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Introducing sun boy, he has the magical ability to maintain a constant body temperature equal to the surface of the sun. What would be his effect on his surroundings? Would he just vapourise everything and fall straight through the ground? Or would he barely be able to burn anything? What would be a safe distance if you had to out run him?

To clarify, the temperature is approximately 5,778 Kelvin (5504.85 Celsius or 9940.73 Fahrenheit). He isn't made from the sun but just has a similar temperature.

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    $\begingroup$ what-if.xkcd.com/35 should answer most of your questions. $\endgroup$
    – Halfthawed
    Commented Aug 4, 2022 at 12:42
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    $\begingroup$ The highest melting point of any material known at this point is about 4400 K. Tungsten is the melting point is about 3695 K so it can’t be a solid. The amount of energy to sustain the temperature would be incredible. You wouldn’t ignite the atmosphere or cause runaway nuclear reactions but you might melt to the center of the earth…. Then if it was always that hot there would be some gradient of temperature from the center of the earth to the surface. Equilibrium would take a long time. $\endgroup$
    – UVphoton
    Commented Aug 4, 2022 at 13:44
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    $\begingroup$ If it's a magically solid object then it's a magically solid object, that's fine. When you say "the size of an adult human", do they also have the weight of an adult human of that size? $\endgroup$ Commented Aug 4, 2022 at 13:53
  • $\begingroup$ @Halfthawed I think the behavior of "loud box" past lava-hot isn't properly calculated, that is, it wouldn't jump rapidly. But what would happen if such an object would stay on the surface without moving sideways, I can't say, and I think this is the only question that XKCD leaves open regarding the described situation. $\endgroup$
    – Vesper
    Commented Aug 4, 2022 at 15:01
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    $\begingroup$ This question doesn't seem to have much to do with worldbuilding, and would be a better fit on the physics stack exchange. $\endgroup$
    – stix
    Commented Aug 4, 2022 at 22:10

1 Answer 1

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Scale of the disaster: anectodical

Sad. I love a good planet-shattering assumption as much as the next guy, but this won't do. I blame your lack of ambition. Yes, the surface of the sun is hot, but it's not that hot. You don't get really get to any of the fun stuff; you're nowhere near the temperature to have nuclear fusion effects, you're not even close to a temperature where you could turn the air to a plasma (you would need to go over 10000K for that). Where is your sense of fun? In fact, typical gases that you'd use on a gas stove (butane, propane, natural gas) burn at around 2000K, which is only a 3rd of the way to your temperature (even tho it's misleading to present a ratio since most temperature-dependent trends are nonlinear)

The problems are essentially going to come from the radiated heat. Let's estimate it (roughly) to get a ballpark idea of the damage. To do that, rather than calculating absolute values (which would be challenging anyway cause you don't give enough data), we will compare it to our good old gas stove:

It will pretty much only burn your house, and maybe a few neighbors but they probably deserved it

Ok, so, my stove burns at 2000K. Sun-boy is at 6000K. That's 3-times as much, and per the Stefan-Boltzmann law, the radiated power varies as the $T^4$, so, at equal surface areas and emissivity, Sun-boy would radiate $3^4=81$ times as much heat. Ok, but he is also bigger. In fact, the body surface area of an adult is around 2m2 (https://en.wikipedia.org/wiki/Body_surface_area). Taking some quick measurements on my stove, I estimate that the flame surface can't exceed 0.03m2. So Sun-guy's surface is about 70 times larger than my stove's flame. Assuming equal emissivities, we can conclude that

$$\text{Sun-guy will radiate roughly }70*81=5670\text{ times as much heat as my stove}$$

According to the inverse-square law, if I stand $\sqrt{5670}=73$m away, I will receive as much heat as if I was standing 1m away from my stove. I can definitely get half a meter away from my stove without feeling pain, so I'd probably still be fine some 30m away from Sun-dude. How disappointing.

It still melts everything it touches tho, isn't that scary?

Well not really. It just takes care of your problem for you really, by burying it 6 feet under. Quite fast, in fact. Some quick and dirty estimates with a latent heat of fusion around 200kJ/kg tell me that you could liquify around 70kg of rock per second (not counting the time to heat it up to fusion temperature, and not considering the fact that you will spend some of that energy further vaporizing the melted rocks). Standing still, you will probably be gone in a matter of tens of seconds to a few minutes. If Sun-man can walk tho, it might actually be feasible to remain on top, since you don't melt the ground that fast (depending on assumptions, you might be sinking from 1 to 20cm/s)

I lack the geology knowledge to tell if a white-hot mass sinking into the ground will cause problems down the line, but I doubt it, seeing the temperature of the interior of the earth. Just in case, I wouldn't stand close to the crater. It is likely going to get trigger happy with all that melted rock getting stirred up with pressurized bubbles

In summary, the damage you will do will be very localized (you will set a couple house on fire), and then the problem should take care of itself

  • ed by Molb0rg: sorry for vandalizing, I can't comment, your answer is great and just one small suggestion, pay attencion to soy boy density and molten rock density he will float, so he so sad that he even can't dive to the core of the planet. As evaporating ability 70kg easily can be slashed 4x (only half of sun guy works on evaporating, and latent heat of evaporation another 2x at least) all in all sinking potencial is quite limited. Great answer, sorry can't upvote either, due old browser.
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  • $\begingroup$ There might be problems if it hits the water table. At least it's not radioactive. $\endgroup$
    – DKNguyen
    Commented Aug 4, 2022 at 16:23
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    $\begingroup$ I think it probably matters how dense it is, perhaps it just floats on the molten rock once it melts it. $\endgroup$
    – UVphoton
    Commented Aug 4, 2022 at 16:52
  • $\begingroup$ @UVphoton true. It will eventually vaporize it anyway and keep sinking, but that will take longer. Giving estimates of the rate becomes too fuzzy at that point $\endgroup$ Commented Aug 4, 2022 at 16:59
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    $\begingroup$ There will be a pillar of flame up from Sun Boy though. That ought to be worth something. And it will probably be windy 30m away because of all the air rushing up above Sun Boy. That wind will help keep you cool. But it might mess up your hair especially if you have it really long on top. You will need to be careful because SunBoy probably will not want to vanish into the earth (how scary!) so he will be running like crazy and he might run towards you and if your hair is in your eyes you might not notice until it bursts into flame. $\endgroup$
    – Willk
    Commented Aug 4, 2022 at 21:44
  • $\begingroup$ Temperature estimates for the inner core are around 5700 K, so once the magically-hot object gets down there, the problem goes away. You'll probably see some impressive phreatic explosions on the way down, though. $\endgroup$
    – Mark
    Commented Aug 4, 2022 at 22:16

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