String theory, which took root in the 1970s, proposes that “all objects in our universe are composed of vibrating filaments (strings) and membranes (branes) of energy.” That’s the ultimate Cool. It unites general relativity (the physics of the very big) with quantum mechanics (the physics of the very small) in one grand unified Theory of Everything, turning current conflicts into harmony.
But string theory offers more. It can undergird the concept of a multiverse: There are more universes than particles in our known universe.
How so? To work at all, string theory requires at least nine spatial dimensions (six of which are curled up out of our sight) plus time. But if our universe (three spatial dimensions plus time) arose randomly among the ten dimensions of possibilities (the “string landscape“), theorists reckon that there should be about 10^500 universes (or more). Literally anything can happen, has happened, and will happen over and over again.
The sheer number suffocates the evidence for fine-tuning. Our universe happens to look fine-tuned? But the theoretical others don’t. New Scientist spells it out: “This concept of a ‘multiverse’ could explain a puzzling mystery — why dark energy, the furtive force that is accelerating the expansion of space, appears improbably fine-tuned for life. With a large number of universes, there is bound to be one that has a dark energy value like ours.”
Well, yes. But what have these fad concepts done for science as we knew it? Dark energy is, at present, a theoretical concept, not an identified type of energy.
There’s more. Supersymmetry, a further development of string theory, offers to create particles just by adding an “s” to their names. We are told that string theory divides all particles in the universe into two types: bosons and fermions, which differ by the nature of their spin. A supersymmetry is a type of connection between the two sorts of particles: Every fermion has a boson, and every boson a fermion. The new zoo of undetected particles is commonly named by adding “s” to its observed partner’s name: “So the electron would be paired with a particle called a selectron, quarks would have corresponding squarks…” Earlier in the decade there were great hopes that the Tevatron collider at Fermilab or the Large Hadron Collider would find these exotic particles but no collider has done so.
Curiously, Peter Higgs, who actually did find a particle (the Higgs boson, consistent with the Standard Big Bang model of physics), is not a believer in either supersymmetry or the multiverse: “It’s hard enough to have a theory for one universe,” he says. As the Economist pointed out in 2016, “Supersymmetry is a beautiful idea. But no evidence supports it.”
Evidence? Increasingly, cosmologists are showing impatience with the traditional search for evidence. There is all too much evidence for fine-tuning, for example, but it is just not philosophically acceptable. Why can’t physics be a showcase for beautiful naturalist ideas, cleverly presented, instead?
Critics, perhaps less imaginative than the theorists, decry string theory’s lack of testability. Science writer Philip Ball complains, “Proposing something as dramatic as seven extra dimensions, without offering the slightest prospect of testing to see if they are there, is a step too far for some physicists.” Indeed. 10^520 universes later, one suspects that science has long since left the building. Physicist Ethan Siegel tells us bluntly at Forbes that string theory is not science: It cannot be tested.
But string theorists offer a number of defences. They seek to change the rules, to allow “non-empirical theory confirmation,” or otherwise loosen the standards for science. At Nature, we learned in 2015 that feuding physicists were turning to philosophy for help, in “a debate over the integrity of the scientific method itself.”
Physicist Frank Close is blunt: “[M]any physicists have developed theories of great mathematical elegance, but which are beyond the reach of empirical falsification, even in principle. The uncomfortable question that arises is whether they can still be regarded as science.” Well, not by traditional standards, to be sure. Science writer John Horgan, even blunter, scoffs, “At its best, physics is the most potent and precise of all scientific fields, and yet it surpasses even psychology in its capacity for bull****.”
One solution might be to start a popular science buzz around the idea that there really is no scientific method after all. That would go down well now that post-modernism is stalking science.
Evidence or no, string theory remains popular. Skeptical Columbia mathematician Peter Woit wonders why: “The result of tens of thousands of papers and more than 30 years of work is that all the evidence is that if you can get something this way that looks at all like the Standard Model, you can get anything. Normally when that happens you simply acknowledge the problem and give up, but for some reason that hasn’t happened.”
If science-based reasoning doesn’t explain string theory, cultural history might: A culture might wish a multiverse into existence despite the facts, to satisfy emotional needs such as making naturalism appear to work. As Philip Ball says, “[N]ailing your flag to the mast of string theory has come to be seen as an expression of faith rather than reason, and physics has become polarised into believers and sceptics.”
In any event, string theorists have grown comfortable with their lack of evidence. At Smithsonian, theoretical physicist Brian Greene admits, “Evidence that the universe is made of strings has been elusive for 30 years, but the theory’s mathematical insights continue to have an alluring pull.” He adds, “I now hold only modest hope that the theory will confront data during my lifetime.”
String theory is possibly best seen as a superstition of naturalism. Superstitions fulfill deep needs that skeptics do not understand and are understandably hard to eradicate.