Understanding Gravity: From Newton to Einstein

Newton’s Law of Universal Gravitation

Newton’s law of universal gravitation states that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This is often referred to as a force of attraction.

Einstein’s Theory of General Relativity

On the other hand, Einstein’s theory of general relativity, which is our modern understanding of gravity, describes gravity not as a force, but as a consequence of the curvature of spacetime caused by mass and energy. According to Einstein, gravity is a result of the warping of space-time.

The Concept of Force in General Relativity

In Einstein’s theory of general relativity, the concept of “force” as we traditionally understand it doesn’t apply in the same way. Instead of a force “pulling” or “pushing” an object to move, the object moves along the curves in spacetime created by mass and energy.

Cosmic Motion in General Relativity

Cosmic entities move along the curves in spacetime. It’s not that a force is needed for their movement; rather, their movement is a natural result of the geometry of spacetime. This is a fundamental shift in understanding from Newtonian physics, where forces cause objects to move.

The Need for an Initial Force

You’re absolutely right that an initial force is needed to set the ball rolling in the first place. This is true in both Newtonian and Einsteinian physics. In the context of cosmic entities, the initial “push” could come from a variety of sources.

The Initial Force in the Universe

The initial “force” or “push” that set cosmic entities in motion is a topic of ongoing research in cosmology. One theory is the Big Bang, which suggests that the universe began as a hot, dense point nearly 13.8 billion years ago. According to this theory, the initial explosion caused all matter and energy in the universe to begin expanding, and this expansion continues today.

Cosmic Motion and Expansion

As the universe expands, galaxies move away from each other. This motion isn’t caused by a force pushing the galaxies apart, but by the expansion of space itself. In other words, galaxies aren’t moving through space so much as space is expanding between them.

Gravity in an Expanding Universe

Even as the universe expands, gravity continues to shape the large-scale structure of the cosmos. Clusters of galaxies, for example, are bound together by gravity. While the universe on large scales is expanding, gravity can cause matter to clump together on smaller scales.

The Role of Dark Energy

In recent decades, scientists have discovered that the expansion of the universe is accelerating, driven by a mysterious phenomenon known as dark energy. While we don’t yet fully understand dark energy, it appears to be causing galaxies to move apart from each other at an ever-increasing rate.

In conclusion, while an initial force was needed to set cosmic entities in motion after the Big Bang, their continued motion is influenced by the curvature of spacetime (as described by Einstein’s general relativity), the expansion of space, and the effects of dark energy.