Einstein thought about the problems with Newton’s theory and finally imagined a new theory called the theory of general relativity to not only explain how gravity works but also to explain what is gravity. Why we need gravity is still an unanswered question. In Einstein’s theory he doesn’t imagine a gravitation force but speculates that space itself is “curved”. What Einstein’s theory says is that gravity isn’t a force at all but rather that matter curves space and that things moving in a curved space just appear to have a force pulling them! Click on the picture to see a video on General Relativity.

Now this is a weird concept and difficult to grasp especially since the proposed curvature is so small. The theory also predicts other weird things like “black holes“. Black holes are regions of space where so much matter has accumulated in such a small volume (this is called a high mass density by physicists - click here to see a video) that nothing can escape the pull of gravity so nothing – not even light – can escape. It’s call a black hole because you cannot see it. You can only measure its gravity effects on nearby bodies.

Unfortunately there are a few flaws in Einstein’s theory as well. A few of these are:

• Einstein’s equations cannot be solved in closed form for even the simplest problem involving only two bodies (there are several single body solutions that have been solved). Even Newton’s simple gravity theory can solve the two-body problem in closed form. Many people including scientists that should know better mistakenly believe that Einstein actually solved this problem but he only found an approximate solution (called a perturbation solution) not a closed form solution.
• Einstein’s equations are usually solved assuming a fixed (that is an unchanging) distribution of matter. A simple astronomy problem is describing the creation of a star like our Sun. Stars are created out of huge clouds of hydrogen gas that collapse or shrink into smaller volumes and then ignite in a nuclear fusion process. Again Newton’s theory explains the dynamic collapse quite well but Einstein’s equations are intractable.
• Astronomers are observing some strange behavior (called anomalies) in the universe that cannot be explained by either Newton’s or Einstein’s theories. As a result astronomers have made up new categories to explain their data. These new categories are called “dark matter” and “dark energy“.

The word dark is used because we can’t see either the matter or the energy. So how do we know these things exist? Well, we don’t! They are assumed or postulated to exist in order to explain the observations but that is just a guess.

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Tags: Physics, Science

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