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It's been said that nobody can actually know how fast the Earth is moving through space, because a reference point is needed to determine absolute velocity. But if mass increases as speed increases, why can't this rule be used to figure out the absolute speed of anything?

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  • $\begingroup$ Mass difference is so less to be measured. Also, it is difficult to measure mass when an object is moving. $\endgroup$ – Anubhav Goel Feb 15 '16 at 10:08
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The problem is that this mass change is also dependent on your frame of reference. The earth seems to stand still in our reference frame (more or less) which is why what we measure is actually its rest mass.

If you were to move in a spaceship at close to light speed away from earth the earth would appear more massive than to an individual that is standing on earth.

So as you see mass is also not absolute but proportional to an observers relative speed.

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    $\begingroup$ Interesting. Assuming mass also decreases as speed decreases, do you know why we can't shoot objects at various speeds in various directions, measure which speed/direction combination yields the lowest mass, and use that as a vantage point of absolute 0 motion? $\endgroup$ – Joe Morano Feb 15 '16 at 9:00
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    $\begingroup$ The problem is we are still observing it from our static view point on earth, so if you fired the objects at the same initial velocity they would all move away from us at the same speed and earth would maintain its status as an inertial frame of reference for us. $\endgroup$ – Jaywalker Feb 15 '16 at 9:03
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    $\begingroup$ Wait, so does that mean that if a spaceship approached light speed, to the people on board it would maintain a normal mass? $\endgroup$ – Joe Morano Feb 15 '16 at 9:05
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    $\begingroup$ thats exactly what it means! $\endgroup$ – Jaywalker Feb 15 '16 at 9:06
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    $\begingroup$ "So as you see mass is also not absolute but proportional to an observers relative speed." The properly defined mass is a Lorentz invariant. $\endgroup$ – dmckee Feb 15 '16 at 21:41

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