Reversed time

Sanjay Manohar

A critical leap forward in understanding physics was statistical mechanics. The tools allowed us to understand how an arrow of time formed from equations which were symmetrical, backwards and forwards. Statistical mechanics introduced the concept of macroscopic states, which describe a system in terms that are less specific than the precise position and momentum of each particle. Macroscopic states might specify the numbers of particles with each position and momentum, but not each particle's exact state. In particular, given this kind of uncertainty in a system's state, we can show that certain future states are more or less likely than others. This allows us to paint a macroscopic picture in which, as time progresses further away from a specified state, particular trends emerge. The most general form of this is the second law of thermodynamics.

There is, however, a different way in which the symmetry is not broken by physics. If time were indeed truly reversed in direction, then the second law of thermodynamics would apply in reverse too. That is, were everything to be put into reverse, we would find the same tendency of things to change into more 'likely' macroscopic states. However we would need to redefine the meaning of likely.

Let us consider a few examples of how things would appear if we, our brains, were kept isolated to observe the universe, while time was put into reverse. The first and most obvious thing to note is that all physical laws would be reversed, for example gravity would be a repulsive force. Stars would appear to suck in photons from all directions, accumulating their energy by splitting helium into hydrogen. The solar system might appear to gradually dissolve into an accretion disc, and stars would be seen to evaporate into primordial dust clouds. The universe might appear to inexorably shrink under its own attraction until it becomes a hot singularity, at the big bang. I will use our conventional time coordinate t to represent time as we know it, with t=0 being the present moment, and a very negative t for the big bang. I will also introduce a convention where green text uses terminology as seen with our current direction of time, and red for terms associated with the world as seen in reverse time. I will always assume that in both red and green worlds, the basic physical states are the same; that is, x(t) and p(t) for every particle is fixed. In quantum mechanical terms, the wave function phi(t) is the same in both worlds. The only difference, in fact, is that the apparent "direction of change" of t is increasing or decreasing. The basic transform is that what we call the future becomes the past, and vice versa.

What does this world look like?

The air is our friend. By a constant bombarding motion, air molecules blow a wind to all objects, setting them in motion. Similarly, surfaces are sticky, and once an object is resting on a surface, it will no longer be thrust upwards by gravity. Surfaces on which objects rest have inner motions which are often imparted to the objects, propelling them along surfaces, or even up into the air. One of the most "directional" phenomena in the world must be the brain. But before addressing this, we must note that all biological processes also appear quite different in reversed time. Let us start in reverse, and consider the process of decay. When an organism decays, its intricate cellular meshes and delicate pipework collapse upon themselves; disorder appears from order. But in the reversed world, we notice a dark homogeneous mush of soil gradually being assembled into an organism. From mud, an infinitude of microbial colonies labour for hours to assemble the veils of flesh until a body emerges. In the case of humans, the heart begins to beat, after a few irregular hiccups, sucking blood through the brain, from the vena cave, and the human mind lights up in electrical glow. A similar but more dramatic process arises from ashes, as the flames draw carbon and dust from the air, from all directions and from the soil, like a great siphon. In the heat of the flames, intense but meticulous forces weld the molecules into organic form, spewing out oxygen fumes, until the body cools and crystallises into its mature form.

This miraculous but natural process is how we are born. From these grand beginnings, our journey carries us through middle age and youth, until we shrivel and shrink. No matter how we began, we will always end up the same, being pulled up into the womb where we will decay, and our cells are gradually resorbed into the body of our successors, our mother. In this way, we do not completely die; our bodily structures are used to seed the next generation - both father and mother will receive our contribution once we have dissolved into mere cells.

Our diet is no less alien. Through the work of a myriad of bacteria, soil is transformed into a carefully prepared aromatic brown paste. Once it is rolled into small packets by the action of water and gravity, we ingesting it through the anus. By further work of gut flora and digestive massage, we synthesise plant matter, leaves and shoots, fruits and roots. Placing them carefully back upon stalks, we feed those plants who vitally purify our air, removing toxic oxygen and replacing it with the carbon dioxide we breathe.

This is nature, operating exactly as it does, but just viewed as a different observer. This perverse view raises the natural question, why do we always perceive the world to be going in the green direction? I hope it is now clear that the second law of thermodynamics cannot help us in this situation, since it is itself reversed.

The answer I would like to put forward is that it's entirely consistent to view the world from the red viewpoint, but there are critical ways in which the mind, as it apprehends the world, will always "see" the green view only. This will arise, I conjectuWre, from properties of how and why mental representations arise.

Thoughts in reversed time

What would thought be like in the red direction? Intuitively you might retort that knowledge and thought would be impossible. Surely information "flows" out of the organism? How can we learn from the past? How can any progress be made? It should surely not be possible to represent the world at all, for red.

But what role do memories play? Before acting I have memories of the act. These memories become more and more vivid as the time of the act draws near. The reminiscences become recollections, which become immediate working memories, just before the act occurs. The causal relation to the act puts them in a rather familiar relation: they function as intentions -- which begin as vague desires, before crystallising into plans, then detailed intentional structures.

Similarly our intentions would appear as memories, with an immediate intent appearing directly after an act, with this intent decaying over time into ill-defined wants and hopes, until events are fully forgotten. Where might freedom of the will fit in the red frame of reference? It appears that the gradual dissolution of memories over time would correspond to the building of goals and intentions leading to an act.

Notice that the way we influence the world is radically different. When I want to communicate, sound waves can be emitted from my ears. These tiny waves are amplified with sympathetic vibrations from all over the room I am in--emerging from the natural vibrations of the walls and ceiling--to produce sound. I can be heard by the high-acuity detectors in peoples throats. Once the sound is funnelled in through the mouth, it causes oscillations in the vocal cords which are transduced into electricity by muscles.

Now, we must consider red epistemology!