If anyone else have said it, people would think he's a crazy sociopath. But when non other than Albert Einstein says that the happiest thought of his life was when he imagined a man falling off the rooftop of a house, then it suddenly becomes a genius idea. Not because he said it, but because it simply is! [1]
What's so special about a man falling of the roof?
Well, it's freedom! Momentarily at least, that man would be in a "free fall" state.For him, during the fall, there is no gravitational field. He won't feel his own weight, he would be completely weightless. And if this said man lets go of anything he is holding, the objects wouldn't fall down, they would stay relatively stationary to him, moving in a uniform motion.
That whole thing is just exactly the same as if you were floating at rest or moving along at a constant velocity in deep space, with no large masses nearby. You'd feel no weight, and all objects in your near vicinity would remain stationary relative to you. Giving them a push, would cause them to move in a straight line at a constant velocity!
You'd be the perfect definition of an inertial observer.
When Einstein looked at those two examples, he said they are both completely equivalent. Physically speaking, they are exactly the same thing.
Even though the man falling of the roof is near the Earth and is accelerating, which would become very apparent once he crashes. Einstein's equivalence principle, however, tells us the one thing to focus on is the experience of the observer. If he feels weightless, then he is in an inertial frame of reference, and would remain so until the instant he crashes!
Our guy who is floating in deep space, wouldn't feel anything have changed if he passes near some planet. An outside distant observer would see the guy drifting towards that planet and accelerating, but the floating guy from his frame of reference wouldn't feel anything until the moment he crashes into that planet.
The Illusion Of Gravity
So why does the floating guy drift towards the planet if there isn't any gravitational force or a gravitational fields?
The answer is:
Curved Spacetime!
From our floating in space guy's observation, he felt the whole time like he was moving at constant velocity in a straight line. And he was actually moving in a straight line through spacetime. However, spacetime around massive objects like planets is curved. So that's why his path appears curved to a distant observer.
This isn't as weird as it sounds, an airplane for example takes the shortest path between cities during its flight, which is a straight line.
But since the Earth's surface is curved, the shortest path doesn't look like a straight line, it would appear as curved.
Geodesics are lines representing the shortest paths on curved surfaces. [2]
It's like when you draw a straight line on a paper, then take that paper and tilt or fold it. The line is still a straight line on that surface, representing the shortest path between two points on that 2 dimentional plane.
Similarly, those are the straight line paths followed by inertial observers through curved spacetime.
Another analogy we can take a look into, is if you imagine two people, a mile apart from each other, moving in straight line towards north.
As they approach north they would seem to be moving closer to eachother, even though both were moving completely straight forward, until they eventually bump into one another once they reach the north pole.
It would appear as though there is a force pushing them closer to eachother, but in fact there is no such thing.
The only reason those two people in our example came together is because they were in straight paths on a curved surface (Geodesic). And gravity is exactly like that!
An astronaut in a space station moving around Earth feels weightless, and is moving in a straight line, but to a distant observer, it wouldn't look like a straight line, because of the curvature around massive objects, Earth in this case. He would appear to be going around it in circles.
"Matter tells spacetime how to curve, and spacetime tells matter how to move"
~ John Wheeler [3]
Now imagine if our guy in deep space was inside a rocket, and that rocket starts accelerating at (9.8 m/s).
An outside observer would see everything inside such a rocket stays stationary while the rocket's floor starts accelerating into them.
From inside the rocket, everything would appear to accelerate down to the ground, you would be feeling a force that is pushing up on your feet. EXACTLY as if you were standing at rest on the surface of Earth.
So, being "at rest" on Earth and accelerating in a rocket, is the exact same thing, we are in fact accelerating, and there is no gravitational field!
Gravitational fields do not exist.
The "gravity" we experience from Earth pushing on us, it is just a diviation from a geodesic path, it's just preventing us from following that path, pushing on us up, stopping us from continuing our path.
If Earth would suddenly disappeare, or if a hole would suddenly emerge through it, we would simply "continue" our path right through it.
We're simply accelerating along with everything else on Earth, and acceleration in general relativity has a new meaning other than the traditional one, it is defined as simply not going in a straight line path through spacetime, deviating from a geodesic!
In curved spacetime, you need to accelerate just so you can "stand still".
This is of course all well known in the physics community, and general relativity has been tested and proven to extreme precisions to be correct over and over again.
What I don't understand, however, is why the insistance on trying to prove things that shouldn't even exist, like the Graviton for example? Which honestly in the light of general relativity, to me at least, doesn't make any sense at all.
ā Thank you for reading ā
ā ā¢1- Einstein's Happiest Thought | 2- Geodesics | 3- John Wheeler | 4- General Relativity |
Note: The purpose of this post is to explore the concept of gravity in general relativity without getting into the technicalities.
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