ABSOLUTELY RELATIVE
1.What is the natural state of a body?(*Explained by GALILEO and NEWTON)
2. Is universe static or not?(*Explained by Hubble)
3.Is distance absolute? (*Explained by Newton)
4.Is time absolute? what about speed of light?what is nature of Space Time and Fourth dimension?(*Explained by General Theory of Relativity[Einstein])
1.NATURAL STATE OF BODY
Before Galileo it was thought that the absolute nature of a body is REST.
It was also believed that a heavy body falls faster than a light body.
By a series of experiment Galileo first showed that all bodies irrespective of their weight falls in same time if air resistance is ignored.
he further showed that the nature of a body is to retain its state.
according to him if a body is in rest then it will tend to be in rest and a body in motion will tend to be in motion.
Newton further used these results and introduced the INERTIA responsible for these.
According to newton A force is needed to change the present state of body.
the required force is is proportional to mass and rate of change of velocity.
So It was accepted that the natural state of a body is to keep its State by virtue of Inertia and to change its state force is required.
2.UNIVERSE STATIC OR NOT
NEWTONS LAW OF GRAVITATION is one of the greatest achievement in classical physics
The law stated that every body in the universe attracts every other body directly proportional to product of the masses and inversely proportional to the square of the distance between them.
The law revolutionaries the view of universe giving a mathematical view towards the universe and its motion.
By this law one can state the path of any planet with much accuracy.(As the law also has some flaws)
At that time the universe was thought to be static.
Newtons realized that according to his theory all star would attract each other and will finally at the center of the universe.
To this Newton stated that this would not happen if the UNIVERSE IS INFINITE.
Because then on every side of a star there would be infinite number of stars. so every point in the infinite universe can be treated as its center and there will be no single point to fall.
Some instead believed that the universe was finite and at large distances gravity was repulsive in nature. This theory did not sustained as it was unable to explain the motion of planets and the equilibrium between stars that the theory explained would be unstable. If a star would get slightly displaced from its position away from a nearby star than repulsive forces would dominate they would further move apart. And if they were brought slightly closer then attractive forces would dominate and they would fall into each other.
Another argument to newton's infinite static universe was stated by OLBERS.
According to him every line of sight in an infinite static universe would end up with a star(as there are infinite number of stars) so the sky should glow as bright as sun even at night.
Even if the light of distant star get absorbed by any interfering material then till now the material would also be at same temperature as the star and would be glowing similar to it.
It might be possible that light from distant stars may not have reached us till now but that raised a question that could have caused the static infinite universe to turn on,
In 1929 HUBBLE changed the view of humans towards universe by observing that DISTANT GALAXIES ARE MOVING AWAY FROM US.
His observation told us that universe is expanding. This further concluded that there would be a time when the universe would be at same place and would have an infinite density.
So it is now known that universe is expanding. It is not static. Universe has a beginning and may have a end too. The very time came to existence at the same moment. the moment is called THE BIG BANG.
3. RELATIVE SPACE
Newton was among the first scientist who discard the unique standard of rest.
According to him the state of rest or motion was relative to observer.
According to him it was possible to a body to be in rest for one observer and in rest for another observer. For example you will observe a mountain to be in rest if you would sit in front of it while an astronaut would see that mountain moving with a quite high speed.
This lack of absolute rest means that it is impossible to determine whether two events took at same place or at same place at different times.
So the idea of absolute position in space was discarded and the state of motion and rest was only believed to be RELATIVE.
4. a. IS TIME ABSOLUTE? ANSWER - ABSOLUTELY NOT.
Newton believed in absolute time.According to him one could not tell the the exact distance between 2 events in space but we could tell the absolute time interval between them. Time was assumed to be totally separate and independent. However this is not correct.
The fact that light travels at a finite speed was first noticed by Astronomer ROEMER by noticing the time taken by jupiter's moon to cross it was uneven .
A proper theory was first given by MAXWELL by describing that there could be wavelike disturbances in electromagnetic fields which would travel in fixed speed.
Maxwell theory predicted that light must travel at a certain fixed rate. but Newton theory said that all velocities are relative to something. So it was believed that the empty space is not quite empty. It was believed to be filled with a substance called ETHER. and light travels through it. The speed of light is also measured relative to ether. According to this if a person travel towards the speed of light then he would measure the speed of light to be more than another person who would move at right angles.
MICHELSON and MORLEY carried out a precise experiment examining the speed of light along earth motion and then perpendicular to earth motion.
What they found was surprising . The speed of light was same in both the cases.
LORENTZ was the first to provide the explanation that clocks slow down when they move through ether.
BUT in 1905 ALBERT EINSTEIN stated that the whole theory of ether was rubbish and unnecessary if one accepts the idea to abandon absolute time.
According to him ALL OBSERVER SHOULD MEASURE THE SAME SPEED OF LIGHT ,NO MATTER HOW FAST THEY ARE MOVING.
The fundamental postulate of GENERAL THEORY OF RELATIVITY was that the laws of physics to be same for all freely moving observers at any speed.
The general theory of relativity have many consequences. one of them was mass energy equivalent (E=MC2) and other was that nothing with a finite non zero rest mass can reach the speed of light.
We know that speed is given by distance traveled devided by time taken or we can say distance traveled is time multiplied by speed.
As we know that there is no absolute distance this means speed multiplied by time is also uncertain and absolute. But as speed of light is constant this means TIME MUST ALSO BE RELATIVE. This ended an era when time was thought to be absolute.
So along with the idea of absolute distance in space the idea if absolute time also came to an end. The only absolute known thing known is speed of light.
4. b. VISUALISING TIME AS DIMENSION
The introduction of relative time changed our view towards universe.
Now we must accept that time is not totally separate and independent from space, but it is combined with it to form a space time.
We usually describe a position as coordinates(x,y,z). similarly we can describe an event in space time as (x,y,z,t). this space time is actually a 4 dimensional system.
An event is something that occurs at a particular point in space and at a particular time.
Visualising a 4dimensional space is quite tough so here is an analogy of that. suppose you drop a stone in a pond and ripples start to form at its surface and the ripple circle got bigger with time.
if we draw a curve representing x and z axis as surface of pond and y axis as time we will get a conical graph (as shown)
Analogous to the ripples in pond surface which is actually a 2 D surface one can imagine of light.
Light is a wave formed in 3D space (x,y,z) and travels with constant speed in all directions forming a 3dimensional cone in 4D space time. giving us a graph like this
The cone above the XZ plane is called future light cone of the event that occurs at present.
The intersection of all three axis is called present.
the cone below theXZ plane is called past light cone.
The region inside the future cone is called absolute future of that event. It is set of all the events that is possibly effected by what happens at present.Nothing outside the light cone can be reached or influenced by the present event because nothing can travel faster than speed of light.
The past light cone is the set of events from which a pulse of light is able to reach the given event.
The region inside the past cone is absolute past, It is set of all events that can effect present event.
If one knows everything that lies inside past cone at a particular time then he can predict the event(or the future).
For example, suppose a star which is one lightyear far from you suddenly stop glowing at time t=0.
Then you would be in the elsewhere of the event at that time(t=0). you will came to know only after one year that the star went out. so at t=a year you enters the future cone.
This also implies that you were actually wathing past. But lets discuss it later.
So time can also be visualised as a coordinate in space time plane and Tthe Elsewhere area in the light cone can't be reached and it has no effect too.
THANKYOU for reading. Your suggestions, questions and comments are welcome.
