Glenn Randall talks about dark matter with Popular Mechanics
PhD Student Glenn Randall is interviewed by Popular Mechanics magazine to discuss Dark Matter and recent discoveries.
There are four fundamental forces of nature: gravity, electromagnetism, the weak interaction and the strong. Or so the common thinking goes. But a team of Hungarian scientists recently rocked the physics community by claiming to find a mysterious fifth fundamental force.
Glenn Randall, a physicist at Arizona State University, is working on a fascinating new experiment called DarkLight that could confirm this game-changing physics discovery. Randall walked Popular Mechanics through what this new force could be, why it could help us understand dark matter, and how DarkLight might prove it exists.
"DARK MATTER MIGHT INTERACT WITH ITSELF VIA SOME YET UNKNOWN 'DARK FORCE.'"
Popular Mechanics: What's behind the hunt for this new physical force?
Randall: One enormous issue we have in modern physics is that when we look out into our galaxy, we can tell that there's a lot more stuff out there than we're able to see. You can figure this out with some fairly simple math, the kind you might assign to a college freshman in physics. You estimate the total mass of all the stars, dust and whatever other matter you can find, and then just compare that with how fast everything is spinning around the center of our galaxy — that spin tells you the actual mass. You'll find that those numbers don't add up. Instead, the universe has about five times more matter than we can account for. We don't know what exactly this stuff is, but we refer to it as dark matter.
We know that dark matter doesn't interact with light, which is why it's invisible to us. And we know it interacts with gravity, because we can see the effect of the extra mass. Some physicists are also currently investigating if dark matter might interact with one of the other two known forces, what we call the weak nuclear force. But the jury's still out.
But there's another possibility. Some theories suggest dark matter might interact with itself via some yet unknown "dark force." A fifth fundamental force we've never documented before. It's what we're hoping to find with the DarkLight experiment, and it's possible that's what the Hungarian physicists may have found. Specifically, we're investigating a certain theory that says this hypothetical force can act kind of like a photon, but between dark matter particles.
Popular Mechanics: So are we talking about a new particle, or a new physical force?
Randall: It may sound strange, but they're actually one and the same. In physics, forces like electromagnetism can be detected as discrete packets at the smallest scale. With electromagnetism these little packets are particles called photons. They're what carries the force. For this hypothetical dark force, the particles could be something we call dark photons. According to the theory, we might expect these dark photons to act quite similarly to normal photons.
"WE COULD GRAB HOLD OF DARK MATTER FOR THE FIRST TIME."
But just to be clear: Even if we can confirm the existence of these dark photons, that's not the same thing as having found actual dark matter. At most, we'd know of a new fundamental force that affects whatever dark matter is.
Popular Mechanics: What could we do with this new force and these dark photons?
Randall: Most importantly, finding these particles could open the door to making dark matter detectable and tangible. If we indeed finally find a force that interacts with dark matter (other than gravity), we might be able to use it to somehow design new types of experiments in laboratories here on Earth. Basically, we could leverage it to grab hold of dark matter for the first time. Now, I don't want to make this sound like it'd be simple. You'd have to start finding and designing various fantastically clever ways to create these dark photons, and experiments to use them, but at least you'd know where to start.
Popular Mechanics: How exactly is your team trying to find dark photons with the DarkLight experiment?
Randall: So, this is pretty cool. We're using one of the most intense electron beams ever created by humans. With that beam we're going to send a stream of electrons through a vacuum into hydrogen gas. We'll then watch for a quite rare event with fantastically precise detectors.
A tiny fraction of the time we send electrons into this hydrogen gas, something really remarkable happens. One of our electrons will smash into the nucleus of a hydrogen atom, creating a photon. As that photon flies off toward our detectors, it can randomly decay into a pair of particles, which we then measure. By studying the momentum on that pair of particles, we can tell if anything strange or inexplicable was going on with that short-lived photon, the one we created by smashing into the hydrogen gas.
The theory says that very, very rarely, the photon we created won't actually have been a photon at all. We'll have randomly created this dark photon instead. We'd be able to tell because unlike normal photons, the theory says, the dark photon would have mass. That's what we're hoping to catch a glimpse of. What's important to know is that this experiment rests on the power of this beam. Because it can rapidly output such a staggering amount of electrons, even this excruciatingly rare event could happen over and over again.
Popular Mechanics: Is this what the Hungarian lab was doing when they found their hint of a new force?
Randall: They actually used an entirely different setup, but you can see some similarities. Their approach was to fire off accelerated protons at isotopes of the element lithium. Occasionally these protons would embed themselves in the nucleus of that lithium, which would be transformed into an excited state of another element, called beryllium. I know, it sounds completely different, but here's where our two experiments start to coincide: Occasionally this beryllium produced a photon, which could decay into a pair of particles which were measured by a detector. By studying that pair of particles, you could see if anything strange or inexplicable was going on with that photon. Sound familiar?
"EVEN IF WE CAN CONFIRM THE EXISTENCE OF THESE DARK PHOTONS, THAT'S NOT THE SAME THING AS HAVING FOUND ACTUAL DARK MATTER."
This lab found a bump in their data, which they think indicates that their photon wasn't a photon at all, but some new particle with a specific mass of 17 Me-V (megaelectron-volts). We're still designing the setup of our DarkLight experiment right now. Where you decide to place your detectors when you're setting up basically constrains the mass of particles you'd be able to look for. We're going to focus on that 17 M-eV region right at the outset, and see if we can confirm what the Hungarians are saying they found.
Being able to confirm this finding is a pretty big deal. In general, with just one experiment, it's very easy to hold some doubt and wonder if something was potentially messed up. If it was a fluke, basically. It's not until two separate experiments have detected something as drastic as a new physical force that we'll be able to have any confidence that what was found was real. And because our DarkLight experiment and the Hungarian study are designed with basically almost no experimental overlap, the probability that we'd both receive a fluke finding for a specific particle of a specific mass, well ... it's not impossible, but it's highly, highly unlikely.