Relativity and Cosmology

Ignoring the fact that black hole physics, negative energy, and well just about every theory regarding black holes has yet to be proven (or for that matter be agreed upon), I fail to see how this sort of structure could possibly ever get even close to breaking even when all factors are taken into account, for example - the vast resources required to make such a structure, or how about even the fuel energy required to get the materials to the black hole. It just could not be made practical.

There is no free lunch, and getting someone else to pay for said lunch does not make it free (i.e. someone still has to pay for it).

Hi Jack, agreed, but it's fun to speculate - how about mining the resources from a handy planet in the vicinity?

One can then ask, why not just colonize the planet? Problem is, us humans will pollute it so much in time that we'll have to keep on looking for new planets to "cannibalize".

Regards, Jorrie

BOY; Every body keeps telling me running a car on free energy or Hydrogen can't be done. Free energy ,running cars on water, and over unity have been or are being, and or will be done long before this pipe dream ever takes off. If they're not being done then somebody isn't paying attention to the things that need to be done here and now. The designers of this pipe dream would do well to come back down to earth and do something constructive in this relm.

The only thing holding us all back from most of our pipe dreams isn't energy production, it's energy storage! We got all kinds of ways of generating energy but we are seriously lacking in the storage department. The best energy storage technologies we have so far are all chemical devices but the nulcei of atoms can hold far more energy than their electron orbitals. A real energy storage breakthrough would be to find a safe and reliable way of pumping large amounts of energy into atomic nuclei and then retrieving the energy as needed. I'm not talking about anything as extreme as nuclear fission or fussion here. We have atomic battery technology but, it's not practical. Maybe some kind of microchip scale nuclear magnetic resonance (NMR) technology would do the trick. I mean, if we could do this then every electrical device in the world could contain it's own lifetime power supply. Even better would be an all-purpose battery that could power anything from your cell phone to your power tools to your electric car. You could keep it on your key chain and take it with you wherever your go. It would be kind of hard to steal someones car when they've got the "gas tank" in their pocket or purse.

My knowledge of black holes is pretty basic. How "big" is this theoretical black hole (golf ball? Jupiter? Sun?). How "big" would the city have to be to be at 1g gravity? I guess this question is similar to the "how much resources" would be required to build this city. Also, I thought that the black hole would consume the garabage, not catapult it, and then get bigger and then bye bye city. It is an interesting thought though.

hi Worden, you wrote: "Also, I thought that the black hole would consume the garbage, not catapult it, and then get bigger and then bye bye city."

The idea is indeed that the garbage is consumed - it's only the empty container that goes back with the extra energy. The other "funny" is that although the black hole consumes the garbage, it gets smaller/lighter because some of its mass is converted to energy!

Regards, Jorrie

The only black holes I know of are the ones that live in the centers of gallaxies. They aren't just wondering about between the stars. That city might then be as big as the "central bulg" of the Milky Way so, it definitely would not be a solid structure as depicted in the cartoon but a Dyson Ring (http://en.wikipedia.org/wiki/Dyson_sphere#Dyson_swarm) of titanic plateforms orbiting the black hole some distance appart from each other. As such, much of the energy derived from the black hole (by whatever means) would need to be used to generate a constant thrust below each platform for gravity.

As I understand it, matter and energy naturally "evaporate" from above the event horizon of black holes. That alone might yield enough energy for us. If not, we could try tapping the two massive jets of particles that naturally spew from the poles. Regardless, by the time we R&D our tech far enough to build such things, there will likely be no such thing as garbage. Indeed, 100% recycling will likely be an essencial prerequisite of such an undertaking. For example, we might need to mine materials from commets, asteroids, moons and small planets to build huge colonial star ships, fly the ships to the black hole and then recycle the ships to build the platforms.

Hi Harbinger, you wrote: "The only black holes I know of are the ones that live in the centers of galaxies. They aren't just wondering about between the stars."

Theoretically, the most common (plausible and usable) type of black holes are the ones "wandering about between the stars". They come from supernovae, where the mass of the collapsing star is from a few tens to a few thousand solar masses. There are more supernovae than there are galaxies which could harbour super-massive black holes in their centers.

It's these "smallish" black holes that may have "hospitable" environments. The smallest black hole that can theoretically exist, after a star has gone supernova, is 1.44 solar masses. At this mass, the mega-city must be at ~4.4 million km from the black hole to enjoy 1g of static gravity (r = sqrt[GM/9.8]).

How "mega" must the city (or at least the girder-work keeping it up) be? Well it will have to be about 28 million km in circumference. Ouch! One can make the circumference smaller by getting closer to the hole and let the whole structure rotate around the hole. It cannot be in normal orbit though, because that will cause weightlessness - probably not good for humans...

Regards, Jorrie

I understand that black holes spew out massive amounts of radiation - X rays and so on. Setting up home anywhere near a black hole would be highly dangerous to human health and we may all end up as deformed mutants in a few generations. The "free lunch" would come with a huge price tag that doesnt sound worth it to me.

Ecologist, writing from a save corner of the Galaxy

Hi Guest, you wrote: "I understand that black holes spew out massive amounts of radiation - X rays and so on."

It is only black holes that are still in their "feeding stage" that spew out massive radiation. It comes from gas, dust, planets, stars, etc. that form an accretion disk around the black hole's equator. The high-energy rays happen when the matter is so compressed that the temperature reaches extremely high temperatures.

Once the neighborhood is swept clean and other matter orbit at safe distances, this "feeding frenzy" stops and the environment is most probably no worse than in other piece of free space. However, the colony should be well shielded, like any long duration spacecraft containing humans.

About the other "implausibilities", as Emjay4119 named it below, well, they are at least based on solid science, whatever that may mean...

Regards, Jorrie

I'm thinking the building phase will be a nice little challenge at least until the ring is closed to balance the gravitational effects.

Hi Emjay4119, yes, a "nice little challenge"!

One way is to construct it far from the black hole and then slowly "rocket" the whole structure into place. Another is to first build the pieces in orbit around the hole and then steer each subsystem by a propulsion system to its "static" position.

Any other ideas?

Regards, Jorrie

I've been thinking the only way to control the Perpetua threads was to use the same method as controlling rumours. If you haven't heard a good one by 10.00am, start one. Well done Jorrie, you must have read my mind. At least this one is interesting and has a whole new range of implausibilities to explore. You'll be able to build it right after you bring John Bradfield back from the dead.

I believe someone said that John Bradford was seen at the Dead Heads last concert!

Jorrie, after all the diversions, back to the science.

If the garbage is fully converted into energy by E=mc² and added to the container's kinetic energy, while the black hole's mass decreases by that same amount, I fail to see how the conservation of mass-energy is satisfied.

Lets say we start with energy E₁ = mc² + Mc², where m is the mass of the garbage and M the mass of the black hole. The kinetic energy of the container is initially zero. After the conversion, we have:

E₂ = 0 + (M-m)c² + mc², where the zero is for the now vanished garbage and the final mc2 is for that energy converted to kinetic energy of the container, giving

E = (M - m + m)c²

= Mc² < E₁

So, the universe has just lost mc² in mass-energy. Or is it me that is missing something?

SL

Hi SL, you wrote:

"E₂ = 0 + (M-m)c² + mc², where the zero is for the now vanished garbage and the final mc2 is for that energy converted to kinetic energy of the container, giving

E = (M - m + m)c²

= Mc² < E₁"

Maybe I have stated things slightly conservatively. The actual final raw kinetic energy gain is double the mass energy of the garbage (2mc²), but obviously all of that cannot be converted into usable electricity. The (perhaps even optimistic) guess is that an advanced civilization may get a 50% yield out of that, i.e., mc².

MTW states it as follows:

"As the garbage flies down the hole, changing the hole's total mass-energy by E_{garbage ejected} < 0..." and

"(Energy gain per trip) = (rest mass of garbage) + (amount, -ΔM, by which the hole's mass decreases)"

I read this as equaling 2mc², because ΔM = -mc², per the above MTW definition. Note that MTW works in units where c = 1, hence mass and energy are in the same units.

Your (SL) equation for the energy conservation then reads:

E² = 0 + (M-m)c² + 2mc² = (M+m)c² = E₁ in your definition, so no lost energy.

Regards, Jorrie