Gravity is truly one of the most enigmatic forces in the universe. It is weak enough that its effect is negligible to a human body at 10,000 km above the earth's surface and yet a black hole, an entire galaxy away, still has some influence on each of us.2 We are familiar with the concept of "weightlessness" inside the International Space Station because of videos of Chris Hadfield and others floating through its various chambers. However, the earth's gravitational influence still has an effect inside the International Space Station (330 km altitude). The apparent weightlessness is due to the fact that, technically, the station is in a free-fall relative to the earth. Centrifugal force from its rotation around the earth nicely balances out the gravitation of the earth giving a relative micro-gravity environment.
On the practical side, gravity is what holds us onto this planet as Earth spins and travels through our portion of the Milky Way Galaxy. It is what gets us into trouble spots resulting in skinned knees and broken bones when we take a big risk on a bike or skateboard. Every object that has mass attracts all other objects that have mass. So, if I stand on a ladder to paint a room and lean so far that I lose my footing, my body, which has a small mass, pulls on the enormous mass of the earth and the mass of the earth pulls on my body. The two masses try to pull each other toward themselves; but, because my mass is small relative to the earth, my body quickly moves toward the large mass of the earth until there is a collision between the two. The collision will most likely be painful for my soft biology but will very likely not harm the floor.
Gravity can also contribute to the "thrill" in our stomach when we ride a roller-coaster at high speed or a car on a hilly road. It both helps and hinders us when we are one year old and learning to walk: it holds us on the floor so that we don't drift into the walls or ceiling; but it also sets us down on our butts when our feet are not in the right place relative to our centre of gravity. It is also what we feel pulling on us as we inch that bicycle to the edge of a ramp and sends us shooting down a ski-hill as we glide off of the chair-lift.
Gravity is something which even the best physicists have trouble conceptualizing, explaining, and fully understanding. For all of its effects that can be measured, experienced, and predicted, we still know very little about what it is and how it has its effect. Some physicists propose gravitons as the mediating particle while others postulate gravitational waves; yet neither has ever been detected. Dr. Pamela Gay, an astronomer and writer has said,
“Having gone from basically philosophical understandings of why things fall to mathematical descriptions of how things accelerate down inclines from Galileo, to Kepler’s equations describing planetary motion to Newton’s formulation of the Laws of Physics, to Einstein’s formulations of relativity, we’ve been building and building a more comprehensive view of gravity. But we’re still not complete, . . . . We know that there still needs to be some way to unite quantum mechanics and gravity and actually be able to write down equations that describe the centers of black holes and the earliest moments of the Universe. But we’re not there yet.”3We are often unaware of gravity in our day-to-day lives; but watch for it, and you will notice its effects all around you. You too may find yourself pondering the mystery of gravity.
1 Some of you are wondering if there even is such a thing as a "Nerd Quotient Scale." I assure you there is. Google it.