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The Theory of relativity was developed by Albert Einstein in the early 1900s. There are two theories of relativity. The first is Special Relativity and the second is General Relativity. Both are based on the Principle of relativity, which was created by Galileo Galilei in the 1600s.
Special Relativity says that every person has their own time. One person's clock says something different from another person's clock. The reason a person's time can be different from another's is because of Time Dilation, which can be thought of more easily by the Twin Paradox.
The Twin Paradox shows how time changes because of speed and mass. The faster someone or something moves or the more mass they have, then the slower time passes for them. This is shown by a pair of twins. Imagine that the first twin moves at a fast speed, near the speed of light for example, while the other twin stays in one general location. When the first twin returns from his trip, he will be younger than the second twin. Thus time and speed are related.
- Speed and mass affect how fast time moves
General Relativity adds gravity to Special Relativity. It says that space and time are joined together. Space is like a grid. Balls (planets) on the grid bend it because of their mass or speed. As one ball moves towards another it falls towards the other ball due to the dent in the grid. It can be shown by putting heavy balls on a trampoline. The dent in the grid is like gravity shown by the planets.
How it works with special relativity
If space and time are one thing represented by a grid and heavy objects stretch out the grid, that means that time passes more slowly near heavy objects. This has been shown in experiments by comparing very accurate clocks on Earth (where Earth's gravity is strong) with very accurate clocks on satellites or rockets in space (where Earth's gravity is weak).
Criticism of relativity
G. O. Mueller wrote a whole encyclopedia arguing against Einstein's relativity. Mueller, and many other philosophers say that space and time are categories of perception, not distortable "things," and so ideas of relativity that treat them as "things" can not be correct.
Paul Dirac and others have said that constants may change over time, including the speed of light. Mueller lists about 4000 Einstein-critical works since 1905, suggesting rethinking relativity.
- Hawking, Stephen. The Universe in a Nutshell. Bantam Books, 2001.
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