About a hundred years ago, Einstein provided humanity with priceless gems of groundbreaking insight regarding gravity, space, and time; unveiling the shroud of mystery previously concealing gravity’s propagation, as well as revealing the true nature of time and it’s inseparable link to space. The truly beautiful and elegant depiction of reality he bestowed to us revolutionized the way we viewed the universe. Einstein viewed the very reality we are engulfed by, space itself, as intertwined with time. He crushed the notion that time was absolute for all and uniformly passed everywhere for everyone, or thing, no matter what. In fact, showing vice versa to be true; that time varied relative to who you asked, and varied based on speeds as well as/or the force of gravity itself. He merged space and time into a singular entity describing reality itself, stating that all energy and matter exists upon spacetime and that the very presence of matter (or energy which, once again Einstein, showed were interchangeable: E=mc2) bends spacetime. The reality we are immersed within can be thought of as a literal sheet of fabric or rubber. The presence of anything upon that sheet, anything with mass, will obviously cause it to warp, deform, bend, and in special cases even twist (rotating masses). Einstein showed us that the presence of matter (mass or energy) bends space and time; this warping of the reality, upon which all we see and know resides, is gravity. All things want to take straight paths through spacetime (unless acted on by external forces), and they do take straight paths, but straight trajectories along four-dimensionally (3 of space and one of time) warped spacetime are perceived, by us, to be curved paths. The path matter takes along gravitationally distorted spacetime is the manifestation of gravity and its effects. The planets orbit the sun because they merely follow stable elliptical paths along the curved space induced by the sun itself.
This picture of reality painted for us by Einstein is truly insightful and is still being backed by observations today. We’ve proven how the rotation of matter drags spacetime with it, not only bending but also twisting it as well. We’ve observed gravitational lensing, where light from objects directly behind stuff in the way (like galaxies, gas clouds, or even our own sun) is visible due to the light travelling along curved space, following paths around the objects in our way. We’ve proven and very well observed the dilation of time predicted by Einstein. We’ve uncovered the existence of black holes, gravitational monsters lurking deep within space and the equations of general relativity. Recently, yet another prediction arising from the brilliant mathematics described to us by Einstein has been observed: the existence of gravitational waves. These recent observations of ripples in spacetime have not only helped to further strengthen general relativity as a pillar in physics, but has also lead to conformations of theories about the origins of heavy elements, as well as unearthing new mysteries for scientists to delve into and solve. The 2017 Nobel prize for physics was given to the observers of special gravitational waves, from a binary system of neutron stars merging. To understand what exactly these waves in spacetime are let us first explore what exactly gravity, black holes, and neutron stars are.
Gravity; it is the manifestation of curvature in reality itself. The heavier an object (the greater its mass) the more intense the curvature in spacetime it induces. Some of the most incredibly massive objects in the Universe are black holes and neutron stars, both of which being the remnants from the deaths of monstrously immense stars; the leftover crushed cores of once vibrant, bright, and enormous spheres of plasma. When the biggest of stars die at the end of their high octane and short lives, they go in the most spectacular explosions we observe gamma ray bursts. The deaths of these stars are violent and bright (so bright that they can even outshine the light from entire galaxies) events that shoot out streams of highly radioactive particulate, gamma rays, racing off into space at the speed of light. The cores of these huge stars collapse in on themselves. Try and take in the magnitude of these cosmic events. The cores of these stars are bigger than our sun itself. When a core this massive implodes in on itself, all that matter is crushed to a tiny point of infinite density (possibly, the true nature and properties of a black hole and the “singularity” are hard to work out and mold into a sensical conceptual image, but I’ll delve deeper into this and other misconceptions later on): a singularity… A black hole. The depressions they cause within spacetime are truly a sight to behold. Black holes, objects with great mass, wielding a force of gravity so strong that the geometry of the curvature of spacetime around them won’t allow for light to take paths that radiate outwards; even light will eventually pass a point of no return. The darkness of a black hole comes from there is a radius around it from which light itself can’t even escape… When I say light can’t escape I have to be careful. There are a few misconceptions about black holes that we must traverse around cautiously. The event horizon is what this threshold is called; a sphere of imminent and pure darkness shrouding the singularity and everything past it. These fascinating structures are forged in the crucibles of the universe’s most violent explosions and live for an unfathomable amount of time. In fact, all that will be left if the universe lives on forever, at some point, will be only black holes and eventually nothing. Slowly, black holes emit energy, Hawking radiation as described by Stephen Hawking, losing their mass over vast periods of time; inevitably, black holes evaporate away.
When dealing with black holes there are a minefield of misconceptions we have to hurdle: the reason to why black holes are dark, what we assign the mass of a black hole too exactly (the singularity? or?), the idea that black holes have the same amount of mass as what they were formed from but seem to have “more” gravity, and how black holes seem to separate the realities of what happens to things that fall into them versus what outside observers say occurred. The equations of classical relativity unveil the quite convoluted properties of black holes. Say you are observing your friend falling into a black hole. As he (or she) slowly descends closer and closer to the sphere of pitch darkness, the space he (or she) is immersed within gets ever more drastically warped. Einstein showed that an object’s speed effects it’s internal clock, but so does the curvature of spacetime (gravity). As your friend strays ever nearer, the time dilation that is felt by them becomes more and more extreme. From your point of view, your friend starts to move increasingly sluggishly towards the event horizon. The closer to his/her demise he (or she) gets, the progressively slower and slower he (or she) moves and ages. Seconds for your friend will be hours, days, weeks, months, years, and eventually, once he (or she) reaches the event horizon, an eternity (infinite amount of time) for you. Once your friend reaches the event horizon, you perceive them to be frozen in that moment forever… In fact, to an observer outside the black hole, your friend will be stuck in the moment just before he or she passes the event horizon. Weird… But even weirder is the fact that if you were to ask your friend to recount his or her descent into the black hole, they would tell you that they merely whizzed by the event horizon no problem all the way down to the singularity. So, to you, and anyone outside the event horizon, your friend’s descent into the black hole never happened, he (or she) froze the moment he (or she) got to the event horizon, but to your friend, his or her journey happened unhindered and infinite time. Black holes seem to forge two realities for observers and victims who strayed too near. It’s exactly these types of seemingly paradoxical conceptions associated with black holes that gives them so many misconceptions.
To outside observers nothing ever really enters a black hole, and this causes us to question the black hole’s mass and exactly what it is that is given that mass. Well, one would logically just say the mass of the black hole is the mass of the singularity, the theoretical point at which all the mass of a black hole is crushed into, but we must again be careful. Since to us here outside of the black hole things never seem to quite pass the event horizon, events, to us, don’t occur past the event horizon. Even from the point at which the black hole was created by the collapsing star core it came from, those outsides of the black hole will agree that no mass really ever entered the black hole… So the question as to what really holds the mass of the black hole is very much hazy in it’s answer, since we can’t really explicitly say that it is accounted for within this theoretical singularity. Though a point with all that concentrated mass would be infinitely dense, it is hard to say anything about that points properties since our mathematics break down when we boil down to such extreme cases.
Another thing to be cautious of about a black hole is why it’s black. A tempting explanation for this is that light can’t escape the black hole’s gravity past the event horizon, but this isn’t entirely true. Sure, the escape velocity for an object at the event horizon is just over the speed of light,
but that’s just a coincidence in the mathematics. The curvature induced by a black hole on spacetime is so dramatic that the possible paths objects can take become restricted. Remember, black holes are created, or can be created, whenever a sufficient amount of mass is crushed to a point at which the force of gravity on the components of that mass itself causes a violent implosion to a point of infinite density. The earth, if squeezed to around the size of an ice cube, can be transformed into a black hole. The space within which the mass once occupied is now influenced by a great force of gravity, but the space around the black hole farther than the radius of the mass from which it was forged by remains afflicted by the same curvature of spacetime as before. Thus, for example, if the sun were compressed to the point of implosion, in other words turned into a black hole, we here on Earth would remain in our exact same orbit, although we’d all freeze as well… Only the space that was occupied by the sun, which is now a tiny fraction of its original size, is heavily warped spacetime. Within this once occupied region of space, paths and orbits, more generally referred to as geodesics, are no longer circular/ elliptical, but very unique. As objects stray closer and closer to the event horizon spacetime becomes more and more warped. At a certain point, the curvature of spacetime is so intense that all paths no matter what velocity an object has are radially inwards; the event horizon. Space is curved in such a way that no matter your speed, you can only travel in one direction: radially towards the center or singularity. A black hole’s veil of darkness has little to do with the light not being able to escape, in fact, even light that can escape the gravity just before it falls into the event horizon is red shifted so intensely that it escapes as non visible light. So not only do things, according to external observers, never actually enter a black hole, they also become frozen and invisible as they stray dangerously close to the event horizon, for the light emitted by them is red shifted out of the visible spectrum and they fall victim, in the perspective of outside observers, to gravity’s intense time dilation… These structures of perplexing properties and immense gravity hold insight to the deepest mysteries we have yet to solve. Yet, they have also provided some of the greatest testaments to our conceptual understanding of the universe. The immense warping in spacetime they cause are the reason why when two of these beasts are in an orbit, they almost always spiral into a decaying orbit (due to energy loss in the form of gravitational waves), eventually colliding and sending out ripping ripples in reality itself just before their demise.
Only the largest stars die and leave behind black holes. Stars that are just barely too small to forge black holes upon their death’s leave behind a type of very interesting and exotic star; they leave behind neutron stars. Comprised of neutronium, the densest and hardest material in the universe (besides of course what ever comprises the singularities of black holes), neutron stars are gravitational beasts in their own right. Ranging from around 10 to 30 km in diameter, yet having a mass of a couple of solar masses (one solar mass is the mass of our sun), these strange stars are at the very limits of density. Taking a mass equivalent/greater than that of our own sun and squeezing it down to a mere 20 km in diameter, is that even possible and if so, how? The structures around us and in space are comprised of countless atoms, congregations of protons and neutrons in a central nuclei and orbiting electrons; the orbiting electrons are located fairly distant from the nuclei of an atom. This space between the electrons and nuclei of atoms is precisely what is squeezed out of what makes up neutron stars. Most of everything we see around us is empty space. The cores of these once massive and alive stars collapse so violently under the force of their own gravity that all the atoms in their cores are crushed to were all the electrons and protons implode in on themselves turning into neutrons. The pauli exclusion principle is what holds neutron stars up against their gravity, basically dis-allowing the neutrons to occupy the same energy and therefore space. Thus these exotic structures are comprised entirely of neutrons: neutronium. Their immense mass and small diameter result in great gravitational forces emanating from them. Dropping a marshmallow on one of these stars would result in an impact force over 20 times stronger than hiroshima… The curvature of spacetime induced by these blue pearls of neutrons is no joke.
Both black holes and neutron stars warp spacetime heavily (pun intended), and it is this feature that they have which allows them to display the ever elusive phenomenon of gravitational waves. So, what exactly are gravitational waves. Well, Einstein showed us that spacetime can be visualised like a rubber sheet that warps and deforms when objects are placed on it. Gravitational waves are ripples in reality itself. Just as pond water ripples when a pebble is thrown into it, spacetime ripples as objects move through it. These ripples, according to Einstein’s equations, disperse outwards through space at the speed of light and whatever comes into contact with them is vertically and horizontally expanded and compressed (stretched and squeezed). These ripples in the fabric of reality dissipate very rapidly the farther they travel though, and thus most of the gravitational waves that emanate from objects or events in space are much too faint for us to ever hope in detecting. This is where black holes and neutron stars come into the picture. Two of the universe’s most dense structures, gravitational monsters, form the strongest gravitational waves we observe, in fact, the only gravitational waves we’ve ever observed. When these small dense remnants of stars find themselves in a decaying spiral orbit around one another, the results are very much so epic. When neutron stars or black holes merge, they emanate violent gravitational waves, getting stronger the closer to merging these exotic astrophysical structures get. With the immense gravitational forces these dense dead star core’s wield, the gravitational waves they produce upon merger are strong enough to traverse unfathomable expanses of space, still being detectable by the time they reach Earth, albeit minutely. As these waves in spacetime pass through us and our planet everything is stretched and compressed over and over again by a fraction of the width of an atom. These once massive crashing waves in reality quickly dissipate over time as they shoot across space at the speed of light. Black hole mergers produce relatively fast intervals of waves; a quick burst of extreme waves are produced as binary pairs of black holes reach their final stages of rapid merger. The detection of gravitational waves we obtained from the merger of two neutron stars (for which the 2017 nobel prize was given) yielded a result much different from those received from black hole mergers. These waves were detected over a much longer time period than ones from black holes, and this data from the merger of two neutron stars provided us with great insight into long standing mysteries and theories.
How exactly do we detect these cosmic waves in spacetime? We have a few detectors of gravitational waves here on earth, and they basically measure the distortions in reality this phenomenon causes by using lasers. Ligo is one of these detecting instruments and it has two facilities, one in Washington while the other is in Louisiana; two detectors so that gravitational waves can be more easily triangulated for their possible points of origin. These detectors each have two perpendicular 2.5 mile long arms inside of which are mirrors that split, redirect, reflect, and funnel laser light causing it to run through the arms and then converge back were the arms intersect. The wavelengths of the laser light cancel eachother out as they reflect off the mirrors backwards through the arms (if the peaks of one wave add with the troughs of another while the peaks and troughs are identical in magnitude then the waves cancel each other completely) , so any gravitational waves that distort the length of the arms will alter the distance the light travels, thus causing calculable changes in the sums of the wavelengths along the arms. These detectors have received a few gravitational waves emitted by black hole mergers, but on August 17th 2017 LIGO and VIRGO (another gravitational wave detector in Italy) detected gravitational waves from the final moments of colliding neutron stars. These pulses in space itself were shortly followed by the detection of gamma rays (thus confirming the predictions of the math, proving that these ripples propagate at the speed of light), and over the preceding days, various other wavelengths of the electromagnetic spectrum. The light observed from this perplexing astronomical event paired with the detection of it’s gravitational waves has helped uncover a world of insight and newfound mystery. Currently, LIGO is not operating for it is being tuned to be even more sensitive. Projected to have 2 times the sensitivity, thus able to scan and receive gravitational waves from 2 times the distance; the volume of space from which we will now be able to detect these ripples in spacetime for has increased 8 times. In the near future we expect to reap tons of data about gravitational waves, allowing us to more deeply and precisely work out the accuracy of our mathematical predictions.
The collision of these dense neutronium pearls result in the most violent explosions observed, gamma ray bursts; raging explosions emitting light enough to outshine the intense combined luminance of hundreds of billions of stars (galaxies). Gamma ray bursts also send out two jets of radioactive gamma rays out into space at the speed of light, which is how the prediction that gravitational waves emanate at the speed of light was essentially confirmed. The interval of ripples in spacetime detected was followed by a surge of gamma rays, only seconds after, as detected by fermilab. The story this data tells us is one that confirms our beliefs of how these monumental events undergo. As these gravitational behemoths spiraled ever closer to one another, gravitational waves violently ripple out into space until the moment the stars collide and fully merge. The resulting gamma ray burst from their merger sent out a shower of light across all spectrums, from visible to gamma rays (the same gamma rays we detected seconds after the gravitational waves here on earth, thus proving that these waves ripple out at the speed of light). Many other wavelengths of light made their imprints on our various detectors, including x rays. Studying the light emitted by this cosmic event provided us with insight into how the heavier metals we see were forged.
We’ve known for a long time that much of the elements we see in nature were formed in the core’s of stars, the universe’s fusion reactors; taking hydrogen and slowly, over their lifetimes, fusing it into heavier and heavier elements like carbon, nitrogen, etc. and even some metals upon their deaths. But stars can’t account for the variety of heavier metals we see in the periodic table. Even the deaths of average stars, supernovae, can only account for heavy elements up to iron and nickel. The pressures present in the fiery hearts of stars just aren’t strong enough to fuse atoms together and create the array of heavier elements we know of. So, it has long been a mystery as to exactly where the vast number of heavier elements in the periodic table came from. The source of the recently detected gravitational waves from this binary neutron star system has shown light onto the once obscure origins of the heavier elements found in nature. By studying light from stars and even the atmospheres of planets, scientists can determine what elements are present in or comprise the source of light. Different elements emit different spectra of light, and studying the spectrums of light emitted from the merger of these neutrons stars millions of years ago has provided quite interesting results; confirming long standing suspicions to the forging of heavier elements. Examination of the light from this merger showed positive for many heavier elements, and thus shows how much of the valuable and pricey metals/elements here are Earth exist thanks to events like this. The pressures and energies involved in the event of two neutron stars merging forms an explosion that not only shoots jets of radiation flying from its poles, but also scatters elements like gold, platinum, and uranium into the vastness of space; conveniently, these events lay the foundation for element rich planets to be formed around the warm light of newborn stars, potentially planets upon which life may one day emerge and thrive as it did with our’s.
Already we’ve seen how much intellectual juice drips from the fruit of this discovery; how it has made proverbial the notion that gravitational waves travel at the speed of light, and given insight into how much of the elements we see have been likely forged. Using the data detected by the three gravitational wave detectors here on earth in combination with the pulse of gamma rays, observers quickly triangulated possible origins for these waves. The light we bare witness too has spent the past 130 million years (as that’s approximately how many light years away from us this event has occurred) inching its way to us, holding the secrets and insights of much knowledge behind its quivering lips; lips that opened the second they reached us, spilling gems of long sought after knowledge, yet also spilling some hints of mystery. Not all the information that we’ve received from this monumental events years in the past and quadrillions of miles away has made crystal clear sense. In science, usually, no matter how much we learn, there’s always an ocean of insight that lay beyond the horizon. Answers spawn questions, and questions yield holes of conception to fill in with our knowledge of the universe.
Fermilab, the laboratory that received the gamma rays using their instruments, found that the intensity of gamma rays received should have been much more than what was actually detected. Most of the gamma ray bursts that we’ve observed occurring do so in regions of space much farther than the one recently observed following the neutron star merger. With the astrophysical event’s vicinity, we should have received much more gamma rays. Theories to explain the data from this event state that the stream of gamma rays ejected from the violent explosion weren’t pointed directly at us. These brilliant events emit massive amounts of deadly radiation in the form of gamma rays, but only in two concentrated beams. Scientists hypothesize that most of the insanely bright and distant gamma ray bursts we observe have one of their streams of gamma rays pointed directly at us, thus we obviously detect a greater concentration of them. The resulting gamma ray burst from this neutron star merger may have shot out gamma rays at an angle away from us rather than directly at us, thus why we detected such low concentrations of gamma rays from this event. As time goes on and scientist further delve into the specifics of this observation, answers to the questions we have will hopefully be accurately solved and described by our knowledge.
The merging of these two massive and dense bodies mustered quite the light show, but curiosity begs the question: what’s left in the aftermath? The merging of these massive compact bodies could either leave behind a massive neutron star, perhaps one of the biggest observed, or a tiny black hole, on the smaller end of the ones we’ve seen (or more accurately, detected). Two of the hardest objects in space clashing together at speeds near that of light, producing the strongest of light shows we know of. This discovery has unlocked so much insight into the world of rippling spacetime, much more due to the light show on various ends of the spectrum this event put on. Being able to see and study the data from this amazing occurrence 100 million years in the past has confirmed predictions that have been patiently waiting for their proofs. This event lit up it’s neighborhood of nearby space, and the future is just as bright, if not, brighter. With the tuning of LIGO’s detectors, we can soon expect gravitational wave data to come streaming in. More than a hundred years ago, the mathematics spawned out of a brilliant mind go to show us just now how amazing our bounds in understanding of the universe and it’s functionality are. A race of intellectually gifted beings residing on a pearl of blue delve deeper and deeper into the most mysterious and mind bending reaches of reality, unlocking more and more of the nature in which the world around us ticks. What a time to be alive, as we are at the forefront of scientific understanding, slowly making shallow the murky waters of what we don’t quite yet know and understand. But don’t get it twisted, spacetime still has a plethora of perplexities and mysteries a human mind has yet to savour.
Sources
https://www.jpl.nasa.gov/edu/news/2016/3/23/modeling-gravitational-waves/
http://www.sciencedirect.com/science/article/pii/S0016003237905830
http://iopscience.iop.org/article/10.3847/2041-8213/aa920c/pdf