Nobel Prize in Physics Awarded for the Discovery of Neutrino Oscillations


Two scientists have been awarded this year’s Nobel Prize in physics for their contributions to experiments showing neutrinos can change identity. Their detection of neutrino oscillations, which allow them to change "flavor," prove that neutrinos have mass, a discovery that the Nobel Prize committee said in a press release “has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe.”

Behind photons, neutrinos are the most abundant particles in the universe. Many are the result of reactions between cosmic radiation and the Earth’s atmosphere, while others are created by nuclear reactions inside the sun. It was long believed that neutrinos were massless, but the groundbreaking discoveries made by Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen’s University, Kingston, Canada, show these particles can change identities and therefore must have mass. 

In the late 1990s Kajita discovered that neutrinos from the atmosphere switched from one identity to another on their way to the Super-Kamiokande detector in Japan. Around the same time, the research group led by McDonald discovered that neutrinos from the sun weren’t disappearing on their trip to Earth, but instead changing identities. 

For decades physicists had been baffled when their theoretical calculations of the number of neutrons came out up to two-thirds higher than their physical measurements on Earth. These two experiments show that the neutrinos haven’t gone anywhere, they’ve just assumed a different form; as a result of these findings, neutrinos are now thought to come in three "flavors." The discoveries also challenge the Standard Model, a popular particle physics theory of the fundamental constituents of the universe that relies on the assumption that neutrinos are massless. 

Intense efforts continue to be made around the world to capture neutrinos and study their properties. Physicists predict any new discoveries could have a huge impact on our understanding of the makeup, history, and future of the universe. 

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Essential Science
What Is a Scientific Theory?
Dean Mouhtaropoulos/Getty Images
Dean Mouhtaropoulos/Getty Images

In casual conversation, people often use the word theory to mean "hunch" or "guess": If you see the same man riding the northbound bus every morning, you might theorize that he has a job in the north end of the city; if you forget to put the bread in the breadbox and discover chunks have been taken out of it the next morning, you might theorize that you have mice in your kitchen.

In science, a theory is a stronger assertion. Typically, it's a claim about the relationship between various facts; a way of providing a concise explanation for what's been observed. The American Museum of Natural History puts it this way: "A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts."

For example, Newton's theory of gravity—also known as his law of universal gravitation—says that every object, anywhere in the universe, responds to the force of gravity in the same way. Observational data from the Moon's motion around the Earth, the motion of Jupiter's moons around Jupiter, and the downward fall of a dropped hammer are all consistent with Newton's theory. So Newton's theory provides a concise way of summarizing what we know about the motion of these objects—indeed, of any object responding to the force of gravity.

A scientific theory "organizes experience," James Robert Brown, a philosopher of science at the University of Toronto, tells Mental Floss. "It puts it into some kind of systematic form."


A theory's ability to account for already known facts lays a solid foundation for its acceptance. Let's take a closer look at Newton's theory of gravity as an example.

In the late 17th century, the planets were known to move in elliptical orbits around the Sun, but no one had a clear idea of why the orbits had to be shaped like ellipses. Similarly, the movement of falling objects had been well understood since the work of Galileo a half-century earlier; the Italian scientist had worked out a mathematical formula that describes how the speed of a falling object increases over time. Newton's great breakthrough was to tie all of this together. According to legend, his moment of insight came as he gazed upon a falling apple in his native Lincolnshire.

In Newton's theory, every object is attracted to every other object with a force that’s proportional to the masses of the objects, but inversely proportional to the square of the distance between them. This is known as an “inverse square” law. For example, if the distance between the Sun and the Earth were doubled, the gravitational attraction between the Earth and the Sun would be cut to one-quarter of its current strength. Newton, using his theories and a bit of calculus, was able to show that the gravitational force between the Sun and the planets as they move through space meant that orbits had to be elliptical.

Newton's theory is powerful because it explains so much: the falling apple, the motion of the Moon around the Earth, and the motion of all of the planets—and even comets—around the Sun. All of it now made sense.


A theory gains even more support if it predicts new, observable phenomena. The English astronomer Edmond Halley used Newton's theory of gravity to calculate the orbit of the comet that now bears his name. Taking into account the gravitational pull of the Sun, Jupiter, and Saturn, in 1705, he predicted that the comet, which had last been seen in 1682, would return in 1758. Sure enough, it did, reappearing in December of that year. (Unfortunately, Halley didn't live to see it; he died in 1742.) The predicted return of Halley's Comet, Brown says, was "a spectacular triumph" of Newton's theory.

In the early 20th century, Newton's theory of gravity would itself be superseded—as physicists put it—by Einstein's, known as general relativity. (Where Newton envisioned gravity as a force acting between objects, Einstein described gravity as the result of a curving or warping of space itself.) General relativity was able to explain certain phenomena that Newton's theory couldn't account for, such as an anomaly in the orbit of Mercury, which slowly rotates—the technical term for this is "precession"—so that while each loop the planet takes around the Sun is an ellipse, over the years Mercury traces out a spiral path similar to one you may have made as a kid on a Spirograph.

Significantly, Einstein’s theory also made predictions that differed from Newton's. One was the idea that gravity can bend starlight, which was spectacularly confirmed during a solar eclipse in 1919 (and made Einstein an overnight celebrity). Nearly 100 years later, in 2016, the discovery of gravitational waves confirmed yet another prediction. In the century between, at least eight predictions of Einstein's theory have been confirmed.


And yet physicists believe that Einstein's theory will one day give way to a new, more complete theory. It already seems to conflict with quantum mechanics, the theory that provides our best description of the subatomic world. The way the two theories describe the world is very different. General relativity describes the universe as containing particles with definite positions and speeds, moving about in response to gravitational fields that permeate all of space. Quantum mechanics, in contrast, yields only the probability that each particle will be found in some particular location at some particular time.

What would a "unified theory of physics"—one that combines quantum mechanics and Einstein's theory of gravity—look like? Presumably it would combine the explanatory power of both theories, allowing scientists to make sense of both the very large and the very small in the universe.


Let's shift from physics to biology for a moment. It is precisely because of its vast explanatory power that biologists hold Darwin's theory of evolution—which allows scientists to make sense of data from genetics, physiology, biochemistry, paleontology, biogeography, and many other fields—in such high esteem. As the biologist Theodosius Dobzhansky put it in an influential essay in 1973, "Nothing in biology makes sense except in the light of evolution."

Interestingly, the word evolution can be used to refer to both a theory and a fact—something Darwin himself realized. "Darwin, when he was talking about evolution, distinguished between the fact of evolution and the theory of evolution," Brown says. "The fact of evolution was that species had, in fact, evolved [i.e. changed over time]—and he had all sorts of evidence for this. The theory of evolution is an attempt to explain this evolutionary process." The explanation that Darwin eventually came up with was the idea of natural selection—roughly, the idea that an organism's offspring will vary, and that those offspring with more favorable traits will be more likely to survive, thus passing those traits on to the next generation.


Many theories are rock-solid: Scientists have just as much confidence in the theories of relativity, quantum mechanics, evolution, plate tectonics, and thermodynamics as they do in the statement that the Earth revolves around the Sun.

Other theories, closer to the cutting-edge of current research, are more tentative, like string theory (the idea that everything in the universe is made up of tiny, vibrating strings or loops of pure energy) or the various multiverse theories (the idea that our entire universe is just one of many). String theory and multiverse theories remain controversial because of the lack of direct experimental evidence for them, and some critics claim that multiverse theories aren't even testable in principle. They argue that there's no conceivable experiment that one could perform that would reveal the existence of these other universes.

Sometimes more than one theory is put forward to explain observations of natural phenomena; these theories might be said to "compete," with scientists judging which one provides the best explanation for the observations.

"That's how it should ideally work," Brown says. "You put forward your theory, I put forward my theory; we accumulate a lot of evidence. Eventually, one of our theories might prove to obviously be better than the other, over some period of time. At that point, the losing theory sort of falls away. And the winning theory will probably fight battles in the future."

Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0
An Ancient Book Blasted with High-Powered X-Rays Reveals Text Erased Centuries Ago
Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0
Farrin Abbott, SLAC/Flickr // CC BY-NC-SA 2.0

A book of 10th-century psalms recovered from St. Catherine’s Monastery on Egypt's Sinai Peninsula is an impressive artifact in itself. But the scientists studying this text at the U.S. Department of Energy's SLAC National Accelerator Laboratory at Stanford University were less interested in the surface text than in what was hidden beneath it. As Gizmodo reports, the researchers were able to identify the remains of an ancient Greek medical text on the parchment using high-powered x-rays.

Unlike the Large Hadron Collider in Switzerland, the Stanford Synchrotron Radiation Lightsource (SSRL) used by the scientists is a much simpler and more common type of particle accelerator. In the SSRL, electrons accelerate to just below the speed of light while tracing a many-sided polygon. Using magnets to manipulate the electrons' path, the researchers can produce x-ray beams powerful enough to reveal the hidden histories of ancient documents.

Scanning an ancient text.
Mike Toth, R.B. Toth Associates, Flickr // CC BY-NC-SA 2.0

In the case of the 10th-century psalms, the team discovered that the same pages had held an entirely different text written five centuries earlier. The writing was a transcription of the words of the prominent Greek physician Galen, who lived from 130 CE to around 210 CE. His words were recorded on the pages in the ancient Syriac language by an unknown writer a few hundred years after Galen's death.

Several centuries after those words were transcribed, the ink was scraped off by someone else to make room for the psalms. The original text is no longer visible to the naked eye, but by blasting the parchment with x-rays, the scientists can see where the older writing had once marked the page. You can see it below—it's the writing in green.

X-ray scan of ancient text.
University of Manchester, SLAC National Accelerator Laboratory, Flickr // CC BY-NC-SA 2.0

Now that the researchers know the hidden text is there, their next step will be uncovering as many words as possible. They plan to do this by scanning the book in its entirety, a process that will take 10 hours for each of the 26 pages. Once they've been scanned and studied, the digital files will be shared online.

Particle accelerators are just one tool scientists use to decipher messages that were erased centuries ago. Recently, conservationists at the Library of Congress used multispectral imaging, a method that bounces different wavelengths of light off a page, to reveal the pigments of an old Alexander Hamilton letter someone had scrubbed out.

[h/t Gizmodo]


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