Once upon a time, a man took a fast rocket to a faraway planet. He soon missed his home world and took a fast rocket back. His twin sister, a physicist, was heartbroken, saying that they were no longer twins and that her sibling was now younger than she due to the phenomenon of time dilation.

But her brother, who was a philosopher, said that they had experienced time equally and so were truthfully the same age. And verily, physicists and philosophers have quarrelled ever since – physicists speaking of clocks and philosophers of time.

This scenario illustrates a famously counterintuitive implication of the special theory of relativity known as the “twin paradox”. It’s a puzzle that two physicists (Adam Frank and Marcello Gleiser) and a philosopher (Evan Thompson) have now taken up in a new book called The Blind Spot. The book shows how bound up philosophy and physics are and how its practitioners can so easily misunderstand each other.

Got time?

Albert Einstein implicitly proposed time dilation in his famous 1905 paper “On the electrodynamics of moving bodies” (Ann. Phys. 17 891), which inaugurated the special theory of relativity. If two identical clocks are synchronized and one then travels at a speed relative to the other and back, the theory implied, then when the clocks are compared one would see a difference in the time registered by the two. The clock that had travelled and returned would have run slower and therefore be “younger”.

For humans to experience the world, time cannot be made of abstract instants stuck together

At around the same time that Einstein was putting together the theory of relativity, the French philosopher Henri Bergson (1859–1941) was working out a theory of time. In Time and Free Will, his doctoral thesis published in 1889, Bergson argued that time, considered most fundamentally, does not consist of dimensionless and identical instants.

For humans to experience the world, time cannot be made of abstract instants stuck together. Humans live in a temporal flow that Bergson called “duration”, and only duration makes it possible to conceive and measure a “clock-time” consisting of instants. Duration itself cannot be measured; any measurement presupposes duration.

These two accounts of time provided the perfect opportunity to display the relation of physics and philosophy. On the one hand was Einstein’s special theory of relativity, which relates measured times of objects moving with respect to each other; on the other was Bergson’s account of the dependence of measured times on duration.

Unfortunately, as the authors of The Blind Spot describe, the opportunity was squandered by off-hand comments during an impromptu exchange between Einstein and Bergson. The much-written-about encounter, which took place in Paris in 1922, saw Einstein speaking to the Paris Philosophical Society, with Bergson in the audience.

Coaxed into speaking at a slow spot in the meeting, Bergson mentioned some ideas from his upcoming book Duration and Simultaneity. While relativity may be complete as a mathematical theory, he said, it depends on duration, or the experience of time itself, which escapes measurement and indeed makes “clock-time” possible.

Einstein was dismissive, calling Bergson’s notion “psychological”. To Einstein, duration is an emotion, the response of a human being to a situation rather than part and parcel of what it means to experience a situation.

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Mutual understanding was still possible, had Einstein and Bergson pursued the issue with rigorous and open minds. But the occasion came to an unnecessary standstill when Bergson slipped up in remarks about the twin paradox.

Bergson argued that duration underlies the experience of each twin and neither would experience any dilation of it; neither would experience time as “slowing down” or “speeding up”. This much was true. But Bergson went on to say that duration was therefore a continuum, and any intervals of time in it are abstractions made possible by duration.

Bergson thought that duration is single. Moreover, the reference frames of the twins are symmetric, for the twins are in reference frames moving with respect to each other, not with respect to an absolute frame or universal time. An age difference between the twins, Bergson thought, is purely mathematical and only on their clocks; it might show up when the twins are theorizing, but not in real life.

This was a mistake; Einstein’s theory does indeed entail that the twins have aged differently. One twin has switched directions, jumping from a frame moving away to one in the reverse direction. Frame-switching requires acceleration, and the twin who has undergone it has broken the symmetry. Einstein and other physicists, noting Bergson’s misunderstanding of relativity, then felt legitimated to dismiss Bergson’s idea of duration and of how measurement depended on it.

The Blind Spot uses philosophical arguments to show how specific paradoxes and problems arise in science when the role of experience is overlooked

Many philosophers, from Immanuel Kant to Alfred North Whitehead, have demonstrated that scientific activity arises from and depends on something like duration. What is innovative about The Blind Spot is that it uses such philosophical arguments to show how specific paradoxes and problems arise in science when the role of experience is overlooked.

“We must live the world before we conceptualize it,” the authors say. Their book title invokes an analogy with the optic nerve, which makes seeing possible only by creating a blind spot in the visual field. Similarly, the authors write, aspects of experience such as duration make things like measurement possible only by being invisible, even to scientific data-taking and theorizing. Duration cannot itself be measured and precedes being able to practise science – yet it is fundamental to science.

The critical point

The Blind Spot does not eliminate what’s enigmatic about the twin paradox but shows more clearly what that enigma is. An everyday assumption about time is that it’s Newtonian: time is universal and can be measured as flowing everywhere the same. Bergson found that this is wrong, for duration allows humans to interact with the world before they can measure time and develop theories about it. But it turns out that there is no one duration, and relativity theory captures the structure of the relations between durations.

The two siblings may be very different, but with help they can understand each other.