The difference between the universe and the solar system
The Sunday Nation
Nairobi,
15 January 2012
John Kamau sends in four questions that have been bugging him, namely: (1) What is the difference between the universe and solar system? (2) Can we calculate the size of the universe? (3) How long would it take to go round the universe? (4) Do we know the starting point and if so where?
The first question is quite easy. The solar system refers to the sun and all the objects that go around it. These include the eight planets (and their moons), asteroids and comets. As explained in a past article, the dominance of the sun’s gravitational force extends to about 2 light years away.
The universe is everything that is in existence. That is, all the stars (including their planetary systems), the interstellar dust and the exotic bodies like so-called black holes.
For that reason, astronomers are careful not to give a definitive answer to this question. They talk about the size of the known universe. Their reasoning is that since universe means everything in existence it would be presumptuous to pretend that they know where all things are.
There is also the idea of the knowable universe. This arises from the fact that the universe is estimated to be about 13 billion years old and therefore we can only see objects that are within 13 billion light years (LY). If there is anything outside that limit, then its light has never reached us and we cannot possibly know about it!
Therefore, the known universe is all the objects that have been seen in our telescopes while the knowable includes unknown objects that might exist up to the 13 billion light year limit.
The third question is quite complicated. One might start by assuming that the universe is a large sphere measuring 13 billion light years (LY) in radius and then calculate its circumference. This would come to about 82 billion LY. At that point, we encounter the first challenge: how fast is the traveller moving?
One might assume farther that it is at the speed of light. Then the answer comes to 82 billion years to go round the universe. But things are not that simple. There is now enough conclusive evidence that the universe is expanding…and the expansion is accelerating.
Now that complicates maters a lot. Imagine that you found yourself at the edge of the universe and then you attempted to circumnavigate it. Since the universe is expanding, it is easy to see that you would never return to your starting point. Thus even after the 82 billion years, you will not have completed your “circuit”.
The answer to the last question (Do we know the starting point [of the universe] and if so where?) is quite puzzling. YES; we know the starting point. It is everywhere!
In essence, the big-bang theory says that about 13 billion years ago, everything in the universe was concentrated at a single point. There was no space and no time. Consequently, the laws of physics did not exist.
Then, BANG! Everything was thrown outwards and they have been flying out ever since. Thus, the universe starts and ends everywhere… I said it is puzzling!
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