Why Must Gravity Exist?

Why must gravity exist?

To begin with, we accept (gratefully) that the laws of physics are the same everywhere in the universe. This didn't have to be the case, but it appears that it is the case. A person throws a ball into the air and it falls back to the Earth. An observer here in Austin would describe the motion of the ball in the same way, using the same equations, as an observer in Boston, Berlin or Bangkok. I push on a rock, and it pushes back on me (with an equal but oppositely directed force), both on Earth and on the Moon. Even on Pluto. A proton is attracted to an electron with a force determined according to a certain equation, valid here in the Milky Way galaxy, but also in other galaxies. An experiment repeated in many different locations may yield different answers, but each time the answer will be calculated by using the same equation. Gravity is weaker at the top of Mount Everest, far from the Earth's center, but a ball dropped from that height falls according to the same equation of motion as a ball dropped from my apartment balcony.

Taking things a step further, the laws of physics are the same whether you are standing still or moving at a constant velocity. (Is standing still even possible? I may be at rest on the Earth, but the Earth is hurtling around the sun, so I'm moving relative to the sun. If anything is moving, everything is moving. It's all relative!) Flying in an airplane, if you were to toss a coin up and forward, it would trace out a parabola on its way to the ground, just as it would do if you were standing firmly on the Earth when you tossed it. If the aisles were wide enough and you didn't fear arrest, you could play catch with another passenger, tossing a baseball back and forth. It would be no different than if you were doing so in Central Park, New York. The ball wouldn't behave differently. Unless the plane hit an air pocket, that is. Or slowed down suddenly. Or turned. Then the ball would appear to move in some arbitrary way. But we're assuming a constant velocity. Moving at a constant speed in a straight line. If the ride was especially smooth, and the all of the windows of the airplane were closed, you wouldn't even be able to tell if you were moving. Just like we can't tell that we (and the Earth) are moving through space right now, orbiting the sun. If everything around you is moving at the same speed you are, then as far as you're concerned, nothing is moving. So in summary, regardless of how fast I'm moving, as long as I'm moving at a constant velocity, I can describe some phenomena using the same laws of physics as someone else that is moving at a different constant velocity. I, standing in the middle of a basketball court, will describe the path of a falling ball using the same equation as a boy sailing across the floor on his skateboard, traveling a constant 5 mph. The universe was just built that way.

Okay, so we're getting closer to answering the original question: why does gravity exist? Hey, if it only took a few sentences to explain it, you'd probably already know it!

Our goal: a world in which the laws of physics are the same to all observers. Are we there, yet? No, not quite. Now we come to acceleration. Say gravity didn't exist. There would be nothing pressing you against your seat. You might float right off the chair as you pressed on your keyboard and it pushed back on you. But what if a rocket was affixed to the bottom of the chair? Upon ignition, it would propel your chair, and your chair would propel you, up towards the ceiling. If you closed your eyes and plugged your ears, all you'd notice was the sensation of the chair pressing against your butt. It would feel no different than if gravity was pulling you into the seat. Einstein hit upon this idea. Gravity and acceleration are the same thing! As far as scientific experiments designed to test this idea are concerned, if you were in a steadily accelerating windowless elevator, accelerating at the same rate forever, you wouldn't be able to tell if you were accelerating or standing still on the surface of a planet (with gravity pulling you to the ground). Were it not for gravity, you'd be able to tell the difference. Without gravity, if you were standing, with feet pressed against a surface, you would know that the surface was accelerating upwards against your feet. Acceleration would be absolute, not relative. (But we want it to be relative. It shouldn't matter whether or not you are accelerating when you choose a set of equations to model some phenomenon. Just like it shouldn't matter where you are or what your relative velocity is compared to someone or something else.) Without gravity, if you held a ball in front of you and released it, you would witness one of two outcomes. The ball might hover there in front of you or it might "fall" towards the ground. Since there's no gravity, the only way to explain its "fall" would be to assert that the ground (and you) were accelerating upwards while the ball was at rest. Once it came in contact with the ground, it would start accelerating upwards with you, resting there at your feet. You see, the ball would behave differently based on whether or not you were accelerating. A person accelerating would describe the motion of the ball using different equations than would a person not accelerating. The laws of physics would not be the same for the two observers. To ensure that the laws of physics are indeed the same for all observers, gravity must exist! With gravity, we can use the same set of equations to describe that ball's path, regardless of whether we are standing on the surface of a planet or steadily accelerating through space in a windowless elevator.

So that's why gravity must exist in a universe in which the laws of physics are the same to all observers, no matter where those observers are, whether they are standing still, moving at a steady velocity, or accelerating.