As 2016 came to a close, a number of publications claimed the confirmation of gravitational waves as the scientific discovery of the year. It truly is huge news, in every single sense of the word, and so it has circulated quite a bit. The NY Times published an absolutely terrific story, detailing some of the individuals responsible for the “discovery” of gravitational waves. Please read it if you haven’t already. As the Times is wont to do, it touched on a lot of big topics extremely eloquently, but rather abstractly. I wish to take this opportunity to go back and give a bit more perspective to just how big some of these topics are. People often feel these topics are outside of their comfort level to understand, and I’m hoping to help in that regard.
First off, you may have noticed I put “discovery” in quotes above. This wasn’t meant in some sarcastic manner to demean the nature of these scientist's work. Einstein discovered these waves when he postulated the ramifications of his new thoughts on gravity. This doesn’t take away from the work of the scientists today, it simply helps explain why what they are doing is so important, and how man achieved something so extraordinary. Let’s start at the beginning.
What is Gravity?
In 1686, Newton published Principia in which he first connected the dots between an objects mass and its gravitational pull. He realized that every single particle had its own gravity and that the planet’s motions in the heavens was determined by their mass.
In terms of pragmatics, Newton’s elegant equation that Gravity = Mass of 1 thing * Mass of 2nd thing divided by the distance between the things squared (G = (M1 *M2)/D^2) effectively solved the questions of gravity. With that equation, it waspossible predict the flight of projectiles in our skies as well as the celestial objects in our heavens. Past that, gravity doesn’t really have any impact on human life.
To highlight the effectiveness of Newton’s theories, astronomers’ calculations using the equation were so accurate that two separate astronomers, Urbaine Le Verrier in Paris and John Couch Adams in Cambridge, were able to simultaneously predict the existence of Neptune with it’s exact size and orbit without ever having seen it. By looking at the orbit of Uranus, and factoring in all the known mass in the area, it was clear that another planet was missing. When they looked for it in the sky, there Neptune was, reflecting the sunlight beautifully just as anticipated.
As time went on, there were some issues arising with Newton’s theories. It turned out, while Newtonian gravity can perfectly predict the orbits of planets very far away from our sun, it was having some issues explaining the orbit of Mercury, the closest object to our star. Emboldened by his success predicting Neptune, Le Verrier decided we must be missing an additional planet closer to the sun. Naming it Vulcan, possibly the coolest name for a planet imaginable, one astronomer even thought he saw it moving across the image of our sun. Unfortunately, the results were not repeatable, but in spite of this, many astronomers continued to assume it must actually be there, somewhere, but it was just so close to the Sun that we couldn’t properly see it.
Newton lived in a world where the fact that light had a finite speed had only just been verified, and he even alludes to it’s confirmation in Principia. People in Newton’s time had so little connection to the concept of electricity that the word itself was only just being coined. So to Newton, it would make perfect sense that gravity acted instantaneously, just as it was suspected that light did for millennia.
After Newton our collective attention dramatically shifted to electricity and understanding the electromagnetic spectrum as a whole. In short order we discovered a number of incredibly useful things like radio waves, microwaves, infrared, the spectrum of light and x-rays. For each one of these we can think of many tangible ways they have dramatically influenced society. Our growth curve for manipulating these waves have effectively defined modern technology.
Einstein was the first smart guy to not get so caught up in this electromagnetic spectrum as to start to probe into the deeper questions Newton's theories on gravity would lead someone to ask. Living in a world where the electromagnetic spectrum was fairly well understood gave Einstein some unique advantages over Newton. If light was the property of some electromagnetic wave and gravity was the property of mass, maybe similar rules would apply?
If you turn on a light, or set fire to a candle the light radiates away from the bulb/wick. If its coming from this source, is it traveling at some speed like everything else, or is actually instantaneous as we perceive it? With no way to test this theory, various scientists shared their own unproven opinions. Al-Haytham is attributed with first thinking the speed must be finite, but with the tools available, it was all just speculation.
In 1638, Galileo recognized that from a distant hill he could observe a cannon firing before he could hear it, and with the distances he had available he could estimate the speed of light was at least 10x faster than the speed of sound. Looking to solve the answer here on Earth, there was nothing further to do. Galileo, did however lay all the ground work necessary for its discovery by first pointing a telescope into the heavens and recording observations on the position of moons orbiting Jupiter.
By 1676, a mere 12 years before Principia, Ole Rømer, using the telescope Galileo had recently invented, observed these same moons and expanding on Galileo’s data was able to definitively observe that the light reflecting off of the moons as they first rounded from behind Jupiter would take longer to appear when Jupiter was furthest from Earth, and would appear earlier when Jupiter was physically closer to us. This finally verified the long held assumption that the speed of light was finite.
So Einstein wondered, what if a huge amount of mass suddenly appeared or disappeared in our solar system. How long would it take for you to see the effects on the other planet’s orbits? It seemed likely to him that it shouldn’t happen instantaneously, but at some finite speed. Convinced that nothing could move faster than light, he postulated that it would likely move at the exact same speed as light. Since light was found to be the property of particle moving as a wave, and gravity was the property of particles’ mass, he hypothesized that gravity must also have some transmitting wave, which he called gravitational waves. And that’s it, that’s when gravitational waves were first discovered.
As Einstein continued to hash out the ramifications of his idea that the speed of light in a vacuum was the absolute speed limit for the universe, he was predicting all sorts of seemingly bizarre results to be reality. So distinct were these calculations, a whole new branch of physics was born, known as General Relativity. As various particle physicists and astronomers began trying to observe the results Einstein predicted, time and time again, his theories were shown to be precisely accurate.
As an example, Mercury’s orbit was perfectly logical according to the rules of general relativity. Einstein’s theory explained that mass was actually warping the surface of the space around it, in the same way that you warp the surface of a trampoline when standing on it. If you think about how it looks when you stand on a trampoline, the area right around your feet is severely warped by your weight. Yet as you move away from where you’re standing, the surface of the trampoline rapidly moves closer to it’s normal elevation. This is why the orbit of Mercury was so obviously distorted by the Sun, whereas the other planets were not so obviously impacted by this warping of space.
Another popular prediction of general relativity is that as objects move closer to the speed of light, time for them actually slows down. You’ve probably heard of the twin paradox, where if you had a pair of 10 year old twins and one twin were to leave the planet on a space ship going 80% of the speed of light and return 50 years later, the twin who remained on earth would have aged 50 years and be 60, while the twin who was on the ship would have only aged 30 years and be 40. Imagine meeting your own twin and seeing yourself basically as you looked 20 years prior!
This result seems fantastical, and no matter how many times you’ve heard it repeated, it probably doesn’t seem possible. Yet every time you use your phone to get you directions, the only reason the whole system works is because we are able to account for the minute changes in time due to the the satellites moving so quickly around our planet. GPS works because at least 3 of the 30 satellites orbiting our planet are always always in direct line of sight of your device. Each of these calculates the distance you are from them at the exact same moment and by using these 3 distances, it is possible to pinpoint your exact location using a process called trilateration. If it wasn’t for general relativity, we could never get these satellites to make an observation at the exact same instant.
These are just two examples of how Einstein’s theory of general relativity has been able to provide real world results in situations where Newtonian gravity has failed us. These theories have been verified hundreds of ways, and have basically been the guiding principles behind an enormous amount of the scientific discoveries in the past 100 years or so. So if everything else Einstein was predicting was coming true, what about those gravitational waves?
Similar to trying to verify the speed of light, trying to prove the existence of gravitational waves seems effectively impossible on the surface. Think about the way that Einstein even came about predicting these waves, that an entire planet would suddenly appear or disappear. How on earth do you make something like that happen? Well, actually, by definition, you could never do such a thing on Earth, because you would literally be destroying the object you were standing on to do so.
Yet again, Einstein’s own theories were there to provide the answers. One of the things general relativity predicts is that if enough mass was in one small area, the strength of gravity near the object would be so intense, that light itself would be unable to move away from it. We now call these fantastical objects black holes, and basically everyone, Einstein included, thought they were ridiculous. A cool mathematical curiosity, but surely too absurd to actually exist in nature.
The thing is, black holes are basically where physics goes to die. All of our previous theories of everything break down when confronted by so much mass in such a tight space. Yet as time was going on, we were discovering other bodies, such as neutron stars, which were getting closer and closer to this mathematical extreme. Since these bodies weren’t quite as dense as black holes, light was able to escape, and we could totally see them sitting there in space, drastically altering the orbits of objects around them and shooting off absolutely brilliant flashes of various electromagnetic waves that would collide with our telescopes with more energy in an instant than the sun would provide over it’s entire lifetime.
Using the information gleaned from these other absurdly dense objects that we could see, the 1960s was filled with mathematicians successfully probing into all sorts of realities for these black holes, such as how fast they would spin in various conditions. By the 1970s, we were starting to predict what sorts of energy would radiate away from the object as long as they weren’t so close as to be unable to escape entirely.
With this knowledge, scientists were once again able to make predictions that were coming true despite not being able to see the object itself, just like back with Neptune. Suddenly it made sense why galaxies formed, and how they held all those stars in the formations we would see across the sky when you simply assumed there was a gigantic (we call them supermassive) black hole sitting in the middle of it all. We could also watch as stars themselves were being deformed and eaten alive by a black hole that had approached too close. Despite by definition not being able to see these black holes themselves, it became very clear they exist, and that they are everywhere.
Well if these black holes are all over the place, and if they have such strong gravitational pulls, then surely two of them must have collided at some point. Finally, astronomers had found a situation where so much mass would be converted to energy so quickly, that gravitational waves would be created on a scale that we could observe them.
Based purely on this intuition, we sunk decades and millions of dollars into building a whole new kind of telescope, called LIGO, the Laser Interferometer Gravitational-Wave Observatory, where we could watch one of these collisions occur. Now that seems like a whole bunch of scary words in one place, but its actually something you can comprehend pretty easily if you take your time and don’t panic.
Understanding a Wave
One of the properties of any wave is that if two waves meet exactly out of phase, they completely cancel each other out. Here is a video of it happening with string, please watch this video as it really makes this whole concept extremely accessible. As you saw, when tuned just right, the string would have points, called nodes, where it was completely stationary and no motion occurs.
Lasers are just highly focused light waves. We know light travels at an absolute speed in a vacuum which never varies. So if you split a laser beam down two paths of the exact same length and reflect them back at one another they reflect back to the original point at the exact same moment, exactly as you saw with the string. This means, that if ever see any amount of motion at that initial spot, then something ha happened causing the length of the two paths to be altered slightly.
This is precisely the setup at LIGO. There are initially two observatories, one in the desert of the state of Washington (LIGO Hansford), and another in Louisiana (LIGO Livingston). Each consists of two 4 km long vacuum tubes. A team of 40 engineers at each location work constantly to ensure that these lasers stay exactly in position and that the vacuum is maintained.
About that vacuum, the pressure inside LIGO's vacuum tubes is one-trillionth of our normal atmosphere. It took 40 days (1100 hours) to remove all 10,000 m^3, enough to inflate 2.5 million footballs, of air from each of LIGO’s vacuum tubes reaching an air pressure one-trillionth of that at sea level. At these lengths of 4 km, the curvature of the Earth causes a 1 meter bend in the tubes. The tubes were designed so precisely as to exactly counteract this bend, giving us the necessary setup to ensure that the lasers precisely interfere with each other at all times.
So why do all this? Well, if a gravitational wave were to pass through earth, the wave would ever so slightly distort the length of each of those tubes, causing the lasers to vary a hair out of phase. By using two of these facilities located 3000 miles apart, these nodes are observed so carefully that they can detect a change in the length of these tubes a mere 1/10,000th the width of a proton. Just the sort of precision necessary to observe a gravitational wave in action.
LIGO was first switched on from 2002 through 2010, and during the whole time, it never was able to spot a gravitational wave. Undeterred, the scientist’s used what they had learned to completely rebuild these structures such that they would be 10 times more accurate and called it advanced LIGO. This increased sensitivity would allow them to observe a gravitational wave coming from 10 times farther way. Not intended to be fully operational until September 18th of 2015, on September 14th at 5:51 am Eastern Standard time, it was observed that the signal in Louisiana arrived 7 milliseconds faster than it had in Washington.
This observation occurred mere days after the equipment had been turned on at all, in stark contrast to the the 8 years of null observations prior. The immediacy of this discovery highlights just how frequent these events occur. While it’s rare for it happen in any one spot in the universe, the universe is so large and has been around so long, it’s going to be trivial for us to find one of these events occurring now that we have the technology to observe them.
Crunching these numbers, the astronomers were able to calculate that 1.3 trillion years ago, a black hole with 29 times the mass of our sun collided with another black hole with 36 times the mass of our sun. When the collision occurred, 3 times the mass of our sun was suddenly converted into energy, effectively disappearing 3 suns worth of mass in an instant.
And there you have it, around 110 years after Einstein had first wondered what would happen if a huge mass was suddenly plucked from existence, we observed it happening. Because, as Einstein had predicted, these waves move at the same speed as light in a vacuum, we know this even occurred 1.3 trillion light years away from earth and took just as long to get to us.
As you can see, this observation is simply astounding as its the culmination of 110 years worth of us blindly stumbling in the dark, only to be able to actually image an object long thought impossible to be seen. Sure, we cannot see them with our eyes, but using these gravitational waves, we can see precisely when and where these objects collided and their exact size as they did so. The fact that this observation occurred so quickly after the experiment was switched on highlights that we now have a whole new way to observe the universe, and all of this occurred because thousands of scientists all stubbornly insisted against all odds the math had to be right and if we just worked harder we could prove it.
In this era of alternate facts and constant second guessing of science and it’s observations, I don’t know if anything could be more powerful, more validating than this observation. It is for this reason I have spilled over 3,000 words and took so much of your time to help you appreciate something so seemingly obscure. Thanks for bearing with me.