A century ago, Albert Einstein became famous.
Sure, he was already well-known among physicists. But the world at large learned his name only after November 1919, when news broke that his theory of gravity had been confirmed—to the dismay of many fans of Isaac Newton.
“Lights All Askew in the Heavens” shouted the headline in the New York Times. “Einstein Theory Triumphs,” a subhead added. As the article recounted, an observation of stars near the sun during a solar eclipse found their apparent position shifted just as Einstein had predicted. Newton’s law of gravity, considered inviolable for over two centuries, had been repealed.
But despite the triumph of Einstein’s theory—general relativity—physicists still wonder whether it will someday face the same fate as Newton’s law.
While Einstein’s gravity has passed every test so far, nobody knows for sure that it applies everywhere, under all conditions. In particular, there is no guarantee that general relativity reigns over the entire expanse of the cosmos.
And several rival theories have been proposed over the years just in case it doesn’t.
The first major test of Einstein’s general theory of relativity came in 1919 from an eclipse, shown here in an image from the scientific paper reporting that light from distant stars was bent by the sun’s gravity just as Einstein’s theory had predicted. Credit: F.W. Dyson, A.S. Eddington and C. Davidson
After Einstein proposed his new theory, it was mostly ignored for a few decades. But in the last half of the 20th century, general relativity became the theory of the universe.
Its equations describe the expansion of the cosmos from its initial high-density, hot big-bang beginning to its current rapidly accelerating expansion.
And today general relativity has earned increasing popular notoriety as scientists have verified its more exotic predictions, including black holes and the vibrations in space known as gravitational waves.
But general relativity’s string of successes may not be endless. It’s true that the theory (along with the theory for nature’s three other fundamental forces) describes the observable universe quite well.
That description includes massive amounts of invisible mass, known as dark matter, along with a peculiar repulsive force, called dark energy, perfusing all of space.
But the dark stuff’s existence is deduced from the assumption that general relativity is correct.
“Given that there is no other (nongravitational) evidence for the dark sector, it is a matter of common sense to question some of the fundamental assumptions that go into the evidence.
And the main assumption is that general relativity is the underlying theory of gravity,” astrophysicist Pedro Ferreira of the University of Oxford in England writes in the current Annual Review of Astronomy and Astrophysics.
If you don’t assume general relativity is in fact correct, then “evidence for the dark sector may signal a breakdown of general relativity on cosmological scales,” Ferreira points out.
In other words, it’s conceivable that there is no dark stuff. If that’s the case, apparent evidence for its existence might actually be a sign that the true cosmic theory of gravity differs from Einstein’s. If so, the current picture of the cosmos would have to be drastically redrawn.
Still, physicists have plenty of reason for confidence in general relativity’s reliability. For one thing, it solved a knotty problem that had perplexed astronomers about the planet Mercury: a discrepancy in its orbit from that forecast by Newtonian gravity. Einstein announced his theory in 1915 as soon as he was able to show that it correctly predicted Mercury’s actual orbit.
Einstein’s key to solving the Mercury mystery was conceiving gravity as an effect of the geometry of space (or technically, spacetime, since his earlier work had shown space and time to be inseparable).
Gravity is not a mutual tug of massive objects, Einstein said, but rather the result of a mass’s distortion of the spacetime surrounding it. Objects orbit or fall into a massive body depending on how strongly the spacetime around it is curved.
Rather than responding to some attractive force, masses just follow the contours of spacetime’s geometry.
Gravity as geometry led to the famous prediction verified in the 1919 eclipse. Einstein pointed out that the curvature of spacetime near the sun would cause light from distant stars to bend when passing nearby, changing the stars’ apparent positions as seen from Earth.
That prediction inspired an eclipse expedition to the West African island of Principe in May 1919, led by British astrophysicist Arthur Eddington. Eddington’s team found that the positions of several stars were shifted by just the amount that Einstein’s math indicated they should be, and twice as much as Newton’s law predicted.
When the eclipse team announced the results in November 1919, one news account heralded them as signaling the need for “a new philosophy of the universe.”
In 2019 the Event Horizon Telescope project produced the first image of a black hole, showing the core of galaxy M87. Details of the distortion of light by the black hole, as revealed in such images, may help test the validity of Einstein’s theory of gravity. Credit: EHT Collaboration
In the century since, Einstein’s gravity has passed many additional tests, such as the spectacular detection of gravitational waves, reported in 2016. But it’s not possible to test the theory under all conceivable conditions.
And experts have long suspected that general relativity can’t be right in realms of extremely high mass density.
At the center of a black hole, for instance, the theory’s equations no longer make sense, because they imply that matter density would become infinite.
Traveling to the interior of a black hole to test general relativity would be a poor strategy, for many reasons. But scientists staying safely home on Earth can probe realms of fairly strong gravity, possibly offering clues.
One project uses a network of telescopes to image the region near the outer edge of a black hole—its “event horizon” (the point of no return for anything falling in).
Such images can provide details of how matter flows into the black hole from its “accretion disk,” a ring of orbiting material outside the event horizon.
“By analyzing the structure of the accretion flow,” writes Ferreira, “it will be possible to probe the structure of spacetime … and test whether it is consistent with general relativity.”
Gravitational waves can also provide details of gravity under extreme conditions, as when two black holes collide. Analyzing the spacetime ripples emanating from such collisions could reveal possible flaws in general relativity’s predictions.
When two black holes merge, gravitational waves are generated, as predicted by Einstein’s general theory of relativity.
If general relativity ever fails, multiple competing gravity theories proposed in recent decades would be waiting in the wings. Most of them boil down to adding a new force to nature’s repertoire of gravity, electromagnetism, and the strong and weak nuclear forces.
Apart from gravity, the three other known forces are accurately described by the “standard model,” a set of equations that obey the requirements of quantum mechanics.
General relativity does not accommodate the quantum math, though, so a major research effort has long been underway to develop a theory combining gravity and quantum theory.
“Unification of general relativity and quantum physics is widely considered as the most outstanding open problem in fundamental physics,” physicist Abhay Ashtekar of Penn State University said at a recent symposium for science writers.
Such a unifying theory, most experts believe, would entail some sort of modification to general relativity.
One way of modifying the theory would be incorporating a new energy field permeating space. The strength of such a field at different locations could alter general relativity’s predictions for matter’s behavior.
Some theorists have proposed instead that an additional source of spacetime warping—an extra layer of geometry—might be a more fruitful approach. Still other proposals, such as superstring theory, could modify general relativity by allowing more dimensions of space than the three commonly encountered. With some mathematical manipulations, all these approaches amount to adding a fifth force.
So far, tests seeking signs of a new fifth force have found nothing. But those tests have been conducted on relatively small scales (compared with the universe as a whole).
It’s possible that general relativity prevails in those tests because other physical effects mask or screen out the deviations that a fifth force would induce. But effects screened out on small scales might be noticeable on large scales, writes Ferreira.
“This is uncharted territory and one of the few pristine arenas in which we might find evidence for new physics.”
Another testable tenet of general relativity is its requirement that gravity travels at the speed of light. Gravitational waves provide a way to test that.
In 2017, the merger of two neutron stars not only sent gravitational waves to Earth (traversing a distance of 130 million light-years) but also released bursts of electromagnetic radiation, including X-rays and gamma rays, which travel at precisely the same speed as light.
Arrival time for the electromagnetic rays and the gravitational waves showed their travel speeds to be identical (within one part in a quadrillion)—ruling out many alternative gravity theories that predicted a difference.
Further such tests, and more refined observations of other cosmological features (such as the remnant microwave background radiation generated when the universe was young), might still someday find flaws in general relativity. If so, some Einstein fans may be disappointed, but most physicists won’t be. They’d relish the excitement of opening a new chapter in the history of physics.
“With the multiple new windows on the gravitational universe…, one would hope that new forces and phenomena are on the verge of discovery,” writes Ferreira. But if Einstein prevails over cosmic distances, Ferreira says, there’s a consolation prize. “In the very least, we will end up with a cast-iron theory for gravity, tested over an enviable range of scales and regimes.”
This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter.
Gravitational Waves Detected, Confirming Einstein’s Theory
Continue reading the main storyAbout a hundred years ago, Einstein predicted the existence of gravitational waves, but until now, they were undetectable.CreditCredit…Artist's rendering/Simulating eXtreme Spacetimes
A team of scientists announced on Thursday that they had heard and recorded the sound of two black holes colliding a billion light-years away, a fleeting chirp that fulfilled the last prediction of Einstein’s general theory of relativity.
That faint rising tone, physicists say, is the first direct evidence of gravitational waves, the ripples in the fabric of space-time that Einstein predicted a century ago. (Listen to it here.
) It completes his vision of a universe in which space and time are interwoven and dynamic, able to stretch, shrink and jiggle.
And it is a ringing confirmation of the nature of black holes, the bottomless gravitational pits from which not even light can escape, which were the most foreboding (and unwelcome) part of his theory.
More generally, it means that a century of innovation, testing, questioning and plain hard work after Einstein imagined it on paper, scientists have finally tapped into the deepest register of physical reality, where the weirdest and wildest implications of Einstein’s universe become manifest.
Conveyed by these gravitational waves, power 50 times greater than the output of all the stars in the universe combined vibrated a pair of L-shaped antennas in Washington State and Louisiana known as LIGO on Sept. 14.
“We are all over the moon and back,” said Gabriela González of Louisiana State University, a spokeswoman for the LIGO Scientific Collaboration, short for Laser Interferometer Gravitational-Wave Observatory. “Einstein would be very happy, I think.”
Members of the LIGO group, a worldwide team of scientists, along with scientists from a European team known as the Virgo Collaboration, published a report in Physical Review Letters on Thursday with more than 1,000 authors.
“I think this will be one of the major breakthroughs in physics for a long time,” said Szabolcs Marka, a Columbia University professor who is one of the LIGO scientists.
The wave nature of simple gravitational waves
- Simple sine oscillations
- From oscillation to wave
- Further Information
What makes a gravitational wave a wave? The standard illustrations (and animations) show the influence of a gravitational wave on a collection of particles floating in space:
In this animation, the red spheres are the free particles, and we have connected them with blue lines. While the resulting impression is that of a solid grid, the blue connections are only there for your visual convenience, to help you keep track of which particle is adjacent to which other particle. In particular, they are not any kind of solid or elastic link between the particles.
The influence of the gravitational waves shows itself in the way that the distances between the particles are changing over time.
In the simple example above, there are two distinct possibilities: Sometimes, the gravitational wave stretches all vertical distances between particles and, at the same time, squeezes all horizontal distances. At other times, all horizontal distances are stretched while all vertical distances are squeezed.
As always in such illustrations, the stretching has been exaggerated to make it visible to the naked eye – in reality, the stretching is more than a trillion billion times less pronounced.
In order to produce this pattern, the gravitational wave must be travelling at a right angle to the image plane, either directly towards or directly away from the viewer.
Simple sine oscillations
Einstein showed Newton was wrong about gravity. Now scientists are coming for Einstein
Albert Einstein can explain a lot, but maybe not black holes. Scientists believe that within the inky depths of these massive celestial objects, the laws of the universe fold in on themselves, and the elegant model of gravity laid out in Einstein’s general theory of relativity breaks down.
They don't know precisely how or where that happens, but a new study brings them closer to the answer.
The study, to be published Aug. 16 in the journal Science, shows that gravity works just as Einstein predicted even at the very edge of a black hole — in this case Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy. But the study is just the opening salvo in a far-ranging effort to find the point where Einstein’s model falls apart.
The first-ever image of a black hole, the dark circle surrounded by a swirling cloud of hot gas.Event Horizon Telescope Collaboration
“We now have the technological capacity to test gravitational theories in ways we've never been able to before,” study co-author Jessica Lu, an astrophysicist at the University of California, Berkeley, said. “Einstein's theory of gravity is definitely in our crosshairs.”
That means we may be closer to the day when Einstein’s relativity is supplanted by some as-yet-undescribed new theory of gravity.
“Newton had a great time for a long time with his description [of gravity], and then at some point it was clear that that description was fraying at the edges, and then Einstein offered a more complete version,” said Andrea Ghez, an astrophysicist at UCLA and a co-leader of the new research. “And so today, we're at that point again where we understand there has to be something that is more comprehensive that allows us to describe gravity in the context of black holes.”
In Newton’s view, all objects — from his not-so-apocryphal apple to planets and stars — exert a force that attracts other objects. That universal law of gravitation worked pretty well for predicting the motion of planets as well as objects on Earth — and it's still used, for example, when making the calculations for a rocket launch.
Gravitational Theory: Newton, Einstein & The Next Wave | AMNH
First off, forget the apple.
One probably didn’t really fall on the head of Sir Isaac Newton in 1665, knocking loose enlightenment about the nature of falling bodies. And while you’re at it, forget what you learned about gravity in school. That’s not how it really works. But don’t take our word for it. Let the main contenders in the history of gravitational theory duke it out themselves.
Round 1: Newton
“Gravity really does exist,” Newton stated in 1687. “[It] acts according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies.” Before Newton, no one had heard of gravity, let alone the concept of a universal law.
Newton could describe gravity, but he didn't know how it worked.
Cambridge University, where Newton studied, was closed due to plague in 1665. Finding respite at his childhood home, the 23-year-old plunged into months of feverish mathematical brainstorming.
This, plus a dubious apple descent in the back orchard, laid the foundation for his masterwork Philosophiae Naturalis Principia Mathematica. In Principia, Newton described gravity as an ever-present force, a tug that all objects exert on nearby objects.
The more mass an object has, the stronger its tug. Increasing the distance between two objects weakens the attraction.
Principia’s mathematical explanations of these relationships were simple and extremely handy. With his equations, Newton was able to explain for the first time why the Moon stays in orbit around Earth.
To this day, we use Newton’s math to predict the trajectory of a softball toss or of astronauts landing on the Moon.
In fact, all everyday observations of gravity on Earth and in the heavens can be explained quite precisely with Newton’s theory.
Okay, we buy it. But how does it work?
Silence from Newton’s corner of the ring.
The truth is, Newton could describe gravity, but he didn’t know how it worked. “Gravity must be caused by an agent acting constantly according to certain laws,” he admitted. “But whether this agent be material or immaterial, I have left to the consideration of my readers.”
For 300 years, nobody truly considered what that agent might be. Maybe any possible contenders were intimidated by Newton’s genius. The man invented calculus, for Pete’s sake.
Ding. Round 2: Einstein
Apparently Albert Einstein wasn’t intimidated. He even apologized. “Newton, forgive me,” he wrote in his memoirs. “You found the only way which, in your age, was just about possible for a man of highest thought and creative power.”
Albert Einstein at the Swiss Patent office in Bern. © Einstein Archives Hebrew University of Jerusalem
In 1915, after eight years of sorting his thoughts, Einstein had dreamed up (literally–he had no experimental precursors) an agent that caused gravity. And it wasn’t simply a force. According to his theory of General Relativity, gravity is much weirder: a natural consequence of a mass’s influence on space.
Einstein agreed with Newton that space had dimension: width, length, and height. Space might be filled with matter, or it might not. But Newton didn’t believe that space was affected by the objects in it. Einstein did. He theorized that a mass can prod space plenty.
It can warp it, bend it, push it, or pull it. Gravity was just a natural outcome of a mass’s existence in space (Einstein had, with his 1905 Special Theory of Relativity, added time as a fourth dimension to space, calling the result space-time.
Large masses can also warp time by speeding it up or slowing it down).
According to Einstein, an object's gravity is a curvature of space.
You can visualize Einstein’s gravity warp by stepping on a trampoline. Your mass causes a depression in the stretchy fabric of space. Roll a ball past the warp at your feet and it’ll curve toward your mass. The heavier you are, the more you bend space.
Look at the edges of the trampoline–the warp lessens farther away from your mass. Thus, the same Newtonian relationships are explained (and predicted mathematically with better precision), yet through a different lens of warped space. Take that, Newton, says Einstein.
Einstein’s theory also triumphantly punched a hole in Newton’s logic. If, as Newton claimed, gravity was a constant, instantaneous force, the information about a sudden change of mass would have to be somehow communicated across the entire universe at once.
This made little sense to Einstein. By his reasoning, if the Sun disappeared suddenly, the signal for the planets to stop orbiting would logically have to take some travel time. And it would definitely take longer to arrive at Pluto than it would Mars.
Nothing universally instant about that at all.
The Secret History of Gravitational Waves
On February 11, 2016, ecstatic scientists worldwide basked in the announcement that the Laser Interferometer Gravitational Wave Observatory (LIGO) had detected gravitational waves produced by the merger of two black holes more than a billion light years from Earth. Many members of the cosmological community had waited, literally, most of their lives to hear that announcement. At least one has confessed that his eyes welled with tears.
LIGO’s achievement rightly captured the public’s imagination, and since that historic February day the newborn science of gravitational wave astronomy has remained in the news. Additional detections have been announced on a regular basis; most spectacularly, on August 17, 2017, the facility recorded gravitational waves produced by the collision of two neutron stars.
In recognition of LIGO’s triumph, the project leaders were awarded the 2017 Nobel Prize in Physics.
Along the way, each press release or media report has been accompanied by a formulaic nod, intended to provide the ultimate seal of approval on the findings: “Gravitational waves were first predicted by Albert Einstein a century ago on the basis of his general theory of relativity.”
That sound-bite may be helpful in selling a complicated story, but it erases essentially the entire historical context.
Contrary to conventional wisdom, Einstein was not the only physicist in the early 20th century attempting to create a modern description of the gravitational field.
With hindsight, we can say that virtually any field theory of gravity will predict gravitational waves, so long as it obeys the fundamental precept that such disturbances must propagate at finite velocity.
Furthermore, Einstein himself did not immediately arrive at a definitive result.
After completing his general theory of relativity in 1916, he initially dismissed the idea of gravitational waves, and his first paper dedicated to the subject got the description very wrong.
Einstein soon hit on the correct formulation, but two decades later he rejected the physical reality of gravitational waves, and he remained skeptical about them for the rest of his life.
Gravitational waves: From Newton to Einstein to LIGO and beyond
News – 30 January 2019 14:01
It is one of the most storied anecdotes in science, one that most of us learnt in school, and that our children and children’s children will probably hear as well — of how in the 17th century, Isaac Newton was resting under a tree in his garden when an apple fell on his head, resulting in the eureka moment that led him to discover the law of gravity.
“Newton’s Theory of Gravity stood for about 200 years…it was probably the most successful theory of physics ever,” says Professor Barry Barish, an experimental physicist from the California Institute of Technology and University of California, Riverside. Prof Barish was in Singapore last week for the 7th Global Young Scientists Summit.
And yet Newton’s theory had a major flaw, he says, one that Newton himself admitted. It assumes that gravitational force is transmitted instantaneously, even if it were acting at huge distances between planets. “But it’s unrealistic to think how gravitational effects would get to us immediately.”
“It’s not that Newton’s theory was wrong,” says Prof Barish. “It was just incomplete and didn’t describe all of nature,” he says.
Enter Albert Einstein in 1915, with his general theory of relativity. Einstein proposed a radical shift in thinking: that gravity isn’t an ordinary force, but rather the result of massive objects distorting space and time around it.
Einstein’s calculations also showed that when massive objects, such as black holes or neutron stars, accelerate, they create ripples in space-time — similar to how placing a bowling ball on a trampoline distorts the space around it.
These ripples are known as gravitational waves.
“But space and time are very stiff so the amount of distortion is incredibly small and Einstein concluded that it would probably never be measured,” explains Prof Barish.
Einstein even rejected his own idea 20 years after he proposed it, writing to his friend and fellow physicist Max Born: “I arrived at the interesting result that gravitational waves do not exist, though they had been assumed a certainty to the first approximation.”
“But what Einstein couldn’t foresee was modern science, where we have powerful lasers and instruments,” says Prof Barish. “So he didn’t believe it was more than a theoretical concept at that time.”
A hundred-year scientific quest
It would take scientists a century after Einstein’s initial prediction to prove that gravitational waves did indeed exist, with Prof Barish playing an instrumental role in its discovery.
In 1994, Prof Barish took on the role of director at the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO comprises two L-shaped detectors, called interferometers, in Washington and Louisiana, which use a series of laser beams and mirrors to pick up distortions caused by gravitational waves.
As director, he helped revive what was until then, a fledgling project. He reorganised the management team, hired more scientists, developed a steady R&D programme for future upgrades, and established the independent LIGO Scientific Collaboration.
He also made technical improvements to LIGO’s interferometers, upgrading the vacuum chambers housing them, making the switch from argon to solid-state lasers, and from analog to digital controls.
On the morning of 14 September 2015, Prof Barish woke to find an email directing him to look at two graphs on an internal LIGO website.
The Washington and Louisiana detectors had both picked up a signal in the early hours of the morning, suggesting a gravitational wave had been detected.
The blip only lasted two-tenths of a second, and caused a movement so tiny it only measured one-ten-thousandth the diameter of a proton, recalls Prof Barish.
“The story actually starts 1.3 billion years ago, when two black holes coalesced and merged, releasing a gravitational wave,” says Prof Barish. “It then passed through the universe and managed to come to Earth…and we detected that distortion.”
Scarce to believe it wasn’t just an instrumental error, Prof Barish and the large team of over a thousand scientists who work on LIGO, spent months meticulously checking to confirm that what the interferometers had detected was indeed caused by a gravitational wave — the first proof of its existence. For his efforts, Prof Barish was awarded the 2017 Nobel Prize in Physics, a prize he shared with two others.
Revealing the secrets of our Universe
Since then, ten other gravitational waves have been detected at LIGO and the Virgo observatory near Pisa, Italy. All but one of the waves were formed from the merger of black holes, with the exception the result of a merger of neutron stars (collapsed core of giant stars).
The most recent announcement of gravitational wave detection in December included an event that to date, is the largest and most distant collision observed — a merger from about five billion years ago that created a black hole 80 times more massive than the Sun, which released an amount of gravitational energy equivalent to five solar masses.
Thanks to these detections, scientists now have a greater understanding of how black holes were created. Gravitational waves allow us to study black holes in a way that was previously impossible using X-rays, optical light, and radio waves.
We can now infer that nearly all stellar-mass black holes weigh less than 45 times the mass of the sun — anything more than that and they’ll be unstable, says Prof Barish — and that heavier black holes were probably created earlier in the Universe and during the Big Bang.