In February 1919, Frank Dyson of the Royal Greenwich Observatory and Arthur Eddington of Cambridge University arranged for two teams of astronomers to observe and photograph a solar eclipse that was to occur in May of that year as it moved across South America, the Atlantic Ocean, and Africa. One team was based in Sobral, Brazil, and the other on the small island of Principe off the West coast of Africa.
The expeditions were meant to put Albert Einstein’s theory of general relativity to the test. Published in 1915 after years of mathematical trial and error, the theory had proven divisive and controversial among the scientific community at the time.
One of the predictions of general relativity was that light passing by an object like the sun would appear to bend. Because of their size, Einstein’s theory posited, astronomical bodies would cause a distortion in space-time so that even light waves – which travel at the absolute cosmic speed limit of 186,000 miles per second (300,000 kilometers per second) and in perfectly straight lines – would distort as well.
The darkness of an eclipse presented the astronomers with an opportunity to observe the stars that appear closest to the sun from Earth’s perspective, as they would otherwise be completely washed out by our star’s brightness.
The teams’ mission was a success. Having taken photos of a cluster of stars known as the Hyades, located in the constellation Taurus, they compared the images with reference shots of the same cluster that had been taken at night, to calculate any potential difference caused by the sun’s presence. In November of that year, both Dyson and Eddington announced that their findings supported the theory. The skeptics had been silenced and Einstein and his work were instantly catapulted to fame.
General relativity: let’s get weird
General relativity can be described without hyperbole as one of the deepest scientific insights into the universe that humanity has ever worked out. After Eddington and Dyson announced their results, the physicist J.J. Thomson reportedly described Einstein’s theory as, “a whole continent of scientific ideas.”
Scientists are still exploring the topography of that continent. The curving of light around massive astronomical objects is just one odd physical manifestation the theory describes.
If you’ve ever found yourself perplexed by the ideas expressed by general relativity, you’re in good company—the theory essentially keys humanity into the fact that space is bendable and the universe is weird. While perhaps an ungraceful description from a technical perspective, that is the essential truth of the environment that everyone and everything inhabits.
The simplest way to explain the theory is that an object’s mass correlates to its gravitational force. The greater the mass, the greater the gravitational force, and the greater the force, the more distortion of space occurs. The strange thing to keep in mind is that the effects of gravity are the visible result of the very distortion of the fabric of space-time. If you drop a marble from the top of a skyscraper, it doesn’t so much freefall through a stable matrix of space as it does follow the bending of space to the ground.
Here’s where things get really trippy. Because that fabric is composed of four dimensions—three dimensions of space and one of time—massive cosmic bodies also bend time. The two are inextricably linked to one another, hence the phrase "space-time".
Back to our marble. If placed in Earth’s orbit, that marble will fall toward the planet's surface due to the curvature of space. But, as we’ve noted, time is a dimension of that marble’s position. As the curvature of space pulls it physically closer to the earth, the curvature of time moves the marble forwards through time as well.
Time is of the essence of space
Professor Eric Poisson is a theoretical physicist and research leadership chair at the Department of Physics at the University of Guelph in Ontario, Canada. His work has helped us better understand some of the most mysterious astronomical bodies in the universe: black holes, neutron stars, and the gravitational waves they produce.
In an interview with Interesting Engineering, Dr. Poisson explained how time won’t always flow the same way due to the effects of general relativity, and how this holds true even when measured at different places on Earth.
“One of the first things that Einstein discovered is that if you have two clocks, and one is moving relative to another, they’re going to tick at different rates. The second thing that he taught us was that it’s not just relative motions between clocks, it’s the relative position in a gravitational field. So, a clock deep in a gravitational field would run slow compared to a clock far out from any sources of gravity. ”
This is why a clock running atop Mount Everest is going to measure time differently than one ticking away at sea level, for example, with the clock on the mountaintop running just slightly faster than the one at lower elevation.
Most clocks aren’t sensitive enough to actually register this difference, however, which amounts to some millionths of a second. While that might seem like a cute, gameshow-ready science fact, it actually reveals how general relativity affects our daily lives.
“The GPS system involves clocks based in orbit,” continued Poisson. “Those timestamps coming to us through radio signals are worked out to reveal a position on Earth. We’ve got a whole bunch of ticking clocks at different places in orbit, clicking at different rates, particularly different than clocks on Earth. If we didn't know about that [temporal] difference due to gravity, we’d find the whole system doesn’t work as planned. It’s a very fundamental aspect of gravity for us on a daily basis.”
Gravitational waves and binary black holes
Poisson and other astrophysicists are now taking general relativity and running with it in new directions. His research deals largely in black holes and the gravitational waves they produce, for which he was awarded the 2005 Herzberg Medal by the Canadian Association of Physicists. And it’s because of his work, alongside that of his former graduate advisor Werner Israel, that we now better understand the internal workings of black holes, the most confounding thing in the observable universe.
Gravitational waves are a cosmic phenomenon predicted by Einstein in 1916 and observed for the first time in 2015 by the LIGO Scientific Collaboration, a group of scientists dedicated to detecting and utilizing the waves as a new tool with which to study the universe.
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Poisson’s contribution to detecting these waves has been crucial. In the early 1990s, he spent three years working under Kip Thorne, one of the founding members of LIGO, who was awarded the Nobel Prize in Physics in 2017, along with Rainer Weiss and Barry C. Barish. Poisson’s way of mathematically solving perturbative gravitational wave equations is a key part of the analyses used by groups like LIGO to detect their existence.
Put simply, gravitational waves are minute ripples in the fabric of space-time produced by the movement of massive bodies like black holes and neutron stars. As LIGO demonstrated when they published their results in the journal Physical Review Letters, scientists now have the technology to actually measure such waves, which involves the displacement of highly-sensitive laser beams.
“We’re hoping that in the 2030s we’re going to be able to launch gravitational wave detectors into space."
“The big challenge is in measuring [them],” Poisson continued. “The hard part is the fact that they’re so tiny. It’s basically using laser beams. If you have two objects that are freely moving in space, what’s going to happen when a gravitational wave passes through is that the distance will start oscillating. What you can do is to use a laser beam to measure that relative distance between the two masses.”
Unfortunately, almost anything can disrupt this measurement. Due to the sensitivity of detectors on Earth, along with the small size of the waves themselves, the technology might register just about anything other than a gravitational wave itself. Seismic activity is just one example of phenomena that can interfere with readings. This is why Poisson and others hope to have detectors in Earth’s orbit in the near future.
“We’re hoping that in the 2030s we’re going to be able to launch gravitational wave detectors into space. We’re pretty hopeful [it will happen] within the next 20 years.”
Poisson is particularly excited about the prospect of gravitational waves helping us gain insight into some of the fundamental mysteries of the universe. “We’re in a world where astronomy is now done with gravitational waves,” he said. “We’re discovering the universe in a completely new way.”
Explaining why that matters, Poisson became animated. “When we look at the universe, for example, through visible light, we see certain things. When we opened up the electromagnetic spectrum to start detecting radio waves, we started to see a universe that was very different than what we had seen before. The visible universe was very quiet, nothing changes much over a human timescale in visible light, but in radio waves, X-rays, gamma waves, you find a universe that’s very agitated.”
Gravitational waves are another window into that universe. This time, however, it's a window to the dark parts of the universe that we know very little about.
“The lesson was, every time you open up a new window into the universe, like gravitational waves, you see something completely different. And now we’re seeing the dark universe. Gravitational waves tend to get produced in areas where very little light gets emitted. We see the merging of black holes, the merging of neutron stars, things that we could not detect in any other way. That [is] the promise of gravitational waves.”
For Poisson, this is one of the most encouraging tools to gain a perspective on the history of some of the largest bodies in the cosmos. Black holes and neutron stars are often found in pairs known as binary systems, which orbit each other at incredible speeds, thus producing gravitational ripples across the universe. The hope is that analyzing those waves will help us learn how those systems formed.
“For astrophysics that’s a big deal. [Gravitational waves] really inform stellar evolution,” explains Poisson.
Playing the longest game
You may remember friends lighting up your social media feed in 2015 when gravitational waves were first detected, but it’s unlikely that they’re something that has continued to excite you as you go about your life. Significant scientific advances, especially in the field of astronomy, can be extremely flash-in-the-pan moments for the general public.
When asked how to get the public to care more about things like gravitational waves and cosmic phenomena they will never have any interaction with, Dr. Poisson readily acknowledged that this work is essentially the concept of delayed gratification taken to its absolute limit.
“We cannot leave humanity in the dark about those very important questions.”
“The study of black holes and gravitational waves may not have an immediate application, we’re not saving lives or inventing technology that will make our lives better anytime soon,” he conceded. “But, to think of our place in the universe, to think of what’s out there on the largest scales, to understand where we are and where we came from—globally speaking, I think those are fundamental questions that all of humanity for all time has been asking.”
The very fact that gravitational waves were observed roughly 100 years after their initial prediction is a testament to the virtues of patience with and consistency within scientific inquiry, something Poisson is adamant we need to advocate for.
“We cannot leave humanity in the dark about those very important questions,” he emphasized. “We may find that it’s going to take a very long time to answer those questions, but as humanity, we have to keep asking [them].”
Thanks to the research and dedication of people like Poisson and his scientific ancestors and contemporaries, we’re inching ever closer to those answers. May they be as marvelously beguiling as the universe has so far shown itself to be.