The flow of time in one direction is so intuitive, so immutable, that we take it utterly for granted. Things don’t fall up; broken pieces don’t reassemble; humans get older, so it goes. Yet time remains a mystery. Physics equations do not seem to have any preference for time directionality — like palindromes, they work equally well in either direction. Indeed, physical processes at the microscopic level are thought to actually be “time symmetric,” and no physical law holds that time can’t flow in the opposite direction. So — maybe it is?

Ludwig Boltzmann was the first person on record to come up with a fairly solid reason for time having a directionality at the macroscopic level. In so doing, the Vienna physicist and philosopher capped some of the greatest minds of the 19th century. He built on the ideas of Nicolas Léonard Sadi Carnot, the French engineer, whose work in heat transfer was initially describing the behavior of steam engines.

Carnot was in the French military under Napoleon when they lost to the British. After this, there was a bit of a rivalry between the two nations, not the only time this has happened. Specifically, the French were upset that the British were so far ahead of them in steam engine technology, thanks to folks like James Watt in the previous century. So Carnot jumped into the race and described a theoretical engine. Carnot’s engine was a perfectly efficient engine that, of course, does not exist, but is very useful for thinking about these concepts.

What Carnot realized is that a perfectly efficient engine is reversible. As long as you don’t lose any energy to heat, you can run it forward or backward as much as you like without any loss. But as soon as the engine is not perfectly efficient, if it even loses a tiny little bit of heat, then you can’t reverse that process anymore. You’ve lost some of the energy forever as heat. This is sort of like saying the best you can ever do is an imaginary unicorn engine that has zero sum entropy, but you can never get negative entropy. And in most real life cases, you’ll only ever get positive entropy (although this word did not exist at the time).

Carnot’s ideas were later codified and made to apply to nature at large by Rudolf Clausius, the German physicist who fathered the concept of entropy and, with it, fundamentals of thermodynamics. Neither of these men was able to explain time with this concept, which ultimately became known as the Second Law of Thermodynamics. But Boltzmann, working later in the century, had an advantage over them. Namely, he believed in atoms.


Atomic theory was not widely subscribed to in Boltzmann’s day. Chemists preferred the theory because it made calculations easier, but it didn’t seem to have as much support in other disciplines. By using atomic theory, though, the laws of physics effortlessly describe our world, because they don’t need to be postulated, they can simply be derived. (Heat is simply the movement of atoms, for example.) Though he struggled intensely to prove his ideas at the time, Boltzmann ultimately showed that entropy is a measure of the number of ways in which the atoms that compose an object can interact — a thing’s “disorder,” as we casually approximate it. More importantly, he showed that entropy has directionality, unlike other things in the universe. He wrote:

“The general struggle for existence of animate beings is not a struggle for raw materials, these for organisms are air water & soil, all abundantly available, nor for energy which exists in plenty in the sun and any hot body in the form of heat, but rather a struggle for entropy, which becomes available through the transition of energy from the hot sun to the cold earth.”

This directionality is such that entropy began as a low value at the beginning of the universe (for reasons unknown), and is continually increasing as the universe ages. The reason for this, apparently, is because there are just so many more ways for atoms to interact as they spread out. It’s true that any physical process can (and probably does) happen in either direction at the microscopic level, but because there are simply so many more options for atomic interactions (especially as they spread out and become less ordered), it is far, far more probable that things will become less ordered. Basically, as each atom breaks its ordered interaction, it has potentially hundreds of other interactions to choose from, and those choices only multiply as other atoms break their interactions. The chances of an atom going right back where it was are very, very low. Entropy, therefore, moves from low to high as the universe ages.

Unlike other physical processes, this means that entropy has a specific direction to it. And that, says Boltzmann, is where the arrow of time comes from. Because things tend to move in one direction and not the other (even though they’re able to move in both), our universe experiences directionality. The very fact that a shattered glass has only an infinitesimally small probability to reassemble means that, at the macroscopic level, there’s not symmetry. Time is the difference between one state and another in our universe. So the reason you can decide what to have for dinner tonight and not yesterday is that entropy has revised the state of the universe in between. The resources that existed yesterday don’t exist anymore, and have given rise to today. It’s sort of like saying that each moment is the decay product of the previous one. So in a weird way, probability gives rise to time.

Enumeration? Or countdown?

But now consider life. A good deal of what defines life is its tendency to resist entropy. So while everything in the universe is moving from lower to higher entropy, we living things do the opposite: We resist equilibrium and build complexity — which, in the simplest physical sense, is about forcing molecules to interact in fewer ways. Note that this does not mean that we violate the Second Law: as long as other elements in the system offset the localized reductions, then the whole system still tends toward higher entropy. Every day that our cells and organelles build and repair themselves, we’re defying the primary feature of physical matter that gives us any reference point for time’s forward march.

So if Boltzmann was right, and time exists because of this entropic change, then perhaps our perception of the passage of time is the opposite of everything else in the universe. The time when you will die is in “the future,” but your molecules will scarcely notice. They’ll be off bonding with oxygen.

Still, time is relative. We’ve agreed that our direction of movement through it is “forward,” but this is, let’s face it, arbitrary. Relative to anything else in the universe, perhaps, this might be backwards. So the “beginning of the universe” might, in fact, be the end of it. Meaning that entropy started high, and is getting smaller. Meaning you might go cross-eyed putting it all together, but that everything still works, just exactly backwards from what we think is going on. It’s difficult to imagine what this might even look like.

What if, then, everything were flipped and reversed? When we build a watch to monitor the passage of time that counts up the seconds, the rest of the universe might see a countdown. What looks like a sunrise in the east and a sunset in the west would be a reversal of Earth’s spin with respect to the perspective of other celestial bodies. And the Infinity War we’re racing towards has already happened. We’re like little blips in the time continuum that reverse all values, and only perceive time moving forward because, eons ago, biology hacked physics.

Ultimately my suggestion here could come down to semantics. If time really is the difference from one state to another, then “time” as we know it is just our experience of the change, however that change occurs. Assigning it a directionality is a bit arbitrary. And furthermore, even though we might be the manifestation of a reversal in entropy, there’s no obvious reason to believe that fact would influence our perception of the directionality of other changes. It should be possible to be made out of the opposite stuff of everything else — that is, to be alive — and still notice the forward direction in the way things change.

The concept of the “illusion of time” is not really relevant to physics, since it’s clear that time solidly exists in the equations. Math is not ambiguous about this. The perception of time, however, is more in the domain of neuroscience (though effectively impossible to test, rendering this nothing more than a fancy thought experiment for the time being). In much the same way that your brain ignores the fact that you can always see your nose, there may also be some sort of processing that filters our perception of time.

In any case, the idea that our feeling and experiences of “things changing” is backwards from what any inanimate object might experience, if it could experience anything, is an intriguing one, if only a thought experiment. It starts to pull at the very concept of time, which might simply be a fancy word for “stuff happening.” So long as anything around you is occurring, at least, you can be assured you’re arriving in the future, in whichever direction you find it.