While I was watching a popular science lecture on YouTube, I came across this sentence
"Sun is giving us a low entropy, not energy"
which was said by Prof. Krzysztof Meissner.
I am not a physicist, but this sounds to me like a huge leap.
I would be pleased if someone could explain to me the difference.
I believe the intended meaning is: It's not the energy of the sunlight that's particularly useful—it's its low entropy.
First, some preliminaries: We always wish to have a system that can do useful work, e.g., run a water wheel, raise a weight, or generate electricity.
The catches are that energy is conserved (which you probably knew about) and also that entropy is "paraconserved" (which you might not have known about). Specifically, entropy can't be destroyed, but it is transferred when one object heats another (and it's also created whenever any process occurs, anywhere).
The problem with producing work arises because work doesn't transfer entropy, but heat transfer does (while also creating some entropy). Therefore, we can't simply turn thermal energy—such as the energy the Sun provides—into work; we must dump the accompanying entropy somewhere as well. This is why every heat engine requires not just a source of thermal energy (the so-called hot reservoir) but also a sink for entropy (the so-called cold reservoir).
In the idealized process, when we pull energy $E$ from a hot reservoir at temperature $T_\mathrm{hot}$, the corresponding entropy transfer is $$S=\frac{E}{T_\mathrm{hot}}.$$
Now we extract some useful work $W$ (by boiling water and running a steam turbine, for example), and we dump all that entropy into the low-temperature reservoir at temperature $T_\mathrm{cold}$ (using a nearby cool river to condense the steam, for example): $$S=\frac{E-W}{T_\mathrm{cold}} .$$
The energy balance works out: $$E-W=(E-W).$$ The entropy balance works out: $$\frac{E}{T_\mathrm{hot}}=\frac{E-W}{T_\mathrm{cold}}.$$ The efficiency is $$\frac{W}{E}=1-\frac{T_\mathrm{cold}}{T_\mathrm{hot}}.$$ And the higher the temperature of the hot reservoir, the more work $W$ we can pull out while satisfying the two conversation laws.
Now to the point: The Sun sends a lot of energy our way: around 1000 W/m² at the earth's surface.
But is this in fact all that much energy?
The heat capacity of water is about 4000 J/kg-°C, so if we simply extracted 1°C worth from 250 g of water per second, we'd match the Sun in energy. Well, per square meter of sun exposure, at least, but there's a lot of water available on the planet, and its absolute temperature is pretty high (283 above absolute zero, more or less, in divisions of °C).
Even better, water is self-circulating, so in this scenario, we could cool seawater and let it recirculate.
As Gerd Ceder memorably put it, we could operate a party boat: pull out thermal energy from water to make ice for our cocktails, and use the extracted energy to cruise around all day.
Unfortunately, the restrictions described above tell us that we can't perform this extraction: there's no lower-temperature reservoir to send the entropy to (here, I'm assuming that most of the earth and atmosphere available to us is at around 10°C). In contrast, the Sun's temperature is enormous—around 5500°C, which makes the denominator of the effective entropy term $S=E/T$ quite small. Thus, it's not the energy of the sunlight that's particularly useful—it's its low entropy.
A conceptual answer in two parts:
First, note that the energy of the Earth is essentially constant. The Earth continuously loses energy to space, and the Sun makes up that loss. (Yes, there are small plus and minuses, but this is basically correct) The Sun’s power is certainly not rapidly increasing Earth’s total energy.
So why does The Sun’s power seem so vital? Well, it makes up the lost power. Surrounding Earth in a Giant Space Comfort Blanket would reduce those losses too, but somehow that seems less great than the concentrated power of the Sun.
So that’s where entropy comes in: the Sun’s energy is concentrated & high temperature, hence low entropy (which is good), unlike diffuse & low temperature high entropy (less good) planetary heat.
Viewed that way, while just making up lost power, the Sun is providing a dose of order (low entropy) which allows life to do its thing by consuming that and giving off the power as disordered low-grade heat.
The entropy of the earth+sun system is lower than a system with the earth surrounded by diffuse energy equivalent to that of the sun. Technically, both systems have the same energy, but the former has much more usable energy.
Obviously, the sentence should be "Sun is giving us a low entropy, not only energy". The Sun's radiation creates an energy flow through Earth which life can utilize. The energy flow is generally utilized to build pockets of order in the surrounding chaos, i.e. to maintain local areas of low entropy, like our bodies, facilitated by a constant flow of energy through it.1 The energy coming from the sun is obviously required for this, but not sufficient, as the following thought experiment shows:
If the Earth were surrounded by a shell with the average radiation temperature of the universe as seen from Earth — dispersed heat, high entropy —, we would receive the same energy by radiation as we do now from the Sun (plus Moon, and stars, and background radiation), but it would be useless.
The energy balance would also be the same as it is right now: We'd radiate away all we get plus some residual radioactivity. But there would be near equilibrium. The only usable energy flux — the only source of energy with low entropy — would stem from the nuclear decay underground. Only that could be used to locally lower the entropy in some places on the surface. The radiation hitting us would be completely useless.
I suppose that's what Meissner meant.
1 Through food which is after a few indirections just stored solar energy.
The sun is not a "source of low entropy." The phrase doesn't even make sense, physically. Think of the analogues "source of low pressure," or "source of cold." This thinking probably comes from the idea pushed by Erwin Schrödinger that animals have to eat low entropy. Being ignorant of the complex chemistry, I cannot say how much the specific entropy (entropy per unit mass) of animal excreta differs significantly from their food. What I can say is that the excess entropy is dumped to the surrounding environment by a combination of raw heat transfer (conduction, convection, and radiation), and gas exchange (sweat, carbon dioxide, and water).
Note the process: energy + entropy in -> energy + more entropy out. More significantly, because the animal has independent access to a low temperature bath, the excreta does not have to be particularly lower entropy than the food.
This is the same basic process the Earth undergoes. The sun acts as a source of both high entropy and energy. In fact, the only thing particularly low entropy about sunlight is it's direction of travel, but that's only true here, 150 million km from the sun. At the surface of the sun, the entropy of the light is higher.
How does the entropy change between here and the sun? The answer is the same as the true answer to the riddle of where we "get our low entropy" from: the cold vacuum of space. As the light travels outward from the sun, the direction of travel becomes more and more certain, dropping the entropy of any particular photon. This is only possible, though, because of some implicit features in the description: there is a cold vacuum into which the photon can spread out from a definite local source.
Note that every other aspect of the light from the sun, that is frequency spread and polarization, remains a source of high entropy. Where we dump more entropy than we receive is primarily a function of the increase in number of photons needed to achieve energy balance.
