Why is gravitational force always attractive?

Gravitational force is always attractive because it pulls objects towards each other due to their mass.

In more detail, the gravitational force is a fundamental interaction that governs the motion of celestial bodies and objects on Earth. It is described by Newton's law of universal gravitation, which states that every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. This force is always attractive, meaning it always tries to pull masses together, never to push them apart.

The reason for this attraction lies in the nature of gravity itself. Gravity is a curvature in space-time caused by mass and energy. When an object has mass, it bends the space-time around it, creating a 'well' that other objects with mass fall into, hence they are attracted towards it. This is the concept behind Einstein's theory of general relativity, which provides a more comprehensive description of gravity than Newton's law.

Moreover, there is no known anti-gravity substance or negative mass in the universe that could cause a repulsive gravitational force. If such a thing existed, it would cause a lot of problems with our current understanding of physics. For instance, it could lead to situations where energy is not conserved, which contradicts one of the fundamental principles of physics.

In quantum mechanics, the still hypothetical quantum gravity is also expected to be always attractive. Although the exact nature of quantum gravity is still unknown, most theories suggest that it will follow the trend of classical gravity and be attractive.

In conclusion, the gravitational force is always attractive because of the nature of mass and space-time. It is a fundamental property of the universe that keeps planets in orbit around the sun, causes tides on Earth, and even holds galaxies together.