It predicts the dimensionality of space, which is the only theory so far to do so, and it also predicts, at tree level the lowest level of approximation for a quantum-relativistic theory , a whole lot of massless scalars that threaten the equivalence principle the universality of free-fall , which is by now very well tested.
If we could trust this tree-level prediction, string theory would be already falsified. But the same would be true of QCD, since at tree level it implies the existence of free quarks. In other words: the new string theory, just like the old one, can be falsified by large-distance experiments provided we can trust the level of approximation at which it is solved.
On the other hand, in order to test string theory at short distance, the best way is through cosmology. Around i. It was followed by a heated discussion and I cannot judge who is right.
I can only add that the absence of a metastable de-Sitter vacuum would favour quintessence models of the kind I investigated with Thibault Damour several years ago and that could imply interestingly small but perhaps detectable violations of the equivalence principle.
Some of the popular coverage of string theory in recent years has been rather superficial. The usual argument is that you need unconceivably high energies.
But, as I have already said, the new incarnation of string theory can be falsified just like its predecessor was; it soon became very clear that QCD was a better theory.
Clearly the enthusiasm of young people is still there. The field is atypically young — the average age of attendees of a string-theory conference is much lower than that for, say, a QCD or electroweak physics conference. What is motivating young theorists? Perhaps the mathematical beauty of string theory, or perhaps the possibility of carrying out many different calculations, publishing them and getting lots of citations.
These are very hard problems and young people these days cannot afford to spend a couple of years on one such problem without getting out a few papers. Today it is much harder to do so. In we had a lot of data to explain and no good theory for the weak and strong interactions. There was a lot to do and within a few years the Standard Model was built.
Today we still have essentially the same Standard Model and we are still waiting for some crisis to come out of the beautiful experiments at CERN and elsewhere.
Steven Weinberg used to say that physics thrives on crises. The crises today are more in the domain of cosmology dark matter, dark energy , the quantum mechanics of black holes and really unifying our understanding of physics at all scales, from the Planck length to our cosmological horizon, two scales that are 60 orders of magnitude apart.
Understanding such a hierarchy together with the much smaller one of the Standard Model represents, in my opinion, the biggest theoretical challenge for 21st century physics. IOP Publishing Jobs. Sign in Register.
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Adding to the conundrum, physicists had come up with five conflicting string theories by the mids. The theory of everything was fractured. Over the next decade, scientists exploring the relationships between the five theories began to find unexpected connections, which Edward Witten, a theorist at the Institute for Advanced Study in Princeton, New Jersey, gathered up and presented at a string theory conference at the University of Southern California.
The novel theory is called M-theory, although to this day no one knows what mathematical form it might take. The "M" is likely inspired by higher-dimensional objects called membranes, Taylor said, but since the theory has no concrete mathematical equations, the "M" remains a placeholder with no official meaning.
Attempts to find those general equations that would work in every possible situation made little progress, but the alleged existence of the fundamental theory gave theorists the understanding and confidence needed to develop mathematical techniques for the five versions of string theory and apply them in the right context.
Strings are far too small to detect with any conceivable technology, but one early theoretical success was their ability to describe black hole entropy in Entropy refers to the number of ways that you can arrange the parts of a system, but without being able to see into the impenetrable depths of a black hole, no one knows what type of particles might lie inside, or what arrangements they can take. And yet, in the early s Stephen Hawking and others showed how to calculate the entropy, suggesting that black holes have some sort of internal structure.
The string framework still faces many challenges, however: It produces an impossible number of ways to fold up the extra dimensions that all seem to fit the broad features of the Standard Model of particle physics, with little hope of distinguishing which is the right one.
Moreover, all of those models rely on an equivalence between force particles and matter particles called supersymmetry that, like the extra dimensions, we don't observe in our world.
The models also don't seem to describe an expanding universe. A number of physicists, such as Peter Woit of Columbia University, view these divergences from reality as fatal flaws. Taylor expects that, while the new era of gravitational wave astronomy may bring new tidbits of information about quantum gravity, more progress will be made by continuing to follow the math deeper into string theory. Regardless of how string theory's Theory of Everything candidacy evolves, its legacy as a productive research program may be assured on mathematical merit alone.
When Witten and others showed that the five string theories were shadows of a single parent theory, they highlighted connections called dualities, which have proven to be a major contribution to mathematics and physics. Untangling the origin of string theory Half a century ago, theorist Gabriele Veneziano visited CERN and wrote a paper that wound up marking the beginning of string theory 28 November, By Matthew Chalmers.
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