It was the year of 1687 when Isaac Newton published “The Principia“, which revealed the first mathematical description of gravity. Newton’s laws of motion along with his description of gravity laid before the world a revolutionary concept that could be used to describe everything from the motions of heavenly bodies to a falling apple. Newton would remain the unequivocal king of gravity for the next several hundred years. But that would all change at the dawn of the 20th century when a young man working at a Swiss patent office began to ask some profound questions. Einstein had come to the conclusion that Newtonian physics was not adequate to describe the findings of the emerging electromagnetic field theories. In 1905, he published a paper entitled “On the Electrodynamics of Moving Bodies” which corrects Newton’s laws so they work when describing the motions of objects near the speed of light. This new description became known as Special Relativity.
It was ‘Special’ because it didn’t deal with gravity or acceleration. It would take Einstein another 10 years to work these two concepts into his relativity theory. He called it General Relativity – an understanding of which is necessary to fully grasp the significance of gravitational waves.
Just What Is Gravity Anyway?
Newton’s laws describes the motions of the planets perfectly, but he never was able to say exactly what gravity was, where it came from, or what it was made of. Einstein posed to himself a simple question – what would happen to the motion of the planets if the sun were to suddenly disappear? Newtonian mechanics say the planets would start moving in straight lines the instant the Sun’s gravity was removed. But this would violate special relativity’s cosmic speed limit. For instance, the Earth would not know the Sun was gone for a full eight minutes after the fact. What could possibly keep the planets in their respective orbits after the Sun was gone?
The answer to this question would reveal the nature of gravity, and what it actually is. And it can be understood quite easily by looking very closely at the similarities between a gravitational field and the force produced by acceleration.
Tale of the Two Elevators
Einstein realized that all physical laws were true despite their frame of reference. The laws that hold true while you are at rest on the surface of the Earth also hold true for the person flying at 30,000 feet at a constant 500 miles per hour. The outcome of experiments done on the plane and on the Earth will be the same. He wondered if the same could be said for accelerating frames and those within a gravitational field.
Let us consider two elevators. One elevator is at rest on the Earth. The other is in deep space accelerating in an upward direction at 9.8m/s2. Is it possible for the occupants to tell the difference between the two frames of reference? If the person in the elevator on the Earth drops an apple, gravity will pull it to the floor with an acceleration of 9.8m/s2. If the person on the accelerating elevator in space does the same, the apple will move to the floor at the rate the elevator is accelerating – 9.8m/s2. Note that the rate the apple will fall is independent of its mass.
Let us now consider the same two elevators in different situations. One of them is in deep space and motionless. The other is in free fall heading toward the Earth. We now ask the same question – is it possible for the occupants to tell which elevator they’re in? When either lets the apple go, it will appear to have no force acting upon it. It will just float there. Einstein realized that the two frames of reference were equivalent. He realized the gravity that we feel on Earth was the same as the pseudo-force we would feel in the accelerating elevator. And that a lack of gravity is no different than free fall. Gravity does not exist as force the way other physical forces do. But if it’s not a force, then just what is it?
To answer this question, let’s take another look at our pair of elevators. Let us imagine that each is equipped with a laser on one side and a detector on the opposite side. They are at identical heights from the floor. Our experiment will consist of releasing a photon from the laser and seeing where it hits on the detector. Photons have no mass, so it should not be affected by gravity. Thus carrying out the experiment in the elevator sitting on the Earth should reveal the photon striking the detector at the same height as its source. But we get a different result when carrying out the experiment on the accelerating elevator in space. The elevator is accelerating upward when the photon is released, so it must hit the detector at a lower point!
If we recognize that the two elevator environments are equivalent, we are forced to conclude that the photon in the elevator on Earth must follow a curved path. Einstein would go on to show that the presence of the Earth’s gravitational field would curve the path of the photon. He would show that gravity is not a force, but simply the geometry of space and time. The “force” that attracts massive bodies together is caused by their curvature of space and time.
Now we know what would happen if the Sun were to suddenly vanish. The curvature of space-time caused by the Sun would spring back and ripple outward. It would send a gravitational wave moving at the speed of light barreling towards the planets. Each planet would keep moving along steadily in their orbits following the still present curvature from the Sun, blissfully ignorant of the incoming doom. Once the wave hits, the curvature is gone. Without the curved space-time to follow, the planets would follow Newtonian mechanics and head out into deep space.
This is all hypothetical of course, and so was the idea of a gravitational wave. Einstein had predicted them, but thought they would be too small to measure here on Earth. This all changed just a few days ago, when two facilities in the US managed to record the merging of two black holes that happened a billion years ago. The merge set off a cascade of gravitational waves, and we were listening.
Now that we know this phenomenon exists and is measurable, more resources will be dedicated to exploring this new field in astrophysics.
Thanks to [PyroChiliarch] for the tip!