The dying of the light

[Vanamonde]

The discussion here is wandering somewhat far afield. The subject of this thread is the observation of stellar decline. If you'd like to discuss the origins and nature of life, please take it a dedicated thread. 

[K^2]

15 hours ago, sevenperforce said:

The solar wind (and radiation pressure, to a lesser degree) is responsible for pushing the dust tail of a comet away from the sun, so it's definitely doing SOME work.

Because there is radial motion, and we're not looking at the full cycle of particles through their orbit.

Lets rephrase the problem. You have a force field that repels things from the center as some smooth, monotonic function of radius. (The function will be different for each body, depending on form factor, but lets focus on just one object in orbit for now.) We can clearly assign a spherically symmetrical potential to it, call it U_p . Hamiltonian for a body in this potential is H = p²/(2m) + U_G+U_p, where U_G is due to gravity, and this Hamiltonian is entirely time-independent. If Hamiltonian is independent of time, total energy is going to be conserved by Noether's Theorem. (You can derive that in other ways, but this is simplest.) Yes, there can be some trade between gravitational potential and this pressure potential and kinetic energy. But the total energy is conserved, which means that orbits will be quasi-periodic. To be precise, quasi-periodicity means that for any finite neighborhood around a starting point, the object will pass through that neighborhood after some finite amount of time. This is true for any central potential problem. So no way for an object to just continually gain energy and escape.

Another example to try and square this away with intuition, imagine a vertical harmonic oscillator in gravitational field. Yes, as the weight moves down, gravity does positive work on it, and the weight will move further down, but this is perfectly canceled as the weight moves up and gravity does negative work. Lacking damping, you'll get periodic movement up and down with exactly the same frequency and amplitude as without gravity, but with gravity, the equilibrium point will be slightly lower. Certainly, the net work done by gravity through a cycle has to be zero.

If planet orbiting a star experiences outward pressure due to solar wind and radiation, that will allow it to orbit very slightly higher at the same energy and angular momentum, but it's still going to be a (nearly) closed orbit that's not going to gain or lose energy over time on average due to that pressure.

Now, the fact that solar wind moves at finite and greatly sub-relativistic speed means that object moving towards the Sun will experience slightly more pressure than object moving away, but because the difference will always oppose motion, it basically acts as an additional drag force. Which means that solar wind is going to help dampen the radial oscillation, forcing orbits to become more circular over time, as well as contributing to planets slowly spiraling in due to drag on angular velocity. But this is no different than moving through a static medium, albeit, with somewhat higher drag coefficient.

[Moach]

Curiously enough, none of our resident sci-fi geeks has so far mentioned the book by George R. R. Martin that shares a title with this thread.

Oh wait, exactly this has now just happened...

This post thus invalidates its own original observation - Good thing the Arrow of Time™ points only in the way where cause has to come before effect, or this post could self-annihilate in an infinitely looping puff of retro-recursive logical anti-vomit.   

That would most likely cause the universe to stop and throw an exception out the window (possibly hitting one of the other universes mentioned in the video)

 

Anyways, GRRMs "Dying of the light" does feature a bunch of folks coping with a planet that's rapidly losing sunlight and will soon no longer support life

During which time his main character deals with some unresolved girlfriend issues that he manages to have escalate to a death-chase type situation on heavily armed flying cars.... 

 

It's really quite a nice read. I do recommend! 

It very much predates the author's best known work, you know: The one with the dragons chick, where various main characters get casually slaughtered by sadistically random plot twists... 

 

Anyways - As to the OP,  the main lesson here clearly is:  be careful when you take the sole copy of the summed knowledge for all mankind with you on a boating trip.  An unknown number of future civilizations can be saved thus, by simply observing safe stowage of personal objects while aboard. 

[eatU4myT]

On 8/23/2020 at 12:42 AM, Moach said:

be careful when you take the sole copy of the summed knowledge for all mankind with you on a boating trip. 

Many and varied are the life lessons KSP has to offer!

[magnemoe]

On 8/18/2020 at 4:19 PM, sevenperforce said:

The solar wind (and radiation pressure, to a lesser degree) is responsible for pushing the dust tail of a comet away from the sun, so it's definitely doing SOME work.

I'm torn on this point. On the one hand, it's true that we can't speculate endlessly without entering the realm of pure science fiction. I do not expect neutronium life to evolve, for example. But I also don't want to limit imagination. For example, there could be sentient clouds on Jupiter, converting solar energy into rotational eddies that maintain their structure. It's unlikely, but it's entirely possible within the realm of real physics.

Life is possible in any place where there is a source of energy and a sink for entropy. Both are required. One reason I don't believe it's realistic to imagine neutronium life is that there is no energy source (other than residual heat) and no entropy sink. Life is a machine that uses energy to artificially decrease its own entropy while increasing the entropy of its surroundings. So you need both to work.

Terran life crawls around on the surface of a giant rock, either breathing or creating corrosive gas. Our energy source is the sun and our entropy sink is the soil beneath our feet. Our ability to selectively convert energy into entropy is totally chemical, completely bound up in a very narrow range of temperatures and pressures. Life does not require these conditions to exist or to thrive, but it does require the existence of persistent structures capable of sustaining low-entropy regions. Intelligent life requires those low-entropy regions to be so thoroughly protected that they are able to hold information. And that is the question.

This, and intelligent life require more energy, you are very unlikely to find something like fish at Europa or other ice shell moons. The size and the energy in that ecosystem is tiny compared to earth, not just the moon is small but only the thermal vents are productive zones so evolution has less to work with and would run slower. 
Many think that the cambium explosion happened at the end of an global ice age, suddenly the ecosystem became much larger and it was about grabbing it. 

 

[sevenperforce]

23 minutes ago, magnemoe said:

This, and intelligent life require more energy, you are very unlikely to find something like fish at Europa or other ice shell moons. The size and the energy in that ecosystem is tiny compared to earth, not just the moon is small but only the thermal vents are productive zones so evolution has less to work with and would run slower. 

Brains (or any other information storage and retrieval mechanism capable of evolving intelligence) are VERY metabolically expensive. They take a long time to grow and therefore they become so very valuable that they must be protected at enormous metabolic cost. 

So while intelligence itself can subsist on very low levels of energy, you're right: you need ENORMOUS selection pressure to evolve an intelligent brain, and therefore you need a very large ecosystem, able to support ongoing competition for resources.

Edited August 31, 2020 by sevenperforce