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From Einstein's principle of relativity space-time and mass are relative to the frame from which they are being observed.

Now would that mean a ball moving at the speed of light wrt to earth would become a black hole for an observer on earth but stay normal for an observer moving with the ball at same speed?

If not then how would you justify the observation of infinite relative mass ball not turning into a black hole from the earth frame of reference?

user1062760
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    See If a 1kg mass was accelerated close to the speed of light would it turn into a black hole? – ACuriousMind Feb 27 '16 at 18:15
  • @ACuriousMind yes I've seen that question but I do not find a justification of the observation

    EDIT: Also if "the laws of physics are the same in all reference frames." then how do you measure and find out that the object is heavier if it'll exert same force as it would at rest?

     – user1062760 Feb 27 '16 at 18:16
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    A justification of what observation? The principle of relativity is not an observation, it's a postulate (and one that's served us well so far). – ACuriousMind Feb 27 '16 at 18:19
  • @ACuriousMind so no object have been experimentally verified to exhibit an increase in relative mass with increase of speed? if it has been verified practically then if you measure mass of an object moving at C from a relative frame it would be infinite, then how would you explain that object having infinite mass and staying normal? – user1062760 Feb 27 '16 at 18:25
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    You should probably read http://physics.stackexchange.com/questions/133376/why-is-there-a-controversy-on-whether-mass-increases-with-speed?, before continuing down that line of thought. The relativistic dependence of momentum and energy have been tested to a fair-thee-well, which means that people who talk about "relativistic mass" will tell you that it has been tested as well. The rest of us use language where the issue doesn't come up (and indeed the non-dependence of the invariant mass has been tested to a similar degree). – dmckee --- ex-moderator kitten Feb 27 '16 at 18:27
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    @ACuriousMind: Let's not go overboard. The principle of relativity (like any other "postulate" in physics is an observation. In this case it's one that was made already by Galileo and that doesn't require Einstein's theory of relativity on any level (for that we need the constancy of the speed of light). The correct answer is given by John Rennie in http://physics.stackexchange.com/questions/28422/can-a-black-hole-form-due-to-lorentz-contraction?lq=1. The answers in the question you linked to are just handwaving and I find them insufficient. – CuriousOne Feb 27 '16 at 18:29

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