Question For Physicists - May Be Kind Of Weird...

Okay. My grasp of Physics is less than optimal but I seem to recall that Einstein said that sufficient quantities of mass would “warp” space-time. Please correct me if I’m wrong.

Now then, would the opposite be true also? If so, what would happen if it were true?

Another weird-ass question - if it were possible to suspend a hemisphere of some exoric material around just half of a black hole so that it stayed tethered a certain distance away, would it act so as to force the opposite side of the black hole to push in the direction of the material? And, if so, what would be the result of, say, four of them being pushed so that they would arive at the same point at the same time.

I know this seems like a couple of ridiculous questions but hey, I’mk not a physicist :slight_smile: Although I have been trying to retrain myself in mathematics…

Let me rephrase that first question…I believe it was that mass would curve space-time. If so, what would compacted space-time act like?

While it’s my brother who is the post-doc physicist rather than myself, I think you need to elaborate more thoroughly. Your question simply does not make sense. If I were to read it literally, it would simply be another gravitational source and so complement the singluarity at some points, and negate it at others, with varying degrees inbetween. And you’d need an interesting device to prevent the two from merging.

But I sense that this is not what you’re after

if it were possible to suspend a hemisphere of some exoric material around just half of a black hole so that it stayed tethered a certain distance away, would it act so as to force the opposite side of the black hole to push in the direction of the material?

You mean would it make the black hole ‘blow’ in that direction instead of ‘suck’?

The problem is that a black hole doesn’t really suck. A vacuum cleaner sucks because it has a motor in it that drives a fan that moves air. When you move air out of a space, other air and stuff (dust, cat hair, cracker crumbs, bugs) in the vicinity rush in to fill the space. ‘Nature abhors a vacuum’ is just a way of saying that if there’s no wall to separate regions of different air pressure, the pressure will equalise between those regions.

Gravity is not the same thing as sucking. A massive object pulls stuff towards it because of that curving of space, or so it’s thought. If you put a bowling ball on a trampoline, it curves the surface. Anything put on that surface will slide or roll towards the bowling ball. If you shoot a marble from one edge of the trampoline towards another edge, and it crosses the part that was curved by the bowling ball, its course will be changed. (That’s how you use a big planet like Jupiter to change the course of a spacecraft that you’re trying to send out of the solar system-- it’s a tried and true method.)

Now then, would the opposite be true also? If so, what would happen if it were true?

Opposite of what? You mean, can you make a mass more massive by curving space-time around it? It wouldn’t be ‘compressing’ the mass or making it denser. A larger mass makes a steeper curve, so you’re asking if an induced steeper curve would make the mass heavier.

If you hook a fishing hook into the trampoline directly under the bowling ball, and pull down on the fishing line, you will artificially curve that space. Any marble on the surface of the trampoline will see a greater curve than would have been there from an unassisted bowling ball. From the marble’s perspective, the bowling ball will look more massive than it is. The marble will have no way of knowing that the bowling ball’s mass hasn’t changed and that it has outside help. :slight_smile: If the marble can observe the size of the bowling ball as well as how much it curves space, it might think the ball’s density has increased, since it’s mass has increased but its size has not.

I should have added, if it isn’t obvious, that you can’t redirect gravity the way you can redirect air flow to change the direction of suction.

Okay, since I seem to not be asking this right, let me re-rephrase…

If it were possible ot have a hemisphere of aome sort of exotic matter that would reflect all forms of radiation, and this were placed at a certain distance from a black hole so that all the radiation of whatever form as redirected inward, would that make said black hole move? If so, what then would happen when four of said black holes were to meet at the same exact point in space?

Granted, I realize that no forms of radiation can escape a black hole but the surrounding area of space would pretty much have to be teeming with various radiations. It would seem to me that if my hypothetical matter shield coould be tethered a fixed distance from said black hole, then the actio-reaction would cause the radiation that was hitting the shield to act as a sort of low-grade propulsion device. Since the shield would be in a fixed state in relation to the black hole (by whatever esoteric means) would this cause the black hole to move?

Christ, I feel like I’m asking for a short lesson in gradute physics…

Stated another way, it would seem to me that a shield of the type that I suggest would be usefull in moving stars. Radiant energy moves out from the star only to hit the shield and then be reflected back into the star. Would this cause the star to move towars the shield since the radiant pressure would be trying to cause the shield to move away from the star?

Now just substitut black hole for star :slight_smile:

And for ghods sake, please ignore my many spelling errors in the preceding posts.


Any amount of mass will cause the intrinsic geometry of spacetime to change. (warp)

You can’t have warped spacetime without something to warp it. As A.J. Wheeler says:

All that would happen is the black hole and the hemisphere would be attrated to each other. If the hemisphere were to be somehow truly suspended then the hole would move towards it and engulf it.

Oh! I get it… you want to take advantage of the radiation coming out from the space around a black hole to push an object.

Well, what kind of radiation is near the black hole… there’s two main things: (1) Hawking radiation, which is the process by which the black hole itself can actually evaporate, but this is incredibly slow; and (2) x-rays and other high energy radiation from the accretion disk, or the stuff swirling around the black hole really fast (the x-rays are part of how we can observe black holes from here).

If you put your ‘sail’ at a distance from the black hole where the outwards force due to radiation pressure was greater than the inwards force due to gravity, sure.

Solar sails have already been proposed, where you use the radiation from the Sun to push you out further into the solar system. Your spacecraft just has to be really lightweight in order for it to work. But we know that the Sun puts out an awful lot of energy… the question would be if the energy (in the form of radiation) from the accretion disk is strong enough that it can push you away stronger than gravity can pull you in or make you go into orbit.

Depending on the radiation strength at that distance, you might not need a giant hemisphere… you could use a little sail. Actually, a giant hemisphere would be bad, because if the black hole were at the centre of where the sphere would be (if it were a whole sphere), the radiation on different parts of the hemisphere would be pushing in different directions. You want a little sail that is small enough that all the radiation hitting it seems to be coming from the same direction (which is what happens when you’re at a great distance from a radiation source, as with the Earth and the Sun).

Fine. Let’s use a flat-plane-sail. Work the same thing from there. Better yet, how about a malformed hemisphere…sort of a flattened one so that more of the radiation hits ans can therefore be turned into usefull work.

Regardless of this though, my big question is still whether or not we could move a black hole. AND if so, what then would happen if we were to force four black hole into each other at the same instant. Better yet, let’s make it six black holes. One from each cardinal direction, i.e., north, south, east, west, left, and right. Since we now have one coming from six basic directions, I could possible see energy being released in the vectors not covered by the incoming black holes but what would happen at the point of impact?

It’s posible that black holes collide. Some galaxies (our own included) have supermassive black holes in their cores, and I think they can collide when two such galaxies interact. As far as I know, they just become a bigger black hole.

I’ll be talking to an extragalactic astronomer tomorrow (my prof, his assignments are confusing me) and I’ll ask him. :slight_smile: