## Why is gravity so different from other forces?

Here’s the relative strengths of the 4 known forces of nature

There are speculative ideas as to why the relative strength of gravity is much weaker compared to the other forces. In my opinion this is not an apples-to-apples comparison. The comparison is not fair in the sense that gravity may not really be a force per se.

The huge gap between the relative strengths of gravity and the other forces is a mystery indeed. This gap is known as the “hierarchy problem.” But, this is a small problem compared to the other mystery: gravity cannot be shielded.

It is interesting that all 3 forces other than gravity are confined to the atom. Outside of the atom there is only a fraction of the electromagnetic force, just enough to facilitate chemistry. Electromagnetic force can be extensive in plasmas as in coronal mass ejections from the Sun but over long distances in space the electromagnetic force is neutral (shielded) because positively charged particles cancel the effects of the negatively charged particles. The strong and the weak nuclear forces are confined to the nucleus of the atom and they are completely shielded. Gravity on the other hand, cannot be concealed or shielded. It operates in the microscopic world. It operates in the macroscopic world. Nothing can stop it and nothing can shield it.

Action-at-a-distance, field, geometry, graviton exchange? Which one is it?

Newton showed us that the gravitational force between two objects is proportional to the masses of those objects and inversely proportional to the square of the distance between them. He could not explain why or how the gravitational force was mediated. Newton declared that the gravitational force is an action-at-a-distance without any further explanation.

Einstein showed us that the gravitational force can be described without invoking the action-at-a-distance. He showed that gravitation is not really a force but an effect caused by the distortion of space-time. He said that a massive object curves/distorts the space-time and the smaller object feels this curvature, the effect manifests as gravitational force. This is the geometrical explanation of gravity. Einstein’s theory describes the gravitational force with an incredible accuracy but it does not explain why the mass distorts space-time.

Another explanatory device in physics is the concept of field. Charged particles like electrons or protons emanate electric fields. Moving charged particles emanate magnetic fields. Other charged particles feel these electric and magnetic fields.

There is yet another explanatory device to describe forces. This is known as interaction-by-particle-exchange. When you throw a ball the catcher feels a force. There is an interaction or force between the thrower and the catcher. The force is transmitted to the receiver by exchanging an object. According to this paradigm all forces work like this. Electromagnetic force is mediated by exchanging photons; the strong nuclear force is mediated by exchanging gluons and and the weak nuclear force is mediated by the exchange of W and Z particles. In this paradigm gravitational force is mediated by the exchange of a particle known as graviton. Why is the gravitational force attractive and not repulsive? There is no explanation.

The graviton-exchange paradigm is problematic. Interaction by particle exchange concept works very well for electromagnetism, and the nuclear forces but it is really problematic in the case of gravitation.  Attempts to unify gravitation with quantum physics have not been successful because these theories use graviton-exchange to explain gravitation.

In my opinion gravitation is special. The biggest evidence for this is the relative weakness of the gravitational force and more importantly the fact that gravity cannot be shielded.

Why is gravity so different from other forces? There are many speculative theories about this but it is fair to say that nobody knows the answer.

If you are interested in learning more I recommend the following:

Theories of the Brane by Lisa Randall

The Hierarchy Problem by Matt Strassler

The Hierarchy Problem by Philip Tanedo

String Theory and the Hierarchy Problem in Physics

Extra Dimensions and the Hierarchy Problem