Gravity is a drag… and Einstein’s right again!
On May 4, scientists announced success after a 50-year quest to measure two key consequences of Einstein’s theory of general relativity. The most perfectly round objects ever created by human hand, spinning aboard a spaceship launched in 2004, have detected infinitesimal disturbances in spacetime, the invisible fourth dimension of the universe:
Earth’s gravity warps spacetime through the “geodetic effect,” which subtracts one inch per year from the circumference of the spaceship’s orbit; and
Earth’s rotation pulls spacetime around with it. Each year, through “frame dragging,” the spinning planet drags spacetime, producing a slight deviation equivalent to the width of a human hair, seen from 10 miles away.
To The Why Files, frame-dragging means that space is no longer flat, or even just warped. It is also twisted. And as a matter of principle, The Why Files likes twisted.
These consequences of predictions made in the early 20th century by history’s archetypal theoretical physicist are yet more proof that Einstein had it right, and are the latest chapters in history’s most compelling scientific detective story; which substantiated the highly theoretical speculation of a brilliant scientist through nuts-and-bolts observations of the universe.
1905: Relatively special
In 1905, the same year he finished his Ph.D. thesis, Einstein published several amazing insights, including papers on Brownian motion and the photoelectric effect (the latter won Einstein his sole Nobel Prize). One of those papers proposed a theory of “special relativity” that said that the speed of light is fixed and independent of the observer’s motion. The 1887 Michelson-Morley experiment convinced Einstein that there was no ether (the supposed physical background that allowed light to move), and that the laws of physics were the same in reference frames moving with a constant velocity relative to each other.
Common sense says that a ball thrown from a moving car will move faster than one thrown by a person standing still – and still faster for someone in another car driving towards it. Common sense, Einstein proved, does not always apply. The speed of light does not depend on whether the light source is mounted on a Stanley Steamer, a space ship or a water tower. The speed of light is constant. And it doesn’t matter whence you observe it. Light speed is light speed. End of story.
1916: General relativity
Einstein’s theory of “general” relativity described how gravity affects space and time. Following his habit, Einstein started a thought experiment — a series of “what-if” questions – related to gravity: “If I were falling through space, I would not feel gravity.” Therefore, the laws of physics did not require gravity in every situation. But since the laws of physics must apply everywhere, then gravity must result from something else, which Einstein concluded was the fabric of spacetime.
The classic explanation for spacetime is this: gravity results when the curved fabric of spacetime causes a massive object (a bowling ball or a galaxy) to distort space-time, causing other objects to fall toward the “valley” it has created in spacetime. To us, this looks like gravity, but to Einstein, it’s more a matter of geometry.
1906: Working on the proof
One year after Einstein published special relativity, scientists got some support for the theory, says Richard Staley, an associate professor of the history of science at the University of Wisconsin-Madison. Einstein and others had predicted, for different reasons, that certain fast-moving electrons would gain mass. German physicist Walter Kaufmann did some experiments, and interpreted his results as proof that the mass gain was due to a competing theory rather than relativity, but “the tests were not accurate enough to make a decisive choice between the different theories,” Staley says.
1919: Sun’s gravity bends light
The first confirmation of general relativity appeared after a highly publicized journey by British astronomer Arthur Eddington. During a total solar eclipse, Eddington observed stars that were almost directly behind the sun. As predicted by general relativity, their starlight was bent by the sun’s gravity.
Gravity, counter to intuition, could bend light, and Eddington, no dunce, became an ardent popularizer of relativity.
Although we may look back on Einstein as an oddball with a zany haircut who stuck out his tongue and rode a bike, he was a serious man who thought about politics as well as physics. Living in Germany during World War I, he was an outspoken pacifist who organized scientists against militarism. “Einstein thought we needed to think across national borders and tried to start a book project to include contributions from people from neutral and enemy countries,” Staley notes. “Most of his colleagues said it was a great idea, but would be counterproductive. They refused to participate, so it did not happen.”
Even before his fame got a boost by the 1919 confirmation of relativity, Einstein was willing to “take stances counter to others,” Staley says. “He was cautioned about going public, but when the war was finished, he decided he’d been right. Even though physics does not give you a particular insight into politics, it was clear that nobody had better insights, so he might as well make his views public.”
1974: Neutron stars and gravity waves
By the 1920s and ’30s, relativity was enshrined as a foundation of physics, but the proofs rolled on. In 1974, researchers found that a pair of neutron stars — phenomenally dense objects formed after regular stars collapse — was losing energy. Neutron stars emit extremely regular radio pulses, and the slowing of the pulses was interpreted to mean they were losing energy through the gravitational waves that general relativity predicts. The discovery won the 1993 Nobel Prize for physics.
Detecting gravity waves remains the object of an expensive, long-term scientific quest.
1979: One weighty lens
In 1936, three years after Einstein emigrated to the United States to escape the Nazis, he predicted that immense gravitation would bend light rather like a lens. Contemporary telescopes were unable to find such a “gravitational lens,” but in 1979, astronomers noticed two surprisingly similar images of a distant quasar and concluded that they were looking at a double image of one giant light source, split in two by a cluster of galaxies along the sight path to Earth.
“As usual, Einstein was ahead of the curve,” Harvard historian of science Gerald Holton told The Why Files in 1997. In 2006, a single quasar appeared in five individual images, again due to the gravity of an intervening cluster of galaxies.
Apparently a trillion stars, more or less, will do strange things…
1997: Neutron stars and frame-dragging
Although the 2011 report from Gravity Probe B was the first to identify “frame-dragging” of spacetime due to Earth’s mass, in 1997, scientists reported that rotating black holes and neutron stars were frame-dragging. The study, by Wei Cui at Massachusetts Institute of Technology, found that the gravity of a black hole spinning several thousand of times per second was distorting spacetime into a funnel shape. “It’s a very abstract thing,” Cui told us.
Black holes are extraordinarily dense points in space with a super-intense gravity that even traps light. Their presence can be deduced from a shower of X-rays produced as matter falls into the hole.
Scientists have long accepted that massive objects distort spacetime much as a bowling ball would distort a web of fabric that supports it. But frame-dragging means a rotating mass has some “sticky” quality that drags spacetime, and frame-dragging was more proof that Einstein was right, Cui said. “These are all results of his theory of general relativity, which described gravity.” In other words, gravity becomes a property of spacetime. “You can take all the facts of gravity and explain them with a certain geometry of spacetime.”
1995: The ultimate chill-out
Back in 1925, when “automobile” meant model A, and “president” meant “Silent Cal” Coolidge, Einstein predicted that a strange phase of matter would exist near absolute zero, a frosty -273°C. Expanding upon the calculations of Indian physicist Satyendra Nath Bose, Einstein calculated that atoms would enter a unified quantum-mechanical state near the coldest possible temperature.
The atoms would become a drill sergeant’s dream — identical in mind and body.
What was dubbed the “Bose-Einstein condensate” would also be a new phase of matter. Since only four phases exist in the universe — gas, liquid, solid and plasma — discovering another phase would pump up a resume.
In 1995, Carl Wieman, a professor of physics at the University of Colorado, and colleague Eric Cornell fulfilled Einstein’s prediction by creating this bizarre phase of matter at just 200-billionths of a degree Celsius above absolute zero. As Wieman told us in 1997, “We wanted to see if real atoms could ever match the ideal system that Einstein was considering, and they did match — really quite nicely.”
Quantum mechanics says that atoms can exist in certain energy states, but not in between. A group of atoms occupies numerous energy states, washing out the quantum-mechanical effects, but in a Bose-Einstein condensate, Wieman said, “You have a bunch of atoms in a single quantum state, obeying the laws of quantum mechanics as a whole. Traditionally, to see a quantum state, you had to look inside a single atom. Now we can look at millions of atoms.”
2011: Sweet success smiles on Gravity Probe B
The insights of the former Swiss patent clerk are impossible to exaggerate, but it took a lot of technical sophistication and ingenuity to detect disturbances in spacetime in the vicinity of Earth. That was the goal of Gravity Probe B.
Francis Everitt, a Stanford University physicist who has devoted his career to sailing Gravity Probe B across technological and financial shoals, compares the “dragging” of spacetime to a giant pot of honey. “As the planet rotated its axis and orbited the Sun, the honey around it would warp and swirl, and it’s the same with space and time.”
Save for the effects of gravity and relativity, the high-tech gyroscopes aboard the spaceship should point forever in one direction. Instead, gravity changes their orientation in subtle but measurable ways.
The rotors in those gyroscopes are the most precise spheres ever manufactured, which is astonishing if you consider that they were measured with “micro-inches” rather than microns.
It is not necessary to offer a practical justification for a proof of relativity – simply explaining the universe is ample. But Gary Shiu, a professor of physics at the University of Wisconsin-Madison, notes that the ultra-precise equipment crafted for the gravity probe helped improve global positioning systems and the gizmos used to map the microwave background radiation that was created shortly after the Big Bang and still pervades the cosmos. “These technologies have already been developed, the spinoff already proven,” Shiu says.
Although some of the previous proofs of general relativity could conceivably be explained with alternate theories, Shiu says, “The frame-dragging detected in Gravity Probe B provides yet another independent test that any alternative to Einstein’s general relativity would have to meet.”
A man apart
A theory must explain the working of some aspect of nature, and it must be tested, generally by trying to disprove its predictions. Does your theory say gravity is an attraction between any two objects? Then, if you can find objects that fail to attract, you need to revise or reject your theory.
After a century of confirmation of Einstein, the obvious remaining question concerns scientific creativity rather than physics: What was Einstein’s secret? “He was very persistent, was the prototypical scientist,” says Shiu, who helped organize an upcoming conference on Cosmology since Einstein. “When he wanted to solve a problem, he could take 10 or 20 years. We cannot figure out the answer in a few months or years, we need to do whatever it takes to solve the problem.”
Kip Thorne, a California Institute of Technology physicist, told us in 1997 that he attributed Einstein’s deep insight to his “conviction that the universe loves simplicity and beauty… His willingness to be guided by this conviction, even if it meant destroying the foundations of Newtonian physics, led him, with a clarity of thought that others could not match, to his new description of space and time. … All new laws that have been successful in describing the real universe have turned out to obey Einstein’s principle of relativity.”
Indeed, Thorne called relativity a kind of super-law that “must be obeyed by all laws of physics, no matter whether they are laws governing electricity and magnetism, or atoms and molecules, or steam engines and sports cars.”
A preference for the simple and universal, and an intuition that the laws of physics should be combined into one set universally applicable
A great ability to visualize interactions in nature through thought experiments
A deep intuition into the essence of a problem
Great power of concentration
Beyond a unique ability to peer inside the universe, Holton says Einstein also wrote about his philosophy and technique. “This man allowed himself to be more public and frank, and in particular about his scientific method, which is very much the method still used by other physicists.”
Yet for all his brilliance, Einstein failed to find the holy Grail of physics –a “grand unified theory” to explain all four physical forces. Electromagnetism and the strong and weak nuclear forces are explained by a single theory called the “standard model,” but to this day, gravitation stands stubbornly apart.
Summing up? Einstein
Einstein’s revolutionary theories grew from his philosophy of nature and insistence that physical laws must be true on Earth, space ships and stars, combined with a phenomenal intuition for nature and enough self-confidence to rewrite Newton’s laws of gravitation and motion. Einstein interpreted experiments from the 1880s, which suggested that the speed of light was independent of the observer’s motion, as meaning that the speed of light is constant throughout the universe. He then proposed that mass would affect light and spacetime, which is the backdrop for all events, atomic, human, cosmic and comic.
Still, everybody makes mistakes. Einstein denied the existence of black holes and loathed the role of chance in quantum theory, saying “God does not play dice with the universe.” He also cooked up a “cosmological constant” because his theories implied that the universe was changing size, which he considered too weird to be true.
When astronomer Edwin Hubble proved that the universe was expanding, Einstein called the cosmo constant “the greatest blunder of his life.” And yet recent discoveries indicating that the universe is, for unknown reasons, expanding ever faster could mean that his “greatest blunder” was not that far off…
Although Newtonian physics still describes what we see every day, more than a century after the young patent clerk brutally shouldered Newton aside, there’s no question Einstein grasped the big picture. And that returns us to this simple question: “How did he do the things he did?”
“Einstein was typically working between several different theoretical approaches,” says Staley, the science historian. “He was looking for places in which the best laws we currently have fail or don’t provide clear guidance, and then was trying to use those critical gaps to provide new insight into connections between different areas. People often think he thought outside the box. I think he thought across several boxes, and saw ways to link theory that others did not recognize. Although others were also looking at the limits of theory and trying to unify different areas, he did it better.”
- Einstein, History and Other Passions, Gerald Holton, Addison-Wesley, 1995. ↩
- The Advancement of Science, and Its Burdens, Gerald Holton, Cambridge University, 1986. ↩
- Gravity Probe B. ↩
- Videos and animations of Einstein’s theories. ↩
- Gravity Probe Btechnology. ↩
- Spacetime 101. ↩
- NOVA: The elegant universe. ↩
- Relativity and the cosmos. ↩
- YouTube: Bose-Einstein condensate. ↩
- Interactive site on black holes. ↩
- Michelson-Morley experiment in motion. ↩
- Einstein’s bio and Nobel speech. ↩
- Einstein archives. ↩
- Gravity basics. ↩
- YouTube: Gravity and spacetime. ↩
Tags: Albert Einstein, Carl Weiman, cosmology, Gary Shiu, Gerald Holton, gravity, relativity, Richard Staley, scientific creativity, scientific process method theory, theoretical physics, University of Wisconsin Madison UW-Madison, Wei Cui