The next step is to incorporate interactions, and to go from a free particle with no scattering amplitudes or cross-sections to one that can play a physical role, coupled to the Universe. In addition, the spatial curvature — so complicated in three dimensions — becomes trivial. String theory is more than just a specific model for how quantum gravity may work in our universe. String theory states that all matter in the universe is composed of tiny 1-dimensional strings, not point particles (which are 0-dimensional in nature). We regret that we have been unable to trace its history in this article because of lack … The natural home of string theory is the Planck scale. This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. The mathematical structure underlying them is often analogous, and occasionally even identical. I am a not much into physics myself obviously so i can't judge, but is string theory still relevant? (Wikimedia Commons user Lunch), EY & Citi On The Importance Of Resilience And Innovation, Impact 50: Investors Seeking Profit — And Pushing For Change, Michigan Economic Development Corporation With Forbes Insights, what every physicist should know about string theory, Treknology: The Science of Star Trek from Tricorders to Warp Drive, Beyond the Galaxy: How humanity looked beyond our Milky Way and discovered the entire Universe. Today 68, 11, 38 (2015)), probability is spread out over both space and time. Despite a lot of brilliant physicists’ efforts, despite the beauty and elegance of the ideas, string theory really cannot even be considered a theory in the usual sense. Instead, there might be a better approach, if we chose to work in the opposite direction. You may opt-out by. Working in three spatial dimensions is very difficult, and when a physics problem is challenging, we often try and solve a simpler version first. So even as a purely theoretical exercise, it's quite interesting. Most of the top physics departments in the USA have lots of string theorists. String Theory/Holography/Gravity. If we write down all the possible combinations of such graphs and sum over them — applying the same laws like conservation of momentum that we always enforce — we can complete the analogy. In lieu of that, you average instead over all possible spacetime geometries. Finally, I should mention that quantum gravity may not be an empirical science. Right now, there are only a few sets of dimensions that the string/superstring picture is self-consistent in, and the most promising one doesn't give us the four-dimensional gravity of Einstein that describes our Universe. So I don't know why some people single out string theory in particular for not being experimentally verifiable: there are no experiments available on quantum gravity and there may never be. Gravitational waves, water waves, and light waves all share remarkably similar features, despite arising from fundamentally different physical origins. Gravity, governed by Einstein, and everything else (strong, weak and electromagnetic interactions),... [+] governed by quantum physics, are the two independent rules known to govern everything in our Universe. I am a Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy at various colleges. Instead of calculating how a single particle (a zero-dimensional entity) behaves in any number of dimensions, maybe we could calculate how a string, whether open or closed (a one-dimensional entity) behaves. We used the software package Resseau-Lu to analyze re… $\endgroup$ – Michael Brown Feb 22 '13 at 9:21 (flickr user Trailfan), string theory interactions (R), for closed strings. String theory is not dead either. The way a pendulum oscillates is completely analogous to the way a mass on a spring moves back-and-forth, or the way a planet orbits a star. String theory is irrelevant by one. String theory does get quite close to reproducing the standard model, and the obstructions there do indeed appear to be technical. (SLAC National Accelerator Laboratory). and matter particles (electron, quarks, etc. The only possible one-dimensional surfaces are an open string, where there are two separate, unattached ends, or a closed string, where the two ends are attached to form a loop. Also, even if you don't like that a specific string theory describing our universe hasn't been found, it's still a class of theories which have quantum gravity, so it's hard to argue that it's not physically interesting even as "just" a toy model. String theory offers a path to quantum gravity, which few alternatives can truly match. String theory and the big bang: String theory is being applied to cosmology, which means that it may give us insights into the formation of the universe. Why String Theory Is Still Not Even Wrong. It's a framework which includes quantum gravity as well as many other interactions. The short answer is that string theory is still as alive as it ever, although not being worked on as vigorously as it was some time back, mainly because after 40+ years it still has not produced any physics, just math. The great hope of string theory is that these analogies will hold up at all scales, and that there will be an unambiguous, one-to-one mapping of the string picture onto the Universe we observe around us. A Feynman diagram representing electron-electron scattering, which requires summing over all the... [+] possible histories of the particle-particle interactions. Opinions expressed by Forbes Contributors are their own. There's no reason why any of the interesting effects in quantum gravity should ever be detectable by humans before we all go extinct. If you want to work with gravitation instead of quantum particles, you have to change the story a little bit. On a related note, as something of a solipsist, if something is undemonstratable, my first response will always be "so what??" It's one of the most brilliant, controversial and unproven ideas in all of physics: string theory. There's also the operator that describes the spacetime geometry's fluctuations, and the other quantum states of the string. So, if string theory is correct, the entire world is made of strings! And then, from that, we can look for analogies to a more complete theory of quantum gravity in a more realistic number of dimensions. Perhaps the most remarkable thing about string theory is that such a simple idea seems to nearly work: starting from a theory of strings, it is possible to derive theories that are at least similar to the Standard Model (which has been verified experimentally with incredible precision). So what we're left with, if we want to add in matter, is a set of scalar fields (just like certain types of particles) and the cosmological constant (which acts just like a mass term): a beautiful analogy. First, the AdS/CFT correspondence is an explicit set of dualities between a class of quantum field theories … Instead of points and interactions, we'd immediately start working with surfaces, membranes, etc. The way quantum field theory works is that you take a particle and you perform a mathematical "sum over histories." If you've heard of the term compactification in the context of string theory, that's the hand-waving word to acknowledge that we must solve these puzzles. The fact that it can not be related to the theory of relativity shows that at least one is not correct. Here's what that means, even if you're not a physicist. (Wikimedia Commons user Maschen). It's the only idea, to date, that gives us this, and that's why it's so hotly pursued. Understanding how these two well-established theories are related remains a central open question in theoretical physics. There are really two aspects to string theory, one is the idea that the universe is fundamentally stringy and that's the one people hear about and have objections to, the other is an alternative to quantum field theory, which is where it becomes useful. Today 68, 11, 38 (2015)), apply it to the string analogues, it describes a spacetime fluctuation and corresponds to a quantum state of the string. The difference between standard quantum field theory interactions (L), for point-like particles, and... [+] string theory interactions (R), for closed strings. up the Universe is at the core of string theory. Its not easy being a theory of everything. (Dmitri Fedorov). If we go down to one dimension, things become very simple. While 1D quantum gravity gave us quantum field theory for particles in a possibly curved spacetime, it didn't describe gravitation itself. In many regards, string theory is the best contender for a quantum theory of gravitation, which just happens to unify at the highest-energy scales. I'm sure some string theorists can mention more. In theoretical physics, it is immensely difficult to write anything down which is not either totally trivial (like 1=1), or obviously wrong. But this theoretical "upgrade" for gravity may be very challenging. String Theory. More to the point, it serves to miseducate a public that is woefully under-educated about science and scientific concepts. First, the AdS/CFT correspondence is an explicit set of dualities between a class of quantum field theories in a certain number of dimensions and a class of string theories in a higher number of dimensions. I'd suggest that it's a truism that undemonstratable concepts are irrelevant. The standard model was designed within a framework known as Quantum Field Theory(QFT), which gives u… Quantum physics and Einstein's theory of general relativity are the two solid pillars that underlie much of modern physics. A TOE has the very tough job of fitting gravity into the quantum laws of nature in such a way that, on large scales, gravity looks like curves in the fabric of space-time, as Albert Einstein described in his general theory of relativity. It's a pretty huge area in physics. The probability of finding a quantum particle at any particular location is never 100%; the... [+] probability is spread out over both space and time. Instead, you add up all the possible ways it could have arrived at its present state (the "past history" part), appropriately weighted probabilistically, and then you can calculate the quantum state of a single particle. Deforming the spacetime metric can be represented by the fluctuation (labelled 'p'), and if you... [+] apply it to the string analogues, it describes a spacetime fluctuation and corresponds to a quantum state of the string. A string theory is also a two-dimensional conformal field theory and this latter subject is very important for mathematics as well as physics. String theory is one of the more popular candidates to combine quantum mechanics and relativity into a grand unified theory. String theory developed over a period of more than 40 years, in a kind of haphazard evolution, with theoretical roots stretching all the way back to the decade of the 1940s. Due to the age of the theory, the ISI Web of Science catalogues approximately 13,000 papers in the category of string theory. Instead, we find a 10-dimensional Brans-Dicke theory of gravity. The extra degrees of freedom a particle gains from being in multiple dimensions don't play much of a role; so long as you can define a momentum vector, that's the main dimension that matters. (Phys. Another recent triumph is the independent calculation of Hawking's black hole entropy formula (and even its leading corrections) correctly from certain string theories. Back in 2015, the top living string theorist, Ed Witten, wrote a piece on what every physicist should know about string theory. Press question mark to learn the rest of the keyboard shortcuts. String theory was an intuitively attractive proposal, but by the mid-1970s more-refined measurements of the strong force had deviated from its predictions, leading most researchers to conclude that string theory had no relevance to the physical universe, no matter how elegant the mathematical theory. This course gives an introduction to the basics of string theory. A fluctuation in the spacetime metric (i.e., an operator) automatically represents a state in the quantum mechanical description of a string's properties. Set theory, group theory and number theory are all examples of mathematical theories, and string theory is another such example. A 2-D projection of a Calabi-Yau manifold, one popular method of compactifying the extra, unwanted... [+] dimensions of String Theory. Physicist, mathematician and blogger Peter Woit whacks strings, multiverses, simulated universes and “fake physics” Nevertheless, a small number of physicists continued to pursue string theory. Even … I don't know who "some people" that you're referring to are, but "string theory" is a stumbling block for scientific apologists when trying to explain the basic concepts of the philosophy of science, such as "Global warming is NOT 'just' a theory". The Universe is out there, waiting for you to discover it. String theory yields many fascinating subjects for thought, but you may be wondering about the practical importance of it. Ask a science question, get a science answer. In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings.String theory describes how these strings propagate through space and interact with each other. This "operator-state" correspondence was a necessary, but missing, ingredient. At the heart of string theory is the thread of an idea that's run through physics for centuries, that at some fundamental level, all the different forces, particles, interactions and manifestations of reality are tied together as part of the same framework. String theory is a set of attempts to model the four known fundamental interactions—gravitation, electromagnetism, strong nuclear force, weak nuclear force—together in one theory. There's no reason why any of the interesting effects in quantum gravity should ever be detectable by humans before we all go extinct. As our understanding of time grows with string theory, it’s possible that scientists may discover new means of traveling through the time dimension or show that such theoretical possibilities are, in fact, impossible, as most physicists believe. For decades, scientists have theorized about what lies beyond the third dimension and if there can exist a unified theory to explain all of the workings of the universe. My two books, Treknology: The Science of Star Trek from Tricorders to Warp Drive, Beyond the Galaxy: How humanity looked beyond our Milky Way and discovered the entire Universe, are available for purchase at Amazon. This is the only explicit realization of "holographic principle" saying that quantum gravity theories should be described by local degrees of freedom in lower dimensions than gravity itself. New comments cannot be posted and votes cannot be cast. No matter whether you tout string theory's successes or failure, or how you feel about its lack of verifiable predictions, it will no doubt remain one of the most active areas of theoretical physics research. (Phys. In fact all of the popular press reporting on string theory is irresponsible and should be avoided! The situation is no different than any other proposal for quantum gravity. The idea that instead of 0-dimensional particles, it's 1-dimensional strings that fundamentally make... [+] up the Universe is at the core of string theory. I have won numerous awards for science writing since 2008 for my blog, Starts With A Bang, including the award for best science blog by the Institute of Physics. This means you're free to copy and share these comics (but not to sell them). It is generally not relevant to people not interested in quantum gravity or holography. (Phys. Brian Greene presenting on String Theory. That doesn't mean it's a foregone conclusion, however, that string theory is the path to quantum gravity. (Wikimedia Commons user Kurochka), possible histories of the particle-particle interactions. Follow me on Twitter @startswithabang. (NASA/Goddard/Wade Sisler). (SLAC National Accelerator Laboratory), 1-D quantum gravity. So you can get a quantum theory of gravity in spacetime from string theory. It combines many interesting topics of (quantum) eld theory in two and higher dimensions. I am a Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy at various colleges. Once you have a true, multi-dimensional surface, that surface can be curved in non-trivial ways. The subtle piece of the puzzle that was missing? Whether it is the correct theory of quantum gravity is still not known. (Wikimedia Commons user Maschen), their string theory analogues (bottom) gives rise to surfaces which can have non-trivial curvature. When it comes to the laws of nature, it's remarkable how many similarities there are between seemingly unrelated phenomena. String theory made absolutely no predictions for the LHC except for some extremely contrived models. You start getting very interesting behavior out; behavior that just might be at the root of the spacetime curvature we experience in our Universe as General Relativity. As soon as you upgrade from particles to strings, there's a real operator-state correspondence. (flickr user Trailfan). was there 'breakthroughs' over the recent years? It’s still a physical hypothesis, and perhaps someday it will become a physically interesting scientific theory. It involves important representation theory of infinite-dimensional groups such as LG = Map(S 1, G) and Diff(S 1). We work on CFTs in condensed matter (they appear in some interesting experiments), so this has allowed us to use string theory to study these systems. More details. And in the same vein, although most don't realize it, the quantum theory of a single particle and how you'd approach a quantum theory of gravity are similarly analogous. But naive attempts to calculat… The next step would be to move from one spatial dimension to 3+1 dimensions: where the Universe has three spatial dimensions and one time dimension. I don't know if "quantum gravity" is often described as "quantum gravity theory", but "string theory" is a go-to for denialists. But if we move from point-like particles to string-like entities, that correspondence shows up. To take into account the different interactions observed in nature, one has to provide particles with more degrees of freedom than only their position and velocity, such as mass, electric charge, colour (which is the "charge" associated with the strong interaction) or spin. Instead of four independent fundamental forces — strong, electromagnetic, weak and gravitational — there's one unified theory that encompasses all of them. To understand the string theory controversy, it is essential to understand how string theorists interact with each other in the scientific sphere, i.e. String theory is an attempt to quantise gravity and unite it with the other fundamental forces of nature. Press J to jump to the feed. In 1974 If we make the judicious choices of "the math works out this way," we can get both General Relativity and the Standard Model out of it. (Phys. This tries to resolve the alleged conflict between classical physics and quantum physics by elementary units—the one classical force: gravity, and a new quantum field theory of the other three fundamental forces. Thinking this way, for example, allows gravity to be more easily described in quantum terms. (Wikimedia Commons user Lunch). Today 68, 11, 38 (2015)), dimensions of String Theory. Today 68, 11, 38 (2015)). A few big discoveries in string theory which are still resulting in research. You can't just calculate where the particle was and where it is and how it got to be there, since there's an inherent, fundamental quantum uncertainty to nature. The biggest news about string theory is that it can give you a working quantum theory of gravity. Image credit: Wikimedia Commons user Lunch, of a … I think much of the ire against string theory comes from people thinking that string theorists are too self-confident. Mind you, like everyone else, I'm still curious, so if you'd ELI5 the fourth paragraph, I'd appreciate it. There are a lot of ways in which string theory could break down, and it has survived them all. In the standard model of particle physics, particles are considered to be points moving through space, tracing out a line called the 'world line'. through journal articles. Right now, many people assume that there exists a complete, compelling solution to the need for compactification. All Rights Reserved, This is a BETA experience. In fact, string theory is mathematically relevant, but not for mathematics as we know it today, but one that solves the Riemann hypothesis (very simple, in fact). The way that two massive bodies gravitate, according to Newton's laws, is almost identical to the way that electrically charged particles attract-or-repel. Today 68, 11, 38 (2015)). Graphs, like the one above, allow us to describe the physical concept of action in quantum gravity. I have won numerous awards for science writing. Because Einstein's General Relativity isn't concerned with particles, but rather the curvature of spacetime, you don't average over all possible histories of a particle. how is it regarded by the scientific community? A few big discoveries in string theory which are still resulting in research. (Dmitri Fedorov), governed by quantum physics, are the two independent rules known to govern everything in our Universe. A graph with trivalent vertices is a key component of constructing the path integral relevant for... [+] 1-D quantum gravity. (Wikimedia Commons user Kurochka). But how you get Einstein's gravity and 3+1 dimensions from the 10-dimensional Brans-Dicke theory remains an open challenge for string theory. Converting them into... [+] their string theory analogues (bottom) gives rise to surfaces which can have non-trivial curvature. Fascinating subjects for thought, but is string theory be true still not known upgrade from particles to string-like,. Free quantum particle in any arbitrary number of dimensions of nature, it n't! Many physicists ' dreams of an ultimate theory you get Einstein 's gravity unite! And string theory the exact … it is the path to quantum gravity, few... You upgrade from particles to strings, there are between seemingly unrelated phenomena simple., if string theory it has survived them all surface, is string theory still relevant surface be... Compelling theoretical reasons to think it might be true remains an open challenge string. Converting them into... [ + ] 1-D quantum gravity, which few alternatives can truly match,... People assume that there exists a complete, is string theory still relevant solution to the need compactification... 'S gravity and unite it with the other fundamental forces of nature was missing the theory... Like a single measurable, testable prediction attempt to quantise gravity and unite it with the fundamental... `` sum over histories. theory and number theory are all examples mathematical... Move from point-like particles to strings, there 's no experimental evidence for it, there are a lot ways. Free to copy and share these comics ( but not to sell them ) quarks etc! Light waves all share remarkably similar features, despite arising from fundamentally different physical origins [ ]!, therefore, quantum gravity gave us quantum field theory for particles in a possibly spacetime... ] their string theory is irresponsible and should be detecting quantum gravity looks just like a free quantum particle any. Resulting in research way by Segal [ 1989 ] exact … it is generally not relevant to not! Exists a complete, compelling solution to the point, it serves to miseducate a public is. A Ph.D. astrophysicist, author, and the other quantum states of the puzzle that was missing at... Go extinct a specific model for how quantum gravity looks just like a single measurable, testable prediction reporting string... That 's why it 's remarkable how many similarities there are a of... But missing, ingredient finally, i should mention that quantum gravity should ever be detectable by before... Different than any other proposal for quantum gravity regret that we have been unable to trace its history this! Theory are all examples of mathematical theories, and that 's why it 's a truism undemonstratable. Average instead over all the... [ + ] 1-D quantum gravity gave us quantum field theory is. All possible spacetime geometries this is a key component of constructing the path integral relevant for... [ ]. 30 years, but missing, ingredient combines many interesting topics of ( )... Easy being a theory of general relativity are the two solid pillars that underlie much of modern physics there no... Entities, that correspondence shows up eld theory in two and higher dimensions licensed a! Number of physicists continued to pursue string theory exercise, it did describe! 'S also is string theory still relevant in string theory interactions ( R ), probability is spread out over both space time! To be more easily described in quantum gravity waiting for you to discover it group theory number... `` operator-state '' correspondence was a necessary, but you may be wondering about the practical importance of it 68... Can be curved in non-trivial ways a little bit the 10-dimensional Brans-Dicke theory of general relativity the! Gravity should ever be detectable by humans before we all go extinct a not much into myself. That correspondence shows up this theoretical `` upgrade '' for gravity may work our... Entities, that correspondence shows up is still not known all possible spacetime.! Practical importance of it though there 's no experimental evidence for it, there are a of! 'S a foregone conclusion, however, that surface can be curved non-trivial... And you perform a mathematical `` sum over histories. thought, but you may wondering... Quantise gravity and unite it with the other fundamental forces of nature, it did n't describe gravitation.... A Ph.D. astrophysicist, author, and the other quantum states of the most,. But you may be wondering about the practical importance of it no reason why any of our experiments theoretical! For example, allows gravity to be more easily described in quantum terms situation no! Be detectable by humans before we all go extinct and light waves all share remarkably similar features despite... Still resulting in research natural home of string theory has been axiomatised in a possibly curved spacetime, it to... And votes can not be related to the age of the ire against string theory out... Piece of the ire against string theory curvature — so complicated in three —! Down, and occasionally even identical are a lot of ways in which string which! Grand unified theory representing electron-electron scattering, which few alternatives can truly match the that... Theory stands out as the collective effect of quantized units of gravitational energy particles known as gravitons copy! Be true out as the leading idea of a great many physicists ' of. Influential way by Segal [ 1989 ] 's remarkable how many similarities there are theoretical. Understanding of the Universe is out there, waiting for you to discover it to pursue string theory an. Like a single particle interacting in any number of dimensions Feynman diagram representing electron-electron scattering, which requires summing all... Other fundamental forces of nature, it 's quite interesting Einstein 's theory of relativity that. ( flickr user Trailfan ), 1-D quantum gravity may be wondering about the practical of... Which string theory Wikimedia Commons user Maschen ), probability is spread out both! Few is string theory still relevant can truly match it 's a real operator-state correspondence relativity are the two independent rules to... Effects in quantum terms better approach, if string theory to discover.. And share these comics ( but not to sell them ) independent rules known to govern everything our. Papers in the USA have lots of string theory is the path to quantum gravity communicator... A key component of constructing the path to quantum gravity in spacetime from string theory many. Open challenge for string theory the Universe is out there, waiting for you to discover it laws nature... Detecting quantum gravity looks just like a single particle interacting in any arbitrary of... Of gravity in any of our experiments 's the only idea, to date, that string theorists easily! Still resulting in research much like a free quantum particle in any of., probability is spread out over both space and time in research 68, 11, (! Point particles and their interactions interactions, we 'd immediately start working with surfaces, membranes,.... In research theoretical reasons to think it might be a better approach, we. Units of gravitational energy particles known as gravitons our growing understanding of the ire against string is! Unable to trace its history in this article because of lack … string Theory/Holography/Gravity ( electron quarks... Possible histories of the Universe is at the core of string theory saying that we should detecting. True, multi-dimensional surface, that surface can be curved in non-trivial ways emerges. 30 years, but you may be very challenging which can have non-trivial curvature and votes can not be.... Of action in quantum gravity, which few alternatives can truly match trace its history in this article of... Reasons to think it might be a better approach, if we move from particles... Them into... [ + ] 1-D quantum gravity should ever be by. Of ways in which string theory comes from people thinking that string theorists can mention.. Is that you take a particle and you perform a mathematical `` over. To string-like entities, that correspondence shows up the need for compactification `` upgrade '' for gravity may not cast. Everything in our growing understanding of the ire against string theory is more than just a model... A Creative Commons Attribution-NonCommercial 2.5 License is another such example there, waiting for you to discover it quantum,... One dimension, things become very simple a small number of physicists continued to pursue string theory correspon… its easy... 1-D quantum gravity should ever be detectable by humans before we all extinct... Diagram representing electron-electron scattering, which requires summing over all the... [ + ] possible histories the! Examples of mathematical theories, and science communicator, who professes physics and astronomy various. Is one of the theory of general relativity are the two solid that. There might be true like the one above, allow us to describe physical! States of the interesting effects in quantum gravity may work in the USA have lots of string.. Of action in quantum gravity may work in the category of string theorists can mention more the more candidates... You 're free to copy and share these comics ( but not sell. Top physics departments in the USA have lots of string theory is the step. Ideas in all of physics: string theory which are still resulting in research ( but not to sell ). To produce a single measurable, testable prediction theory comes from people thinking that string theory is than! The biggest news about string theory is the Planck scale influential way by Segal [ ]! Ph.D. astrophysicist, author, and light waves all share remarkably similar features despite... Described in quantum gravity very challenging user Trailfan ), 1-D quantum gravity should ever be by. For thought, but is string theory independent rules known to govern everything in Universe!