How does gravity work, and what is the general theory of relativity?

Gravity is one of the four fundamental forces of nature, and it is the force that we are most familiar with in our daily lives. It keeps us anchored to the ground, causes objects to fall, and maintains the Earth’s orbit around the Sun. The story of our understanding of gravity involves many significant figures in the history of science and is rooted in the development of our understanding of the universe.

Understanding of Gravity Prior to Einstein

Before Albert Einstein presented his General Theory of Relativity, the most accepted theory of gravity was Isaac Newton’s Law of Universal Gravitation. This law, proposed in 1687, posits that every particle of matter in the universe attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. In mathematical form, this can be expressed as:

F = G * (m1*m2) / r^2

Where: F is the force of attraction between the two bodies, G is the gravitational constant, m1 and m2 are the masses of the two bodies, and r is the distance between the centers of the two bodies.

This law served us well for more than two centuries and can accurately predict the motion of planets in the solar system and the trajectory of a ball when you throw it. However, Newton’s theory had a significant conceptual issue: it did not explain how gravity works, i.e., how the force is transmitted across empty space. This is where Einstein’s theory comes into the picture.

Einstein’s General Theory of Relativity

In 1915, Albert Einstein presented the General Theory of Relativity, revolutionizing our understanding of gravity, space, and time. Instead of describing gravity as a force transmitted through space, as Newton did, Einstein described gravity as the curving of space (and time) around massive objects.

According to Einstein, mass and energy distort spacetime, creating a “dimple” or “bending” of space. When a less massive object comes near this distortion, it travels along this curved space, creating the appearance of the object being attracted to the larger body. So, when the Earth orbits the Sun, it is following a curved path in the warped spacetime caused by the Sun’s mass. And when an apple falls from a tree, it’s following a curved path in the warped spacetime caused by Earth’s mass.

In simple terms, Einstein’s theory tells us that gravity is not a ‘force’ transmitted across space but is instead a warping of spacetime by mass and energy. Objects moving in this warped spacetime have their paths curved, and this curvature of paths is what we observe as gravitational attraction.

Testing the General Theory of Relativity

One of the first confirmations of Einstein’s theory came in 1919, during a total solar eclipse. Einstein’s theory predicted that light from distant stars would be bent as it passed the Sun. Sir Arthur Eddington led an expedition to measure the positions of stars near the Sun during the eclipse, and the displacement of star positions confirmed Einstein’s prediction.

Further strong evidence for General Relativity came from the study of the orbit of Mercury. The point of closest approach to the Sun in Mercury’s orbit shifts slightly with each orbit, a phenomenon known as the precession of the perihelion. While Newtonian gravity predicts this precession, it can’t account for all of the observed precession. However, the predictions made by General Relativity match the observations exactly.

More recent tests have come from observations of binary pulsar systems (two neutron stars orbiting each other). These systems are losing energy over time, causing the two neutron stars to spiral in towards each other. The rate of this energy loss matches perfectly with the predictions of energy

carried away by gravitational waves, as predicted by General Relativity.

Concluding Thoughts

Einstein’s theory of General Relativity is currently our best description of gravity. It has passed numerous tests and made predictions that have since been observed. However, like all scientific theories, it’s not the final word. It doesn’t fit neatly with quantum mechanics, the other major pillar of modern physics. Finding a theory that can reconcile these two is one of the biggest unsolved problems in theoretical physics and is the current focus of much research.

So, our understanding of gravity, as with all scientific knowledge, is continually evolving. Who knows what future insights into this fundamental force of nature await discovery as we push the boundaries of our understanding of the universe.