Buzz
In everyday language, the word theory often refers to a mere "hunch" or "guess." For instance, if you regularly see the same man on the northbound bus each morning, you might theorize that he works in the northern part of the city. Similarly, if you forget to store your bread in the breadbox and notice chunks missing the next morning, you might suspect the presence of mice in your kitchen.
In the realm of science, however, a theory is much more than a simple guess. It usually refers to a well-established explanation of how different facts are related and offers a clear and concise interpretation of observed phenomena. As the American Museum of Natural History puts it: "A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses, and facts."
Take, for instance, Newton's theory of gravity, also known as his law of universal gravitation. This theory asserts that every object in the universe responds to gravity in a uniform way. Observations of the Moon's orbit around Earth, the movement of Jupiter's moons, and the fall of a hammer all support Newton's theory. Thus, Newton's theory succinctly encapsulates our understanding of how objects interact with gravity, regardless of their location in the universe.
A scientific theory "organizes experience," according to James Robert Brown, a philosopher of science at the University of Toronto, speaking to Mytour. "It arranges it into a systematic framework."
A THEORY THAT SUCCEEDS EXPLAINS
For a theory to be accepted, it must be capable of explaining existing facts. Let’s examine Newton's theory of gravity as an illustration.
In the late 1600s, it was understood that the planets followed elliptical orbits around the Sun, but the reason behind the elliptical shape was unclear. Similarly, the motion of falling objects had been understood since Galileo’s work about fifty years earlier. He had formulated a mathematical equation explaining how the speed of a falling object increases over time. Newton’s significant achievement was linking all these observations. As the story goes, his epiphany came when he saw an apple fall in his hometown of Lincolnshire.
According to Newton's theory, each object in the universe is drawn to every other object by a force that is directly proportional to their masses and inversely proportional to the square of the distance between them. This is referred to as the “inverse square” law. For example, if the distance between the Earth and the Sun were to double, the gravitational attraction between them would be reduced to one-quarter of its current strength. Through his theories and some calculus, Newton was able to demonstrate that the gravitational forces between the Sun and the planets caused their orbits to be elliptical as they traveled through space.
Newton's theory is remarkable because it accounts for so much: the falling apple, the Moon's orbit around the Earth, and the orbits of all the planets—and even comets—around the Sun. Everything now had a logical explanation.
A THEORY THAT SUCCEEDS PREDICTS
A theory becomes even more compelling when it predicts new, observable phenomena. The English astronomer Edmond Halley applied Newton's theory of gravity to calculate the orbit of the comet that now bears his name. In 1705, by considering the gravitational influence of the Sun, Jupiter, and Saturn, Halley predicted that the comet, last observed in 1682, would return in 1758. And indeed, it did. Unfortunately, Halley passed away in 1742 and did not live to witness the comet's return. Brown calls the predicted reappearance of Halley’s Comet "a spectacular triumph" of Newton’s theory.
In the early 20th century, Newton's theory of gravity was surpassed—so to speak—by Einstein’s theory of general relativity. (Where Newton had imagined gravity as a force between objects, Einstein described it as the warping or curving of space itself.) General relativity could explain phenomena that Newton's theory could not, such as an anomaly in Mercury’s orbit, which slowly shifts—a phenomenon known as "precession"—so that while Mercury’s orbit remains elliptical, over time it traces out a spiral path, much like the designs made on a Spirograph toy.
Crucially, Einstein’s theory also predicted phenomena that diverged from Newton’s ideas. One such prediction was that gravity could bend light, a theory dramatically proven during a solar eclipse in 1919, catapulting Einstein to fame. Nearly a century later, in 2016, the detection of gravitational waves confirmed another of his predictions. Over the course of the century, at least eight predictions made by Einstein’s theory have been validated.
A THEORY CAN EVOLVE, COMBINE, OR BE REPLACED
Nevertheless, physicists believe that Einstein’s theory will eventually be supplanted by a more comprehensive theory. It already appears to be at odds with quantum mechanics, the theory that best describes the subatomic realm. The two theories present very different views of the universe. General relativity depicts the universe as being made up of particles with specific positions and velocities, which move in response to gravitational fields that fill all of space. On the other hand, quantum mechanics offers only the probability that a particle will be located at a certain position at a particular time.
What might a "unified theory of physics"—one that merges quantum mechanics with Einstein’s theory of gravity—look like? It would likely integrate the strengths of both theories, enabling scientists to understand both the immense and the minuscule aspects of the universe.
A THEORY CAN ALSO BE CONSIDERED A FACT
Let’s shift focus from physics to biology for a moment. It is precisely because of its vast explanatory power that biologists hold Darwin’s theory of evolution in such high regard. This theory helps make sense of data across many scientific fields, including genetics, physiology, biochemistry, paleontology, and biogeography. As biologist Theodosius Dobzhansky famously said in his influential 1973 essay, "Nothing in biology makes sense except in the light of evolution."
Interestingly, the term evolution can refer to both a theory and a fact—something Darwin himself recognized. "When Darwin discussed evolution, he made a distinction between the fact of evolution and the theory of evolution," Brown explains. "The fact of evolution was that species had indeed evolved [i.e. changed over time]—and Darwin had abundant evidence to support this. The theory of evolution, however, is an effort to explain this process." Darwin's eventual explanation was natural selection—the idea that organisms' offspring will vary, and those with more advantageous traits are more likely to survive and pass those traits on to the next generation.
WE PLACE CONFIDENCE IN THEORIES
Many theories are well-established: Scientists have just as much confidence in the theories of relativity, quantum mechanics, evolution, plate tectonics, and thermodynamics as they do in the fact that the Earth orbits the Sun.
Other theories, particularly those at the forefront of current research, remain more speculative, such as string theory (the concept that everything in the universe consists of tiny, vibrating strings or loops of pure energy) or the various multiverse theories (which propose that our universe is just one of many). String theory and multiverse theories are controversial due to the lack of direct experimental evidence, and some critics argue that multiverse theories are not even testable in principle. They contend that no conceivable experiment could be conducted that would confirm the existence of these other universes.
Sometimes, multiple theories are proposed to explain natural phenomena; these theories may "compete" with each other, with scientists evaluating which one offers the best explanation for the observations.
"Ideally, this is how it should unfold," Brown explains. "You present your theory, I present mine; we gather substantial evidence. Over time, one theory may clearly emerge as superior to the other. At that point, the weaker theory gradually fades away. Meanwhile, the stronger theory will likely face challenges in the future."
