What would happen if you could remove the event horizon from Sgr A*? [closed]

Question

There is a mind-bendingly massive black hole at the center of our galaxy, and our hyperdimensional idiot antagonist just dropped his wallet in it. He figures he'll just turn it off for a bit and fish it out, much to the chagrin of - well, everybody? I guess?

The assumptions -

1. The event horizon is gone.
2. Physics works, except for the bit about the event horizon being gone.
3. The reason the event horizon is gone should amount to an application of energy, charge or force that overcame the gravitational force of the singularity.
4. Once the event horizon 'opens', the idiot stops doing the thing.
5. Whatever he did, he does not take measures to undo it directly.

I think it has to unleash a speed-of-causality wall of super-supernova proportions, like when a black hole evaporates below the mass limit. But that's the thing - this hole is so far above the limit, I'm pretty sure it'll burp (very, very loudly) and then immediately collapse again.

So assuming, for instance, he spun this black hole to an absurd angular momentum, what might the outcome be, for the black hole and for the galaxy it anchors? Can the black hole immediately reform, and start reincorporating the lost mass? Does the matter have to finish expanding first? Or is the situation so far out of bounds that the result cannot be approximated?

Answer 1

We cannot say what will happen because one of your assumptions is typically considered to be invalid:

Physics works, except for the bit about the event horizon being gone.

You can't add energy to make the event horizon go away. Paradoxically, more energy means more gravity. You can, however, add charge. The Reissner–Nordström metric defines the behavior of a charged non-rotating black hole. Such a black hole has two event horizons at $\frac{1}{2}(r_s\pm\sqrt{r_s^2-4r_Q^2})$, where $r_s$ is a radius based on the mass of the black hole and $r_Q$ is a radius based on the charge (the exact radii are in the linked article).

Objects with a charge greater than their mass can exist in nature, but they can not collapse down to a black hole, and if they could, they would display a naked singularity.[9] Theories with supersymmetry usually guarantee that such "superextremal" black holes cannot exist.

The other way to get rid of an event horizon is with angular momentum. The Kerr metric describes a spinning chargeless black hole, and it can exhibit the lack of an event horizon under some circumstances, at least mathematically. No such black hole has ever been observed.

The result of stripping the event horizon is a naked singularity, where one can observe mass being crushed to infinite density. This breaks general relativity, and that is our best model as to what happens around such massive objects. In fact, it's so bad that it is hypothesized that there is a "cosmic censorship" which ensures all singularities have an event horizon around them. Loop gravity suggests they may exist, but it is still a "toy" model by comparison with limited evidence to go on.

So the real answer is not only do we not know, we don't have a clue. Science's ability to predict what happens if naked singularities are afoot is currently no better than that of a Zoroastrian priest divining the future. At some point, we may know more.

Answer 2

It would be possible to deform the event horizon of a black hole when you have anti-gravity technology.

The event horizon of a black hole is defined as the points where the gravity becomes so strong that the escape velocity exceeds the speed of light. If you could counteract the gravity with an anti-gravity field, then the event horizon would retract. When you have enough anti-gravity power to reach the singularity at the center of the black hole, you could retrieve the mass which entered it.

However, modern physics are not yet able to fully explain in what kind of state that matter would be. The answer by Cort Ammon goes into more detail in this regard. If you dropped your wallet in a black hole, then it is unlikely that it would still be recognizable as one.

Answer 3

Very Little for Earth: Our hero's biggest worry is the planet/spaceship they disembarked from to retrieve their wallet being flung out into space.

Removing the event horizon means you lower the force of gravity. Let's look at an extreme case. Suppose the whole black hole disappears for a few days and then reappears. The lack of gravity propagates across the galaxy at the speed of light. That takes $100,000$-ish years to reach the outer rim and $30,000$-ish years to reach the Earth. It is followed by a wave of normal gravity when the blackhole reappear.

The general effect is things move to wider orbits of the black hole. See this question for a similar scenario where the Sun is removed for a few days. The conclusion was the orbits change very litttle in the grand scheme of things.

The Sun weighs 1 solar mass and Earth is about 9 light minutes away. The order of magnitude is $10^{-5}$ and the ratio of mass:distance is $1/10^{-5} = 10^5$. That ratio determines how big of an effect the event causes. Larger mass means bigger change and larger distance means smaller change.

Saggitarius weighs on the order of $10^6$ solar masses and our solar system is about $30,000$ light years away. On the order of $10^4$ light years. The ratio of mass:distance is $10^6/10^4 = 10^2$. Compared to last time the effect on the solar system is $0.001$ of before.

Of course objects closer to the center experience a much bigger effect. Some of them have orbits changed dramatically. Though all less than a few light days which is minuscule on the galactic scale. Even closer objects fall out of orbit and are hurled outwards from the galactic centre. An errant star could cause some damage to whatever it passes. However it most likely won't pass close enough to do much harm, and will take thousands of years to clear the galaxy.