People debate a lot about
this question, why does the sun shine from early days
of science. And towards the end of the 19th century,
there was a famous physicist called Lord Kelvin,
addressed this question. Of course people also
wanted to know-how long the sun is going to last!
Then once people learned about the conservation of energy,
it's clear the sun wouldn’t go on forever. Even the sun would have some limit
when it exhausts all the energy it got. Lord Kelvin
said, well the sun can’t
possibly last more than 20,000 years. That’s
a kind of shocking statement. Human beings
have been around more than a million years. So it sounds almost like tomorrow.
But there
was also counter argument for example like Darwin considering evolution
of the biological systems, and some
record ago, geological data coming from sampling rocks from around the world. They believed
Earth must be at
least300 million years old. Here’s a big difference in numbers.
If the sun can
possibly last more than say, hundreds of thousands
of years, but Earth is much older than that, does it
mean that Sun was
born much later more recently after the long history of the Earth? That doesn't quite sound right?
So what Lord Kelvin was missing here,
and so Lord Kelvin was actually
wrong, was that he estimated the life of the Sun based on the
assumption that the Sun shines basically
out of the chemical reaction.
So if we imagine the Sun shining is like an explosion
of dynamite or some kind of burning fuels
and stuff like this. Then certainly you can compute
how long it can possible last because we know
how much energy it can
produce for a given amount of mass of a Sun fuel. So, that's what he did.
But the big mistake
he did was to
assume that the chemical reaction was
the only source of energy.
And indeed, that was the most source of powerful energy back in those days, so it was a natural
assumption for him. But he was not aware of this famous equation, E =m c squared proposed by Albert
Einstein. And he (Lord Kelvin) proposed his equation which was quite controversial at that point.
contd....
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