Two scientists as different as could be — one a bookish astrophysicist who formerly served as NASA's chief scientist, the other a charismatic mathematician who moonlights as a painter — have teamed up to unlock the secrets of dark matter.
From his Washington, DC office at NASA headquarters, Dr. Jim Green admitted that although he retired as NASA's top scientist in January, he was already back as a consultant. He told Futurism the story of meeting up with his friend, Yeshiva University mathematician Ed Belbruno, when the latter invited the former to speak at the University of Augsburg in Germany.
Over lunch, they got to talking about the Pioneer Anomaly, the astrophysics-speak term for the bizarre slowing down effect witnessed by Pioneers 10 and 11. One thing led to another, and the pair soon found themselves with a long shot concept for an "Interstellar Probe" mission that they say could gather unprecedented data about dark matter and its place in the cosmos.
The following interview has been edited for length and clarity.
Futurism: What is dark matter?
Ed Belbruno: So the short of it is, no one knows.
There seem to be three components of our universe. One component is this mysterious energy that is pushing the universe apart and making it expand. I don't want to call it anti-gravity, but it's a repulsive force called dark energy. I mentioned this one first because it's the biggest one: 70 percent of all the energy in our universe is from that, and we don't know what it is, which is a pretty amazing statement.
The other big component is the visible, measurable matter in the universe. Your coffee in the morning, the stars and galaxies, and everything else out there. That's the matter. That's the other 30 percent of our universe.
Of this 30 percent that's left, 80 percent of it is invisible. We don't know what it is. And that 80 percent is called dark matter. It's dark because it's invisible, the same way we call dark energy by that name. It's invisible also, but it's matter, meaning it makes a gravitational field just like our Earth makes a gravitational field.
Jim Green: Yeah, it's very hard to visualize or to really understand what dark matter is. From a scientific perspective, what we do is we try to measure its properties. Now, astronomers see what its influence does, but what we want to do is to really understand its properties through an experiment that shows us what the dark matter force is. If we can measure that, and it's there — it may be greater than we thought, it may be less than we thought — it folds into an overall idea of what it could possibly be.
Now, there's a lot of discussion that ranges from dark matter being mini black holes to, potentially, matter that we may be familiar with, with special types of particles, axioms and neutrinos and, and a number of other things. The experiment is not designed to measure any of those, but to just create another set of input as to its properties.
But I have to tell you, it's a very fascinating topic, because it may have been there before the Big Bang.
F: Why are you pushing for the Interstellar Probe mission now?
JG: So right now, we have the two Voyagers and New Horizons, which are the only operating spacecrafts that are in the process of leaving the solar system. But in reality, they're not moving very fast. They only have a certain capability in terms of the power that they're taking with them. And therefore, they're only going to last so long. In fact, the Voyagers are at about 140 AU, which is beyond what we call the influence of the solar magnetic field or the heliosphere. In other words, when the power is gone, it's gone.
The power and the speed of the spacecraft are so important that a new mission is being devised, and that mission is called the Interstellar Probe. The concept is that they want to go 500 astronomical units out, as fast as they possibly can, with the right set of instruments, and with the amount of power that they need to be able to make it that far.
So here's the spacecraft that is being designed by humans today, that will be a significant spacecraft that will last for a couple generations of scientists, as it explores the outer reaches of our heliosphere. And that ends up pushing the limits of where you could begin the process of starting to measure dark matter.
So the "why now" is we're now developing a mission that could go farther, because we want to know what's out there. And now we have an opportunity to explore this idea of making the dark matter measurement.
F: Is it possible we could harness dark matter as an energy source?
EB: If you've got this substance permeating our galaxy, 90 percent of it, and it gives gravity, then therefore, there's a huge energy source out there. And it's so huge, it swamps anything that we could possibly imagine, with our wildest imaginations.
So 50 percent of what's permeating our solar system is precisely this material, whatever it is. So therefore, this means that it has unknown properties. It's anyone's guess what you can do with it, once we find out what it is. My guess is because it causes light to deflect when you're looking at other galaxies, that it's a pretty potent material, and it can be used as an energy source, probably. And how we would focus that energy source would maybe be hundreds of years or thousands of years away. But if you could focus it, you would have an unlimited supply of energy in outer space to power spacecraft, and this would just be absolutely beyond what we could imagine at the moment, what you could do something like that.
My guess is that the potential material that dark matter is made of would revolutionize our understanding of science, and may very well allow us to make star drives, which could be used in spacecraft that could that could traverse interstellar distances.
F: Is there dark matter in the room with me right now?
EB: Yeah, absolutely. It's here. Where you're sitting, on some level beyond our senses. It's going through you right now. The only reason it's not anything to be concerned about is because the gravity of the Earth is so much, it drowns it out. It's noise, but it's there.
JG: What we see when we sit here, of course, is reflected light. Light is being affected by baryonic — visible — matter, not being affected by dark matter. So we have no inkling there's dark matter among us. Now, that doesn't mean that it's not here at such a low density that the measurement of things that we do in laboratories haven't found it yet.
F: Some people think dark matter might be a misinterpretation of flawed observations. Do they think there’s a chance they’re right?
EB: I say this with no hesitation whatsoever: they're absolutely incorrect. This is equivalent, to me, to people that doubt climate change. It's clearly here and they have their own issues where they don't want to believe.
JG: There's such a large body of work that it's clear something is affecting, in particular, orbits of stars around galaxies in the outer regions of spiral arms that keep the arms together. And they don't follow Newtonian mechanics, because the arms stay together, they flow together. So something is taking the stars and moving them at the velocity of the motion of the interior stars that are closer to the center of the galaxy. That itself is powerful evidence. There's such a large body of evidence and we see that in other galaxies, because we can measure the velocity of those stars. It's pretty irrefutable. I think that train has left the station.
F: Is it possible that dark matter is something really out there, like ghosts or alternate dimensions?
EB: Once people get over the hump of being a skeptic, and they admit there's something there, then the next question is, what could that possibly be? And we don't know a lot about the universe, we don't know a lot about our own world. It could be wild, something we can't even imagine.
Our science, don't forget, is based on deductive reasoning. And that deductive reasoning can be backtracked to the laws of mathematics. Because after all, all of physics makes use of the laws of mathematics, right? And if you look at that, what are you really saying? You're saying that our laws of physics are based on the commutative law for addition, the associative law of addition, and multiplication, the basic things you learn in grade school about the number system. Those basic rules — there's only about a handful of them — those basic rules are so far behind our math, and our math is behind the laws of physics. So therefore, you're requiring that we understand the universe based on our laws, which are very limited, of math and physics that go into deductive reasoning. And that is really limited, obviously.
So what this dark matter problem could be telling us is that we don't have enough laws of the universe available to us right now to understand the problem. So, in other words, our ways of thinking using deductive reasoning may not be sufficient for this problem. And if that's the case, it could be anything that we can't even imagine, really. So if you want to say it's ghosts, or whatever, I'm not going to get metaphysical and go out on the limb with that one. But it could be things that we don't even imagine now. And we're trying to fit it with our limited thinking. It may not be that limited. It may be something really unusual that that we're just not ready to understand at the moment.
JG: Well, scientifically, when we talk about it, those things that you mentioned don't really enter into the conversation [laughs]. We have no preconceived idea that it has a connection with human life as it as it evolves, lives and dies. So scientifically, we want to explain it to the point where we can develop mathematics.
You know, the description as to what science really is, is our attempt to describe the nature of things, for which dark matter, we believe, is one of those things. And for us to be able to describe it, we start with equations. It's really the same way 110 or 120 years ago with, with Einstein and his concepts of the theory of relativity, that a new set of physics was determined. It started from conceptual ideas, that then mathematically could be expressed, and then you take the mathematics and you push them to the limits, and we have found black holes. You know, black holes emerged from the mathematics of relativity.
So this is why getting adequate descriptions of dark matter, mathematically, is critically important for us because we want to be able to then understand its nature.
F: How would you describe dark matter to a kindergartener?
JG: I might start with a ball on a string and then have it spin. And then have them see that as you shorten the distance, the ball speeds up, but then tell them imagine that doesn't happen, that the outer part of the string moves as fast as the object on the inner part. And that's due to a different force that we are starting to measure but don't have the ability to do that yet.
EB: If there's a little kid that didn't understand much of anything, because they're so little, they understand imagination, little kids are always imagining, they have imaginary friends. And I would say to them that most of our world is imaginary, because they don't understand a galaxy or anything like that. But most of what's out there — and they can interpret that however they want — most of what's out there is imaginary, we don't know what it is.
More on dark matter: NASA Proposes Sending Spacecraft to Measure Mysterious Dark Matter