This simulation shows the merging of two stellar-mass black holes, their gravity warping the starlight in the background.
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
We now live in a universe that we know is humming with gravitational waves.
Before the historic announcement on Thursday morning at a National Science Foundation (NSF) meeting in Washington D.C., there were only rumors that the Laser Interferometer Gravitational-Wave Observatory (LIGO) had discovered this key component of Albert Einstein's General Relativity, but now we know that the reality is even more profound.
With stunning clarity, LIGO was able to “listen in" on the moments before a black hole binary system (two black holes orbiting one another) merged as one, producing a gravitational wave signal that was so clear, so in keeping with our theoretical models, there was little room for speculation. LIGO had witnessed a powerful black hole “re-birthing" that occurred around 1.3 billion years ago.
Gravitational waves have always been there and always will be, washing through our planet (indeed, washing through us), but only now do we know how to find them. We've now opened our eyes to a different kind of cosmic signal — the vibrations caused by the most energetic events known — and we are therefore witnessing the birth of a brand new field of astronomy.
“We can now hear the universe," said LIGO physicist and spokesperson Gabriela Gonzalez during Thursday's triumphant meeting. “The detection is the beginning of a new era: The field of gravitational astronomy is now a reality."
Our place in the universe has changed profoundly and this discovery's impact could be as transformative as the discovery of radio waves or the realization that the universe is expanding.
Making Robust Theories Even Stronger
Trying to explain what gravitational waves are and why they're so important is almost as complex as the equations that describe them, but finding them not only strengthens Einstein's already robust theories as to the nature of spacetime; we now have a tool that can probe into a layer of the universe that was once invisible to us. We can now sample the spacetime ripples generated by some of the most energetic events that occur in the universe and, perhaps, use gravitational waves to reveal new physics and discover new astrophysical phenomena.
“Now we have proven that we have the technology to go after and detect gravitational waves, this opens up many possibilities," Luis Lehner, of the Perimeter Institute for Theoretical Physics, Ontario, told me during an interview soon after Thursday's announcement.
Lehner's research focuses on compact objects (such as black holes) that generate powerful gravitational waves. Though not affiliated with the LIGO collaboration, Lehner was quick to realize the ramifications of this historic discovery. “This signal couldn't be better," he said.
This is an aerial photograph of the LIGO Livingston Observatory in Louisiana. LIGO SCIENCE COLLABORATION
The discovery is profound in 3 ways, he argues. First, we now know that gravitational waves exist and we know how to detect them. Second, the signal detected by the LIGO stations on Sept. 14, 2015, is the strongest evidence yet of the existence of a binary black hole system — each black hole “weighing in" at a few tens of solar masses. The signal is exactly what we'd expect to see during the violent merger of two black holes, one 29 times the mass of our sun and the other 36 solar masses. Thirdly, and possibly even more important, “short of sending someone to a black hole," this is the strongest direct evidence of the existence of black holes.
This event was also very lucky, as many scientific discoveries tend to be. LIGO is the biggest project funded by the National Science Foundation and it was originally put online in 2002. It turned out that, after many years of seeking out the elusive signal of gravitational waves, LIGO simply wasn't sensitive enough and in 2010 the observatory went offline while its international collaboration worked on a huge sensitivity upgrade. Five years later, in September 2015, “Advanced LIGO" was born.
At the time LIGO co-founder and theoretical physics heavyweight Kip Thorne was positive that Advanced LIGO would be a success, telling the BBC: “We are there; we are in the ball park now. It's clear that this is going to be pulled off." And sure enough, within days of the upgrade, a surge of gravitational waves rippled through our planet and LIGO was at last sensitive enough to observe them.
This binary black hole merger isn't thought to be particularly special in its own right; it is calculated that these kinds of events happen once every 15 minutes somewhere in the universe. But this merger happened in the right place (1.3 billion light-years away) at the right time (1.3 billion years ago) for LIGO to be listening. It was a clear signal from the universe that Einstein got it right and his gravitational waves were real, revealing a cosmic event that unleashed a peak power 50 times the power output of all the stars in the universe combined. This huge blast of gravitational wave energy was recorded as a high-frequency “chirp" by LIGO as the black holes rapidly spiraled into one another, merging as one.
To confirm the propagation of gravitational waves, LIGO is comprised of 2 observing stations, one in Louisiana and the other in Washington. To rule out false positives, a candidate gravitational wave signal needs to be detected by both stations. And the Sept. 14 event was detected first in Louisiana and then 7 milliseconds later in Washington. The signals matched and, through triangulation, physicists were able to learn that it originated in Southern Hemisphere skies.
Gravitational Waves — What Are They Good For?
So we have a confirmed black hole merger signal, what now? This discovery is historic, that much is clear — one hundred years ago, Einstein wouldn't have dreamed that these waves would be detectable, but here they are.
General relativity was is one of the most profound scientific and philosophical realizations of the 20th Century and it forms the basis of some of our most intellectual investigations into reality itself. Astronomically, the applications of general relativity are clear; from gravitational lensing to measuring the expansion of the universe. But what's not so clear are the everyday applications of Einstein's theories, but much of today's technology uses lessons from general relativity and things we take for granted. Take, for example, global positioning satellites: they wouldn't be the precise tools that they are if simple corrections for time dilation (a general relativity prediction) weren't considered.
It's clear that general relativity has real-world applications, but when Einstein presented his new theory in 1916, it's highly doubtful that any application would have seemed obvious. He was simply piecing together the universe as he saw it and general relativity was born. So now another component of general relativity has been proven, how might gravitational waves be used? Well, astrophysicists and cosmologists are obviously thrilled.
“Once we've collected data from pairs of black holes, they will be like lighthouses scattered through the universe," said theoretical physicist Neil Turok, Perimeter Institute Director, in a video presentation on Thursday. “We will be able to measure the rate the universe is expanding, or how much dark energy there is in the universe to extraordinary precision, far, far greater than what we can do today
“Einstein developed his theory with some clues from Nature but made basically on the grounds of logical consistency. One hundred years later you're seeing its predictions confirmed at exquisite precision."
What's more, the Sept. 14 event has some peculiarities physicists are looking forward to investigating. For example, Lehner pointed out that from analysis of the gravitational wave signal, the “spin" or angular momentum of the merged black hole can be measured. “If you've worked on the theory for long enough, you'll know that spin the black hole has is very, very peculiar," he said.
For some reason, the final spin of the black hole is slower than expected, indicating that the two black holes collided at a low speed, or they were in a collision configuration that caused their combined angular momentum to counteract each other. “That is very curious; why would nature do that?" said Lehner.
This early puzzle could be down to some basic physics that hasn't been considered, but more excitingly it could reveal some “new" or exotic physics that is interfering with the predictions of general relativity. And this highlights another use for gravitational waves: as they are generated by strong gravity phenomena, we have a means to probe these environments from afar, perhaps turning up some surprises along the way. Also, we might combine observations of astrophysical phenomena with the electromagnetic signals to add more dimensions to our understanding of what makes our universe tick.
Naturally, when huge announcements are made of complex scientific discoveries, many people outside of the scientific community ask how it affects them. The profundity can be easily missed and this is definitely the case when it comes to gravitational waves. But consider this: When X-rays were revealed by Wilhelm Roentgen in 1895 during his experiments on cathode ray tubes, few would have known that in only a few years these high-energy electromagnetic waves would become a key component in everyday medicine from diagnosis to treatment. Likewise, the first experimental production of radio waves in 1887 by Heinrich Hertz confirmed predictions by James Clerk Maxwell's famous electromagnetic equations. Only years later, in the 1890′s, a series of demonstrations by Guglielmo Marconi, who set up radio transmitters and receivers, proved they had a practical use. Also, Schrodinger's equations describing the unfathomable world of quantum dynamics are finding an application right now in the development of super-fast quantum computing.
A LIGO engineer assesses the interferometer for contaminants.LIGO
All scientific discoveries are profound and many eventually have everyday applications that we take for granted. For now, the practical applications of gravitational waves may seem restricted to astrophysics and cosmology — we now have a window into a “dark universe" where no electromagnetic radiation is required. There is little doubt in my mind that scientists and engineers will find other uses for these spacetime ripples besides the awesome application of probing spacetime. That said, to detect these waves in the first place huge advances in optical engineering had to be performed by LIGO that will inevitably spawn new technologies.
Ultimately, the detection of gravitational waves is a triumph for humanity that will continue to teach us new things about our universe for generations to come. This is most definitely a golden age for science, where historic discoveries are commonplace. These discoveries drive our culture forward, making us all richer and more aware that our universe is a beautiful and complex place. And we know we have the intellectual capability to create models of how we think the universe works and then perform experiments to prove we are right.
But for me, I'm most excited to see the first “live" gravitational maps of the cosmos, where the periodic humming of neutron stars orbiting one another and the impulsive eruptions of supernovas are plotted, revealing a new universe, a universe humming with ripples in spacetime.