Remembering the future
Struck by time’s arrow
Trevor Bekolay, Volunteer Staff
On Feb. 28, 2008, the University of Manitoba hosted a talk by Nobel laureate Sir Anthony J. Leggett entitled “Why can’t time run backwards?” as part of the Robert and Elizabeth Knight distinguished visiting lecturer program. In true scientific fashion, Leggett’s talk about how theoretical physics views time’s progression raised more questions than answers. I have been pondering one of those questions since attending the talk last month.
In short, the question is the same one posed in the lecture’s title: can time run backwards? Though I am no physicist, I am a well-versed science-fiction nerd. I’ve been exposed to a myriad of made-up ways of travelling back in time, and, to date, none have struck me as even remotely possible. My understanding of physics (Einstein’s theory of relativity, to be specific) is that, to go back in time, one must go faster than the speed of light. While we thought that was an impossible feat for a long time, some have since posited methods of surpassing the speed of light. Unfortunately, those methods work in theory only — in practice, they require exorbitant amounts of energy or hypothetical particles that exist only in mathematical models.
So when Leggett introduced a concept that could, in my mind, make backwards time travel possible, I was understandably shocked. Before the physics department sicks their wild dogs on me, no, it does not encroach upon Einstein’s theory of relativity. It all has to do with the arrow of time.
While it sounds like a trite poetic device, the arrow of time has a strict definition in the natural sciences: it is the direction in which time progresses on a four-dimensional model of the universe. The concept sounds abstract, yet we are all able to identify the arrow of time.
Imagine a single drop of water falling into a calm, serene pond. The drop hits the water with a little splash, then waves ripple out from that point. Now, imagine that same scene, but played in reverse. First the waves ripple towards a centre point. There is an awkward splash, and then the drop lifts up into the air. It looks unnatural. In most situations, we are able to determine if a video is being played forwards or backwards; that is, we can clearly identify if a scene follows the arrow of time.
In addition, most would agree that we can remember things that have already happened and that we cannot remember events that have not happened. Our innate perception of linear time is known as the psychological arrow of time.
In his talk, Leggett discussed four other arrows of time.
Most physicists are familiar with the thermodynamic arrow of time. The second law of thermodynamics states that in an isolated system entropy will only increase with time. Entropy can be thought of as a measure of disorder. While entropy is said to increase with time, note that systems can fluctuate and that it is possible to have entropy decrease; however, on average, over any nontrivial amount of time, entropy will increase.
A good way to visualize the thermodynamic arrow of time is to think about a deck of cards. Let’s say that it’s a new deck, so the cards are all in ascending order and grouped by suit. This would be a closed system with a very low amount of entropy because the cards are in a specific order. Now, start playing a game with those cards. As you deal cards randomly and shuffle between turns, it is clear that the deck of cards is going to become unordered. Sometimes you will get lucky and the cards will occur in a favourable order. This corresponds to the fluctuations in entropy: there are times when the cards will be more ordered than others, yet, as you play and shuffle the cards, you can be pretty sure that the deck will be much more unordered (have more entropy) than when you began with the ordered deck.
Third, we have the biological arrow of time. Simply put, this refers to evolution: the process in which, over long periods of time, organisms evolve from simpler forms to more complex ones. At first, it seems like this phenomenon contradicts the thermodynamic arrow of time in that complicated organisms would exhibit less entropy than simple ones.
Literature on the arrow of time discusses this paradox without any clear conclusions. In my opinion — and this is only my line of reasoning, not the scientific consensus — the biological arrow of time is not a paradox because natural selection exploits fluctuations of entropy. If I may continue the contrived example of decks of cards, we can visualize reproduction as taking a whole bunch of decks of cards and shuffling them. Clearly, over the whole group, entropy will increase; any decks that had order before are likely to have that order destroyed. However, due to fluctuations in entropy, some decks of cards will have randomly assembled into a favourable order. Natural selection dictates that those well-ordered decks of cards have a better chance of persisting and passing on their ordering to further generations. Whether you buy that explanation or simply accept a paradox between evolution and the second law of thermodynamics, the biological arrow of time undoubtedly exists and is observable.
The fourth arrow of time is the electromagnetic arrow of time. It is likely the hardest arrow to describe simply, so I apologize in advance to physicists I offend in my oversimplification. We see the electromagnetic arrow of time through the prevalence of retarded waves over advanced waves.
Let’s think about an antenna that is transmitting radio waves. Typically, we expect that radio signals we transmit will emanate outwards to the future. This is the behaviour associated with retarded waves. If the antenna was transmitting advanced waves, on the other hand, then we could transmit a signal to the future and to the past. As it turns out, the advanced waves that are travelling to the past get absorbed by matter and turned into heat, leaving only the future-travelling part that follows the same laws as retarded waves. The absorption I just described is also a part of the thermodynamic arrow of time, so it is commonly said that the electromagnetic arrow of time derives from the thermodynamic arrow time.
Finally, we have the fifth arrow of time — the one that got me thinking about time travel. It is the cosmological arrow of time, which points in the direction of the universe’s expansion. As you may already know, the universe is expanding. It started off in a small, hot, dense state. Then there was a big bang and the universe started expanding. As time as we know it moves forward, the universe continues expanding.
It is this fifth arrow that brings up the issue of time travel. It seems quite reasonable to assume that the other four arrows will hold forever. There’s no reason to believe that we will start to remember future events. The expansion of the universe, however, is not quite as certain.
There are three main hypotheses on how the universe will progress. The first is that our universe is open; that is, it will continue expanding indefinitely. The second is a flat universe, in which the universe will expand to a certain size and then stay relatively static. The third is a closed universe, in which the universe expands to a certain point, then begins shrinking, eventually collapsing in on itself.
So far, we have discussed the five arrows of time independently. And this makes sense; all of these arrows come from different observations. However, as we progress in science and attempt to move towards unified theories that can encompass ideas in many different fields, it seems natural to consider what would happen if all five arrows were connected. What if the reason we remember past events and can discern when movies are being played backwards is actually related to entropy and the expansion of the universe? Then, the question of what model our universe actually follows becomes far more important. If our universe is flat, then will time as we perceive it simply stop? If our universe is closed, will we eventually play out our lives in reverse? Or is the truth even more complicated?
In 1998, scientists studying the violent explosions of white dwarf stars and the variable stars created unexpectedly observed that the expansion of the universe is accelerating. While that doesn’t guarantee that our universe is open, it means that it is probably not an immediately pressing concern to us. So rest easy for now. But if we do end up repeating this life in reverse, then I would like this article to serve as me saying “I told you so” to the world. Of course, since I don’t recall being congratulated for this article in my past, my guess is that things didn’t quite work out that way. I suppose only time will tell.
