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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

When your speed exceeds light, how does your mass become 'imaginary'?

In the relativistic equation for mass:

                  Rest Mass

  mass    =    ------------------

                              2   1/2

               (  1  -  ( V/c)   )

You will note that for V greater than the speed of light, you end up taking the square root of a negative number...what mathematicians call an imaginary root. All this is saying physically, is that the mass of a body increases to infinity asymptotically as V APPROACHES the speed of light. Notice that just AT V = C, the denominator becomes Zero, and the observed mass becomes infinite. For any continuous acceleration, you will have to first deal with getting to this point, before you have to worry about stepping over it to get imaginary mass!!!

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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.