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By now we’re used to the idea that the world has four dimensions: three spatial and one temporal. But what if there were a fifth dimension – what would that dimension look like, and how
would it relate to time? One of the central threads running through the philosophy of time concerns the idea that time flows. This doesn’t sound controversial: for most people, it flows in
much the same way as a river. But there are problems with this view. If time flows, it’s surely reasonable to wonder about the rate it flows at. But rates of flow are construed as ratios
over time – a river flows at one metre per unit of time, say – so it would seem time should also flow at some rate over time: one second per second. This doesn’t work, though: it’s like
saying that, for every dollar you give me, I give you a dollar back: what could be gained in such a transaction? We commonly think of time flowing into the future, away from the past, but
time wouldn’t “go anywhere” if it flowed at one second per second. WHAT TO DO? One option might be to construe the flow of time as a ratio of time over space, so that time flows, for
example, at one second per metre. But this too would be pretty odd: treating time as dependent on space in this fashion flies in the face of our intuitive understanding of its nature.
Another alternative might be to invoke a further temporal dimension – a fifth dimension – which can then be used as the yardstick for measuring temporal flow. Call the ordinary temporal
dimension “A” and this new temporal dimension “B”. In this view, time flows at one second of A per second of B. Thus, time now “goes somewhere” in that it charts a path through a higher
dimension. Appealing to a fifth dimension in this fashion is often seen as a strategy of last resort by philosophers of time: the idea is simply too wild to take seriously. THE LHC It has
been predicted that the Large Hadron Collider in Switzerland might generate particles that time-travel by taking shortcuts through a fifth dimension. SPATIAL TREATMENT Unfortunately,
according to the physicists responsible for the relevant experimental predictions, the fifth dimension is spatial, not temporal. Even if the fifth dimension _were_ temporal, there’d still be
a problem. This is because the experimental predictions conducted so far are produced against the backdrop of a particular physical theory: general relativity. General relativity undermines
any basis we might have for believing that time flows at all, as it’s portrayed as a space-like dimension. This means that, very roughly, we should think time flows only if it’s coherent to
think of space flowing, which seems implausible. ON THE PLUS SIDE … Perhaps we have reason to take heart. Although general relativity plays a role in relevant experimental predictions,
those predictions are actually coming from a particular theory of quantum gravity: string theory. This theory reconciles our best theories of the very big (general relativity) with the very
small (quantum theory). Some versions of string theory posit as many as 11 dimensions. With so many, surely it’ll be possible to make some sense of the idea that time flows … won’t it?