Is G too high?

[SlyReaper]

Been having a bit of a think about the Kerbal universe. How its planets and moons are minuscule yet somehow have comparable surface gravities to real-life planets and moons. Would this be realistic in a universe where all physical laws are the same except for a higher value of G, the gravitational constant? Planetary bodies would be able to form with less material, stars would be able to maintain fusion pressures at their cores with less mass. I've given this maybe 5 minutes of thought, so can anyone think of an objection I haven't thought of? I'm good at maths, but I have had considerable to drink this evening, so it's possible I've forgotten something.

[Deadweasel]

Been having a bit of a think about the Kerbal universe. How its planets and moons are minuscule yet somehow have comparable surface gravities to real-life planets and moons. Would this be realistic in a universe where all physical laws are the same except for a higher value of G, the gravitational constant? Planetary bodies would be able to form with less material, stars would be able to maintain fusion pressures at their cores with less mass. I've given this maybe 5 minutes of thought, so can anyone think of an objection I haven't thought of? I'm good at maths, but I have had considerable to drink this evening, so it's possible I've forgotten something.

One could possibly make the argument that the planets and moons in the kerbals' system are simply much more dense. This could not only explain the higher gravity relative to their size, but also why a rocket impacting the surface leaves nary a divot!

[Red Iron Crown]

I think the smaller size was a concession to playability from before we had time compression in the game.

It has the side effect of making the planets improbably dense, maybe they have a baseball sized piece of neutronium at their cores.

[Horn Brain]

Guys,

He's just asking about whether G, the gravitational constant, being higher in this universe could explain the same thing you're explaining by saying the planets are more dense. The density thing is another explanation, not the one he's asking about.

I think a higher G would make sense, to answer the OP. Certainly more sense than the planets being made of some fundamentally different kind of matter than the kerbals and their ships. If Kerbin has Earth's density, then G is 10.6 times higher (from solving 1g = G * 4/3 * pi * Kerbin Radius) than it is in real life.

According to this slideshow: http://www.thphys.may.ie/staff/mshadmehri/AstroNotes/MINIMUM.pdf

The minimum mass for a star to ignite fusion should go as G^(-1.5). The real world minimum star mass is about 0.08 solar masses. If we scale with our new G and compare with the mass of Kerbol (for which we must divide the wiki value by 10.6 to account for the different G), then Kerbol has about 3% the minimum solar mass in the kerbal universe. It should therefore be a brown dwarf still, maybe just a plain old gas giant. I think that means that (if we are going to persist in trying to figure out the physics of a game designed by devs that don't care about physical consistency) for Kerbol to be a star, G cannot be the only physical constant that we adjust. We could also adjust the Boltzmann constant, or perhaps the charge of a proton.

Edited February 21, 2014 by Horn Brain
corrected numerical error that did not affect the conclusions

[problemecium]

I've always figured Kerbol is a brown dwarf. It shows up yellow and doesn't output nearly the kind of blinding white light that our sun does. And brown dwarfs aren't actually brown, so it isn't impossible they look kind of like Kerbol.

[vexx32]

Man, this ain't a question I could answer with any degree of certainty. I'd think the folks in the Science Labs section might be able to. They're usually good at finding odd little details here and there.

[MaverickSawyer]

Part of the general consensus a while back is that Kerbin is made of depleted uranium. It predated the release of 0.17 and the arrival of planets other than Kerbin, but it made a heck of a lot of sense at the time.

[Brotoro]

You can't have the gravitational constant G change without a change in one or more of the fundamental dimensionless constants like the fine structure constant or the number of Plank masses per kilogram... and trying to figure out all the changes that would result from that makes my head hurt. A universe anything like what you are used to may not be possible if you go changing the fundamental dimensionless constants by even a little bit.

[NERVAfan]

The masses and radii of the planets are given in the Knowledge Base in-game. So it's definitely the density thing, not different laws of physics.

[SlyReaper]

If it's the density thing, then how could such dense planets form in the first place? Could this be a far distant future, trillions of years away, where stellar nucleosynthesis has generated a far greater abundance of heavy elements? If that were the case, I'd think the kerbals wouldn't just be green, they'd glow green.

As for Kerbol being a brown dwarf, I guess that kinda kills my idea of G being the only change required. But then I never expected it would be that easy. Maybe instead of (or as well as) changing G, you could increase the strength of the strong nuclear force or electromagnetism to cause all materials to be more dense? So I'll just go with "all the physical constants differ in such a way as to form a solar system that resembles ours, only much smaller"? If you consider the fundamental physical constants as variables, it forms a fairly huge multi-dimensional parameter space - it's not beyond the realm of possibility there exists a combination that would result in the kerbalverse.

[Red Iron Crown]

I think you're making a mistake trying to mold the Kerbin system into real planet formation theory. Its parameters are what they are for gameplay reasons, it's no more complicated than that. Any conjured up explanation of why it is so is justification after the fact.

[Fiddlestyx]

-insert scientific lingo here so I sound smart-

[Commander Jebidiah]

Maybe kerbins scientists decided that kerbin gravity was 1G and if we were to find a planet like kerbin we would probably say that kerbin has 1/10 the gravity of earth.

[Xavven]

That could be, but then their definition of meter or second would also be different, since their 1G is 9.8 m/s² which is what it is on Earth.

Maybe kerbins scientists decided that kerbin gravity was 1G and if we were to find a planet like kerbin we would probably say that kerbin has 1/10 the gravity of earth.

[SlyReaper]

I think you're making a mistake trying to mold the Kerbin system into real planet formation theory. Its parameters are what they are for gameplay reasons, it's no more complicated than that. Any conjured up explanation of why it is so is justification after the fact.

So what? It's fun to think about.

[NERVAfan]

If it's the density thing, then how could such dense planets form in the first place? Could this be a far distant future, trillions of years away, where stellar nucleosynthesis has generated a far greater abundance of heavy elements?

Nah, there aren't any elements that dense -- osmium, the densest, is 22.59 g/cm^3 which is only about five times denser than Earth.

I think Brotoro suggested that the planets are shells around micro-black-holes... there was a funny thing he wrote about Magratheans building them...

[Brotoro]

NERVAfan said:

...I think Brotoro suggested that the planets are shells around micro-black-holes... there was a funny thing he wrote about Magratheans building them...

That would be this piece (with apologies to Douglas Adams):

Designing Planets

BROTORO: Tonight on Kerbal Space Program Forum I'd like to welcome our special guest, Mr. Slartibartfast, who has consented to talk to us this evening about solar system design and construction. Thank you very much for being with us today!

SLARTIBARTFAST: Well, I don't normally do interviews, but when a very important client asked for a representative from the Commercial Council of Magrathea, it happened that I was the only one who'd ever talked to an Earthman, so I got the job. It's all a bit of a bother, really.

BROTORO: A very important client? Who would this very important client be?

SLARTIBARTFAST: I'm sorry, but we do not disclose information about our clients. They are all very wealthy and powerful beings, and I'm sure you'll understand that they enjoy their anonymity. But I will say that you'd never have gotten this interview if the Mice were not very big fans of space travel.

BROTORO: Mice are big fans of space travel?

SLARTIBARTFAST: Certainly. Why else would they have arranged to be some of the first passengers ever flown in a rocket? But I've said more than enough on that subject already.

BROTORO: Ah. OK, let's get right to tonight's topic. Is there any truth to the rumor that Magratheans did the design and construction work for Squad on the Kerbol planetary system?

SLARTIBARTFAST: I've already told you, we don't discuss our clients.

BROTORO: Well then, hypothetically speaking, could you tell us if it's possible to design a solar system with these specifications?

(BROTORO hands Slartibartfast a sheet of paper. Slartibartfast glances over it briefly.)

SLARTIBARTFAST: Ah, yes... Miniature planets are adorable, aren't they? They were all the rage with a certain portion of our clientele.

BROTORO: So it's possible to construct a solar system matching those specifications?

SLARTIBARTFAST: Certainly. We Magratheans pride ourselves on being able to build any kind of planet.

BROTORO: But look at the densities of those planets. Most of them have densities greater than any known element.

SLARTIBARTFAST: That's not a problem. We use black holes.

BROTORO: Black holes? But wouldn't a black hole inside a planet just swallow it?

SLARTIBARTFAST: No, no. We use spherical force field bubbles centered around small black holes. We can adjust the mass of the black hole and radius of the force shell to get any planet density desired, then cover it with the mantle and crust material of the client's choice. It's a very flexible system.

BROTORO: So you're saying that Kerbin is mostly hollow inside, with some old rocky material thrown on top?

SLARTIBARTFAST: No. More like the inner one third is hollow. And I assure you we use only the finest ingredients when building our planets.

BROTORO: Wouldn't these force field bubbles require energy to maintain their structure?

SLARTIBARTFAST: Certainly they require energy. We just allow a small amount of matter from the lower mantle to trickle through openings in the sphere. As the matter falls into the gravity well of the black hole, the energy equivalent to a sizable fraction of its rest mass is released as radiation before it crosses the event horizon. That radiated energy is captured and used by the force field bubble, which also acts as a Dyson sphere.

BROTORO: Dyson sphere? So do Magratheans know about Freeman Dyson?

SLARTIBARTFAST: Who? You misunderstood me. I used the Magrathean term for "spherical mega-structure that captures all radiated energy from a central power source," and that little fish I put in your ear translated the concept into a term you are familiar with. I've never heard of a freemandyson.

BROTORO: Ah, so that's what the fish was for. I thought it was a quaint Magrathean greeting or something.

SLARTIBARTFAST: You're sitting there wearing a digital watch, and you think my people are quaint?

BROTORO: But wait... If Kerbin has no liquid iron core inside, how could it have a magnetic field?

SLARTIBARTFAST: Does Kerbin have a magnetic field?

BROTORO: Um... well, I'm not sure. The developers haven't given us any magnetometers yet, but I just assumed it did.

SLARTIBARTFAST: And it well might. Magnetic fields are easy. We would just need to orbit a ring of charged matter around the black hole inside the force sphere. Adjusting the charge on the matter can give the client any magnetic field desired, and the inclination of the ring can offset the magnetic pole from the rotational axis if desired. Any field strength can be tailored for the client, although it's easier for us if they want a simple dipole. Haven't you wondered why Jool has such a weak magnetic field for a jovian planet?

BROTORO: It does?

SLARTIBARTFAST: Of course. If it didn't, the planet would have powerful radiation belts, and any kerbal you sent in there for aerobraking would be late.

BROTORO: Late?

SLARTIBARTFAST: Yes. As in, "The late Jebediah Kerman." Ah hum.

BROTORO: . . .

SLARTIBARTFAST: I don't understand why Earthmen always stare at me blankly when I use that line.

BROTORO: I'm just wondering how you know about Jebediah Kerman.

SLARTIBARTFAST: It's the Babelfish again. I just used the Magrathean word for "bad-ass space pilot," and the fish did the rest in conjunction with your primitive brain.

BROTORO: Well. What about Kerbol, the sun of the system? How could something with that low of a mass possibly support thermonuclear fusion?

SLARTIBARTFAST: I'm not part of our stellar division... I work on the planets, and I specialize in doing coastlines... but there are two different ways we handle miniature stars.

BROTORO: One way involves black holes?

SLARTIBARTFAST: Both ways do. In one method, we simply make the force sphere around the black hole small enough that the density and pressure in the overlying hydrogen-rich material is sufficient to maintain thermonuclear reactions at the rate needed for the desired luminosity. The second way involves simply letting sufficient matter flow through the force sphere and into the black hole to provide the desired luminosity.

BROTORO: Which method did you use for Kerbol?

SLARTIBARTFAST: I didn't say we made Kerbol. I was speaking hypothetically. Why don't you just measure the neutrino flux coming out of Kerbol... then you could tell if there are fusion reactions going on inside of it.

BROTORO: Well... again... we don't have any neutrino detectors as yet.

SLARTIBARTFAST: You really do need to take more interest in your greater environment. If you're not paying attention to things outside your planet, terrible things could happen to it.

BROTORO: Yes, thank you. But what about Minmus?

SLARTIBARTFAST: What about it?

BROTORO: The developers tell us that it's made of ice, but that can't possibly be the case given its distance from the sun, can it?

SLARTIBARTFAST: Kerbin, like your own planet Earth, is located at a distance from the sun where the equilibrium temperature is below the freezing point of water. If not for their natural greenhouse effects, the oceans on both bodies would be frozen over. The temperature is even lower when you are dealing with a surface that has a very high albedo, like ice, since that reflects away most of the incoming energy.

BROTORO: Yes, yes, I know all that. But Minmus has no atmosphere. Its surface is in vacuum. And water ice would sublime directly into a gas and escape into space at the equilibrium temperature of Minmus. Minmus should be a gigantic comet!

SLARTIBARTFAST: Ah, I see. Young Hargledertfirst of our small moon department solved that problem.

BROTORO: Did he?

SLARTIBARTFAST: She. She engineered a microorganism that was active in the small moon's water during its early warm phases. The organism excretes a clear polymer as a waste product...kind of like a resin. All of the ice on the small moon contains a small percentage of this polymer. When the ice is exposed to vacuum, the water will indeed sublime away, but the polymer is left behind and forms a barrier to further sublimation. And any future impacts or landing rocket flame scars on the surface are self-healing once the vapor clears.

BROTORO: Really? And that's what you did for Minmus?

SLARTIBARTFAST: I was speaking hypothetically.

BROTORO: And I suppose you'll tell me that Eve is purple because of a little food coloring.

SLARTIBARTFAST: Indeed. It takes a very small percentage of certain chemical compounds to give a planetary surface or atmosphere a desired color. You'd be surprised how many clients are upset if they don't get just the right shade of purple or some other color in their skies. We are very good at this, and all the chemical stains and particulates are USDA approved.

BROTORO: You know about the USDA? Oh, wait...it's the Babelfish again, isn't it.

SLARTIBARTFAST: Indeed.

BROTORO: So... if you had to do it all over again, are their any changes you'd make to the Kerbol system?

SLARTIBARTFAST: Well, yes, now that you've asked. I think Kerbin would look much better with more fjords on the coastlines. I always try to put a lot of fjords on a planet, but the managers always say it's too much. A lot of my work got erased. I was quite incensed about that.

BROTORO: Hypothetically speaking, you mean.

SLARTIBARTFAST: Um. Yes. Of course.

BROTORO: Well, our time is up. We really appreciate you coming here today. So long, and thanks for the fish.

SLARTIBARTFAST: No, I'm afraid I must take the Babelfish back with me.

BROTORO: Oh, please? I think it would be very useful *arrgh* Hey!

SLARTIBARTFAST: Flerti slark tilligert sibilas fer torrn. Ser fasto gerhs.

BROTORO: Yeah. Whatever.

Edited February 19, 2016 by Brotoro

[Felsmak]

Not if you believe.

[Vilheim]

Actually, we can be fairly certain that their definition of a meter and a second are at least fundamentally different.

Take note that special relativity does not apply in the Kerbal Universe.

Since the definition of meter per second in our world comes from the constant speed of light in all reference frames, which is a result of special relativity, and Kerbalkind have no such relativity in their universe, they must define the meter in some fundamentally different way.

[SlyReaper]

Actually, we can be fairly certain that their definition of a meter and a second are at least fundamentally different.

Take note that special relativity does not apply in the Kerbal Universe.

Since the definition of meter per second in our world comes from the constant speed of light in all reference frames, which is a result of special relativity, and Kerbalkind have no such relativity in their universe, they must define the meter in some fundamentally different way.

The meter was originally one ten thousandth of the distance from the north pole to the equator through Paris. It got retconned to be related to light speed when it turned out they got the measurement wrong.

[problemecium]

Maybe the Kerbals simply have different measurement units. Including those for time. All the equations work out if you just change the numb- okay that sounds a lot stupider than it should, but my point stands.

[SpaceSphereOfDeath]

Maybe the Kerbals simply have different measurement units. Including those for time. All the equations work out if you just change the numb- okay that sounds a lot stupider than it should, but my point stands.

Then why is the clock in the top corner in seconds, and the planets stats in km and mass in tonnes?

[Noticeably FAT]

Then why is the clock in the top corner in seconds

It isn't.

Ok, that could just be due to my laggy laptop.

[Horn Brain]

I was trying to figure out earlier if we could define new Kerbal units of mass and length that would fix the problems we see (time I held constant because unless we do everything in fast or slow motion a second is obviously the same in the kerbal universe as in ours). I don't think I was able to find enough equations that related back to real life to fix the units, though.