Time Travel. A History

To me, the most effective physicists are the ones who retain a degree of modesty about their program. Bohr said, “In our description of nature the purpose is not to disclose the real essence of the phenomena but only to track down, so far as it is possible, relations between the manifold aspects of our experience.” Feynman said, “I have approximate answers and possible beliefs and different degrees of certainty about different things, but I’m not absolutely sure of anything.” Physicists make mathematical models, which are generalizations and simplifications—by definition incomplete, stripped down from the cornucopia of reality. The models expose patterns in the messiness and capitalize upon them. The models themselves are timeless; they exist unchanging. A Cartesian graph plotting time and distance contains its own past and future. The Minkowskian spacetime picture is timeless. The wave function is timeless. These models are ideal, and they are frozen. We can comprehend them within our minds or our computers. The world, on the other hand, remains full of surprises.

William Faulkner said, “The aim of every artist is to arrest motion, which is life, by artificial means and hold it fixed.” Scientists do that, too, and sometimes they forget they are using artificial means. You can say Einstein discovered that the universe is a four-dimensional space-time continuum. But it’s better to say, more modestly, Einstein discovered that we can describe the universe as a four-dimensional space-time continuum and that such a model enables physicists to calculate almost everything, with astounding exactitude, in certain limited domains. Call it spacetime for the convenience of reasoning. Add spacetime to the arsenal of metaphors.

You can say the equations of physics make no distinction between past and future, between forward and backward in time. But if you do, you are averting your gaze from the phenomena dearest to our hearts.^*11 You leave for another day or another department the puzzles of evolution, memory, consciousness, life itself. Elementary processes may be reversible; complex processes are not. In the world of things, time’s arrow is always flying.

One twenty-first-century theorist who began to challenge the mainstream block-universe view was Lee Smolin, born in New York in 1955, an expert on quantum gravity and a founder of the Perimeter Institute for Theoretical Physics in Canada. For much of his career Smolin held conventional views of time (for a physicist) before, as he saw it, recanting. “I no longer believe that time is unreal,” he declared in 2013. “In fact I have swung to the opposite view: Not only is time real, but nothing we know or experience gets closer to the heart of nature than the reality of time.” The rejection of time is itself a conceit. It is a trick that physicists have played on themselves.

“The fact that it is always some moment in our perception, and that we experience that moment as one of a flow of moments, is not an illusion,” Smolin wrote. Timelessness, eternity, the four-dimensional spacetime loaf—these are the illusions. Timeless laws of nature are like perfect equilateral triangles. They exist, undeniably, but only in our minds.

Everything we experience, every thought, impression, intention, is part of a moment. The world is presented to us as a series of moments. We have no choice about this. No choice about which moment we inhabit now, no choice about whether to go forward or back in time. No choice to jump ahead. No choice about the rate of flow of the moments. In this way, time is completely unlike space. One might object by saying that all events also take place in a particular location. But we have a choice about where we move in space. This is not a small distinction; it shapes the whole of our experience.

Determinists, of course, believe that the choice is an illusion. Smolin was willing to treat the persistence of the illusion as a piece of evidence, not to be dismissed glibly, requiring explanation.

For Smolin, the key to salvaging time turns out to be rethinking the very idea of space. Where does that come from? In a universe empty of matter, would space exist? He argues that time is a fundamental property of nature but space is an emergent property. In other words, it is the same kind of abstraction as “temperature”: apparent, measurable, but actually a consequence of something deeper and invisible. In the case of temperature, the foundation is the microscopic motion of ensembles of molecules. What we feel as temperature is an average of the energy of these moving molecules. So it is with space: “Space, at the quantum-mechanical level, is not fundamental at all but emergent from a deeper order.” (He likewise believes that quantum mechanics itself, with all its puzzles and paradoxes—“cats that are both alive and dead, an infinitude of simultaneously existing universes”—will turn out to be an approximation of a deeper theory.)