Special & General Relativity Questions and Answers
If we see a galaxy moving near the speed of light, as they would also see ours, whose mass is getting bigger?
Relativity says that there are two reference frames to deal with in any physical problem, the reference frame of the moving object, and the reference frame of the one that is 'at rest' relative to you. The one you are in is called the 'proper' frame. If you had two identical masses in your frame, and you accelerated one of these to nearly the speed of light, the moving mass would appear to increase. However, if you were following along in the moving frame, you would not notice anything unusual happen. However, if you now looked at the mass you are leaving behind, its mass would appear to be increasing.
As astronomers look out into the universe towards distant galaxies, they measure them to be receding at greater and greater speeds. The problem is that according to general relativity, the 'Mother of All Relativity Theories' this is not motion as we know it, because it consists not of the motion of galaxies through space, but the dilation or stretching of space itself. We cannot use special relativity as a guide because it is not designed to handle curved space-time, or space that is expanding in scale. The result is that since galaxies are not moving in accordance with special relativity, the predictions of mass increase and time dilation do not apply.
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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.