Physics philosopher David Z Albert talks to cosmologist Sean M. Carroll in this 2008 Bloggingheads interview. They defend string theory as legitimate science, even tho they concede that a particle accelerator the size of the galaxy might be unable to test the theory. That is, there is no conceivable observation that would be contrary to the theory.

Nevertheless Albert argues that gravitation is a "spectactular predictive success of string theory."

This is nonsense, of course. If string theory said anything about gravity, then it would be testable. Gravitation is well understood, but there are some anomalies such as dark matter and dark energy. String theory does not resolve these, as one would expect if it really predicted gravity. String theory does not predict gravity.

The only connection between string theory and gravity is that string theory predicts a spin-2 particle, and gravity waves have characteristics of a spin-2 field. But gravity waves have never been observed, and there is no hope of observing those spin-2 gravitons. Also, string theory models are usually assumed to be Ricci-flat, as general relativity requires of empty space.

A recent SciAm article claimed that unitarity and supersymmetric gravity could be used to show that a spin-2 graviton resembles two spin-1 gluons, ressurecting ideas that had been abandoned in the 1980s as unworkable. There are fewer divergences that previous thought, the article says. But there are still infinities that prevent predictions.

They go on to discuss the arrow of time. But there are no testable ideas there either.

Albert also discusses being sucked into the 2004 movie What the Bleep Do We Know!?. He claims that his views were misrepresented. The movie is New Age drivel about quantum consciousness. Of course his ideas were misrepresented. They always are. Physicists themselves cannot express themselves coherently on this subject. Philosophers never say that their views are accurately represented. I have listened to him for a couple of hours, and I still don't think that I could summarize his views in a way that he would accept.

Albert said that physicist Eugene Wigner once said that a dog could make a quantum observation, but not a rat, because dogs are more conscious. Wigner was brilliant and one of the creators of quantum mechanics. If physicists say stuff like this, then they should not be surprised that New Age hippies go further.

Albert previously trashed Krauss's book on nothing, and so Krauss badmouthed all philosophers. Albert was supposed to be one of those philosophers who actually understands physics. They both understand physics but they also say some wacky things.

"The only connection between string theory and gravity is that string theory predicts a spin-2 particle, and gravity waves have characteristics of a spin-2 field. But gravity waves have never been observed, and there is no hope of observing those spin-2 gravitons. Also, string theory models are usually assumed to be Ricci-flat, as general relativity requires of empty space."

ReplyDeleteI'm sorry, but this is just not right. First of all, gravitational waves are predicted by general relativity, and general relativity has passed many detailed tests. It's true that we have not quite seen gravitational waves directly, but we have indirect evidence for their existence coming from observations of a certain pulsar. By measuring the change in the frequency of this pulsar, Hulse and Taylor confirmed that it was losing energy in exact agreement with the prediction of gravitational waves. They were awarded the 1993 Nobel prize for this work.

If we assume that gravitational waves exist, then the existence of gravitons is forced on us by quantum mechanics. This is the starting point for all research programs seeking to quantize gravity. It is not in any way specific to string theory.

I also think you're not giving string theory the credit it deserves. The theory reproduces general relativity on large distance scales, and it eliminates many of the difficulties associated with nonrenormalizability of gravity. That's not such a trivial thing. Also, string theory models are Ricci flat not because space is empty but because string theory is formulated by perturbing around a fixed background metric. This background dependence is often cited as a problem for string theory, but the problem has been solved to some extent with the discovery of the AdS/CFT correspondence.

You are right that pulsars have been observed to dissipate energy in accordance with general relativity, but no one has observed the gravity waves. LIGO has tried, and failed.

ReplyDeleteSaying that qauantum mechanics forces gravitons is speculative and probably unobservable. String theory only reproduces general relativity to the extent that string theory assumes general relativity in the first place. String theory does get rid of some of the infinities in its renormalization, but unless it gets rid of all of them, what good is it?

"LIGO has tried, and failed."

ReplyDeleteYes, as I said, we have not observed gravitational waves directly, but their existence is supported by very good indirect evidence.

"Saying that qauantum mechanics forces gravitons is speculative and probably unobservable."

Actually, it's a very simple consequence of the fact that any observable associated with a monochromatic wave must change periodically in time. In particular, the energy expectation values of a monochromatic gravitational wave are periodic, and this implies that the energy spectrum of such a wave is discrete.

"String theory only reproduces general relativity to the extent that string theory assumes general relativity in the first place."

Not true. There is no sense in which gravity was put into string theory by hand, and in fact this is why people sometimes say that string theory predicted gravity. It was a complete surprise that string theory contained a theory of gravity.

"String theory does get rid of some of the infinities in its renormalization, but unless it gets rid of all of them, what good is it?"

I am not an expert on this topic, but I know that there are strong arguments that string theory is perturbatively finite, and arguments that the theory is consistent non-perturbatively as well.

I should also say a few more words about the role of flat spacetime in string theory; the explanation in my previous post was a bit sloppy. The point is that in string theory, spacetime is not assumed to be flat. In string theory, you're starting with the flat metric of Minkowski spacetime and introducing gravitational effects by perturbing this metric. A priori, this means that string theory is background dependent. But this turns out to be incorrect because string theory has a dual description in terms of a conformal field theory on the boundary of spacetime.

The thread continues here. (I accidentally responded to the wrong thread.)

ReplyDelete