How do we realize the unseen changes in the universe?
We don’t. If they are not observable, they are not changes. You may not see how a tree or a plant grows, still, the change is a fact.
There are, however, six changes that can be realized in the universe, and which are the symptoms of life – we are born, we grow, we procreate, we stay for some time, we dwindle, and then we die.
The universe is material, so any change in it is material. The universe and anybody within it undergo a constant cycle or creation, maintenance, and destruction.
The soul, however, does not go through such changes. The soul, like God, is unborn and unchanging.
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By human calculation, a thousand ages taken together form the duration of Brahma’s one day (4.32 billion years). And such also is the duration of his night. (Bg. 8.17)
At the beginning of Brahma’s day, all living entities become manifest from the unmanifest state, and thereafter, when the night falls, they are merged into the unmanifest again. (Bg. 8.18)
Again and again, when Brahma’s day arrives, all living entities come into being, and with the arrival of Brahma’s night they are helplessly annihilated. (Bg. 8.19)
Yet there is another unmanifest nature, which is eternal and is transcendental to this manifested and unmanifested matter. It is supreme and is never annihilated. When all in this world is annihilated, that part remains as it is. (Bg. 8.20)
That which the Vedantists describe as unmanifest and infallible, that which is known as the supreme destination, that place from which, having attained it, one never returns—that is My supreme abode. (Bg. 8.21)