September 14, 2018 marked the three year anniversary of one of science’s most monumental achievements to date: the detection of gravitational waves. It’s an achievement that Albert Einstein believed mankind would never accomplish and it marks the beginning of a new era in astrophysics and maybe even humanity itself. Since the first detection in 2015, there have been five other conclusive detections. These results are leading to a bright future for some of science’s darkest mysteries. Before we get into why this matters so much, let’s discuss what the hell a gravitational wave even is.
Imagine throwing a stone into a massive lake, except this stone represents two colliding black holes and the lake is the fabric of spacetime. Now imagine the ripples traveling away from that stone at the speed of light. Finally, imagine standing on the opposite side of the lake and trying to measure these ripples, but by the time they reach you, they are a billion times smaller than an atom. Keeping with this metaphor, would you believe me if I said the lake was 1.3 billion light years across? That’s approximately 7,642,212,985,000,000,000,000 miles and it means that, even traveling at the speed of light, it took the waves 1.3 billion years to reach you!
At this point you may be wondering how it’s possible that scientists are able to measure a wave that’s 1 billionth the size of an atom. As I said before, even Einstein thought the task was impossible, and he’s the one that predicted the existence of these things in the first place. Simply stated, a bunch of scientists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) were able to do it with some mirrors and some laser beams.
Before I get into a tale of two (neutron) star-crossed lovers, laser beams, and black holes, let’s take a step back and sum up centuries of hard work in one paragraph to provide some context to the discovery (these next discoveries are all incredible and important in their own right but are another story, for another article).
Our story begins with some cavepeople discovering fire. Now let’s fast forward thousands of years to the 17th century with Isaac Newton getting hit on the head with an apple. From here, he derived a crapload of calculus and calls it a day with three laws of universal motion that supposedly dictate everything that happens in the entire universe everywhere. Now fast forward another century and a half or so to the 1800s when James Clerk Maxwell theorized that light travels as an electromagnetic wave (which encompasses visible light, X-rays, radio waves, etc). Scientists figured, “hey, if light is a wave, then it needs some sort of medium to travel through, right? Let’s call this medium the ether!” They were wrong. By the time 1887 rolled around, Albert Michelson and Edward Morley conducted an experiment with this new fangled apparatus known as an interferometer that Michelson had invented. It allowed them to get super sensitive measurements (*wink wink nudge nudge* this is important) and with this apparatus, they concluded, “What the @#$%?! There is no ether?! How is this possible?” Don’t worry Michelson, you’re still gonna get your Nobel Prize. At this point in the story, scientists everywhere were reeling. Chaos ensued and the end of days was upon humanity. Doomsday had begun….okay, maybe it wasn’t that extreme, but still, this was such an incredible failure; so incredible, in fact, that it’s regarded as one of the most important failed experiments in science.
It took almost two decades for one hero to rise out of the ashes of the ether and explain how light travels throughout the Universe. This hero is none other than Albert Einstein. We’ve finally reached the 20th century and with this arrival ushers in a new era of science that sets the tone for the rest of the century and into the present day. Einstein was basically able to determine that the laws of physics are identical to an observer in a non-accelerating reference frame and that the speed of light in a vacuum is constant, no matter what. Going along with this, Einstein determined that the speed of light is the speed limit of the Universe and nothing can exceed it . He called this the theory of special relativity. The main problem with special relativity was that it omitted gravity which is kind of a huge deal because as far as we know gravity is universal. After almost a decade of work, Einstein was eventually able to include gravity into his theory of relativity, which finally brings us to the foundation that gravitational waves rests upon, Einstein’s theory of general relativity.
Preceding the theory of general relativity, it was believed that gravity was a force (thanks to Newton) and we basically took it for what it was. In the context of general relativity however, the concept of gravity becomes slightly more complicated, but exponentially more interesting. Immediately following the advent of special relativity, a German mathematician (as well as one of Einstein’s past professors) by the name of Hermann Minkowski theorized that space and time are actually fused into one geometric entity, known as the space-time continuum. As quoted by Minkowski “henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.” As a side note, imagine being the guy that taught Einstein? Another notable quote from Minkowski is “oh, that Einstein, always skipping lectures… I certainly never would have thought he could do it.” So don’t feel bad about skipping a lecture once in a while, Einstein did it too and look how he turned out, am I right? Anyways, Einstein was able to take Minkowski’s theory of space-time and combine it with a geometric interpretation of gravity, theorizing that objects cause curvature in the space-time continuum, which we feel as gravity.
Einstein’s theory of general relativity implied that massive, catastrophic events in the Universe create distortions in space-time that radiate outward from the source of EPIC DESTRUCTION (or creation I guess, if you’re an optimist.) First imagine one black hole. Now imagine two black holes orbiting each other. Now imagine these two black holes SLAMMING INTO EACH OTHER. Sounds pretty catastrophic right? It’s events like these that create gravitational waves and it was a situation not so dissimilar to this that scientists were able to measure (from the safe distance of Earth).
As I said earlier, the gravitational waves that reach Earth are 1 billionth the size of an atom and are impossible to notice by standard means. You aren’t gonna be hanging out one day and be like, “wow, I just felt the spatial dimension stretch and then compress”… or maybe you will say that. I don’t want to make assumptions about your life, but I can assure you that if this happens you should see a doctor because those feelings are not from gravitational waves. To put it in perspective: if an atom was the size of the Earth, a gravitational wave that we can measure would be the size of a marble!
Remember Michelson and his interferometer from before? And then remember when I subtly winked at you and nudged you saying it was important? Well, scientists built a GIANT Michelson interferometer for the sole purpose of directly measuring gravitational waves. The laser interferometer cleverly utilizes lasers, mirrors, and the properties of light to get precise measurements, whether they’re on the scale of your annoying lab partner smacking the table while you’re trying to take measurements (you know who you are) or on the scale of gravitational waves rippling through Earth at the speed of light. When I say giant, I mean that each arm of this interferometer is two and a half miles long! It’s so sensitive in fact that engineers had to build two, on almost directly opposite sides of the country, to get any sort of meaningful measurement (one interferometer is able to factor out random noise from the other and the two together can cross-reference and fact check to ensure they’re actually receiving gravitational waves and not just a car crash on a nearby highway.)
The first gravitational wave that the Laser Interferometer Gravitational-Wave (LIGO) was able to measure has been so aptly named GW150914. Wow! What is this classification system? Probably something super technical, right? Nope! It means it’s a Gravitational Wave discovered September 14, 2015 a.k.a 09/14/15 thus… GW150914. In the three years since, LIGO has been able to detect five other conclusive gravitational wave events, four of which being binary black hole mergers. The most recent discovery LIGO has made was of a binary neutron star merger. The incredible thing about this event was that, unlike a black hole merger, the merger of the neutron stars emitted electromagnetic radiation (which includes visible light) as well as gravitational waves. This means that not only did LIGO get gravitational wave readings, but other telescopes all across the Earth (and orbiting the Earth) were able to get electromagnetic readings that corresponded almost exactly to the reading made by LIGO!
Black holes get their names from the fact that they’re, you guessed it, black holes. With ordinary telescopes, it’s really difficult to observe them because the gravitational pull is so strong that electromagnetic radiation cannot escape from its grasp and telescopes are literally there to (greatly) enhance our ability to perceive and interpret electromagnetic readings. Without these readings, scientists had been forced to make indirect measurements of black holes either by measuring the light AROUND the black hole providing a silhouette, or through careful calculations of the gravitational influence they have on surrounding objects. This is where LIGO becomes key in discovering and measuring these nearly invisible black holes. Using the gravitational wave readings, we can now make direct measurements that further our understanding of the Universe.
Laser interferometry is only going to get better from here and will dramatically further our understanding of the Universe. Not only is work being done on LIGO to make it increasingly more sensitive, but there is now talk of building an interferometer in space in the decades to come. The future of gravitational wave research is a promising one and more and more scientists across the globe are hopping on board (or in this case….the wave). A new chapter in multi-messenger astronomy is opening and combined with light-based telescopes, there are going to be some incredible observations made regarding the Universe, and our place in it.
Therefore, the advent of measuring gravitational waves is incredibly exciting because it not only provides strong evidence for Einstein’s theory of general relativity, but it also opens up a whole new window to observing the Universe around us. This discovery is a testament to the hundreds of thousands of successful and (maybe more importantly) failed experiments that humanity has conducted in the name of science and the progression of mankind. If telescopes allow us to turn our heads up to the skies and see, then LIGO has given us the ability to put our ears down and listen to what the Universe is trying to tell us.