Is gravity a cosmic fluid?

Part 1: The weight of history
By Marcelo Fontinele

"What goes up, must come down."

Since we were children, we have heard this phrase that simply expresses one of nature's most fundamental phenomena: gravity. However, behind this apparent obviousness lies one of the deepest mysteries of the universe. Throughout the history of science, we have tried to decipher its nature: Is it a force? A curvature? Or perhaps something even more fundamental, like a fluid that permeates the cosmos?

In this first part, we revisit the visions of two geniuses: Isaac Newton and Albert Einstein. Understanding them is essential to taking the next step toward a new interpretation.

Newton: Gravity as a Universal Force of Attraction

Isaac Newton, in his work Philosophiæ Naturalis Principia Mathematica (1687), postulated the Law of Universal Gravitation, formulated as:

$$F = G \cdot \frac{m_1 m_2}{r^2}$$

Where:

• F is the gravitational force of attraction between two bodies;
• G \(\approx 6.674 \times 10^{-11} \text{ Nm}^2/\text{kg}^2\) is the universal gravitational constant;
• \(m_1, m_2\) are the masses of the bodies;
• r is the distance between their centers of mass.

Newton also formulated the Laws of Motion, especially the Second Law:

$$\vec{F} = m \cdot \vec{a}$$

By uniting both, he described the motion of celestial and terrestrial bodies with admirable precision. His greatest triumph was applying the same laws that govern a falling apple on Earth to explain planetary orbits.

However, Newton never explained why gravity exists. Its force acted "instantaneously at a distance"—an idea he himself found unsettling.

"That gravity should be innate, inherent and essential to matter... is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it." — Newton

Einstein: Gravity as the Geometry of Spacetime

Two hundred years later, Albert Einstein revolutionized the understanding of gravity with his General Theory of Relativity (1915). He proposed that gravity is not a force, but a consequence of the curvature of spacetime caused by mass and energy.

The key idea is expressed by Einstein's Field Equations:

$$R_{\mu\nu} - \frac{1}{2}g_{\mu\nu}R + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}$$

Where:

• \(R_{\mu\nu}\) is the Ricci tensor (local curvature);
• R is the scalar curvature;
• \(g_{\mu\nu}\) is the metric tensor of spacetime;
• \(\Lambda\) is the cosmological constant;
• \(T_{\mu\nu}\) is the energy-momentum tensor (mass/energy content);
• c is the speed of light.

Einstein showed that: "Matter tells spacetime how to curve; spacetime tells matter how to move."

The Connection: Einstein's Newtonian Limit

Einstein did not deny Newton—he generalized him. When gravitational fields are weak and velocities are small compared to light, General Relativity recovers Newton's Law.

Taking the weak-field and static-time limit, the gravitational potential \(\Phi\) is:

$$\Phi = -\frac{GM}{r}$$

The Schwarzschild metric approximates to:

$$ds^2 \approx \left(1 + \frac{2\Phi}{c^2}\right)c^2dt^2 - dx^2 - dy^2 - dz^2$$

Limitations and Provocations

Despite all this, we still do not understand the fundamental origin of gravity. Bold ideas are beginning to emerge: perhaps gravity is not fundamental, but emergent. A collective effect, just as temperature emerges from the movement of molecules.

What if we are swimming in an invisible fluid, and what we call "gravity" is just the behavior of this cosmic medium?

In Part 2, we will explore this hypothesis: gravity as an emergent effect of a "spacetime fluid." From gravitational whirlpools to density bubbles, get ready to enter a new paradigm.

To be continued...