Let's finally attempt to put it all together, Space, Time, and Gravity
In part 2 of this series, we saw that gravity is the effect of the curvature of Spacetime around massive objects, so in that sense, we understand what is gravity. In this part, let's attempt to dig even deeper and answer the ultimate question of why is there gravity and what is its relation to time.
Your time versus my time
Let us begin with a simple observation: Although initially synchronized, a precise clock on the surface of the Earth does not match that on the International Space Station. This is a fact and there is no debate on it. But the International Space Station is moving at a very high speed and we know that motion in space affects the passage of time. (Kinematic Time Dilation)
So let's try and remove motion. Experiments have proved that very precise and synchronized stationary clocks at different elevations do not agree with each other. It turns out that the clock near the surface of the earth ticks slower than the clock at an elevation. To stretch this even further, it means that your head is older than your feet! So gravity is affecting Time. The closer we are to a massive object, the slower the time flows. This is called Gravitational Time Dilation.
The closer we are to a massive object, the slower the time flows.
As illustrated by the image below, time flows slower at the base of this tall building than on the top floor ( The effect is highly exaggerated in the illustration below but it is indeed true as proved experimentally ) The image shows the same time arrives a bit later at the base of the building when compared to the top. Time is therefore relative. Your time and my time are not the same if you are on the top floor and I am on the ground floor.
As shown in the illustration, the degree to which time is slower at the bottom compared to the top traces a curved path. And yes, this is the curvature of the Spacetime fabric. Therefore just like space, time is also bent or curved.
But still, the question remains, why does this happen? Why does gravity slow down time?
To answer this deep question, we must ask ourselves an even deeper question. What is time?
What is Time?
We measure time with a digital or a mechanical clock. Something in the clock ticks at a regular interval or frequency. This is also the same concept in much more precise atomic clocks where we define a tick as the resonant frequency of atoms. In its deepest sense, therefore, time is a period of interaction of atomic or subatomic particles.
So something must be happening differently to these particles on the surface of Earth compared to a distance from Earth. The frequency of vibration of these particles is slower on the Earth's surface compared to further away. Further, for massive objects with much higher spacetime curvature, this effect must be much more. Imagine the center of a black hole with an extremely dense mass equivalent to many suns compacted to a point, the curvature of Spacetime is also massive, so much so that the vibration of these particles must come to a halt i.e time stops at the center of a black hole.
The best way we can imagine this and get a sense of intuition in my view, is that of a river. Imagine a flowing river. What happens to the flow of water near the shore versus in the middle? Indeed the water flows slower near the shore compared to the middle.
This is because the particles of water near the shore experience more resistance due to interacting with other mediums like rocks and sand. The layer of water closest to the shore experiences the most resistance. This layer of slowing water also interacts with the next layer of water and slows that down as well but a bit less. This layer slows down the next layer as well but even less, till we reach the middle of the river where the effect of slowdown is minimal or zero. This is similar to the influence of Gravity. At a sufficient distance, an object can go out of the influence of gravity of the Earth but the closer it gets to Earth the stronger the influence of its interaction with the layer above Earth.
Here is the final analogy: As the water particles flow they have a forward motion through "space" but their interaction periodicity at a molecular level (which can be thought of as "time" ) also is impacted as they interact with different media near the shore. Hence it is all about the interaction of matter with the medium and their gradual slowdown as we get closer to the surface of massive objects ("Shore") that traces curvatures resulting in what we call and experience Gravity.
Image Source: https://youtu.be/wrwgIjBUYVc?si=ocoygNV61Fzhm-q1
So if we define time as the interaction frequency of particles as they interact with a medium, then this is indeed what results in the effect we call Gravity. In other words, Time causes Gravity.
This brings us to the end of this series. I have attempted to unify and make sense of this complex dance of Space, Time, and Gravity and in that endeavor have proposed an intuitive way to visualize the true root cause of Gravity. I urge you to continue your own thought experiments and keep asking one simple question - Why?
If you want more of Gravity and love fiction, then you may enjoy my graphic novel from the series "Ventures in Space and Time" : Kohiti's Anomaly
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