Saturday, November 17, 2007

Effective Field Theory Verses String Theory

It's my turn for Physics Philosophy. You had your quantum hermeneutics and your MOG/dark matter philosophy this week, time for String Theory vs. Effective field theory.

If a theory is not renormalizable it blows up at high energies, so it can't be a true theory of everything. However, if below some energy it doesn't blow up but yields good results at that low-energy it is called an effective filed theory. At a certain energy, it is effectively a correct theory.

To many these are the quantum-gravity theories we should be working on because they work well at energies we expereince. Furthermore, every renormalizable theory is replaced by a new one at high energies anyways. For example, QED is a renormalizable field theory, but who cares? At high energies it has to be replaced by QCD anyways. Who would care if QED was only good up to a certain energy if it needed to be replaced at a higher energy anyways? (Since new physics emerges)

String theory has the appeal that it is remormalizable at all energies so it can be a true theory of everything. It's problem is it is so complex it can't accurately describe low-energy physics in a way that gives testable predictions.

So physics philosophers what do we do?:
  1. Work on a theory which has the possibility of working and describing the whole Universe with the con that is is hard to describe low-energy physics, the physics we see in real life everyday?
  2. Work on a theory that works perfectly at a certain energy scale we want it to with the con it will eventually blow up outside that scale and will therefore need to be replaced?
So there you go. What do we do? Go for the gold now, even though it is really hard, and may not work or second, do something we know from the outset is flawed but for all intents and purposes works well where we need it to work.

(The sound of one hand clapping is now heard off in the distance.)


  1. I guess it just depends on what you're trying to accomplish. I just went to a major conference here at UIUC celebrating the 50th anniversary of the BCS theory of superconductivity (yes, that was discovered here in Urbana) where one of the keynote speakers was a particle physicist (random, I know). Anyways, he was some really big name in particle physics (I can't remember it right now or I'd mention it -- you'd probably know him), but even he could see that it was a bit odd having him speak at a superconductivity conference. Basically one of his major points was that asking whether high energy physics (like particle physics) or condensed matter physics (like superconductivity) is more important is an ill-posed question. They are different undertakings with different aims. Particle physicists don't generally go to work thinking "How is what I am going to do today going to improve peoples' lives here in 20 years?" But that is exactly the type of thing that condensed matter physicists often think about. Particle physicists are more likely to ask "How is what I am going to do today going to help reveal the deeper mysteries of the fundamental nature of how the universe works?"

    So I guess after that incredibly long diatribe what I'm trying to say is whether you "go for the gold" or *how* you go for the gold depends on what you're trying to accomplish. QED has led to all sorts of new advances in electronic technology and will likely continue to do so. However, as you said, it's not really a good "theory of everything" -- there has to be more. I think string theory is an exciting possibility for the "theory of everything" (I still have my doubts about it, but ok), but I'm not holding my breath for it to make it into my everyday life.

  2. Bill, I think you are right in a lot of ways. Good to hear from you.


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