THERE is a reason we say time goes by: it seems to flow. No matter how still we stand in space, we move inexorably through time, dragged as if in a current. As we do, events steadily pass from the future, via the present, to the past.
Isaac Newton saw this as a fundamental truth. 鈥淎ll motions may be accelerated and retarded, but the flowing of absolute time is not liable to any change,鈥� he wrote.
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So how does time flow, and why always in the same direction? Many physicists will tell you that鈥檚 a silly question. 鈥淭he idea that time can in some meaningful sense be said to flow, it鈥檚 just a complete non-starter,鈥� says , a philosopher at the University of Cambridge.
For time to flow, it must do so at some speed. But speed is measured as a change over time. So how fast does time flow? , a cosmologist from the University of Cape Town, South Africa, has an answer: 鈥淥ne second per second.鈥� Price says that鈥檚 meaningless. Even if time were standing still, it could be said that for every second that passes, one second passes. Indeed, if that鈥檚 a measure of flow, we could say that space flows: it passes at one metre per metre.
Ellis is up against one of the most successful theories in physics: special relativity. It revealed that there鈥檚 no such thing as objective simultaneity. Although you might have seen three things happen in a particular order 鈥� A, then B, then C 鈥� someone moving at a different velocity could have seen it a different way 鈥� C, then B, then A. In other words, without simultaneity there is no way of specifying what things happened 鈥渘ow鈥�. And if not 鈥渘ow鈥�, what is moving through time?
Rescuing an objective 鈥渘ow鈥� is a daunting task. But of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, has given it a go by tweaking relativity. He argues that we can rewrite physics in a way that includes 鈥渘ow鈥� if we sacrifice some of our objective notions of space.
Most physicists aren鈥檛 having it. The general consensus is that time is more or less just like space 鈥� an immutable dimension, stretched out through a four-dimensional 鈥渂lock universe鈥�.
鈥淓very moment in that universe has a past, present and future,鈥� says Sean Carroll from the California Institute of Technology in Pasadena. 鈥淎 person is described as a history of moments, and those moments all have a feeling that they鈥檙e moving from the past to the future.鈥�
That doesn鈥檛 answer the question so much as shift it. If time does not flow, what makes us think it does?
Read more: 鈥�10 mysteries that physics can鈥檛 answer鈥� yet鈥�
This article appeared in print under the headline 鈥淲hy do we move forwards in time?鈥�
