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12 Illuminating Facts About General Relativity

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Wikimedia Commons // Public Domain

This year marks the 100th anniversary of a scientific breakthrough that fundamentally changed our world. 

In 1915, Albert Einstein presented his theory of general relativity, which proposed that gravity itself was the result of a warping of spacetime by massive objects like stars and planets. He was 36 years old and already quite famous in the world of theoretical physics, most notably for his theory of special relativity, which proposed that the laws of nature are the same for all nonaccelerating observers everywhere—and that the speed of light is constant (also, E=mc2!). At the time, these ideas rocketed Einstein to worldwide fame. Today, they're the basis for much of our understanding of the universe.

At the World Science Festival last week, the premiere of the stage performance Light Falls: Space, Time and an Obsession of Einstein shed new—well, you know—on Einstein's historic 1915 discovery. Led by physicist Brian Greene, the show featured a dramatic (and historically accurate) account of Einstein’s journey toward the incredible breakthrough. In celebration, here are a few things we learned.

1. A Compass Provided Early Inspiration.

When he was 5 years old, Einstein’s father gave him a compass. The instrument enthralled his curious young mind, as the needle always pointed north regardless of its position. The boy asked himself, "How?" And thus began Einstein's lifelong journey to understand unseen forces. "That experience made a deep and lasting impression on me," he later wrote. "Something deeper had to be hidden behind things."

2. So Did Clocks.

Another common instrument inspired Einstein too. At the turn of the 20th century, while young Albert was a clerk at a patent office in Bern, the world was becoming more technologically advanced and connected. It became increasingly important for clocks in faraway cities to agree on the time. Figuring out a way to synchronize the world’s timepieces led to many proposals that likely passed through Einstein’s hands. His own take on the problem was inspired by his lifelong fascination with light. He reasoned that if you could used light signals to coordinate and account for the infinitesimal travel time for the light to deliver the message, you could synch clocks pretty easily. But Einstein realized that two clocks moving at two different speeds—say, on two moving trains—wouldn't be able to precisely synchronize. This understanding of the relativity of time was an integral step in the development of his later theories.   

3. The Constancy of the Speed of Light Was a Huge Breakthrough.

While clocks can travel at different speeds, light can't. That's what Einstein postulated in 1905 with the special theory of relativity, which says the speed of light is constant. We take it for granted now, but at the time, this theory was radical. While supported by James Maxwell’s equations, the idea flew in the face of Newtonian physics. The concept that anyone in the universe, regardless of their own speed, would measure the speed of light as 300,000 km/s, meant that light behaves unlike anything else we know of. This core insight took him a step closer to the theory of general relativity, which essentially simply adds gravity to the equation. Special relativity put the burgeoning scientist on the map.

4. He Found Happiness in Strange Things.

In 1907, just two years after Einstein published the special theory of relativity, he had the “happiest thought of his life.” It wasn’t about a loved one, a remembrance, some sense of self satisfaction, or even the poetry of the cosmos. It was about a man falling from a building. Einstein realized that a man falling alongside a ball would not be able to recognize the effects of gravity on the ball. Again, it’s all relative. This connection between gravity and acceleration became known as the equivalence principle.

5. His General Relativity Drafts Are Contained in a Notebook.

When Einstein died in 1955, a small, brown notebook was found among his papers. It contained within it the notes he was taking while working through the ideas of general relativity from the winter of 1912 when he moved from Prague to Zurich. The Zurich notebook contains amazing bits like a modified four-dimensional Pythagorean theorem to account for the curvature of spacetime. The notebook also contains traces of Einstein’s mistakes (yes, even he made them). Wrong assumptions and dead ends are all contained in the pieces of aged graph paper. All were part of the path to greatness.

6. He Had Friends Who Helped Him Refine the Theory …

Marcel Grossmann and Einstein met in school, and they remained friends for the rest of their lives. Grossmann helped Einstein get hired at the patent office, and Einstein later called on him to help through some ideas. Grossmann was a mathematics professor at the Swiss Polytechnic when Einstein visited him in 1912, and the academic helped his old classmate with the math that would prove this new take on gravity. When the theory of general relativity was finally published, Einstein praised his collaborator: “Grossmann supported me through his help, not only in sparing me the study of the relevant mathematical literature, but also in the search for the gravitational field equations.”

7. ... and a Frenemy Who Accused Him of Stealing It.

David Hilbert was a fellow scientist and friend of Einstein’s—until their relationship took a negative turn leading up to the publication of the theory of general relativity. Hilbert too developed a theory of general relativity—and even published it five days before Einstein. What started as camaraderie and a supportive exchange of ideas turned into a bitter rivalry that included accusations of plagiarism. Since then, historians have examined the proofs and say that Hilbert’s lack certain key ingredients to make the theory work. In other words, history got it right: the cred belongs to Einstein. Oddly, a portion of Hilbert’s proofs are missing, with no indication of what they might have held.

8. The Introduction of the Theory Was Huge. 

In November 1915 Einstein presented his masterwork to the Prussian Academy of Science, wherein he introduced general relativity and what are now known as the Einstein field equations. The paper was published the following year, and while the man and the concepts received great attention (after all, Einstein was already a well-regarded figure), it wasn't until he was able to confirm the predictions that he became a towering figure in scientific achievement and a worldwide celebrity. It was a big moment for Einstein. He'd synthesized the ideas he'd been working on for 10 long years. Now he had to show the world he was right.

9. The Sun Helped Prove Him Right. 

As any good scientist knows, an unproven theory isn’t science, it’s philosophy. Einstein needed his equations to make accurate predictions about the behavior of objects in space. One of his conjectures held that light traveling near a large gravitational field should curve. To test it, Einstein needed the help of a solar eclipse, which would facilitate the view of starlight passing through the sun’s gravitational field. On May 29, 1919, in a test conceived by astronomer Sir Frank Watson Dyson, and with the help of Sir Arthur Eddington, astronomers were able to take pictures to compare with their "true" location and measure the bend of light of 1.75 arcseconds—the very number Einstein’s theories predicated.  “LIGHTS ALL ASKEW IN THE HEAVENS” read the November New York Times headline. From that moment on, Einstein was a superstar.

10. General Relativity Explained Mercury's Weird Behavior.

“The discovery was, I believe, by far the strongest emotional experience in Einstein’s scientific life, perhaps in all his life. Nature had spoken to him.”

-Abraham Pais

The general theory of relativity’s ability to explain the precession of the perihelion of Mercury—the change in orbital orientation the planet experienced when closest to the sun—gave Einstein another opportunity to test his theory. When it neared the sun, Mercury didn't behave as Newtonian physics predicted it should. The problem had baffled scientists for years. The behavior of gravity as laid out in the general theory explained these discrepancies. His understanding of how mass warps space ended a 200-year-old mystery about our celestial neighbor. 

11. His Scientific Papers Became Front Page News.

Once general relativity theory had been proven, Einstein skyrocketed to fame in a way that’s hard to imagine today. His papers were published in their entirety on the front page of newspapers like the Herald Tribune and pasted in department store windows where people would clamor to read them.

12. The Discovery Made So Much More Possible.

One hundred years later, the impact of the general theory of relativity is almost too massive to quantify. It’s why we have GPS, and it’s paved the way for our understanding of black holes and dark matter, the Big Bang and its immediate aftermath, and the discovery of our expanding (and accelerating) universe. It doesn’t stop there: we’re still waiting to see things like gravitational waves—little ripples in the fabric of spacetime—predicted by general relativity. Perhaps most importantly, the theory was a step that may one day lead to a grand unified theory that will complete the picture of the universe that humans have been trying to piece together since the beginning of our existence. Einstein’s one small step was a giant leap that we’ll spend perhaps another 100 years trying to match.

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15 Subatomic Word Origins
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In July 2017, researchers at the European Organization for Nuclear Research (CERN) found evidence for a new fundamental particle of the universe: Ξcc++, a special kind of Xi baryon that may help scientists better understand how quarks are held together. Is that Greek to you? Well, it should be. The names for many of the particles that make up the universe—as well as a few that are still purely theoretical—come from ancient Greek. Here’s a look at 15 subatomic etymologies.

1. ION

An ion is any atom or molecule with an overall electric charge. English polymath William Whewell suggested the name in an 1834 letter to Michael Faraday, who made major discoveries in electromagnetism. Whewell based ion on the ancient Greek verb for “go” (ienai), as ions move towards opposite charges. Faraday and Whewell had previously considered zetode and stechion.


George Stoney, an Anglo-Irish physicist, introduced the term electron in 1891 as a word for the fundamental unit of charge carried by an ion. It was later applied to the negative, nucleus-orbiting particle discovered by J. J. Thomson in 1897. Electron nabs the -on from ion, kicking off the convention of using -on as an ending for all particles, and fuses it with electric. Electric, in turn, comes from the Greek for “amber,” in which the property was first observed. Earlier in the 19th century, electron was the name for an alloy of gold and silver.


The electron’s counterpart, the positively charged proton in the nuclei of all atoms, was named by its discoverer, Ernest Rutherford. He suggested either prouton or proton in honor of William Prout, a 19th-century chemist. Prout speculated that hydrogen was a part of all other elements and called its atom protyle, a Greek coinage joining protos ("first") and hule ("timber" or "material") [PDF]. Though the word had been previously used in biology and astronomy, the scientific community went with proton.


Joining the proton in the nucleus is the neutron, which is neither positive nor negative: It’s neutral, from the Latin neuter, “neither.” Rutherford used neutron in 1921 when he hypothesized the particle, which James Chadwick didn’t confirm until 1932. American chemist William Harkins independently used neutron in 1921 for a hydrogen atom and a proton-electron pair. Harkins’s latter application calls up the oldest instance of neutron, William Sutherland’s 1899 name for a hypothetical combination of a hydrogen nucleus and an electron.


Protons and neutrons are composed of yet tinier particles called quarks. For their distinctive name, American physicist Murray Gell-Mann was inspired in 1963 by a line from James Joyce’s Finnegan’s Wake: “Three quarks for Muster Mark.” Originally, Gell-Mann thought there were three types of quarks. We now know, though, there are six, which go by names that are just as colorful: up, down, charm, strange, top, and bottom.


Made up of a quark and an antiquark, which has identical mass but opposite charge, the meson is a short-lived particle whose mass is between that of a proton and an electron. Due to this intermediate size, the meson is named for the ancient Greek mesos, “middle.” Indian physicist Homi Bhabha suggested meson in 1939 instead of its original name, mesotron: “It is felt that the ‘tr’ in this word is redundant, since it does not belong to the Greek root ‘meso’ for middle; the ‘tr’ in neutron and electron belong, of course, to the roots ‘neutr’ and ‘electra’.”


Mesons are a kind of boson, named by English physicist Paul Dirac in 1947 for another Indian physicist, Satyendra Nath Bose, who first theorized them. Bosons demonstrate a particular type of spin, or intrinsic angular momentum, and carry fundamental forces. The photon (1926, from the ancient Greek for “light”) carries the electromagnetic force, for instance, while the gluon carries the so-called strong force. The strong force holds quarks together, acting like a glue, hence gluon.


In 2012, CERN’s Large Hadron Collider (LHC) discovered a very important kind of boson: the Higgs boson, which generates mass. The hadrons the LHC smashes together at super-high speeds refer to a class of particles, including mesons, that are held together by the strong force. Russian physicist Lev Okun alluded to this strength by naming the particles after the ancient Greek hadros, “large” or “bulky,” in 1962.


Hadrons are opposite, in both makeup and etymology, to leptons. These have extremely tiny masses and don’t interact via the strong force, hence their root in the ancient Greek leptos, “small” or “slender.” The name was first suggested by the Danish chemist Christian Møller and Dutch-American physicist Abraham Pais in the late 1940s. Electrons are classified as leptons.


Another subtype of hadron is the baryon, which also bears the stamp of Abraham Pais. Baryons, which include the more familiar protons and neutrons, are far more massive, relatively speaking, than the likes of leptons. On account of their mass, Pais put forth the name baryon in 1953, based on the ancient Greek barys, “heavy” [PDF].


Quirky Murray Gell-Mann isn't the only brain with a sense of humor. In his 2004 Nobel Prize lecture, American physicist Frank Wilczek said he named a “very light, very weakly interacting” hypothetical particle the axion back in 1978 “after a laundry detergent [brand], since they clean up a problem with an axial current” [PDF].


In ancient Greek, takhys meant “swift,” a fitting name for the tachyon, which American physicist Gerald Feinberg concocted in 1967 for a hypothetical particle that can travel faster than the speed of light. Not so fast, though, say most physicists, as the tachyon would break the fundamental laws of physics as we know them.


In 2003, the American physicist Justin Khoury and South African-American theoretical physicist Amanda Weltman hypothesized that the elusive dark energy may come in the form of a particle, which they cleverly called the chameleon. Just as chameleons can change color to suit their surroundings, so the physical characteristics of the chameleon particle change “depending on its environment,” explains Symmetry, the online magazine dedicated to particle physics. Chameleon itself derives from the ancient Greek khamaileon, literally “on-the-ground lion.”

For more particle names, see Symmetry’s “A Brief Etymology of Particle Physics,” which helped provide some of the information in this list.

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NSF/LIGO/Sonoma State University/A. Simonnet
Astronomers Observe a New Kind of Massive Cosmic Collision for the First Time
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NSF/LIGO/Sonoma State University/A. Simonnet

For the first time, astronomers have detected the colossal blast produced by the merger of two neutron stars—and they've recorded it both via the gravitational waves the event produced, as well as the flash of light it emitted.

Physicists believe that the pair of neutron stars—ultra-dense stars formed when a massive star collapses, following a supernova explosion—had been locked in a death spiral just before their final collision and merger. As they spiraled inward, a burst of gravitational waves was released; when they finally smashed together, high-energy electromagnetic radiation known as gamma rays were emitted. In the days that followed, electromagnetic radiation at many other wavelengths—X-rays, ultraviolet, optical, infrared, and radio waves—were released. (Imagine all the instruments in an orchestra, from the lowest bassoons to the highest piccolos, playing a short, loud note all at once.)

This is the first time such a collision has been observed, as well as the first time that both kinds of observations—gravitational waves and electromagnetic radiation—have been recorded from the same event, a feat that required co-operation among some 70 different observatories around the world, including ground-based observatories, orbiting telescopes, the U.S. LIGO (Laser Interferometer Gravitational-Wave Observatory), and European Virgo gravitational wave detectors.

"For me, it feels like the dawning of a next era in astrophysics," Julie McEnery, project scientist for NASA's Fermi Gamma-ray Space Telescope, one of the first instruments to record the burst of energy from the cosmic collision, tells Mental Floss. "With this observation, we've connected these new gravitational wave observations to the rest of the observations that we've been doing in astrophysics for a very long time."


The observations represent a breakthrough on several fronts. Until now, the only events detected via gravitational waves have been mergers of black holes; with these new results, it seems likely that gravitational wave technology—which is still in its infancy—will open many new phenomena to scientific scrutiny. At the same time, very little was known about the physics of neutron stars—especially their violent, final moments—until now. The observations are also shedding new light on the origin of gamma-ray bursts (GRBs)—extremely energetic explosions seen in distant galaxies. As well, the research may offer clues as to how the heavier elements, such as gold, platinum, and uranium, formed.

Astronomers around the world are thrilled by the latest findings, as today's flurry of excitement attests. The LIGO-Virgo results are being published today in the journal Physical Review Letters; further articles are due to be published in other journals, including Nature and Science, in the weeks ahead. Scientists also described the findings today at press briefings hosted by the National Science Foundation (the agency that funds LIGO) in Washington, and at the headquarters of the European Southern Observatory in Garching, Germany.

(Rumors of the breakthrough had been swirling for weeks; in August, astronomer J. Craig Wheeler of the University of Texas at Austin tweeted, "New LIGO. Source with optical counterpart. Blow your sox off!" He and another scientist who tweeted have since apologized for doing so prematurely, but this morning, minutes after the news officially broke, Wheeler tweeted, "Socks off!") 

The neutron star merger happened in a galaxy known as NGC 4993, located some 130 million light years from our own Milky Way, in the direction of the southern constellation Hydra.

Gravitational wave astronomy is barely a year and a half old. The first detection of gravitational waves—physicists describe them as ripples in space-time—came in fall 2015, when the signal from a pair of merging black holes was recorded by the LIGO detectors. The discovery was announced in February 2016 to great fanfare, and was honored with this year's Nobel Prize in Physics. Virgo, a European gravitational wave detector, went online in 2007 and was upgraded last year; together, they allow astronomers to accurately pin down the location of gravitational wave sources for the first time. The addition of Virgo also allows for a greater sensitivity than LIGO could achieve on its own.

LIGO previously recorded four different instances of colliding black holes—objects with masses between seven times the mass of the Sun and a bit less than 40 times the mass of the Sun. This new signal was weaker than that produced by the black holes, but also lasted longer, persisting for about 100 seconds; the data suggested the objects were too small to be black holes, but instead were neutron stars, with masses of about 1.1 and 1.6 times the Sun's mass. (In spite of their heft, neutron stars are tiny, with diameters of only a dozen or so miles.) Another key difference is that while black hole collisions can be detected only via gravitational waves—black holes are black, after all—neutron star collisions can actually be seen.


When the gravitational wave signal was recorded, on the morning of August 17, observatories around the world were notified and began scanning the sky in search of an optical counterpart. Even before the LIGO bulletin went out, however, the orbiting Fermi telescope, which can receive high-energy gamma rays from all directions in the sky at once, had caught something, receiving a signal less than two seconds after the gravitational wave signal tripped the LIGO detectors. This was presumed to be a gamma-ray burst, an explosion of gamma rays seen in deep space. Astronomers had recorded such bursts sporadically since the 1960s; however, their physical cause was never certain. Merging neutron stars had been a suggested culprit for at least some of these explosions.

"This is exactly what we'd hoped to see," says McEnery. "A gamma ray burst requires a colossal release of energy, and one of the hypotheses for what powers at least some of them—the ones that have durations of less than two seconds—was the merger of two neutron stars … We had hoped that we would see a gamma ray burst and a gravitational wave signal together, so it's fantastic to finally actually do this."

With preliminary data from LIGO and Virgo, combined with the Fermi data, scientists could tell with reasonable precision what direction in the sky the signal had come from—and dozens of telescopes at observatories around the world, including the U.S. Gemini telescopes, the European Very Large Telescope, and the Hubble Space Telescope, were quickly re-aimed toward Hydra, in the direction of reported signal.

The telescopes at the Las Campanas Observatory in Chile were well-placed for getting a first look—because the bulletin arrived in the morning, however, they had to wait until the sun dropped below the horizon.

"We had about eight to 10 hours, until sunset in Chile, to prepare for this," Maria Drout, an astronomer at the Carnegie Observatories in in Pasadena, California, which runs the Las Campanas telescopes, tells Mental Floss. She was connected by Skype to the astronomers in the control rooms of three different telescopes at Las Campanas, as they prepared to train their telescopes at the target region. "Usually you prepare a month in advance for an observing run on these telescopes, but this was all happening in a few hours," Drout says. She and her colleagues prepared a target list of about 100 galaxies, but less than one-tenth of the way through the list, by luck, they found it: a tiny blip of light in NGC 4993 that wasn't visible on archival images of the same galaxy. (It was the 1-meter Swope telescope that snagged the first images.)


When a new star-like object in a distant galaxy is spotted, a typical first guess is that it's a supernova (an exploding star). But this new object was changing very rapidly, growing 100 times dimmer over just a few days while also quickly becoming redder—which supernovae don't do, explains Drout, who is cross-appointed at the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto. "We ended up following it for three weeks or so, and by the end, it was very clear that this [neutron star merger] was what we were looking at," she says.

The researchers say they can't be sure if the resulting object was another, larger neutron star, or whether it would have been so massive that it would have collapsed into a black hole.

As exciting as the original detection of gravitational waves last year was, Drout is looking forward to a new era in which both gravitational waves and traditional telescopes can be used to study the same objects. "We can learn a lot more about these types of extreme systems that exist in the universe, by coupling the two together," she says.

The detection shows that "gravitational wave science is moving from being a physics experiment to being a tool for astronomers," Marcia Rieke, an astronomer at the University of Arizona who is not involved in the current research, tells Mental Floss. "So I think it's a pretty big deal."

Physicists are also learning something new about the origin of the heaviest elements in the periodic table. For many years, these were thought to arise from supernova explosions, but spectroscopic data from the newly observed neutron star merger (in which light is broken up into its component colors) suggests that such explosion produce enormous quantities of heavy elements—including enough gold to put Fort Knox to shame. (The blast is believed to have created some 200 Earth-masses of gold, the scientists say.) "It's telling us that most of the gold that we know about is produced in these mergers, and not in supernovae," McEnery says.

Editor's note: This post has been updated.


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