I just started reading Penrose's new book, Cycles of Time.

One thing that struck me was this idea: For the purpose of life, we shouldn't think of the sun as a source of energy, but rather we should think of our system (the hotness of the sun against the backdrop of cold outer space) as a source of low entropy. It's the low entropy that life essentially feeds off of.

Penrose notes that the earth stays at a relatively constant temperature (with the minor exception of Global Warming), so the sun actually isn't really a net source of energy for the earth. The energy eventually makes its way back into space. However, most of the energy released back is in the form of low frequency electromagnetic radiation which is in contrast to the yellow spectrum of the sun. Penrose seems to imply that this conversion of energy from high frequencies to low frequencies means that there is a net-reduction of entropy on earth. It isn't completely clear, but it appears that he's further implying this entropy reduction is a result of life processes (the push towards life attaining greater complexity requires low entropy I guess).

I'm not sure what I think about all of this. This whole argument seems to mean that we could someday just do some spectral analysis on planets in other solar systems to determine if they're reducing entropy and therefore harboring life (though I guess there are plenty of other types of entropy reducing mechanisms that could be happening). Anyway, I'm curious if anybody has any thoughts on this view.

This part is just not true. There's no conservation of entropy rule/law, it can increase on the whole

Also, I don't know why he thinks the sun is not a source of energy. The sun emits energy, the Earth gets some of it, seems pretty simple

Ok, well I'm sure I didn't do the best job at paraphrasing what he said... Hopefully someone else has read this and can do better.

But the gist is that the sun is important to life only as a source of low entropy. If all of the space surrounding our planet were at the same temperature, even if it was very hot/energetic, the energy would quickly become unusable.

Anyway, yeah I guess it is clear that the conversion of energy can never imply, by itself, a reduction of entropy.

Here's some quotes:
"""
One tends to think of the Sun as supplying the Earth with an external source of energy, but this is not altogether correct, as the energy that the Earth receives from the Sun by day is essentially equal to that which the Earth returns to the darkness of space! If this were not so, then the Earth would simply heat up until it reaches such an equilibrium. What life depends on is the fact that the Sun is much hotter than the darkness of space.
...
[A buncha stuff about the photons from the sun having higher frequency than those returned by the earth to the sun --> individual photons from the sun have greater energy than individual photons the earth returns to space --> earth releases more photons than it absorbs]. Accordingly, Boltzmann's S=klog V, tells us that energy coming in from the Sun carries a considerably lower entropy than that returning to space.
...
So what the Sun does for us is not simply to supply us with energy, but to provide this energy in a low-entropy form, so that we (via the green plants) can keep our entropy down, this coming about because the Sun is a hot spot in an otherwise dark sky. Had the entire sky been of the same temperature as that of the Sun, then its energy would have been of no use whatever to life on Earth.
"""

So while Penrose may have not exactly stated some of the things I did, all of this seems to me to be rife with implications. Is any of this useful or meaningful?

Even if I agree here I still can't find many implications. I see it as simply:

The sun provides us with our source of energy and order. Living beings need to increase disorder in the rest of the Universe to sustain themselves and grow etc.

And as a side note, I am unsure what it would be like if the sky and Earth and Sun were all the same temperature. My first thought is we can still charge up some solar cells from the radiation, hence we still have our source of order.

However everything being the same temperature, and emitting and absorbing the same sprectrum of light, kind of implies no order for us, like Penrose says. Could anyone else weigh in on this?

But what is happening to the entropy of the planet itself then!!! Obviously our actions in the planet have consequences and the thermal radiation to space is not the only one of them! This is a ridiculous topic without a detailed study of the entropy of all subsystems involved.


Also expect second law and first law to survive statistically only as fundamentals after the revolution this decade that is coming...Those are only classical approximations of the real world. Expect second law of course to play a key role in the development of the breakthrough.

wikity what? Are you expecting a perpetual motion machine to be built soon?

I think he might be trying to make an explanation of effective temperature of a body as difficult to understand as possible, possibly with some purpose that is missing me at the moment.

Put a thing in front of a fire then it will heat up until it is about the same heat as the air coming from the fire. At that point there will be no gain or loss of heat unless something happens.

Similarly the effective temperature of a planetary body is the equilibrium temperature derived from say the suns heat. The earth is not exactly at its effective temperature due to green house gasses.

Not really, nothing that violates in a profound macroscopic way current laws with no other consequence, unless you view equally interesting the possibility to produce energy from exotic means that involve properties of spacetime. Understanding things at a deeper level and explaining current laws with more universal ones more economically is definitely always opening doors to possibilities for connections never realized before that may eventually mean something for energy production too why not. Often after a breakthrough a period of intense production of derivative results follows. E=mc^2 is definitely not the last profound universal magnificent equation we have seen but its the first major derivative result of relativity. It becomes possible only due to a seemingly unrelated breakthrough originating from ideas not motivated from energy arguments. In the end even mass/energy may be the wrong way to look at it. For energy is clearly still a concept of the old paradigm that may survive only as a special case of something broader.

Ok, well I'm still not too far through the book. But I'll give the gist of where he's going... He discusses the Second Law of Thermodynamics and entropy quite a lot. And this is primarily to show that our universe is currently in a state of relatively low entropy (at least low enough for solar systems and life to exist). So the universe must have had extremely low entropy during the big bang. That is, the state of the big bang was somehow very special. Nothing new in any of this... Somehow this idea that the big bang had extremely low entropy is to help strengthen some cosmological model he develops later on.

Anyway, I was just trying to figure out if there was any relevance or importance to this relation between life and entropy, anything worth thinking about. I don't think it completely fit into the direction of the book, but he was using it more or less to show that solar systems must be a source of low entropy if we consider life and things like an unbroken egg to have low entropy. And then solar systems too must have some kind of source of low entropy, etc...

Just calculate the entropy of various systems of same mass/energy and see how it grows as you move away from some well understood order (like in solids to gases say). It will then become obvious to you how entropy/mass (of course "volume" the system extends matters too) say correlates to various things you observe in the universe life included.

I may have to track this book down and read it. This is the area of mathematical physics that I studied in grad school so it would be interesting to see where it is going.

Entropy is roughly a measure of the randomness of a system. In general, the more ordered a system, the lower its entropy. In any process entropy can never decrease, it can only remain the same or increase. It remains the same in a process that is reversible, but that is actually an unattainable ideal. In any real process, it always increases. It can appear to decrease locally, for example if I trap a gas in a cylinder with a piston and compress it by pushing the piston to decrease the volume, I decrease the entropy of the gas. But somewhere I have produced work to move the piston, and that subsystem must have an entropy increase which will at least offset the decrease in the cylinder/piston system.

If there were no sun, then the earth would be at about 4 degrees K. which is the temperature of empty space. The sun is at 3000 K or so at its surface so it radiates energy in all directions in space. The earth absorbs radiation from the sun and reflects some. It heats up due to the energy it absorbed and as it warms it begins to radiate that energy back into space. It reaches a steady state temperature when the energy it is radiating back into the blackness of space equals the energy that it absorbs from the small hot spot of the sun. Obviously that temperature is much lower than the sun's temperature. If the entire sky were the temperature of the sun, there would be no cool space in which to shed heat, and the earth would quickly reach 3000 K also.

Green house gases slightly decrease the emissivity of the earth so that its ability to radiate heat into space is slightly decreased. This causes a slight increase in the steady state temperature of the earth. The term "effective temperature" has no meaning.

The absorption and radiation of heat by the earth results in an increase in entropy for the universe. Life through photosynthesis causes a small amount of the energy to be captured as chemical energy in the sugars created during the photosynthetic process. This slightly decreases the entropy increase of the overall process, at least for a time. However, at steady state decomposition and respiration release that stored energy back into the earth's atmosphere and eventually back into space. In the end, if there is any change in the entropy increase due to life it is either very small, or indetectable. I cannot imagine how it could be used to tell if distant planets had life on them or not.