The public perception of string theory has definitely changed over the last few years, with the latest evidence this week’s cover story in New Scientist, which begins:
It’s the theory everyone loves to hate.
The article (available fully only to subscribers, I fear) is entitled String Theory: The Fightback, and its story line is that, because of all this criticism, after nearly 25 years of work, finally:
string theorists themselves have realised they must find ways to put their models to the test. They may still be far from being able to observe a string in a laboratory, but experiments planned for the near future – and even one currently under way – could provide tantalising evidence either for or against string theory…
Now the string community is fighting back by devising creative, if indirect, ways to look for signs of strings – from hidden dimensions to ripples in space-time and other potential signatures of a stringy universe. The time has come to put string theory to the test…
Critics should take heed. Experiments now show that string theory may be testable after all. One study at a time, string theorists seem to be homing in on models that will make specific, falsifiable predictions.
What follows is the usual misleading hype of bogus “tests” of string theory, of the sort I’ve written about extensively here. They are:
For one of my postings about this, see here. More than three years ago Polchinski and the KITP at Santa Barbara issued a press release about this trumpeting the idea that such superstrings could be observed by LIGO “over the next year or two.” The problem with this kind of “test” of string theory is that you can easily come up with string theory models that produce lots of cosmic superstrings (already falsified), no observable amount of cosmic superstrings (can’t use to test string theory), or precisely the amount of cosmic superstrings such that no hint of them would be seen until now, but there would be networks of the things lurking just below the threshold of measurability, to be discovered by the next generation of experiments (highly unlikely, pretty much pure wishful thinking). Polchinski has now stopped talking about LIGO and a year or two, and instead is promoting measurements of pulsars and five to ten years:
According to Polchinski, though, our best bet for observing gravitational waves emanating from strings is to use pulsars. A pulsar is a rapidly spinning neutron star that fires out a beam of electromagnetic radiation as it rotates, like a lighthouse. These flashing beacons act as some of the most accurate clocks in the universe, and a gravitational wave rippling between a pulsar and Earth would disturb the otherwise precise timing of the pulses arriving here. The most likely cause of such fluctuations would be black holes colliding, but waves from strings would yield a unique timing pattern that would make them stand out. “Over the next five to 10 years,” Polchinski says, “these [pulsar observations] will probe the most interesting models.”
My colleague Brian Greene is quoted about this, making the essential point about this kind of “test”:
Sure, catching sight of a cosmic string would be a boon for string theory, but is there any observation that would serve as a death knell? For many sceptics, it’s not that string theory is so hard to prove correct that puts them off, but rather that you can’t falsify it. “I’m not aware of any test that if it fails will prove string theory wrong,” says physicist Brian Greene of Columbia University in New York. “That’s a real headache. You’d like to have a situation where you have a prediction, and if it’s right the theory is right, and if it’s wrong the theory is wrong.”
The problem is that the quoted paper doesn’t actually say that. Here’s the paper’s concluding paragraph:
However, a possible discovery of tensor modes may force us to reconsider several basic assumptions of string cosmology and particle phenomenology. In particular, it may imply that the gravitino must be superheavy. Thus, investigation of gravitational waves produced during inflation may serve as a unique source of information about string theory and about the fundamental physics in general.
Given the wealth of possible string theory scenarios, I have no doubt that if an imprint of gravity waves is found in the CMB, there will be string theory models that would “predict” it. As for the idea that not seeing such gravity waves would be evidence for string theory, here’s what Glashow has to say:
Not everyone thinks these tests will be useful, however. “Not seeing something is hardly evidence for string theory,” says Nobel laureate Sheldon Glashow of Boston University, Massachusetts, an outspoken critic of string theory. He feels that such a result would mean very little. “String theorists are very wise. They can come up with a way to explain anything.” String theory is simply not testable, he says. “There are an enormous number of string theories and they describe zillions and zillions of universes, none of them observable in any way. It sounds to me like angels dancing on the head of a pin.”
“We’re still very far from being able to say, here is the exact string theory that describes QCD,” Susskind says. “But the connections that show up between nuclear physics and string theory are fascinating. Sceptics won’t consider this evidence for string theory, but nuclear physicists will use string theory and in time discover how accurately it describes these experiments.”
What Susskind is neglecting to mention here is that these are tests of whether string theory is a useful way to do calculations in an already tested theory of the strong interactions, and has nothing to do with the question of testing the idea of string theory as a unified theory of quantum gravity and particle physics.
Also in New Scientist, there’s a story about Neil Turok’s recent talk at PASCOS entitled “Is the Cold Spot in the CMB a Texture?” which links to the string theory article. Evidently there’s a patch of CMB where the temperature is anomalously low (the probability of this happening supposedly being 2 percent) and one can speculate that this may be due to a topological defect of some kind. Amusingly, the online version of the New Scientist article includes an interpolated editorial comment that someone forgot to take out before publication:
Turok presented the findings at a conference on particles Hi Anil. This threw me a bit. We say the team noticed previous work by Turok, and then before we know it Turok is the one presenting the work. When did he join them, strings and cosmology at Imperial College London last week.