--- title: "5 Things You Don't Understand about Gravity" description: "What goes up, must come down. Gravity needs no introduction—it's the familiar force that keeps our feet on the ground and our earth locked in a stable orbit around the sun.\n\nBut the mechanisms behind gravity are far more complicated than meets the eye. Gone are the days when we believed that an apple falling onto Newton's head represented just about everything there was to unpack about this invisible aspect of our universe. In this article we're going to break down five things about gravity that are often oversimplified, commonly misunderstood by the public, or even still a complete mystery.\n\n## It's Not Exactly a Force\n\nIt's commonly taught that all interactions in the universe can be explained by the four fundamental forces: the strong and weak nuclear forces, electromagnetism, and gravity. However, you'll sometimes see these listed under a slightly different name—the four fundamental interactions—which is often considered to be slightly more accurate, because gravity isn't exactly a force like the other three.\n\nTo explain this idea, we need to start from the beginning.\n\nFor over 200 years, the prevailing theory of gravity had been laid out by Isaac Newton. In Newtonian gravity, the attraction between two objects is described as an equal and opposite pull that can be calculated based on the objects' masses and relative distance. The earth pulls on you, keeping you tethered to the ground, and, likewise, you pull on the earth—it's just that you have so much less mass that it doesn't visibly move the earth. This model worked wonders; it was easy to understand and experimentally verified. However, even Newton himself was a bit troubled with one part. You see, he'd figured out how to measure and predict gravitational attraction, but he still had absolutely no clue what caused it or how this action could occur over such large distances. In a letter to Richard Bentley, he wrote:\n\n> \"That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it.\"\n\nThis frustration would be shared by scientists around the world until it was finally resolved by none other than Einstein. In his theory of general relativity, Einstein revolutionized our understanding of gravity by showing that it isn't actually a force acting between two objects, but rather is a byproduct of the way mass distorts spacetime.\n\nAs you may have learned in a physics class, an object in motion stays in motion unless it is acted upon. This means if you are cruising through a vacuum in space, your speed and direction would remain unchanged until something affected you. Newton was spot on when he came up with this idea, and Einstein built upon it to explain his new understanding of gravity. In a Newtonian model, your path through space would be a straight line, and this straight line would be bent by a gravitational attraction, curving your path toward the object as you are pulled in its direction.\n\nWhat Einstein did is he showed that it isn't really your path through space that is curved, but rather space itself—and this may seem like a pointless distinction to make, but it's actually important. Mass bends and stretches spacetime, and as a consequence, what you perceive as a straight path is curved along this warped terrain, moving you toward the attracting mass. This straight yet curved path is called a **geodesic**, and it's easier to understand with a comparison to passenger airlines. Airplanes want to take the shortest route to their destination, which is a straight line, and from the pilot's perspective, they indeed take off and fly straight with no deviation. However, when you zoom out, you can see that their overall path was actually curved, because despite the surface of the earth being two dimensional, it curves into a third dimension on the surface of a sphere, and thus what they perceived as a straight line followed this curve.\n\nIt's a bit harder to visualize this in three dimensions because we live in three dimensions ourselves, but the concept is similar. Objects travel in a straight line, guided by the curves of spacetime. This gives the impression that there is a mysterious force pulling a distant planet toward a heavy star, but in reality, there is no force—no invisible push or pull that is shoving the planet in a new direction—it is simply a consequence of the star's mass warping spacetime.\n\nAnd on a more advanced note, it's not just about mass. The distortion of spacetime is actually created by any form of energy-density, which is why even though photons are generally believed to have a rest mass of zero, they still interact with gravity, and you can even technically create a black hole out of enough radiation focused in a small region in space.\n\nWhat you've just read is really just the tip of the iceberg when it comes to how curved space gives rise to gravity. We won't get into the deeper details, but we will leave you with the best explanation of it all, a quote from legendary theoretical physicist John Archibald Wheeler, who simplified this whole concept in a single phrase:\n\n> \"Spacetime tells matter how to move; matter tells spacetime how to curve.\"\n\n## Our Theories Fall Apart at the Quantum Level\n\nGeneral Relativity refined Newton's findings and provided us with our most accurate model of gravity to date. However, it is not perfect, and has one glaring issue.\n\nOf the four fundamental interactions, gravity is the only one that has yet to be reconciled with quantum mechanics. Put simply, relativity is used to describe larger objects, and quantum mechanics is used to describe the universe on the subatomic level. So, when working in the domain of one, usually you can safely ignore the other because they describe such different aspects of the universe.\n\nHowever, there are times when we can't just ignore one of them, and need both an understanding of quantum mechanics and gravity working together to fully describe what's going on. This is the case in the highest levels of energy, such as the moments immediately following the big bang, and near the smallest regions of space, such as near the singularity of a black hole. If we truly want to fully understand every aspect of the universe, these two will eventually need to find common ground.\n\nThis has led to the decades-long search for a more comprehensive theory of quantum gravity, or, in other cases, a so-called \"theory of everything\" that unifies all domains, gravity included.\n\nWithout getting into the specifics, there have been many different models put forward over the years, such as loop quantum gravity, M-theory, string theory, Twister theory, and several others, but one problem that they all share is that because they are all predicting results on these extremely high energy levels or tiny, tiny regions of space, they are next to impossible for us to verify with real-world experiments using current technology. For instance, Loop Quantum Gravity predicts that spacetime is not continuous and smooth, but is actually made of distinct pieces or bits that can't be divided further. As a result, this theory makes the shocking prediction that the speed of light is not constant, but rather varies ever so slightly depending on a photon's wavelength, with higher energy photons moving slightly slower. However, this difference is so indescribably tiny that we really have no hope of detecting it. Likewise, for several predictions of string theory, our particle accelerators simply can't reach the levels of energy required to either prove or disprove the prediction.\n\nStill, that's not to say that these theories are simply numbers on a chalk board. Even without experimental validity, they have still yielded interesting scientific and mathematical contributions—it's just that without observable evidence, they can't yet be solidified as accepted science. And as a result, there is no consensus in the scientific community concerning quantum gravity, and the field is currently developing at this very moment. It's certainly something to look out for in the coming decades, as fresh ideas and technological improvements increase our experimental capabilities; we may finally crack this case once and for all.\n\n## It's Also Limited by the Speed of Light\n\nOne of the most famous findings of Albert Einstein is the relationship between energy and mass, summed up in the equation E = mc2, where E is energy, m is mass, and c is the speed of light. However, the value c isn't just the speed of light—it's actually just the fastest possible speed in the universe at which anything can travel, including light, massless particles, and even information.\n\nCrucially, when Einstein formulated his theories of relativity, he found that this cosmic speed limit also applies to gravity, implying the existence of gravitational waves that ripple outward through spacetime. When there is a change of energy density—say, if our sun were to suddenly disappear without a trace—the earth would remain in its current, stable orbit for another 8 minutes without any clue that the sun had just vanished, because that's how long it would take for the last gravitational waves to reach us.\n\nBut despite his prediction of the existence of gravitational waves well over a hundred years ago, they were so small and difficult to directly measure that scientists at the time doubted if there would ever be technology precise enough to detect them.\n\nFortunately for us, that technology does exist, and in 2015, scientists announced the first ever direct detection of gravitational waves, using the Laser Interferometer Gravitational-Wave Observatory, or **LIGO**. LIGO is one of the most delicate instruments ever created, comprised of two long tunnels in an L shape with an array of carefully constructed lasers and mirrors inside. The idea is that if a gravitational wave of sufficient strength moves through the system, it will ripple through and, for a brief moment, make one of the arms slightly longer and the other slightly shorter. This can be detected because the laser inside the machine is split into two beams that then bounce back and forth nearly 300 times in the separate arms before recombining back in the center. Because the beams are of the exact same strength and arriving at the same time, when they collide with each other they are completely destroyed by interference. However, if a ripple of a strong enough gravitational wave comes through, stretching the arms of the detector ever so slightly, it will disrupt the timing of these beams so that they don't perfectly recombine at the center as expected, and instead of destroying themselves, some energy will be left over and detected.\n\nIt's obviously far more complex in reality, because scientists also have to account for background interference of things like the earth and the sun, but that's the basis of it. To understand it on the simplest level: if spacetime is nice and calm, the lasers will destroy each other, and an indicator stays dark; but if spacetime is messed with, some energy from the lasers survives, and an indicator lights up.\n\nDetecting this tiny, minuscule change is a process so extremely precise that it's hard to even put it into perspective. The arm of the LIGO detector—the long tunnel through which the lasers travel—is 4 kilometers in length, or 2.5 miles, and a strong gravitational wave will change the length of this arm by smaller than a thousandth the diameter of a proton. On the official LIGO website, they compare this change to measuring the distance to the nearest star, 4.2 light years away, to the accuracy of the width of a human hair. Simply incredible.\n\nIn 2015, after getting some modernized upgrades, the detectors gave the first indication that a gravitational wave had been measured, with the main terminal giving off a short, audible chirp. This tiny beep not only earned several people a Nobel Prize in Physics, but was also evidence that Einstein had indeed been correct over a hundred years ago, long before the technology was around to pull off an experiment like this.\n\nTo determine where these gravitational waves had originated from, a joint effort was launched across the astronomical community, combining the findings of LIGO with sky surveys and gamma ray detections. To give them a head start on their search, though, scientists at least knew what general part of the sky to start looking in, because the waves had been detected by two separate LIGO systems in the United States—one in the state of Washington and the other in Louisiana. Because the waves traveled at the speed of light, there was about a 7-millisecond delay between the two sites, giving researchers a general direction of the source. There is one other gravitational wave system on earth that is believed to be sensitive enough to detect it, located in Italy, but unfortunately, this third one was shut off and undergoing maintenance during the event.\n\nThe consensus is that the origin of the waves was the merging of a pair of binary black holes, each around 30 times the mass of our sun, most likely located about 1 billion light years away in the direction of the Magellanic Clouds. And remember, because these gravitational waves travel at the speed of light, the actual event that created them happened in the distant past, long before vertebrates had even evolved on earth.\n\n## Gravity Warps the Passage of Time\n\nPerhaps the strangest of Einstein's predictions are those related to time dilation. There are two types of time dilation—one that arises as a result of velocity, and one that arises as a result of gravity, which is the one we'll be going over. Gravitational time dilation is a crazy concept that our brain just isn't wired to understand intuitively, but we'll do our best to break it down.\n\nAccording to relativity, the closer an observer is to a gravitational source, the slower time passes for them relative to an observer that is further from the gravitational source. This means that technically, a person living on the surface of the earth would age more slowly than someone living on the moon, because the person on earth experiences a stronger gravitational effect—though when we're talking about time dilation with things like the earth, this difference is super small, on the scale of mere fractions of a second for a human lifespan. However, it is interesting to note that because the strength of gravity is much stronger at the center of the planet, if you had some stopwatches running from the moment the earth formed, you'd find that its core is about two and a half years younger than its surface.\n\nTime dilation was displayed perfectly in the movie *Interstellar*, where one hour spent on Miller's Planet is equal to seven years spent back on earth, due to the fact that Miller's Planet is in close proximity to a large black hole. The time dilation caused by this black hole is also how the main protagonist ends up younger than his own daughter by the end of the film.\n\nConfirming time dilation experimentally was only limited by the accuracy of clocks, which improved greatly throughout the 20th century and led to multiple confirmations of Einstein's theories. Even the simplest of experiments yielded the expected results, such as placing one atomic clock at sea level and the other on top of a mountain. If your clocks are accurate enough, you'll find each and every time that the clock taken to the higher elevation ticks a fraction of a second faster.\n\nNow it's easy enough to take this at face value and move on, but a bit more analysis is needed to truly understand the \"why.\"\n\nThe key here is remembering the \"time\" half of spacetime. According to Einstein, the two are inseparable. When a large mass stretches and distorts space, it is also stretching and distorting time, which is what causes the difference in how quickly time passes. Think of time like a one-way road, on which you're always driving forward at a constant speed. Now imagine that every mile, there is a sign indicating that one year has passed. A pair of twins will go their lives passing by these mile markers at the exact same time, reaching ages 5, 10, and 20 simultaneously because they both live on the surface of earth, and their roads of time are identical.\n\nNow picture that one of the twins is teleported near the event horizon of a black hole, a place with immense gravitational force compared to the earth. In this environment of intense gravity, his road will be stretched and lengthened, making the distance between each mile longer than those of his twin back on earth. This means that even though they are moving forward through this time road at the same speed, the twin near the black hole has more distance to cover in order to age a single year, and thus it takes longer compared to his twin on earth, and he ages more slowly.\n\nStill, the point of relativity is that each twin doesn't notice a difference in the passage of time on their own. For each individual observer, time appears to pass at the normal rate, and time dilation is only noticeable when you compare two different locations.\n\n## It Is Most Extreme in Black Holes\n\nThe Einstein field equations are a group of equations published as part of General Relativity that describe the geometry of spacetime. Only a few months after they were published in 1915, Karl Schwarzschild found a peculiar solution to them—a point in space where when matter reached a certain density, some of the terms in the field equations became infinite. Today, we know this as a **singularity**, around which the force of gravity is so strong that not even light can escape, creating the infamous black hole.\n\nYou could fill a library with books dedicated to understanding black holes, singularities, and the strangeness that occurs at such high levels of gravity, so we're going to go through just a few things that most people aren't aware of.\n\nFor starters, the popular perception of black holes is that they \"suck\" matter into them as they move around the universe, like a hungry cosmic vacuum cleaner. But this isn't true. As long as you haven't crossed the event horizon—the point of no return—the gravitational pull exerted by a black hole is no different than the gravity exerted by any other object of that size. For instance, if you were to replace our sun with a small black hole of the same exact mass, the planets would continue orbiting as usual, as if nothing had changed. Of course, our planet would become inhospitable without the warmth of the sun, but the point is that we wouldn't suddenly be sucked into the center of the solar system and eaten by the black hole, which is a very popular misconception. Instead, we would just keep cruising around on our stable, yearly orbit.\n\nIn fact, it's not just objects that can orbit a black hole, but even light itself. There is a specific radius from the black hole at which light is trapped in a perfectly circular orbit, balanced on the perfect trajectory to avoid both falling past the event horizon or escaping into free space. This point is called the **photon sphere**. It leads to some interesting thought experiments, such as standing in the perfect spot so that light bouncing off your head travels all the way around the black hole and into your eyes, allowing you to look at the back of your own head—but in reality, the photon sphere is not only a one-dimensional line, having no thickness, but also orbits on the line would be highly unstable, as even the slightest interaction would disrupt the perfect distance.\n\nNow let's say you were trying to set up this little experiment and accidentally fell past the event horizon. Most people understand that this is the point from which you cannot return to the outside world, but let's break down exactly why that is.\n\nFor every celestial object, there is something known as **escape velocity**. This is the speed you need to reach in order to escape its gravitational pull. For the earth, it's about 25,000 miles per hour; for the sun, it's closer to 1.4 million miles per hour. That's really, really fast, but still within the realm of possibility if you have a nice spaceship with a powerful enough propulsion system. The issue with black holes is that once you pass the event horizon, the escape velocity becomes higher than the speed of light, and since we know that this speed is impossible to reach or surpass, you are now trapped.\n\nBut it gets even stranger than simply not being able to go fast enough. Once you've passed the event horizon, spacetime becomes increasingly stretched and warped beyond anything imaginable, to the point where no matter which path you take, the singularity is unavoidable—not just in space, but also in time. It becomes an inevitable point in your future. To understand this, think back to our analogy of a time road. When we discussed these time roads before, their length was infinite; all we cared about was how stretched it became under intense gravity. But once you're past the event horizon of a black hole and under the grip of its gravitational pull, spacetime becomes so curved that your time road now has a concrete end, a finish line at the singularity. No matter which direction you face or how fast or slow you try to move, every single path you could possibly take through spacetime ends with you meeting the singularity. It is inevitable—all thanks to gravity.\n\n## Key Takeaways\n\n- Gravity is not a force but a consequence of mass distorting spacetime.\n- General relativity and quantum mechanics have not been reconciled.\n- Gravitational waves travel at the speed of light, confirmed by LIGO.\n- Time passes more slowly in stronger gravitational fields, a phenomenon called gravitational time dilation.\n- Black holes have extreme gravity, making escape impossible beyond the event horizon.\n\n## Frequently Asked Questions\n\n### What is gravity according to Einstein's theory of general relativity?\n\nGravity is not a force but a byproduct of the way mass distorts spacetime. Mass bends and stretches spacetime, causing objects to move along curved paths, known as geodesics.\n\n### How does gravity affect the passage of time?\n\nGravity warps the passage of time, a phenomenon known as gravitational time dilation. The closer an observer is to a gravitational source, the slower time passes for them relative to an observer further away.\n\n### What are gravitational waves and how were they detected?\n\nGravitational waves are ripples in spacetime caused by changes in energy density. They were first detected in 2015 using the Laser Interferometer Gravitational-Wave Observatory (LIGO), which measures tiny changes in the length of its arms caused by these waves.\n\n### What is the event horizon of a black hole?\n\nThe event horizon is the point of no return around a black hole. Once an object crosses this boundary, the escape velocity becomes higher than the speed of light, making it impossible to escape the black hole's gravitational pull.\n\n### What is the photon sphere?\n\nThe photon sphere is a specific radius around a black hole where light is trapped in a perfectly circular orbit, balanced to avoid falling past the event horizon or escaping into free space.\n\n### What is the difference between Newtonian gravity and Einstein's theory of general relativity?\n\nNewtonian gravity describes the attraction between two objects as an equal and opposite pull, while Einstein's theory shows that gravity is a consequence of mass warping spacetime, causing objects to move along curved paths.\n\n### What is the speed limit of the universe?\n\nThe speed of light is the fastest possible speed in the universe, at which anything, including light, massless particles, and information, can travel.\n\n### What is the significance of the Einstein field equations?\n\nThe Einstein field equations describe the geometry of spacetime and led to the discovery of singularities, which are points in space where the force of gravity is so strong that not even light can escape, creating black holes.\n\n### What is the current status of quantum gravity theories?\n\nThere is no consensus in the scientific community concerning quantum gravity. Various theories like loop quantum gravity, M-theory, and string theory exist, but they are difficult to verify with current technology.\n\n## Sources\n\n- [Original Side Projects video: 5 Things You Don't Understand about Gravity](https://www.youtube.com/watch?v=NEtilzJOWgs)\n- [Hero image source](https://upload.wikimedia.org/wikipedia/commons/8/87/LIGO_at_Hanford%2C_Washington_%28LIGO_Pic_09-CC%29.jpg) by NOIRLab/LIGO/NSF/AURA/T. Matsopoulos / openverse, by.\n\n## Related Coverage" url: https://sideprojects.pub/article/5-things-you-dont-understand-about-gravity.md canonical: https://sideprojects.pub/article/5-things-you-dont-understand-about-gravity datePublished: 2026-06-17 dateModified: 2026-06-17 author: - name: Simon Whistler url: https://sideprojects.pub/author/simon-whistler publisher: Side Projects image: "https://media.sideprojects.pub/cdn-cgi/image/width=1600,height=900,fit=cover,quality=80,format=auto/articles/NEtilzJOWgs/hero.jpg" type: Article contentHash: 68c393b6e97d58129700d0512dc1f7f824646f94d599c3f8c061addd8ae2debe tokens: 6207 summaryUrl: https://sideprojects.pub/article/5-things-you-dont-understand-about-gravity.md.summary.md --- What goes up, must come down. Gravity needs no introduction—it's the familiar force that keeps our feet on the ground and our earth locked in a stable orbit around the sun. But the mechanisms behind gravity are far more complicated than meets the eye. Gone are the days when we believed that an apple falling onto Newton's head represented just about everything there was to unpack about this invisible aspect of our universe. In this article we're going to break down five things about gravity that are often oversimplified, commonly misunderstood by the public, or even still a complete mystery. ## It's Not Exactly a Force It's commonly taught that all interactions in the universe can be explained by the four fundamental forces: the strong and weak nuclear forces, electromagnetism, and gravity. However, you'll sometimes see these listed under a slightly different name—the four fundamental interactions—which is often considered to be slightly more accurate, because gravity isn't exactly a force like the other three. To explain this idea, we need to start from the beginning. For over 200 years, the prevailing theory of gravity had been laid out by Isaac Newton. In Newtonian gravity, the attraction between two objects is described as an equal and opposite pull that can be calculated based on the objects' masses and relative distance. The earth pulls on you, keeping you tethered to the ground, and, likewise, you pull on the earth—it's just that you have so much less mass that it doesn't visibly move the earth. This model worked wonders; it was easy to understand and experimentally verified. However, even Newton himself was a bit troubled with one part. You see, he'd figured out how to measure and predict gravitational attraction, but he still had absolutely no clue what caused it or how this action could occur over such large distances. In a letter to Richard Bentley, he wrote: > "That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it." This frustration would be shared by scientists around the world until it was finally resolved by none other than Einstein. In his theory of general relativity, Einstein revolutionized our understanding of gravity by showing that it isn't actually a force acting between two objects, but rather is a byproduct of the way mass distorts spacetime. As you may have learned in a physics class, an object in motion stays in motion unless it is acted upon. This means if you are cruising through a vacuum in space, your speed and direction would remain unchanged until something affected you. Newton was spot on when he came up with this idea, and Einstein built upon it to explain his new understanding of gravity. In a Newtonian model, your path through space would be a straight line, and this straight line would be bent by a gravitational attraction, curving your path toward the object as you are pulled in its direction. What Einstein did is he showed that it isn't really your path through space that is curved, but rather space itself—and this may seem like a pointless distinction to make, but it's actually important. Mass bends and stretches spacetime, and as a consequence, what you perceive as a straight path is curved along this warped terrain, moving you toward the attracting mass. This straight yet curved path is called a **geodesic**, and it's easier to understand with a comparison to passenger airlines. Airplanes want to take the shortest route to their destination, which is a straight line, and from the pilot's perspective, they indeed take off and fly straight with no deviation. However, when you zoom out, you can see that their overall path was actually curved, because despite the surface of the earth being two dimensional, it curves into a third dimension on the surface of a sphere, and thus what they perceived as a straight line followed this curve. It's a bit harder to visualize this in three dimensions because we live in three dimensions ourselves, but the concept is similar. Objects travel in a straight line, guided by the curves of spacetime. This gives the impression that there is a mysterious force pulling a distant planet toward a heavy star, but in reality, there is no force—no invisible push or pull that is shoving the planet in a new direction—it is simply a consequence of the star's mass warping spacetime. And on a more advanced note, it's not just about mass. The distortion of spacetime is actually created by any form of energy-density, which is why even though photons are generally believed to have a rest mass of zero, they still interact with gravity, and you can even technically create a black hole out of enough radiation focused in a small region in space. What you've just read is really just the tip of the iceberg when it comes to how curved space gives rise to gravity. We won't get into the deeper details, but we will leave you with the best explanation of it all, a quote from legendary theoretical physicist John Archibald Wheeler, who simplified this whole concept in a single phrase: > "Spacetime tells matter how to move; matter tells spacetime how to curve." ## Our Theories Fall Apart at the Quantum Level General Relativity refined Newton's findings and provided us with our most accurate model of gravity to date. However, it is not perfect, and has one glaring issue. Of the four fundamental interactions, gravity is the only one that has yet to be reconciled with quantum mechanics. Put simply, relativity is used to describe larger objects, and quantum mechanics is used to describe the universe on the subatomic level. So, when working in the domain of one, usually you can safely ignore the other because they describe such different aspects of the universe. However, there are times when we can't just ignore one of them, and need both an understanding of quantum mechanics and gravity working together to fully describe what's going on. This is the case in the highest levels of energy, such as the moments immediately following the big bang, and near the smallest regions of space, such as near the singularity of a black hole. If we truly want to fully understand every aspect of the universe, these two will eventually need to find common ground. This has led to the decades-long search for a more comprehensive theory of quantum gravity, or, in other cases, a so-called "theory of everything" that unifies all domains, gravity included. Without getting into the specifics, there have been many different models put forward over the years, such as loop quantum gravity, M-theory, string theory, Twister theory, and several others, but one problem that they all share is that because they are all predicting results on these extremely high energy levels or tiny, tiny regions of space, they are next to impossible for us to verify with real-world experiments using current technology. For instance, Loop Quantum Gravity predicts that spacetime is not continuous and smooth, but is actually made of distinct pieces or bits that can't be divided further. As a result, this theory makes the shocking prediction that the speed of light is not constant, but rather varies ever so slightly depending on a photon's wavelength, with higher energy photons moving slightly slower. However, this difference is so indescribably tiny that we really have no hope of detecting it. Likewise, for several predictions of string theory, our particle accelerators simply can't reach the levels of energy required to either prove or disprove the prediction. Still, that's not to say that these theories are simply numbers on a chalk board. Even without experimental validity, they have still yielded interesting scientific and mathematical contributions—it's just that without observable evidence, they can't yet be solidified as accepted science. And as a result, there is no consensus in the scientific community concerning quantum gravity, and the field is currently developing at this very moment. It's certainly something to look out for in the coming decades, as fresh ideas and technological improvements increase our experimental capabilities; we may finally crack this case once and for all. ## It's Also Limited by the Speed of Light One of the most famous findings of Albert Einstein is the relationship between energy and mass, summed up in the equation E = mc2, where E is energy, m is mass, and c is the speed of light. However, the value c isn't just the speed of light—it's actually just the fastest possible speed in the universe at which anything can travel, including light, massless particles, and even information. Crucially, when Einstein formulated his theories of relativity, he found that this cosmic speed limit also applies to gravity, implying the existence of gravitational waves that ripple outward through spacetime. When there is a change of energy density—say, if our sun were to suddenly disappear without a trace—the earth would remain in its current, stable orbit for another 8 minutes without any clue that the sun had just vanished, because that's how long it would take for the last gravitational waves to reach us. But despite his prediction of the existence of gravitational waves well over a hundred years ago, they were so small and difficult to directly measure that scientists at the time doubted if there would ever be technology precise enough to detect them. Fortunately for us, that technology does exist, and in 2015, scientists announced the first ever direct detection of gravitational waves, using the Laser Interferometer Gravitational-Wave Observatory, or **LIGO**. LIGO is one of the most delicate instruments ever created, comprised of two long tunnels in an L shape with an array of carefully constructed lasers and mirrors inside. The idea is that if a gravitational wave of sufficient strength moves through the system, it will ripple through and, for a brief moment, make one of the arms slightly longer and the other slightly shorter. This can be detected because the laser inside the machine is split into two beams that then bounce back and forth nearly 300 times in the separate arms before recombining back in the center. Because the beams are of the exact same strength and arriving at the same time, when they collide with each other they are completely destroyed by interference. However, if a ripple of a strong enough gravitational wave comes through, stretching the arms of the detector ever so slightly, it will disrupt the timing of these beams so that they don't perfectly recombine at the center as expected, and instead of destroying themselves, some energy will be left over and detected. It's obviously far more complex in reality, because scientists also have to account for background interference of things like the earth and the sun, but that's the basis of it. To understand it on the simplest level: if spacetime is nice and calm, the lasers will destroy each other, and an indicator stays dark; but if spacetime is messed with, some energy from the lasers survives, and an indicator lights up. Detecting this tiny, minuscule change is a process so extremely precise that it's hard to even put it into perspective. The arm of the LIGO detector—the long tunnel through which the lasers travel—is 4 kilometers in length, or 2.5 miles, and a strong gravitational wave will change the length of this arm by smaller than a thousandth the diameter of a proton. On the official LIGO website, they compare this change to measuring the distance to the nearest star, 4.2 light years away, to the accuracy of the width of a human hair. Simply incredible. In 2015, after getting some modernized upgrades, the detectors gave the first indication that a gravitational wave had been measured, with the main terminal giving off a short, audible chirp. This tiny beep not only earned several people a Nobel Prize in Physics, but was also evidence that Einstein had indeed been correct over a hundred years ago, long before the technology was around to pull off an experiment like this. To determine where these gravitational waves had originated from, a joint effort was launched across the astronomical community, combining the findings of LIGO with sky surveys and gamma ray detections. To give them a head start on their search, though, scientists at least knew what general part of the sky to start looking in, because the waves had been detected by two separate LIGO systems in the United States—one in the state of Washington and the other in Louisiana. Because the waves traveled at the speed of light, there was about a 7-millisecond delay between the two sites, giving researchers a general direction of the source. There is one other gravitational wave system on earth that is believed to be sensitive enough to detect it, located in Italy, but unfortunately, this third one was shut off and undergoing maintenance during the event. The consensus is that the origin of the waves was the merging of a pair of binary black holes, each around 30 times the mass of our sun, most likely located about 1 billion light years away in the direction of the Magellanic Clouds. And remember, because these gravitational waves travel at the speed of light, the actual event that created them happened in the distant past, long before vertebrates had even evolved on earth. ## Gravity Warps the Passage of Time Perhaps the strangest of Einstein's predictions are those related to time dilation. There are two types of time dilation—one that arises as a result of velocity, and one that arises as a result of gravity, which is the one we'll be going over. Gravitational time dilation is a crazy concept that our brain just isn't wired to understand intuitively, but we'll do our best to break it down. According to relativity, the closer an observer is to a gravitational source, the slower time passes for them relative to an observer that is further from the gravitational source. This means that technically, a person living on the surface of the earth would age more slowly than someone living on the moon, because the person on earth experiences a stronger gravitational effect—though when we're talking about time dilation with things like the earth, this difference is super small, on the scale of mere fractions of a second for a human lifespan. However, it is interesting to note that because the strength of gravity is much stronger at the center of the planet, if you had some stopwatches running from the moment the earth formed, you'd find that its core is about two and a half years younger than its surface. Time dilation was displayed perfectly in the movie *Interstellar*, where one hour spent on Miller's Planet is equal to seven years spent back on earth, due to the fact that Miller's Planet is in close proximity to a large black hole. The time dilation caused by this black hole is also how the main protagonist ends up younger than his own daughter by the end of the film. Confirming time dilation experimentally was only limited by the accuracy of clocks, which improved greatly throughout the 20th century and led to multiple confirmations of Einstein's theories. Even the simplest of experiments yielded the expected results, such as placing one atomic clock at sea level and the other on top of a mountain. If your clocks are accurate enough, you'll find each and every time that the clock taken to the higher elevation ticks a fraction of a second faster. Now it's easy enough to take this at face value and move on, but a bit more analysis is needed to truly understand the "why." The key here is remembering the "time" half of spacetime. According to Einstein, the two are inseparable. When a large mass stretches and distorts space, it is also stretching and distorting time, which is what causes the difference in how quickly time passes. Think of time like a one-way road, on which you're always driving forward at a constant speed. Now imagine that every mile, there is a sign indicating that one year has passed. A pair of twins will go their lives passing by these mile markers at the exact same time, reaching ages 5, 10, and 20 simultaneously because they both live on the surface of earth, and their roads of time are identical. Now picture that one of the twins is teleported near the event horizon of a black hole, a place with immense gravitational force compared to the earth. In this environment of intense gravity, his road will be stretched and lengthened, making the distance between each mile longer than those of his twin back on earth. This means that even though they are moving forward through this time road at the same speed, the twin near the black hole has more distance to cover in order to age a single year, and thus it takes longer compared to his twin on earth, and he ages more slowly. Still, the point of relativity is that each twin doesn't notice a difference in the passage of time on their own. For each individual observer, time appears to pass at the normal rate, and time dilation is only noticeable when you compare two different locations. ## It Is Most Extreme in Black Holes The Einstein field equations are a group of equations published as part of General Relativity that describe the geometry of spacetime. Only a few months after they were published in 1915, Karl Schwarzschild found a peculiar solution to them—a point in space where when matter reached a certain density, some of the terms in the field equations became infinite. Today, we know this as a **singularity**, around which the force of gravity is so strong that not even light can escape, creating the infamous black hole. You could fill a library with books dedicated to understanding black holes, singularities, and the strangeness that occurs at such high levels of gravity, so we're going to go through just a few things that most people aren't aware of. For starters, the popular perception of black holes is that they "suck" matter into them as they move around the universe, like a hungry cosmic vacuum cleaner. But this isn't true. As long as you haven't crossed the event horizon—the point of no return—the gravitational pull exerted by a black hole is no different than the gravity exerted by any other object of that size. For instance, if you were to replace our sun with a small black hole of the same exact mass, the planets would continue orbiting as usual, as if nothing had changed. Of course, our planet would become inhospitable without the warmth of the sun, but the point is that we wouldn't suddenly be sucked into the center of the solar system and eaten by the black hole, which is a very popular misconception. Instead, we would just keep cruising around on our stable, yearly orbit. In fact, it's not just objects that can orbit a black hole, but even light itself. There is a specific radius from the black hole at which light is trapped in a perfectly circular orbit, balanced on the perfect trajectory to avoid both falling past the event horizon or escaping into free space. This point is called the **photon sphere**. It leads to some interesting thought experiments, such as standing in the perfect spot so that light bouncing off your head travels all the way around the black hole and into your eyes, allowing you to look at the back of your own head—but in reality, the photon sphere is not only a one-dimensional line, having no thickness, but also orbits on the line would be highly unstable, as even the slightest interaction would disrupt the perfect distance. Now let's say you were trying to set up this little experiment and accidentally fell past the event horizon. Most people understand that this is the point from which you cannot return to the outside world, but let's break down exactly why that is. For every celestial object, there is something known as **escape velocity**. This is the speed you need to reach in order to escape its gravitational pull. For the earth, it's about 25,000 miles per hour; for the sun, it's closer to 1.4 million miles per hour. That's really, really fast, but still within the realm of possibility if you have a nice spaceship with a powerful enough propulsion system. The issue with black holes is that once you pass the event horizon, the escape velocity becomes higher than the speed of light, and since we know that this speed is impossible to reach or surpass, you are now trapped. But it gets even stranger than simply not being able to go fast enough. Once you've passed the event horizon, spacetime becomes increasingly stretched and warped beyond anything imaginable, to the point where no matter which path you take, the singularity is unavoidable—not just in space, but also in time. It becomes an inevitable point in your future. To understand this, think back to our analogy of a time road. When we discussed these time roads before, their length was infinite; all we cared about was how stretched it became under intense gravity. But once you're past the event horizon of a black hole and under the grip of its gravitational pull, spacetime becomes so curved that your time road now has a concrete end, a finish line at the singularity. No matter which direction you face or how fast or slow you try to move, every single path you could possibly take through spacetime ends with you meeting the singularity. It is inevitable—all thanks to gravity. ## Key Takeaways - Gravity is not a force but a consequence of mass distorting spacetime. - General relativity and quantum mechanics have not been reconciled. - Gravitational waves travel at the speed of light, confirmed by LIGO. - Time passes more slowly in stronger gravitational fields, a phenomenon called gravitational time dilation. - Black holes have extreme gravity, making escape impossible beyond the event horizon. ## Frequently Asked Questions ### What is gravity according to Einstein's theory of general relativity? Gravity is not a force but a byproduct of the way mass distorts spacetime. Mass bends and stretches spacetime, causing objects to move along curved paths, known as geodesics. ### How does gravity affect the passage of time? Gravity warps the passage of time, a phenomenon known as gravitational time dilation. The closer an observer is to a gravitational source, the slower time passes for them relative to an observer further away. ### What are gravitational waves and how were they detected? Gravitational waves are ripples in spacetime caused by changes in energy density. They were first detected in 2015 using the Laser Interferometer Gravitational-Wave Observatory (LIGO), which measures tiny changes in the length of its arms caused by these waves. ### What is the event horizon of a black hole? The event horizon is the point of no return around a black hole. Once an object crosses this boundary, the escape velocity becomes higher than the speed of light, making it impossible to escape the black hole's gravitational pull. ### What is the photon sphere? The photon sphere is a specific radius around a black hole where light is trapped in a perfectly circular orbit, balanced to avoid falling past the event horizon or escaping into free space. ### What is the difference between Newtonian gravity and Einstein's theory of general relativity? Newtonian gravity describes the attraction between two objects as an equal and opposite pull, while Einstein's theory shows that gravity is a consequence of mass warping spacetime, causing objects to move along curved paths. ### What is the speed limit of the universe? The speed of light is the fastest possible speed in the universe, at which anything, including light, massless particles, and information, can travel. ### What is the significance of the Einstein field equations? The Einstein field equations describe the geometry of spacetime and led to the discovery of singularities, which are points in space where the force of gravity is so strong that not even light can escape, creating black holes. ### What is the current status of quantum gravity theories? There is no consensus in the scientific community concerning quantum gravity. Various theories like loop quantum gravity, M-theory, and string theory exist, but they are difficult to verify with current technology. ## Sources - [Original Side Projects video: 5 Things You Don't Understand about Gravity](https://www.youtube.com/watch?v=NEtilzJOWgs) - [Hero image source](https://upload.wikimedia.org/wikipedia/commons/8/87/LIGO_at_Hanford%2C_Washington_%28LIGO_Pic_09-CC%29.jpg) by NOIRLab/LIGO/NSF/AURA/T. Matsopoulos / openverse, by. ## Related Coverage