MULTIVERSE
MULTIVERSE
GLOSSARY
brane Short for ‘membrane’. A concept used in string theory to generalize objects in our universe into higher dimensional space. Branes can have more than four dimensions – a ‘p-brane’, for example, has p dimensions. Brane cosmology is the idea that our visibly four-dimensional universe (three space, and one time), plus a number of hidden dimensions are found on a brane in a higher dimensional space (called the ‘bulk’).
bulk Sometimes popularly referred to as ‘hyperspace’, the bulk is a higher dimensional space that is theorized to contain the brane on which our four-dimensional universe exists.
compactification The process by which in string theory, dimensions can be hidden (or curled up).
interferometer A piece of equipment that is able to take light from two sources and superimpose the two waves to create an interference pattern. This technique allows for very tiny displacements to be measured (for example, those caused by gravitational waves passing by) or to combine light from separate telescopes to make very detailed images of the sky.
LIGO Laser Interferometer Gravitational-Wave Observatory, see here.
membrane See brane.
multiverse The idea that our universe might just be one of many multiverses (for example, found on different branes in the bulk) is sometimes invoked to explain why we have the physical constants we have, and why fundamental particles have the masses they have. In a multiverse, all values are possible, and we just happen to live in the universe that has the values it has (partly because this set of values allows stars and planets to form and life to evolve – not all do).
MWI (Many Worlds Interpretation) An interpretation of quantum mechanics, where instead of an observation making a particle choose which part of its waveform it is in (‘collapsing the waveform’), all possible states happen in different universes, and an observation just selects which of those universes we are part of.
Randall–Sundrum model A particular type of brane-world theory in which the universe is a brane in five-dimensional space.
sparticle A symmetric theory of particle physics. Sparticles are also known as ‘superpartners to particles’ in the Standard Model. In supersymmetry, all fermions should have an sparticle pair, which is a boson (but otherwise the same mass) and all bosons should have fermionic sparticles. Particle physicists had some fun naming sparticles. For example, the sparticle pair to the top quark is known as the ‘stop squark’.
superdimensionality Having more than the typical number of dimensions.
Tevatron A particle accelerator (inactive since 2011) at Fermilab, in Illinois, USA, which is the second largest in the world (after the Large Hadron Collider, see here).
INFINITE UNIVERSE
the 30-second blast
The universe has been expanding at approximately the same rate for almost the entire time since the Big Bang. It has been hypothesized that during a very brief moment, very early in the history of the cosmos, the universe far exceeded that expansion speed in a burst of hyperpowered inflation. Within 10–32 (a hundredth of a millionth of a trillionth of a trillionth) of a second, the diameter of the universe inflated at least 1026 (one hundred trillion trillion) times! If that is correct, the inflationary model could help explain several important observations about the uniformity and geometry of the universe. In the decades since its initial proposal it has come to light that the inflationary model could produce a remarkable cosmic scenario. An infinite number of regions of the universe could undergo hyperinflation, one by one, throughout all of space and time. Each inflationary event would create its own new set of cosmic horizons and observable universes, which could have physical characteristics and laws that are completely unpredictable, different and essentially inaccessible from those created by other inflationary events. Inflation would occur eternally, and the universe will become an infinite multiverse, where every possible observable universe that could ever exist will inevitably exist.
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In the inflationary cosmology model, our universe could be a multiverse filled with an infinite number of expanding observable universes with an infinite variety of properties.
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Some astrophysicists regard the inflation-induced multiverse as scientifically problematic. For them, the remit of science is to reduce the number of possible explanations of the universe with observations and experiments until we have one correct answer; so, if inflation produces every single possible universe, it has no useful predictive or discriminatory power and is thus not a meaningful scientific theory. Others disagree and think the inflationary model will someday be observationally confirmed.
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ALAN GUTH
1947–
ANDREI LINDE
1948–
American and Russian-American theoretical physicists and, with Paul Steinhardt, pioneers in the development of the original inflationary model, which has since evolved and been shown to lead to the possible existence of the multiverse
Eternal inflation could create an infinite number of universes, none of which could ever communicate with any of the others.
MANY WORLDS
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Quantum mechanics as a scientific theory has built-in uncertainty. Any measurement has a margin of error at the subatomic level, and every event has a chance of either occurring or not. In our universe, however, only one outcome out of all the possible outcomes ultimately occurs; the final result can be observed and measured and will forever affect everything that happens afterwards. One mathematically valid way to deal with that uncertainty is to assert that all the possible outcomes actually do occur, but their different effects on the future also occur. For every event in the history of time, therefore, an entirely new timeline is created. Every new timeline is as valid as every other timeline, so instead of one world experiencing a single past, present and future, many worlds exist in parallel – known as the ‘many-worlds interpretation’ (MWI) – none of which can communicate with any of the others. The one that matters to us, of course, is the one in which we share our existences and experiences. You may well exist at this very moment in a parallel universe reading these very words, but you might be wearing different clothing, or sporting a different haircut, or maybe living in a yellow submarine – you’ll never know for sure.
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One interpretation of quantum mechanics implies the existence of a vast number of parallel universes being created at every instant in cosmic history.
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If the MWI of quantum mechanics is correct, the number of parallel timelines created since the Big Bang is staggering, and more are being produced continually. Every possible thing that could have happened in the history of our universe but did not, actually did happen in another universe. The total number of timelines, however, is not necessarily infinite, and some things that could never have happened in our universe might indeed never have happened anywhere else either.
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HUGH EVERETT
1930–81
American physicist who, in 1957, first proposed the idea of MWI, or the ‘relative state’ formulation
BRYCE SELIGMAN DEWITT
1923–2004
American theoretical physicist who, from the 1970s onwards, promoted Everett’s formulation
Every choice you have ever made may have spawned a whole new universe with a whole new history – and a whole new you.
STRING THEORY
the 30-second blast
The quantum universe still has so many mysteries that seem inexplicable using the conventional idea of particles interacting in space and time. Physicists have therefore turned to a deeper, all-encompassing idea of the subatomic realm: the particles we observe are not merely occupying the four dimensions of space-time but are also simultaneously occupying additional dimensions that we cannot yet see or measure. Particles in our universe would thus be partial manifestations of superdimensional ‘strings’ of energy. Interactions between these ever-vibrating strings – in up to 11 dimensions – produce all the particles in our universe. Consider the following simplified analogy. Pluck a guitar string. The string has its own length, thickness, composition, etc; depending on how you view the string, it could look like a dot, a line, even a circle, perhaps shimmering as the string vibrates. As it does so, it rapidly changes shape in a repeated back-and-forth motion, disturbing the air around it and making sound waves that we can hear. Now imagine that string touching other guitar strings vibrating at different rates. If all the strings were producing their own tones in their own motions, what kinds of new tones, beats or harmonies might result? Guitar strings are only three dimensional; what amazing variety of behaviours might 11-dimensional strings produce? Quite possibly, everything in the universe.
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Each particle in our universe may merely be the four-dimensional projection of an 11-dimensional vibrating ‘string’.
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How can dimensions be hidden from detection? One possibility is called ‘compactification’ – an idea pioneered by Swedish physicist Oskar Klein and others – in which dimensions could be so small that they are compacted within others, making them invisible. From a distance, a piece of wire might look like a one-dimensional line, but to an ant walking around it the wire would appear to have three dimensions of length, width and depth.
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THEODOR KALUZA
1885–1954
German physicist who, in 1919, showed how adding dimensions to space-time could help connect the fundamental forces of nature
OSKAR KLEIN
1894–1977
Swedish theoretical physicist who came up with the idea that other dimensions might exist but are too small for us to detect
Tiny multidimensional vibrations of matter undetectable in our space-time may underlie everything we see in our universe.
SUPERSYMMETRY
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The Standard Model of particle physics is a shining star in the firmament of modern science. Like any believable scientific theory, it has its limitations – for example, it does not explain what the particles of cosmological dark matter are. One way to address some of these mysteries is to propose the existence of a set of fundamental subatomic particles that symmetrically mirror the well-known particles of the Standard Model but are just different enough to manifest themselves to be measured only in certain extreme environments. The term ‘supersymmetry’ is used to describe the relationship between these so-called ‘sparticles’ and the familiar fermions and bosons. When supersymmetry is meshed with string theory and its accompanying idea that numerous dimensions may exist beyond length, width, height and time, some mysteries of our cosmic origins and the history of the universe can also be addressed. It may be possible, for example, to tie all four fundamental forces of the universe – gravity, electromagnetism and the weak and strong nuclear forces – together as a unit, one that does not begin to break apart until the moment of the Big Bang. Thus, supersymmetric string theory (or, simply, superstring theory) may be the way to unify quantum mechanics and general relativity, the two great theoretical frameworks of modern science.
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If every subatomic particle has a unique supersymmetric partner, many important fundamental mysteries about the universe could be unified beyond what we have with the Standard Model.
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Experimental confirmation of superstring theory has been pursued since the beginning of the twenty-first century at major high-energy particle accelerators, such as the Tevatron at Fermilab in the USA and the Large Hadron Collider at CERN in Europe. Thus far the data have not confirmed the existence of sparticles, so the theory may need to be re-evaluated – either that or perhaps even more powerful experimental facilities might be required.
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MARY K. GAILLARD
1939–
S. JAMES GATES
1950–
EVA SILVERSTEIN
1970–
SHAMIT KACHRU
1970–
Four of the many prominent physicists working on the theoretical foundations of supersymmetric string theory
Particle–sparticle pairs extending into multi-dimensional symmetries could be knitting the universe together.
M-THEORY
the 30-second blast
With the combination of string theory and supersymmetry, known as ‘superstring theory’, physicists had, by the end of the twentieth century, developed at least five mathematical formulations that all appeared to show that general relativity and quantum mechanics were connected at the most fundamental cosmological level. Each formulation involved ten dimensions of space and time and were related in complex ways but seemed distinct from one another. Then, in 1995, a conceptual breakthrough occurred: each of these superstring theories, it was proposed, could be seen as a limiting case of an even more complex theory that involves an 11th space-time dimension. Key to this 11-dimensional theory is the mathematical idea of a membrane. In the same way that a string is a flexible extension of a single point, imagine a two-dimensional membrane as a flexible extension of a string – like a flag that can flap in the wind along both its width and its height. By considering the multidimensional universe as a structure where particles and membranes can move and interact, passing varying amounts of energy back and forth, four-dimensional space-time structures like our universe can be created and destroyed and possibly coexist with other universes with different natural laws, limits and lifespans.
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Merging superstring theory with membranes makes the existence of multiple universes like ours – a four-dimensional cosmos moving within an 11-dimensional supersymmetric structure – possible.
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The name of this 11-dimensional theory has remained intentionally ambiguous as a capitalized first initial: ‘M-theory’. Although its original premise included the concept of membranes, subsequent theoretical calculations have suggested that the use of membranes to describe its properties might be unnecessary. Its proponents have wryly suggested that, other than M for ‘membrane’, it could also stand for ‘mystery’ or even ‘magic’.
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EDWARD WITTEN
1951–
American physicist, also recognized for his discoveries in pure mathematics, who proposed M-theory; in so doing he launched what has become known as the ‘second superstring revolution’.
Our universe may owe its existence to cosmic membranes touching as they billow in a multiversal breeze.
COSMIC MULTICONNECTIONS
the 30-second blast
If one version of M-theory is correct, and our universe of one temporal and three spatial dimensions is a membrane that exists within a vast 11-dimensional structure that can accommodate myriad possible universes, how might our universe be related to other universes in this super-multidimensional realm, what extradimensional rules govern the behaviour of our universe and might we somehow be connected to other universes? Although the possibilities seem limitless, the scientifically measured parameters of our universe – for example, the disparate strengths of gravitational and electromagnetic forces – can help reduce the huge number of choices. One example of what might be possible is the Randall–Sundrum model. Gravity is extremely weak compared with electromagnetism on subatomic scales but is dominant at interstellar distances. If our universe is a brane embedded in a five-dimensional bulk, a second brane with strong gravity could interact with us through the fifth dimension. If the bulk is strongly warped in that fifth dimension, then our size and space could grow tremendously compared with the other brane, but our gravity would weaken just as much. Although we cannot sense either the bulk or the other brane, our connection with them would be the cause of the cosmos we experience today.
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At least one plausible model of how cosmic forces work posits that our universe is continually interacting with a different space-time within a structure of even higher dimension.
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Experimental verification of any theory of connected membranes or warped space-time dimensions will be exceedingly challenging. One possibility is that very high-energy collisions of subatomic particles might cause gravitons – the hypothesized carrier of the gravitational force – to be detectable in our space-time for fleeting moments in time. However, years of collisions in the Large Hadron Collider at CERN have thus far yielded no such detections.
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LISA RANDALL
1962–
RAMAN SUNDRUM
1964–
Respectively American and Indian-American physicists who formulated two versions of the five-dimensional warped-cosmic-geometry model that today bears their name
The puzzling properties of gravity may be caused by the way our universe interacts with another brane.
UNIVERSE UNKNOWN
the 30-second blast
With all the fascinating possibilities of hidden dimensions, or parallel timelines, or multiple universes that may exist, it can be easy to overlook the myriad mysteries within our own space-time, along our own timeline, about our own universe and the remarkable work being done to try to solve them. One spectacular example is the detection of gravitational waves. Many millions of years ago colliding neutron stars and black holes in distant galaxies rang our universe like the clapper of a cosmic bell. The resulting waves of space-time travelled until they passed through Earth, momentarily warping the diameter of our planet by less than the width of an atomic nucleus. In 2015 and 2017 astrophysicists were able to detect and measure this fleeting, tiny change, settling a key mystery of the general theory of relativity first published by Albert Einstein a full century earlier. Even as new discoveries are made, long-standing puzzles deepen still further, such as whether these gravitational waves finally confirm the existence of gravitons, the hypothesized subatomic particles that carry the force of gravity. Meanwhile, what new puzzles might spring forth in the wake of these cosmic waves? In the end, the questions we will have to answer are the ones we have yet to ask.
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Even if we restrict ourselves to discoveries within the observable part of our own universe, we are left with a seemingly endless stream of unanswered cosmic questions.
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The detection of gravitational waves was made possible only after nearly half a century of effort by thousands of dedicated scientists and engineers to develop, construct and operate LIGO, a continent-sized facility consisting of a pair of 4-kilometre (21/2-mile) long underground facilities in the USA, 3,000 kilometres (1,865 miles) apart. Direct detection of gravitons, however, will likely require a far larger observatory, thousands of times the mass and volume of our entire planet Earth.
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RAINER WEISS
1932–
BARRY BARISH
1936–
American physicists who have devoted their careers to detecting gravitational waves; they were honoured for their efforts alongside Kip S. Thorne with the 2017 Nobel Prize in Physics
Astronomers have discovered cosmic ripples in space-time caused by black holes crashing together in the distant universe.
