I have been thinking a lot about the question: If a mass moves close to the speed of light, does it turn into a black hole?

But I didn't find the answers satisfactory. I have two options:

  1. Ask the question "WHAT happens if a mass moves close to the speed of light", cite this question, and then see if it gets a response, if not then create a bounty.

  2. Is there some way to place a bounty on THIS question, with the added rule that I want a more rigorous mathematical derivation/description of what the observer sees and what the relativistic object sees.

If there was a way to accomplish (2) I think that would be better for the community because:

a. There are fewer questions asking almost but not quite the same thing.

b. Almost anyone that reads my hypothetical question would be interested in the OP's and vice versa.

  • 2
    $\begingroup$ Whatever course you choose, you need to clearly explain why the existing answers are not satisfactory for you. There's not much point in asking for rigorous mathematics if you don't properly understand the underlying principles. $\endgroup$
    – PM 2Ring
    Aug 27, 2019 at 7:25
  • $\begingroup$ The thing is, the bricks in the walls of your house are travelling close to the speed of light, as viewed by a passing neutrino. And as far as I know, they haven't turned into black holes yet. What the question seems to show is that you haven't understood the basic ideas of relativity yet. $\endgroup$
    – alephzero
    Sep 5, 2019 at 21:20
  • $\begingroup$ the bricks in my wall look probably extremely strange to a passing neutrino $\endgroup$ Sep 5, 2019 at 21:21
  • 1
    $\begingroup$ @frogeyedpeas Yes, but a brick is either a black hole or it isn't. If doesn't change to and from being a black hole depending on who is looking at it. $\endgroup$
    – alephzero
    Sep 5, 2019 at 21:22
  • $\begingroup$ Probably move to chat soon, but is the apparent density of the bricks different from a neutrinos frame of reference versus ours? $\endgroup$ Sep 5, 2019 at 21:23

2 Answers 2


I feel like it might get bad reception either way. The accepted answer of the question you link to seems to be satisfactory to your question as well. The answer is "nothing special", since relativity says so. I don't think putting a bounty on the question will help get answers that will say anything beyond this, and just posting your "WHAT happens if a mass moves close to the speed of light?" question would probably result in being marked as a duplicate of the question you link to.

If you are actually interested in what happens when this mass interacts with something that is at rest relative to you, then you might as well just post a new question asking about this specific scenario (unless it has already been asked in separate question as well). Just make sure you ask about a specific scenario. Just asking "what happens when this thing happens" is probably too broad of a question, and it will most likely be closed as such.

However, there are no rules against this. If you want to post the bounty, then you can do it. If someone answers to your satisfaction you can award the bounty. If you choose to ask a new question, make sure to explain why the question you linked to is not a duplicate, give specific details to set the question apart, etc. A common thing I see on this site is for users to just say "this is not a duplicate of this question" without explaining or showing why. Make sure this is not you!


If you want to ask a new question, you should be very explicit about the difference with the old one. And if it is not convincing enough, it might end up closed as a duplicate. If you want to set a bounty on the old question, you can e.g. choose the "Improve Details" option in the bounty menu. The next step will offer you to write a custom subtext to the bounty, where you can explain what you want exactly. This is not an uncommon procedure.


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