One of the fundamental concepts we study in school is gravity. We all know that gravity keeps us on the ground, keeps planets in their orbits, causes ocean tides, the formation of stars, and other things. It has always been defined as a force that holds everything together, but what if, in reality, it isn't a force? Imagine if I spill an apple. According to Isaac Newton, the ground can be thought of as being at rest, and the earth exerts gravitational force on the apple, which causes it to accelerate downward. However, according to Albert Einstein, the apple is stationary, and the ground along with everything on it is accelerating upwards into the apple now that may sound preposterous and maybe even moronic but it’s not sophistry there’s something actually substantive here. To analyze motion, one needs a "frame of reference," which is really just some X-Y-Z axes to label points in space and a click to track time. Let's start with some physics 101 and Sir Newton's laws of motion. You can only quantify motion relative to other things, which is why you require a frame of reference. If you are unfamiliar with the term "frame of reference," you should stop reading this article right away and go watch this great 1960s black-and-white MIT film about frames of reference on YouTube. Since the difference between a frame actually being at rest and one that is actually moving at a constant speed is meaningless and only a matter of perspective, Newton's laws of motion cannot really tell you if a frame is actually doing either. Interesting though it may seem, Newton's rules can actually inform you whether or not your frame of reference is indeed accelerating. Take an object that is not subject to any forces and release it to see how that works. If the object stays in place, your frame of reference is said to be a "inertial frame of reference" and it is not moving forward. Newton's second law, F=ma, only holds true in inertial frames, meaning that the net force acting on an object will equal that object's mass times its acceleration ONLY IF YOU ARE MEASURING THAT ACCELERATION USING AN INERTIAL FRAME. This makes inertial frames unique in Newtonian physics. For example, suppose youre in a train that starts accelerating uniformly forward along a flat track, relative to the trains interior, you would accelerate backwards even though you can’t really identify any horizontal forces on you, so inside the train car, F decidedly does not equal MA and the train’s frame of reference is not inertial. In contrast a frame attached to the tracks pretty much is inertial —atleast if you disregard earths rotation, because relative to that frame, you don’t accelerate at all instead the train accelerates forward underneath you. Now more generally, any frame that accelerates relative to an inertial frame will not be inertial. inertial frame and non accelerating frames are synonyms in Newtonian physics. In fact, one can think of inertial frames as the standard against which you measure true acceleration and from the perspective of inertial frames, motion simply obeys a rule ~ F=ma. Alright let’s go back to that accelerating train car and look at things from the train cars frame of reference, inside the train car not only does everything accelerates backwards for no apparent reason everything accelerates backwards uniformly, with the same acceleration remember from the preferred point of view of the inertial gram that’s attached to the tracks, everything else is all stationary and only the train is accelerating forward to intercept everything else and ofcourse everyone in the train moves in lockstep in the train’s frame. But hold on a second, there’s something else familiar that makes people accelerate in lockstep— The Newtonian force of gravity. In fact in the absence of air resistance, that’s the actual defining feature of gravity. So in the train cars frame of reference which is accelerating forward, it’s as if there’s an additional gravitational field that points “backwards” so accelerating frames of reference mimic gravitational field in the opposing direction of the frame’s acceleration. That’s pretty interesting. If one combines that extra fake gravitational field with the actual gravitational field of the earth, which points down, it looks like there’s a net gravitational field inside the train that points at some angle down and back. “Smarter Every day” channel on YouTube has a pretty famous and solid video of a helium balloon in an accelerating car that happens to illustrates this point really well

Follow the above link to watch and get a better understanding on what’s about to go on.

Well Welcome back. Now as you saw, when destin hit the accelerator, a pendulum hanging from the ceiling tilts back that’s tied to the floor while a balloon that’s tied to the floor tilts forward, He(Destin) explains that air is piling up in the rear of the car and getting slightly denser there, so the balloon is just trying to go towards the less dense air near the front. All of that is quite true, but there’s another way to think about the situation, one can also think that the car’s forward acceleration is mimicking some extra gravity pointing backwards. Combine that with earths “real” gravitational field then its as though the total gravity inside the car points down and back at around a 30 degree angle. That’s the new vertical and the pendulum string and the balloon string are just aligning with the vertical the way they always do. The pendulum Hangs “down” and the ballon hangs up because air is denser on the ground and less dense at higher altitudes, in fact the accelerating frame of reference of Destins car is completely indistinguishable from having that car stationary on the surface of some other planet with slightly bigger “gravity” than earth and tilt it upwards by about 30 degrees. If you blacked out the windows of the car and put perfect shock absorbers in the minivan, then for all Destin and his kids know, they’re completely at rest, tilted upwards on another planet in a perfectly inertial frame of reference. Huh Lmaoo. Now In Newtonian physics this is just an accounting trick that has no broader significance. “Really” destins car is accelerating and this extra backwards gravity is “fake”. But Einstein asked Hold on, what if the so called “real downwards gravity” from Earth is also fake. A side effect generated because earths surface is really accelerating upwards? Now Newton and yourself would say that’s just ridiculous, reminding Einstein that inertial Frames are the standard for measuring true acceleration, so you can only say earth is really accelerating upwards of you can identify an inertial frame relative to which earths surface accelerates upward, and obviously there’s no inertial frame like that right? Well hold what there is? What about a frame that’s in free fall? Think about it, if I put a person in a box and drop them off a cliff, then I’m the frame of the box, everything is just floats, weightless. The falling frame of the box behaves just like a stationary inertial frame that’s way out in intergalactic space where there’s no gravity. So why can’t the boxes frame be inertial? Well because newton says that frame can’t be inertial it’s really accelerating downwards in 9.8 meters per second squared. The interior just “seems” like zero G because the downwards acceleration acts like a fake extra upward gravitational field that, from the perspective of the box, just happens to exactly cancel the “real” downward gravitational field of earth by coincidence. Really? I mean really? Really newton🙄. Einstein says look feller I’m just following your rules. You established the test for what an inertial frame is —— release a force-free object and it stays put. Stationary frames intergalactic space pass that test but really falling frames on earth also pass that test if your so-called gravity is fictitious. More to the point, Newton, if you’re inside the box there’s no way for you to know that you’re not In intergalactic space. This inability to distinguish free fall from lack of gravity has a name by the way, Einstein calls it “The Equivalence Principle” and if you buy it, then maybe falling frames really are inertial if so then it’s the falling frames that established the standard of non-acceleration, in which case it’s really the ground that’s accelerating upwards and that we’ve and what we’ve always been calling it gravitational force is an artifact of being in an accelerated frame of reference. It’s not different from the weird, backward jolt you experience on the train that you know perfectly well isn’t being caused by anything so why insist that the downward jolt that we experience every day on Earth has a physical origin? Maybe gravity, just like that backward jolt on the train, is an illusion. Now newton would say that earth is really flat it’s round Down isn’t really down, it’s radially inward and this creates two problems with thinking about freely falling frames as inertial or thinking about gravity as an illusion. First, two objects in a falling box or falling toward Earth on not-quite-parallel radial spokes. So from the perspective inside the box they won’t actually remain stationary. They accelerates towards each other slightly, even though there are no forces on them, in seeming violation of F=ma. Second, Einstein, orbiting frames of reference on the space station should also be considered inertial. but those frames accelerate relative to frames that are just falling straight down and if you recall to the beginning of this article inertial frames aren’t supposed to accelerate relative to each other. That’s a really good point and kinda looks like game over for Einstein right?? Well not quite. It turns out that there’s a loophole that makes Einstein”s viewpoint self-consistent. The rule that inertial frames can’t accelerate relative to each other turns out to be true only if the what’s called a “flat geometry” if instead the world is a non-Euclidean and curved space-time, then straight line at constant speed doesn’t mean what you think it is and it turns out that inertial frames in a curved space-time can do almost anything they want. It took Einstein about 8 years to realize that. But once he did a beautiful model of the world emerged called “general relativity” it makes several predictions that newtons theory of gravity does not and so far it’s passed all its experimental tests and one of the central precepts of general relativity is that we inhabit a curved space-time and it that curved space-time the orbit of the ISS is a constant speed straight line. The arc of basketball throwing a 3-point shot? Constant speed straight line. You, my friend sitting down or laying down reading this article is accelerating, giving you the impression that there’s a force of gravity when in fact, no such thing exists.

So what exactly is this so called gravity?

The theory of general relativity by Albert Einstein describe gravity as a curvature in space-time rather than a force, something that warps the space-time fabric causing things to accelerate towards each other.

Space-time fabric We actually know what the three dimensions in our world are that is length width and height we is perceived all around us however there’s said to be a fourth dimension which is Time. The theory of general & special relativity treats time as a dimension along side the three other dimensions. Under these circumstances, physicists have formed a mathematical model which includes all three dimensions of space and the dimension of time which is the space-time fabric. To put in simpler terms, one cannot move in space without moving through time as long as one is moving time is passing even if one is moving faster than the speed of light and time is moving backwards, time is still moving therefore these four dimensions exist and physicist use the model of space-time to understand the concept of gravity, in many hypotheses the space-time is often compared to a trampoline with a bowling ball lying in it’s center and another small ball away from it. The bowling ball causes a depression/dent in its center due to its weight the rest of the trampoline will curve around it now the small ball lying at the end of trampoline will start slowly moving down towards the bowling ball at the middle. There’s no real force attracting these two balls it’s merely acceleration so the trampoline is said to be the space-time fabric and the bowling ball at the center is an object with mass so technically gravity is actually a space-time curvature rather than a force to understand this concept a lil better one can look at the earth and the moon. One knows that the earth is a lot bigger than the moon and has a greater mass therefore it can be referred to as the bowling ball that bends the space-time fabric one always knows that the moon orbits around the earth and due to the warp created by earth it travels around the bend. There’s no actual force that’s pulling the moon to the earth it’s just a distortion in the space-time fabric causing the moon to follow a certain path.

How do we know Einstein’s theory is actually correct?

Over the years, physicists have found gazillion evidence that actually proves this theory for example one of the most famous proof of this theory, Gravitational lensing. Gravitational lensing is a tool used to study and understand the distribution of matter in our universe, it allows astronomers to view distant galaxies as they were in the past and helps them gain an idea of how the universe was formed. The mathematics and physics behind this concept proved that any massive celestial object such as a planet or even a quasar can bend any passing light rays, these celestial objects are said to warp the space-time fabric and it acts as a lens that bends and magnifies the light passing by it this way we are able to receive multiple images of the celestial object. There is also another effect known as the Shapiro time delay effect that confirms the theory of general relativity, one knows that the sun is extremely massive and a lot bigger than any of our planets it causes a bend or a dent in the space-time fabric, any light that passes by won’t travel in a straight line as it would bend as well as a result, light from any planet on the other side of our solar system will reach us a little bit later than expected. We can’t really see the bend in light passing through the sun as the sun is bright enough but the bend of light passing by the sun was in fact confirmed during the total eclipse of the sun. Einstein’s theory of general relativity and gravity is actually a lot more complex although we have quite a few proving his theories we haven’t yet found all the answers but it does give us a pretty good idea of how gravity works and how our stars and galaxies were formed.