Gravity is incredibly weak. Just think: You can lift your foot despite the mass of the entire Earth pulling against it. Why is it so weak? That’s unclear. And it might take a very, very big science experiment to find out.
James Beacham is a physicist at Duke University who works on the ATLAS detector at the famous Large Hadron Collider (LHC) in Switzerland. He recently described his dream physics experiment to Gizmodo: an unfathomably large atom smasher—an Ultra Hadron Collider—around the outer edge of the Solar System. Such an experiment could solve most of the mysteries of physics at once, such as the true nature of dark matter or whether time travel is possible.
“To understand what was going on the moment after the Big Bang, getting closer and closer to the moment itself, we need to achieve higher energies in collider experiments, and to do that we need to build larger collider experiments,” Beacham said. “We currently understand pretty well what was going on when the universe was about the size of an apple; this is what we can achieve with the energies at the LHC. But smaller than that, farther back in time than that, is a mystery.”
Physicists are pretty sure they know the universe’s basic principles. Particles interact through forces, of which there are four known ones: electromagnetism; the “weak” force; the “strong” force; and gravity. Each force has rules that have been figured out by hundreds of years of experiments. Each one has a different strength.
Compared to the other three, “gravity is not just weak, it’s nearly negligible,” Beacham said. As he described it:
At the Large Hadron Collider, where I work, we investigate the basic, elementary rules of nature by slamming protons together at high energies. The rules we explore are described in terms of particles and forces, and gravity is the only one of the four known forces that we don’t even care about when we do our calculations of very high-energy proton collisions... if we give the strong force a strength of 1, the strongest force we know of, then gravity has a strength of 10-39. That’s a decimal with 39 zeros after it. That’s almost nothing.
This is one of the big, baffling mysteries of science. Why are the strengths of the forces arranged this way? Why is gravity so weak?
Nature is the way that it is, regardless of how humans feel it should be. But experiments have shown that, at high enough energies, electromagnestism and the weak force merge together into the same force. At even higher energies, physicists have reason to suspect that the strong force would unify with them, too. But gravity seems to be different. Scientists don’t know if gravity would unify with the rest of the forces at high enough energies.
“Gravity is indeed a force of nature, but its rules—the mathematics upon which its best, most accurate description rests—are somehow very different from the others,” Beacham said. He went on:
Gravity is best described by Einstein’s general relativity, and the other three forces are described by the Standard Model of particle physics, based on quantum field theory, and although there are some similarities... they’re very different. In fact, some would say fundamentally so: When we try to naively stitch the two together, we get nonsense answers.
And in our current universe, with our current technology, “it’s essentially impossible for us to answer this question empirically,” said Beacham. Why’s that? “We can’t get to such large collider energies, primarily because we currently can’t build a particle collider large enough to get there.” He said some theorists think that there’s extra stuff (like other particles or extra spatial dimensions as proposed by the string theory and its extensions) that might show up in more feasible experiment that could unite gravity with the other forces. But to know for sure, we have to get to the size of the Solar System.
Even the 27-kilometer-round (16-mile) Large Hadron Collider, which uses superconducting magnets to accelerate and collide beams of protons to 99.9999990 percent of the speed of light, isn’t big enough to answer these questions. It can only probe what the universe was like when it was the size of an apple. Scientists would need more energy, and therefore a larger collider, in order to probe what it was like at smaller sizes.
How big? You could maybe unify the strong and weak nuclear force with a collider built around Mars. But to add gravity to the fray, “by some naive estimates, we’d need a collider around the orbit of Neptune. Worse, some physicists claim that such an estimate is hopelessly naive, and that we’d need to go bigger.” The benefits would be enormous—such a collider would probe the Planck scale, the smallest scale we could possibly look at as allowed by quantum mechanics. “We’d understand everything about gravity, quantum mechanics—and, by the way, we’d also get the unification of the electroweak and strong forces, for free, and also time travel, string theory, dark matter, dark energy, the measure problem, multiverse theory, etc.” Time travel? As Beacham explained, we’d have such a detailed understanding of the universe and how space and time work that perhaps we’d be able to implement our knowledge in a future time-manipulating technology.
“It’s entirely possible that gravity and the other forces of nature unify at some extremely high energy, but to investigate the question we’d need to build an LHC-style collider circling around the outer edge of the Solar System, or larger. An ideal summertime project.”
Unfortunately, Beacham’s dream experiment is currently impossible:
The technology, person-power, and resources to construct a particle collider circling around the outer edge of the Solar System simply don’t exist. Even if we just use the existing accelerator and detector technologies like those we use at the LHC, the challenge is scale, in a very practical sense: It’s unclear as to whether there is even enough accessible material available in any Solar System sources—the Earth, the Moon, the planets, asteroids, etc.—to build such a structure.
And to accelerate protons to such high energies, even those at the LHC, we use superconducting magnets. Magnets only attain the property of superconductivity if you make them very cold. You might think that that would be a benefit of building a particle accelerator in space. Space is very cold! Alas, it’s not cold enough for superconductivity, though. Outer space is at 2.7 K, but the magnets need to be at 1.9 K. Close doesn’t cut it. At the LHC we achieve these temperatures with liquid helium. It’s unclear as to whether there is enough accessible liquid helium anywhere in the nearby universe to cool a circular accelerator that stretches around the outer edge of the Solar System.
At these energies, the detectors would need to be enormous. You’d need to train the physicists and come up with the unfathomable amount of computing power. You’d need advanced robotics, shielding from asteroids, comets, and other debris. And you’d need to power the dang thing. You can’t use solar power, since the machine surrounds the Sun at Neptune’s distance. “Something of this size will require advances in power that likely haven’t been envisioned and that someone will only envision far in the future.”
Such an experiment would change physics. Ultimately, these experiments serve to help physicists understand things, and such an accelerator would provide conclusive answers to plenty of questions. It would change the way humans think. It would change what we mean by “understanding.”
If we were to build a particle collider around the outer edge of the Solar System, the knowledge we would gain—about the nature of gravity, of how quantum mechanics and general relativity fit together, about time travel, about what happened at the moment of the Big Bang, about whether our universe is just one of an almost infinite number in a multiverse—would so fundamentally alter our perception of reality, alter our relationship to nature, that this language, the understanding of the world, of humanity, of how anything happens at all, of our place in this universe, will seem so radically inadequate that a new kind of understanding would need to be invented to take its place.
Obviously no humans are currently working on such an experiment, though CERN has pitched a Future Circular Collider that would contain a 80- to 100-kilometer ring of tunnel. But maybe, somewhere in the universe, someone or something is hard at work on this project. Said Beacham:
It would be fantastic if some remote civilization somewhere else in the universe were already working on this, and one of the most enticing aspects of the possibility of identifying and communicating with an alien society is to ask them about their elementary physics experiments. Did they measure the same mass for the Higgs boson? Did they discover the X and Y bosons, the particles that would demonstrate the unification of the electroweak and strong forces? Did they reach the Planck scale? What is dark matter? Can we travel backward through time?
The universe will continue to work by whatever rules it follows. The real question is whether humans will ever have the wherewithal to truly understand these laws.