Introduction to Gravitation
Gravitation is another type of force acting on two bodies which might have the least impact on the bodies with smaller mass and larger separation between them. However, it turns out to be the strongest one when the matter is about celestial bodies. The discipline of the universe is the action of gravity which holds everything at its point. Although being a universal force, physicists are still unable to solve many paradoxes of gravity.
General theory of relativity is a branch of physics that deals with the cause and effect of gravity. The range of gravity goes to infinity, but to see its effect, there are certain criterias. The earth is attracted to the sun, but because of our negligible mass compared to the earth, we may not feel its effect. However nothing remains free from gravity, inside or outside the space.
Newton’s Law of Universal Gravitation
Nicholas Copernicus initially developed the concept of gravity by giving a heliocentric model of planets. Before Ptolemy had proposed the geocentric model, which failed to explain the motion of planets. Copernicus correctly predicted the motion and position of planets but many queries like the seasonal changes, time factors etc. remained unanswered. Later, Kepler solved all the confusions in Copernicus theory and gave the law of planetary motion. Combining both theories, Newton formulated a universal law of gravitation in the 17th century, which was accurate for every corner of the universe.
Newton’s Law of Universal Gravitation states that every single particle is attracted to every other particle with a certain amount of force. This force is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. It can be expressed as;
F = G Mm/R2 [Equation 1]
Where:
- F is the gravitational force,
- G is known as the universal gravitational constant whose value is 6.67 โ 10-11 Nm2/Kg2.
- M and m are the masses of the two objects,
- R is the distance between their centers.
Whole classical mechanics is based on Newton’s law of Gravitation. The law has made it easier to calculate the gravitational force and compare the forces at different regions.
Gravitational Fields and Potential
Gravitational field is the area surrounding a body, up to which it has its effect of gravity on another object. However, one should understand that the force of gravity remains negligible when the separation of two objects is larger. Mathematically defining, the strength of a field at a point becomes a force per unit mass, which is experienced by a small test mass placed at some point. The calculation of gravitational field strength helps to visualize the effect of gravity within its range.
On the other hand, gravitational potential is the amount of work to be done to remove that unit test mass from the influence of gravity of that body. It is the potential energy per unit mass at a place in a gravitational field. If V is the potential required, at a distance r, against a body of mass M, then the potential can be calculated as,
V = -G M/r [Equation 2]
Here, as the work has to be done against gravity, it is indicated by negative signs to show the opposing nature of work.
Einstein’s Theory of General Relativity
The general theory of relativity is a revolutionary theory which revamped the concept of gravity. It was proposed by Einstein in the early 20th century which disregarded the concept of Newton on gravitational force. Einstein regarded gravity as the consequence of the bending of space-time rather than a force which occurred due to the mass, energy and momentum of celestial objects present in the universe. He confirms that gravity is an indication of how objects move in geodesic curved space-time.
Theory of relativity describes that heavy objects cause spacetime to curve, and other moving objects follow those curved trajectories, which we experience as gravity. It opposed that gravitational force acts at a distance as suggested by Newtonian mechanics and stated gravity itself as a distortion of spacetime. A piece of the evidence of gravity might be the case of solar or lunar eclipse, where the starlight is bent by the sun or the moon.
Einstein gave an equation to denote this curvature according to the energy and momentum of a matter which is expressed as;
Gฮผฮฝ + ฮgฮผฮฝ = 8ฯG/c4 Tฮผฮฝ [Equation 3]
Here, Gฮผฮฝ represents the curvature of space-time, Tฮผฮฝ represents the energy and momentum of a matter and ฮ is the cosmological constant.
Gravitational Waves: Detection and Significance
The field equation stated that the gravity of super-massive objects bends space-time. These gravitational waves propagate as the solution of his field equation. When the galaxies started forming just after the big bang, two distant galaxies merged due to the gravitational pull. The black holes lying at the center of these galaxies also combined and formed ripples in the fabric of space-time. These ripples created a moving distortion in the curvature of space-time and flowed violently throughout the space. They travelled at the speed of light and finally arrived on earth. The waves continuously compress and expand the space-time whose first detection was made on September 14, 2015.
The Advanced LIGO Observatory located in Louisiana, Livingston was able to make the first direct detection of gravitational waves. Another detector is the VIRGO interferometer (a Michelson interferometer) located in Pisa, Italy. It is kept isolated from external interference to increase the detection sensitivity of gravitational wave signals.
Only waves emitted by compact bodies may travel at the speed of light and reach the planet. Thus gravitational waves are the best choices to study about those compact bodies and their effect of gravity. Also, the gravitational waves evolving since the big bang are still roaming in space. Therefore, they can provide important insights on the dynamics of the very young universe
Gravitation in Astrophysics: Black Holes and Orbits
When a massive star collapses, it remains as a dot in space due to its own gravity. However, this dot is not something negligible but a very mysterious one with such a strong gravity, not even letting light pass. On approaching closer to the singularity, time is said to stay steady. Thus, gravity and time are the opponents. Sad to say, no one has the idea about what actually lies inside a black hole due to its compact nature.
In astrophysics, the big bang is regarded as the reason for the current universe. Although being purely hypothetical, it has a strong theory and the majority of the masses believe in it. This theory states that gravity took birth after the big bang. Till then gravitation is governing the space where the motion of planets, satellites, moon etc. all rely on gravity. Keplerโs law of planetary motion plays a major role in describing the nature of the orbits. They follow an elliptical path due to the inverse-square nature of gravitational force, as suggested by Kepler. It has been later derived in Newtonian mechanics. Gravity is also a main cause for phenomena like tidal forces and binary and trinary star systems. Hence, we can say nothing is untouchable by gravity.
The Role of Gravity in the Universe’s Expansion
Since the Big Bang, the cosmos has been expanding. The “Lambda-CDM model,โ is a cosmological model, which clearly explains the existence and structure of the universe as well as the cosmic microwave background. The continuous spreading of the universe is also studied under the light from distant galaxies and supernovae. Dark matter, which does not emit, absorb, or reflect electromagnetic radiation, is thought to be present in large quantities in galaxies. Thus, gravity not only governs the attraction between masses, but it also opposes the cosmological redshift.ย
The cosmos has neither a limit nor a core at the broadest scale, where galaxies are spread equitably and evenly in all directions. These several galaxies are also gravitationally bonded together, which are frequently observed as clusters or groups. These clusters can hold hundreds or thousands of galaxies due to the presence of strong gravity.
Densely populated regions in the cosmos draw matter to them and become denser. On the other hand, under-dense places are rarefied as matter travels away from them. This phenomenon is known as gravitational instability. Structures can develop as a result of gravitational instability. Knowing the formation of these structures and their composition is very important in order to get details about their evolution.
Gravitational Lensing: Observing Distant Galaxies
Gravitational lensing shows the image of outer space. It shows the composition of galaxies and other massive objects by creating their brighter and fainter image. It occurs when a massive structure, like a galaxy cluster or a super cluster, bends light coming from distant sources as it moves towards the observer. Gravitational lensing has been well-described by Einstein’s general theory of relativity. Newtonian physics also predicts the bending of light, if it is considered as a particle. The gravity of the surrounding object curves the light coming from the point of origin and the image of the object can be observed which periodically gets magnified and distorted. This phenomenon of gravitational lens is widely applied in astrophysics to extract signals from distant galaxies and dark matter, to study the structure of compact bodies, quasars and other multiple purposes.
There are strong, weak, and microlensing categories. Strong lensing produces several images, curves, or arcs of the background object. Weak lensing causes small deviations in the shapes of background galaxies. It shows the distribution of dark matter. Microlensing is a type of lensing occurring when a single star or planet quickly magnifies a background star.
Microgravity and Its Effects on Human Spaceflight
Microgravity is a condition of weightlessness, experienced during freefall. In space, microgravity can be considered as the measurement of the degree of acceleration an object can experience. While orbiting around earth, astronauts go through microgravity. Although being under earth’s gravity, they are in a state of constant freefall. When the object keeps falling faster and faster, the gravitational force will become significantly smaller, about 10^-6 times the gravity of earth. This phenomenon makes them appear like floating around space.
While looking at the effects of microgravity on the human body, there are lots of complications seen on an astronaut. A short exposure to microgravity can change a human’s motor control system while prolonged exposure to freefall or weightlessness can cause bone loss. It may cause the muscles to weaken. It also has an effect on the neurons which balance and direct the body. Fluid distribution gets disturbed which may cause variations in blood cholesterol and blood pressure. Also, fluctuations are seen in muscle fiber composition, cardiovascular function, and immune system function which are some of the other negative effects of microgravity on the human body.
These effects must be highly considered before launching an astronaut for a long-term mission in space. The best research on this topic can help to plan a daily life on the most favorable regions like mars and moon. Thus medical studies and biology can also reach to a next extent on studying microgravity.
Conclusion
Gravity is a fundamental aspect of nature whose existence is universal. It leads to the development of two major branches of physics viz Classical or Newtonian mechanics and Cosmology. These two branches define gravity in totally different ways. Two compact binary bodies also come closer due to their gravity and combine, creating ripples of their interactions whose impacts are never ending. Gravity is also responsible for cosmological redshift. The further study of gravity and microgravity can make it possible to reside in space outer from earth. To sum up we can say all the mysteries of the universe are lying within the topic โGravitationโ. Its study can deepen our exploration on how everything exists and where it begins and ends.
References
Thorne, K. S., Misner, C. W., & Wheeler, J. A. (2000).ย Gravitation. San Francisco: Freeman.
Ehlers, J., Bรถrner, G., Cohen, I. B., Weyl, H., DeWitt, B. S., Freedman, D. Z., … & Silk, J. (1987). Gravitation.
Ohanian, H. C., & Ruffini, R. (2013).ย Gravitation and spacetime. Cambridge University Press.
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