Introduced by Albert Einstein in the early 1900s, the theory of relativity provides insight into the workings of the universe. This theory is divided into two parts: the special theory and the general theory. Each part offers explanations and comprehension to a certain extent. The special theory focuses on understanding atoms and small objects, while the general theory is applicable to the study of larger objects, like planets. With Einstein’s creation of the theory of relativity, he surpassed Isaac Newton’s centuries-old mechanics, establishing himself as a forward-thinking and adaptable scientist.
According to Einstein, the concept introduced by Relativity is that there is no absolute motion in the universe. Instead of existing in a flat, absolute time, humans exist within a curved space-time. To better understand this idea, let’s consider Earth. With a circumference of around twenty-five thousand miles, it can be traveled in approximately twenty-four hours, indicating that Earth rotates at slightly over one thousand miles per hour. One might assume that there is an object in the solar system which remains still and could serve as a reference point to measure Earth’s velocity. However, it turns out that every chosen reference object is also moving, demonstrating that nothing is fixed and everything continually moves. The Theory of Relativity encompasses mechanics, gravitation, and space-time to illustrate how all things are interconnected and cannot be considered separate entities.
The Theory of Relativity comprises two parts: special relativity and general relativity. Special relativity, published in 1906, applies to microscopic physics, such as atoms and small objects. In contrast, general relativity, published in 1916, is intended for astrophysics and cosmology involving solar systems, planets, and large objects.
Sir Arthur Eddington was a British Astronomer who was among the first to fully comprehend the Theory of Relativity. When asked about how many people understood it, he cleverly responded with “who is the third?”
To assess the accuracy of the Theory of Relativity pertaining to gravity, various phenomena have been examined. These include quasars, the 3 kelvin microwave background radiation, pulsars, and potentially black holes. The exploration of these phenomena has resulted in advancements in space programs as well as improvements in technologies like telescopes and computers. These developments aim to enhance the precision of calculations based on this theory.
The Theory of Relativity consists of two main parts: the special theory and the general theory. The special theory focuses on reference frames, particularly in a laboratory setting where objects move in straight lines with uniform velocities. In this type of frame, if everything within the lab is falling, it would appear as if nothing is moving. However, the movement of the lab is significant to an observer within it. The principle of relativity proposes that experiments conducted within an internal frame are independent of the frame’s uniform velocity. A prime example of this concept is the constant speed of light, which remains the same for all observers within the internal frame, regardless of their own speed. Simultaneous events in one frame may not be simultaneous when observed from a frame moving relative to the first one. The perceived motion varies depending on the observer’s location, velocity, and direction. Interestingly, the mechanical foundations of special relativity were researched in 1908 by Hermann Minkowski, a German mathematician. Minkowski’s work influenced Albert Einstein to theorize the absence of gravity during free fall. According to the EEP (Einstein’s Equivalence Principle), the laws of physics should exhibit special relativistic properties during any free fall scenario.
A consequence of EEP is that space-time must be curved. It is technical to consider two frames falling freely, but on opposite sides of the Earth. According to Minkowski, spare time is valid locally in each frame. However, since the frames are accelerating towards each other, the two Minkowski space-times cannot be extended until they meet. Therefore, with gravity, space-time is not flat locally but curved globally. Any theory of gravity that fulfills EEP is called a metric theory.
The theory has two sides: a special side and a general side. The general side involves the principle of demonstrating how space-time curves in the presence of matter. To determine the curvature, a specific metric theory of gravity, such as general relativity, is necessary. Einstein aimed to find the simplest equations and discovered a set of 10. In order to test the general theory, he conducted three tests: gravitational red shift, light deflection, and perihelion shift of Mercury.
For the light deflection test, Einstein utilized the curve of space-time caused by the sun’s gravity. If the light comes close to grazing the solar surface, it should be deflected by 1.75 seconds of arc. Gravitational lensing works based on the concept that when light passes near a massive body like a star, its path is altered due to deflection. The amount of deflection relies on the mass of that body. This led to an idea that galaxies could serve as rough optical lenses for distant objects’ light.
The identification of the first gravitational lens occurred in 1979.Galaxies could act as crude optical lenses for light from more distant objects.The first gravitational lens was identified in 1979.Another success achieved by general relativity was its ability to explain Mercury’s orbit peculiar shift even after considering other planets’ influence.The shift amounted to 43 seconds of arc per century at Mercury’s perihelion point.In late nineteenth century astronomers were perplexed with this phenomenon.However,this occurrence was later clarified through general relativity which attributed it to natural effect resulting from Mercury’s motion within Sun’s surrounding curved space-timeRecent radar measurements have confirmed this explanation, showing a consensus of about 0.5%.
General relativity satisfies the Equivalence Principle (EEP) for all objects, including the Earth and Moon. If the Nordtvedt effect occurs, there would be slightly different accelerations in their attraction to the Sun. This discrepancy would cause a minor disruption in the orbit of the Moon, which can be measured through lunar laser ranging. Initially applied in cosmology, general relativity predicted the expansion of the universe from a condensed state known as the big bang. The steady state theory initially contested this idea by proposing continuous creation of matter throughout the universe. However, new knowledge strongly supports the big bang theory and is consistent with relativity. Crucial evidence was provided by the discovery of background radiation in 1965. This electromagnetic radiation fills the universe at a temperature approximately 2.7 K above absolute zero and is considered as a remnant trace of an early hot phase after the big bang, as proposed by general relativity theory. Furthermore, it is necessary for helium abundance observed in cosmos (ranging from 20 to 30 percent by weight) to align with conditions predicted by relativity theory for the big bang.
General relativity has introduced various celestial phenomena, including neutron stars, black holes, gravitational lenses, and gravitational waves. Neutron stars are compact stellar bodies of small size. For example, a neutron star with the same mass as the Sun would only have a radius of 10 km (6 mi). These stars form through violent astronomical events like supernovae, in which gravity compresses them to the point where their density becomes comparable to atomic nuclei, primarily composed of neutrons. Since their proposal in the 1930s, numerous celestial objects with similar characteristics have been observed. The discovery of the first pulsar in 1967 revealed rapidly rotating neutron stars emitting regular radiation pulses. The pulse period indicates the rotation period of these stars.
Black holes, which are celestial bodies with an incredibly strong gravitational field, are considered one of the most remarkable predictions of general relativity. The concept of black holes predates the 20th century, and these objects possess such a potent gravitational force that nothing, including particles, radiation, or light, can escape from them. This property is what gives them their name. It is believed that black holes primarily form when highly massive stars collapse inwardly and over time they can grow larger by attracting more material towards themselves. Some scientists have suggested the existence of supermassive black holes within star clusters and galaxies, potentially even in our own galaxy. While there is no definitive proof for their presence, compelling evidence supports their existence in various known locations.
Relativity offers a perspective for understanding the constant motion and inherent uncertainty of velocity in everything. It dispels misunderstandings about planetary movement with established equations. However, this approach falls short when studying tiny elements such as atoms because it neglects the complexity of celestial bodies like stars, planets, and solar systems. Additionally, advancements in technology have allowed us to make accurate measurements that confirm the trustworthiness of relativity since its beginning.
