THE ORIGINAL VERSION of this story appeared in Quanta Magazine.
Of the 100 trillion neutrinos that pass through you every second, most come from the sun or Earth’s atmosphere. But a smattering of the particles—those moving much faster than the rest—traveled here from powerful sources farther away. For decades, astrophysicists have sought the origin of these “cosmic” neutrinos. Now, the IceCube Neutrino Observatory has finally collected enough of them to reveal telltale patterns in where they’re coming from.
In a paper published in June in Science, the team revealed the first map of the Milky Way in neutrinos. (Usually our galaxy is mapped out with photons, particles of light.) The new map shows a diffuse haze of cosmic neutrinos emanating from throughout the Milky Way, but strangely, no individual sources stand out. “It’s a mystery,” said Francis Halzen, who leads IceCube.
The results follow an IceCube study from last fall, also in Science, that was the first to connect cosmic neutrinos to an individual source. It showed that a large chunk of the cosmic neutrinos detected so far by the observatory have come from the heart of an “active” galaxy called NGC 1068. In the galaxy’s glowing core, matter spirals into a central supermassive black hole, somehow making cosmic neutrinos in the process.
“It’s really gratifying,” said Kate Scholberg, a neutrino physicist at Duke University who wasn’t involved in the research. “They’ve actually identified a galaxy. This is the kind of thing the entire neutrino astronomy community has been trying to do for forever.”
Pinpointing cosmic neutrino sources opens up the possibility of using the particles as a new probe of fundamental physics. Researchers have shown that the neutrinos can be used to open cracks in the reigning standard model of particle physics and even test quantum descriptions of gravity.
Yet identifying the origin of at least some cosmic neutrinos is only a first step. Little is known about how the activity around some supermassive black holes generates these particles, and so far the evidence points to multiple processes or circumstances. READ MORE...
Newton’s third law tells us that for every action, there’s an equal reaction going the opposite way. It’s been reassuring us for 400 years, explaining why we don’t fall through the floor (the floor pushes up on us too), and why paddling a boat makes it glide through water.
When a system is in equilibrium, no energy goes in or out and such reciprocity is the rule. Mathematically, these systems are elegantly described with statistical mechanics, the branch of physics that explains how collections of objects behave. This allows researchers to fully model the conditions that give rise to phase transitions in matter, when one state of matter transforms into another, such as when water freezes.
But many systems exist and persist far from equilibrium. Perhaps the most glaring example is life itself. We’re kept out of equilibrium by our metabolism, which converts matter into energy. A human body that settles into equilibrium is a dead body.
In such systems, Newton’s third law becomes moot. Equal-and-opposite falls apart. “Imagine two particles,” said Vincenzo Vitelli, a condensed matter theorist at the University of Chicago, “where A interacts with B in a different way than how B interacts with A.” Such nonreciprocal relationships show up in systems like neuron networks and particles in fluids and even, on a larger scale, in social groups. Predators eat prey, for example, but prey doesn’t eat its predators.
For these unruly systems, statistical mechanics falls short in representing phase transitions. Out of equilibrium, nonreciprocity dominates. Flocking birds show how easily the law is broken: Because they can’t see behind them, individuals change their flight patterns in response to the birds ahead of them.
So bird A doesn’t interact with bird B in the same way that bird B interacts with bird A; it’s not reciprocal. Cars barreling down a highway or stuck in traffic are similarly nonreciprocal. Engineers and physicists who work with metamaterials — which get their properties from structure, rather than substance — have harnessed nonreciprocal elements to design acoustic, quantum and mechanical devices. READ MORE...
