This week’s passing of famed British physicist Stephen Hawking has sparked reflection around the globe — and perhaps, dreamily, even throughout the universe that he dedicated so much of his life to understanding better.
It’s worth revisiting Hawking’s appearance at the Seattle Science Festival on June 16, 2012, when he gave a talk at the Paramount Theater titled “Brane New World.”
In that talk, visible in the Pacific Science Center’s video above and transcribed below, Hawking laid out his views on what he thought could be the ultimate theory of the universe, known as M-theory.
GeekWire’s Alan Boyle covered the event for NBCNews.com and wrote at the time that Hawking’s “crazy smarts and his sharp wit” were both in evidence that night.
Here is the transcription:
Brane New World
“In this talk I want to describe an exciting development that may transform our view of the universe and of reality itself, whatever that may be. The idea is that we may live on a brane, or surface, in a larger space. The word ‘brane,’ spelled b-r-a-n-e, was introduced by my colleague Paul Townsend to indicate decentralization of the membrane to more dimensions.
“I suspect the pun on ‘brain’ was quite deliberate.
“We think we live in three-dimensional space that is weak and specifies the position of an object in the room by three numbers. There could be five feet from the north wall, three feet from the east wall, and two feet above floor level.
“Or on a large scale, there could be latitude, longitude and height above sea level. On an even larger scale, we can specify the position of a star in the galaxy by three numbers: galactic latitude and longitude, and distance from the center of the galaxy.
“As well as the three numbers that specify position, we can add a fourth number that specifies time. Thus we can describe ourselves as living in a four-dimensional spacetime, where events are labeled by four numbers: three for the the position of the event and the fourth for its time.
“It was Einstein’s stroke of genius to realize that spacetime isn’t flat but is curved and warped by the matter and energy in it. According to this general theory of relativity, objects like planets try to move on straight lines through spacetime, but because spacetime is curved, their paths appear bent as if by a gravitational field. It is as if you placed a heavy weight to represent a star on a sheet of rubber. The weight will depress a sheet and cause it to curve near the star.
“If you now roll little ball bearings on the sheet to represent planets, they can orbit a star.
“We now have confirmation that spacetime is curved, from the GPS navigation system which is fitted to boats, airplanes and some cars. This operates by comparing signals from several satellites. If one assumed spacetime was flat, one would calculate the wrong position.
“Three dimensions of space and one of time are all that we see. So why should we believe in extra dimensions that we can observe? Are they just science fiction, or do they have detectable consequences?
“The reason we take the idea of extra dimensions seriously is that, although Einstein’s general theory of relativity agrees with all of the observations we have made, the theory predicts its own downfall. Roger Penrose and I showed that it predicted that time had a beginning and a big bang, and an end in black holes. At these places, the general theory of relativity would break down, so one could not use it to predict how the universe began, or what would happen to someone who fell in a black hole.
“The reason general relativity would break down in the big bang and black holes is that it does not take account of small-scale behavior of matter. In normal situations, the warping of spacetime is very slight over comparatively long length scales. But at the beginning and end of time, spacetime will be scrunched up to a single point.
“To read this we need to combine general relativity, the theory of the very large, with quantum mechanics, the theory of the very small. This would create a TEO, or Theory of Everything, that would describe the universe from beginning to end.
“We have been searching for the Theory of Everything for the last 30 years, and we now think we have a candidate called M-theory. In fact, M-theory isn’t a single theory but rather a network of theories that all seem physically equivalent. It allows us to have several different descriptions of the universe, all of which predict the same observations and are useful in their own right.
“Remarkably enough, in many of the theories in the M-theory network, spacetime has more than four dimensions we experience. Are these extra dimensions real? I must admit I have been reluctant to believe in extra dimensions, but the M-theory network fits together so beautifully and has so many unexpected correspondences that I feel to ignore it would be like claiming that God put fossils in the rocks that tricked Darwin into believing in evolution.
“So how is it that spacetime appears four-dimensional to us, but is 10- or 11-dimensional in M-theory? Why don’t we observe another six or seven dimensions? The conventional answer to this question, which was generally accepted until recently, was that the extra dimension is like a human hair. If you look at it from a distance, it looks like a one-dimensional line. But if you look at it under a magnifying glass, you’ll see the thickness and that the hair is really three-dimensional.
“However, there has recently been a more radical suggestion: that one or two of the extra dimensions may be much larger or even infinite. Because these large extra dimensions have not been seen in particle accelerators, it is necessary to suppose that all particles of matter are confined to the brane or surface in spacetime, and are not free to propagate through the large extra dimensions.
“Light will also have to be confined to the brane, or we would already have detected large extra dimensions, and the same is true of the nuclear forces between particles: Gravity, on the other hand, is a universal force between all forms of energy or mass that cannot be confined to the brane but will permeate the entire space.
“Because gravity spreads out into the large extra dimensions as well as along the brane, it ought to fall off more sharply with distance than the electric force which is confined to the brane. Yet, we know from observations of the orbits of the planets that the gravitational pull of the sun goes down the further one is from the sun, in the same way that electric forces fall off with distance.
“So if we do live on a brane, there has to be some reason why gravity doesn’t spread far from the brane but is confined to a neighborhood around it. One possibility is that the large extra dimensions end on a second, shadow brane not far from the brane on which we live.
“We would not see the shadow brane because light could travel only along the branes and not through the space between. However, we would feel the gravity of matter of the shadow brane. There could be shadow galaxies, shadow stars and even shadow people who might wonder about their gravity they feel from matter on our brane.
“Instead of the extra dimensions ending on the second brane, another possibility is that they are infinite but highly curved like a saddle. Lisa Randall and Raman Sundrum showed that this kind of curvature would act rather like a second brane. The gravitational influence of an object on the brane would be confined to a small neighborhood of the brane, and would not spread out to infinity in the extra dimensions.
“But there is an important difference between this Randall-Sundrum model and the shadow brane model. Bodies that move under the influence of gravity will produce gravitational waves: ripples of curvature that travel through spacetime at the speed of light. Like the electromagnetic waves of light, gravitational waves should carry energy, a prediction that has been confirmed by observations of the binary pulsar.
“If we indeed live on a brane in a spacetime with extra dimensions, gravitational waves generated by the motion of bodies and the brane would travel off into the other dimensions. They would be reflected back if there were a second shadow brane, and would be trapped between the two branes.
“On the other hand, if there was only a single brane, and the extra dimensions went on forever, like in the Randall-Sundrum model, gravitational waves could escape altogether and carry away energy from our brane world.
“It is only the short gravitational waves that would escape from the brane, and the only sources of significant amounts of short gravitational waves are likely to be black holes. A black hole on the brane will extend through a black hole in the extra dimensions. If the black hole is small, it will be almost round. That is, it will reach about as far into the extra dimensions as its size on the brane.
“On the other hand, a large black hole on the brane will extend to a black ‘pancake,’ which is confined to a vicinity of the brane and which is much less thick in the extra dimension than it is wide on the brane.
“Some time ago I discovered that black holes won’t be completely black. They will emit particles and radiation of all kinds as if they were hot bodies. The particles and radiation like light would be emitted along the brane, because matter and electric forces would be confined to the brane. However, black holes also emit gravitational waves. These would not be confined to the brane but would travel in the extra dimension as well. if the black hole was large and pancake-like, the gravitational waves would stay near the brane.
“This would mean that the black hole would lose energy and mass at a rate one would expect for a black hole in four-dimensional spacetime. The black hole would therefore slowly evaporate and shrink in size until it became small enough that gravitational waves that radiated could begin to escape freely into the extra dimensions.
“To someone on the brane, the black hole would appear to be emitting dark radiation — radiation that cannot be observed directly on the brane, but whose existence can be inferred from the fact that the black hole was losing mass. It would mean that the final burst of radiation from an evaporating black hole would appear less powerful than it actually was.
“This could be why we have not observed bursts of gamma rays that can be ascribed to dying black holes, though the other explanation would be that there aren’t many black holes with mass low enough to evaporate in the age of the universe so far. That would be a pity, because if a low-mass black hole were discovered, I would get a Nobel Prize.
“So how could a brane world have originated? In my opinion, an attractive explanation of the origin of a brane world would be that it was spontaneously created as a fluctuation of the vacuum. The creation of a brane would be a bit like the formation of bubbles of steam and boiling water.
“Liquid water consists of billions and billions of H2O molecules, packed together with couplings between nearest neighbors. As the water is heated up the molecules move faster and bounce off each other. Occasionally these collisions will give molecules such high velocities that a group of them will break free of their bonds and form a little bubble of steam surrounded by water.
“Most small bubbles of steam will collapse to liquid again. But a few will grow to a certain critical size, beyond which the bubbles are almost certain to continue to grow. It is these large expanding bubbles that one observes when water boils.
“The behavior of brane worlds would be similar. Fluctuations in the vacuum would cause brane worlds to appear from nothing as bubbles. The brane would form the surface of the bubble, and the interior would be the higher-dimensional space.
“Very small bubbles would tend to collapse again to nothing, but a bubble that grew by quantum fluctuations beyond a certain critical size would be likely to keep on growing. People such as us, living on the brane, the surface of the bubble, would think the universe is expanding.
“It would be like painting galaxies on the surface of a balloon and blowing it up. As the brane expanded, the volume of the higher dimensional space inside would increase. Eventually there would be an enormous bubble, surrounded by the brane on which we live.
“Matter on the brane, the surface of the bubble, will determine the gravitational fields in the interior of the bubble. Equally, the gravitational fields in the interior will determine the matter on the brane.
“It is like a hologram. A hologram is an image of a three-dimensional object that is encoded on a two-dimensional surface. I knew all about holograms, having been one myself along with Newton and Einstein on an episode of ‘Star Trek.’
“In a similar way, what we think of as four-dimensional spacetime may be just a hologram for what is happening in the five-dimensional interior of the bubble. We take it as obvious that we live in the world of three space dimensions and one time dimension, but maybe we are just shadows cast by a flickering fire on the wall of the cave in which we have our existence. Let’s hope that any monsters we encounter are also shadows.
“Brane world models are a hot topic of research. They are highly speculative, but they offer new kinds of behavior which can be tested by observation. They could explain why gravity seems to be so weak. Gravity might be quite strong in the fundamental theory, but the spreading of the gravitational force and the extra dimensions would mean it would be weak at large distances on the brane on which we live.
“If gravity is stronger in the extra dimensions, it would make it much easier to form little black holes in collisions of high energy particles. It might be possible in the LHC, the Large Hadron Collider, that is based in Geneva. A tiny black hole wouldn’t gobble up the Earth, as newspaper scare stories would have one believe. Instead, the black hole would disappear in a puff of Hawking radiation, and I would get a Nobel Prize.
“So on the LHC we may discover a brane new world.”
Editor’s note: Less than a month after Stephen Hawking gave this talk, scientists using the LHC reported their discovery of the Higgs boson — but they haven’t yet seen evidence of extra dimensions or microscopic black holes, as Hawking hoped. The quest continues.
Also, there’s not yet been any sighting of a gamma-ray burst conclusively associated with the death of a black hole. But over the past few years, gravitational-wave observatories have picked up the signatures of black holes and neutron stars crashing together. When the first detection was announced in 2016, Hawking noted with pleasure that it was “consistent with predictions about black holes that I made in 1970.”
GeekWire aerospace and science editor Alan Boyle contributed to this report.