Why can’t you fall into a white hole? Part II: Fire, Aim, Ready!

Continued from Part I.

So, the question is, what happens if you shine your flashlight into a white hole? Let us look at the black hole again, first. Shining light into a white hole is equivalent to shining light out of a black hole. Unfortunately, you cannot shine light from out of a black hole! Not from inside the horizon, anyway. But you can, from just outside.  As far as an outside observer is concerned, it takes a very long time for the light to get out far from the gravity well. Just like it takes a long time for the light going in to get very close to the horizon. Light also blue-shifts a lot as it climbs out, just like it red-shifts when going in.

So it takes almost forever until you can see something coming out from near the horizon. But how long does it take for that something to get out by its own internal clock? After all, if you are falling into a black hole, you can cross the horizon pretty quickly by your own clock, even though it looks like it takes forever to anyone who is watching you fall in. And the answer is rather surprising: it takes exactly the same time to get out as it takes to fall in, assuming you have the same speed after you come out as you had when starting to fall in.

This is pretty easy to understand in the familiar Newtonian case: a ball bouncing against the floor doesn’t take any longer to bounce up than to fall down. The situation is the same in General Relativity: the bounce takes just as much time as the fall, if there is no energy loss. Of course, it is much harder to construct something to bounce off if you are dealing with an object so dense, it is on the verge of collapsing into a black hole. The only known objects with a hard surface which almost as dense as black holes are neutron stars. And you cannot bounce a ball off them, not really.

Now, back to the white hole case. What’s the counterpart of an object falling into a black hole? It’s an object coming out of the white hole. Nothing really special there. Just like something thrown into a black hole appears to merge with the horizon for an outside observer while crossing through the horizon and hitting the singularity shortly afterwards by its own clock, an object emerging from the white hole’s singularity and being hurled through the horizon and into the outside world, while appearing to slowly unglue from the horizon and zip outward.

Now consider the opposite case: an object leaving a black hole from close to the horizon, straight up, without engines. It is quite possible, if the object has enough speed to achieve the escape velocity. To escape, it would have to start traveling close to the speed of light relative to something hovering just above the horizon. Or it can be a light ray emitted by that something.

So far, so good. Now, again, let’s consider the time-reversed situation: a white hole and an object or a ray of light falling into it. To the outside observer, the object disappears near the horizon, just like it did in the case of falling into a black hole. But what happens from the point of view of the object itself? Remember, it takes as long to reach the white hole horizon as it takes to climb out of the horizon of a black hole, which is also as long as the time to fall to the horizon of a black hole. So, an person falling into a white hole will reach its horizon after a fairly short time… and then what? By definition of the white hole, which is a time-reversed black hole, nothing can fall through its horizon, no matter how hard it tries. But… what happens to that poor schmuck who reaches the white hole horizon at full speed and has nowhere else to go? What prevents him from going forward? Join us next time, for a surprising answer!

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3 responses to “Why can’t you fall into a white hole? Part II: Fire, Aim, Ready!”

  1. Warg Franklin says :

    Ok again I’m taking the skeptical position here:

    >Just like something thrown into a black hole appears to merge with the horizon for an outside observer [taking infinitely long to do so]

    Does this not imply that a black hole can never form? If, from the outside perspective, nothing every crosses an event horizon, how does the horizon go from not existing and enclosing no matter, to existing and enclosing matter?

    I’m going to thus take it that black holes can’t exist in reality.

    Anyways, as for our alternative universe where we’ve defined our white hole/black hole into existence, don’t they simply look the same, from the outside?

    Both are just a superdense region of spacetime thus having curvature such that from the outside, nothing can ever fall in, because it takes forever to get to the event horizon, and nothing can ever come out, because it takes infinite time to come from the event horizon.

    So black holes and white holes are the same thing, no? Is there any observable difference?

  2. Edge of Gravity says :

    > If, from the outside perspective, nothing every crosses an event horizon, how does the horizon go from not existing and enclosing no matter, to existing and enclosing matter?

    Relativity is very counter-intuitive this way. The event horizon does not exist for an outside observer because it’s not a slice through space that exists for all time (or until destroyed) once formed, like, say, the earth surface. It’s more like an expanding sphere of light from a single flash in all directions. Only, because of gravity, the expansion rate of this sphere of light is zero, so no outside observer can ever see it. But any observer crossing this sphere inwards would see it expanding outward at, well, the speed of light. Hence they can never catch up and get out.

    > So black holes and white holes are the same thing, no? Is there any observable difference?

    There is no observable difference in a “quiescent” state, no. But no black hole can emit light/mater, and no white hole can absorb light/matter. So, if you notice that something falls into a supercompact object, it must contain a black hole, and if something comes out of a supercompact object, it must contain a while hole.

    If you know that the object was formed by stellar collapse, then there is definitely no white hole there. We do know as much about most stellar-sized holes, but we are not 100% sure of the origins of the black holes in the centers of galaxies, there is an outside chance that they formed during the Big Bang, and potentially also contain, or once contained, a white hole.

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