Quotes from astronauts on seeing the Blue Marble

These quotes were given in the EOS issue of Dec. 4, 2012, published by the American Geophysical Union:

Astronaut Russell Scheickhart, in describing his experience from space on the Apollo 9 mission said, “When you go around the Earth in an hour and half, you begin to recognize that your identity is with the whole thing. And that makes a change. You look down there and you can’t imagine how many borders and boundaries you cross, again and again and again, and you don’t even see them. And from where you see it, the thing is a whole, the Earth is a whole, and it’s so beautiful. You wish you could take a person in each hand, one from each side in the various conflicts, and say, Look. Look at it from this perspective. Look at that. What’s important?’”

Apollo 17 astronaut Eugene Cernan has reflected about his experience on that flight: “You have to literally just pinch yourself and ask yourself the question, silently: Do you know where you are at this point in time and space, and in reality and in existence, when you can look out the window and you’re looking at the most beautiful star in the heavens—the most beautiful because it’s the one we understand and we know, it’s home, it’s people, family, love, life—and besides that it is beautiful. You can see from pole to pole and across oceans and continents and you can watch it turn and there’s no strings holding it up, and it’s moving in a blackness that is almost beyond conception.”

See this post on my own impression of the "Blue Marble" picture shown above.

Some comments on entropy

Definition

Since I reviewed the basic teachings concerning entropy, I see entropy everywhere. The most helpful definition of entropy is the one regarding the state of order. The highest the order, the lowest the entropy. Thus, I declare myself an enemy of entropy, in the sense that I, we, always try to create some order, to put things in order: putting the plates away in the kitchen, entangling an electric wire, etc. Why is it actually easier to put things in disorder and to create entropy than the reverse? One useful explanation to comprehend this difference is to see that a state in order is an improbable state, while a state in disorder is a probable one: the electric wire is more likely to be tangled after so many years than to stay untangled, and there are many ways to put a mess in a room, but only one to put things at their places.

Entropy and time

There is some controversy concerning the relationship between entropy and time. The problem is that the fundamental laws of classical physics (from Newton) are reversible in time; that is whatever happens in one direction (toward to the future) can very well happen in the other direction (toward the past). Thus, as we see a drop of milk in a tea cup spreading and diffusing throughout the volume, we should see all the milk particles to come back and form the initial drop of milk. This is indeed possible according to Poincaré's recurrence theorem, although, because the state of the drop is very unlikely compared to all the states where the milk is spread, the probability that this happens is tiny (but in theory, it could happen!).

On the other hand, the second law of thermodynamics says that for a closed system, the entropy has to increase: the spreading of the drop of milk within the cup is a perfect example of entropy increase. Some, such as Prigogine, argues that entropy carries with itself the so-called arrow of time: because entropy increases, we can make the difference between past and future. But the question then remained, is the second law compatible to the reversible laws of classical physics? Roger Penrose, in his book The emperor's new mind, argues that the second law is not only compatible, but also, contrary to Prigogine's view, that the entropy does not carry the arrow of time with it. Whatever the direction, toward the past or the future, the entropy has to increase within a closed system, in particular within a system where there is no constraint on the entropy. In the case of the drop of milk, although toward the future there is no constraint and the entropy increases indeed, toward the past, there is the constraint of the initial conditions saying that the entropy is low at the beginning: thus, if you run the experiment backwards, there is the constraint that at the end, the entropy is lower than at the beginning. Because of this constraint, the second law does not apply as such and in consequence, the entropy does not itself carry the arrow of time. The arrow of time exists because our system started with a state of low entropy. The remaining question is thus, why and how did we start with a state of low entropy?

Source of low entropy

Both L. Botlzmann and R. Penrose describe the struggle for life as a struggle for low entropy, with the ultimate source of low entropy being the sun. L. Botlzmann writes

"The general struggle for existence of animate beings is therefore not a struggle for raw materials [...] nor energy [...], but a struggle for entropy, which becomes available through the transition of energy from the hot sun to the cold earth."

and R. Penrose says

"We do not need to gain energy from our environment because energy is conserved. But we are continually fighting against the second law of thermodynamics. Entropy is not conserved; it is increasing all the time. To keep ourselves alive, we need to keep lowering the entropy that is within ourselves."

Thus, how do we get this low entropy? The ultimate source of entropy is the sun, and plants are the organisms which are using directly this entropy source, transforming it into molecular structures, themselves ready to be eaten. We, humans, via the food web, are eating plants or animals who themselves eat plants, to get low entropy for our body.

In this aspect, I am then wondering if we can class the food web in terms of entropy content. The plants would get a source of low entropy S_i, some of it would be used such that the entropy content gained in the eaten plant would be actually larger, S_i < S_plant . This process would repeat in that the higher in the food web, the higher the entropy content. In that respect, I would conclude that 1) humans would be organisms with some of the highest entropy (the most disorder) and 2) we should all be vegetarians in order to efficiently get low entropy in our diet. Do you agree with these conclusions?

Why the sun?

R. Penrose also explains why the sun is a source of low entropy and I was very surprised to learn that the reason is nearly a geometrical one. The sun is a hot spot, a small disk of light compared to the entire sky. Because of this geometrical configuration, the energy we receive from the sun has a much lower entropy that the energy sent back to space by earth because this energy is sent into all directions. Thus, if I understand correctly, if the earth was surrounded by many suns so much so that they would cover the entire sky, there would not be any source of low entropy and life would be unable to exist? Of course, one still needs to explain why the sun, itself a compact star and thus a source of low entropy, exists but the explanation goes on with cosmological arguments that I understand much less.

References
Ludwig Boltzmann, The second law of thermodynamics, in Theoretical physics and philosophical problems
Jean Bricmont, Science of chaos or chaos in science?
Roger Penrose, The emperor's new mind

The Blue Marble

This is one of the first image of the Earth ever taken (this one was taken on December 24, 1968). It reveals that from space planet Earth is seen as a beautiful blue planet recovered by a turbulent sea of white clouds over a background of darkness. I am too young to have lived without such picture of Earth in mind. What about the wiser generations? What were their reactions the first time they realize how blue Earth is? Did they ever imagine such lifefull picture? If you can answer, please leave me a comment. I will be glad to read it.

Mahalo