The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or mathematician to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time here.
Most of us complain about nothing from time to time. Like when you open the fridge, see the empty shelves, and sigh, “There’s nothing to eat” – even though there’s a half-eaten yogurt and some suspicious looking lettuce.
Of course, that’s a rather subjective form of nothing. There are two more precise definitions we might consider. First, there is nothing with a lowercase n. This is what you get when you start with something and take everything away: the yogurt, the lettuce, the air molecules in the fridge and more besides. But there is also Nothing, with a capital N. This is far more absolute. It ľ±˛ő˛Ô’t what you get when you empty the fridge. A true Nothing can never have been something, and it can never become something. It just ľ±˛ő˛Ô’t. If such a Nothing can exist – and it’s hard to imagine how – then everything, including us, must be disconnected from it.
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As physicists, we are more interested in things we can connect to, in the lesser form of nothing with a lowercase n. This vacuum once triggered a war that lasted over 1000 years. The conflict began with Ancient Greek philosophers in the 5th century BC, specifically Leucippus and his pupil, Democritus. They proposed that matter was made up of tiny little indivisible pieces, or “atoms”, frolicking about in an infinite void – in nothing.
It was a philosophy that Aristotle was later determined to destroy. He believed that matter was a continuous fluid, expanding and contracting, forever shifting between the four basic elements of earth, air, fire and water. He built a model of the universe divided up into concentric spheres, with terrestrial layers made up of the four elements and celestial layers made from a fifth element known as the aether. Each of the celestial spheres was moved by its larger neighbour: the lunar sphere moved by Mercury, Mercury by Venus and so on, until you reached the final layer of stars, fixed at the edge of the material world. According to Aristotle, this final layer was moved by something beyond the material world, by the “prime mover”, by God.
In connecting his model to the existence of God, Aristotle triumphed over the atomists in the hearts of many – in particular, medieval Christian philosophers such as Thomas Aquinas. The atomist philosophy became a heresy. To embrace the void was to embrace the absence of God, to embrace evil. Eastern philosophers took a very different view. In many Eastern religions, the void is a source of enlightenment. Buddha, for example, was said to have realised Śū˛Ô˛â˛ąłŮÄĺ, the emptiness of emptiness, freeing himself from all suffering and misery. With nothing at the heart of their spirituality, Eastern mathematicians weren’t afraid to develop radical new ideas like zero, the numerical representation of nothing. These ideas were violently rejected in the West. In 1299, Florentine officials went as far as to ban the use of Hindu-Arabic numerals, especially zero, to prevent fraud. After all, adding a few zeros to your accounts could make them look a lot healthier.
While Thomas Aquinas had championed Aristotle’s ideas in the early part of the 13th century, by the time of the Florentine decree, the Ancient Greek’s grip on Western philosophy was already under attack. In 1277, Bishop Étienne Tempier asked whether it was possible for God to move the heavens, for which the answer was obvious: God could do anything. And yet, Aristotle had argued that it wasn’t possible, since moving the heavens would leave behind a vacuum and that was forbidden. To Tempier, it was Aristotle and his followers who were guilty of heresy. God could certainly create a vacuum or a void, and so it became blessed. As for zero, it also received a pardon in the Western world, not from the Bishops of the Catholic Church, but from the accountants. They invented double entry bookkeeping and zero became an essential tool in balancing the books.
By the 17th century, it wasn’t just God who could create a vacuum. Physicists Evangelista Torricelli and Blaise Pascal had also managed it using a mercury barometer. But had they really created nothing?
With the advent of quantum mechanics in the early part of the 20th century, we learnt that the answer was a resounding “no”. In a quantum world, you cannot say with precise accuracy where a particle is or how fast it is moving. There is always a quantum wiggle.
The same is true of the vacuum. You might imagine it as a silent, empty place, but it still contains that quantum wiggle. In this case, it’s the wiggle of virtual particles popping in and out of existence. They push and pull on the vacuum, giving it enough energy to bend the shape of space and time. Many astronomers believe that so-called dark energy – a concept used to explain why the universe is expanding at an ever-increasing rate, as measurements suggest – is really the energy of empty space. This energy from the void could be driving the accelerated cosmic expansion, growing faster and faster as we head towards a universal vacuum.
So, if nothing is not the vacuum, then where is nothing? Can it even exist in our physical realm? In 1982, physicist and Fields medallist Ed Witten found the answer. He discovered a terrifying new form of nothing, far more aggressive than a void or a vacuum. It revealed itself as an instability of space-time in which a bubble of literally nothing can appear and begin devouring the universe from the inside out. This ľ±˛ő˛Ô’t just empty space. It is the absence of space itself. If a bubble of nothing were to appear in your house, it wouldn’t leave behind a gaping hole – it would also erase the hole. It would take everything: your TV, your sofa and even the space they once occupied. This spectre of destruction would quickly spread, a runaway instability in which space itself just gives up and disappears.
With the prospect of space and time devoured by nothing, it is no surprise that physicists sought to better understand what Witten had found. In 2011, it led two academics at Princeton University, and , to discover an unexpected new way to think about nothing: as a . We’ve already seen how a vacuum can store energy that can bend the shape of space and time. When the energy is positive, you get positive curvature and a universe that expands at an accelerated rate. When the energy is negative, you get negative curvature and a universe that initially expands before collapsing back in on itself, disappearing into an apocalyptic crunch. Brown and Dahlen imagined bubbles filled with a vacuum of negative energy that bends space-time by a negative but finite amount. As they ramped up the curvature inside the bubble to larger and larger negative values, they noticed that the physics was drawing closer and closer to what Witten had seen. They concluded that Witten’s bubble of nothing could be understood as a bubble of vacuum space-time curved to negative infinity.
Brown and Dahlen’s perspective is backed up by the “holographic principle”. This is the idea that our universe might be like a projection, a three-dimensional reality encoded on a lower-dimensional boundary. I like to think of this as two different languages: on the one hand, I have the three-dimensional language of the bulk; on the other, the language of the lower-dimensional boundary. Both languages can describe the same physical phenomena, albeit in a very different way. As it happens, we are more used to speaking the language of the bulk, but that is just a choice. We could choose to speak the language of the boundary, and that would be no more strange than my Spanish dad choosing to speak Spanish from his home in Liverpool.
But what does this have to do with bubbles of infinitely negative curvature? The holographic principle is best understood for space-times that are negatively curved. It is in this case that we have a proper dictionary relating the bulk and boundary languages. But here comes nothing. It turns out that the number of words in the boundary language depends on the curvature of the bulk. As that curvature plummets towards negative infinity, the number of words goes to zero! The boundary language completely disappears and you are left with nothing. Brown and Dahlen were right.
Using a vacuum of infinite negative curvature may open up new ways to think about how the universe was born from nothing, or how it might end up as nothing. This ľ±˛ő˛Ô’t just something we might like to think about – it’s something we have to think about. You see, just as Witten showed that nothing is possible, a team from Harvard showed that it is inevitable.
Physicists and have argued that there are no islands in the . This is the landscape containing all the theories of quantum gravity that make sense. Some of them are wild and wonderful and have nothing in common with the universe we see around us, but at least they are logically consistent. Suppose you reach into the landscape and take your favourite theory of quantum gravity, and I take mine. Then McNamara and Vafa are saying that there should be a way to dynamically transition between the two. These transitions can sometimes occur by passing through a wall, a portal separating your world of quantum gravity from mine.
But there is more, or perhaps less. In addition to all the fancy theories of quantum gravity, there is the simplest quantum gravity theory of them all: the theory describing nothing. Since this trivial theory is part of McNamara and Vafa’s landscape, there must always be a way to get to and from nothing. Nothing ľ±˛ő˛Ô’t an island cut off from the rest of physics. It is right there, waiting for us to find it.
Feared by some and worshipped by others, we now know that nothing is inevitable. It has haunted history for millennia, contaminated your bank account and one day, it’ll eat the universe for breakfast.
Should we be worried?
Or is it just nothing?
Spend a weekend with some of the brightest minds in science, as you explore the mysteries of the universe in an exciting programme that includes an excursion to see the iconic Lovell Telescope.
Mysteries of the universe: Cheshire, England
