February 10, 2024 Issue #3
Introduction
Astrophysics is the fundamental science of studying where essentially everything comes from and where everything will go someday. One question that still has scientists cringe their heads around is the question of what essentially causes the universe to evolve. Classical mechanics tried to explain this phenomenon in the way that forces and movement are present in a static time flow that is completely separated from everything else. Today, however, we answer this question a little differently by using something we call ENTROPY.
The Thermodynamic Arrow of Time
But what really is entropy anyway, and what is thermodynamics?
Thermodynamics studies the relations between heat and other forms of energy (such as mechanical or electrical energy). One known thermodynamical concept is expressing heat in terms of the momentum and hence the kinetic energy of particles. The more movement, the higher the kinetic energy is, and we hence expect higher temperatures. A common example of this is water.
Depending on the vibrations and the movement of the molecules is the temperature of the medium. But what else can we see in this image?
On the very left we can see water in its solid state and on the very right in its gaseous state. What can we see besides the difference in particle movement?
The molecules on the left are obviously more organised and have more structure, while the ones on the right and in the middle are moving around pretty chaotically. We can define a degree of disorder from the right to the left, meaning there is more order on the left and less order on the right:
What rule can be derived from this?
Naturally, when you put a ball of ice into a glass, it slowly transforms into water over time. We may say that a system of order tends to move to disorder. And here is where entropy kicks in. Entropy essentially is a scientific concept that is most commonly associated with a state of disorder, randomness, or uncertainty.
But there must be some limit to disorder. A ball of ice would not just turn into gas by sitting around in a space at room temperature. The ball of ice doesn’t turn into gas because there are no favourable conditions other than to do so. The ball of ice would need to generate and use a lot of energy to turn into water vapour if it isn’t in a room with the fitting conditions.
And there is the catch of entropy. Although thermodynamics states that in an isolated system entropy (the degree of disorder) tends to increase over time, it doesn’t imply that it cannot remain constant because in the case of water turning into ice that is precisely what it does.
So our final definition of entropy for today is:
In isolated systems, the total entropy tends to increase over time, following the second law of thermodynamics. This law states that the entropy of an isolated system never decreases; it either remains constant in reversible processes or increases in irreversible processes.
QUICK INTERLUDE REVERSIBILITY
In thermodynamics, the terms "irreversible" and "reversible" refer to the nature of processes that a system undergoes.
Irreversible Processes: These are one-way processes that cannot be undone to restore the system and its surroundings to their original states. They typically involve an increase in entropy and are common in real-world systems.
Reversible Processes: These are idealised processes that can be undone step-by-step to restore the system and its surroundings to their original states. Reversible processes minimise entropy generation and are often used in theoretical analyses, but are rarely achievable in practice. (For example, ice melting and becoming water.)
Entropy in Stellar Evolution
But what does entropy have to do with the evolution of the universe?
The universe rapidly accelerated after the Big Bang, which is an event that occurred roughly 13.8 Billion years ago. Ever since then, matter has started to clump together through gravitational attraction, creating stars, planets and eventually galaxies.
The formation of molecules is actually due to entropy. Similar to the water example, molecules flew around increasing entropy which eventually led to stellar evolution. But there is more to it.
When the first stars started to form, they generated their energy from fusion reactions in their cores. Essentially, this fusion reaction increases the entropy of the stellar interior. As the star ages, it one day reaches the end of its lifetime when the degree of disorder is too high to keep the star alive. In those cases, some stars explode in a supernova explosion. These processes release immense amounts of energy into space, contributing to entropy increase in the surrounding interstellar medium.
And not only that. Stellar explosions such as supernovae are not just incredibly violent but artistically beautiful events, they are also one big reason why you and I are alive. We are carbon-based organisms and when massive stars reach the end of their lives, they undergo supernova explosions, releasing tremendous amounts of energy and scattering newly synthesised elements, including carbon, into the surrounding interstellar medium.
These ejected materials then contribute to the formation of subsequent generations of stars, planets, and ultimately, life as we know it. Therefore, the carbon atoms that make up our bodies and the organic molecules essential for life likely originated from the ashes of ancient supernova explosions.
So, essentially, the increase in entropy in one system can lead to the existence of a new stable system, which, for example, is your body. But also your body as its inevitable end due to the increased disorder.
Entropy and the Fate of the Universe
Ultimately, entropy is also linked to one crucial thing: the entire fate of the universe. We have learned that entropy describes the measure of the amount of disorder within a given system and that it tends to increase over time. Entropy is actually what implies an intrinsic arrow of time, with the universe progressing from states of lower entropy (more ordered) to states of higher entropy (more disordered).
As the universe continues to expand and evolve, entropy steadily increases, driving irreversible changes in cosmic structures and processes. Eventually, this relentless rise in entropy will have profound implications for the fate of the universe.
How the universe ends is ultimately undecided, but entropy certainly will play a role in the process, no matter if it crunches together, freezes or repeats itself.
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Thanks again and I'll see you soon.
Victor (@observethecosmos)
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