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Can String Theory Accommodate Inflation?
David Shiga writes "String theory is the leading contender for a "theory of everything" that could unite all the forces of physics. But a recent study suggests that it may be more difficult than scientists had hoped to square string theory with inflation — the widely accepted notion that the early universe had a period of especially rapid expansion. Some say this could even lead to the abandonment of either string theory or inflation, though no one is ruling out a possible resolution yet."
I've never understood how string theory could ever be validated, except through funky math invented for the purpose. It's my understanding that if you enlarged an atom to the size of the universe a string would be about 50 feet long (about a planck length). How is that ever going to be testable in practice? From all I've read, the energy required is just not possible, ever.
Um...physics has completely turned upside down in the last century and has changed pretty dramatically over the last 20 years. What kind of remote island are you living on that you're so out of touch and think that 'different' approaches never get funding? If you've never bothered to look at current research then you really don't have any right to speak, and it's obvious that you haven't. For example there has been ongoing debate for many years now between people who are searching for dark matter and proponents of MOND. There's nothing more annoying than pontification from ignorant armchair physicists.
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I'm not a string theorist, but I am a cosmologist. Here are some thoughts:
Inflation has not been "confirmed" in away way. It's the best explanation for a very very limited number of datapoints we have on the "early" Universe. Very smart people (e.g., Sean Carroll, now at CalTech) have made convincing cases that inflation is actually incoherent in important ways. I have spent quite a bit of time trying to come up with alternatives to inflation, and it's damn hard -- it "works" very well, in the sense that it solves a bunch of problems all at once that are hard to solve individually. But it does invoke plenty of nonstandard physics we've never seen in the Universe, let alone the lab.
Inflation and dark energy are deeply connected. They both require something called "negative pressure". Negative pressure is bizarre, and actually is from a Newtonian perspective a violation of the conservation of energy (in General Relativity, energy is not conserved -- rather a complicated combination of numbers some of which refer to what we'd measure as energy is conserved.) Negative pressure means that if you take a box of the stuff, and let it expand, at the end of the day there's actually more stuff in there than you started with.
String theory should better be known as "a collection of approaches." It does not have the coherence of, say General Relativity, which is a mathematically closed system. Talking about "giving up string theory" is kind of dumb -- essentially what you are saying is "do not try to do the following large class of calculation." There are definitely competitors to string theory, but none have captured the attention of a highly fractious community the way string theory has.
Not sure if anyone's still reading this thread, but I'm happy to talk more about it. Reply with questions if you like!
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Some versions of string theory do make a genuinely interesting prediction. They predict a particle that hasn't actually been observed. A large class of the most studied string theories require adding just this one particle to the standard model, and it has to have spin 2, and mass 0.
That's a great candidate for a graviton, which is also being predicted for some other reasons. When some of the most aestetically interesting versions of string theory turned out to predict not just any particle or whole family of particles, but that specific one, many physicists got more interested in those theories.
However, general and special relativity don't actually predict gravitons - Einstein was able to treat gravity as a strain inherent in space-time and not as something mediated by a particle at all, and get some very testable results. Quantum Mechanics doesn't really require Gravitons either. Actual particle accelerator experiments have satisfied various symmetry theories from just the particles observed, and this again doesn't include gravitons.
There's no practical way to build an accelerator that could even theoretically reach the energies needed to test unification of all four fundamental forces, and gravity is the odd man out that we have no expectation will be integrated by either accelerator experiments or astronomical observations.
Proof of a mediating particle for gravity would still not prove any of the string theorys, but it would give the likelyest of them some fairly strong support. For now, we're stuck - a theory looks mathematically beautiful, and actually makes a prediction, but we aren't sure yet if that prediction is ever going to become testable, and on the other hand we have no categorical proof the prediction is fundamentally untestable. A test would be nice, but so would a stronger reason for saying there could be no test than just that we aren't yet a type 2 civilization, with the energy of a whole galaxy to use, so we are limited by the economics of it.
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