# Using a black hole as a weapon

1. May 14, 2010

### Dr Chaos

Would it be possible to use a black hole as a weapon?

What I was thinking is that you could use a very powerful gun and some very dense material to make a black hole projectile. All you would need is a bullet in the shape of an elongated sphere with some extremely dense material inside. What would happen, is that the bullet would be fired at such a speed that it lorentz-contracts into its Schwarzchild radius and becomes a very micro black hole. Is this idea plausible? I am quite sure that you cannot move a black hole but can it maintain original momentum? What it is I am struggling with is trying to find the perfect ratio of speed (and lorentz factor) to mass (and lifetime).

Useful formula:

$$\gamma \equiv \frac{1}{\sqrt{1 - v^2/c^2}}$$

Where M is mass in grams.

Last edited: May 14, 2010
2. May 14, 2010

### tiny-tim

Hi Dr Chaos!
The bullet's width stays the same.

3. May 14, 2010

### DocMuffinZ

Not a good weapon, even theoretically. Try not to hurt your back loading a bullet that massive XD

4. May 14, 2010

### Jonathan Scott

No, it isn't plausible. It's the Schwarzschild radius in the bullet's own rest frame which matters.

The total energy in a projectile moving near c is essentially equal to whatever kinetic energy you put into it (to the extent that the rest mass can be ignored). Even if you had the means to make the projectile into a micro black hole that would make it a LESS effective weapon, as it would minimize its cross-section to the extent that it would do much less harm than the original bullet, passing right through most materials.

5. May 14, 2010

### Count Iblis

I agree with Jonathan, you really have to somehow compress the object to within its own Schwarzchild radius. Now, if you make a micro black hole, then that black hole will emit lethal amounts of Hawking radiation, so it could be a used as a weapon...

The momentum will actually not stay zero because of the stochastic nature of the emitted radiation. So, while the photons are emitted with equal probability in all directions, you will have statistical fluctuations. The black hole will thus move like a random walker.

6. May 14, 2010

### PaulS1950

From an action / reaction point of view this would never work.
If your "gun" weighs as much as the black hole you are going to shoot then the black hole will move at 1/2 the expected velocity and the gun will move in the reverse direction at the same speed. If you are talking about just 3000 fps then both the gun and bullet would move in opposite directions at 1500 fps.
How would you make a gun that has the weight of a black hole? what would you use to propel a bullet that absorbs all matter and energy that gets close enough to act upon it?

Not even conceptually possible.

7. May 14, 2010

### Dr Chaos

I know that; that is why I said the bullet would be in the shape of an elongated sphere - so it lorentz contracts into a spherical shape.

Now, for the mass of the bullet and its practicality, here is what I propose:
The bullet is fired at an extremely high speed. Thus, the mass increases by a substantial amount while the bullet is moving and the Schwarzchild radius is then increased. This is good because it doesn't really need to have a ridiculously large mass to begin with.

I just need to find some plausible values to satisfy all of this.

8. May 14, 2010

### Jonathan Scott

Making something move very fast has no effect on its self-gravitation in its own rest frame. Lorentz contraction is only from other viewpoints, not its own viewpoint.

9. May 14, 2010

### Dr Chaos

So you can't lorentz-contract something to make it more dense?

10. May 14, 2010

### IcedEcliptic

If you had such energy at your disposal, kinetic weaponry is best, for explosion, and depending on the material, radiation.

11. May 14, 2010

### IcedEcliptic

Perhaps if you spun it while compressing it from all angles? It would take unthinkable energy. Perhaps you create MBHs with an accelerator, and use some kind of Alcubierre drive mechanism, to move the hole and fire that?

12. May 14, 2010

### DaveC426913

Yes, but as with all solutions that are going to come out of this, once you're harvesting the vast enegies required, why bother with the BH? Just turn your gun on your target and open er up. Whatever's in there to tame a BH is surely enough to put a hole in someone's gunwale.

13. May 14, 2010

### IcedEcliptic

I agree, this is why I first said: "If you had such energy at your disposal, kinetic weaponry is best, for explosion, and depending on the material, radiation."

14. May 14, 2010

### DaveC426913

Yeah. That was aimed at the OP, not you.

15. May 15, 2010

### Jonathan Scott

Correct.

16. May 15, 2010

### LURCH

Try this simple thought experiment:

Start with a neutron star that just barely lacks enough mass to become a black hole. Accelerate that object to near light speed, and its apparent mass is multiplied thousands of times. The neutron star now becomes a black hole with a an event horizon at a certain radius, R.

Orbiting around that neutron star there is a satellite with a highly elliptical orbit. This elliptical orbit goes from ½R out to 2R. You now have a satellite repeatedly falling into and then exiting from the event horizon of a black hole. This is a situation that cannot exist in the real world, and it is the situation that must necessarily exist if one could turn an object into a black hole by accelerating it.

Therefore, one cannot turn an object into a black hole by accelerating it.

17. May 15, 2010

### Dr Chaos

Is it not the behaviour of the satellite that is impossible? Why does it exit the event horizon? Also it would be extremely difficult to maintain an orbit like that at such speeds anyway. I don't understand how you can make something smaller and increase its mass without altering its density.

18. May 15, 2010

### IcedEcliptic

Make it smaller yes, but that requires adding energy to compress. Contraction due to acceleration is a matter of the frame of reference, I think. You need more than a relativistic effect: added mass/compression.

19. May 16, 2010

### LURCH

It is indeed the behavior of the sattelite that is impossible. But keep in mind that all motion is relative, so if you are travelling past the nuetron star at near lightspeed, that is the same as saying the star is travelling past you. So it is no m,ore difficult to maintain the orbit "at such speeds" than it would be at rest, because the star is in fact at rest in its own refference frame.

So again, the sattelites behavior is impossible, and would inevitably arrise from the original premise. Therefore, the premise is incorrect.

20. May 17, 2010

### Dr Chaos

So is there a relativistic Schwarzchild radius for a moving object? If our sun is accelerated to 0.9 c, what is its Schwarzchild radius?