I have one question for all of you: why? Seriously, I mean: this seemingly simple and straightforward word - ‘why’ - encapsulates a massive breadth of everything we could ever imagine, marvel, and wonder about. Why am I here, today, talking to all of you? Why ‘this’. Why ‘that’. In fact, there is such a vast amount of these mysteries that the word ‘why’ brings about, that I’m going to ask you one myself: if you could explore, investigate, and perhaps even answer one question, concept, or mystery that human curiosity has to offer - what would it be? And, as you might have guessed, why?
For me, personally, there’s one question that trumps all others and that, is the great puzzle of the universe. How do things work? What universal forces are at play? Now, today we aren’t going to dive into the philosophical aspect of these topics, but rather analyze them through a concrete and physical point of view.
Indeed, we’ll be learning about the foundation of modern physics, the most elegant and important theory of all - this is, of course, none other than Albert Einstein’s Theory of General Relativity.
Some of you may have already heard about Relativity. Still, today we’re getting a look at the principles, intuition, and frameworks at play - in order for you to get a meaningful understanding of this rather intricate theory.
Let’s start off with the base - the backbone of any physical interpretation of our universe. What is Gravity? Simply put, it is a fundamental force that tends to bring objects closer together. We are standing on the ground, the moon orbits the Earth, and the Earth orbits the Sun - these are consequences of this phenomena. It operates in between these objects and prevents them from moving further from one another. Now, after hearing this, you might think that gravity is simply a force that attracts objects relatively to their respective masses. The more one object is massive, the more mass it would pull towards it. However, in reality gravitation doesn’t function as simply as a force. For example, if gravity operated as a force, all the satellites around the Earth would fall towards the center of the planet. If we would execute this experiment, the satellites would fall in a slightly bent manner, following the direction of the Earth’s rotation. So what truly is gravity? Well, defining it means redefining the concepts of space and time into a single, unified spacetime, which science has achieved thanks to Einstein’s theory of General Relativity.
In the words of famous American physicist John Wheeler : “Spacetime tells matter how to move. Matter tells spacetime how to curve.”
That’s it. That’s General Relativity in a nutshell.
One day, as legend has it, Einstein had one of his famous thought experiments that would go on to change the course of scientific history forever. He imagined - what would happen if a man, high up on a ladder, were to fall? But, he didn’t think of it the same way you and I would think of it. Instead, he put himself in the perspective of the man, and imagined not what would happen as he would splatter on the ground below, but rather what he would experience as he was falling.
Now, put yourself in Einstein’s shoes and tell me : what do you think would happen ?
Einstein realized that gravity would be the only force acting upon him. He would be accelerating towards the ground, but since the ground would not be pushing up against his body, he would feel no weight. With no weight resistance, he would be in free fall. And this, as you might have guessed, would be no different than being weightless in the vacuum of space.
In a way, gravity and acceleration were just different ways of describing the same phenomenon.
The foundation of General Relativity is based upon what is known as the Equivalence principle. It states 2 things: all masses must fall in the same way and free fall isn’t an object accelerating but is the ‘natural movement’ for all bodies in the universe. During free fall, our body follows its natural movement and it is only when we have touched the ground that we can feel our weight, which corresponds to our feet putting a stop to our fall. In General Relativity, gravity therefore is no longer a force. When objects such as the planets in our solar system appear to ‘attract’ each other, this is just them moving naturally but in a part of space whose geometry is curved. Masses never attract one another, what does happen though, is that the greater the mass of an object, the more it will curve the fabric of spacetime that surrounds it. This is what influences the trajectories of all nearby objects. The most fundamental concept of General Relativity is the idea that all objects that possess a mass distort spacetime around them.
This may seem daunting at first glance, so let’s take a look at an example.
Think of our universe as a huge grid of space and time, a massive object will indeed have the effect of contorting the notions of distance, direction, and the flow of time. If we were to throw a ball into an empty part of space, it will continue its movement in a straight line at a constant speed; this is known as Newton’s first law of motion. If we now throw that same ball next to the sun, the presence of that massive object will bend the previously straight trajectory of the ball towards the Sun, creating an ‘attraction’ effect.
Now that you have a better idea of what General Relativity puts forward, let’s analyze how this affects the function of black holes. To remind you, a black hole is an infinitely massive object whose whole mass is contained within an infinitely minuscule point; known as the gravitational singularity. At that point, spacetime is distorted infinitely and the laws of physics break down. Outside of what we call a black hole’s event horizon, the limit to where matter can go without entering the black hole, the black hole behaves similarly to any other object. When we are near the event horizon, it is impossible to stay in a stable orbit around the black hole, the distortion of spacetime is too great which makes the trajectories of objects chaotic and impossible to predict. However, as soon as we find ourselves far enough from the event horizon, as is the case with stars or planets, it is possible to gravitate in a stable fashion around the black hole (this is a bit more complicated, but it’s what we call a Geostationary Lagrange transfer orbit).
In particular, light rays from faraway stars change direction when they are close to a black hole. For this reason, if we try to observe a black hole, we can see a distorted image of distant stars which are actually behind it. This is known as gravitational lensing.
One of the most significant phenomena that arises from General Relativity is gravitational time dilation. When we find ourselves close to massive objects, the straight lines that we follow are distorted not only in space, but also in time.
With that being said, what do you think would happen to our way of perceiving the world around us?
The flow of time that we perceive will be altered according to the distance at which we are from the massive object. Far away from a black hole, where the curvature of spacetime is practically imperceptible, time flows, more or less, like anywhere else. However, the closer we move to a black hole, the more our subjective flow of time slows down. This revolutionary concept discovered by Einstein shatters the major postulate upon which Newtonian physics was built. Basically, what Einstein’s saying is : “get out of here Newton, and let me show you how it’s done. Time isn’t absolute. Time is relative.”
You may have heard of the Twin Paradox, which is a famous example of time dilation in action. I’m going to take a similar example.
If we compare the internal clocks of two people, one who’s on the surface of the Earth, and the other on top of the Eiffel Tower. The person standing on the Earth, being closer to its center of gravity, will feel time passing slightly slower than the one on the Eiffel Tower. In a year, the person at the bottom will age a microsecond less than the person at the top. Although it is subtle on this scale, it is crucial when, for example, programming GPS satellites to ensure the synchronization of their internal clocks.
There’s a famous quote by Einstein that goes like this: “If you can’t explain something to a 6-year-old, you don’t understand it yourself.” The next step for you to truly get a grasp of General Relativity is to explain it to someone yourself - the ball is in your court.
Comentarios