That is right. Such a black hole could be used to power things far away from stars, e.g. interstellar spacecraft .OmCheeto said:A billion tonnes yields a black hole with a lifetime 200 times that of our universe.
No, the Planck energy should be sufficient. It just won't last long.OmCheeto said:It would take a billion tonne equivalent energy to smoosh two protons into a black hole.
There are two problems with the idea that low mass black holes are not affected by gravity. First, if we stick to the Newtonian model, free fall acceleration is unaffected by the mass of the falling object. Decrease the mass of the object and you reduce the gravitational force, but you also reduce its inertia. The result is a wash. Second, if we adopt the idea that gravity is geometry and that freely falling objects follow geodesic paths then the mass of a falling object is largely irrelevant to the path it follows through space-time.thetrellan said:Without enough mass, it should neither suck matter in nor be sucked to gravitational centers. No gravity, no force.
A small mass doesn't mean zero mass and a black hole can get very close to objects (there is nothing that would repel it).thetrellan said:As an average guy with an average education, I have trouble buying all of this 'embedded in matter' stuff. Black holes are dangerous because of gravitational attraction, right? Its gravity pulls things in. Without enough mass, it should neither suck matter in nor be sucked to gravitational centers. No gravity, no force. Why should being embedded in matter change that?
Yeah, but aren't we talking something on a subatomic scale here? At that size, surely even something as exotic as a black hole would behave differently, perhaps even behave like subatomic particles do. Do things on that scale still react normally to gravity?mfb said:A small mass doesn't mean zero mass and a black hole can get very close to objects (there is nothing that would repel it).
The model that predicts that black holes exist (i.e. General Relativity) predicts that they are affected by gravity. So yes, they do still react normally to it.thetrellan said:Yeah, but aren't we talking something on a subatomic scale here? At that size, surely even something as exotic as a black hole would behave differently, perhaps even behave like subatomic particles do. Do things on that scale still react normally to gravity?
thetrellan said:aren't we talking something on a subatomic scale here? At that size, surely even something as exotic as a black hole would behave differently, perhaps even behave like subatomic particles do. Do things on that scale still react normally to gravity?
True. But why should such a thing grow, if what makes it do so is its intense gravity? There is no reason to think it would ever find the center of the planet, either, when not even light does this. The only particle I know of that can move through solid matter is the neutrino. I admit I don't know much about them, and that what I do know is pretty suspect. But they go clean through planets, don't they?PeterDonis said:It depends on what "subatomic scale" theory of physics you are talking about.
Classically, everything that has energy responds to gravity. That's what our best classical theory of gravity, General Relativity, says. Black holes have energy (because they have mass), so they respond to gravity according to this theory.
If you want to bring in quantum mechanics, as your allusion to different behavior on the "subatomic scale" suggests, then we don't know the answer for sure, theoretically speaking, because we don't have a quantum theory of gravity. But all the indications we have are that the rule from classical GR will still hold: everything that has energy will respond to gravity. So even with quantum effects included, there is no reason to think that a very small black hole will not respond to gravity.
Experimentally, as far as we can tell, gravity does affect subatomic particles, or at least their energy levels:
https://www.nature.com/news/bouncing-neutrons-probe-dark-energy-on-a-table-top-1.15062
I think the operative word is "normally". Meaning it lacks to mass to grow the way a naturally formed BH would, and so would behave like any other subatomic particle. There is no event horizon, because it hasn't the mass to attract inescapably. In fact, isn't that why they dissipate? Not enough mass to sustain an event horizon?jbriggs444 said:The model that predicts that black holes exist (i.e. General Relativity) predicts that they are affected by gravity. So yes, they do still react normally to it.
There's no such thing as not having the "mass to attract inescapably". If the mass is non-zero the Schwarzschild radius is non-zero; if that mass is all contained within the Schwarzschild radius the event horizon will form and nothing at the event horizon will be able to escape. Whether the hole evaporates or grows depends on whether it is "hotter" than its surroundings, which determines whether it loses more energy by Hawking radiation than it absorbs frames from outside.thetrellan said:I think the operative word is "normally". Meaning it lacks to mass to grow the way a naturally formed BH would, and so would behave like any other subatomic particle. There is no event horizon, because it hasn't the mass to attract inescapably. In fact, isn't that why they dissipate? Not enough mass to sustain an event horizon?
How about the crust of the Earth? As statedmfb said:A small mass doesn't mean zero mass and a black hole can get very close to objects (there is nothing that would repel it).
Schwarzschild radius. This is what I needed to understand. So basically any matter that collapses has an event horizon where the original diameter was, and nothing can escape crossing that. Thanks.Nugatory said:There's no such thing as not having the "mass to attract inescapably". If the mass is non-zero the Schwarzschild radius is non-zero; if that mass is all contained within the Schwarzschild radius the event horizon will form and nothing at the event horizon will be able to escape. Whether the hole evaporates or grows depends on whether it is "hotter" than its surroundings, which determines whether it loses more energy by Hawking radiation than it absorbs frames from outside.
What we don't know, because we do not have a complete theory of quantum gravity at extremely small scales, is what if any as-yet-undiscovered physics might show up at these scales. However, it is somewhat pointless to speculate without a candidate theory that will make quantitative predictions.
Once in a while matter falls in, that makes it grow. Where "it" is a hypothetical stable microscopic black hole, so we are in a science fiction scenario.thetrellan said:True. But why should such a thing grow, if what makes it do so is its intense gravity?
A black hole has nothing in common with light. It would be in free-fall to a very good approximation (see above: There is nothing stopping it motion), only deviating from that when a particle falls into the black hole. This process makes it lose kinetic energy over time and it settles in the core of Earth over time.There is no reason to think it would ever find the center of the planet, either, when not even light does this.
Neutrinos at not too high energies can go through planets, sure, but they are not the topic here.The only particle I know of that can move through solid matter is the neutrino. I admit I don't know much about them, and that what I do know is pretty suspect. But they go clean through planets, don't they?
No, the event horizon is much smaller.thetrellan said:So basically any matter that collapses has an event horizon where the original diameter was, and nothing can escape crossing that.
jbriggs444 said:A small black hole dropped from rest on the surface of the Earth will fall toward the Earth's center. Being subject to negligible resistance by the Earth's crust, mantle or core, it will enter a very eccentric orbit about that center with a period of around two hours and an apogee at the Earth's surface. [At least if we hand wave away the likelihood that it will evaporate first].
mfb said:Where "it" is a hypothetical stable microscopic black hole, so we are in a science fiction scenario.
thetrellan said:why should such a thing grow, if what makes it do so is its intense gravity?
thetrellan said:There is no reason to think it would ever find the center of the planet, either, when not even light does this.