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Name: Christos
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Question:
Good day from Greece. When our solar system was formed 5 billion years ago the local entropy of the interstellar space was obviously decreased. How exactly the total entropy of the universe increased by the formation of our solar system? If it has been increased due to the fact that another star had to die as a supernova in order for the solar system to be formed, you need to take into account that there were no stars at the time of the big bang. Which brings the question that when the first star was formed after the big bang, the total entropy of the universe was increased, thus violating the second law of thermodynamics. What exactly I am thinking wrong as it is impossible that the above statement is correct?


Replies:
Hi Christos,

I think you might be confused about what is entropy. It is common to think of entropy as 'disorder', but this simple analogy will lead you astray in many instances. For example, a crystal (highly ordered) may have more entropy than a solution of salt (which seems less ordered). So "disorder" is not always a good way to think of entropy.

A better way to think of entropy is the probability of a system of being in a given configuration. Over time, systems will evolve into their most probable configuration -- but they may not start there, and the random motions they take may require a long time to get to the most probable configuration. The laws of physics govern what configuration is probably -- things like quantum physics and gravity determining how systems end up being.

So, in the context of the universe, I think there are still a lot of questions to be answered about what happened at the very early ages, and I am not an expert in early-universe physics, but I think it is safe to say that going from a highly-energetic but uniform state to a lower-energy, more distributed state (solar systems, etc.) is not a violation of the second law. The system is progressing toward its most probable configuration, and human perceptions of "order" or "disorder" do not necessarily relate back to entropy.

Hope this helps,

Burr Zimmerman


What you are missing is particles sent into space. Their dispersal produced an entropy increase.

When the solar nebula condensed to form our solar system, matter became more localized, true. But in the process, energy was released into space, mostly by radiation of photons.

If that had not happened, then the solar nebula could not have collapsed. The gas molecules and dust motes of the nebula would collide with each other and rebound with no loss of relative speed. But since they could lose energy by radiating photons into space, they could gradually cluster into the bodies now composing the Solar system.

Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Be very careful with the word "obviously" especially applied to a cosmic scale, and with a situation and state of matter such as the "Big Boom" and the uncertainty that followed that event(s)??? How do you know it was a single event. Maybe it was a process, not an event. The statistical definition of entropy, S, is: S = k x ln (N) where k is Botzman's constant and N is the density of quantum mechanical states. But even here it is not clear how "N" is calculated, or even defined. These are very complicated and arcane topics. It is not even obvious what the 'entropy' of the Universe means. In addition, you assert that: "Which brings the question that when the first star was formed after the big bang, the total entropy of the universe was increased, thus violating the second law of thermodynamics." I challenge you to prove your assertion.

Vince Calder


Christos,

The laws of science are determined by what seems to work, not by what is always true. We do not know what is always true because we cannot test every object in every situation. Also, some laws apply to average effects but not individual particles. You use two premises that might not be correct.

Our universe is all we know how to measure, but it is not necessarily all that there is. Some theories include many universes that can transfer energy between each other. If this is so, we have never measured it. Laws for transfer between universes do not have to fit the limits of energy transfer within a universe. We like to think they do, but laws can change as we understand more things.

Entropy is part of thermodynamics, and thermodynamics is a part of physics that applies to quantities and effects for large groups of particles. Even temperature is an average quantities. If the universe reduces to many particles and waves, then thermodynamics might apply. If the universe reduces to one huge super-particle, then quantum physics will have to take over. If you read about string theory and its problems, you will see that we do not know what happens within a black hole. General relativity and quantum physics disagree. So far, we have no way to test it.

Science and engineering are perfect laws to always decide exactly what happens in the universe. They are fields of study that allow us to reduce billions of facts to just a few patterns. These patterns are then reduced to equations whenever possible. The equations allow us to talk about the patterns and to make better use of the patterns. The patterns allow us to use what has already been measured to predict future measurements. So long as the predictions work, the science and engineering laws work. If they fail enough times, we adjust the patterns to allow for the new measurements. The laws of science are very important, but they only apply to what we can measure and are based only on what has already been measured. They are wonderful, but they are not everything.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


You have to be very careful of the word "obviously". In cosmology, nothing (or almost nothing) is "obvious". You leave yourself open to the response, "Show me why it is obvious?" I give you that challenge. Why is it obvious that when the first star was formed after the big bang, the total entropy of the universe was increased? After the "big bang" space itself was being created, not things floating around in space. If this is so, and it seems to be the case, it is not clear how the second law ( a macroscopic variable ) does, or does not apply. You are tackling a problem that has evaded resolution for decades, or more.

Vince Calder



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