Friday, January 7, 2011

The Scale Of The Universe and And It's "Best Theory".

Many of you have heard the phrase "use the right tool for the right job", and when it comes to physical theories the story is no different.  For example, I often hear that quantum mechanics is more fundamental and thus a better theory than Newtonian physics.  But is it always the better theory?  For example, does quantum mechanics describe the solar system better than Newtonian physics?  For all practical purposes the answer is a big "No Way!".

And, further, can Newtonian physics describe the large scale properties of the universe as well and general relativity?  Again the answer is no.

Look at the flash game above.  As you move the cursor back and forth, you see the universe at different scales.  And for each separate scale, a different physical theory becomes the best theory to use to describe that scale.  It really is the case that scientists are well advised, when describing the universe, to use "the right tool for the right job."

Question: But aren't the more fundamental theories are telling more about what is really going on?

Actually, it's hard to say!  For example, I've already posted on how some of the theoretical machinery going into our most fundamental theories of nature could just be clever mathematical models that just so happen to fit nature.  Not necessarily what is actually going on.   Furthermore: I'll give another example: is spacetime really curved, like general relativity says, or is something else going on like the interaction of a spin-2 graviton?  (Or something else entirely and yet the math just happens to work out making them clever models as opposed to the true reality!)

So, my advice to those who want to classify (and many do!) which physical theory is most superior or "most correct": I advise you to first ask what scale of the universe you are trying to describe.  Because, it turns out that each scale of the universe has it's own best theory.

A best theory for describing the cosmos at large... a different best theory for describing how a planes and rockets fly through the air or how bridges stand... a different best theory for describing how elementary particles interact... a different best theory etc...

Finally: It is this observation that allows cosmologists to think there may be a better theory than general relativity for describing scales larger then have been examined thus far.   Or: one reason why string theorists have good case for why there might be a better theory than standard quantum theories for describing the smallest of scales.

In short: the idea of a best theory is really scale dependent!

Click on the image to the right from XKCD.


  1. The best theory to describe the Universe at the scale of Stan taking a physics test is the 'Wild Guess' theory. =:) My professors tended to disagree. Thank you for confirming in my own mind that they were wrong all along.

  2. Loved this! What a great Flash game.


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