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I Questions I have been thinking of regarding black holes

  1. Jul 27, 2017 #1
    So, as one does, I have been pondering the nature of the universe and specifically black holes. I apologize for the rambling nature of this post...I haven't really put it into a coherent format because time is a bit limited. Also, English is not my first language and I apologize for any mistakes. Also, I am not a physicist and my understanding of physics I know is pretty limited, hence why I am asking these questions. Anyhow, here goes.

    As most people who find astrophysics interesting knows, black holes are bloody weird. It is the only place that we know of that matter moves faster than the speed of light.

    According to general relativity, as I understand it, you need more than an infinite amount of energy in order to accelerate matter past the speed of light, hence why achieving this is a physical impossibility, except in black holes. This would then mean, according to my logic (which may be faulty) that a black hole must consist of a mass beyond an infinite amount in order to generate the energy required to accelerate a particle to beyond the speed of light. However, we know this is not the case for two reasons. We can relatively accurately measure the mass of a black hole and secondly, the universe still exists with black holes in it. If there was a black hole with more than an infinite amount of mass, it would have more than an infinite amount of gravity and thus suck in the entire universe (once again, this is as I understand it and I may be wrong here). So, how can matter be accelerated to past the speed of light without a larger than infinite amount of energy?

    So this leads me to the only conclusion...the theory of general relativity is either wrong or incomplete. I prefer to think of it more as incomplete than wrong.

    Now, let's move that aside. Another thought that has intrigued me is that general relativity does not say that matter be accelerated to a speed faster than light. It does allow for a particle to be created already moving at this speed, however this has never been detected and we just don't know if it happens. However, a particle moving at beyond the speed of light would according to general relativity be moving backwards in time.

    So, my question is, if a particle inside a black hole gets accelerated beyond the speed of light, would it then be hurled out of the black hole via time travel? Would it then slow down or keep going backwards in time until the universe was only a singularity? Would it be moving backwards as a "seperate" particle than the one in the past, or would it be like rewinding an old vcr from the perspective of the particle?

    Another thing about black holes is that as I understand it, is if matter approaches a black hole and comes upon the event horizon, it would appear to stay there as time due to the massive gravity would for all intents and purposes stop (from the perspective of an outside observer). This would mean that there should be a massive amount of matter appearing on the event horizon of every black hole made from all the matter it has ever "swallowed". I do not think that we are observing this. I admit that my understanding may be incorrect here and if it is I would appreciate it if someone corrects me. If I am correct though, why are we not observing this?

    If anyone can answer any or if possible all of my questions I would greatly appreciate it.
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  3. Jul 27, 2017 #2


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    No it doesn't. I would say you need better sources for your knowledge on black holes.
  4. Jul 27, 2017 #3
    Then I apologize. I must have misunderstood what was said when Dr. Tyson said something to this regard when I saw a documentary featuring him. I apologize as I am not English and I should have thought first that I might have misunderstood this.
  5. Jul 27, 2017 #4


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    Popular science is for getting the general public interested in science and for teaching them about science. It is normally useless for learning actual science as it is usually full of simplifications and rewritings into ordinary language rather than mathematics with the intention to make it "understandable". Generally, you should not try to reason based on what is presented in popular scientific literature and documentaries.
  6. Jul 27, 2017 #5
    Thank you. This makes sense because I have seen a few logical "gaps", but thought I must be wrong because I am definitely not trained in physics. I will read up more about it. Do you have any suggestions on where to go to get a better understanding for someone who doesn't really "know" anything. lol
  7. Jul 27, 2017 #6


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    If you want to understand the logical reasoning and be able to reason about physics yourself, then there is no substitute for an actual textbook in physics or (even better) a university course in the subject you are interested in. Be aware that these will generally require relatively advanced - often university level - mathematics.
  8. Jul 27, 2017 #7


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    Well, that's not the only conclusion. Another possible conclusion worth considering is that you've misunderstood it in some way. In fact:
    - Black holes don't involve anything moving faster than the speed of light, so even if that required infinite energy black holes wouldn't require infinite energy.
    - It's not exactly wrong to say that it requires infinite energy to accelerate something to faster than the speed of light, but it is a potentially misleading oversimplification. You'll only find it in books and videos that have to oversimplify because they're trying to explain without using the math that is required for a proper explanation. There's really no substitute for a proper textbook that does the explanation right: Taylor and Wheeler's "Spacetime Physics" is good.
    - Likewise the idea that a particle moving faster than light is moving backwards in time is not really right. If you google for "Tachyonic antitelephone" you'll find some good explanations (and, this being the internet, plenty of bad ones too).
    -Time does not slow down and stop as you approach a black hole, so things fall through the event horizon quite quickly. It just takes longer and longer for light to get from the infalling object to a distant observer, so that distant observer never sees it happen. I don't know of any math-free explanation of what's going on, but once you understand special relativity at the level of that Taylor and Wheeler book, then https://preposterousuniverse.com/wp-content/uploads/2015/08/grtinypdf.pdf will get you started.
  9. Jul 27, 2017 #8
    Once again thank you for your response. I have mostly read Dr. Tyson's books because they are easier to understand. But I will try to read some physics textbooks as I currently don't have the time to really study the subject.
    But I would like to thank you for enlightening me that not even science can be taken at face value once Hollywood gets their hands on it. Though I should have assumed as much. lol
  10. Jul 27, 2017 #9
    Thank you for your response. As Orodruin has done, I fully now realize that my understanding is VERY faulty. Thank you for taking the time to enlighten me.
  11. Jul 27, 2017 #10

    What is your definition of "infinite"?

    I do share your confusion here.

    "As the eminent American physicist Kip Thorne describes it, it [the singularity at the center of a black hole] is 'the point where all laws of physics break down.' So it would seem hard to disprove any assertion or theory, including FTL matter (or that long after this universe has expanded to where it's so large and empty that every celestial object in it is receding via Hubble expansion from every other faster than c (as many already are) and even baryonic matter's molecules and atoms have begun to separate, black holes' singularities will then "explode" into new universes), and speculation is fine, but, to me, serves no useful or verifiable/predictive scientific purpose, falling then into the realm of philosophy and, even worse, religion/fantasy/SF.
  12. Jul 27, 2017 #11
    I suppose the light from the accretion disk feeding the BH would hide any "swallowed matter", and if the accretion disk was gone, any light from the infalling matter would be undetectable relative to light coming at us from behind the BH. I thought this was interesting:

    Last edited: Jul 27, 2017
  13. Jul 28, 2017 #12


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    The accretion disc is a separate issue. It forms because matter tends not to be falling straight in to the black hole.

    With an isolated non-rotating black hole, seen from a distance, radially infalling matter never appears to cross the horizon. The light from it is ever more redshifted and its clocks tick ever more slowly as it approaches the horizon. Classically this goes on for ever. In reality light is quantised and there is, in fact, a last photon emitted by the matter before it crosses the horizon. But you'd have to wait an awful long time for it to arrive and you would never be 100% confident that there wasn't just one more photon to come.

    However, seen from the perspective of the infalling matter there's nothing unusual about the event horizon. It crosses perfectly happily in finite time.

    So there's no matter accumulating at the horizon. You can see this directly locally, and remotely the apparently-not-quite-swallowed matter fades out of sight.

    This is all according to general relativity. I gather that some (but not all) possible future theories do predict matter not making it to the horizon, but I don't know anything more about that. And we don't know which (if any) of our beyond-GR theories is correct.
  14. Jul 28, 2017 #13


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    I don't believe that anybody predicts that bound systems will ever be pulled apart by expansion.
    I'm not sure that this is a correct description of black hole evaporation, either. @PeterDonis would know more.
  15. Jul 28, 2017 #14
    Thanks for your thoughts here, Ibix. I was only offering it as an example of a "crackpot" hypothesis that cannot be disproved, as too little is known. However, couldn't the rapidly expanding primordial universe be considered a bound system? In any case, this expansion wasn't evaporation.
  16. Jul 29, 2017 #15


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    No. Heuristically, the concept of a "bound system" only makes sense if there is a "rest of the universe" outside the system. For the whole universe that is obviously not the case.
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