# What Does “Happy New Year” Even Really Mean?

When Albert Einstein’s good friend Michele Besso died in 1955, just a few weeks before Einstein’s own death, Einstein wrote a letter to Besso’s family in which he put forward a scientist’s consolation: “This is not important. For us who are convinced physicists, the distinction between past, present, and future is only an illusion, however persistent.”

The idea that time is an illusion is an old one, predating any Times Square ball drop or champagne celebrations. It reaches back to the days of Heraclitus and Parmenides, pre-Socratic thinkers who are staples of introductory philosophy courses. Heraclitus argued that the primary feature of the universe is that it is always changing. Parmenides, foreshadowing Einstein, countered by suggesting that there was no such thing as change. Put into modern language, Parmenides believed the universe is the set of all moments at once. The entire history of the universe simply is.

Today we would call this the “eternalist” or “block universe” view—thinking of space and time together as a single four-dimensional collection of events, rather than a three-dimensional world that evolves over time. Besides Parmenides and Einstein, this picture is shared by the Tralfamadorians, an alien race who appear in Kurt Vonnegut’s novel Slaughterhouse-Five. To a being from Tralfamadore, visiting the past is no harder than walking down the street.

This “timeless” view of the universe goes against our usual thinking. We perceive our lives as unfolding. But it has adherents even in contemporary physics. The laws of nature, as we currently understand them, treat all moments as equally real. No one is picked out as special; the laws simply say how any moment relates to the previous one and to the next.

Perhaps the most energetic and persistent advocate of the claim that time is illusory is the British physicist Julian Barbour.

Impressively, Barbour has managed to do interesting research in physics for decades now without any academic position, publishing dozens of papers in respected journals. He has supported himself in part by translating technical papers from Russian to English—in his spare time, tirelessly investigating the idea that time does not exist, constructing theoretical models of classical and quantum gravity in which time plays no fundamental role.

We have to be a little careful about what we mean by “time does not exist.” Even Parmenides or Barbour would acknowledge the existence of clocks, or of the concept of being late. At issue is whether each subsequent moment is brought into existence from the previous moment by the passage of time. Think of a movie, back in the days when most movies were projected from actual reels of film. You could watch the movie, see what happened and talk sensibly about how long the whole thing lasted. But you could also sneak into the projection room, assemble the reels of the film, and look at them all at once.

The anti-time perspective says that the best way to think about the universe is, similarly, as a collection of the frames.

There has, predictably, been some push back. Tim Maudlin, a philosopher, and Lee Smolin, a physicist, have argued vociferously that time is real, and that the passage of time plays what we might call a generative role: It indeed brings the future into existence. They think of time as an active player rather than a mere bookkeeping device.

Whereas traditional topology uses regions of space as fundamental building blocks, Maudlin takes worldlines (paths of particles through time) as the most basic object. From there, time evolution seems like a central feature of physics.

Both researchers have been developing new mathematical tools and physical models to buttress their views. Maudlin’s novel approach focuses on the topology of spacetime itself—how different points in the universe are sewn together.

Whereas traditional topology uses regions of space as fundamental building blocks, Maudlin takes worldlines (paths of particles through time) as the most basic object. From there, time evolution seems like a central feature of physics.

Smolin, in contrast, has suggested that the laws of physics themselves are evolving with time. We wouldn’t notice this from moment to moment, but over cosmological time scales, the parameters we think of as fixed may eventually take on very different values.

There is, perhaps, a judicious middle position between insisting on the centrality of time and denying its existence. Something can be real—actually existing, not merely illusory—and yet not be fundamental. Scientists used to think that heat, for example, was a fluid like substance, called “caloric,” that flowed from hot objects to colder ones.

These days we know better: Heat is simply the random motions of the atoms and molecules out of which objects are made. Heat is still real, but it’s been explained at a deeper level. It emerges out of a more comprehensive understanding.

Perhaps time is like that. Someday, when the ultimate laws of physics are in our grasp, we may discover that the notion of time isn’t actually essential. Time might instead emerge to play an important role in the macroscopic world of our experience, even if it is nowhere to be found in the final Theory of Everything.

In that case, I would have no trouble saying that time is “real.” I know what it means to grow older or to celebrate an anniversary whether or not time is “fundamental.” And either way, I can still wish people a Happy New Year in good conscience.

By Sean M. Carroll for Smithsonian Magazine

Sean M. Carroll is a research professor in physics at the California Institute of Technology. He is the author of From Eternity to Here, Spacetime and Geometry and The Particle at the End of the Universe, which won the Winton Prize from the Royal Society.

Credit: Smithsonian Magazine

# Study of masses interacting via gravity challenges the idea that special initial conditions are needed to give time a direction(PDF)

The fundamental laws of physics, we believe, do not depend on the direction of time. Why, then, is the future so different from the past? The origin of this “arrow of time” has puzzled physicists and philosophers for more than a century, and it remains one of the fundamental conceptual problems of modern physics [1]. Although a preferred direction of time can occur in models of physical systems, this typically happens only if one inserts very special initial conditions. Julian Barbour at the University of Oxford and his colleagues [2] have now shown this tinkering isn’t necessary to produce an arrow of time in a system of masses interacting via Newtonian gravity. They demonstrate that the evolution of this surprisingly simple system almost always contains a unique moment of lowest “complexity,” a point they identify as a “past” from which two distinct (and more complex) “futures” emerge.

In their gravitational model, Barbour and his colleagues find a state of “low complexity” that is analogous to Boltzmann’s low-entropy fluctuation. But in their case, no rare statistical fluctuation is necessary to explain this state; instead, it arises naturally out of simple physical laws that have no explicit dependence on the direction of time. The authors study one of the simplest possible systems: a collection of N point particles interacting through Newtonian gravity. Their only assumptions are that the total energy (potential plus kinetic) and the total angular momentum of the system are zero. From earlier numerical simulations and analytic analysis, it is known that in the distant future, such a system tends to break up into weakly interacting subsystems—typically, pairs of masses in Keplerian orbits [8]. Starting with such a dispersed system and running time backwards, one might expect that it would coalesce in the past into a state of high density. Barbour and his coauthors show analytically that this expectation is right: for almost every initial configuration of masses, there is a unique moment of minimum size and maximum uniformity. From this point, the system expands outward, approximately symmetrically in both directions of time (Fig. 1). The system is therefore globally symmetric in time, as the equations dictate, and yet has a local arrow of time.

As a key step in their argument, the authors analyze the evolution of the masses in “shape space,” a space of observables that describe the shape of the system but are independent of its size and orientation. Three bodies, for instance, form a triangle, and their shape space is the space of similar triangles. Shape space contains a natural dimensionless measure of complexity, denoted 𝒞S, which is determined by the moment of inertia and the total Newtonian gravitational potential. 𝒞S describes the degree of nonuniformity and clustering; it has a minimum at the moment of minimum size and grows approximately monotonically from that minimum in both directions of time. Barbour and his colleagues provide a fairly simple and intuitive explanation for this behavior by showing that the dynamics of the N-body system in shape space has an effective friction term, which provides a sort of dissipation even though the underlying equations of motion are symmetric in time. (Credit:

via physics.aps.org