Can strings be the ultimate constituents of the universe–more fundamental than matter or energy, and even than space or time? If they’re not made of matter or energy, what are they, then?
If you look for some light fare for your placid Saturday afternoon, here’s an idea: ponder these ultimate questions–on life, nature, the universe and everything–with my fellow SciAm network bloggers George Musser and John Horgan, by watching the debate they had last night at Bloggingheads.tv.
George and John make an especially compelling duo to watch because their personas are almost diametrically opposed. George’s is that of an unabashed optimist, and toward the end he states his belief that science will eventually discover the ultimate simple laws of everything that will make everyone smack their forehead and go, “why didn’t I think of it?”
John on the other hand has for decades fashioned himself as the curmudgeon of science journalism, poking fun at the cultural idiosyncrasies of science and essentially declaring game over in his 1997 book The End Of Science: Facing The Limits Of Knowledge In The Twilight Of The Scientific Age (as well as in numerous articles for Scientific American, where he used to be a senior writer).
Both of them however reason with much more nuance and subtlety than those simplistic descriptions would have you think, as they demonstrate in more than an hour of discussion intertwining the grand themes of fundamental physics: string theory, the multiverse, cosmic inflation and the anthropic principle–and even the issue of what science itself is.
The part I found most intriguing is toward the beginning, when John takes issue with the idea that strings, those infinitesimally thin and frustratingly enigmatic loops that many of our readers either hate to love or love to hate, aren’t made of matter or energy.
Instead, some physicists say, strings would somehow occupy a deeper layer of reality. It is matter and energy that are made of strings, not the other way around. Similarly, John laments, some physicists contend that strings do not exist in space or time, but instead that space and time themselves may be made of strings.
“My question was,” John says, “What is a string then? If it’s not something that can be situated in space and time and if it’s not constituted of matter or energy, what the hell is it? Is it some kind of pure mathematical form?”
(Mathematical? Now you’re talking, I said to myself.)
George’s answer: let’s not get ahead of ourselves. “It is perhaps premature to talk about what a string might be made of since it’s already a big jump to talk about strings.”
I suspect however that John’s objection is more of a philosophical nature. It strikes at heart the same dispute between realism and phenomenology that has divided physicists ever since Albert Einstein and Niels Bohr were arguing over the implications of quantum mechanics. Simplifying, one could trace that tension all the way back to the philosophers of ancient Greece, starting with Parmenides and Heraclitus.
Immanuel Kant on the other hand observed that science cannot access the ultimate nature of reality–the noumenon (or “thing in itself”). Our eyes and ears, or our finest scientific instruments for that matter, only detect phenomena, not noumena.
In that view, a scientific theory cannot say what the ultimate constituents are: only that they behave according to the predictions of certain mathematical rules. Thus, the question of what the string is made of is not relevant as long as we can experimentally verify that it acts the way that a theoretical string does.
And theoretical strings are geometrical objects; in modern geometry–and by modern I mean ever since Carl Friedrich Gauss, 1777-1855, and Georg Bernhard Riemann, 1826-1866–geometrical objects can be defined intrinsically, that is, it is not necessary to think of them as occupying a larger “space.”
But strings may forever be beyond the scope of science, as John points out. His objection is that strings are too small to detect with any conceivable experiment.
George (who is the author of The Complete Idiot’s Guide to String Theory) had a great answer: that smallness is not specific to strings. Any theory that unifies all forces of nature will have to include gravity. But gravity is extremely weak compared to the other forces, which implies that detecting its quantum behavior requires experimenting with energies that are completely outside of our reach. Any test of a quantum theory of gravity “will have to be indirect and none of them is going to be decisive.”
One thing that fascinates me about the focus on phenomena versus noumena is that in my view, on a very different philosophical level, it is beautifully paralleled in the formal structure of mathematics. Modern math is entirely rooted in the theory of sets, in the sense that any mathematical object and construction can be defined starting from sets. But in any description of set theory that I’ve seen, you never say anything about what the sets ultimately are made of. There’s never a set that contains anything except other sets.