Entry 1435, on 2012-09-05 at 13:08:52 (Rating 1, Science)
On a few occasions in this blog I have talked about the big questions, but sometimes little questions are just as interesting as (or even more interesting than) the big ones. In Carl Sagan's book, "the Demon-Haunted World", he mentions some seemingly simple questions which are often asked by kids, but can be deeply meaningful in many ways.
In the book Sagan discusses how children are lost to science because adults don't want to - or don't know how to - answer questions about the world. He gives some examples of apparently trivial questions which reveal interesting truths about the universe. So here's the questions along with my answers to them (which may or may not agree with other people's answers to the same questions)...
Why is the Moon round?
Why is grass green?
What is a dream?
How deep can you dig a hole?
When is the world's birthday?
Why do we have toes?
These are some really cool questions but I would add a few more that I have heard...
Why is the sky blue during the day?
Why is the sky dark at night?
What is a star?
How much does the sky weigh?
What is time?
Why is water wet?
Why did God let my kitten die?
So, let's get started on these questions: why is the Moon round? Anyone with a knowledge of astronomy or physics should know the answer, but I suspect a large proportion of people won't. The simple answer is the Moon is round because it's big. All big things tend to be round, but why?
First let me give some examples. Look at the moons in our solar system. The Earth's moon is quite big (although not the biggest) and it is round (more specifically it is spherical or even more precisely it is most like an oblate spheroid). So are the other big moons and all the planets (although Saturn is noticeably flattened). The Sun is also very round, in fact it is a bit of a mystery why it is as round as it is. But the small moons are not round. Phobos and Deimos, the two small moons of Mars for example, are both "potato shaped".
So there certainly seems to be a link between size and roundness. Why?
There are 4 basic forces in our universe: electromagnetism, the strong force, the weak force, and gravity. All other forces are just manifestations of these. For reasons not fully understood, but possibly concerned with extra dimensions, the forces are of wildly different strengths. The weakest force (gravity) is a trillion trillion trillion times weaker than the strongest force (the strong force).
The two forces which affect most of what we normally see are electromagnetism (which is 100 times weaker than the strong force) and gravity. Based on these figures a single teaspoon of material has more electromagnetic force than the whole Earth has gravitational force! So you would wonder why gravity ever has any effect at all.
There are two attributes of gravity which allow it to dominate in many situations: unlike the strong force which only works over incredibly small distances gravity has an effect which stretches out forever (it obeys the inverse square law); and unlike electromagnetism (which creates positive and negative charges, north and south magnetic poles, etc) it only has a positive effect - there is no negative force to balance the positive (although according to Einstein, negative gravity should exist and this could be related to dark energy and the increasing expansion of the universe, but these ideas are currently poorly defined).
So big objects are mainly affected by gravity because the other forces either only work over short distances or the positive and negative effects cancel each other out. As more mass is present gravity gets stronger and stronger but the electromagnetic force stays the same. That force holds material like rock in a particular shape but eventually gravity becomes strong enough to overcome the other forces and pulls all the material into the most compact shape: a sphere.
So the Moon is round because of the attributes of the fundamental forces of nature. It didn't have to be that way. If the relative strengths of the forces were different, the distance the forces were active on were different, or the forces had different polarities then the whole situation would be completely different to what it is now.
The fact that the Moon is round is deeply significant yet many people never even think about it because it is taken for granted. The Greeks thought that the circle was the perfect shape so naturally heavenly bodies, such as the Moon and Sun, would be round. Maybe that sort of thinking has remained with us after thousands of years. But we should question everything, even things which are "obviously just meant to be that way" because some of the most basic questions are the most interesting.
I seem to have used quite a bit of space talking about the round Moon so I guess I'm not going to get on to the other little questions today. That will be a subject for a future blog entry. If you like this stuff, stay tuned!
Comment 1 by Doug Mackie on 2012-09-05 at 21:12:43:
Neil Armstrong's death has reminded me that while some people have an idea of how far away the moon is few know how tiny the moon is - only 1/81 the mass of Earth. (But 3/11 the diameter).
For such a tiny mass you might expect the surface gravity to be tiny. And certainly, Armstrong inelegantly bounced along. But he certainly could be seen to experience more than 1/81 of the gravity on Earth. What is the explanation?
As noted in OP, gravity obeys an inverse square law. For calculation purposes it is also convenient to assume the mass is a point source. That is, you pretend that you are standing on a thin shell at the surface of the object (e.g. Moon, Earth) of a mostly empty sphere with mass all located at one tiny point at the centre of the sphere.
In the case of the Earth although the mass is 81 x larger you are 11/3 (=3.67) times further away. Plug that into an inverse square (mass over distance squared) and you get 81 / (3.67 ^2) = 81 / 13 = 6 for the Earth compared to 1 /1 = 1for the Moon. That is, gravity on the surface of the Moon is 1/6th that on Earth.
And come to that, why is Haumea so oddly shaped?
Comment 2 by OJB on 2012-09-05 at 22:49:04:
The mass of the Moon relative to the Earth is actually very large - much more than other major planets and their moons. Yes, your explanation of surface gravity does illustrate a point many people don't realise. It also works in reverse: many massive planets, such as Jupiter, have much lower "surface" gravity than their mass would suggest because of their large diameter.
Haumea is thought to be oddly shaped because it has low density (half that of Earth) and a high rotation speed (3.9 hours).
Comment 3 by Anonymous on 2012-11-16 at 09:27:49:
As shown by Einstein in 1921, space-time has a number of interesting properties. One of them is the fact (verified various times) that space is compressed between things that have mass, so that they tend to move together. There is no such force we call gravity!
Comment 4 by OJB on 2012-11-16 at 09:28:06:
Yes, I understand your point. In many ways gravity is an anomaly and one of the major aims of physics is to integrate the current relativity explanation with quantum theory so that it can be explained in the same way as other forces. But even if that hasn’t happened gravity is still seen as one of the four fundamental forces. Either it’s a different type of force than the others or it’s the same and we just haven’t figured out how it fits with quantum theory yet. Thanks for your comment.
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