Gravity. It is relentless. It’s all around you. It impacts everything you do, every moment of every day. Gravity is holding you in the chair you are sitting in right now. It’s holding the chair on the floor. It’s holding the building on the ground. Gravity built the Earth by pulling matter together, it holds the moon in orbit, it keeps us close to the sun. Without gravity, the universe would just be a sea of free-floating atoms. Nothing, as we know it, would exist.
If someone turned gravity off like a light, we would not survive for long. If the force of gravity suddenly relented, the universe would instantly fall out of order. The Earth would spin off into space, far from the Sun. The Sun itself would stop burning as its atomic components dissipate into space. You would fall off the Earth and begin flying through space where the extreme cold and the lack of oxygen would compete to see which could kill you first. The Earth would stay intact for awhile, but it would eventually break apart with nothing to hold it together. All life would die.
But don’t worry, gravity is not going away, ever.
Several years ago, I attempted to re-create an experiment performed by Galileo in the late 16th century in the city of Pisa, Italy. * Galileo stood on top of the Leaning Tower of Pisa and dropped two balls simultaneously (the balls were identical in size/ volume – very different in weight). The point of the experiment was to observe which of the balls hit the ground first - the heavy ball, the light ball, or maybe they would hit at the same time.
I performed the experiment by climbing to the top of the 30-foot-high water tower at my cabin on Manitoulin Island, with two rocks (one much larger than the other) in hand and various children watching from below. As I stood at the top, arms outstretched, the gravity from the rocks pulling at my arms, the greatest challenge I had was keeping the children from standing in the drop zone below. Once I cleared them out with my voice, my next challenge was ensuring I dropped the rocks at exactly the same time. Eventually I let go and watched from above as they hit the ground several feet from the kids.
The result of my experiment matched that of Galileo’s. Both rocks (in his case, both balls) landed at precisely the same time.
In the late 17th century Isaac Newton taught us in his law of universal gravitation that every object in the universe attracts every other object in the universe through the force of gravity. The force of attraction is proportional to the mass of the two objects and inversely proportional to the square of the distance between the two objects. Simply put, the heavier the objects and the closer the objects, the stronger the gravitational pull.
What is the largest object that is very close to us? Our planet, Earth. So, everything around us, including our own bodies are strongly attracted to the Earth. Nearby objects that are larger than Earth, such as the Sun, are far enough away that the size and proximity of the Earth produces a stronger pull. The Sun is trying to pick us up off the Earth and pull us in, but the Earth has a stronger pull, because it is so much closer.
Do two rocks sitting on the ground beside each other have a gravitational attraction to each other? Yes, but the attraction to the Earth is far stronger because the Earth is far heavier.
But wait, if the law of gravitation is accurate – that the force of attraction is proportional to the mass of the objects - in my experiment and Galileo’s experiment, wouldn’t the heavier objects land first? After all, the Earth’s gravity should have pulled harder on the larger rock and heavier ball. Why didn’t it?
Was there another object’s gravitational pull pulling up on the objects, thus slowing the larger one? Was the distance to the ground too short to register a difference? The answer to both questions is no, the two rocks and two balls fell at exactly the same speed. In fact, any two objects would fall at the same speed as long as they were heavy and sleek enough for air resistance to be a non-factor. Even a car and a marble would fall at the same speed. (A feather would not, because of air resistance.)
But if the gravitational pull on the car, or the heavier ball, or the larger rock, is stronger, why doesn’t it fall faster?
The answer to this perplexing question is found in the law of inertia. Inertia (which Galileo knew about and was testing that day) is the tendency of matter to remain in its existing state of rest or motion. An object that is moving will continue moving in the same direction, at the same speed, unless a force is applied to it. An object at rest, wants to remain at rest, and will until a force is applied to it.
Imagine a 10-pound dumbbell and a 25-pound dumbbell sitting on the rack at the gym. Both are at rest. Both require a force to move them. Which one requires a greater force? You guessed it – the heavier one. Your back will agree with me when you lift them. Now imagine them both rolling along the floor at the same speed. The law of inertia wants them to keep rolling forever until some force stops them. They crash into the wall – the wall provides the force. Which one puts a bigger dent in the wall? Correct - It took more force to overcome the inertial resistance of the 25-pound weight.
Back to the rocks and balls on the towers, they were both at rest as Galileo and I held them up waiting to drop them. Both of them, according to the law of inertia, wanted to remain at rest until a force was applied to them, just like the dumbbells on the rack. When Galileo and I let go, the force of gravity was applied to them. (Remember the dumbbells on the rack - weight matters.) The inertial resistance on a heavy object is stronger than the inertial resistance on a lighter object. Therefore, the heavier object required more force to get it up to speed. Fortunately, the gravitational attraction between the earth and the heavier rock and ball was stronger, thereby applying more force. The smaller objects required a smaller force to overcome their inertial resistance, a smaller force was applied (by gravity), and the two rocks fell and hit the ground at the same time, as did the two balls. Inertial resistance and the gravitational force required to overcome it, are equal. So… both rocks and both balls fell at the same speed.
More than 200 years after Newton, Albert Einstein revised our thinking about gravity. Newton’s law still applies mathematically, but Einstein changed it from thinking of gravity as a force to thinking of gravity as a warping of space. This sounds crazy, right? Well, a lot of what Einstein came up with sounded crazy at the time and still might sound crazy today. But experimentation has proven him correct.
How can an object warp space? Even Einstein didn’t attempt to explain the how and why, he just explained what is happening.
Heavier objects warp space to a greater degree than lighter objects. The planet Earth warps space far more than the rock lying on the ground. The Sun warps space far more than the Earth.
Einstein tells us that the Earth, as it orbits the sun, is attempting to travel in a straight line, but space around the sun has been warped to such a degree that the Earth ends up travelling around the sun. Similar to a car driving around a bend in the road if the road is highly tilted. The car, if the driver is sleeping, would attempt to go in a straight line. The road, if tilted enough, would save the drivers life. Please remember roads are NOT tilted enough, so don’t fall asleep while driving.
Let’s explain it a different way. I will use a common object as a metaphor for space. The backyard trampoline for example. Imagine the trampoline’s surface is space. Now imagine a bowling ball is the Sun and it is sitting in the middle of the trampoline. The bowling ball is at rest, not moving. What do we imagine is happening to the trampoline? The trampoline is warped by the weight of the bowling ball. The closer you get to the ball, the greater the warp. Out around the edge of the trampoline there is a slight warp, and in the center of the trampoline there is a very dramatic warp. This is also what is happening with the sun in space.
Now imagine you are standing beside the trampoline with a tennis ball in your hand. The tennis ball is Earth. Roll the tennis ball across the trampoline at a high rate of speed, aiming two feet to the right of the bowling ball. Does the tennis ball travel in a straight line? No, it curves a bit to the left as it makes its way across the trampoline. Now roll it slowly in the same direction. What trajectory does the ball take? If you rolled it slowly enough, it would circle the bowling ball and eventually get pulled in until the tennis ball and the bowling ball meet.
What you are seeing with the tennis ball is very similar to what is happening when the Earth orbits the Sun. There is an equilibrium speed where the tennis ball would circle the bowling ball in perpetuity. This speed is almost impossible to attain because it’s so precise, but the Earth is travelling at this precise speed around the Sun. Gravity, or the warping of space, holds the Earth from spinning off into space. Momentum, combined with the centrifugal force (the force pulling objects in a straight line) keep the Earth from being pulled into the Sun.
So, if gravity is a warping of space, how does it hold you in your chair? Einstein would tell us that like the tennis ball, you are travelling through space in an attempted straight line, but the Earth, like the bowling ball, has warped space and pulled you in. It is the Earth’s gravity that holds you in the chair. If you lean too far to the left, you’ll fall out of the chair and onto the floor. And if the floor isn’t strong enough, you’ll fall through the floor onto the ground. And if the ground weren’t so strong, you would get sucked into the center of the Earth. The Earth’s gravitational pull is always trying to pull you in.
Gravity impacts almost everything in the universe. The only things not impacted by gravity would be abstract objects, such as thoughts, facts and numbers. Anything with zero mass is free of gravitational attraction.
Gravity tries to crash every airplane, it pulls the blood in your body downward away from your head (unless you are hanging upside down), forcing your heart to pump it up. Gravity holds galaxies together, it allows hot air to rise (cold air is heavier and falling, forcing hot air up), it keeps a black hole black – even light can’t escape from a black hole because it is held in by gravity.
Another trick of gravity is the lifting and dropping of the world’s oceans, in other words, the tides. There are numerous forces affecting the rising and falling of the tides, but the two most significant by far are the gravitational pull of the moon and the gravitational pull of the Sun. The moon has an even greater impact than the Sun because of its proximity to Earth.
If you were sitting in Vancouver or Halifax by the ocean and the moon was in the sky above your head, it would mean you could look out at the water and the tides would be high. The gravitational pull of the moon is lifting the ocean. Strange, isn’t it? Why does the moon lift the ocean, but it doesn’t lift the land? Well, actually it does lift the land ever so slightly. The rise of the land is so slight, you don’t notice, but the water, being liquid, is easier to move.
So then, why doesn’t the gravitational pull of the moon lift you up in the air? Or lift the houses around you off the ground? What is the difference between a house and the ocean?
The difference is the ocean is much heavier than a house. Remember, the law of gravity states that the force of attraction is proportional to the mass of the two objects. The weight of BOTH objects matters equally. The moon and the ocean combined have a much greater mass than the moon and a house. However, their distance is the same. The law of gravity also states that the force of attraction is inversely proportional to the square of the distance between the two objects, meaning the farther away an object is, the less the attraction. The house and the ocean are equally far away from the moon, so the difference in weight becomes the only limiting factor.
It is also true that you would experience high tide in Vancouver or Halifax when the moon is above the Indian Ocean – on the opposite side of the Earth. Low tide, comparatively, occurs when the moon is at a 90-degree angle, perhaps on the horizon.
One final trick of gravity occurs in the aforementioned black hole. Quite literally, black holes are only black because no light can escape. The material in a black hole is not actually black – it could be any colour- it appears black due to a lack of light. Black holes are only visible because they block us from seeing what is behind them. **
How does gravity keep light from shining off the material in a black hole? The key is the immense weight of the material in a black hole. Black holes are made from among the densest materials in the universe – the remnants of a collapsed star. The material is so dense and so massive that the weight of the black hole is enough to impact the light trying to escape. The light begins its journey outward (as it always does), gets to a certain point, and, just like a bullet shot straight upward, eventually slows and bends backward returning into the massive black hole. To a person standing far outside the hole looking in, the light would never arrive at their eyes and they would not see anything except darkness. Yes, light is subject to gravitational forces even though it has no mass – it does have energy, and energy and mass have an equivalency, as Einstein tells us (E=MC^2).
Black holes are terrifying places that should be avoided at all costs. They sit there doing nothing except consuming and destroying anything that comes close to them. Fortunately, the closest one is hundreds of light years away at the center of our galaxy.
Gravity has been a constant in the universe since the beginning of time and it will always be here. This is a good thing for us because without it we would never have existed, the Earth would never have formed, the Sun would not be there to warm the solar system which would not exist. The universe would be nothing but a sea of freely floating atoms.
BT
*There is some debate as to whether Galileo did this, or he had someone else do it, or he used the idea as a thought experiment (as geniuses do).
**There is only one black hole that has ever been viewed and it happened only recently. Before the most recent telescopes were launched, we didn’t have the technology to see dark objects so far away. Black holes have been, however, known about for a long time before this because their gravitational impact is so immense that it impacts the movement of nearby stars.
