Posts Tagged ‘string theory’
“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.)
“The truth is not always beautiful, nor beautiful words the truth.”*…
Does anyone who follows physics doubt it is in trouble? When I say physics, I don’t mean applied physics, material science or what Murray-Gell-Mann called “squalid-state physics.” I mean physics at its grandest, the effort to figure out reality. Where did the universe come from? What is it made of? What laws govern its behavior? And how probable is the universe? Are we here through sheer luck, or was our existence somehow inevitable?
In the 1980s Stephen Hawking and other big shots claimed that physics was on the verge of a “final theory,” or “theory of everything,” that could answer these big questions and solve the riddle of reality. I became a science writer in part because I believed their claims, but by the early 1990s I had become a skeptic. The leading contender for a theory of everything held that all of nature’s particles and forces, including gravity, stem from infinitesimal, stringy particles wriggling in nine or more dimensions.
The problem is that no conceivable experiment can detect the strings or extra dimensions…
John Horgan examines physicist Sabine Hossenfelder‘s claim that desire for beauty and other subjective biases have led physicists astray: “How Physics Lost Its Way.”
* Lao Tzu, Tao Te Ching
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As we contemplate certainty, we might recall that it was on this date in 1595 that Johann Kepler (and here) published Mysterium cosmographicum (Mystery of the Cosmos), in which he described an invisible underlying structure determining the six known planets in their orbits. Kepler thought as a mathematician, devising a structure based on only five convex regular solids; the path of each planet lay on a sphere separated from its neighbors by touching an inscribed polyhedron.
It was a beautiful, an elegant model– and one that fit the orbital data available at the time. It was, nonetheless, wrong.
Detailed view of Kepler’s inner sphere
“Doubtless we cannot see that other higher Spaceland now, because we have no eye in our stomachs”*…
An ” Amplituhedron“, an illustration of multi-dimensional spacetime
Our architecture, our education and our dictionaries tell us that space is three-dimensional. The OED defines it as ‘a continuous area or expanse which is free, available or unoccupied … The dimensions of height, depth and width, within which all things exist and move.’ In the 18th century, Immanuel Kant argued that three-dimensional Euclidean space is an a priori necessity and, saturated as we are now in computer-generated imagery and video games, we are constantly subjected to representations of a seemingly axiomatic Cartesian grid. From the perspective of the 21st century, this seems almost self-evident.
Yet the notion that we inhabit a space with any mathematical structure is a radical innovation of Western culture, necessitating an overthrow of long-held beliefs about the nature of reality. Although the birth of modern science is often discussed as a transition to a mechanistic account of nature, arguably more important – and certainly more enduring – is the transformation it entrained in our conception of space as a geometrical construct.
Over the past century, the quest to describe the geometry of space has become a major project in theoretical physics, with experts from Albert Einstein onwards attempting to explain all the fundamental forces of nature as byproducts of the shape of space itself. While on the local level we are trained to think of space as having three dimensions, general relativity paints a picture of a four-dimensional universe, and string theory says it has 10 dimensions – or 11 if you take an extended version known as M-Theory. There are variations of the theory in 26 dimensions, and recently pure mathematicians have been electrified by a version describing spaces of 24 dimensions. But what are these ‘dimensions’? And what does it mean to talk about a 10-dimensional space of being?…
Experience says we live in three dimensions; relativity says four; string theory says it’s 10– or more… What are “dimensions” and how do they affect reality? Margaret Wertheim offers a guide: “Radical dimensions.”
* Edwin A. Abbott, Flatland: A Romance of Many Dimensions
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As we tax our senses, we might spare a thought for Robert Jemison Van de Graaff; he died on this date in 1967. A physicist and engineer, he is best remembered for his creation of the Van de Graaff Generator, an electrostatic generator that creates very high electric potentials– very high voltage direct current (DC) electricity (up to 5 megavolts) at low current levels. A tabletop version can produce on the order of 100,000 volts and can store enough energy to produce a visible spark. Such small Van de Graaff machines are used in physics education to teach electrostatics; larger ones are displayed in some science museums.
Boy touching Van de Graaff generator at The Magic House, St. Louis Children’s Museum. Charged with electricity, his hair strands repel each other and stand out from his head.
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