Gravity

Original post date: August 02, 2018

"You don't understand the gravity of the situation!" But do we understand the concept of gravity at all?

"Newton discovered gravity when an apple fell on his head." The apple didn't fall on his head, and he had to work for twenty years straight to formulate the concept of gravity.

The attractive force by the virtue of mass is called gravitational force. But what is force? F = m * a? A push or pull that brings motion to an object when unopposed? Force is a broad term. In everyday language, "put more force" simply means spending more energy on an object to bring about a physical change. Force can also act without physical contact, like magnetic force or nuclear force. Each definition originates from the same idea but is used in its own unique way. But is gravity really a force?

The first theory of gravity was stated by Aristotle in the 4th century BC. He argued that there can be no effect without a cause, and that bodies fall downward because it is their tendency to move toward the centre of the universe. Earth, of course, was considered that centre. Other astronomers, including Bhaskaracharya, also theorized a geocentric model and defined gravity as an attractive force directed only toward Earth.

In the modern era, scientists like Galileo, Copernicus, Kepler, and Newton made significant contributions to this deeply misunderstood concept. According to Newton's theory, gravity is a force caused by matter due to its mass. It is the weakest force in nature and exists between all bodies in the universe. This concept was so ahead of its time that, with minimal flaws, it was accepted and is still in use. Newtonian gravity holds up well in practical and classical mechanic. However, when it comes to the cosmos, the picture breaks down. The theory does not account for black holes or other complicated cosmic phenomena. But why should mass give rise to a universal attractive force? There must be a reason.

In recent times, gravity is understood through the theory of space-time. Space-time is a four-dimensional concept that treats space and time collectively to explain gravity. According to this framework, matter wraps an invisible fabric around itself. Any matter obeying the space-time continuum creates a bend proportional to its mass. The heavier the matter, the deeper the bend. This curvature influences an attractive force on every other object in the universe. A useful analogy is a trampoline: objects create a depression, and that depression draws other objects toward it. On a cosmic scale, the distances are enormous, so the attractive force is weaker and takes longer to act.

Does this mean heavier bodies inevitably collide with lighter ones? Not necessarily. Gravity is two-directional. The lighter object also exerts a force on the heavier one, causing it to orbit rather than collide. This is why Earth never falls into the Sun. Earth has its own gravitational influence, which keeps it in orbit. This understanding breaks down at extremes, however. Something as dense as a black hole warps space-time so severely that it collapses into a singularity, pulling in everything nearby.

Black holes, then. What makes them absorb everything, including light? No one has directly observed a black hole or entered one. So what happens to matter that falls in remains unknown. Since black holes absorb everything, including radio waves, the only reliable way to confirm their presence is by detecting gravitational waves, which are distortions in the space-time continuum.

How does Einstein's theory differ from Newton's? Both struggle to fully explain black holes. The advantage of the space-time framework is that it incorporates time as a dimension to explain relative motions of cosmic bodies, allowing the laws of physics to be verified from different perspectives. Consider two people moving at different speeds, each projecting a light ray at the other. The calculated speed of light differs between them, which would break the laws of physics. To resolve this, Einstein stitched three-dimensional space together with time, forming space-time. With that, the speed of light became a constant and the laws of physics held. This is a simplified analogy, but it captures the core importance of the continuum.

So, what is gravity? We began with force, moved through centuries of theory, and arrived at the space-time continuum. Every framework we have is true within its own terms. And yet, all of this is barely the surface. The more we discover, the more there is to discover. That is the beauty of science. It never exhausts.

Originally posted on thedopplerdude.blogspot.com.