“You still don’t get it, do you? He’ll find her! That’s what he does! That’s ALL he does! You can’t stop him!” -Kyle Reese, the Terminator
Now that we’ve all survived Judgment Day, we can stop looking for ways to stop the Terminators, and go back to the search for dark matter. Let’s back up a bit, though, and take a look at normal matter first.
As you well know, normal matter here on Earth is always subjected to the force of gravity. You throw something up into the air, and the Earth’s gravity always works to pull it back down. In general, something moving under only the influence of the Earth’s gravity will make a parabolic shape, like the motorcycle above.
But is that really what’s going on? After all, if I launched this motorcycle fast enough, it wouldn’t make a parabola at all. Perhaps, if I even gave it a big enough starting speed, I could put it into orbit around the Earth!
And this, of course, is true. It’s basically what we do to get satellites, shuttles and space telescopes into orbit around the Earth. And — just like the Moon — they obey the laws of gravity too, and orbit about the Earth in an elliptical shape.
But what about those trajectories that don’t make it off of Earth? Are they somehow, fundamentally, any different? The (perhaps surprising) answer is no! What you see as a parabola is really just one small part of a giant ellipse, centered on the Earth!
Of course, if you’re made out of normal matter, you don’t complete the ellipse: the Earth is in the way! And normal matter like you, as you may have noticed, doesn’t pass through the Earth so easily. It doesn’t pass through any normal matter easily, in fact, as you may have noticed. If you take your thumb and press down on any normal matter, like in this fingerpaint turkey below,
you’ll find that there’s something stopping your finger from simply passing through to the other side. There is, of course, a very fundamental physics reason behind this. If I go all the way down to the atomic level, I find, for any type of solid matter, the following type of structure.
In other words, matter is made up of atoms. And you know what happens when you bring two different things together: the atoms push back against one another, and don’t allow you to simply pass one through the other. Practically, atoms take up a finite amount of space. Fundamentally, your atomic nature is why you don’t fall through the Earth when you stand. Gravity would love to pull you down towards the center of the Earth, but the force of the atoms on the ground push back on the atoms on the soles of your feet, keeping you from falling.
The force behind it is electromagnetism, and along with gravity and the two types of nuclear forces, they make up the fundamental interactions of nature.
Atoms, of course, are subject to all four of these forces. Gravity, perhaps, is just the most common one that we talk about. But not all types of matter feel all four of these interactions. Take, for instance, the neutrino. We can take a neutrino and shoot it through hundreds, thousands, or even millions (if we had millions) of kilometers of solid matter, like we do all the time!
Why can we do that? Because neutrinos only interact gravitationally and through the weak force. They have no electromagnetic interactions (because they’re electrically neutral) and they have no strong nuclear interactions. In fact, the only reason we can detect them at all is because of the weak force. Despite its name, the weak force is some 1025 times stronger than gravity is. If you want to detect a neutrino, you need to build a huge tank full of stuff for neutrinos to interact with…
…and a huge number of neutrinos in order for you to have a chance to just see a few.
Now, neutrinos — as elusive as they are — can not be the dark matter we see in our Universe, because they interact too much! That’s right, whatever dark matter is, it interacts gravitationally, and less strongly in every other way than all known particles in the Universe.
Which explains why it’s so hard to find! In other words, if you had a dark matter particle that you threw up into the air (somehow), its path would look like this:
In other words, it would simply pass through the Earth, because it doesn’t interact by the strong, electromagnetic, or weak forces, as far as we can tell. Which is maddening, because we want to be able to find it!
But we make our best models for a galaxy, and what do we find?
That the dark matter exists in a huge, diffuse halo around the galaxy. Because it isn’t made out of atoms, it doesn’t “collide” in the conventional sense. It simply falls freely under the influence of gravity, and takes the plunge in these giant elliptical paths, undaunted by all the stars, planets, gas, dust, etc., in its way.
It also doesn’t collide with itself! If we were to take two of these giant dark matter halos and try to smash them into each other, know what would happen?
They’d simply pass through one another. Which is great for astrophysics, because it aligns with what we see in an entirely consistent way. But which is lousy for dark matter hunters, because it makes the stuff incredibly difficult to detect directly!
And in the meantime, you realize, the Earth is orbiting the Sun, which is spinning around the galaxy, which is filled with dark matter.
Do we have a chance at detecting it? Well, yes, of course we do. All we need to do is build a detector — in principle — that’s sensitive to it. And there are a few groups looking for it in different ways. There are groups like CDMS and Xenon (shown below), which are trying to detect a collision between dark matter and normal matter directly.
These groups don’t see anything, and are placing tighter and tighter constraints on what dark matter could be. There are also groups looking for any general sort of thing, like DAMA or COGENT, and looking for differences between the times of year where the Earth is moving faster through the galaxy and slower through the galaxy.
As reported, these groups see stuff! Well, they see differences, anyway. But is what they’re seeing dark matter, or is it something different? They don’t know; at this point, either explanation works. What we do know is that direct detection experiments and attempts to create these particles using accelerators has failed so far.
It could be dark matter, of course. And if it is, all we have to do is figure out a way to create it in the lab and we’ll have solved our great dark matter mystery! But if this “annual modulation signal” is due to anything else, such as an asymmetry in emitted particles from the Sun, a modulation in zodiacal background, or anything else that involves “normal” matter, then we are fooling ourselves by looking for dark matter in this fashion.
It’s very exciting for the teams that are seeing something, but they’ve got a long way to go to demonstrate that it’s actually dark matter, rather than something much more mundane. Here’s hoping, though!