Astro2110 – The Solar System

Ancient Greek philosophers such as Plato and Aristotle deduced that moving heaven bodies makes circular motion because a circle is the perfect heaven path. Later scientists such as Tycho claimed that heavenly bodies are pushed by angels. Nobody previous to Isaac Newton was aware of the concept “Gravity.” Perhaps the most famous legend about gravity is that the idea of gravity came to the mind of Isaac Newton when he was struck by a fallen apple under the apple tree (though it might not be true at all).

Newton’s formula is given below:

Where G is the gravitational constant; r is the distance between two objects; m1 and m2 are mass correspond to two masses. It can be seen from the formula that gravity exists between any two objects. It would be safe to conclude that modern science and humanity’s understanding of the Universe took flight after Newton discovered gravity.

Nonetheless, though Newton’s classical law works perfectly in most cases, it failed to explain some phenomena. For example, Newton’s physics cannot explain why a clock on a high-speed airplane might be slightly slower than a clock on the ground. Here enters Albert Einstein:

Einstein developed a whole new idea of gravity: general relativity and specific relativity. According to Einstein, a 4-dimensional spacetime is a unified entity of space, time, and gravity resulting from the spacetime curvature. Since light travels through spacetime, massive objects will also be bent. Einstein’s theory was substantiated by discovering “Einstein Cross,” which is perfectly predicted by his general relativity theory. There are four images of a distant star because the light emitted was bent a massive object. 

Nevertheless, modern particle physics claimed that gravity resulted from the exchange of elementary particles between two objects. Gravity, along with EM force, the strong-interaction force, and the weak interaction force, are four fundamental forces in the Universe. All objects with mass exchange graviton and thus creating attraction force within. The range of this exchange is infinite.

Human beings have been working on gravity for hundreds of years, yet we still have conflicting (also complementary) theories regarding what causes gravity. Our endless exploration is ahead of us. 

Gravity has been a mystery for most of human history, and it was only recently(about 350 years ago) that the first gravitational theory was published. Gravity was always thought of as a force that brought things to their “natural place.” But when Copernicus came up with his idea for a heliocentric solar system, a theory of gravity was needed to explain things. Galileo hypothesized that gravity depended on mass and not much else. Then in 1687, Issac Newton was hit on the head by a falling apple, and his gravitational theory was born. He saw an apple falling, and thought, how could these objects start falling from rest without a force pulling them? He also realized that the moon wouldn’t stay orbiting earth if there wasn’t some force keeping it in orbit. He found that the force between two objects would be the produced of their masses, and would be inversely proportional to the square of the distance between the two objects. This gravitational theory became the basis for science, physics, math, and astronomy for the next few hundreds of years. For example, Newton’s idea of gravity led to the discovery of Neptune, as scientists found that the orbit of Uranus was influenced slightly by an outside source.

In 1916, Albert Einstein presented another level to Newton’s gravitational theory, The General Theory of Relativity. While Newton believed space and time to be fixed in place, Einstein believed them to be malleable. Einstein hypothesized that gravity is actually the warping of space and time together. The more massive an object, the more space-time would be warped. This theory explained many things that Newton couldn’t, like why light bends around massive objects, and why clocks run at different speeds based on their positions relative to Earth’s surface. His theory also predicted black holes, which are regions of mass that are so dense that even light cannot escape them, hence the name “black holes.”

Einstein’s theory had two main points: The first was that the laws of physics are the same as long as an object is not accelerating. For example, when inside of a moving train, the laws of physics are the same. And second, that the speed of light is always the same speed of 300,000km/sec, no matter how fast the observer is moving. He also described what is now known as “time dilation.” So if you are traveling in a fast rocket, and pass a slower rocket with a clock inside, that other clock would be tickling slower than yours. Likewise, if you brought a clock to the top of a mountain, it would tick slower since the strength of gravity is slightly less. Back to the rocket scenario– the slower rocket would seem shorter in comparison to yours, and the faster you go, you and your rockets mass will continually increase. Essentially, as your speed increases, so does mass, needing you to provide increasing energy. This is why we cannot ever reach the speed of light. To reach the speed of light, you’d need an infinite amount of energy. Einstein also created his famous matter-energy equation, e=mc^2. Because the speed of light is huge, a small amount of mass can be converted into a large amount of energy. This theory has been proven over the years through experiments involving synchronized clocks in planes, and the fact that ripples in the fabric of spacetime(gravitational waves) were found in 2016.

Einstein’s theory has been incredibly essential to our modern world. Some things that would be impossible without the idea that matter can be converted into energy include Nuclear Power Plants and nuclear weapons. Additionally, GPS systems could never work without knowledge of general relativity.