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Loop of pumped fluid slowly falling into black hole.

  1. Dec 10, 2012 #1
    You have this loop of water (or some other "incompressible" fluid) with a nuclear powered water pump that constantly pumps the water in a circle. This loop is slowly lowered near the event horizon of a black hole, lets even make it a super duper massive black hole to minimize tidal effects, it's "slowly lowered" so we don't have a super fast moving loop of water, and my pump is nuclear powered so it can pump against massive head pressure :)

    So what happens when part of the loop crosses the event horizon? Specifically what does the fluid do?

    Now obviously the fluid can't get pumped back out past the event horizon, and in an ideal case fluid wouldn't be pumped into the now missing part of the loop because the fluid would occupy that space and there'd be no way to put more in there... however since it's within the event horizon no information about there being fluid can make it back outside, so how would the fluid outside of the event horizon "know" that it can't go into that other section of pipe. I know that it would be pulled into the SMBH eventually, but what happens to the fluid which the pump is trying to move through the loop.

    Reason for question:
    I was watching Stargate and was thinking about this and all these shows that have this "portals" to move you from one location to the next, more specifically that these portals are one way only (yeah you can go both ways in Stargate but you have to reopen it from the other side IIRC). So I started wondering about how that would affect the physiology of life going through these one way portals, now it's glossed over as "fictional" but still had me wondering, blood is pumped in different directions in the body, nerve endings send electrical signals in different directions too, and blood (or electrical signals) pumped "forward" in your body wouldn't be able to come "back" until you completely went through said portal. So rather than come up with this question, I simplified using the only one-way portal I know of (event horizon) and used a loop of fluid.
  2. jcsd
  3. Dec 10, 2012 #2


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    If you somehow manage to support the outer parts, the loop will simply break apart. There is no way the parts inside can be supported from outside. You don't need a fluid, the pipe along will give the same result. Once the pipe is broken, fluid can flow into the BH at both sides.

    You can get a pipe through the event horizon, if you just let it fall (and hope that tidal forces are small enough).

    That is very problematic, as molecules in our body constantly randomly move in all directions due to their temperature. For water, this is about ~1km/s. You always have atoms going "back" in your body, unless you are very quick compared to the gate. This does not even take the electromagnetic force into account which allows the formation of those molecules.
  4. Dec 10, 2012 #3
    Well I don't mean you're physically trying to "hold" the loop of fluid outside of the event horizon and prevent it from falling in. I simply mean slowly lower it to the point just outside then let go and let gravity do the rest. Being as it'd "slowly" get pulled in, I was trying to imagine it pumping water in a loop outside of the event horizon, then as soon as one part of the loop entered what would happen to the pumping of fluid.

    This is an angle that I didn't consider, if a pipe can in fact fall through (in one piece) then obviously electromagnetic forces are still holding the pipe outside the event horizon to the inside as it's being pulled in, so in fact information is being transferred between molecules just inside the event horizon that says "hey I'm here and electromagnetically attracted to you" and molecules just outside the event horizon saying "yup I read you loud and clear and am still connected to you". If this is the case, then there is information coming out of a black hole such that the water in the loop outside would in fact know there's no where to go and would stop flowing. I'm guessing because one of the properties of a black hole is it's charge it can in fact convey information outside when it relates to charge...
    *sigh* ok my head hurts and I still need to write a final for one of my astronomy classes.

    Hence why I tried to simplify my question with a loop of water and a black hole. Although I'm sure with a black hole you can't neglect time dilation effects and what not. Plus it IS fictional in the first place and maybe shouldn't be thought about too much :D
  5. Dec 10, 2012 #4
    I think it's an interesting question even if the formulation is a little.. odd.. ;) Why not simplify. I'm inside the EH of an SMBH holding one end of our carbon nanotube rope, you're on the outside holding the other. I give it a yank. Do you feel it?

    The science says "nope" so the question is, what happens to the rope. Does it just stretch or break apart?
  6. Dec 10, 2012 #5
    An object crossing the event horizon of a black hole must be traveling at C relative to the surface of the event horizon so there is no such thing as slowly crossing the event horizon. Imagine a space ship falling in nose first. There is a light on the nose set to come on when its computer calculates that it has crossed the event horizon. We know that the light cannot escape the event horizon so we know that an observer on the tail of the craft must be inside the E.H. by the time he sees it. We also know that the speed of light measured by a scientist on the ship must be C in his reference frame. Put together, these facts mean that the time for light to travel from the front of the craft to the rear must equal the time it takes for the rear of the craft to get to where the front was when the light came on.

    Now we all know that is impossible because nothing can travel at C. It won't make sense until you re-visualize motion through space and time as a single path through 4-space. Imagine a clock moving through 4-space, leaving an imaginary line behind it. Every time the clock ticks it leaves a dot on the line. The length between the dots is always the same. If the dots are at the same x coordinate they are 1 second apart on the t coordinate, if they are 300,000 km apart on the x coordinate then they are at the same t coordinate. If both the x and t coordinates change then the Pythagorean theorem describes the relationship between how much they each change.

    Intuitively we separate time from space and measure time in seconds and space in meters, but this distinction is purely artificial. 1 second actually does equal about 300,000,000 meters. Seconds and meters are different units for the same quantity.

    Gravity twists these coordinates so that what looks like a space coordinate from a distance (like distance from a black hole) becomes a purely time coordinate at the event horizon. At that point the ship is not actually moving toward the black hole at 300,000 km/s which would be impossible, it is moving toward it at 1 second per second. The B.H. is no longer in front of the ship, but in it's future.
    Last edited: Dec 10, 2012
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