# Monkey Hunter demonstration in GR

1. Feb 9, 2010

### flatmaster

I'm not sure if this has been discussed before.

We all know about the Monkey Hunter Demonstration. Assuming a uniform gravitational field, a projectile shot directly at a target will strike the target as long as the target has zero velocity at t=0 and both the projectile and target begin to accelerate under gravity at t=0. Of course, the physical demonstration is a hunter shooting a monkey with the monkey dropping from the branch the instant the gun is fired. I think we worked through the math in undergrad.

My question is how would this thought experiment change within GR?

Also, Would this experiment be possible? Is there any place close enough to the GR - classical barrier that has a huge, but classically uniform value for g over the three spacial coordinates? Is there a place where you can say F = mg*yhat with a large value for g?

2. Feb 9, 2010

### cesiumfrog

How do you propose to define "directly"?

(And will you be symmetric in applying the "uniform" requirement?)

Last edited: Feb 9, 2010
3. Feb 9, 2010

### bcrowell

Staff Emeritus
Totally cool question!

Seems to me that the only sensible way to define "directly" is that the gun has a laser sight, and the laser sight is making a red dot on the monkey's chest.

Suppose the gun is located outside the event horizon of a black hole, at a radius the permits circular orbits for rays of light. Then the monkey could actually be right *behind* the gun, but the laser sight would be hitting the monkey in the chest.

Now you run into a problem with defining the notion of dropping the monkey simultaneously with the firing of the gun. But GR does allow simultaneity to be well defined for spatial positions that are close to one another, and the monkey is close to the gun -- it's right behind the gun.

So you fire the gun and drop the monkey behind it simultaneously. Clearly the monkey will disappear below the event horizon very quickly, and so will the bullet.

Well, the bullet is certainly going to hit the monkey, because they will both impact on the singularity.

4. Feb 10, 2010

### Physics Monkey

Save the monkeys! :tongue2: