C. Henze/NASA Ames Research Center
This graphic shows the results from a numerical simulation of 2 spinning black holes orbiting one another, generating powerful gravitational waves -- the phenomenon that was discovered by LIGO's super-sensitive detectors on Sept. 14, 2015.
Exactly 100 years ago on Nov. 25, 2015, physicist Albert Einstein, then 36, presented a fourth and final lecture to the Prussian Academy of Sciences about his new general theory of relativity. The idea not only redefined the concept of gravity, but also ended up reshaping humanity’s perspective on reality. Here’s a look at the theory in thought and action.ANALYSIS: Event Horizon Telescope Will Probe Spacetime's Mysteries
Max Planck Institute for Gravitational Physics
Einstein was famous for his thought experiments, which often played out for years in his imagination. From the gedankenexperiment, as it is known in German, Einstein grasped fundamental concepts about the physical world that could be verified by observation and experiments. One of his most famous ones began in 1907 when Einstein pondered if a person inside a windowless elevator could tell if he was in a gravitational free-fall, or if the elevator was being hauled up by a constant acceleration. Einstein decided the laws of physics must be the same in both cases. The mathematical equation he derived to explain this so-called principle of equivalence, which equated the effects of gravitation with acceleration in zero-gravity, became the basis for general relativity.NEWS: Einstein's Theory Passes Extreme Gravity Test
A total solar eclipse on May 29, 1919, gave astronomers an opportunity to verify Einstein’s general theory of relativity by proving that the sun’s gravitational field was bending the light of background stars. The effect was only observable during time when the sun’s light was dim enough for stars to become visible. British astronomer Arthur Eddington led an expedition to the island of Principe, off the West Coast of Africa, to photograph the eclipse, which lasted nearly seven minutes. The images of stars in the region around the sun proved that Einstein’s interpretation of gravity trumped the 200-year old Newtonian model, which interpreted gravity as a force between two bodies. Einstein saw gravity as warps and curves in space and time.Time Probe: Crowdsourcing Einstein's Relativity
In 1917, Einstein amended his general relativity theory to introduce what he called the “cosmological constant,” a mathematical way to counter the force of gravity on a cosmological scale and stave off the collapse of the universe, which the general relativity theory posited. At the time, astronomers believed that the Milky Way was surrounded by an infinite and static void. In 1923, Edwin Hubble and other astronomers find the first stars beyond the galaxy and by 1929 Hubble provides evidence that space is expanding. Einstein realized the cosmological constant was a blunder. Or perhaps not. In 1998, scientists made the startling discovery that the expansion of the universe is speeding up, driven by an anti-gravity force called dark energy, which in many ways acts like Einstein’s cosmological constant. Pictured here is the Hubble Space Telescope’s extreme deep field view, which contains about 5,500 galaxies. The telescope is named after Edwin Hubble.ANALYSIS: Hawking Tries to Find Black Hole's Emergency Exit
One of the first implications of the general relativity theory was the realization that if an object is compressed enough, the dimple it generates in the fabric of space and time will be too strong for even photons of light to escape. Thus, the idea of black holes was born. Though they can’t be directly observed, astronomers have found black holes of all sizes by measuring how they affect nearby stars and gas. Pictured here is an artist’s rendering of a black hole named Cygnus X-1, siphoning matter from a nearby star.ANALYSIS: Is the Wormhole in 'Interstellar' Possible?
NASA/Swinburne Astronomy Productions
Like ripples in a pond, scientists believe that gravity transmits in waves, deforming space and time across the universe. It is similar to the movement of electromagnetic radiation, which propagates in waves, except that gravitational waves are moving the fabric of space and time itself. So far, attempts to find gravitational waves, such as those caused by two black holes colliding for example, have been unsuccessful. Next week, the European Space Agency plans to launch a prototype space-based observatory called the evolved Laser Interferometer Space Antenna (eLISA) to test a technology to find gravitational waves. Pictured above is an artist's rendering of two merging galaxies rippling space and time.NEWS: Advanced LIGO Resumes Quest for Gravitational Waves
The Feb. 11 announcement that scientists had, for the first time, proof that space itself vibrates is expected to unleash a bevy of discoveries about things that go bump in the proverbial darkness.
The initial detection of so-called gravitational waves occurred in September when a pair of black holes, each about 30 times more massive than the sun, spiraled in toward each other and then merged into a new, larger black hole more than 1.3 billion light years away.
In a flash, the crash released the equivalent of 50 times the energy of all the stars in the universe, powerful enough to ever-so-slightly jiggle L-shaped, 2.5 mile-long laser beams on Earth that comprise the heart of the Laser Interferometer Gravitational-Wave Observatory, or LIGO.
LIGO observatories in Louisiana and Washington had just been upgraded when the detection was made. Scientists spent months verifying the gravitational waves’ footprint, which changed the length of the laser light arms an amount 10,000 times smaller than the diameter of a proton. Meanwhile, LIGO continued to monitor for other space-shaking cosmic booms.
“Before this, we didn’t even know that black holes existed in pairs,” University of Florida physicist David Reitze, now serving as LIGO director at the California Institute of Technology, told the House Science Committee last week.
“It’s the start of a new astronomy,” added Massachusetts Institute of Technology physicist David Shoemaker.
The LIGO detectors collected data for another three months, then shut down to prepare for an even larger boost in sensitivity.
Additional findings have yet to be released, but Louisiana State University physicist Gabriela González, a spokeswoman for the LIGO Scientific Collaboration, hinted to legislators that the detection of the merging black holes was not a solitary event.
“We saw one event in one month … so we can only predict from that data. But we have taken data for three more months, which we are still analyzing and everything that we see is consistent with what we saw there,” Gonzalez said.
From theoretical models, scientists expect to be able to detect at least a few gravitational wave events per year, she added.
Merging black holes aren’t the cosmic events likely to vibrate the fabric of space and time.
Scientists are hopeful that LIGO will sense the rumblings of neutron stars, which are the dense remnants of collapsed stars so packed with matter that a single teaspoon would weigh 10 million tons.
Typically, neutron stars are magnetized and spinning, though how that works is not fully understood. They also can exist in pairs, giving scientists an opportunity to detect not only gravitational waves set off by their interactions, but X-ray, radio waves and other electromagnetic radiation they produce.
“We can put together all this information … and know more than we could have ever known without gravitational waves or without this combination, this synergy of information,” Shoemaker said.
LIGO also may be able to ferret out supernova explosions, collapsing stars, cosmic strings and even what Shoemaker calls “defects” in the interwoven fabric of spacetime.
“There certainly will be surprises. Every time we open up a new window to the universe, we see new things,” Shoemaker said.
By the time LIGO returns to work late this summer or early fall, it may be joined by the first of several planned laser interferometers outside the United States.
Virgo, a French- and Italian-backed project, located near Pisa in Italy, will add a third ear to detect and verify gravitational waves and pinpoint their source.
Virgo also will serve as a backup if either of twin U.S. LIGO detectors is offline. Having at least two simultaneous detections is key to being able to rule out possible terrestrial sources of vibrations.
Japan is developing a gravitational wave detector as well, and last week the Indian government gave its approval to move ahead with the LIGO-India project.
Europe also is testing a space-based gravitational wave detector called LISA Pathfinder.
“In space, instead of having 2.5-mile long arms (to detect gravitational waves), you can have 2.5-million mile long arms. Our sensitivity grows with the length of those arms,” Shoemaker said.
Since gravitational waves, like electromagnetic radiation propagate at different lengths, scientists expect that multiple gravitational wave observatories will be needed to study different phenomena.
“We are looking at the dark side of the universe, about which we know very little,” Gonzalez said.