Earth Size

Earth Size
Relative Size of the Sun and Earth

Monday, January 13, 2014

TAU Introduction


Our home as seen from the International Space Station
Roughly 2.5 million years ago, our ancestors began using tools to improve their lives and, in some (perhaps implicit) sense, also began learning about the world in which they were embedded.  Only very recently, with the aid of new “tools”, have we been able to peek out and learn about our nurturing blue home and its proper context within the vast unknown of the outside Universe.  

At least as important as any particular discovery we’ve made is the realization that the Universe is seemingly accessible --- by performing experiments and applying rational arguments, we can actually learn about its history and evolving nature.  Here, I'll try to outline the modern understanding of the workings of the Universe.

In my mind, there are at least 3 high-level goals one should pursue while reading something like this:

You should emerge on the other side with some appreciation of your cosmological context, your relative place in the Universe at large and the real relationships in scale between the cosmos as a whole and its constituent parts.  We tend to live our lives ignorant of the realities and relationships of nature on scales very much smaller than we and those much larger (and not just spatial scales, but timescales too; some things happen very much more quickly or slowly than is easily observed).  You will hopefully develop an appreciation for these frameworks that bind all objects and processes together into one intimately woven history and future.  To paraphrase Neil deGrasse Tyson, we are a small part of, and live in, the larger Universe; it's important to understand, though, that the Universe and the products of 14 billion years of cosmic evolution are also within us.  We are a consequence of what has come before and what continues to happen around us.

An understanding of the physical scales involved should also be paired with an understanding of the evolution of the Universe in time.  A crucial point is that, as a consequence of the natural laws we've discovered, the Universe must change over time.  It is not the same today as it was yesterday, and is certainly different from a few billion years ago.  If it really evolves, where’s the evidence?  Shouldn't these changes be observable somehow?  Yes!  Read on...

The engine that drives our understanding about how the Universe behaves is the process of science.  You will (hopefully) develop an appreciation of this human enterprise while keeping in mind not only the chains of logic and inference we use to make sense of the cosmos around us, but the assumptions we tacitly use to learn about the Universe.
For example, to do science, we typically make the following assumptions about the Universe at large:

The Universe does not operate via "Magic"

There is a consistent set of underlying rules and principles that governs the behavior of all objects or phenomena.  To investigate something scientifically means, in part, to believe that it can be explained as a natural consequence of either known laws or new principles that may extend the known ones.  

Humans can understand these rules

This is not as obvious as it sounds.  Naively, we might think that we can understand any set of rules of behavior, but let's be cautious.  You'll learn, for example, that we've largely solved the mystery of how the moon orbits the earth; it is, in fact, the same force that causes apples to fall out of trees.  We think we understand that.  But just try explaining that to a dog.  It's not just that they're not interested --- they're not.  More fundamentally, the dog's brain is made of the same stuff as ours is, as far as we know, but it seems safe to say that dogs are somehow limited in their capacity to understand the Universe compared to a human.  Perhaps the number of neurons and connections in our brains is sufficient to appreciate abstract concepts that might escape the dog.  This works all right for our current model of gravity, but you could imagine some natural process operating through a set of rules that are far too abstract or complex for us to understand.  There may be creatures elsewhere with fantastically more complex brains that can understand these new rules, but we might forever find them incomprehensible.  Perhaps quantum mechanics, the description of the rules followed by very small particles, is already fundamentally beyond our conceptual understanding.  In any case, all I'm saying is that just because we assume everything operates according to rules doesn't mean that we will be able to always understand all the rules that govern the Universe.  To do science, though, we assume that we can. 

The same rules apply everywhere

This breakthrough is commonly first attributed to Galileo and Newton.  It is tremendously important since it allows us to perform experiments here on Earth and then assume that the same rules and processes apply in the same way even in the most distant galaxy.

Now, it's possible that at least one, or all, of the above assumptions are not true.  We still don't know, for example, if consciousness has a purely mechanical or physical explanation.  To investigate it scientifically, we have to assume that it does.  Even if such a model for consciousness is developed, it's important to keep in mind the following caveat regarding scientific theories:

No scientific theory can ever be proven "true"

(Really, I like to say “model” rather than the overused “theory”).  They can certainly be proven false, but never true. The best we can ever say is that all the available evidence supports a given model.  Indeed, as we’ll see, most initial models that have ever been developed have yielded to modification and replacement over time.  New models are ever more precise and can better explain new observations gathered over longer times and with better technology.  We'll see many examples of this as we go along.  People often make mistakes on both sides; to say that some model is “just a theory” (evolution, gravity) is ignorantly dismissive of the evidence in favor of its wide adoption (a theory in science is much stronger than a “guess” in popular language. A reasonable “guess” in science is called a hypothesis.)  But to assume that a given theory is “true” in some ultimate sense is also naive. Any theory is subject to further testing and refinement, or perhaps replacement; however, new theories are still constrained to be consistent with everything we’ve observed so far.

Evolutionary history of mammals
Finally, I should draw a distinction between “fact” and “theory” since they are frequently confused in popular media.  Facts are observable outcomes of experiments.  Anywhere you go on the surface of the Earth, a dropped object will fall towards the center of the Earth.  You can even precisely measure how objects fall, at such-and-such a rate, and find that it is roughly constant no matter where you go on the surface of the Earth.  
Newton noted that the path of the Moon around the Earth also seems to obey the same general principle.  We imagine now that it “falls” towards the center of the Earth.  As we accumulate facts, then, we then may generate some abstract set of rules that explain these observations.  In this case, that would originally be Newton's gravitational theory.  It generalizes these facts into a unifying mathematical framework that is able to reproduce the observations and then further predicts the outcome of some new set of observations, like the motion of the planets around the Sun.  Later we might find that there are facts that contradict our theory, so it will need to be modified.  Einstein's modification of Newton's theory is just such an example, as we'll see.  So there are the “facts” of gravity and the “theory” of gravity that knits the known facts together in an explanatory model.  In another familiar case, there are the facts of evolution (the genetic code of organisms change from generation to generation, organisms compete with each other for limited resources in a changing environment and may pass on to their offspring any traits that give them an advantage), and the theory of evolution attempts to explain these facts using concepts such as genetic mutations and natural selection.

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