Posts Tagged ‘string theory’
“For what man in the natural state or course of thinking did ever conceive it in his power to reduce the notions of all mankind exactly to the same length, and breadth, and height of his own? Yet this is the first humble and civil design of all innovators in the empire of reason.”*…
A “theory of everything” (a Grand Unified Theory on steriods)– a (still hypothetical) coherent theoretical framework of physics containing and explaining all physical principles– is the holy grail of physicists. Natalie Wolchover checks in on the most recent front-runner in the hunt…
Fifty-eight years after it first appeared, string theory remains the most popular candidate for the “theory of everything,” the unified mathematical framework for all matter and forces in the universe. This is much to the chagrin of its rather vocal critics. “String theory is not dead; it’s undead and now walks around like a zombie eating people’s brains,” the former physicist Sabine Hossenfelder said on her popular YouTube channel in 2024.
String theory is a “failure,” the mathematical physicist and blogger Peter Woit often says. His complaint is not that string theory is wrong — it’s that it’s “not even wrong,” as he titled a 2006 book. The theory says that, on scales of billionths of trillionths of trillionths of a centimeter, extra curled-up spatial dimensions reveal themselves and particles resolve into extended objects — strands and loops of energy — rather than points. But this alleged substructure is too small to detect, probably ever. The prediction is untestable.
A further problem is that uncountably many different configurations of dimensions and strings are permitted at those tiny scales; the theory can give rise to a limitless variety of universes. Amid this vast landscape of solutions, no one can hope to find a precise microscopic configuration that undergirds our particular macroscopic world.
These issues are profound indeed. Yet in my experience, the typical high-energy theorist in a prestigious university physics department still thinks string theory has a good chance of being correct, at least in part. The field has become siloed between those who deem it worth studying and those who don’t.
Recently, a new angle of attack has opened up. An approach called bootstrapping has allowed physicists to calculate that, under various starting assumptions about the universe, a key equation from string theory naturally follows. For some experts, these findings support the notion of “string uniqueness,” the idea that it is the only mathematically consistent quantum description of gravity and everything else.
Responding to one bootstrap paper on her YouTube channel, mere weeks after the “undead” comment, Hossenfelder said it was “string theorists do[ing] something sensible for once.” She added, “I’d say this paper strengthens the argument for string theory.”
Not everyone agrees, but the findings are reviving an important question. “This question of ‘Does string theory describe the world?’ has just been so taboo,” said Cliff Cheung, a physicist at the California Institute of Technology and an author of the paper discussed by Hossenfelder. Now, “people are actually thinking about it for the first time in decades.”
Getting wind of this work, I wanted to drill down on the logic and examine how the string hypothesis is faring these days…
And so she does: “Are Strings Still Our Best Hope for a Theory of Everything?” from @nattyover.bsky.social in @quantamagazine.bsky.social. Eminently worth reading in full.
Compare/contrast with: “Where Some See Strings, She Sees a Space-Time Made of Fractals.”
* Jonathan Swift, A Tale of a Tub
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As we grapple with Godel, we might spare a thought for Hermann Rorschach; he died on this date in 1922. A psychiatrist and psychoanalyst, his education in art helped to spur the development of a set of inkblots that were used experimentally to measure various unconscious parts of the subject’s personality. Rorschach knew the human tendency to project interpretations and feelings onto ambiguous stimuli and believed that the subjective responses of his subjects enabled him to distinguish among them on the basis of their perceptive abilities, intelligence, and emotional characteristics. His method has come to be known as the Rorschach test, iterations of which have continued to be used over the years to help identify personality, psychotic, and neurological disorders.
Perhaps his insight that we humans tend “to project interpretations and feelings onto ambiguous stimuli” can inform our understanding of physicists trying to construct mental/conceptual models of our reality, which they’ve been doing for a very long time, and of the limitations of that quest.
“In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.”*…
In the face of cosmologists who are trying to combine string theory with the theory of cosmic inflation to understand the universe-as-we-find-it, Neil Turok suggests a much simpler explanation. Frank Landymore reports…
Our understanding of the universe, as advanced as it is, remains riddled with paradoxes and huge question marks. Physicists have come up with some pretty heady ideas to explain them — we’ll get to those later — but there just might be a far “simpler” solution to all those holes in cosmology.
As Higgs Chair of Theoretical Physics at the University of Edinburgh Neil Turok explains in an essay for The Conversation, there could be a “mirror” universe that existed before the Big Bang and is a reflection of our own, moving backward in time.
It’s a trippy concept to wrap your head around, but simpler on the physics side of things. It would neatly balance out some of the asymmetries we observe in the universe, provide an answer to dark matter, and supplant some of what Turok would characterize as clumsier leading theories in cosmology, like cosmic inflation and string theory.
“Picturing the big bang as a mirror neatly explains many features of the universe which might otherwise appear to conflict with the most basic laws of physics,” wrote Turok, who published his team’s findings in the journal Annals of Physics. “The progress we have already made convinces me that, in all likelihood, there are alternatives to the standard orthodoxy — which has become a straitjacket we need to break out of.”
The physical laws of the universe should exhibit charge, parity, and time reversal — collectively known as CPT — symmetry, which essentially means every physical interaction can be mirrored. So to break down its implications: every particle should have an anti-particle of the opposite charge, every space has its inversion, and time can be reversed.
Except that’s not what we actually observe. Time only goes forward, and there are more particles than anti-matter particles. As far as we can tell, our universe is not symmetrical.
But: “Our mirror hypothesis restores the symmetry of the universe,” Turok argued. He compared it to looking at your reflection: “The combination of you and your mirror image are more symmetrical than you are alone.”
Extrapolating our universe backward in time through the Big Bang, “we found its mirror image, a pre-bang universe in which (relative to us) time runs backward and antiparticles outnumber particles,” Turok wrote.
This could also solve the mystery of dark matter, an invisible substance thought to make up 85 percent of all matter in the universe. Under the mirror hypothesis, weak, subatomic particles called neutrinos would be the ideal candidate to explain it.
Since we’ve only observed left-handed neutrinos, perhaps yet-unseen right-handed could even exist in the mirror universe.
What’s more, this could also tidily explain why the universe appears to be so uniform and flat. The prevailing theory is that a period of accelerated, faster-than-light expansion called cosmic inflation was responsible for shaping how the universe is today — but we’re yet to observe the large gravitational waves this would have produced.
With a handy mirror universe, however, “statistical arguments explain why the universe is flat and smooth and has a small positive accelerated expansion, with no need for cosmic inflation,” Turok wrote.
Of course, there’s a lot more needed to bear out this intriguing hypothesis. But Turok argues that, even if disproven, it demonstrates that there could be more straightforward explanations than what the Standard Model offers…
A mirror of our own, going backwards in time: “Physicist Says There’s Another Universe Hiding Behind the Big Bang,” from @futurism.
Turok’s full essay– longer and more detailed, but very accessible– is here.
* Douglas Adams, The Restaurant at the End of the Universe
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As we size up symmetry, we might send lofty birthday greetings to another ponderer of the cosmos, Carl Edward Sagan; he was born on this date in 1934. An astronomer, cosmologist, astrophysicist, astrobiologist (his contributions were central to the discovery of the high surface temperatures of Venus), he is best remembered as a popularizer of science– via books like The Dragons of Eden, Broca’s Brain and Pale Blue Dot, and the award-winning 1980 television series Cosmos: A Personal Voyage (which he narrated and co-wrote), the most widely-watched series in the history of American public television (seen by at least 500 million people across 60 different countries).
He is also remembered for his contributions to the scientific research of extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation.
(Readers can enjoy a loving riff on Cosmos here.)
“Men knew better than they realized, when they placed the abode of the gods beyond the reach of gravity”*…
In search of a theory of everything…
Twenty-five particles and four forces. That description — the Standard Model of particle physics — constitutes physicists’ best current explanation for everything. It’s neat and it’s simple, but no one is entirely happy with it. What irritates physicists most is that one of the forces — gravity — sticks out like a sore thumb on a four-fingered hand. Gravity is different.
Unlike the electromagnetic force and the strong and weak nuclear forces, gravity is not a quantum theory. This isn’t only aesthetically unpleasing, it’s also a mathematical headache. We know that particles have both quantum properties and gravitational fields, so the gravitational field should have quantum properties like the particles that cause it. But a theory of quantum gravity has been hard to come by.
In the 1960s, Richard Feynman and Bryce DeWitt set out to quantize gravity using the same techniques that had successfully transformed electromagnetism into the quantum theory called quantum electrodynamics. Unfortunately, when applied to gravity, the known techniques resulted in a theory that, when extrapolated to high energies, was plagued by an infinite number of infinities. This quantization of gravity was thought incurably sick, an approximation useful only when gravity is weak.
Since then, physicists have made several other attempts at quantizing gravity in the hope of finding a theory that would also work when gravity is strong. String theory, loop quantum gravity, causal dynamical triangulation and a few others have been aimed toward that goal. So far, none of these theories has experimental evidence speaking for it. Each has mathematical pros and cons, and no convergence seems in sight. But while these approaches were competing for attention, an old rival has caught up.
The theory called asymptotically (as-em-TOT-ick-lee) safe gravity was proposed in 1978 by Steven Weinberg. Weinberg, who would only a year later share the Nobel Prize with Sheldon Lee Glashow and Abdus Salam for unifying the electromagnetic and weak nuclear force, realized that the troubles with the naive quantization of gravity are not a death knell for the theory. Even though it looks like the theory breaks down when extrapolated to high energies, this breakdown might never come to pass. But to be able to tell just what happens, researchers had to wait for new mathematical methods that have only recently become available…
For decades, physicists have struggled to create a quantum theory of gravity. Now an approach that dates to the 1970s is attracting newfound attention: “Why an Old Theory of Everything Is Gaining New Life,” from @QuantaMagazine.
* Arthur C. Clarke, 2010: Odyssey Two
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As we unify, we might pause to remember Sir Arthur Stanley Eddington, OM, FRS; he died in this date in 1944. An astrophysicist, mathematician, and philosopher of science known for his work on the motion, distribution, evolution and structure of stars, Eddington is probably best remembered for his relationship to Einstein: he was, via a series of widely-published articles, the primary “explainer” of Einstein’s Theory of General Relativity to the English-speaking world; and he was, in 1919, the leader of the experimental team that used observations of a solar eclipse to confirm the theory.
“Supersymmetry was (and is) a beautiful mathematical idea. The problem with applying supersymmetry is that it is too good for this world.”*…
Physicists reconsider their options…
A wise proverb suggests not putting all your eggs in one basket. Over recent decades, however, physicists have failed to follow that wisdom. The 20th century—and, indeed, the 19th before it—were periods of triumph for them. They transformed understanding of the material universe and thus people’s ability to manipulate the world around them. Modernity could not exist without the knowledge won by physicists over those two centuries.
In exchange, the world has given them expensive toys to play with. The most recent of these, the Large Hadron Collider (LHC), which occupies a 27km-circumference tunnel near Geneva and cost $6bn, opened for business in 2008. It quickly found a long-predicted elementary particle, the Higgs boson, that was a hangover from calculations done in the 1960s. It then embarked on its real purpose, to search for a phenomenon called Supersymmetry.
This theory, devised in the 1970s and known as Susy for short, is the all-containing basket into which particle physics’s eggs have until recently been placed. Of itself, it would eliminate many arbitrary mathematical assumptions needed for the proper working of what is known as the Standard Model of particle physics. But it is also the vanguard of a deeper hypothesis, string theory, which is intended to synthesise the Standard Model with Einstein’s general theory of relativity. Einstein’s theory explains gravity. The Standard Model explains the other three fundamental forces—electromagnetism and the weak and strong nuclear forces—and their associated particles. Both describe their particular provinces of reality well. But they do not connect together. String theory would connect them, and thus provide a so-called “theory of everything”.
String theory proposes that the universe is composed of minuscule objects which vibrate in the manner of the strings of a musical instrument. Like such strings, they have resonant frequencies and harmonics. These various vibrational modes, string theorists contend, correspond to various fundamental particles. Such particles include all of those already observed as part of the Standard Model, the further particles predicted by Susy, which posits that the Standard Model’s mathematical fragility will go away if each of that model’s particles has a heavier “supersymmetric” partner particle, or “sparticle”, and also particles called gravitons, which are needed to tie the force of gravity into any unified theory, but are not predicted by relativity.
But, no Susy, no string theory. And, 13 years after the LHC opened, no sparticles have shown up. Even two as-yet-unexplained results announced earlier this year (one from the LHC and one from a smaller machine) offer no evidence directly supporting Susy. Many physicists thus worry they have been on a wild-goose chase…
Bye, bye little Susy? Supersymmetry isn’t (so far, anyway) proving out; and prospects look dim. But a similar fallow period in physics led to quantum theory and relativity: “Physics seeks the future.”
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As we ponder paradigms, we might send insightful birthday greetings to Friedrich Wilhelm Ostwald; he was born on this date in 1853. A chemist and philosopher, he made many specific contributions to his field (including advances on atomic theory), and was one of the founders of the of the field of physical chemistry. He won the Nobel Prize in 1909.
Following his retirement in 1906 from academic life, Ostwald became involved in philosophy, art, and politics– to each of which he made significant contributions.
“The Universe is under no obligation to make sense to you”*…
Uppsala University researchers have devised a new model for the Universe – one that may solve the enigma of dark energy. Their new article, published in Physical Review Letters, proposes a new structural concept, including dark energy, for a universe that rides on an expanding bubble in an additional dimension.
We have known for the past 20 years that the Universe is expanding at an ever accelerating rate. The explanation is the “dark energy” that permeates it throughout, pushing it to expand. Understanding the nature of this dark energy is one of the paramount enigmas of fundamental physics.
It has long been hoped that string theory will provide the answer. According to string theory, all matter consists of tiny, vibrating “stringlike” entities. The theory also requires there to be more spatial dimensions than the three that are already part of everyday knowledge. For 15 years, there have been models in string theory that have been thought to give rise to dark energy. However, these have come in for increasingly harsh criticism, and several researchers are now asserting that none of the models proposed to date are workable.
In their article, the scientists propose a new model with dark energy and our Universe riding on an expanding bubble in an extra dimension. The whole Universe is accommodated on the edge of this expanding bubble. All existing matter in the Universe corresponds to the ends of strings that extend out into the extra dimension. The researchers also show that expanding bubbles of this kind can come into existence within the framework of string theory. It is conceivable that there are more bubbles than ours, corresponding to other universes.
The Uppsala scientists’ model provides a new, different picture of the creation and future fate of the Universe, while it may also pave the way for methods of testing string theory…
(For a different emerging new theory– that may or may not be contradictory– see “Our universe has antimatter partner on the other side of the Big Bang.”)
* Neil deGrasse Tyson
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As we fumble with the fundamentals, we might send carefully-deduced birthday greetings to Richard Bevan Braithwaite; he was born on this date in 1900. A Cambridge philosopher who specialized in the philosophy of science, he focused on the logical features common to all sciences. Braithwaite was concerned with the impact of science on our beliefs about the world and the appropriate responses to that impact. He was especially interested in probability (and its applications in decision theory and games theory) and in the statistical sciences. He was president of the Aristotelian Society from 1946 to 1947, and was a Fellow of the British Academy.
It was Braithwaite’s poker that Ludwig Wittgenstein reportedly brandished at Karl Popper during their confrontation at a Moral Sciences Club meeting in Braithwaite’s rooms in King’s. The implement subsequently disappeared. (See here.)
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