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« **on:** November 15, 2015, 12:06:00 PM »
Next, let us say that we had taken two accurate clocks, and placed one of them in the above spaceship

before it left us, and kept the other with ourselves for reference. And suppose that we now observe

the clock in the moving spaceship from our position, and measure by our clock, how much time it

takes to move forward by 1 hour. Normally, we expect it to be 1 hour too, but our measurement

would actually show it much longer -- 2 hours and 17 minutes. A person travelling in the vehicle will,

however, find nothing unusual, with his meter rod or his clock. But will find a meter rod held by us

to be precisely that much shorter, and our clock that much slower. These observations have nothing

to do with limitations of measurement, but are inherent in the very way our universe is structured.

Space and time are thus relative to an observer, and have no absolute basis.

The special theory' has another interesting consequence. Suppose the spaceship of our example

fires a small space probe directly in front at a speed of 0.6 billion kmph, as measured from it.

Now, let us say we measure the speed of that probe from our position on earth. Common sense

tells us that we should find it to be the sum of the two speeds (0.9 and 0.6) i.e., 1.5 billion kmph.

But actual measurement would show it to be far less -- only 0.974 billion kmph!! Einstein's theory

in fact tells us that no matter what we do, we can never make an object move at a speed exceeding

1 billion kmph, which is the speed of light in vacuum. These consequences are again not due to any

practical limitations on our part, but only because our universe is structured that way!!

Einstein's 'general theory of relativity' goes a step further to say that space and time are not two

separate entities as we normally take them to be, but are components of a four dimensional

matrix, called 'spacetime'. This theory explains gravitation as due to a 'curvature' of this spacetime

in the vicinity of objects. We find the concept of spacetime itself not so easy to conceive; that of

a curvature in it beats our imagination. Empirical reality of the macrocosm is thus entirely aline

to our common sense notions of the world.

contd.,

Arunachala Siva.