If gravity were stronger, the universe would collapse into a black hole

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We discover the laws of nature by comparing phenomena and asking why things happen one way rather than another. One of the longest-standing questions in physics is why gravity is so much weaker than the other fundamental forces. Gravity is the most familiar force in the universe. It keeps our feet on the ground, guides the Moon in its orbit, and shapes the evolution of galaxies. And yet, compared to the other fundamental forces of nature, gravity is astonishingly weak. This suggests that something strange is at play. So why is gravity so feeble?

Fig. 1. A thought experiment of the Moon crashing into the Earth.

The four forces of nature

Physics recognizes four fundamental forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. The last two can be eliminated almost immediately, as they are both short-ranged and play no role in shaping planets, stars, or galaxies. The strong nuclear force is immensely powerful; it binds protons and neutrons together inside atomic nuclei. But step outside the nucleus, and it effectively vanishes. The weak nuclear force, responsible for radioactive decay, is even shorter-ranged. Since both forces fade away over subatomic distances, they aren’t in competition on cosmic scales. That leaves just one force to compare with gravity: electromagnetism.

Gravity versus electromagnetism

Gravity and electromagnetism have important similarities: both are long-range forces that operate over great distances and follow inverse-square laws. In theory, they are directly comparable. In the realm of elementary particles, electromagnetism is approximately 10³⁹ times stronger than gravity. This difference is typically obscured because electric charges exist in positive and negative forms that cancel each other out. As a result, large objects are generally electrically neutral. By contrast, gravity is always attractive and never cancels out. Its weakness is balanced by its universal nature. But what if it weren't weak?

The universe maintains a delicate balance at every scale, from subatomic to macrocosmic. It contains a lot of matter, all of which generates gravity. Since negative gravity doesn't exist, the combined effect is strong enough to hold cosmic structures, yet not too strong to cause gravitational collapse. We can explore a familiar example of the Moon-Earth system to illustrate what would happen if gravity had a strength comparable to electromagnetism.

A thought experiment that explains the universe

The Moon orbits Earth at a comfortable distance, at approximately 3,680 km/h (around 2,237 mph). Earth's gravitational pull is strong enough to keep the Moon bound, yet not so strong that it causes it crash into Earth. If we scale gravity up to the strength of the electromagnetic force, the Moon would need to move at a speed exceeding the speed of light by trillions of times. In such a scenario, no stable orbit could exist because it would require orbital speeds far exceeding the universal limit.

This problem wouldn’t be limited to moons and planets; atoms could not remain immune. Matter would clump violently at every scale; stars and galaxies could never form as stable structures. Instead, gravitational collapse would dominate immediately and everywhere, forming horizons and singularities long before the stars and galaxies could even emerge. In practical terms, the universe would rapidly collapse into what amounts to one enormous black hole.

The Power of Gravity Lies in Its Weakness

Gravity’s strength diminishes with distance, but it never vanishes. Over vast cosmic scales, it is the only force that continues to act coherently across space. That is why gravity, despite its weakness, is the force that shapes the universe, forming stars and galaxies, guiding their motions, and holding vast structures together. Gravity does not win by power, but by persistence, and that is what makes it dominant on cosmic scales.

It is tempting to view gravity’s relative weakness as a flaw or an unresolved problem. However, this disparity in strength is a prerequisite for a universe like ours to exist. If gravity were even slightly stronger, the universe would be short-lived and largely structureless. If it were much stronger, it would never have existed in any meaningful form at all. Gravity’s weakness is not an accident; it is what allows large cosmic structures to be stable and long-lived, making complexity and ultimately life possible.

Fig.2. Animation of the Moon orbiting Earth. Video by ydvisual (YouTube). Source: “Moon Orbit Animation,” ydvisual, YouTube, 2018

Gravity also does not need to be balanced by opposing charges in the way that electromagnetism does; there is no "negative" gravity that cancels it out. Instead, gravity is finely tuned with other fundamental aspects of the universe, most notably the universal speed limit set by the speed of light. These constraints determine how fast matter can collapse, how structures grow, and how the universe avoids the instant catastrophic end. This subtle coordination, rather than sheer power, enabled matter to organize into the complex cosmic structures we observe today.