Grace Weir
Published for Time tries all things, Institute of Physics, London 2019


David Berman

Copernicus had it right. He took humanity from the centre of the universe. All our senses and all our experiences comes from one perspective, that of ourselves. Our intuition, our “common sense” comes from that single point of view. As we developed theories of Nature and how the world works, this bias remained and became culturally ingrained. From the Ancient Greeks through to the Renaissance, the Earth was the centre of Nature and the universe and everything implicitly converged around the human view of the world. Copernicus broke this homocentric view. For the first time we were removed from the centre and placed nowhere special, rotating around a giant ball of fire. Today we take for granted that we are not at the centre of the solar system, never mind the universe; but to the medieval mind it was shocking. How strange to think of the world in constant motion through the solar system when our experience every day of our lives seems to contradict this.

Progress in physics has come by replaying Copernicus again and again, by challenging our common sense perspectives and by removing our human viewpoint further and further from our description of Nature.

When we look at the two great pillars of scientific achievement of the 20
th century, quantum mechanics and relativity, they have dismantled all the notions we have about how Nature works. So called “classical” physics remained quite human. When we learn about (or indeed teach) Newton’s laws, the examples appeal to every-day experiences such as spinning ice skaters, billiard balls and rolling wheels. When we look in the quantum world of the very small we can have no experience or intuition of that world except through elaborate experiments that bring out the randomness, the nonlocality and the pure “strangeness” of the quantum realm.

The same is true of relativity. Einstein’s great insight was to unify space and time into one thing, a “spacetime”. The split between what was space and what was time is then down to an individual’s orientation in this spacetime. Remarkably, this orientation is determined by the speed at which we travel. In other words, relativity showed that what we think of as the time part in spacetime depends on how fast we move. Since we can all move at different speeds, there isn’t a single universal notion of time, no universal future or past.

When one first hears this description of relativity it’s tempting to think of Plato’s cave. In the Republic, Plato describes a rather unfortunate set of people, imprisoned in a cave from birth and tied such that they are permanently facing the interior cave wall. They perceive the world only through shadows on the wall. A two-dimensional world projected from the three-dimensional reality. The unfortunate prisoners cannot change their perspective and they cannot imagine beyond the two-dimensional shadows. We are Plato’s prisoners. Unable to change our orientation in spacetime, though this time our bonds come from the energy costs of moving at speeds near that of light. Thus just like the prisoners we do not see Nature as it is but as a projection with a single fixed perspective. Plato’s description was a metaphor for how philosophy can free the mind, and he imagined that he could liberate the prisoners and provide them with new perspectives of the world. Physics and relativity in particular do this in a less metaphorical sense. We live in four dimensions, not the three that we see as space. Although I cannot move to change my perspective in spacetime, we have the mathematical description of the geometry of the four dimensions that allows us to imagine spacetime from different perspectives. That is the theory of relativity. Its magic is to liberate us from the shadows.

Physics still proceeds by challenging our everyday assumptions and then seeing the consequences. String theory takes this further, by reimagining the most basic building blocks of the universe to have extension, like loops of string. The consequences of this though are still not fully understood even after 40 years or so. One thing that we do know is that string theory provides us with a new minimum distance in space and time, which is given by the string length. At very short distances Einstein’s old idea of spacetime often modelled by a smooth rubber sheet bending and stretching becomes something entirely different. It’s something we can’t visualise but we rely on mathematics to describe it.

As a somewhat amazing corollary string theory, introduces the idea of a “duality” in spacetime. This is the idea that apparently different spacetimes are secretly equivalent. It is as if we have two very different pictures that we have just now discovered are simply complementary views of the same object. The traditional Einsteinian view of space and time would mark these “dual” spaces as being distinct but in the world of the string they are one and the same. That means Einstein’s language can’t be the right one to describe spacetime in string theory. In reality, rather than having an equivalence between two spacetimes we should think of there being a single object behind it all. That is an object behind space and time that we perceive from different Einsteinian perspectives and that these different perspectives are the “dual” descriptions nature.

We still do not know what that object really is and this is the topic of my research, to provide a better description of spacetime that takes into account all of the complementary views of space and time and produces a single object unifying these dual descriptions into one thing. Then we don’t see the shadows but what lies behind.

And so our notion of time and space seems to be more and more distant from our human experience of it. But to be human is not to be limited by “common sense”, but to embrace different perspectives of the universe and appreciate the gift of being able to see and, through our imaginations, experience the world in a new way.