Michael Doser / senior research physicist at CERN, the European particle physics laboratory, and science communicator
A limited view of the world
As humans, we are used to apprehending our environment through our senses, most prominently through our eyes, but of course, other senses as well: “seeing” through the sense of touch, building models of our surroundings through our ears, with multiple inputs using different modalities in parallel resulting in a more complete picture of the world.
Our senses are just sensors that feed information to our brain, which does the real work of combining their inputs, building models, refining them, testing them against reality. And of course, even under optimal conditions, our senses are limited: we hear neither infrasound (used by whales) nor ultrahigh frequencies (used by bats); we can’t see ultraviolet (bees can) nor infrared (snakes do), and our sense of smell is woefully inadequate in comparison to that of many mammals.
A wide range of technologies has greatly expanded what can be made visible or audible. In order to sense those realms, they must be translated into what our senses and brain can deal with, mapping the information obtained by those sensors onto our sensorium: false color thermal images that use the colors of our visual range, radio emissions of the planets or the stars that are frequency shifted into our auditive range. Here, it becomes important to clarify what we mean by “see” and what we mean by “invisible”. Detector technologies change the meanings of these two concepts: “seeing” must be taken to mean “detecting” using any technology whose signal can be mapped onto the original meaning of “seeing”, while invisible is then expanded to mean that no existing technology can detect such an object.
In some sense, this is a trivial expansion and translation: it builds on known physics and thus technology and only expands our sensorium into realms where living cells are not the optimal sensor technology. It is furthermore noteworthy that, in one way or another, all our senses (and more generally, those of all living things on this planet) rely on electromagnetic interactions: light, electric fields, magnetic fields. The other three known forces: gravity, the strong interaction, the weak interaction, don’t feature in our senses, and we are to all effects blind to them.
Seeing through not seeing, hearing through not hearing
Using light is however sufficient to indirectly “see” large invisible astro-physical objects, since extended galaxy-mass entities can act like huge lenses for light emitted by galaxies behind them. By measuring the distribution of the images formed by such a gravitational lens of a single or multiple sources of light behind it, it is possible to calculate the parameters of the invisible lens, and thus to measure the distribution and mass of that which is invisible.
This and other observational techniques show that there exist objects that exert gravitational influence, but neither emit nor absorb light, and which have been dubbed “dark matter”. Many hypotheses on what dark matter might be have been proposed, ranging from supermassive black holes down to particles so light that their quantum mechanical wavelength is equivalent to the size of a galaxy. Furthermore, the Universe contains even more invisible entities. Again from a wide range of observational inputs, it appears that the universe also contains something dubbed “dark energy”, which counteracts gravity and is responsible for an increasingly rapid expansion of the Universe.
What this means is that there are a lot of things out there that – as they do not emit or absorb light – are effectively invisible, but affect, through gravity, the motion of the light-emitting objects we can see. The vast majority of the Universe (96% to be precise) appears to be made of dark matter (21 %) and dark energy (75 %), and unlike invisible neutrinos, which at least very occasionally bump into electrons or quarks via the weak interaction, no hypothesized dark matter particle has yet been detected.
One single heavy invisible pink elephant or billions of light pink gnats?
Although a wide range of astrophysical observations thus underline its existence, dark matter’s nature is unknown. Over the last decades, there has been a broad hunt to detect them, and since there is no way to know what the quarry looks like or how strongly (if at all) it interacts electromagnetically, the hunt has focused on the two types of candidates that might have a chance of being detected using present-day technologies: on one hand, massive compact halo objects (“MACHOs”), and on the other, ultralight weakly interacting massive particles (“WIMPs”). The former are assumed to be made of normal matter, too cold to emit detectable infrared light, or to consist of a range of black holes with different masses. Naturally, not interacting with electromagnetic radiation, neither have any color (are are thus certainly not pink). After a number of years spent observing with many telescopes, and the discovery of only a handful of MACHOs, it is now clear that they are not what dark matter is.
Swimming in a dark sea
The hunt has thus focused on much lighter sub-atomic objects, with names as exotic as the quarry. WIMPS could come in form of axions, dark photons, photinos, sterile neutrinos, WIMPzillas, Winos, chameleons, squarks or even stranger ones, almost all of which do not exist. As experiments search for them and come up empty handed, the list grows only imperceptibly shorter, since their characteristics can often be adjusted, “explaining” why they have not been seen. The hunt for these certainly almost (and perhaps completely) invisible – and mostly inexistent – types of particles triggers the imagination and creativity of researchers around the world, who are driven by the possibility that we are immersed in a sea of WIMPs that pass through us without interacting except via the most tenuous link, one that it takes the utmost sensitivity to detect and make visible.
A nightmare
There is however a nightmare scenario: that dark matter interacts with ordinary matter only through gravity, that there is in effect no “portal” between ordinary and dark matter, and that dark matter can thus not be detected otherwise than via its gravitational interaction. If that were the case, all attempts at seeing dark matter, irrespective of how ingenious they are, would be doomed, would in fact remain blind to the vast amounts of dark matter that must surround us. As 80% of the mass of the Milky Way should be in form of dark matter, it would be a resounding intellectual defeat if, in spite of our best efforts, its nature could not be elucidated.
Blinders, interdisciplinarity and eclecticism
Indeed, there is a different way of being blind to what certainly exists invisibly, and that has nothing to do with the sensors, and everything to do with the processing unit that assembles, interprets and understands the sensor’s information, and that is the brain. This too is a form of blindness: the inability to step out of a narrow trench and contemplate the landscape in which it is embedded.
In the end, this is the real challenge in attempting to see the invisible: to expand on what is known, it is necessary to explore regions that have never been explored before, and for that, we need to dare to set out, with incomplete tools and with a high risk of failure, not just once, but repeatedly, and we must tolerate that most who have the courage and the imagination to set out will return empty handed. Not because they are incapable hunters, but because their quarry is elusive, because the techniques needed to hunt them have not yet been invented, because the quarry may not even exist. To set out into the unknown with hope against hope requires learning from other’s techniques, world views and expertise to chart courses that do not simply head where others have already been and have returned empty handed.
Rather than cursing the darkness, we should be trying to invent new types of light: it is with the mind that one can truly see the invisible world, one that doesn’t necessarily exist, but might.
