Cosmological Models
Evolution of the Cosmological Model
Until the Copernican Revolution, it was believed that the universe was geocentric. Prominent figures such as Thales, Eudoxus, Anaximander, and Aristotle generally thought that the Earth was at the center of the universe and that other celestial bodies revolved around it.
Thales of Miletus
Thales, one of the leading scientists and thinkers of Ancient Greece, was the founder of the Ionian School and one of the Seven Sages (Sophos). He was also the first Greek mathematician and introduced geometry to the Greeks. By measuring the shadow of a pyramid at the moment when his own shadow equaled his height, he calculated the height of the pyramid.
Anaximander
An Ionian philosopher from Miletus who lived in the pre-Socratic period. A student of Thales, Anaximander is historically recognized as the first person to have written down his teachings, and his work is considered the first book written in Greek prose.
Geocentric Model
Thales suggested that the Earth originated from water and that it floated like a wooden plank on the surface of water.
Anaximander, however, critically approached this view and rejected it, claiming that the Earth was "suspended in space."
Eudoxus proposed that the universe was composed of nested spheres. He was the first to model the universe geometrically.
Aristotle further developed Eudoxus' model into a mechanical system, asserting that the universe was finite and spherical. According to Aristotle, Earth was at the center of the universe, surrounded by the spheres of the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The outermost sphere contained the fixed stars.
Heliocentric Model
With the publication of Copernicus' On the Revolutions of the Celestial Spheres, the geocentric view of the universe was dramatically challenged, introducing the idea that the Sun was at the center of the universe. However, Copernicus still believed that planetary orbits were circular.
Kepler's Laws of Planetary Motion
Kepler abandoned the idea of perfectly circular orbits and, in 1609, discovered that planetary orbits were elliptical. He formulated the following laws:
- Law of Orbits: All planets move in elliptical orbits with the Sun at one focus.
- Law of Areas: A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
- Law of Periods: The square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit.
Gravitation
Newton observed that a spinning conker, when released, would fly off in a straight line. This led him to theorize that the Moon must be held in orbit by some force, just as an apple falls to the ground due to gravity. He demonstrated that the force keeping the Moon in orbit was the same force that caused objects to fall to Earth—the force of gravity.
Theories of Relativity
Einstein's major breakthrough in physics was influenced by Maxwell’s equations of electromagnetism:
"What would happen if we could move at the speed of light and chase after a beam of light?"
According to Newtonian mechanics, one would expect to catch up with the light beam, making it appear stationary. However, Maxwell’s equations implied that stationary light was impossible, leading to the development of Special Relativity.
Special Relativity
The Special Theory of Relativity states:
- The laws of physics are the same in all inertial frames of reference.
- The speed of light in a vacuum is constant and independent of the motion of the source or observer.
Predictions of Special Relativity
- Time slows down for an object as its speed increases; at the speed of light, time would stop.
- Objects moving at high speeds experience an increase in their mass and cannot reach the speed of light.
- Moving objects contract in length along the direction of motion.
- Nothing can travel faster than the speed of light.
General Relativity
General relativity, recognized as the modern explanation of gravity, generalizes Newton’s law of universal gravitation and Special Relativity by describing gravity as a geometric property of space-time.
Einstein Field Equations
These equations describe how matter and energy influence the curvature of space-time. Any massive object causes a distortion in the space-time fabric.
Gravitational Waves
Any object with mass causes ripples in the fabric of space-time, known as gravitational waves. Einstein first predicted their existence in his General Theory of Relativity.
Detection of Gravitational Waves
In early 2016, the LIGO Observatory directly detected gravitational waves for the first time. These waves were generated by the collision of two black holes 1.3 billion light-years away. Because the speed of light is finite (~300,000 km/s), LIGO detected an event that actually occurred 1.3 billion years ago.
References
- MASON, Stephen F. A History of the Sciences, Macmillan General Reference; Reissue edition, 1962
- D'INVERNO, Ray, Introducing Einstein's Relativity, Clarendon Press Oxford University Press, 1992
- MURRAY, Carl D., DERMOTT, Stanley F., Solar System Dynamics, Cambridge University Press, 2000
- TOPDEMİR, Hüseyin Gazi, Tarih Boyunca Geliştirilmiş Evren Modelleri, Bilim ve Teknik, TÜBİTAK, 2011
- WITZE, Alexandra, CASTELVECCHİ, Davide, Einstein's Gravitational Waves Found at Last, Nature, 2016
- Wikipedia, Theory of Relativity
- ŞENGÖR, A.M Celal, Bilgiyle Sohbet, İş Bankası Kültür Yayınları, 2013
- WALKER, Jearl, HALLIDAY, David, RESNICK, Robert, Fundamentals of Physics, Wiley Global Education US, 10th Edition, 2013