Newton’s laws of motion
Scientists come up with theories to explain known phenomena of nature and predict future outcomes. A significant aspect of study of natural phenomena is the idea of force. Scientists study reaction of bodies under the influence of force. This study got a major boost in 1687 when Newton published his book “Philosophiae Naturalis Principia Mathematica” which contained the three laws of motion. The first and the second law of motion define the concept of force qualitatively and quantitatively respectively. The three laws of motion defined Mechanics which itself is the first chapter of Physics.
Law of Universal Gravitation
Scientists like to describe nature with as few laws as possible, in particular they want to reduce the number of types of forces. Newton proposed the Universal law of Gravitation which was the first significant unification of force laws as he reasoned that the same Gravitational force that governs movement of planets also governs motion of bodies on the earth. As of now there are only two more fundamental forces i.e. electro weak and nuclear strong. All other forces are manifestations of these. Scientists apply mathematical models and techniques to analyse natural phenomena. Newton contributed to the development of Calculus, the branch of Mathematics most widely used in Physics. All this cemented Newton’s position as father of Classical Physics. His laws explained almost all of the known observations, were simple to understand and brought science close to our common sense view.
Einstein’s 4 papers
In 1905 Einstein published 4 papers which challenged the basic assumptions of Physics itself. He estimated the size of molecule based on the observation of a simple phenomenon, Brownian motion just as Newton started working on Law of Gravitation after seeing a simple event i.e. fall of an apple. He provided a correct explanation to photo electric effect based on particle model of light which in turn contributed to the idea that the nature is more easily explained if we apply different mathematical models in different circumstances rather using one model to explain all observations. This was also his biggest contribution to Quantum Mechanics
Special Theory of Relativity
However, it was his Special Theory of Relativity and Mass Energy equivalence equation that rewrote the laws of Physics. Special Theory of Relativity is based on an unusual assumption that the speed of light is same regardless of movement of the source or the observer, an assumption that breaks the idea of relative motion of Classical Physics.
It predicts that mass, length and time are not constant but a function of velocity of the body. In Classical Physics, they are absolute quantities. Since at least one of these quantities appear in all formulae of Physics either explicitly or implicitly it impacts almost all the branches of Physics. It overrides Newton 2nd law of motion, which is the basic equation of Mechanics. One of most widely used equation of Physics, F = ma is not valid as per the Special Theory of Relativity. It is not a theory of Mechanics and was proposed to explain discrepancies in laws of Electromagnetism.
Its predictions defy commonsense. It says that nothing can move faster than the speed of light in free space. It suggests that clocks slow down in a moving reference frame (time dilation), masses of bodies increase and lengths contract. Yet another prediction of the Special Theory of Relativity is about simultaneity i.e. two events separated by space happening at the same time in one reference frame appear to happen at different times in a frame moving with respect to the first frame.
The assumption that speed of light is constant has been used to define unit of length, i.e. meter. It is now defined as a distance travelled by light in certain amount of time. Earlier it was defined by markings on a bar which was almost impossible to maintain at constant length. No experiment has been performed so far that contradicts Special Theory of Relativity but other outcomes like Twin’s Paradox or Pole in the Barn Paradox etc. are still thought experiments i.e. very difficult to verify physically. The fact that it impacts almost all branches of Physics making it virtually impossible to say that the theory is valid is all circumstances. Partly due to this and partly due to historical reasons, it is referred to as theory though it is more accurate than Newton’s laws of motion which are referred to as laws.
Mass Energy Equivalence
One of the papers that Einstein published in 1905 refers to the formula of equivalence of mass and energy i.e. E = mc2, sometimes referred to as the most famous equation of Physics. By writing this formula, Einstein carried the unification process further as it reduced the two different concepts of mass and energy to one. Though the formula was known in a different format earlier, Einstein derived it based on the Special Theory of Relativity. If the theory is not valid, then this formula is also not valid. The formula governs all energy formation of Universe. Energy of Earth comes from Sun, from its interior and from radio activity. All the 3 mentioned sources derive energy from the destruction of mass.
General Theory of Relativity
In 1916 Einstein published the General Theory of Relativity and assumes that gravitational and inertial masses are same. The former refers to the mass that appears in Newton’s law of Gravitation and creates force while the latter appears in Newton’s second law of motion and is an attribute of a body due to which it experiences force. The theory removes the restriction of inertial reference frames that is used in Special Theory of Relativity, besides incorporates the effect of Gravitation, hence is called General theory.
Though it predicts movement of particles under the influence of Gravitation, it is not written in the form of a force law where the word force appears on the Left Hand Side of an equation. In fact, rather than force, the theory is written in terms of of distortion of the space time curvature near masses. It is a complex mathematical theory based on field equations that relate the space time curvature to energy (and hence masses). It takes a completely different approach to Gravitation than from Newton’s law. Though it does not directly override it, it predicts things e.g. time dilation, bending of light due to gravitational fields etc. that Newton’s law does not do. Some of these e.g. gravitational red shifts or lensing etc. are used to detect various objects in Universe by detecting changes in frequency or paths of light as it passes through intense gravitational fields. It predicts formation of Black holes, the stars with intense gravitational force that pull everything into them including light,
Like Special Theory of Relativity, General Theory of Relativity was ahead of times. One of its predictions of Gravitational waves which are travelling distortions in space time. They were detected for the first time by LIGO experimentin Dec 2015 and announced in Jun 2016. So far the lab has detected 4Gravitational waves, the last one in September 2017. Infact, US scientists Rainer Weiss, Barry Barish and Kip Thorne won the 2017 Nobel Prize for Physics for their pioneering role in the detection of gravitational waves. Though considered to be a fundamental theory of Astrophysics, its predictions are slowly getting verified. Partly due to this, historical connotations and its mathematical complexity, it is referred to as theory though it is more accurate than Newton’s law of Gravitation which is referred to as law.
Modern Physics
Year 2005 was declared as Year of Physics to acknowledge the immense contribution that Einstein’s four papers published in 1905 made to Physics. Relativity and Quantum Mechanics are the two pillars of Modern Physics. The former explains things more accurately when bodies are moving at speed comparable to the speed of light and later when their sizes are at atomic scale. It was Einstein’s far sightedness that his Relativity theories are still getting verified and referred to as theories though they explain observations better than Newton’s laws. Often scientists discover something at variance with these theories, but only end up retracting later. One recent example being that in 2011, scientists at CERN discovered neutrinos moving faster than the speed of light. However further analysis negated the results. Einstein also made contribution to Quantum Mechanics including explanation of Photoelectric effect, Bose Einstein statistics etc.
Tomorrow, the Relativity theories may become laws or theories that explain phenomena even more accurately may emerge. However, Einstein’s contribution to Relativity and Quantum Mechanics has cemented his position as father of Modern Physics.