becoming brightly lit.
That's why dark matter is a lot like the light inside your closed fridge.
It's been more than 60 years since the first evidence for dark matter appeared. That evidence did not come in the direct trace of a particle in a cloud chamber. Instead, it came through the behavior of stuff we already knew about — stuff in the visible world — pointing beyond itself to the presence of something else that was invisible.
First, there were spiral galaxies rotating so fast they should have spun apart if only the gravity of the visible stars were holding everything together. There had to be more mass — invisible mass — present, adding to the gravitational census and keeping the galaxies together as they spun like mad. Then there were the vast clusters of galaxies buzzing around a common center like bees in a swarm. The individual galaxies were moving far too fast through the swarm to be accounted for by the matter we can see. The conclusion, once again, was that there must be matter we can't see tugging the visible galaxies up to their high speeds, while also holding the whole cluster together for eons upon eons.
The spin of individual galaxies and the motion of galaxy clusters — these are just two separate lines of evidence that, like our imagined experiments on the closed refrigerator, move us from what is seen to what lies beyond "sight." There are now many other lines of evidence for dark matter. There are, now, many converging experiments all pointing to the remarkable fact that at least 70 percent of the stuff making up the cosmos is invisible to us. In this sense, we've used science's remarkable capacity to carry us past our senses to see that, for the most part, it's the ghosts that have won. The unseen world is, pretty much, the entire world.
Think about that the next time the refrigerator closes.