Newton's law of universal gravitation and Einstein's theory of general relativity are two fundamental ideas about gravity in physics. But in a battle between classical mechanics and modern physics, which one wins out? Read more to find out!
Newtonian Gravity
Legend has it that Isaac Newton thought of the concept of gravity when an apple fell on his head while he was sitting under an apple tree.
While this isn’t the exact story (sorry to burst your bubble!), the falling apple helped Newton realize that a force must exist on falling objects. Otherwise, they would not move from rest. He would also use this concept to celestial bodies, like planets, to explain why they orbit around each other. This discovery was crucial to science at the time, as it was unknown as to why objects in space moved in conic section pathways (think hyperbolas, parabolas, and ellipses).
Newton called this force “gravity” and determined that it exists between any two given objects in the universe. His universal law of gravitation states that this force only depends on the distance between the objects and the masses involved. Gravity will always be attractive.
Newton’s equations are simple and remain handy, even to this day. Despite being formulated over 300 years ago, it is still used today to precisely calculate gravity on Earth and in the universe.
However, Newton couldn’t explain how gravity worked. All he knew was that it was a constant, instantaneous force, and left it up to other scientists to figure it out.
General Relativity
Enter Albert Einstein.
Einstein and his theory of general relativity covered ideas about gravity not covered by Newton. By Einstein’s theory, gravity was a result of a mass simply existing and warping spacetime. Gravitational force does not depend on the distances or masses involved, as Newton predicted, but rather on how strong the spacetime around an object is curved. Gravity does not even need a mass at all! Based on Einstein’s theory of special relativity, mass and energy are interchangeable, meaning that even massless particles, such as photons (light), can generate gravity.
One way to think about general relativity is using a trampoline. If you were to stand in the middle of a trampoline, your weight would pull the trampoline fabric downward. This indent would be stronger by your feet, but weaker near the edges. If you were to place a small ball into the trampoline, it would roll towards you in the middle. By Newton’s law of universal gravitation, it would be said that the ball was attracted to you, but by general relativity, the ball would be following the curve of spacetime.
General relativity would triumph over Newtonian gravity when it solved the mysterious case of Mercury’s orbit. Before Einstein, Mercury’s orbit could not be properly described by Newtonian gravity; its orbit shifted slightly and it could not be fully explained with gravitational effects from other objects in space. When Einstein was able to use his theory of general relativity to precisely explain the orbit’s discrepancy, it was key to proving his theory correct. General relativity would continue to thrive when it predicted the bending of light from a distant star more accurately than Newton’s gravity during a solar eclipse.
Then why do we still use Newton’s law of universal gravitation?
While general relativity has been more precise, Newtonian gravity is still incredibly accurate and can be used for simpler situations. General relativity uses complex calculus that makes it difficult to compute. But Einstein has opened up the world to more complex physics, like black holes and gravitational waves that continue to develop our understanding of the universe.
Thanks for reading!
Written by Kaylee Barrera
Sources
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“Gravitational Theory: Newton, Einstein & The Next Wave | AMNH.” American Museum of Natural History, www.amnh.org/explore/videos/space/gravity-making-waves/newton-einstein-gravity. Accessed 12 May 2021.
Mattson, Barb. “100 Years of General Relativity.” NASA Blueshift, 25 Nov. 2015, asd.gsfc.nasa.gov/blueshift/index.php/2015/11/25/100-years-of-general-relativity.
Siegfried, Tom. “Einstein’s Genius Changed Science’s Perception of Gravity.” Science News, 13 Mar. 2020, www.sciencenews.org/article/einsteins-genius-changed-sciences-perception-gravity.
Thompson, Hobie, and Sarah Havern. “Gravity.” The History of Gravity, web.stanford.edu/%7Ebuzzt/gravity.html. Accessed 12 May 2021.