Over 100 years after its first publication in 1915, Einstein’s theory of general relativity has finally been completely verified by experimental evidence. But what was the final piece of the puzzle, why did his theory take so long to verify, and why should you care?
General relativity is Einstein’s interpretation of gravity. To date, it is the most accurate and elegant way to describe the complexity of the universe at astronomical and cosmological scales, and it does so in relatively (pun intended) simple terms. All of the burning balls of plasma and massive clouds of cosmic dust are described in perfect detail by 12 – or rather 16 – equations that represent the dynamics of existence.
At first glance, this would be absolutely terrifying to most people: all those subscripts and Greek letters replacing the more pedestrian of high-school physics. But upon closer inspection, it reveals a rather beautiful simplicity. The left side of this equation describes the geometry of the fabric of the universe and how it ‘bends’ or ‘warps’. The right side describes how the matter and energy within the universe moves. This beautiful equation allows us to understand many of the wondrous events that occur within our universe: star-forming nebulae, the orbits of the planets, and even cataclysmic events like the merging of black holes. Space and time are described as one and the same; components of Einstein’s fabric: space-time. Space-time is made of the three dimensions of space – up and down, left and right, forward and back –and a fourth dimension of time. What my main man Einstein explained with the above equation is that gravity is simply a manifestation of matter warping space-time.
The merging of two black holes is a perfect example of a cosmic event that releases so much energy that it produces periodic ripples in space-time, i.e. gravitational waves: the last unverified prediction of general relativity. As of February 2016 however, these waves have been discovered.
What is a gravitational wave?
Imagine throwing a stone into a pond. The ripples travel across the surface of the pond transferring energy as they go. Now we replace the water with space-time and the ripples become gravitational waves. In the same way that there are troughs and peaks in the water waves, there are compressions and expansions of space-time as a gravitational wave travels. Any mass moving through space-time will produce waves as it does so, just like a boat in the ocean. These waves are space itself compressing and expanding.
That should be pretty easy to detect, right? People should be getting noticeably shorter and fatter when a star goes supernova half a million light years away? Wrong. Because the effects of gravitational waves are so incredibly, unimaginably small. The change in height of a person is on the scale of tiny fractions of a proton. It seems almost impossible to even feasibly be able to measure such a distance; this is where LIGO comes in.
LIGO stands for Laser Interferometer Gravitational wave Observatory. It features two perpendicular lasers bouncing back and forth within the 4km-long arms of the detector. Initially the beams are perfectly in sync, but if a gravitational wave passes through the detector the beams become out-of-sync, as it compresses one of the 4km arms and expands the other. If the beams become out of sync, there is a variation in the light output.
The output signal is then compared against theoretical signals calculated using Einstein’s equations. Analysing the raw output signal itself would be a nightmare as a tremor on the other side of the world is ‘loud’ enough to be detected by LIGO.
What was the smoking gun that proved gravitational waves?
When two black holes are orbiting around one another, it is called a binary black hole system. After millions of years, this orbit can deteriorate, and when the two black holes get close enough to one another, something interesting happens. They begin to orbit each other tightly, speeding up and warping space-time as they do. As their orbital distance contracts, the black holes emit massive amounts of their mass as energy in the form of gravitational waves. In the moment of merging, the black holes are orbiting one another at the unimaginably fast speed of half the speed of light.
Once they merge, a mass equivalent to about three times that of our sun is converted into gravitational energy. It’s fitting that it would be the first event to produce gravitational waves that we could detect, as it is so ‘loud’. It’s also our first evidence of such a binary black hole system, which would have been unable to detect using light astronomy as this is a ‘dark’ event.
Einstein, using no more than a pen and paper, was able to use mathematics to derive the secrets of the universe. His ideas have not only had profound impacts on physics and the philosophy of science, but have enabled humanity to grasp far further than we ever could have without them. No theory is as well-known or as accurate. It is now completely and unequivocally supported by experimental evidence. Where do we go from here? There are still plenty of unknowns in the universe: from dark matter to dark energy. We may need a new theory in order to understand them, but, regardless, Einstein will always be among the likes of Newton or Dirac; the giants, whose shoulders we stand on.
Dr. Eric Thrane, of Monash University was tasked with inserting false readings into the machine in order to test its efficacy:
What was the process that LIGO went through in order to ensure that the gravitational wave they detected wasn’t simply random noise or a misreading of the equipment?
Extraordinary claims require extraordinary evidence, and so the vetting of this detection was extremely thorough. The LIGO Collaboration (and our partners the Virgo Collaboration) wrote an entire paper devoted just to this topic. To make a long story short, there are no known detector artefacts that mimic the very particular signature of binary black hole system, and the fact that we see the same signal in two detectors on opposite ends of the USA very strongly suggests that it was astrophysical.
Can you describe how it felt when you saw the results that proved gravitational waves?
It was a roller coaster ride: excitement, disbelief, relief. It’s also important to point out that, at first, we didn’t know that the signal was real. It took weeks of detective work to gain confidence. Thus, the initial excitement was tempered with caution.
Einstein has now been completely vindicated. Where does astrophysics and cosmology go from here?
As a gravitational-wave astronomer, vindicating Einstein is just the beginning. With the first detection of gravitational waves, we are opening up a new window on the Universe. Now we can probe a side of the Universe we’ve never seen before. Who knows what we’ll find.
How do you feel personally now that Einstein’s theoretical legacy is completed?
It’s a source of pride to have any connection to Einstein. He’s such a towering figure in physics.