We live in a world of shadows. We live amidst unseen forces that influence the universe even as we are blind to their presence. In other words, we live amidst ghosts.
Dark matter has not been found ... again.
Last week an international collaboration of scientists working with an ultrasensitive particle detector a mile underground announced that, after a few months of watching, not a single dark matter particle had been discovered. So, was this a major setback, as some news media reported? What exactly was the real importance of this result?
More importantly, what does it tell us about how we "moderns" have come to live so comfortably with so many invisible and dark entities?
Physicists over the last few decades have become convinced of dark matter's existence due to gravitational effects. They can see luminous matter (our kind of stuff) moving at high speeds, but often can't find enough other luminous matter around to account for the gravity that sets things in motion. From the absence of visible mass driving those motions, astronomers infer the existence of some form of invisible (dark) material.
The LUX experiment was designed to find one very specific candidate for dark matter. To understand this you have to begin with the fact that here are only four known forces in the universe: gravity, electromagnetism, the strong nuclear force and the weak nuclear force. That means the best bet for dark matter are so-called "weakly interacting massive particles" (WIMPs). WIMPs can only push or pull other kinds of mass via gravity and the weak force. Since they are immune to the electromagnetic force, they can't emit light and are therefore invisible to astronomers' telescopes.
If WIMPs constitute dark matter then a tenuous wind of these particles should constantly be streaming through the Earth (and me and you) right now. The supersensitive LUX experiment was hoping to catch the rare collision between a passing WIMP and an atom of regular matter in the form of Xenon (LUX stands for Large Underground Xenon experiment).
But it never happened.
After a few months of running their experiment, the LUX team announced that it had come up empty-handed. That is what drove the news stories about "setbacks."
But if the headlines implied physicists were reassessing their belief in dark matter, they were wrong.
All the LUX results tell about is the stuff dark matter can NOT be made of. In particular, all they tell us is that WIMPs with a certain range of masses have now been ruled out. There are other possible kinds of WIMPs and there are other possible kinds of dark matter particles that are not WIMPs.
These days there are many, many reasons why physicists and astronomers believe some form of dark matter must exist. From our basic understanding of cosmology to precise measurements made of the distortions of space-time (gravitational lenses), dark matter pops up all over the place in modern astrophysics. Given its importance, it would take a lot more than a single experiment, eliminating a single potential candidate, for astrophysicists to throw up their hands and give up on dark matter.
Which brings us back to ghosts (of a sort).
When was the last time you saw a radio wave? For hundreds of thousands of years, human beings wandered the Earth and were blind to the presence of radio waves, infrared light, X-rays and gamma rays. Then, a century and half ago, physicists recognized that we move through a sea of electromagnetic radiation, with visible light being the only directly perceived version of these waves. Fast forward to 2013 and those unseen electromagnetic entities shape our world in ways so countless we could hardly imagine our world without them (think Wi-Fi, cellphones, microwave ovens and, of course, good old car radios).
And what about the curvature of the four-dimensional-space-time-continuum that lies at the heart of Einstein's relativity? You don't hold it in your hands and you can't taste it on your tongue. And yet, every time you call on the GPS capacities of your smartphone, you rely on the existence of space-time and its "invisible" curvature.
We are awash in the unseen and science has made us aware of the flood.
While revealing hidden realities is one core promise of science, as a culture we have only become comfortable with calling these "things" real because so many of them are implicated in our technologies. It's our devices that make so much of the unseen manifest.
But we should never forget that each and every unseen truth — from electromagnetic waves to atoms themselves — began as a proposed entity. They began as invisible solutions to very visible problems. And while some proposed solutions actually fail in the long run (for example, the never-found luminiferous aether) enough of these ghosts have become "real" for us to understand why physicists are ready to stand by their dark universe.
For now ...