The Force of Gravity
Gravity is a force that we learn to cope with even before birth. It is always present and often ignored until we trip and fall, or an object slips from our hands and hits the floor. Astronauts do not feel the effects of gravity while in orbit, but it is there, too. They are in a state of constant free-fall as they orbit Earth, the same as when a parachutist jumps from an airplane and free-falls before opening their parachute. If you were to construct a platform that reached up to the attitude of the International Space Station (about 250 miles, or 402 km) and had the astronauts step on a scale, you would see that the astronaut’s weight would be about 87% of what it is on the Earth’s surface.
Gravity emanates from matter. If an object has mass, it has a gravitational field. Gravity propagates through the cosmos at the speed of light, and as Einstein showed in his Theory of General Relativity, it can bend light as it distorts the fabric of space. The force of gravity permeates all of space, as every object interacts with every other object by their intrinsic gravitational fields. This can be seen in Newton’s law of gravity:
F = GM1M2/R2
The formula shows that the gravitational force between two masses is equal to the product of the two masses divided by the square of the distance separating them times G—the Gravitational constant (G = 6.67 x 10-11 m3/kg-s2). So, while you sit comfortably at your desk, or on your favorite chair and read this article, you are being pulled and tugged by every object in the Universe…
Think about it. As Jupiter moves about the Sun, it is pulling on you with a force of about 3 x 10-8 N, or about 7 x 10-9 pounds-force (this value is for Jupiter at its closest to Earth). A small force, to be sure, but nevertheless it is there. The force between the Sun and a person with a mass of 80 kg is about 0.5 N (0.1 lbs). If we consider the force between our Sun and the most massive object in our galaxy, the black hole at the galactic core, we find the there is a force of almost 2 x 1016 N between them. This is for a black hole with an estimated mass of 4 million solar masses at a distance of 26,000 light-years! The black hole pulls on our test person with a force of 7 x 10-13 N.
These forces are ever-present, so when you consider the massive black hole at the center of any galaxy constantly tugging on everything in the galaxy, you can see that over time (and we’re talking billions of years here) it influences everything in the galaxy to some extent. The closer the object is to the black hole the stronger its influence. Galaxies pull on other galaxies as their gravitational forces reach across the vast intergalactic space and can form massive galactic clusters like the Abell 1689 cluster, which gravitationally distorts the space around it forming a gravitational lens.