Introduction
Gravity is one of the four fundamental forces in the universe, and its exploration has been a focal point of both scientific and philosophical inquiry for centuries. For hundreds of years, Isaac Newton's laws of motion and gravitation were held as gospel, offering a coherent and simple explanation of how celestial bodies interact with each other. This was the only established theory solving gravitation until more complex theories such as General and Special Relativity emerged in the 20th century superseding Newton's law by a mile. These Relativity Theories were able to elegantly explain peculiar phenomena like the precession of Mercury’s orbit and gravity lensing. But, even with the rise of these theories, the Modified Newtonian Dynamics (MOND) continues to gain traction as an alternative theory of gravity. Invented in the 1980s by physicist Mordehai Milgrom, MOND proposes that Newton's laws of motion and gravitation are only visible on large scales and for small accelerations, while in the regime of low accelerations, there are deviations that diverge noticeably from the predictions of these laws. This article aims to explore the Modified Newtonian Dynamic (MOND), its theoretical development and its consequences.
Background
For the past few decades, cosmologists have posited the existence of 'dark matter,' which refers to the hypothetical form of matter that doesn't emit light or other forms of electromagnetic radiation but is believed to be present in our universe. Scientists posit that dark matter accounts for the observed gravitational perturbations among celestial bodies and accounts for 95% of the universe's total mass. The missing mass problem is what led to the initial development of dark matter theories. Until the 1960s, the beauty and elegance of Newton's laws of motion and gravitation had effectively solved all gravitational problems posed. But with increased technological gains, more sophisticated scientific instruments began to observe properties that could only be explained by the existence of more mass in the universe than what we could see with our conventional means. According to gravitational theorists, when two celestial bodies orbit each other, they move under the influence of gravity. Hence, it is established that the visible matter of any celestial body, labeled baryonic matter, exerts sufficient gravitational force to hold itself and the entire system in place. However, these observed gravitational perturbations can not be explained through conventional theories focused on baryonic matter; therefore, many researchers presume that there are other components or forces that scientists haven't detected yet.
The MOND Hypothesis
The MOND theory suggests that dark matter does not exist but rather that the baryonic matter doesn't obey the laws of gravitation as we know them. In essence, MOND proposes that our perception of gravity may be an artifact of our perception of the universe's universe based on accelerating objects within our immediate environment. Milgrom's idea as explained in his 1983 paper posits that the motions of stars in galaxies are different from one another "in a way that cannot be explained by the laws of gravitation of Newton and Einstein" with this effect kicking in once the acceleration becomes too weak (below c.a 10^-10 m/s^2). Thus, Milgrom proposed that in this regime, the gravitational force must increase at a slower rate than what we observe for Newton's laws to still hold. This phenomenon is termed "modified dynamics" or MOND. It explains the dynamics of galaxies, which are stable as they rotate around their centers without separating, as their velocity cannot be explained by the generally accepted theory of gravity. Milgrom proposed that MOND could replace dark matter theories.
MOND and Its Development
The initial skepticism with Milgrom’s proposed theory was derived from the fact that it contains the ‘magical formula’ a0, that appears arbitrary and without basis. a0 represents the relevant acceleration scale within which the perturbations of gravity begin to be observed. In Milgrom’s words “a0 seems to be the transition scale between two regimes of gravitation, one being Newtonian, the other governed by some emergent and special law of nature". Critics have also argued that MILGROM's empirical model may be valid for rotation curves, but yet to be tested in other areas of astronomy. However, over the years, as more observation technologies emerged, evidence began to emerge that lent credence to the MOND theory. Whereas expected results based on a dark matter paradigm have become increasingly complex with detailed simulations accounting for multiple forms of static and dynamic interactions needed to simulate galactic motions, MOND provides reliable fitting at the observed scales. In the early 21st century, technology saw another breakthrough with the development of the Hubble Space Telescope, which enabled a better observation of astronomical phenomena, particularly those which could have been obscured in the former days by our atmosphere. The ability of HST to observe the entire galaxy and not just its center proves to be useful in MOND's predictions. It brings in the previously observed "Tully-Fisher relation" (TFR), which relates the brightness of a galaxy and the speed at which the stars are moving in that galaxy’s disk. MOND predicts this relationship without requiring massive amounts of dark matter, even in the cases where models based on baryonic matter and Newtonian dynamics fall short.
Additionally, with the advent of more high-energy physics discoveries, particularly those from the Large Hadron Collider, scientists began to make headway on coalescing and understanding dark matter, dynamical dark energy and gravity better. While particle physicists continue to search for particles that can account for dark matter beyond the WIMP, other scientists are exploring the MOND theory further.
One of the most significant developments in recent times is the work of Jacob Bekenstein and his modified entropy theory. Bekenstein brings the MOND theory into cold waters by exploring one of the universes’ fundamental laws. The entropy of a black hole is the bar setting constant for the whole universe; as no other object in the universe can have a value exceeding that of entropy in a black hole. Bekenstein worked on modifying the black hole's entropy law to account for DM mass missing, which MOND theory takes on by modification of the law of gravity. Bekenstein's work on the entropy modification seems to show that what we know today to be MOND theory may be a necessary correction to an incomplete general relativity law. In essence, by tweaking classical physics, Bekenstein showed that MOND could just be the tip of the iceberg and that even more information on gravity could still surface.
Conclusion
The Modified Newtonian Dynamic (MOND) theory probes the fundamental question of what lies beyond our current understanding of gravity. While the classical Newtonian laws of motion and gravitation have been effective in explaining celestial objects’ movements, there are discrepancies that MOND captures. While it is yet to be subjected to the rigors of scientific testing, MOND continues to drive scientific inquiry into the complexities of gravity, revealing the possibility that our existing theories may not be sufficient to describe the behavior of the universe effectively. The study and understanding of MOND is important as it can help to bridge the gap between classical Newtonian law and more comprehensive theories like Einstein's general relativity while potentially advancing our understanding of current open questions in particle physics. An open scientific mind is needed to move beyond our one-century-old understanding of Gravity to answer current mysteries of the universe, and MOND provides a perfect avenue for embarking on that quest.
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