What are the strings in String Theory made of?


Layman: What are the strings in String Theory made of?

Physicist: Well, they are not made of anything. They are fundamental.

Layman: Like how sometime back protons were fundamental, and then how quarks were fundamental?

Physicist: You see, physics usually advances gradually, building upon our earlier understanding. Sometimes, we have breakthroughs: times such as when Newton published his Principia, when Einstein published his Theory of Gravitation or when Quantum Mechanical Laws were published. New models of reality that change our conception fundamentally are found and we begin again. The earlier theories are still true but they are now a small part of a bigger picture. Scientific progress is this continuous illumination of newer vistas, the lighting of hidden parts of an inexhaustibly large picture.

Layman: Nothing is really fundamental then. Everything is provisional? There will never be a Theory of Everything?

Physicist: Well, a Theory of Everything is a linguistic construct, not a physical one. We can never be sure if a theory explains everything in the Universe because we don't and can't possibly know all that the Universe is made of. A Theory of Everything is a grand way of saying we know a lot.

Layman: I suppose this is your personal view. Hawking says a Theory of Everything is just around the corner. I don't think he meant to say we know a lot. I think he said we would know Everything.

Physicist: He said it before, not anymore. Gödel's Incompleteness Theorem has seen to that.

So, I re-read Physicist Edward Witten's beautiful article on String Theory yesterday night and kept thinking about what Wittgenstein said: Knowledge is in the end based on acknowledgment. In many ways, it surely is (and personally for me, to acknowledge this is to make peace with myself). Consider the question: Are the Strings in String Theory fundamental? By definition Strings are fundamental. To me, 'by definition' is equivalent to a big acknowledgment - atleast among peers - that is not based on a proven fact. String Theory is beautiful (so I hear, unfortunately, the math is beyond me). Often, the beauty has proved to be a reliable guide to Reality. But guidance is not a guarantee. Anyho, I made-up the above dialogue between a layman and a physicist to keep my wife from dozing-off while I waxed on and on about cats and strings. I don't understand Gödel's Incompleteness Theorem so I've stopped at that. You are welcome to extend it.

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"I don't understand Gödel's Incompleteness Theorem..."

It'll be fully comprehensible when it's complete....

Godel's incompleteness theorem in a nutshell:

Consistent; complete; concise. Pick two.

If you leave out consistent, you can prove that 0 = 1, so that's not very useful.

You might be tempted to leave out concise (because completeness is a tempting thing to have!), but then, as Godel proved, you literally need an infinite number of axioms. (See below.)

So you leave out complete, and when someone asks "Is the sentence 'this sentence is false' true or false?" you say "I can't know." Not "I don't know," that implies you might figure it out someday; rather, you know that you will not ever know. (The wonder of Godel's proof was that he was able to make a simple formal logic system look back upon itself and be able to ask the question from within the system, thereby proving that there will always be something we don't (can't) know.)

(If you really wanted completeness, when you find such an undecidable sentence, you could make a new axiom defining its truthfulness one way (or the other), but there will always be another undecidable statement, and you'll need another axiom, and another, and another.)

By Dale Sheldon (not verified) on 14 May 2009 #permalink

Well, strings could also have an internal substructure, except that the typical length scale associated with strings - the string scale - is comparable to the Planck scale, the minimum distance where the concepts of geometry make sense. So there can't really be anything shorter, and in this sense, there can't be any further substructure inside strings. Once strings are understood properly, that's the end of the story.

In perturbative string theory, strings are truly fundamental. For finite couplings, nonperturbatively, they're not. It's "M", whatever it is (imagine a versatile and fuzzy object that can behave as strings, branes, or black holes), that is fundamental in general. At any rate, the hierarchy of matryoshkas within matryoshkas does ultimately end.

Q: Is the sentence 'this sentence is false' true or false?
A: No.

Luboš, I think the confidence that 'the hierarchy of matryoshkas within matryoshkas does ultimately end' is reasonable and justified. The basic quantum mechanical underpinnings have stood the test of time. But, it would be hubristic to think that we would figure out Everything (which includes the fundamental building blocks). I would not be surprised if, in a few decades, String Theory becomes the equivalent of Newtonian Mechanics (assuming String Theory wins out among the competing ones) and - behold the blasphemy - plank's constant is canned. If history is any indication, something like this would probably happen. My reading of history is at best limited, and you can clearly tell that I am not a physicist, so it is quite possible that I may be talking nonsense here.

Physicist: He said it before, not anymore. Gödel's Incompleteness Theorem has seen to that.

I thought Goedel's theorem was well known much before Hawking even became a physicist?

sri, Hawking revised his position after considering the implications of Goedel's theorem.

Q: Is the sentence 'this sentence is false' true or false?

A: true, because true or false is true. ;-)

By Pascal Bourguignon (not verified) on 20 May 2009 #permalink

May be we perceive the universe as mass as we have similar properties as it has ?
What if there is stuff which passes right through, but we can not perceive it, neither that matter knows we exist ?
May be matter is just an illusion, and we are a part of it, we just can perceive it ?
What if there are parallel worlds passing right through you, but they can not perceive, you neither can you see them ?
What if matter is really not matter, its just an illusion of matter as perceived by us ?
What of matter is just space vibrating ? or what else ? who knows ?