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Let's Teach Gravity: It's About (Space)Time

Over a hundred years ago, Einstein revolutionized the way the human species views gravity, space and time. And yet, with a typical 21st century high school education, you will only have a hint that these revolutions ever took place.

Astonishingly, we now have visual evidence of black holes, and we recently discovered the gravitational wave background of the universe (both predictions of Einstein’s General Theory of Relativity). Our GPS equipment uses relativity theory in order to operate. Today Einstein’s Theory of Relativity stands atop a large mountain, looking down upon all of the evidence supporting it. It’s about time we started teaching this in our schools.

If asked about gravity, a high school graduate will explain how the force of gravity acts between any two massive objects, the closer masses are to each other the larger the force. The more massive they are, the larger the force. This is Newton’s Law of Gravity, published over 330 years ago in 1687.

Newton, Law of Gravity, Moon Orbit, Force
Newton's Force of Gravity

Given, Newton was a genius of scientific thought; even Einstein agreed. During Einstein’s time, there was a lot of evidence that showed Newton’s Law of Gravity to be correct.

Even so, Einstein’s General Theory of Relativity replaced Newton's Law of Gravity over 100 years ago, in 1915. Einstein’s General Theory of Relativity treats gravity not as a force, but as a result of the geometry of the universe: curved spacetime. Einstein’s new theory had a large hill to climb in order to replace Newton’s Law. He began the process in 1915 with a correct prediction of Mercury’s orbit, and a few years later in 1919, showing how our Sun bends light from passing stars. As the decades rolled by, prediction after prediction of Einstein’s Theory proved to be valid. Newton’s Law of Gravity had been useful, but failed these same tests. In fact, even Newton

Spacetime Curvature, General Relativity, Mass effect on spacetime
Einstein's Curved Spacetime

himself admitted to not knowing how gravity would operate across vast distances of space, “You sometimes speak of gravity as essential and inherent to matter. Pray do not ascribe that notion to me, for the cause of gravity is what I do not pretend to know, and therefore would take more time to consider of it.” He never did find a mechanism to explain it.

Now ask a high school graduate about time. A typical (or even high level) response is: Time is a measurement of events. Clocks used to be based on celestial events like the Moon’s cycles, or the Sun’s cycles, but now we set our clocks to the decay of radioactive isotopes, and can synchronize our clocks down to the billionth of a second. Seconds are seconds and do not change based on where you are in the universe, so we should be able to set our clocks here on Earth and go to the Moon and back and they will still be in synch.

Time dilation, time is relative

The first part of the answer you will notice is correct. Unless you are a student of relativity theory, you would not know that the second part is wrong. It is exactly how Newton would have answered the question. But Einstein said seconds on Earth are not the same as seconds measured on the Moon. Einstein’s General Theory of Relativity dictates that clocks slow down in curved spacetime. The larger mass (Earth) curves spacetime more and therefore slows down time more, so a second on Earth lasts longer than a Moon second. The clock that remained on the Earth would show less elapsed time than the one that travels to the Moon and back.

Ask a high school student what the fastest thing in the universe is and they will probably tell you about light. Some even know that the speed of light is close to 3.0 x 10^8 m/s, and that it is constant. Unless they took an advanced physics class, they would not be able to answer the next question: How is the speed of light related to space and time?

Special Theory of Relativity, Einstein
Einstein's Special Relativity

Newton also believed the speed of light was fast, but did not think this changed space or time. It took Einstein’s particular genius to understand how the speed of light affected space and time. In his Special Theory of Relativity he showed how objects which travel close to the speed of light see the world differently. Time is dilated and lengths contract in the direction of motion. Put another way, time itself passes more slowly on fast-moving objects. For fast-moving objects, if there was a way for an observer to measure their length, the observer would see them as shorter than their length at rest. When we observe really fast-moving objects, we see evidence of time and space distortion, just as Einstein predicted.

Gravity. Space. Time. These are fundamental concepts in science. Students have a right to learn the modern view of each of these topics, not just what we thought hundreds of years ago. Science teachers owe it to their students to do this. Go ahead and teach Newton's Law of Gravity, but do it as a history lesson. Then, move on to what we currently understand about the curvature of spacetime. Let's teach gravity; it's about (space)time!


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