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Consider:

The supermassive black hole in a galactic core. A neutron star of nearly maximum mass does a very close flyby (remember, galactic black holes don't have the incredibly vicious tides of an ordinary one--it can get awfully close without crossing the Roche limit.)

Now, a neutron star can easily have an escape velocity of half of lightspeed. What happens when it flies past at 86% of c (Lorentz factor = 2). An observer sees it with half the radius in the line of motion and twice the mass--it's escape velocity is above lightspeed. Thus the observer sees the object as a black hole.

It completes it's flyby and slows back down--the observer sees it go from being a black hole to being a neutron star--but that can't be.

Qmechanic
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Loren Pechtel
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  • You're correct: that scenario cannot occur. FWIW, modern treatments of relativity avoid the concept of relativistic mass, because it can be misleading like that. See https://physics.stackexchange.com/q/133376/123208 – PM 2Ring Sep 21 '19 at 03:16
  • Loren, I've downvoted your question for the "does not show any research effort" reason. Your question is an example of a frequently asked question here (which is why it was closed as a duplicate so quickly). The community does expect that users thoroughly search for an answer here and elsewhere before posting. – Alfred Centauri Sep 21 '19 at 03:44
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    @Alfred Please note that Loren cast the final duplicate vote. See https://meta.stackexchange.com/q/250981/334566 – PM 2Ring Sep 21 '19 at 03:53
  • @PM2Ring, thank you for bringing that to my attention. – Alfred Centauri Sep 21 '19 at 03:54
  • @AlfredCentauri Note that I envisioned a quite different scenario than the question it was a dupe of even though the fundamental issue is the same. – Loren Pechtel Sep 22 '19 at 04:38

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