How does the force of gravity slow down time?
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In special relativity, Einstein showed that objects in motion change shape. But that pace was balanced. If an object is moving at the same speed, then the laws of special relativity apply. Moreover, these formulas are developed in the absence of any fundamental force.
But what happens if the speed of the object is accelerated rather than balanced, i.e. it increases moment by moment?
Then the kinetic force will act on that object. This momentum ball is again born from the belly of the gravitational ball. That is, how do the laws of relativity behave in an accelerated context or in the presence of gravity? What mass will increase, how will it increase? Will the length of the object decrease, or will time slow down? These questions were not answered in special relativity. So Einstein thought of extending relativity.
But the task was not easy.
Einstein took his time, doing several thought experiments. Ten years later, in 2015, he published the General Theory of Relativity.
The definition of gravitational force is rewritten in this theory.
The doors of the unknown mysteries of the universe are suddenly opened. Where our Milky Way was once thought to be the only galaxy in the universe, within a century it has grown to two hundred billion. Even cosmic objects like black holes, neutron stars, quasars, pulsars, white dwarfs, red giants have been discovered. They would not have been discovered with the help of advanced telescopes, if they had not been predicted from various solutions of the General Theory of Relativity.
A theory of such great impact will naturally take time to build – 10 years is not a long time.
The General Theory of Relativity was developed three hundred years ago from the principles of the Italian scientist Galileo. Galileo is said to have seen a piece of stone and a feather thrown together from the Leaning Tower of Pisa, both falling to the ground simultaneously. There is no way to confirm the truth of this story, but Newton tested Galileo’s principle by dropping a guinea and a feather into an airtight jar. Galileo’s principle was correct, mass is no barrier to falling objects, heavy, medium heavy, light—all objects fall to the ground with the same acceleration.
Here too Einstein did a thought experiment – there was a man inside an elevator. Suddenly the rope of the elevator breaks, the elevator starts falling freely. Einstein could see in his mind that the man had a hammer in his hand and a pen in his pocket. The man dropped the hammer from his hand, dropped the pen from his pocket. The hammer and pen did not fall on the floor of the elevator. As if floating in the air. The man’s chest floated down to the ground with the broken rope elevator.
Why is this the case? Einstein thought. His predecessor Galileo-Newton thought both. Newton showed that two types of matter have equal mass. One is its inertial mass, which it experiences when it hits an object, or the mass an object experiences when an object accelerates. The other is gravitational mass, the mass that objects feel due to gravitational pull. Neither Galileo nor Newton explained why this would be the case. They couldn’t figure out the obvious reason.
Einstein, however, took a different path. Just as in special relativity the speed of light was constant, i.e. axiom, in general relativity the same was assumed. That is, he called the fact that inertial mass and gravitational mass are equal as a natural principle, which cannot be changed. This principle of nature is called ‘Principle of Equivalence’.
He also stopped thinking of time and space as separate. He said, this is our universe in space and time. Time without space and space without time has no value.
As much as there is space in what we call the universe, i.e. where the events are happening, all the events there cannot be explained only in terms of space or only time. Both must be thought of together. He shows that what we experience as gravity in spacetime is just another form of acceleration.
Einstein showed that gravity is not a ball of attractive character as we think of it. Rather, when a heavy object is at a point in spacetime, it bends the spacetime around it. So it creates a hole in the space around the object. Now if a relatively small object passes around this space-time hole, then the object will tilt towards the hole. Because the trajectory of the object is on space-time. If that path bends, then the object will no longer be able to move in a straight path. The smaller object will lean towards the larger object. That is, the acceleration of the smaller object towards the larger object. Then it will seem that the big object is attracting the small object towards itself. If the larger object is the Earth and the smaller object is the apple, then the acceleration of the apple will be entirely towards the center of the Earth. Then the apple will fall towards the center of the earth. We would think that the earth is pulling the apple towards its center.
As mentioned earlier, the mass of matter bends spacetime. As a result, any other object coming into this curved space-time will bend its trajectory. Let’s say, an alien spaceship from a distant planet is coming towards Earth. Coming straight. But since space-time is bent due to the Earth, the closer the spaceship gets to the Earth, the more its path will feel bent. It will gradually tilt towards the Earth due to the gravitational curvature of the Earth.
Let’s say that the speed of the spaceship is greater than the velocity of the earth (the velocity of the earth is 11.2 km/h). Then it wants to be able to escape the gravitational curvature of the earth and go to a distant space. But if its velocity is less than the Earth’s escape velocity, it cannot move past the gravitational curvature. Then it will be forced to descend towards the earth.
Let’s say that the speed of the spaceship is much faster than the free velocity of the earth. So if it wants it can go without ignoring the gravitational curvature of the earth. But that does not mean that the spaceship will be able to pass the gravitational curve without any problems. Since the path is curved, it is a long distance to cross this area.
Let’s say a motorbike is moving in a straight line. The driver will pass a crop field. It has crossed this field before. Let’s say the length of the field is 200 meters. Let’s say the speed of motorbike is 72 km per hour. That is 20 meters per second. That is, the bike will take 10 seconds to cross the crop field.
The driver knew that. But this time he came and saw that the field is not as flat as before. The entire field has been dug up and the soil removed. The field has turned into a rough ditch. There is no other way a biker would use it. He has to cross the field like that ditch. That’s what the driver did. After crossing the field, he saw that this time he took more time. Because this time his path was no longer smooth. It took more than 5 seconds to cross the field full of small winding ditches.
Einstein showed that matter bends space-time, preventing light from traveling around heavy objects, i.e. at the very core of the gravitational field. The path of light is bent. So light takes longer to cross the gravitational field. From the outside, it would appear that the gravitational field of heavy objects is slowing down time.
This was one of the main points of Einstein’s General Theory of Relativity. That is the same thing here. Time is not absolute. Observer and positioning time is slower for some and faster for others.
Source: New Scientist