Similarly there is no absolute time
and even that is relative. What is time? It is durationbetween 2 events. If
there are no events, then there is no time either.
What is a day? It is the time; the
earth takes to rotate once. Similarly, a year is the time in which the earth
revolves around the sun. If there is no earth, then this time is not there
either.
What happens to time when we leave
the earth? Different planets have different rotation times and different
revolution times. Therefore, their days and years are very different from ours.
If we move further into space into stars and galaxies, then time becomes much
more confusing.
Suppose an event takes place in the
US, then we can tell exactly when that event would take place as per our time
in the country. But as we travel into interplanetary space, the word
simultaneity loses its meaning.
For example the star Sirius is 8.5
light years away from us. So, the light that we see today emanated from Sirius
8.5 years ago. Sirius is one of the nearest stars to us. If we move further
there are many galaxies which are even billions of light years away.
This means we are now seeing that
galaxy as it was billions of years ago. Whether today that galaxy exists or not
would be known to us billions of years later. This means we are virtually
seeing through time. Looking to this
Einstein postulated that even simultaneity is relative.
For this Einstein gives an analogy. Imagine
that a person is standing near a railway track and a train is passing through.
Suddenly the sky became cloudy and two bolts of lightning fell simultaneously.
One in front of the train at a place called “A” and another at the back of the
train at a place called “B”. Another person “C” is located exactly in between
“A” and “B”. As he had two mirrors in his hands he could see both the bolts of
lightning simultaneously and concluded that they have fallen simultaneously.
Another person “D” is traveling on
top of the train exactly in front of “C”. He too had two mirrors in his hands
and he too is watching the two bolts of lightning. “D” sees things differently.
As the train is traveling from “A” to “B”, it would appear to “D” that the
first bolt in the front has fallen first as the speed of the train is added.
Similarly, the second bolt in the back appears delayed as the train moves away
from it. To “D” it is the speed of train that determines the duration between
the two lightning bolts. So, the same event appeared different to two different
observers. This proves that simultaneity is a myth and time is relative.
As per the theory of Relativity a
time shows correct duration as long as it is stationery. As soon as it starts
moving, time starts slowing down. In normal speeds this slowing down is not
perceived. But when we start moving closer to the speed of light, then we
perceive this change.
A person who is traveling along with
the watch would not be able to recognize the slowdown in time, but to a stationary
observer it is clearly visible.
For example a person goes to the
station and gets into a train that travels with 90% the speed of light that is
270,000 Km/Sec. In the station before getting in he corrects the time as per
the clock in the station. He gets down from the train after half an hour as per
his watch, but when he compares his watch with the clock in the station already
an hour has elapsed. At 90% the speed of light times slows down by half. As one
approaches the speed of light, times starts further slowing down and at the
speed of light time stands still. If we cross the speed of light which is
impossible as per the theory of Relativity, then time starts moving backwards.
Similarly objects traveling would
constrict in the direction of travel. Even this constriction is not visible to the
person or the object that is traveling but is clear to a stationary observer.
Again, at 90% the speed of light the
object would shrink by 50% and as it approaches the speed of light it starts
shrinking further till its length becomes zero at the speed of light.
Similarly the mass of the object
traveling would increase gradually and at the speed of light, the mass of the
object becomes infinite.
Okay, we may not come across persons
or objects that travel at such high speeds in our daily life but in the
Universe such speeds are not uncommon.
All the above narrated is the Special
Theory of Relativity published by Einstein in 1905. In his Special Theory of
Relativity published in Einstein redefined gravitational attraction.
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