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Physics, Astronomy & Cosmology / Re: How long could humanity survive if the sun went out?

« on: 02/08/2021 11:33:08 »

I would say that the submarine would stay there, in a very uncomfortable state, for a very long time.
 It gets quite cold after a while

They might be OK if they can find some thermal vents to keep them warm.
But I think that wouldn't last forever.
It would probably outlast the crew.

If half the crew killed the other half then the stores would last (slightly more than) twice as long.
Calling that "humanity" would be debatable, but the only people left alive to debate it...

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Physics, Astronomy & Cosmology / Re: What spun-down the inner planets ?

« on: 26/09/2018 00:39:03 »

The time it takes for a body orbiting another body to tidally lock is found by the equation:
T = wa^6 mQ/(3GM^2 K R^3)
w is the starting angular velocity of the body's rotation
a is the semi-major axis of its orbit
Q is the dissipation factor
m the body's mass
G the  universal gravitational constant
M the mass of tht body it is orbiting
K the "Love" number.
R the radius of the body.
Q and K are not well known except in the case of the Earth and Moon system.  So for the sake of argument, we will assume they are the same for all three planets and the Sun.
Since we are comparing planets orbiting the Sun G and M will be the same.
If we assume that w starts out as the came for all three then the difference in tidal locking times can be simplified to the relationship of
a^6 m/R^3   
It also turns out that the ratio of m/R^3 comes out to be fairly close to each other for all three planets.  So in the end, the major deciding factor is a^6
 Earth is 2.58 times further from the Sun than Mercury, so it would take nearly 300 times longer to tidal lock to the Sun.
The Earth is 1.38 times further than Venus and would take 7 time longer to lock.  Assuming all other things are equal.

One thing to keep in mind is that tidal forces fall of by the cube of the distance.  So even though the Moon's tidal effect of the Earth is ~ twice that of the Sun's, At the distance of Mercury, the Sun's tidal force has increased by a factor of 17, and be 8.5 times stronger than the Moon's effect on the Earth and at the distance of Venus it would be 1.3 times stronger than the Moon on the Earth. 

But we don't know that all things started out equal or stayed that way. For example, the Earth is believed to have been struck by a Mars sized body in its past which initially formed the Moon, and likely spun the Earth up quite a bit.  This would have given it a lot more rotational energy to shed.
Venus actually rotates slower than it orbits.  This also may have been due to a collision; one that robbed it of spin rather than giving it more spin. 

So without knowing the full history of each planet it is hard to say what all the influences were that contributed to their present rotations.

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Physics, Astronomy & Cosmology / Re: How hot would a man inside a sealed, heat-proof bottle become?

« on: 05/04/2017 04:27:45 »

I'll give this a shot, since this is exactly the kind of eccentric physics question that I would find myself thinking about.

2,000 kilocalories a day is 8,368,000 joules a day, 348,667 joules per hour, 5,811 joules per minute or 97 joules per second. 97 joules per second is, of course, the same as 97 watts. Since I've read (a long time ago) that mammals require about 10 times as much energy as reptiles of similar size thanks to the waste heat produced by their metabolism, I'll assume that 90% of that energy is turned into waste heat. This results in our human releasing about 87 watts of heat energy (given that I've seen other estimates between 80 and 100 watts for human waste heat, this seems to be a reasonable estimate).

So now you can easily calculate how many joules this "man-in-a-bottle" would accumulate over different time spans. After a day, it is 7,531,200 joules, after a year, it is 2,750,770,800 joules, and after a million years, it is 2,750,770,800,000,000 joules.

Converting that into a temperature measure is trickier, since human beings have a complex composition. However, I'll simplify it by assuming that our human has a heat capacity equal to their own mass in water (humans are mostly water anyway). Although heat capacity changes with temperature, I think we can still get some meaningful results here. Thankfully, I saved a calculation that I did a while ago where I found that the average heat capacity of water over its entire liquid range is 4.19555 joules per gram times Kelvins.

Our "man-in-a-bottle" has a starting body temperature of 310.15 Kelvins and a mass of 100,000 grams. It therefore takes about 419,555 joules to heat him up by 1 Kelvin. On day one, 7,531,200 joules of heat energy are added, resulting in an increase in temperature of 17.95 Kelvins (a total temperature of 328.1 Kelvins, 54.95 degrees Celsius or 130.91 degrees Fahrenheit). If this was a normal person, they would be quite dead on day 1. However, our super-man can keep going, so now on to a full year...

Before we can add a year's worth of energy, we have a new problem to face: we need to find out how much energy it will take to get our man to the boiling point. Why? Because the heat capacity of water vapor is different than that of liquid water. Also, energy will be consumed in the very act of boiling water (heat of vaporization). Without taking these into consideration, we can't expect our estimate to be accurate. Take note that this assumes that our man has enough room inside the bottle to vaporize into steam. If not, the water he is composed of will remain liquid for longer (due to the pressure increase) and probably throw off the calculations.

The boiling point of water is 373.15 Kelvins, so that's 63 Kelvins that would have to be added to our human from an unheated start. This results in a requirement of 26,431,965 joules, which will be subtracted from the total energy input over a year's time (2,750,770,800 joules - 26,431,965 joules) to give 2,724,338,835 joules left over after reaching the boiling point. The heat of vaporization of water is 2,256 joules per gram, so now we need 225,600,000 joules to take the water from liquid to gas. The remainder, 2,498,738,835 joules, will go into heating up the resulting water vapor.

The heat capacity of water vapor varies quite a bit depending upon the temperature. Right at water's boiling point, it's about 1,890 joules per gram times Kelvins, or 189,000,000 joules to heat up our steam man by one Kelvin. This would increase his temperature by 13.22 Kelvins for a final temperature at the end of one year of 386.37 Kelvins, 113.22 degrees Celsius or 235.8 degrees Fahrenheit. Since this is not significantly above water's boiling point, the given heat capacity of water vapor is probably accurate enough for the sake of this calculation.

A million years is going to be a doozy and I probably cannot calculate it accurately. I can, however, place somewhat of an upper limit on it (since heat capacity tends to increase for water vapor as its temperature goes up). If we assume (wrongly) that the water vapor's heat capacity remains at 1,890 joules per gram times Kelvins, we can calculate the resulting temperature after adding 2,750,770,547,968,035 joules (the energy remaining after boiling the water at its boiling point). The result is about 14.55 million Kelvins after a million years. This is almost certainly wrong. The thermal behavior of such a high temperature plasma would no doubt be very, very different from that of water vapor. I suspect our immortal man would be nowhere near that hot in actuality. I would need some kind of heat capacity for plasma at high temperatures to get a good ballpark estimate.

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Physics, Astronomy & Cosmology / Re: Does the atomic nucleus have nuclear orbitals occupied by protons and neutrons?

« on: 29/10/2016 11:48:53 »

Lets look at this a slightly different way. Electron orbitals are just the shape that the wave function of the electron takes on when bound to a nucleus. Therefore if the wave function of the electron is real then the electron orbitals should also be real. It would take some serious evidence to show the opposite.

There are certainly reasons to believe that the wave function is real.

At best you can say that according to some interpretations of Quantum Mechanics the wave function isn't "real/physical" but the question is unsettled. Although there is evidence pushing for the real camp.

Additionally,
The phase of a wave function (the complex bit) can have physical effects beyond just participating in the squared magnitude (the extra rotation of the photons in the link).

You can make qubits by exploiting the phase (imaginary) parts of wave functions.

The things above and others like single particle diffraction and tunneling are less strange if you simply accept that the wave function is real.

But to reiterate one should never state as fact that the wave function is not real. At best the question is exactly as unsettled as the question about which interpretation is correct with a bit of evidence pulling for the wave function is real camp.

What is the form of hydrogen used in the experiment? Is atomic or molecular?
If it is atomic, how to prevent them from forming diatomic molecule?
If it is molecular, how can it produce rotationally symmetrical pattern?

It was atomic hydrogen. You can break a fraction of hydrogen molecules apart and keep that fraction constant by adding energy so more are broken at the same rate others reform. Then you can build a trap big enough for only one atom and eventually if you wait long enough you'll have a trapped hydrogen. If you keep the gas dilute enough collisions between things will be very rare.