Branes and multiple universes
One insight emerging from M-theory is the concept of branes - spatially extended objects arising from the mathematics of the theory which stretch into extra dimensions, for example the two-dimensional two brane and the three-dimensional three brane. In the original mathematics of string theory this extended all the way up to all up to 9 branes, strings being really one-branes. The original mathematics which produced this result were perturbative (that is, approximate) with the result that one of the spatial dimensions was too small to be seen based on the assumption that the string coupling was small.
The more precise mathematical methodology of M-theory revealed a universe with 10 spatial dimensions and one of time, in other words 11 spacetime dimensions. So by this stage, string theory was not just a theory that contained strings. It also included membranes (2-branes) with 2 spatial dimensions and 3 branes with 3 spatial dimensions and so on. In the braneworld scenario, our universe exists on a 3 dimensional brane, which floats in a higher dimensional expanse potentially populated by other branes, in other words other parallel universes.
Parallel universes are the subject of Brian Greene’s latest book (at the time of writing) The Hidden Reality subtitled Parallel Universes and the Deep Laws of the Cosmos (2011) in which he postulates the existence of no less than 9 parallel multiverses, each stemming in its own way from many of the laws of the cosmos we are familiar with in other contexts: the quilted multiverse, the inflationary multiverse, the brane and cyclic multiverse, the landscape multiverse, the quantum multiverse, the holographic multiverse, and the simulated and ultimate multiverses.
The more precise mathematical methodology of M-theory revealed a universe with 10 spatial dimensions and one of time, in other words 11 spacetime dimensions. So by this stage, string theory was not just a theory that contained strings. It also included membranes (2-branes) with 2 spatial dimensions and 3 branes with 3 spatial dimensions and so on. In the braneworld scenario, our universe exists on a 3 dimensional brane, which floats in a higher dimensional expanse potentially populated by other branes, in other words other parallel universes.
Parallel universes are the subject of Brian Greene’s latest book (at the time of writing) The Hidden Reality subtitled Parallel Universes and the Deep Laws of the Cosmos (2011) in which he postulates the existence of no less than 9 parallel multiverses, each stemming in its own way from many of the laws of the cosmos we are familiar with in other contexts: the quilted multiverse, the inflationary multiverse, the brane and cyclic multiverse, the landscape multiverse, the quantum multiverse, the holographic multiverse, and the simulated and ultimate multiverses.
By way of example, in the cyclic universe, collisions between braneworlds can manifest themselves as big bang like beginnings, yielding universes that are parallel in time. Another suggestion is that, in view of the similarity between conditions at the big bang and at the centre of black holes, each characterised by a colossal density of crushed matter, every black hole is the seed for a new universe that erupts into existence through a big bang like explosion, but is forever hidden from our view by the black hole’s event horizon.
The existence of braneworlds may go some way towards explaining the apparent weakness of gravity in our own universe. (I can counter the gravitational pull of the entire planet by picking up a coffee cup. A small magnet uplifting a paper clip can do likewise).
Gravitons, the hypothetical particle for the gravitational force, can enter or leave a braneworld. When some of the streaming gravitons leak off the brane and flow into the extra dimensions, the gravitational attraction between objects will be diminished. The more the dilution, the weaker gravity appears. Data attesting to missing energy is explained by positing that our universe exists on a brane and arguing that debris with the capacity to fly off our brane - gravitons – had carried the energy away[1].
In another beguiling allusion, Gavin Hesketh compares brane worlds with the floors of an office building. Our entire four-dimensional universe comprises one floor – a putative brane, where the Standard Model works well. The other floors in the building are composed of other brane/universes of whose existence we are blissfully unaware and gravity freely takes the stairs up and down, roaming the building. “Suppose that gravity appears weaker because it covers the whole building, whereas the other forces are limited to one floor.... (M)ost of the force of gravity is disappearing into these extra dimensions”. [2]
Another example: for decades Schrödinger’s 1926 wave equation was thought to be relevant only to small things, but in 1957 Hugh Everett, working from a strict interpretation of Schrödinger’s maths, argued that his wave equation should apply to everything because all things material, regardless of size, are made from molecules, atoms and subatomic particles. This led to the many worlds approach to Quantum Mechanics and ultimately to what is now described as the Quilted Multiverse, as the universes roll themselves out like repeated patterns on a quilt [3].
A coincidence between the theory of cosmic inflation and the many worlds interpretation of quantum mechanics
More recently (2011, 2017), a coincidence has been identified between these two theories[4] . The theory of cosmic inflation suggests that our universe is one of infinitely many, comprising a multiverse that formed when the early cosmos expanded exponentially. During this expansion, some regions would have halted their rapid expansion sooner than others, forming so-called “bubble universes”. One of the problems with this concept is that it destroys the theory’s ability to make predictions because “in an eternally inflating universe, anything that can happen will happen; in fact, it will happen an infinite number of times”.
It has lately been postulated that the cosmological picture of the eternally inflating multiverse may be the mathematical equivalent of the “many worlds” interpretation of quantum mechanics, which endeavours to explain how particles can seem to be in many places at the same time. As we have already seen, in the quantum world the outcome of any process is always probabilistic, and things have only a certain chance of ending up here or there. In the meantime, things are in a so-called superposition of states between outcomes, but “once we look”, the outcome crystallises into one state or the other, and thereafter all subsequent measurements will find the same result as the first. The Copenhagen interpretation explains this by saying that the first measurement changed the system from a state of superposition to a particular state. However, in 1957, Everett came along with his “many worlds” interpretation of quantum mechanics, the central insight of which was that the state of a quantum system reflects the state of the whole universe around it, with the effect that we must include the observer in a complete description of the measurement. Under this scenario, the quantum state after the measurement is still a superposition, but a superposition of two entire worlds. A corollary of this is that everything in nature obeys the laws of quantum mechanics, whether large or small.
If this is accepted, the eternally inflating multiverse and the quantum mechanical “many worlds”’ theory become one and the same., and the many bubble universes of inflation do not all exist in a single real space but represent probable different branches on the tree of probabilities. If this is true, the many worlds interpretation of the multiverse would mean that the laws of quantum mechanics do not operate solely on the microscopic realm. They also play a crucial role in determining the global structure of the multiverse even at the largest distance scales.
Yasunori Nomura uses the event horizon of a black hole and the different perspectives of observers inside and outside that boundary as an analogy, and compares that scenario to the so-called “cosmological horizon”: the finite boundary of the spacetime region within which we are capable of receiving signals from deep space.[5] Space is expanding so rapidly that the signals from the furthest outlying regions will forever be beyond our reach because the expansion of space causes them to recede faster than the speed of light. In other words, any description of the quantum state of the universe should include only the region within and on the horizon. For these purposes, the cosmos should not be regarded as infinite.
So where does the multiverse, which we thought existed in an eternally inflating infinite space, stand in all this? The answer is that the creation of bubble universes is probabilistic, just like any other quantum process, and inflation could produce many different universes, each with a different probability of coming into being. Having started from an initial state, the multiverse then evolved into a superposition of many bubble universes, so the theory goes. Because each of these universes is finite, the problem of predictability that was raised by the prospect of an infinitely large space encompassing all possible outcomes is avoided, and each universe (each possible outcome) retains a specific probability of coming into being.
“Quantum mechanical probabilities therefore determine outcomes in cosmology and in microscopic processes. The multiverse and quantum many worlds are really the same thing; they simply refer to the same phenomenon - superposition – occurring at vastly different scales”. [6] Micro meets macro, as it were.
This scenario may actually be capable of being tested, because if the multiverse theory is correct, this should lead to a small amount of “saddle-shaped” negative spatial curvature in our universe, one where objects would travel through space not along straight lines as in a flat cosmos but along curves, even in the absence of gravity. Experiments studying how distant light bends as it travels through the cosmos are currently under way, and results should be achieved in the next two decades. If these experiments find any amount of negative curvature, they will support the multiverse concept. Conversely, the discovery of positive curvature would falsify the notion of a multiverse altogether.
The subject of parallel universes is highly speculative and, speaking generally, no one should be convinced of anything not supported by hard data [7]. However, all of the parallel universe proposals to be taken seriously emerge naturally and unbidden from the mathematics of theories developed to explain conventional data and observations (my emphasis): inflationary cosmology, the cosmological constant, quantum field theory and black holes. They are not simply the armchair theories of an idle mind.
[1] Greene (2011), 118.
[2] The Particle Zoo - The Search for the fundamental nature of reality, Quercus, Hachette, London, 2016, 249-250.
[3] Greene (2011), 321.
[4] What follows is an edited summary of Yasanori Nomura's article "The quantum multiverse", in the Scientific American, June 2017, 23-29.
[5] Ibid, 28.
[6] Ibid, 29
[7] Greene (2011), 8
The existence of braneworlds may go some way towards explaining the apparent weakness of gravity in our own universe. (I can counter the gravitational pull of the entire planet by picking up a coffee cup. A small magnet uplifting a paper clip can do likewise).
Gravitons, the hypothetical particle for the gravitational force, can enter or leave a braneworld. When some of the streaming gravitons leak off the brane and flow into the extra dimensions, the gravitational attraction between objects will be diminished. The more the dilution, the weaker gravity appears. Data attesting to missing energy is explained by positing that our universe exists on a brane and arguing that debris with the capacity to fly off our brane - gravitons – had carried the energy away[1].
In another beguiling allusion, Gavin Hesketh compares brane worlds with the floors of an office building. Our entire four-dimensional universe comprises one floor – a putative brane, where the Standard Model works well. The other floors in the building are composed of other brane/universes of whose existence we are blissfully unaware and gravity freely takes the stairs up and down, roaming the building. “Suppose that gravity appears weaker because it covers the whole building, whereas the other forces are limited to one floor.... (M)ost of the force of gravity is disappearing into these extra dimensions”. [2]
Another example: for decades Schrödinger’s 1926 wave equation was thought to be relevant only to small things, but in 1957 Hugh Everett, working from a strict interpretation of Schrödinger’s maths, argued that his wave equation should apply to everything because all things material, regardless of size, are made from molecules, atoms and subatomic particles. This led to the many worlds approach to Quantum Mechanics and ultimately to what is now described as the Quilted Multiverse, as the universes roll themselves out like repeated patterns on a quilt [3].
A coincidence between the theory of cosmic inflation and the many worlds interpretation of quantum mechanics
More recently (2011, 2017), a coincidence has been identified between these two theories[4] . The theory of cosmic inflation suggests that our universe is one of infinitely many, comprising a multiverse that formed when the early cosmos expanded exponentially. During this expansion, some regions would have halted their rapid expansion sooner than others, forming so-called “bubble universes”. One of the problems with this concept is that it destroys the theory’s ability to make predictions because “in an eternally inflating universe, anything that can happen will happen; in fact, it will happen an infinite number of times”.
It has lately been postulated that the cosmological picture of the eternally inflating multiverse may be the mathematical equivalent of the “many worlds” interpretation of quantum mechanics, which endeavours to explain how particles can seem to be in many places at the same time. As we have already seen, in the quantum world the outcome of any process is always probabilistic, and things have only a certain chance of ending up here or there. In the meantime, things are in a so-called superposition of states between outcomes, but “once we look”, the outcome crystallises into one state or the other, and thereafter all subsequent measurements will find the same result as the first. The Copenhagen interpretation explains this by saying that the first measurement changed the system from a state of superposition to a particular state. However, in 1957, Everett came along with his “many worlds” interpretation of quantum mechanics, the central insight of which was that the state of a quantum system reflects the state of the whole universe around it, with the effect that we must include the observer in a complete description of the measurement. Under this scenario, the quantum state after the measurement is still a superposition, but a superposition of two entire worlds. A corollary of this is that everything in nature obeys the laws of quantum mechanics, whether large or small.
If this is accepted, the eternally inflating multiverse and the quantum mechanical “many worlds”’ theory become one and the same., and the many bubble universes of inflation do not all exist in a single real space but represent probable different branches on the tree of probabilities. If this is true, the many worlds interpretation of the multiverse would mean that the laws of quantum mechanics do not operate solely on the microscopic realm. They also play a crucial role in determining the global structure of the multiverse even at the largest distance scales.
Yasunori Nomura uses the event horizon of a black hole and the different perspectives of observers inside and outside that boundary as an analogy, and compares that scenario to the so-called “cosmological horizon”: the finite boundary of the spacetime region within which we are capable of receiving signals from deep space.[5] Space is expanding so rapidly that the signals from the furthest outlying regions will forever be beyond our reach because the expansion of space causes them to recede faster than the speed of light. In other words, any description of the quantum state of the universe should include only the region within and on the horizon. For these purposes, the cosmos should not be regarded as infinite.
So where does the multiverse, which we thought existed in an eternally inflating infinite space, stand in all this? The answer is that the creation of bubble universes is probabilistic, just like any other quantum process, and inflation could produce many different universes, each with a different probability of coming into being. Having started from an initial state, the multiverse then evolved into a superposition of many bubble universes, so the theory goes. Because each of these universes is finite, the problem of predictability that was raised by the prospect of an infinitely large space encompassing all possible outcomes is avoided, and each universe (each possible outcome) retains a specific probability of coming into being.
“Quantum mechanical probabilities therefore determine outcomes in cosmology and in microscopic processes. The multiverse and quantum many worlds are really the same thing; they simply refer to the same phenomenon - superposition – occurring at vastly different scales”. [6] Micro meets macro, as it were.
This scenario may actually be capable of being tested, because if the multiverse theory is correct, this should lead to a small amount of “saddle-shaped” negative spatial curvature in our universe, one where objects would travel through space not along straight lines as in a flat cosmos but along curves, even in the absence of gravity. Experiments studying how distant light bends as it travels through the cosmos are currently under way, and results should be achieved in the next two decades. If these experiments find any amount of negative curvature, they will support the multiverse concept. Conversely, the discovery of positive curvature would falsify the notion of a multiverse altogether.
The subject of parallel universes is highly speculative and, speaking generally, no one should be convinced of anything not supported by hard data [7]. However, all of the parallel universe proposals to be taken seriously emerge naturally and unbidden from the mathematics of theories developed to explain conventional data and observations (my emphasis): inflationary cosmology, the cosmological constant, quantum field theory and black holes. They are not simply the armchair theories of an idle mind.
[1] Greene (2011), 118.
[2] The Particle Zoo - The Search for the fundamental nature of reality, Quercus, Hachette, London, 2016, 249-250.
[3] Greene (2011), 321.
[4] What follows is an edited summary of Yasanori Nomura's article "The quantum multiverse", in the Scientific American, June 2017, 23-29.
[5] Ibid, 28.
[6] Ibid, 29
[7] Greene (2011), 8