One of the nice things about being in the science writing business for a while is that, even when the science of a given topic doesn't make a whole lot of sense, it at least starts to look familiar. One of these topics is the arrow of time, which Sean Carroll (Caltech physicist, not Sean Carroll, Madison biologist) discussed at AAAS. Briefly, time's a bit of an annoyance to physics. For relativity, time is just another dimension in space-time. But, as Carroll pointed out, while we often find we've made a wrong turn and wound up going right when we meant to go left, we never find that we wind up in yesterday.

The panel Carroll organized was about equally divided between physicists who are dealing with time, and people who are working on understanding how different aspects of biology may reflect an arrow of time.

There was also a philosopher of science to tie it all together. Huw Price of Sydney's Center for Time mentioned that the consensus of the modern physics community seems to be that an arrow of time is largely illusory. But Price noted that it was apparently a very persistent illusion, since it was already bothering Arthur Eddington, a physicist who was around for the birth of quantum mechanics. Eddington had apparently challenged his peers to actually act like it was an illusion, and start writing papers as if time were flowing backwards. The challenge was widely ignored.

Price suggested that there was evidence that physicists were starting to take Eddington's challenge bit more seriously, though. And the physicists in the session went on to prove him right. (We've covered an earlier attempt to deal with time's arrow, for example.)

Time in physics

Carroll set up the problem by noting that, based on thermodynamics alone, we shouldn't expect that the Universe should be anything more than a momentary dip from maximum entropy. Unfortunately, that means that anything outside of the room we're in, and all of past history, is likely to be unreliable, an illusion of our specific entropy dip. We avoid this thermodynamic awkwardness because the big bang provides us with a low entropy boundary state.

But that creates an issue of something like the anthropic issues associated with our Universe's particular physics—it seems to grant our particular direction of time a privileged position, as compared to an equally attractive alternative direction. Physicists, egalitarians that they are, tend to hate indications of privilege. Carroll, who is a theoretical cosmologist, has been looking for ways to remove our Universe from a privileged position.

The idea he's currently focusing on is whether a universe that looks much like ours eventually will, given a few hundred billion years—a high entropy state of diffuse cold matter—spawn child universes. In a universe-sized, high-entropy field, small individual regions will randomly fluctuate into lower entropy. Carroll thinks it might be possible for these to pinch off from the larger universe, at which point they act as their own, distinct universe. The temporal direction of these baby universes will be random, making our own Universe's arrow of time a matter of chance, rather than privilege.

(For those of you who remember our coverage of multiverses from the World Science Festival, we mentioned that Adrei Linde thinks of our Universe as a region in a larger fabric that's undergoing inflation at different rates. When I asked, Carroll said that this fabric is already low-entropy, so could be viewed as a process that's happening within his bubbles of mini-universe.)

Nobel Prize winner Anthony Legett was interested in whether we might already be looking at temporal violations in our own Universe. His talk was a bit hard to follow, but involved the sorts of experiments that we use to test Bell's inequalities, which are used to demonstrate how quantum entanglement seems to violate local realism (information can't be transmitted between isolated systems, at least not any more quickly than light can travel between them). Legett is intrigued by a possible alternative interpretation: local realism remains intact, but causality runs into problems.

In this interpretation, when we measure things on one entangled photon, the effect doesn't propagate instantaneously across space through some sort of spooky action at a distance. Instead, it propagates backwards in time, influencing events when the photons are first produced.

Both Carroll and Legett emphasized that they're a long way from figuring out how to identify tests of their ideas, but either of them would have profound implications for how we view time. And, in any case, Price (the philosopher) is happy that someone is at least taking up Eddington's challenge.

Time as an evolutionary phenomenon

If physics isn't sure what to make of the arrow of time, biology faces no such issues. Its foundational theory, evolution, seems to be a one-way street: once something useful evolves, like photosynthesis, there seems to be no going back without sterilizing the planet. Evolution has also produced brains that seem extremely adept at navigating a world in which time's arrow is real.

Apparently, at least one physicist, Michael Lässig, has taken note, and is looking into whether we can map evolutionary concepts onto thermodynamics. From what he can tell, some global measures of fitness always increase, much like entropy. But, unlike the thermodynamic landscape on which entropy increases, the fitness landscape is always shifting—Lässig suggests that we need to think of it as a seascape, rather than a landscape.

Using the classical population genetics equations, Lässig focused on deriving a figure that he called the fitness flux. It's not entirely clear what this is analogous to in the natural world, but, as it appears in the equations, it's squared, which means that the impact of fitness flux can never be negative. Lässig argues that this helps drive the system to ever increasing fitness. He also thinks that it can be measured at the genomic level, as a signal within the background of random base changes in non-conserved sequences.

Time on the brain

If the arrow of time is an illusion, you'd expect it to reside in the organ where we perceive it: the brain. And that's where Kathleen McDermott is looking. She performs functional MRI imaging, and looks at how the brain handles different points in time. As she noted, the brain is adept at both remembering detailed scenes of the past and envisioning equally detailed scenes in the hypothetical future. So, how do these processes differ in terms of brain activity?

On some levels, they simply don't. If you look at the areas that light up when the brain is recalling a past event, they all light up when a future one is imagined, to largely the same levels. (Imagining the future engages additional areas of the brain, but their levels of activity simply don't appear to be as high as the activity of the areas involved in memory.)

What's going on here? McDermott said that it appears that people construct future events using familiar people and items drawn out of their memory, and so the process engages the same brain structures that pull those items out when past events are remembered. To support this claim, she asked subjects to envision both familiar future events, like a New Year's Eve party, and then imagine themselves in an unfamiliar context—in this case, at a bullfight.

Envisioning the familiar scene engaged the same areas as memory. The overlap was much, much smaller when the bullfight was envisioned. So, at least in the brain, the more significant distinction seems to be one between familiar and unfamiliar, not future or past.

Not that this tells us anything about the physics, but it's still pretty interesting.