You may have read here and there about the genteel discussions concerning the status of string theory within contemporary theoretical physics. We’ve discussed it on CV here, here, and even way back here, and Clifford has hosted a multipart discussion at Asymptotia (I, II, III, IV, V, VI).
We are now very happy to host a guest post by the man who wrote the book, as it were, on string theory — Joe Polchinski of the Kavli Institute for Theoretical Physics at UC Santa Barbara. Joe was asked by American Scientist to review Peter Woit’s Not Even Wrong and Lee Smolin’s The Trouble With Physics. Here is a slightly-modified version of the review, enhanced by footnotes that expand on some more technical points.
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This is a review/response, written some time ago, that has just appeared in American Scientist. A few notes: 1) I did not choose the title, but at least insisted on the question mark so as to invoke Hinchliffe’s rule (if the title is a question, the answer is `no’). 2) Am. Sci. edited my review for style, I have reverted figures of speech that I did not care for. 3) I have added footnotes on some key points. I look forward to comments, unfortunately I will be incommunicado on Dec. 8 and 9.
All Strung Out?
Joe Polchinski
The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next. Lee Smolin. xxiv + 392 pp. Houghton Mifflin, 2006. $26.
Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law. xxi + 291 pp. Basic Books, 2006. $26.95.
The 1970’s were an exhilarating time in particle physics. After decades of effort, theoretical physicists had come to understand the weak and strong nuclear forces and had combined them with the electromagnetic force in the so-called Standard Model. Fresh from this success, they turned to the problem of finding a unified theory, a single principle that would account for all three of these forces and the properties of the various subatomic particles. Some investigators even sought to unify gravity with the other three forces and to resolve the problems that arise when gravity is combined with quantum theory.
The Standard Model is a quantum field theory, in which particles behave as mathematical points, but a small group of theorists explored the possibility that under enough magnification, particles would prove to be oscillating loops or strands of “string.” Although this seemingly odd idea attracted little attention at first, by 1984 it had become apparent that this approach was able to solve some key problems that otherwise seemed insurmountable. Rather suddenly, the attention of many of those working on unification shifted to string theory, and there it has stayed since.
Today, after more than 20 years of concentrated effort, what has been accomplished? What has string theory predicted? Lee Smolin, in The Trouble With Physics, and Peter Woit, in Not Even Wrong, argue that string theory has largely failed. What is worse, they contend, too many theorists continue to focus their efforts on this idea, monopolizing valuable scientific resources that should be shifted in more promising directions.
Smolin presents the rise and fall of string theory as a morality play. He accurately captures the excitement that theorists felt at the discovery of this unexpected and powerful new idea. But this story, however grippingly told, is more a work of drama than of history. Even the turning point, the first crack in the facade, is based on a myth: Smolin claims that string theorists had predicted that the energy of the vacuum — something often called dark energy — could not be positive and that the surprising 1998 discovery of the accelerating expansion of the universe (which implies the existence of positive dark energy) caused a hasty retreat. There was, in fact, no such prediction [1]. Although his book is for the most part thoroughly referenced, Smolin cites no source on this point. He quotes Edward Witten, but Witten made his comments in a very different context — and three years after the discovery of accelerating expansion. Indeed, the quotation is doubly taken out of context, because at the same meeting at which Witten spoke, his former student Eva Silverstein gave a solution to the problem about which he was so pessimistic. (Contrary to another myth, young string theorists are not so intimidated by their elders.)
As Smolin charts the fall of string theory, he presents further misconceptions. For example, he asserts that a certain key idea of string theory — something called Maldacena duality, the conjectured equivalence between a string theory defined on one space and a quantum field theory defined on the boundary of that space — makes no precise mathematical statements. It certainly does. These statements have been verified by a variety of methods, including computer simulations [2]. He also asserts that the evidence supports only a weak form of this conjecture, without quantum mechanics. In fact, Juan Maldacena’s theory is fully quantum mechanical [3].
A crucial principle, according to Smolin, is background independence — roughly speaking, consistency with Einstein’s insight that the shape of spacetime is dynamical — and Smolin repeatedly criticizes string theory for not having this property. Here he is mistaking an aspect of the mathematical language being used for one of the physics being described. New physical theories are often discovered using a mathematical language that is not the most suitable for them. This mismatch is not surprising, because one is trying to describe something that is different from anything in previous experience. For example, Einstein originally formulated special relativity in language that now seems clumsy, and it was mathematician Hermann Minkowski’s introduction of four-vectors and spacetime that made further progress possible.
In string theory it has always been clear that the physics is background-independent even if the language being used is not, and the search for a more suitable language continues. Indeed (as Smolin belatedly notes), Maldacena duality provides a solution to this problem, one that is unexpected and powerful. The solution is still not complete: One must pin down spacetime on the edges, but in the middle it is free to twist and even tear as it will, and black holes can form and then decay. This need to constrain the edges is connected with a property known as the holographic principle, which appears to be an essential feature of quantum gravity. Extending this principle to spaces with the edges free will require a major new insight. It is possible that the solution to this problem already exists among the alternative approaches that Smolin favors. But his principal candidate (loop quantum gravity) is, as yet, much more background-dependent than the current form of string theory [4].
Much of Smolin’s criticism of string theory deals with its lack of mathematical rigor. But physics is not mathematics. Physicists work by calculation, physical reasoning, modeling and cross-checking more than by proof, and what they can understand is generally much greater than what can be rigorously demonstrated. For example, quantum field theory, which underlies the Standard Model and much else in physics, is notoriously difficult to put on a rigorous foundation. Indeed, much of the interest that mathematicians have in physics, and in string theory in particular, arises not from its rigor but from the opposite: Physicists by their methods can obtain new results whose mathematical underpinning is not obvious. String theorists have a strong sense that they are discovering something, not inventing it. The process is sometimes messy, with unexpected twists and turns (not least the strings themselves!), and rigor is not the main tool.
Woit covers some of the same ground, although his interests are more centered on particle physics and on the connection with mathematics than on the nature of spacetime. His telling is more direct, but it is rather stuffed with detail and jargon, and his criticisms of string theory are simpler and somewhat repetitious.
A major point for Woit is that no one knows exactly what string theory is, because it is specified only through an infinite mathematical series whose sum is ill-defined. This assertion is partly true: With new physical theories there is often a long period between the first insight and the final mathematical form. For quantum field theory, the state of affairs that Woit describes lasted for half a century [5]. In string theory the situation is much better than he suggests, because for 10 years we have had tools (dualities) that give us in many cases a precise definition of the theory. These have led in turn to many new applications of string theory, such as to the quantum mechanics of black holes, and there are hints to a more complete understanding.
But what about the lack of predictions? This is the key question, for Woit, for Smolin and for string theory. Why have the last 20 years been a time of unusually little contact between theory and experiment? The problem is partly on the experimental side: The Standard Model works too well. It takes great time, ingenuity and resources to try to look beyond it, and often what is found is still the Standard Model.
A second challenge was set forth by Max Planck more than a century ago. When one combines the fundamental constants of special relativity, general relativity and quantum mechanics, one finds that they determine a distance scale at which these theories appear to come together: the Planck length of 10-33 centimeters. To put this number in perspective, one would have to magnify an atom a billion times to make it the size of a coffee cup, and one would have to magnify the Planck length a trillion trillion times to make it the size of an atom. If we could probe the Planck length directly, we would be able to see the strings and extra dimensions, or whatever else is lurking there, and be done with it. But we cannot do that, and so instead we must look for indirect evidence. And, as was the case with atomic theory, one cannot predict how long such a leap will take.
Smolin addresses the problem of the Planck length (“It is a lie,” he says). Indeed, Planck’s calculation applies to a worst-case scenario. String theorists have identified at least half a dozen ways that new physics might arise at accessible scales [6], and Smolin points to another in the theories that he favors [7], but each of these is a long shot. As far as experiment yet shows, Planck’s challenge stands.
Or it may be that string theory has already made a connection with observation — one of immense significance. Positive dark energy is the greatest experimental discovery of the past 30 years regarding the basic laws of physics. Its existence came as a surprise to almost everyone in physics and astronomy, except for a small number, including, in particular, Steven Weinberg.
In the 1980s, Weinberg had been trying to solve the long-standing puzzle of why the density of dark energy is not actually much greater. He argued that if the underlying theory had multiple vacua describing an enormous number of potential universes, it would not only explain why the density of dark energy is not high, but would also predict that it is not zero. Weinberg’s reasoning was contrary to all conventional wisdom, but remarkably his prediction was borne out by observation a decade later.
The connection between string theory and dark energy is still a subject of much controversy, and it may be that Weinberg got the right answer for the wrong reason. However, it may well turn out that he got the right answer for the right reason. If so, it will be one of the great insights in the history of physics, and the multivacuum property of string theory, seemingly one of its main challenges, will, in fact, be just what nature requires.
A second unexpected connection comes from studies carried out using the Relativistic Heavy Ion Collider, a particle accelerator at Brookhaven National Laboratory. This machine smashes together nuclei at high energy to produce a hot, strongly interacting plasma. Physicists have found that some of the properties of this plasma are better modeled (via duality) as a tiny black hole in a space with extra dimensions than as the expected clump of elementary particles in the usual four dimensions of spacetime. The prediction here is again not a sharp one, as the string model works much better than expected. String-theory skeptics could take the point of view that it is just a mathematical spinoff. However, one of the repeated lessons of physics is unity — nature uses a small number of principles in diverse ways. And so the quantum gravity that is manifesting itself in dual form at Brookhaven is likely to be the same one that operates everywhere else in the universe.
A further development over the past few years, as our understanding has deepened, has been the extensive study of the experimental consequences of specific kinds of string theory. Many of these make distinctive predictions for particle physics and cosmology. Most or all of these may well be falsified by experiment (which is, after all, the fate of most new models). The conclusive test of string theory may still be far off, but in the meantime, science proceeds through many small steps.
A central question for both Smolin and Woit is why so many very good scientists continue to work on an idea that has allegedly failed so badly. Both books offer explanations in terms of the sociology of science and the psychology of scientists. These forces do exist, and it is worth reflecting on their possible negative effects, but such influences are not as strong as these authors posit. String theorists include mavericks and contrarians, strong-willed individuals who have made major contributions — not just in string theory but in other parts of physics as well. The borders between string theory and other areas of physics are not closed, and theorists would emigrate if they did not believe that this was the most promising direction in which to invest their time and energies.
In fact, the flow of intellectual talent has been in the other direction: In recent years, leading scientists in particle phenomenology, inflationary cosmology and other fields have found ideas generated by string theory to be useful in their disciplines, just as mathematicians have long done. Many have begun to work with string theorists and have in turn contributed their perspectives to the subject and expanded the view of how string theory relates to nature.
This convergence on an unproven idea is remarkable. Again, it is worth taking a step back and reflecting on whether the net result is the best way to move science forward, and in particular whether young scientists are sufficiently encouraged to think about the big questions of science in new ways. These are important issues — and not simple ones. However, much of what Smolin and Woit attribute to sociology is really a difference of scientific judgment.
In the end, these books fail to capture much of the spirit and logic of string theory. For that, Brian Greene’s The Elegant Universe (first published in 1999) or Leonard Susskind’s The Cosmic Landscape (2005) do a better job. The interested reader might also look to particle-phenomenologist Lisa Randall’s Warped Passages (2005) and cosmologist Alexander Vilenkin’s Many Worlds in One (2006) for accounts by two scientists from other fields who have seen a growing convergence between string theory and their ideas about how the cosmos is put together.
Joseph Polchinski is a professor of physics at the University of California, Santa Barbara, and a permanent member of the Kavli Institute for Theoretical Physics. He is the author of the two-volume text String Theory (Cambridge University Press, 1998).
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[1] It is obvious that there could have been no such prediction. From 1995-98, string theorists were discovering a host of new nonperturbative tools: dualities, branes, black hole entropy counting, matrix theory, and AdS/CFT duality. These were at the time studied almost exclusively in the context of supersymmetry. The problem of moduli stabilization, necessary for any nonsupersymmetric compactification (and positive energy density states are necessarily nonsupersymmetric) was left for the future; there were no general results or predictions. Page 154 refers to no-go theorems. There was a prominent no-go theorem two years later due to Maldacena and Nunez. However, not only the timing but also the physics is misstated. This paper makes several restrictive assumptions, and gives a long list of well-known papers, some as early as 1986, to which its results simply don’t apply. So this was never a broad constraint on string theory.
[2] On the string theory side, all calculations of anomalous dimensions and correlators represent precise statements about the strong coupling behavior of the gauge theory. However, it is argued on page 282 that the gauge theory is not known to exist. For the purpose of this discussion it is sharpest to focus on the gauge theories in 1+1 and 2+1 dimensions, which were shown by Itzhaki, Maldacena, Sonnenschein, and Yankielowicz to also give background-independent constructions of quantum gravity. These theories are superrenormalizable – their couplings go to zero as powers at short distance — so they are even better-defined than QCD, and one can calculate to arbitrary accuracy on the lattice. Even the supersymmetry is no problem: the lattice breaks it, but because of the superrenormalizability one can calculate explicitly the counterterms needed to restore the symmetry in the continuum limit, and so all the predictions of AdS/CFT can be checked algorithmically.
This has already been done, not by Monte Carlo but by using discrete light-cone quantization, which has the nice property of preserving SUSY and also not paying an extra numerical penalty for large N. The present results of Hiller, Pinsky, Salwen, and Trittman are notable. The error bars are still large (but again, the issue is whether there are predictions in principle, not what can be done with today’s technology) but it does appear that the gauge theory Hilbert space, truncated to 3 x 1012 states, is in fact describing a graviton moving in a curved spacetime. Possibly less algorithmic, but numerically impressive, is the four-loop calculation of Bern, Czakon, Dixon, Kosower, and Smirnov: the Pade extrapolation to strong coupling agrees with the prediction of AdS/CFT to one or two percent.
[3] The gauge theory is a consistent and fully quantum mechanical theory, so if it contains classical gravity then it is by definition a solution to the problem of unifying Einstein’s theory with quantum mechanics. Moreover, the gravitational field must itself be quantized, because the duality relates gauge theory states to correctly quantized graviton states.
It is very difficult to define a `weak form’ of the duality which accounts for all the successful tests and is not actually the strong form. I am taking the definition here from page 144, which refers to classical supergravity as the lowest approximation, and talks about the duality being true only at this lowest order.
However, to get more background I have looked at the relevant papers by Arnsdorf and Smolin and by Smolin. The central arguments of these papers are wrong. One argument is that AdS/CFT duality cannot describe the bending of light by a gravitational field because there is a dual description with a fixed causal structure. If true, of course, this would invalidate the duality, but it is not. The gauge theory has a fixed causal structure, but signals do not move on null geodesics: there is refraction, so signals slow down and bend, and it is this that is dual to the bending of light by a gravitational field. Indeed, this duality between ordinary refraction and gravitational lensing is one of the fascinating maps between gravitation and nongravitational physics that are implied by the duality.
The second argument is that the tests of AdS/CFT duality are consistent with a weaker notion of `conformal induction,’ whereby a boundary theory can be defined from any field theory in AdS space by taking the limit as the correlators approach the boundary. This misses an important point. In general this procedure does not actually define a self-contained field theory on the boundary. Consider a signal in the bulk, which at time t is moving toward the boundary so as to reach it at a later time t’. According to the definition of conformal induction, the existence of this signal is not encoded in the boundary theory at time t, so that theory has no time evolution operator: the state at time t does not determine the state at time t’. In AdS/CFT the boundary is a true QFT, with a time evolution operator, and the signal is encoded even at time t. As a rough model of how this can work, imagine that every one-particle state in the bulk maps to a two-particle state in the boundary, where the separation of the particles plays the role of the radial coordinate: as they come close together the bulk particle move to the boundary, as they separate it moves away. Something like this happens even in real QCD, in the contexts of color transparency and BFKL diffusion.
[4] I am referring here to the problem of the constraints. Until these are solved, one does not really have background independence: there is an enormous Hilbert space, most of which is unphysical. In AdS/CFT, not only the bulk spacetime but also the bulk diffeomorphism group are emergent: the CFT fields are completely invariant under the bulk diffeomorphisms (this is also what happens in the much more common phenomenon of emergent gauge symmetry). In effect the constraints are already solved. One of the lessons of duality is that only the physical information is common to the different descriptions, while the extra gauge structure is not, it is an artifact of language not physics. (The CFT has its own SU(N) gauge invariance, but here it is straightforward to write down invariant objects.)
[5] I am counting from the mid-20’s, when the commutation relations for the electromagnetic field were first written down, to the mid-70’s when lattice gauge theory gave the first reasonably complete definition of a QFT, and when nonperturbative effects began to be understood systematically.
[6] The ones that came to mind were modifications of the gravitational force law on laboratory scales, strings, black holes, and extra dimensions at particle accelerators, cosmic superstrings, and trans-Planckian corrections to the CMB. One might also count more specific cosmic scenarios like DBI inflation, pre-Big-Bang cosmology, the ekpyrotic universe, and brane gas cosmologies.
[7] I have a question about violation of Lorentz invariance, perhaps this is the place to ask it. In the case of the four-Fermi theory of the weak interaction, one could have solved the UV problem in many ways by violating Lorentz invariance, but preservation of Lorentz invariance led almost uniquely to spontaneously broken Yang-Mills theory. Why weren’t Lorentz-breaking cutoffs tried? Because they would have spoiled the success of Lorentz invariance at low energies, through virtual effects. Now, the Standard Model has of order 25 renormalizable parameters, but it would have roughly as many more if Lorentz invariance were not imposed; most of the new LV parameters are known to be zero to high accuracy. So, if your UV theory of gravity violates Lorentz invariance, this should feed down into these low energy LV parameters through virtual effects. Does there exist a framework to calculate this effect? Has it been done?
Joe Polchinski wrote:
Positive dark energy is the greatest experimental discovery of the past 30 years regarding the basic laws of physics.
Indeed, there are several evidences indicating that the expansion of the Universe is accelerating.
However, I would be more confident when some details are completely understood. The devil is in the details. I cite here two papers as examples of what I mean.
– The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star — Nature 443, 308-311 (21 September 2006) — Although this remarkable SN can be easily seen as an outlier, an hence promptly removed from cosmological investigations, there could be contamination from other “super-Chandra” SNs in the current SN Type Ia data, although less evident (because of lower mass) than SNLS-03D3bb. A thorough study on these objects and their possible contamination in the SN data used for cosmology must be clearly understood.
– The Uncorrelated Universe: Statistical Anisotropy and the Vanishing Angular Correlation Function in WMAP Years 1-3 (astro-ph/0605135). Or see their homepage. I quote from their paper: “If indeed the observed l=2 and 3 CMB fluctuations are not cosmological, there are important consequences. Certainly, one must reconsider all CMB results that rely on low ls (…) Moreover, the CMB-galaxy cross-correlation, which has been used to provide evidence for the Integrated Sachs-Wolfe effect and hence the existence of dark energy, also gets contributions from the lowest multipoles (…)”
Details like these might turn out to be just mere “details” that will not affect the current evidences. Or not. Some would rather adopt a radically skeptical position, some others would bet on the current evidences with no worries. That is a personal choice.
In any case, all independent evidences must eventually fit into the same picture, and the gaps in our knowledge hopefully will be erased with further data. For that matter, careful and independent astrophysical investigations (like, e.g., the current efforts to test the evidence for dark energy from clusters of galaxies at high z) will be very interesting.
[Some references on this are:
A Cooray et al. 2004 Growth rate of large-scale structure as a powerful probe of dark energy Phys. Rev. D 69 027301
J Weller et al. 2002 Constraining dark energy with Sunyaev-Zel’dovich cluster surveys Phys. Rev. Lett. 88 231301]
The connection between string theory and dark energy is still a subject of much controversy, and it may be that Weinberg got the right answer for the wrong reason. However, it may well turn out that he got the right answer for the right reason. If so, it will be one of the great insights in the history of physics, and the multivacuum property of string theory, seemingly one of its main challenges, will, in fact, be just what nature requires.
I do not understand the logic in the above paragraph, I believe it would be interesting to further elaborate. What if the details that I have mentioned previously turn out to be important, or otherwise, if future investigations with new and better data indicate something else? Will it mean to be a clear evidence to falsify string theory? (Or for that matter, any theory based on a multivacuum scenario?) What are the specific predictions of these theories?
Or, alternatively, if it turns out that indeed there is an acceleration caused by a dark energy component with clear and specific numbers at hand, why would this imply univocally a signature of a multivacuum? What about other proposals? How are we going to be able to discern “what nature requires” among them except with the aid of clear experimental/observational predictions from these theories?
I do not mean here to flame this already naturally hot thread, but just to indicate some of my concerns, which I believe are also shared by some.
Best regards,
Christine
What is the probability that given we exist and given that our existence is equally probable in any spiral galaxy, that we’d actually exist in a rather small grouping of galaxies instead of being in the thick of a supercluster?
Christine, the fact that the universe is accelerating is by now extremely well-established. Even if you don’t believe in the supernova data at all (and there’s no reason not to), the CMB plus some very weak constraint on the Hubble constant implies acceleration — and that doesn’t depend on the low-l multipoles at all. The CMB plus constraints on the matter density strongly implies the existence of dark energy, or perhaps modified gravity. And independent observations of large-scale structure, cluster counts, gamma-ray bursts, and baryon acoustic oscillations all provide additional evidence.
It makes all the sense in the world to keep an open mind about any particular explanation for the acceleration, but the universe is definitely accelerating.
Dear Sean,
Thanks for the comment. Yes, as I wrote previously, there are several evidences, and if it was not clear from my previous comment, let me add that I see these evidences as very interesting results. It is not the case that I do or do not believe in one dataset or another, in any particular sense, but thanks for pointing out the low-l results as having no effect on deriving the acceleration. I’ll review that. In any case, the fact that I very much would prefer the situation in which all details were completely understood still makes sense to me. Perhaps that is a too much conservative position, I don’t know. For instance, I do place a considerable distinction between the two sentences: “there are several evidences for the acceleration” and “the universe is definitely accelerating” (for instance, you seem to use both sentences interchangeably). Maybe that is not the point of view of many, and I respect that.
Best regards,
Christine
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I’ve got an even better one, Christine:
Hey, Joe. The balance points that define the anthropic coincidences appear to be self-regulating in areas where we can make this distintion, so the implication is… what you see, is what you’ve got, and that doesn’t change with expansion. I wonder how that could happen, unless accelerating matter generation counterbalances accelerating expansion…
Anthropic reasoning also indicates that characteristics/traits/asymmetries… are inherent, and are evolved to higher orders of the same basic structure, which also supports the first implication:
http://www.amazon.co.uk/Goldilocks-Enigma-Universe-Just-Right/dp/0713998830/ref=sr_11_1/026-8934482-6040423?ie=UTF8
In their Quantum McSilly rebuttal of the anthropic principle, Starkman and Trotta decided that, “in order to live and thus view the universe, humans need to collect and expend energy, so humans should prefer a universe that is flying apart as slowly as possible, making it easier to go out and collect energy to expend. In such a universe, the cosmological constant should be as low as possible, even lower than the value seen now.”
The valuable connection to energy consumption that they’ve made is arrogantly maligned by Wheeler’s interpretation, because we’re here to work, not watch, (necessity being the mother of invention, and all that), and “pound-for-pound”, we are magnitudes more energy-efficient at generating matter/antimatter pairs than Black Holes or Supernovae… speaking of counterbalancing effects.
we cannot declare final victory over the c.c. until we can define the stability mechanism with real first principles, instead of excuses.
“Even if you don’t believe in the supernova data at all (and there’s no reason not to), the CMB plus some very weak constraint on the Hubble constant implies acceleration … It makes all the sense in the world to keep an open mind about any particular explanation for the acceleration, but the universe is definitely accelerating.” – Sean
“…the flat universe is just not decelerating, it isn’t really accelerating…” – Philip Anderson, http://blogs.discovermagazine.com/cosmicvariance/2006/01/03/danger-phil-anderson/#comment-10901
The fact that it is Philip Anderson in the last quote should not matter. Just concentrate on the science:
GR is not the final theory of gravity, which will have to take account of quantum effects. GR predicts that expansion there is a departure from Hubble’s law at extreme redshift due to deceleration caused by gravity.
The supposed “acceleration of the universe” observation that there is no gravitational retardation in evidence, not that there is acceleration.
You can explain this either by dismissing long-range gravity or you say there is an acceleration due to dark energy which cancels out the effect of long range gravity.
Any Yang-Mills quantum gravity however predicts the lack of gravitational acceleration precisely so there is simply no room for any significant amount of ad hoc dark energy in explaining the result: the quantum gravity coupling (effective charge) for gravity falls off due to the redshift of the gauge bosons being exchanged when the masses are receding at relativistic velocities. There are different analyses to this problem, but all lead to the same conclusions!
If such changes were capable in the accelerating universe, and a time, different speeds dominated, then how would that take place? How could we ever had known that micro perspective would allow us peer into this nature without considering particle reductionism.
A “cross over point” in LHC presents an interesting opportunity for that speed to change? Saids, increased activity of suns could cause such changes? Qui Non?
Dark energy is a mysterious repulsive force that causes the universe to expand at an increasing rate. Investigators used Hubble to find that dark energy was already boosting the expansion rate of the universe as long as nine billion years ago. This picture of dark energy is consistent with Albert Einstein’s prediction of nearly a century ago that a repulsive form of gravity emanates from empty space. Data from Hubble provides supporting evidence to help astrophysicists to understand the nature of dark energy. This will allow them to begin ruling out some competing explanations that predict that the strength of dark energy changes over time.
“This picture of dark energy is consistent with Albert Einstein’s prediction of nearly a century ago that a repulsive form of gravity emanates from empty space.” – Plato
Einstein was falsifying a steady state cosmology by adding a cosmological constant to general relativity without mechanism, and he used a much greater amount of dark energy (enough to cancel gravity effects at a distance equal to the average separation of galaxies in the universe). What needs today isn’t a reversion of prejudiced ad hoc “explanation” using “dark energy”. What is needed is quantum gravity that predicts what is seen.
Einstein was falsifying [the facts by inventing] a steady state cosmology… What is needed today isn’t a reversion to prejudiced ad hoc “explanation” using dark energy. Sorry, I’ll call it a day.
Dear Joe:
Regarding your question in footnote [7], it seems to me there is some confusion here about violations of Lorentz invariance (which single out a preferred restframe) and deformations of Lorentz-transformations (which are observer independent, but have a modified functional dependence on the boost parameter). I believe Lee Smolin referred to the latter, since he has worked on the topic for quite some while. You find a nice introduction e.g. in
gr-qc/0207085.
I have put together some more references (that also address Robert’s question from above) on a post on my blog (my comment got too long):
Deformed Special Relativity
I sincerely hope that this clarifies at least some points.
I should admit though that the status of a quantum field theory which successfully includes these deformations of special relativity is presently very unsatisfactory. Many publications on the topic use arguments based on a couple of equations, with a consistent framework still lacking, which is very frustrating. I am currently working on cleanly formulating a quantum field theory with DSR, and I know that others are as well. I am very optimistic that there will be some progress soon.
It is however possible to arrive at some general predictions by making use of the deformed transformations, or by using kinematical arguments. Many examples of this can be found in papers by Giovanni Amelino-Camelia et at, see e.g. gr-qc/0412136.
Best regards,
Sabine
About the “suggested reading” at the end of the review, there’s a huge problem with atleast Greene’s book (it’s the only one of them I’ve read so I can only comment it), because it has only claims (such as particles being tiny vibrating strings of which amplitude and wavelength corresponds to different masses and force charges of them and that 11 dimensions of which 7 of them are curled up in Calabi-Yau shapes are required, etc.) and absolutely nothing to back these claims up, other than “according to string theory…”. Even something obviously as important to the theory as Calabi-Yau shapes, the only explanation I can recall from the book of it was a picture of how they might look like.
Add to that, there’s not even a single equation in the book and that the author happily concludes that it could be all wrong.
I mean, a reader basicly has to take his word for it and based on his credentials hoping he’s not making it all up. Although nicely written, it could have aswell been written by a science fiction writer and there would be no difference if the author did his homework on basics of physics.
So my question is, is there any book out there that would explain string theory with actual equations, from the beginning, building up with any previous insights, to the present and how it actually connects to real world physics?
Ari, the difference between The Elegant Universe and science fiction is that the former is backed up by large books full of equations — one of them is linked right at the top of this review! Here’s another, and another, and another.
Sean, thanks for the links! Which one would you consider to be the most self-contained and up-to-date? According to Woit, Zwiebach’s book would only cover a small part of the string theory story not taking the reader very far into the issue of how to connect strings to real world physics.
You should look at the books for yourself; Zwiebach’s is aimed at undergraduates, so goes more slowly and doesn’t get as far, whereas the others are for graduate students.
Hey, Great links to:
String Theory and M-Theory: A Modern Introduction
Supersymmetry and String Theory: Beyond the Standard Model
These books do not seemed to be released yet. Does anybody know how these will be different then Polchinski’s books? Are they just more up to date or do they cover more ground or different topics? Also, any other good books on Supersymmetry besides Weinberg’s and Wess and Bagger’s? Thanks.
These quantum processes reveal the nature of curvatures as well…until….anomalistic behaviour of the superfluid?
NCWhat is needed today isn’t a reversion to prejudiced ad hoc “explanation” using dark energy
Do you think only particle collisions happen in the colliders? That the “cross over” point “only” happens there as well?
What use “particle shower” from cosmic particle collisions?
For the lay people here.
What is Dark Energy?
Some “thing” had to cause it? We needed a microscopic theory?
If superfluid anomalies are considered, then relativistic realization of a “flat space” at such an “energy extreme” could have encouraged, the change to the universe speeding up? Of course I speculate.
The “curvature parameters” had to be encouraged some how?
I’d like to be your next guest blogger, ok thx.
Guest Blogger: Clay Aiken
I just wanted to wish all my friends here a truly blessed holiday season!! And here’s hoping to more good times ahead!! What a rollercoaster ride we’ve been on together, haven’t we?? But I believe that it is only as we view the earth from high above, and then come crashing down to earth, with the dualistic winds of betrayal and good cheer blowing upon our faces, that we truly understand that our lives are but a glimmer of hope, a ray of sunshine, in a dark world of destruction. Ah, but a glimpse!! And so I ask you to pause, and to consider for one brief moment the true meaning of Christmas, which was revealed to me in a dream, a vision I shall now share with you.
Waking up in the middle of the night and startled by a chill that overcame me completely, I sought to nestle closer to my dog Jane. In vain I grasped at her torso. Where had she gone?? Oh, travesty, oh perplexity divine!! And so I left my room, only to see that she had escaped through the open back door. Distressed, I chased after her, following the fresh foot prints down a hill into a gully, a gully that was the scene of many toboggan rides, where Clay and his dog shared an unadulterated joy together. Orgasms of the mind!!
As I called out her name, I heard a voice from a tall tree, beckoning me to approach. This tree spoke to me in a deep, soothing voice, saying “To be a tree is to be what I am!! I would not have it any other way. It is only as we accept our lot that we can be truly free, and to enjoy all the bounty that is given to us. Clay, I’ve seen you grow up before my eyes, and I am amazed at the man you have become!! So full of life, and an instrument of love and faithful devotion to reveal and ascertain all that the Creator of the Universe has bestowed upon us all!! And so in this holiday season, understand that the following is what Christmas is really all about: Sometimes a smile is all that is needed to bring peace and harmony to the earth, a firm handshake and a well-timed wink can topple totalitarian governments, and giggle of glee can wrest Lucifer’s control of this sad world, and give it to the spirit of a small child!! Spread this message Clay, this trifecta of joy and dazzling delights, for it is as we seek to understand, that we truly know completely. This I truly and honestly believe!!”
And so, on that night, I learned the true meaning of Christmas, and I ask that you will now pause and consider what you can do. Whether it be a smile, giggle of glee, or a handshake and wink, understand that everyone is needed in the battle!! What will you do to save humanity in its darkest hour??
Wonderful!!
Joseph Smidt,
About SUSY: it kind of depends on what level you want and what aspects you are most interested in. There are many reviews online, for example the 2001 BUSSTEPP Lectures on SUSY by Figueroa-O’Farrill, and the more comprehensive An Introduction to Global Supersymmetry by Argyres. If you’re just starting out, my advice is to just *pick a convention* (I like Wess & Bagger’s) and try to work out as much as you can yourself following any and all notes.
invcit,
Thanks, I am currently taking quantum field theory and wanted to have a full selection of texts to learn SUSY from since often one text helps with some things better than others. Thanks for the online review tips too.
Having read all the above comments on Joseph Polchinski’s reviews (and referred back to various parts of his two volumes on string theory and Weinberg’s Quantum Theory of Fields) it seems that one needs to step back a little to consider the fast-moving sub-quantum universe as a rather small correction to our manifestly positive reality. Who will list the micro-subtleties, in plain English, which have led to this? Of course, this goes to those most macroscopic of phenomena, dark energy and the cosmological constant…
“Richard Feynman and others who developed the quantum theory of matter realized that empty space is filled with “virtual” particles continually forming and destroying themselves. These particles create a negative pressure that pulls space outward. No one, however, could predict this energy’s magnitude.” – Plato
No, pair production in only occurs above the IR cutoff. (Collision energies of 0.511 Mev/particle.) Space is thus only filled with particle creation-annihilation loops at distances closer than 1 fm to a unit charge, where the electric field strength exceeds 10^20 v/m. This is the basis of renormalization of electric charge, which has empirical evidence. http://arxiv.org/abs/hep-th/0510040 is a recent analysis of quantum field theory progress that contains useful information on pair production and polarization around pages 70-80 if I recall correctly. For an earlier review of the subject, see http://arxiv.org/abs/quant-ph/0608140.
Glast and high energy photons, or LHC?
What use “Calorimetric designs” if one does not record where that “extra energy” is going? Is it insignificant?
Of course I could be mistaken, but in seeing “Gran Sasso” such developements one has to wonder what was missed? Maybe a “bulk perspective” where “the medium” is the message?:)
I would say its about time for that new policy 😉
Elliot