Space and Time

Most widely understood ideas about motion of bodies come from Newton and Galileo Aristotle believed that natural state of a body was to stay at rest and it will move only when force is applied. Galileo conducted experiments (which manifested themselves in a fictitious but interesting story of dropping weights from the tower of Pisa), were used by Newton to formulate famous 3 newton’s laws of motion. Newton completely contradicted Aristotle’s idea of natural state of the body. According to Newtonian law, everybody  will continue to be in state of rest or state of motion unless it is acted upon by an unbalanced force. This idea was published in 1687 in newton’s Principia Mathematica.

Though Newton and Aristotle disagreed on the concept of Absolute space (famous ping pong example, where ping pong ball bouncing  at the same location on a table on a moving train would appear to be hit two different positions for an observer who is stationary, hence there is no absolute space), they do believed in Absolute time. So Newton introduced the concept of relativity of a position of the object in space but it still believes in absolute time meaning that time between two events would be measured same for all observers. e.g though place at which ping pong ball hits differs for two different observers, the time it takes the bounce the ball would be the same.

Apparently Newton’s law of physics fail to explain the motions of objects that travel at or near the speed of light. In 1676, Danish astronomer,Ole Christensen Roemerr, discovered the fact that light travels at finite but high speed while observing eclipses for Jupiter moons.

In 1865, James Clerk Maxwell, a British physicist, proposed a theory which unified partial theories used to explain electricity and magnetism . Interestingly since newton’s law had eliminated the idea of absolute rest, to explain the constant speed for the light (electromagnetic waves) that he invented to a matter called ether so that he can say that speed of light is constant relative to ether that fills up all the empty space.

In   1887, Albert Michelson (Later to become First American to win Nobel prize in sciences) and Edward Morley conducted experiment where they tried to measure speed of light in the direction of earth’s motion as well as right angle to it. If earth was orbiting around sun in ether, speed measure while in the direction of earth’s motion would be higher than the one measured while at right angle. BUT, experiments found the speed to  be exactly same!

Between 1887-1905 there were several attempts to explain this observation. In 1905, Albert Einstein, pointed out in his famous paper that idea of ether was unnecessary if we abolish the idea of absolute time. Einstein chose to disagree with fundamental beliefs of Aristotelian/Newtonian beliefs of absolute time. Similar point was made by leading french mathametician Henri Poincare.

With famous equation, E=Mc^2, there is direct relation between energy and mass of the body. So as energy of the object increases, it would also increase the mass of the energy since c=speed of light is constant. So as objects starts accelerating, it would add to its mass and in turn it will get difficult to accelerate.

Now here is the part I did not get from the book, Hawking mentions that at 10% of the speed of light there would be 0.5% increase in mass and at around 90%, mass would be more than double and as it approaches speed of light, mass would be so huge that it would be impossible to move the object and hence it is impossible to go beyond speed of light. So according to theory of relativity only ‘things” that can achieve speed of light are the things that don’t have any mass such as electromagnetic waves. I don’t understand why M would grow infinitely if the speed of light itself is finite.

So if the beam of light was observed by two observers (one stationary and one moving at X speed), then since…

  1. Distance = Speed * Time
  2. Distance is different

According to Newton’s law, since distance is different and time is absolute there would be two different speeds of light but in relativity theory if we make speed of light constant then guess what needs to change, TIME. That means same event would take two different times for two different observers.


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