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How is it possible to live past the event horizon?

  1. Jul 16, 2012 #1
    In the book "The Black Hole War" by Leonard Susskind, he states that a person can live past the event horizon (to a certain point, of course) in a massive black hole because "the horizon of the larger black hole would be so large that it would almost appear flat. Near the horizon, the gravitational field would be very strong but practically uniform."

    Although the gravitational field would be uniform, wouldn't the person be accelerating too fast into the black hole that he would be compressed, either horizontally or vertically depending on the position of the person? (Gravity's strength is so strong that even light can't escape past the event horizon, meaning that acceleration by gravity exceeds that c?) I thought a person can "feel" acceleration although there is no air?

    I may be thinking too much.

    Thank you in advance!
  2. jcsd
  3. Jul 17, 2012 #2


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    No, like the astronauts in orbit who feel nothing, the sensation of being pulled by a uniform gravitational field is one of weightlessness. Only tidal forces, differences in accelerations, will be detectable.
  4. Jul 17, 2012 #3


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    For a supemassive black hole the distance to the singularity is so great the difference in gravitational force over the length of your body is negligible.
  5. Jul 17, 2012 #4
    Okay. So in this case, the only force that matters is the tidal force (difference of gravitational force on your feet and head)? What I thought was that although the gravitational field is uniform, the acceleration (like when the rocket first launches from the ground) is really high that the person would get squished. Or is the acceleration really not that high? I was thinking this way because inside the horizon, the gravity is strong enough to keep the light from escaping, while on the outside, the light can still escape. Isn't there a huge difference in gravitational field, thus accelerating a lot when you pass the horizon?
  6. Jul 17, 2012 #5
    Is this because you are giving in to gravity, cancelling out gravity? I think I understand now.
  7. Jul 17, 2012 #6
    The acceleration is high but if you're in free fall, you won't feel gravity- just like astronauts in orbit. As Chronos posted above, tidal forces are negligible at the event horizon of a supermassive black hole.

    For smaller black holes- a few solar masses- the gravity gradient is very steep at the event horizon; the horizon is much closer to the center of smaller black holes. Parts of your body closer to the black hole are going to have a much greater acceleration then the parts farther away.
  8. Jul 17, 2012 #7


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    Perhaps I can help. Let's look at astronauts in orbit. Let's say one orients his feet towards the Earth and his head away from it. His head will be attracted less strongly than his feet will. This of course is obvious if we look at the math for gravity, as it falls off with distance. So the 5-6 feet between his feet and his head mean that gravity gets weaker by a very small amount at his head than his feet. This difference in strength is called a gradient. The strength of gravity is so low for the Earth that the gradient is not noticeable to people, so for nearly all purposes we can simply say that it is a uniform field and ignore the gradient in every day life. Also, remember that gravity attracts ALL of your body, so in a uniform field every single subatomic particle that makes up your body would be pulled in a certain direction equally as hard. When a gradient exists the field isn't uniform and will pull parts of your body at different strengths.

    Now we must look at the differences between the Earth and a black hole. First, the Earth is not very massive compared to a black hole, so it's gravitational strength is not very high. Second, a black hole with the mass of a few suns is VERY small. The event horizon is only 60 km across! (That's about 37 miles) In contrast, a supermassive black hole with a mass around 1 billion suns has a diameter of about 10 au. (1 au is the distance between the Sun and the Earth)

    Now why does the size of the black hole matter? Well, it doesn't really. The size is actually the diameter of a spherical event horizon. Remember that event horizon is NOT a physical object. If you were to fall into a black hole you would encounter an increasing force from gravity. At a certain distance this force would be strong enough to keep light from leaving. However if you keep falling in, which of course you will, the force simply gets stronger as you continue to fall.

    Let's assume that the model of a black hole where we have all mass packed into an infinitely small point is correct. For supermassive black holes around 1 billion solar masses that means that as you fall towards it the distance where light cannot escape is much greater than a black hole of around 10 solar masses. Which of course is as it must be if we remember that more mass equals a larger force at any given distance.

    But that doesn't explain the whole story yet. For that we need to discuss the gradient. Now a gradient, as I said before, is a difference in strength between two points in a field. (Such as a gravitational field) The thing about a gradient is that it is steeper as we get closer to the singularity. (that means that the difference in strength between points of equal distance is greater as we get closer) For a supermassive black hole we are VERY far away from the singularity when we pass the event horizon, so the gradient is very shallow, like walking up long slope to the top of a hill. For a small black hole, since we are much much closer to the singularity when we pass the event horizon, the gradient is very very steep, like trying to walk up the steep slope of a mountain. However, if you were to fall into a supermassive black hole, you would pass the event horizon just fine. But remember that the force just keeps on increasing, as does the gradient, as we get closer to the singularity. So when you get very very close to the singularity the gradient would be extremely steep and you would be ripped apart.

    So in either case, falling into a singularity of the type we looked at would eventually result in you being ripped apart.
  9. Jul 17, 2012 #8
    Please also realize that the event horizon is the limit at which gravity exerts a counter accelerating force on light such that light moving at c cannot cross that threshold. Light literally cannot travel fast enough in any vector to not fall back in. This literally means that if two particles interact on the inside of the blackhole and produce a photon and its trajectory was directly away from the center moving at c, it would not pass the event horizon. even if it were produce just on the inside of the event horizon.

    The event Horizon represent a point where the acceleration due to gravity is at least c toward the cent of the gravity sphere.

    You died a long time before you got close to the event horizon.

    g = 9.81 m/s^2 = 32.2 ft/s^2 as felt at the surface of the planet.

    An astronaut in orbit actually feels around 90 percent of the gravity that someone at sea level would experience.

    If you are a fan of Evolution or God either way your body is made to handle about 9.81 m/s^2 = 32.2 ft/s^2 . More than that and stresses to your body start to slowly kill you. Your heart is only so strong. Blood pressure is a concern.

    Basically if you are anywhere near a black hole you died before you got to the singularity -- not because of gravity gradients but because your blood was to heavy to move through your body. remember a few thousand miles from a supermassive black hole light can escape -- but that does not mean the gravity is 'weak'
  10. Jul 17, 2012 #9


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    If you die, acceleration is not the reason.

    Your body is able to handle the corresponding force from the floor. Without floor, there is no force to handle. While this has other issues you can observe in space travel, the gravitation does not influence the body at all in free fall. In fact, this is the central assumption for General Relativity: You cannot tell that you are in a gravitational field, unless you observe some events elsewhere.
    Even behind the event horizon, an astronaut is in free fall, similar to astronauts in the ISS. They won't see the difference, unless gravity gradients become significant.

    Unrelated: The central black hole in the Milky Way has a Schwarzschild radius of about 10 million kilometers. It is wrong to simply divide this by the speed of light, but it should give the correct order of magnitude: You would have something like 30 seconds inside (+- factor 5), until you hit the singularity.
  11. Jul 17, 2012 #10
    you are assumming you are in free fall -- that was not stated in the problem description.

    However to be in free fall close to a blackhole you have to be coasting along near c.

    I make the same assumption that the author of the question does... you are on a direct line toward the black hole.

    To orbit in free fall you need to be going at a speed that takes you tangent and would have created a distance to the center equal to that which you would have lost due to gravity. I get it. I understand free fall -- but if you try to move...
    Last edited: Jul 17, 2012
  12. Jul 17, 2012 #11
    You don't feel any acceleration as you free fall. Wanna try? http://www.gozerog.com/
    What you feel is tidal force, which in a nearly uniform g field is nearly zero.
  13. Jul 17, 2012 #12


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    A black hole has no solid surface. Unless you are in an accelerating rocket or similar device, you are always in free fall. And in terms of the accelerating rocket: If this can kill you, it can do so everywhere, the black hole ist not required at all.
  14. Jul 17, 2012 #13


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    I would also note to the OP that the event horizon is NOT a physical thing, and your body (if you close your eyes) just inside and just outside of the EH is not aware of the EH.

    What you SEE, apparently will give you a clue that you are inside or outside but you won't FEEL any different (I'm talking about a small amount inside or outside).
  15. Jul 17, 2012 #14
    How does what you see give you a clue? You don't see the horizon! In any moment you have photons hitting you from every direction. And for the falling observer, the horizon is just a surface moving out at the speed of light, just one among infinite parallel trivial event horizons. There must be another shrinking event horizon staying ahead of the observer, which is the real event horizon for him (particles that fell into the BH too long ago can no more send signals to him and are beyond this horizon). Is there a reason to think something must change in the visual landscape once the EH is behind you?
  16. Jul 17, 2012 #15


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    There's a nifty "video" (action graphic) somewhere on the internet that shows what you see as you fall into a supermassive BH and pass the EH, and it does show a change in what you see. Sorry I don't have a link. Sounds like you wouldn't believe it anyway.
  17. Jul 17, 2012 #16
  18. Jul 17, 2012 #17


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  19. Jul 17, 2012 #18
    Why? I don't have any preconceptions, and my knowledge of the matter is very limited. I just posed a question, sorry if I sounded arrogant. And I don't want to "believe" anything, I want to understand. I think I saw that video some time ago. I'll look for it.
  20. Jul 17, 2012 #19
    Just to confuse the issue about the event horizon because I can't find a definitive answer about black holes.

    Are black holes an actual singularity, or are black holes some kind of thickening "soup"?

    My current visualization is a singularity that forms when the blackout is first created surrounded by something like a Dyson sphere(???) where time & in falling matter basically freezes at the event horizon.
  21. Jul 18, 2012 #20
    Black holes are defined as a region of spacetime.
    This region contains a singularity according to GR, which is often interpreted as the symptom that the description is not completely correct. There's no "soup" in the way GR describes them: infalling matter is obliged to hit the singularity. But who knows what a quantum description of a black hole will look like. Let's wait for the experts' answers about this.
    What has the Dyson sphere to do with it?
    Time doesn't "freeze". Proper time always flows at its own rate, so to say. The "freezing" you mention depends on your choice of coordinates, it only happens in certain (pathological) coordinate frames. Have a look at Painlevé coordinates, which avoid this problem, and then at Kruskal-Szekeres diagrams and Penrose diagrams which are a different way to map spacetime and show clearly why there's no room for a standing "thick soup".
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