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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

If Einstein says that everything is relative, how can there exist any firm constants upon which to base a Unified Theory of Nature?

Because relativity doesn't say that EVERYTHING is relative, only the way that we describe them. Relativity has as its core the concept that there are some quantities that remain 'invariant' regardless of which observers are making a measurement. This is because the 'thing' that is invariant has to be described by its complete 4-dimensional properties. For example, if you were to take a yard stick and rotate it, the length would still read one yard when you compute the distance between its endpoints in 3-d space. But suppose you only computed its endpoints in terms of 2 of its coordinates? The length of the yard stick would change depending on its orientation in space, just as its shadow on a wall will have any length. What special and general relativity show is that all observers will agree on the value or 'length' of a quantity if it is expressed in its complete 4-dimensional form. The search for a Unified Theory is now rooted in the search for these 'invariants' and their 'covariant' descriptions which lead to consistent field theories.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.