What is the reason behind Gravity is not a Force?

Hello everyone. As we all studied in our school life, many types of powerful forces exist in this universe, like electric force, magnetic force, friction force, viscous force, etc. Many are attraction forces by which two or more bodies come closer. And the rest of them are different types of troops. But they all are not mysterious at all. But that’s what I’m going to talk about today is the most popular force, “Gravity.”

Definition of gravity:

To understand the definition of gravity, one must first understand the three laws of motion. These laws, put, describe how objects move. The first law, the law of inertia, states that an object will remain at rest or in motion unless acted on by an outside force. The second law, known as the law of acceleration, states that the acceleration of an object is proportional to the force acting on it. The third law, known as the law of action and reaction, says that for every action, there is an equal and opposite reaction.

Now that we have a brief understanding of the three laws of motion, we can better understand the definition of gravity. Gravity is the force by which a planet or other body draws objects toward its center. The strength of gravity keeps all of the planets in orbit around the sun. Earth’s gravity keeps you on the ground and makes things fall.

The strength of gravity depends on two things: the mass of the object and the distance between the objects. The more mass an object has, the more gravity it has. The closer you are to an object, the stronger its gravity is.

Earth’s gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That’s what gives you weight. And if you were on a planet with less mass than Earth, you would weigh less than you do here.

The force of gravity also gets weaker with distance. So, if you were twice as far away from an object, the force of gravity would be only one-quarter as strong. And if you were 10 times as far away, the force of gravity would be one-hundredth as strong.

A final thing to know about gravity is that it is the weakest of the four fundamental forces. The other three are a strong force, a weak force, and electromagnetism.

Gravity is not a force:

Now, what if I say that till now whatever you’ve read is false. Gravity is not a force at all.

Now here’s why-

According to “The General Theory of Relativity“, gravity is not a force. That means there is no gravitation force also. But you might be thinking “How is this possible?”

Well, let us assume a simple example. Think of yourself on a rooftop of a large building like Burj Khalifa and you started falling downward {may be freely( i.e. there is no air resistance)}. Now, if you are good at imagining, you can see that whatever is with you is falling with the same acceleration as yours. That means the objects with you remain stationary relative to you. And the most interesting feeling is you can’t feel your weight. Now assume yourself in a spaceship traveling in deep space. Not a single large mass is there. Now, in this image, again you feel as massless whether your spaceship is at rest or coasting along at constant velocity.

These two situations are exactly similar to each other. And you will be the inertial observer. But if you know why I underlined that part only?

Because Albert Einstein said these two situations are equivalent, not just similar.

 According to Einstein, these two scenarios are physically exactly the same thing. That means if you are falling from the rooftop, then it is not for gravity and there is no gravitational field and you will not be accelerating. But you must say that if a man is falling from a rooftop then it must be to gravity and he must be in a gravitational field. And he is accelerating his speed by 9.8 m/s².

These two situations look very different, but Einstein’s equivalence principle tells us one thing, i.e. the experience of the observer. If they feel weightless then they are in an inertial frame of reference. Now think if there is a planet so close to the spaceship and the spaceship is traveling with a constant velocity, and the man is not paying attention to the planet. Now if you are an external observer, you might notice that the path of the spaceship bends over slightly towards the planet. But inside the spaceship, the man would remain oblivious. The man still feels no force, no acceleration, and feels completely weightless. As the ship gets closer to the planet it continuously increases its speed. But inside the spaceship, the man still can’t feel anything. For him, nothing has changed. So, he is an inertial observer until he crashes into the planet.

But still, you can say, “How do you define the curve path of the spaceship without Gravity and gravitational field?”.

Well, I will define the curve path using curved space-time.

Einstein introduced space-time to us in his theory of special relativity.

So, he was moving in a straight line through spacetime. But around massive objects spacetime curves itself. The more the object will be massive, the more it curves the spacetime. So, that’s why his path appeared curved to you(or any distant observer).

Now imagine, you are in your car and you are going from one end to another end of the Earth (or you can imagine any other two places also) in a short path. Now you think that your path is a straight line but since the Earth’s surface is curved, the shorted path also be curved and this is called a geodesic. Astronauts in ISS also feel weightless. That means they too are the inertial observer traveling on a geodesic. But remember earth curves spacetime around it. So, they will travel on a path that looks like a helix. It only looks like a circular orbit if you forget the time dimension.

You can do an experiment in your house, i.e. bend sheet experiment.

I’ve tried to do that experiment here. If you don’t understand from here you can go to youtube and search there. Many videos are there.

As you can see here how the planets and the sun curve space-time.

Now go back to the spaceship and let’s boost the rocket and accelerate it at 9.8 m/s². Now, what will be happened?

Now, he(the spaceman) just feels like we feel on Earth.

So, the moral of the story is- there is no gravitational field we all are just accelerating. But you can say “Then why I’m not going upwards?” Well for what? All the objects around you are also accelerating with the same magnitude. So, suppose you want to see whether this “theory of acceleration” is correct or not. In that case, you can jump from the rooftop of a large building and tell your friend to stand on a particular floor, and then you can see your friend accelerating upward at the value of 9.8 m/s². Now you must be thinking that if we all are accelerating upwards, then why is the Earth not expanding. Well, it is possible for you to be accelerating, even though your spatial coordinates are not changing.