I’ve long puzzled over the idea that physics is reversible. That its laws, with some caveats, work the same if time runs forwards or backwards. It’s even been suggested that, except for entropy, time could run backwards just as easily as forwards.
But this seems contrary to our everyday experience. With some exceptions, we can tell if a film or video clip is shown in reverse. Objects that fall, break, or grow (such as plants or crystals), look different seen in reverse.
I think there is more going on there than just entropy.
To begin with, and for the record, I see time as fundamental and axiomatic. Nothing comprises time; it just is. And it only moves forward; its “arrow” is built in. (Also, it’s not a spatial dimension. Even Einstein’s theories, which give us that concept of spacetime, do distinguish space from time.)
I’ve written plenty about this view of time and won’t explore it here. I’ll mention two points. Firstly, if the Big Bang was an event that happened, that seems to imply a before in addition to the obvious after. Secondly, every particle’s proper time always moves at one second per second; it’s impossible to directly (objectively) experience time slowing down or speeding up.
Reversing time might seem, as with space or charge, as simple as changing a plus sign to a minus sign. Yet there are physical properties with only positive (or zero) values. For instance, physicists imagine negative mass and negative energy, but the former is exotic (like unicorns) and the latter mostly an accounting device. And note that with mass and time (and energy), some and zero are significantly different states. Massless and timeless are special states.
Negative time would be like negative weight. Possibly useful for tracking debits, but not real. (Horses and forward time are real. Unicorns and reverse time are ideas.)
More importantly, I think reverse time raises some serious questions regarding causality and locality.
In its math and fundamental dynamics, physics does work the same regardless of the direction of time. Particle interactions, quantum and classical, do work the same way forwards and backwards.
And sideways. Feynman diagrams, which describe particle interactions, are valid descriptions of interactions in all four 90° rotations Which means they’re valid forwards and backwards (180° rotations).
Our experience of quantum wavefunction collapse adds a wrinkle. As far as we can tell, collapse is random and nonlocal (the latter arguably what Einstein actually meant by “spooky action at a distance”). But a basic tenet of physics says information is never lost, so, at least in principle, any interaction can be recovered, if not rewound.
[I think our faith in the conservation of information should be reexamined, but that’s another matter.]
In many regards it does seem that, at a small enough scale and except for wavefunction collapse, we can’t tell whether the movie runs forwards or backwards.
But at only slightly higher levels there are many things are immediately obvious when viewed in reverse. The higher the level, the more obvious it is. Contrast that with another form of reversal, reality in a mirror. Without text or other clues, mirror reality is largely indistinguishable (except for baseball games).
A common view is that entropy accounts for our experience of time’s apparent arrow. Some see it as responsible for it; the arrow emerges from entropy.
Rather than seeing entropy as time’s “ratchet” — a one-way mechanism — I see it as a consequence, an abstract measure of the dynamics of a closed system. It’s what happens with physics, particles, and time. It’s statistics. [I’ve written plenty about entropy, too, and won’t belabor it here.]
It seems to me there is something deeper, more fundamental. Time is only rational when it goes forwards.
Consider the difference between making a tree and burning (or otherwise destroying) it. The former inescapably takes a lot of time, but the latter is accomplished quickly. Likewise making glass versus breaking glass. Or eggshells. Or making gunpowder (or gasoline) versus burning it.
Some processes, chemical usually, require time but the reverse of that process may not. One can compress a spring slowly or quickly with the same result, but what process creates an instant tree? Or egg? (The ability to make gasoline as quickly as we burn it would certainly be a boon.)
A canonical example is the falling, breaking glass or egg. It’s claimed that the right application of forces — the exact reverse of forces expelled in the breaking — will reverse this, resulting in an intact glass or egg back on the table.
I don’t buy it. Not even in principle.
When a glass falls and shatters, the individual pieces go flying due to their individual momenta (their mass, directions, and speeds). As these pieces strike the floor, each other, and any objects on the floor, their momenta diffuse, eventually becoming zero relative to surrounding objects. The energy of the fall dissipates into the environment.
Supposedly, if the environment gave back that energy in just the right places at just the right times, the still pieces would move towards each other and reform the glass with the right momentum to leap back up onto the table where it lands perfectly upright.
The first objection: What combination of physical forces fuses glass pieces seamlessly into whole glass? The formation of glass requires a slow phase change from molten to solid. Pushing two solid pieces together isn’t that.
A second objection involves causality. A single cause pushed the glass off the table. That started a series of diverging events. The initial nudge may only have been enough to tip the glass over but falling converts potential energy into kinetic, providing the force to shatter the glass. The process of a large force dissipating is very different from a collection of tiny forces conspiring towards a specific result.
What causes those tiny pushes? How do they act in perfect concert towards a specific end result? Who or what conducts that symphony? There is also a question of locality. Widespread points acting in synch implies communication between those points.
In contrast, in the forward direction, there is no need or expectation of synchronization. Each moving piece is on its own interacting with the local environment, all driven by the single cause of the falling glass. The process after the cause is asynchronous. The reverse process is necessarily perfectly synchronized.
Let’s consider something that, at least on the large scale, we can deliberately reverse: a rocket taking off. In fact, reversing rocket takeoff is all the rage these days.
Because of its ballistic nature, as far as its flight profile is concerned, a rocket taking off looks the same as a rocket landing. Both go slow at the bottom and fast at the top, both have an orbit on one end of things and standing still on the ground at the other. And in both cases, a large amount of force from the business end is responsible for it all.
The normal version of a rocket landing involves firing the engine to generate the necessary force. As with takeoff, the force comes from the engine exhaust leaving the engine. Newton’s third law pushes the rocket in the other direction. On takeoff, that accelerates; on landing, it decelerates. In terms of forces, these look identical.
But in the time-reversed version, once again, causality is weirdly intentional and nonlocal. Somehow, certain cold particles drifting in a very large region of space, must begin moving towards a common location. Along the way they collide with other particles, gaining their momentum. These collectively get faster and hotter until they combine to a jet of fire where the forces align just right to slam it backwards into the oncoming rocket.
Inside the rocket engine, this hot exhaust undergoes a chemical reaction that robs it of its heat and converts it to very cold fuel which the rocket pumps suck into large tanks. This process slows the rocket, which has been in stable orbit, such that it descends and lands. It turns out the timing of this allows it to land in a desired spot (given some reverse aerodynamics to trim the flight).
Once it lands, the energy of its orbit is stored in the cryogenic fuel. Presumably it could be used to launch the same rocket back into the same orbit.
Which, in suggesting perpetual motion, shows something must be wrong with this picture (or with thermodynamics). Obviously, entropy has to play a role in preventing time from running both ways. But it’s not entirely clear to me it plays a role in which way that is. Issues of causality and locality seem more significant in disallowing reverse time.
A key difference between forward and reverse time, with regard to causality, involves light cones and the difference between our past and future ones.
We all have one light cone extending into our past and another extending into our future. The former contains all the events that could affect our present moment, the latter all the future events our present actions might affect (or effect). Events outside these two cones can neither affect us nor can we affect them.
They’re called “cones” but that’s due to the simplified views of spacetime (the 1D and 2D views). In reality we’re each at the center of two sets of nested expanding spheres. One set is the past, the other the future. The closer the sphere surface, the closer in time. (The scale is one nanosecond per foot.)
In terms of causality, our past light cone converges on us while our future light cone diverges from us as our effects dissipate into the environment. (“For want of a nail…”)
These are significant differences when it comes to time reversal. As illustrated with the breaking glass and rocket, reversing time requires turning divergence into convergence. But the causal rules of those aren’t the same. A backwards movie is obvious.
One could rightfully chalk this up to entropy. In both cases it’s the massive unlikeliness of moving from diffuse random states to highly organized states.
What I think is missing is the roles played by causality and locality. Given some random state, it’s highly unlikely to move spontaneously to any ordered state, but especially unlikely to move to a desired ordered state.
Why would a collection of widely dispersed cold molecules begin to move towards the specific end goal of coalescing into exactly the right sort of fireball to slow down a rocket — an event many days in their future?
Keep in mind that some fraction of the energy is in the form of photons that are a long distance away. Once in the atmosphere, sound waves are part of the picture as well. A lot has to come together to reverse divergence, and the necessary cause seems to be in the future.
So, bottom line:
Forward time; divergent; from one to many; from a known state to a random state. Calculation is serial and parallel. Each state begets the next based on local conditions.
Backward time: convergent; from many to one; from a random state to a known state. All calculation must be done in advance. Each part acts in synchronization. Each requires specific inputs at specific times.
Because I believe time is fundamental, there is no question about time’s arrow emerging. It’s axiomatic. And therefore, no question about reversing time. But beyond that, I think the issues with causality and locality, and that some chemical processes are time-dependent, suggest that reverse time is a unicorn.
Stay divergent, my friends! Go forth and spread beauty and light.