Exactly one hundred years ago, Albert Einstein presented the correct version of his general theory of relativity. Evidence for that theory has piled up over the past century. From solar eclipses to flying atomic clocks, scientists took everything they could to disprove Einstein's theory. Without success.
A solar eclipse is a rare and fantastic sight, but the one on May 29, 1919 added to the excitement. Especially for the British scientist Arthur Eddington. He had devised a plan to settle a scientific dispute between two titans. Not that Isaac Newton and Albert Einstein were really arguing, after all, they lived centuries apart, but their theories made different predictions about how strongly gravity deflects light rays.
Although the general theory of relativity had made good predictions about strange anomalies in the orbit of the planet Mercury, the theory was far from universally accepted. Many scientists still adhered to Newton's 17th-century laws in that regard.
Eddington had the perfect experiment in mind. He wanted to test how the sun — by far the heaviest object in the solar system — bends light from stars in the background. Stars just next to the sun would shift a little bit due to its gravity. And since Einstein's theory predicted a shift twice as large as Newton's, an accurate measurement would be the deciding factor.
A solar eclipse was the perfect opportunity:normally the brightness of the sun makes such a measurement impossible. But that problem disappears the moment the moon moves in front of the sun. And that was about to happen on May 29, 1919.
In his experiment, Eddington wanted to observe the gravitational influence of the sun on starlight from the bright star cluster of the Hyades. General relativity predicted a minuscule shift of about 1.75 arcseconds (that's the thickness of a hair nearly 40 meters away). But Newton's laws predicted an even smaller effect.
Two scientific teams set out to see the solar eclipse. One team went to Sobral in Brazil, the other to the island of Principe off the coast of West Africa. Ultimately, the expeditions recorded shifts of 1.6 and nearly 2 arc seconds. Eddington published his results on November 6 of that year, and they proved to many scientists that Einstein was right. The news even made the front pages of newspapers around the world. General relativity had made Einstein a celebrity.
Mercury's Crazy Orbit
Although the solar eclipse of 1919 is considered the first real test for general relativity, there was already an older problem that Einstein could solve with it. In 1859 the French mathematician Urbain Le Verrier published a book in which he stated that Mercury was not doing what it was supposed to do according to the laws of classical mechanics:orbiting the sun in a neat ellipse.
Le Verrier had studied the times of the transits of Mercury – when the planet passes in front of the sun – for the previous 150 years and noticed that the transitions started just a little faster than expected. Some of this anomaly could be explained by gravitational perturbations from the other planets, but in the end it was general relativity that came up with a complete solution. Einstein stated that due to the curved space close to the sun, deviations occur in the 'classic' planetary orbits. This enabled him to explain his publication the deviation of one-hundredth of a degree per century.
Huge calculator
But what exactly had Einstein come up with? To explain that, we cannot avoid a small lesson in physics. The now exactly one hundred years old general theory of relativity actually builds on Einstein's ten years older special theory of relativity. In it he stated that it doesn't really matter how fast an observer moves, the laws of nature are always the same.
On board a train traveling almost at the speed of light, nothing is substantially different:a clock ticks (to the observer) just as fast, a scale indicates the same. That seems logical, but even more strange is that the unwavering speed of light is still the speed of light, even in a vehicle that itself is almost at the speed of light. The theory therefore has strange implications for time, which ticks faster or slower for observers at different speeds.
With Einstein's elegant special theory of relativity, a lot suddenly seemed to fall into place, but there was a problem:accelerated observers. “They actually completely messed up the special theory of relativity,” says Gijs Nelemans, astronomer at the astrophysics department of Radboud University Nijmegen. But Einstein found a solution. He realized that acceleration and gravity are very similar. In fact, they are indistinguishable. For example, you cannot determine whether you feel gravity or an acceleration in a closed elevator.
Einstein argued that this had far-reaching consequences for gravity:it is not a classical force, such as the electromagnetic force, for example. On the contrary, it is a result of the distortion of space. Objects that normally move in a straight line bend simply by following the curvature of space. The Earth that makes a circular motion around the sun is, in Einstein's view, a planet that describes a straight line in a space curved by the sun. “This approach ensured that accelerated observers also fit in with the theory of relativity,” says Nelemans. “It took a very long time to get this mathematically correct on paper. That was a huge calculation.”
Einstein's waves
Nelemans himself is involved in research that will probably put general relativity to the test again. He and colleagues are looking for gravitational waves. Einstein's theory predicts that when two extremely compact and heavy objects – for example neutron stars or black holes – orbit each other, they not only distort space but also create waves, so-called gravitational waves.
When such a gravitational wave passes the earth (at the speed of light), the theory states that this changes the distances between objects for a moment, although these are minuscule influences:a meter only becomes 10 -20 meters longer or shorter. But how can you measure such changes? “One problem is that such a wave affects all matter, including a 'physical ruler' with which you want to measure changes in space,” says Nelemans. “You don't actually measure anything at all!”
This can be solved by using a ruler that does not change with the space:a ruler of light. That too is actually an 'invention' of Einstein. “His special theory of relativity states that the speed of light is always the same,” says Nelemans. “The moment a gravitational wave passes, only the frequency of the light will change for a while.”
Gravitational wave detectors such as LIGO (Laser Interferometer Gravitational-Wave Observatory are looking for such a change). ) in the United States and Virgo in Italy. The detectors consist of two kilometers long and perpendicular arms with powerful lasers that extinguish each other exactly in the middle. The moment the frequency of one of those laser samples changes even a fraction, you can notice that at the point that the rays touch each other:they no longer extinguish each other for a moment.
Both observatories have already had a measurement campaign for many years, in which they have not detected a single gravitational wave. But the scientists involved are not going to stop there. “This year, an improved LIGO will already be measuring, and a year later the Virgo upgrade will also be ready,” says Nelemans. “In 2019 they have their maximum sensitivity. Then they really have to start measuring something, because it is unlikely that we will need even more sensitive detectors.”
I'm going on a trip and take with me:an atomic clock
Perhaps the strangest implication of Einstein's work is the stretching of time. Is that really true, Joseph Hafele and Richard Keating must have wondered more than forty years ago. Together they did the most practical experiment for the theory of relativity in 1971. At a time when the theory of relativity had long been widely accepted among scientists.
Both special and general relativity predict the so-called time dilation. The first theory states that time will slow down for someone traveling at great speed through the universe.
Not that that person notices anything:everything and everyone on board the spaceship is subject to the stretching of time. Clocks on board tick slower, computers slow down, and because natural processes cannot escape time dilation, an observer's perception is also slower. According to the general theory of relativity, exactly the same happens when someone is in a strong gravitational field.
Hafele and Keating wanted to test both theories with an experiment. They would take four extremely precise atomic clocks on a plane trip, twice around the earth. That should already yield a measurable time dilation, due to the aircraft's speed and the fact that they experience a slightly smaller Earth gravity at altitude. Of course, the scientists also left an atomic clock in their lab in Washington for verification. At 7:30 p.m. on October 4, the first shipment of atomic clocks departed, eastward, with Earth's rotation. The double world trip took just over 65 hours. Just over a week later, on October 13, the same shipment of bells was sent west. On a journey of more than 80 hours, also twice around the planet.
When they returned to Washington, they compared their atomic clocks with clocks left on the ground. And what turned out? The clocks that had traveled east were on average 59 nanoseconds behind. The clocks that had gone west were 273 nanoseconds ahead. It was only billionths of a second, but the effect was unmistakable and, moreover, in accordance with the theory. The most precise clocks in the world hadn't ticked as fast – a boost for Einstein.
Relativity on a large scale
Those experiments provided important evidence for Einstein's theories, but also show that the relativistic effects hardly play a role in our life on earth. However, if you look at cosmic scales or at extreme objects, that changes. For example, if light is not bent by the sun, but by an entire galaxy, with hundreds of billions of stars. That can create a special effect:Einstein rings.
When two galaxies line up exactly as seen from Earth, the leading object can bend light from the farthest galaxy so that it appears as a ring around the leading object. Einstein wrote in a 1936 publication:"Of course there is no hope that we will ever directly observe this phenomenon." He was proved wrong, because he only took this effect into account with stars. But nowadays a dozen (partial) Einstein rings are known, caused by (clusters of) galaxies.
The general theory of relativity is also noticeable in the 'colors' of distant galaxies. As light moves from a strong gravitational environment to a low gravitational field, the wavelength of the light increases. The light is, as it were, slightly shifted to the red, hence the name gravitational redshift. We therefore see light from the heavy center of galaxy clusters somewhat 'redder' than light from the edge of such a cluster.
Worthless GPS without relativity
You might think that the theory of relativity has little practical use, after all it makes predictions for extremely fast and heavy objects.
Yet there is one system that would not work without regard for relativity:satellite navigation such as the US Global Positioning System (GPS).
That navigation system uses atomic clocks on board satellites that orbit around the earth at roughly 14,000 km/h at an altitude of about 20,000 km. Both the speed and the fact that Earth's gravity is nearly halved at that altitude mean that the clocks aboard the satellites would be about 38 microseconds a day faster than Earth clocks. If engineers did not take this into account, the system would build up an error in the positioning of about 10 kilometers per day!
General relativity has passed numerous tests over the past hundred years, and time and again Einstein passed with flying colors. Yet detectors such as LIGO and VIRGO are once again examining the theory. Nelemans says it will test the theory in areas where it has never been tested.
“Basically, all the tests to date have been done with objects traveling less than one percent of the speed of light,” he says. “Einstein says that it becomes interesting when objects move towards the speed of light, because then the effects become much greater. The real character of the theory of relativity shows itself at high energies, which we should be able to measure with extreme objects such as fast-moving neutron stars or black holes. Perhaps these measurements will give us hints about areas where the theory is beginning to crumble.”