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.

  • 14
    Please post a link; on its face, the statement is extravagantly false. – bapowell Apr 12 at 19:10
  • I do think sun gives us energy (heat). And if the earth receives heat, its entropy increases. Isn't it? So the earth gets entropy. I don't quite under "low entropy". Is it comparing with the entropy loss of the sun? The entropy is $\delta Q/T$. – user115350 Apr 12 at 21:56
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    This idea is discussed at some length in Sean Carroll's From Eternity To Here. Carroll's version of the argument is that you could extract more heat from the Sun if you moved to live in the photosphere, but you couldn't do more useful work with that heat because there's not any access to a low-temperature reservoir. – rob Apr 13 at 1:15
up vote 59 down vote accepted

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 the hot reservoir at temperature $T_\mathrm{hot}$, the unavoidable 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 soil is about 1000 J/kg-°C, so if we simply extracted 1°C from a kilogram of soil per second, we'd match the Sun in energy per square meter. And there's a lot of soil available, and its absolute temperature is pretty high (about 283 above absolute zero in divisions of °C).

And the heat capacity of water is four times as high! Even better, water is self-circulating, so in this scenario, we could cool seawater and let it recirculate. 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 $S=E/T$ relatively small. Thus, it's not the energy of the sunlight that's particularly useful—it's its low entropy.

  • Comments are not for extended discussion; this conversation has been moved to chat. – ACuriousMind Apr 14 at 9:40
  • Actually, work does transfer entropy via mass transfer. – hyportnex May 1 at 10:46
  • Whether one classifies $\sum\mu_i\,dN_i$ as work depends on one's field. I'm excluding it here, obviously. – Chemomechanics May 1 at 13:07

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.

  • I respectfully disagree with both your statements "the energy of the Earth is essentially constant" and "The Earth continuously loses energy to space, and the Sun makes up that loss." The Earth loses its energy (e.g. its core is cooling down) and the Sun certainly doesn't make up for that energy loss. – calocedrus Apr 16 at 3:09
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    @calocedrus true, but that’s 0.03% of the Earth’s energy budget: one of the small pluses and minuses I mentioned. See en.m.wikipedia.org/wiki/Earth%27s_internal_heat_budget – Bob Jacobsen Apr 16 at 5:06
  • Bob, the 0.03% figure is for the Earth's energy budget at the surface, not for the Earth's total energy budget. But well, granted that we can restrict the discussion to only the Earth's surface here. – calocedrus Apr 16 at 8:34

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 temperature of earth's surface — dispersed heat, high entropy —, we would receive the same energy by radiation as we do now from the sun, 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 equilibrium, no flux which could be used to locally lower the entropy. 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.

protected by Qmechanic Apr 13 at 6:15

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