Our scientific understanding of gravity was established by Isaac Newton in 1687. Newton's theory of gravity stood the test of time for two centuries until Albert Einstein proposed his 'General Theory of Relativity,' filling in the gaps left by Newton's theory of gravity.
Despite the many successes of Einstein's theory of gravity, many phenomena, such as gravity inside a black hole and gravitational waves, can't be explained.
Recently, a new study has found direct evidence for a modified theory of gravity at low acceleration. Prof. Kyu-Hyun Chae carried out the study from Sejong University in Seoul, Korea. Chae observed the orbital behaviors of cosmic structures called wide binary stars from data collected by the European Space Agency's Gaia space telescope.
These findings are significant as they point towards a new theory of gravity, different from the Newton-Einstein theory.
Newton's & Einstein's theories of gravity
Newton's theory of gravity was revolutionary at the time. It successfully explained the attraction between bodies on Earth and beyond, granting us a deeper understanding of planetary motion.
However, as the scope of technology expanded, Newton's framework revealed gaps in its ability to account for complex gravitational phenomena. Anomalies in Mercury's orbit were one of them, which puzzled astronomers and showed that the theory falls apart in explaining extreme gravitational conditions.
Then, in 1915, Einstein published his opus magnum, "The General Theory of Relativity." This transformative theory reimagined gravity as the curvature of spacetime itself, unifying mass and energy in a cosmic dance.
Einstein's theory bridged Mercury's orbit and explained the bending of starlight around massive bodies during a solar eclipse. However, even Einstein's visionary insights fell short in the face of the cosmic abyss—black holes, where gravity becomes infinitely intense.
To fill in these gaps, scientists proposed the concept of dark matter. This elusive form of matter is invisible as it doesn't interact with light, but its effects can be seen through its gravitational pull. It was postulated to explain the discrepancies between observed gravitational effects and predictions.
But we don't know what form dark matter takes and if it even exists.
MOND: Modified Newtonian dynamics
Although dark matter could potentially explain discrepancies, many scientists have been skeptical because of the lack of evidence. This has led to alternate theories.
Modified Newtonian dynamics, or MOND, was first proposed by Israeli scientist Mordehai Milgrom in 1983 and could explain these galactic anomalies, including the ones observed by Chae.
The basic premise of MOND is that Newtonian gravity, which works well for most everyday situations, might behave differently at extremely low accelerations.
This deviation from Newtonian physics is suggested to occur when the gravitational fields are weak. MOND suggests that at these low accelerations, the force of gravity no longer follows the familiar inverse square law but exhibits a different functional form.
The idea behind MOND is to modify Newtonian gravity to explain gravitational anomalies, like galaxy orbital velocities, without requiring dark matter.
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