*Introduction: This article is translated from the space.com article "Why is gravity so weak? The answer may lie in the very nature of space-time" by Paul Sutter, published on July 29, 2022.*

Why is gravity so weak compared to the other four fundamental forces?

Even if the strength of gravity were increased by a billion times, it would still be the weakest force, less than a trillionth of the strength of the other forces. The weakness of gravity is truly surprising and begs for an answer.

Interestingly, the reason for the weakness of gravity may not lie in gravity itself, but in the mechanism of the Higgs boson and the nature of space-time.

## The Hierarchy Problem#

Lift a piece of paper. Congratulations, you have successfully countered the combined gravitational force of the entire Earth.

This requires almost no effort because gravity is the weakest of the four fundamental forces in nature (at least as far as we know). In terms of magnitude, gravity is weaker than the strongest force (the strong nuclear force) by a trillion trillion times.

There is another way to perceive the weakness of gravity. There is a minimum limit for the mass of a possible black hole, known as the Planck mass. It is calculated by multiplying the reduced Planck constant by the speed of light and dividing by the gravitational constant (G), and then taking the square root. This mass is approximately 10⁻⁸ kilograms. If gravity were stronger, that is, if G were larger, it would be possible to create smaller and lighter black holes.

In comparison, the W and Z bosons (the carriers of the weak nuclear force) are about a trillion trillion times lighter than the Planck mass. Therefore, the weak nuclear force, which is only slightly stronger than gravity, is actually billions of trillions of times stronger than gravity.

To most physicists, this "hierarchy problem" seems peculiar. Of course, the universe itself may be peculiar and not require an explanation. But that is not a very satisfying explanation. It rather suggests that there is more to discover in the physics of the fundamental forces.

## What is the Higgs Boson?#

Let us temporarily ignore the electromagnetic force and the strong nuclear force and compare gravity to its closest counterpart, the weak nuclear force. Perhaps, if we can understand why the weak nuclear force is so much stronger than gravity, we can grasp the overall picture.

We have no idea why gravity has the magnitude it does. There seems to be no physical theory attempting to explain this. But there is something that seems to explain the nature of the weak nuclear force, namely the Higgs boson.

The Higgs boson is a field that permeates all of space-time and forces many other particles (such as electrons) to interact with it. This interaction process gives particles mass. The more interactions with the Higgs boson, the greater the mass.

The W and Z bosons can also interact with the Higgs boson and acquire mass through this interaction. It is the mass of the W and Z bosons that determines the nature of the weak nuclear force, as these particles are the ones involved in the force.

So what determines the mass of the particles that interact with the Higgs boson? It is none other than the mass of the Higgs boson itself. If its mass were different, the masses of all other particles, including the W and Z bosons, would also change.

*This image shows a test conducted by the Compact Muon Solenoid (CMS) detector at the European Organization for Nuclear Research (CERN) in 2012, where a Higgs boson decayed into a pair of photons (yellow dashed line and green cylinder). (Image source: CERN)*

Now, it is time to point out this: the mass of the Higgs boson is peculiar. It is large - about 250 GeV, similar to the mass of many other particles - but not huge. Its size is also not small. In fact, for the way the Higgs boson works, a naive quantum mechanical understanding predicts that the many interactions it participates in (which are very numerous) would either cancel each other out and make its mass zero, or reinforce each other and make its mass tend to infinity.

What causes the Higgs boson to be precisely in a delicate range - just enough to ensure order in the universe? But the Higgs boson restricts the W and Z bosons to very small values, making the weak nuclear force much stronger than gravity.

In other words, the reason gravity is the weakest force in the universe is not because there is something wrong with gravity, but because the weak nuclear force is "cheating."

## A New Idea about Space-Time#

There is currently no widely accepted answer to the unnatural state of the mass of the Higgs boson, and therefore, we do not have an answer to our hierarchy problem, let alone the strangeness of the weakness of gravity.

But all these discussions assume that we have correctly calculated everything - the mass of the Higgs boson, the Planck mass... Perhaps, there are some fundamental elements of this universe that are missing from our calculations.

In many potential answers, there is an idea that calls into question our understanding of the fundamental structure of space-time. String theory has laid the groundwork for such ideas. To properly describe this theory mathematically, we need new compact dimensions of space.

*Conceptual depiction of string theory. (Image source: Getty Images)*

However, in string theory, these extra dimensions are extremely small, curled up into tiny shapes smaller than the Planck length.

Nevertheless, some of these extra dimensions may be slightly larger. These dimensions are often referred to as "Large extra dimensions," although calling them "large" is not in the everyday sense - they are still on the millimeter scale.

In these theories, the other three forces are confined to our ordinary three-dimensional universe, sometimes referred to as "branes." However, gravity can reach all dimensions and is referred to as the "bulk." In this case, the strength of gravity would be the same as the other forces, or even stronger, but it would be spread out over more dimensions. Therefore, in a three-dimensional experiment, gravity seems much weaker.

We have tested gravity with fairly high precision, but not on such small scales. If our universe has these extra "large" dimensions, we might observe some strange phenomena on scales smaller than a millimeter.

For example, we might observe stronger gravity at small distances because it cannot "leak out" into those extra dimensions. Or we might be able to create miniature black holes in particle colliders because at these tiny scales, creating black holes might be easier than we imagine.

So far, no experimental evidence has been found for extra dimensions, and gravity remains incredibly weak.