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I Black Holes - Spaghetification countered by time dilation?

  1. Oct 6, 2015 #1
    Here is an oddball that I am wondering -
    could the spaghetification be countered by time dilation? Because as you approach a black hole (assuming you go in legs first) not only do your legs experience higher gravity than your torso, but they are also subjected to more time dilation - as they stretch, they also get stuck in an ever more "compressed" area of time, essentially slowing down while the rest of the body catches up.

    Is the ratio of these phenomena powerful enough to cancel each other out?

    Also I have a follow up question, if you don't mind -
    This is all postulated under the assumption of the existence of Hawking radiation; as you approach a black hole, you experience exponential time dilation, assuming you survive the approach long enough, you could see the entire universe unfold above you...

    But the question is, as the black hole you're falling into evaporates, accounting for time dilation, could the black hole decay to the point where it can no longer sustain itself and "blows up" before you even fall in?

    So if my second question is even appropriate, it raises some interesting notions about black holes, one of which being how could it have even formed in the first place as time compression wouldn't even get matter to hit the event horizon across the entire black holes lifetime.

    Am I making any sense? Or are you going to start tossing rotten eggs at me and chase me off the stage?o:)

    :nb)
     
  2. jcsd
  3. Oct 6, 2015 #2

    phinds

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    You completely misunderstand time dilation and you do so in a way that is VERY common so don't feel bad. Time dilation is NEVER anything that you experience, it is only a perception from an object that is moving in your frame of reference.

    To see the truth of this, consider: right now, as you read this, you are MASSIVELY time dilated according to a particle in the CERN accelerator and you are mildly time dilated relative to a speeding asteroid somewhere out in space, and you are trivially (but measurably) time dilated according to the ISS, and you are not at all time dilated according to the chair you are sitting in. Now how could you possibly be all of those things at the same time? Obviously, you can't. It's just what is seen from the other objects that are moving relative to you. You, of course, see THEM as time dilated in exactly the same way and they don't care any more than you need to.
     
  4. Oct 6, 2015 #3
    Eh... time dilation is actually a physical, quantifiable phenomena, we even had to adjust our GPS satellites to account for it. If you get ever closer to a black hole, whatever is outside will go ever faster. According to Einstein, time is relative to the observer, but it is definitely not an illusion. While others from the outside will see you as frozen, you on the inside may experience normal progression of time, but everything on the outside will move ever faster.
     
  5. Oct 6, 2015 #4

    phinds

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    In that case, you now have to answer my question. How can you be time dilated to several different degrees all at the same time? Clearly you are saying you ARE, I just want you to explain how it is possible.
     
  6. Oct 6, 2015 #5
    This is what I'm saying - you have 2 positions, position "A" and position "B", where time dilation at position "B" slows time to 50% of normal.

    - If you are in position A, 30 seconds will pass in position A, while only 15 seconds will pass in position B.
    - If you are in position B, 30 seconds will pass in position B, while an entirety of 60 seconds will pass in A.
    - Throw a clock in position A then wait in position B for 1 hour. After you go to position A, the clock will show 2 hours.

    This is what I'm talking about!
     
  7. Oct 6, 2015 #6

    phinds

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    Yes, the AMOUNT of time that passes in different locations can be different but that does not mean that their clocks tick at different rates and you can only verify it if they get back together again and they can only do that by taking different paths through space-time which is what causes their clocks to tick at the same rates and yet show different amounts of time.

    And you still have not answered my question.
     
  8. Oct 6, 2015 #7
    I am not speaking about multiple dilations here, only multiple points of view. If you are outside the black holes' sphere of influence, you will see matter spiral in ever slowly. If you are ridiculously close, you will see the entire universe speed across its life cycle.

    Now, assuming that the hawking radiation actually exists:

    1) If you look at it from the outside
    How could anything have fallen in the black hole in the first place if time dilation causes matter falling in to crawl to a near perfect halt? The black hole would evaporate before anything could even have time to fall in!

    2) If you're falling inside
    The whole universe, you being inside and everything outside going ever faster, revolves to the point that the black hole evaporates before you even have time to fall in.

    Both address the same issue - black hole evaporates before anything actually hits the event horizon!
     
  9. Oct 6, 2015 #8

    phinds

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    Well, since timed dilation isn't real to the infalling observer, this does not happen so it not a problem.
    The infalling observer sees nothing amis as he falls in (until he gets to the point of spheghetification, at which time things go badly amis).

    You continue to believe that the outside observer's observations are real to the in-falling observer. This simply is not true and you are not going to get anywhere until you come to terms with that.
     
  10. Oct 6, 2015 #9
    I am not talking about observers, I am talking about matter in general. Any references I made to the observer was not as an actual person but merely a point of view, like in from where you look at it. But both my questions are sepparate from each other. The spaghetification is an entirely different thing.

    If we go back to the examples of A and B -
    If point B is ridiculously close to the event horizon and time dilation becomes so extreme, that the entire progression of the universe passes in point A to point B's one second, either way you put it, in both cases, regardless whether you're in point A or point B, the black hole will reach a point that it will evaporate before anything actually falls into it. Will it?

    I request somebody else also join this conversation.
     
  11. Oct 6, 2015 #10

    phinds

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    No, it most emphatically will not. It may appear that way to an external observer, but that has nothing to do with the reality of the in-falling person just falling in with no notice of the event horizon

    You have continually evaded my question. If you believe time dilation is real, you need to explain how one "person" can be dilated to an infinite number of degrees all at the same time.
     
    Last edited: Oct 6, 2015
  12. Oct 6, 2015 #11

    PeterDonis

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    It's true that there is a physical, quantifiable phenomenon that is often called "time dilation", yes. But it's also true that that physical, quantifiable phenomenon plays no role in what a person free-falling into a black hole experiences locally. In particular, the "time dilation" phenomenon has nothing at all to do with spaghettification, and it certainly doesn't change the way spaghettification is experienced by the person falling in.
     
  13. Oct 6, 2015 #12

    PeterDonis

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    No, they aren't, at least not locally. Locally, your legs and your torso act just like they would if you were in free fall anywhere else in spacetime; you can't even tell, locally, that you are falling through the horizon of a black hole.

    There are ways to define the "time dilation" of your legs and torso relative to a distant observer (someone at rest far away from the hole), and if you were to turn around and go back up to that distant observer, that "time dilation" would have a direct physical meaning--it would show up as you having aged much less than the distant observer. But none of that is detectable to you, locally, as you're falling in; and if you fall inside the horizon and on in towards the singularity, you can never go back up and meet up with the distant observer anyway, so you can never make the direct observation that gives "time dilation" a real, physical meaning.

    Actually, if we allow quantum gravity into the picture (Hawking radiation is a quantum phenomenon), we don't have a single, well-defined model to use, since we don't have a good theory of quantum gravity. We don't even know if spaghettification still happens if you fall inside a quantum black hole that is emitting Hawking radiation. So if you're asking about spaghettification, it would be best to stick to the purely classical model, where there is no Hawking radiation and the mass of the black hole is the same for all time.

    Not if you fall in. If you "hover" just above the horizon, yes, this could happen. But you are talking about someone who free-falls inward through the horizon. That person would not see the entire future of the universe; they would fall through the horizon and hit the singularity pretty quickly (on the order of seconds, by their clock, for a black hole of a million solar masses or so, the sort we think is at the center of our galaxy).

    This is not correct; the "time dilation" you are talking about does not keep a black hole from forming. We have had plenty of previous PF threads on this topic; see, for example, these:

    https://www.physicsforums.com/threads/how-can-black-holes-actually-ever-exist.338103/#post-2353574

    https://www.physicsforums.com/threa...-surface-of-a-black-hole.718648/#post-4549672
     
  14. Oct 6, 2015 #13
    You will pardon me if I haven't seen this before, the forum has so many old topics, its difficult to find what you're looking for, especially for somebody who doesn't attend it much like I do.:confused:

    Well once again we are in the subjective interpretation of what the black hole's properties actually are, but what we do know is the presence of time dilation, and according to yourself from the link you provided, time dilation factor reaches infinity as something approaches the black hole... which could suggest that everything the black hole grabbed after its formation could have accumulated on a bubble at the very edge of the event horizon. Some say that if you drop into one and somehow survive the drop, all the matter that the black hole ever pulled in would suddenly hit you in a near timeless zone.

    You say that one doesn't experience time dilation while falling down to earth, granted, but one does also not experience spaghettification either. But in black holes, both things are exaggerated. I believe that there is an effect involved.
     
  15. Oct 6, 2015 #14

    PeterDonis

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    There is no subjective interpretation in any of the things I said; I was talking about direct observables.

    If "time dilation factor" is given an appropriate physical interpretation, yes. But you are using the term "time dilation" in ways that are not justified by that physical interpretation.

    No, I said one doesn't experience time dilation when falling into a black hole. One does experience spaghettification in that case.

    You never experience time dilation locally; to you, your own clock always ticks normally, at one second per second. Time dilation, where it is present, is something other people perceive--they perceive your clock to be ticking slower (or, in some cases, faster) than theirs. Or, if you and another person separate and then come back together, your clocks might show different elapsed times; but neither of you will have observed any difference in the way your clocks tick while you were traveling. The difference in this case is just a difference in path length; the two of you took different paths through spacetime, and those paths have different lengths (elapsed times). It's no different than two people traveling from, say, New York to Los Angeles by different routes, and their odometers showing different distances traveled when they meet up again.
     
  16. Oct 6, 2015 #15

    PeterDonis

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    I am closing this thread since the original question has been answered. #Thomas#, before posting again on this topic, please take the time to learn about how GR models black holes. It might also help if you learned about how the twin paradox is modeled in SR, since the term "time dilation" is often used when what is really meant is the difference in elapsed times between two people who separate and later meet up again.
     
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