The Physics of the Present

Alejandra Miranda
4 min readJan 29, 2020


What happens when you see something? Say, for the sake of argument, this sentence you are now reading. Dismissing the whole business of how the brain processes visual stimuli, let’s set our focus on the travel time of the information you receive. And to simplify matters even further, let’s also ignore how atoms absorb and reemit light, taking only into account the classical propagation of photons (or the basic units of all light).

We can then agree that photons of light are bouncing around the room you’re currently at. Either because a window is open or a lamp is on (or maybe a combination of both). And in their bouncing they happen to hit things, such as the surface of the device you’re using right now to read.

Some of this prancing light then gets absorbed, while some is reflected outwardly in many directions. After all, electronic gadgets absorb and emit light in different ways that remain encoded in the reflected light. A fraction of this reflected light then travels from the device to your eyes, and thanks to the brain’s wondrous ability to decode sensory information, you are able to read these words.

All this seems to be immediate, right? You might even go so far as to assert, “I’m reading this sentence right now.’’

But, are you really?

Believe it or not, you’re not.

Allow me to explain.

Since light travels at a finite speed, it takes time for it to bounce from your gadget to your eyes. If you are looking at your device at a distance of 30 cm from your eyes, for example, the travel time of light from said gadget to your eyes is about one nanosecond (or one billionth of a second). As a result, when you see a word, you are seeing it as it appeared some time in the past. And this same process repeats itself for every object you observe and every person you talk to.

For instance, take a look at the objects around you. The light bouncing from each one of them takes a different time to get to your eyes. The brain then has the task of integrating these different sources of visual information. And it is so good at doing this that it fools you into thinking that you are looking at everything “at once”, in spite of the different distances said objects have from you.

In reality though, there is a contrast. Although one that we can’t distinguish, since the differences in light’s arrival time are much smaller than what our eyes can discern and our brains process. Thus, the present (understood as the sum total of the sensory input we say is happening “now”) is nothing but a mere illusion.

To illustrate this a bit better, here’s an experiment:

Have an observer watch two lights flashing at slightly different times. Setting up the time interval below 20 milliseconds or so would lead the observer to believe that both lights are flashing concurrently. However, raising that time interval would allow the observer to perceive the variations in the flashing. Hence, we could state that this timescale sets the limit of visual simultaneity for the observer

Which would then lead us to conclude that:

The present exists because our brain blurs reality.

Think about this way. A hypothetical brain endowed with ultra-fast visual perception would catch the difference between the two flashing lights much earlier. “Now” would turn into a much narrower experience for this brain, differing from the “now” we currently perceive. Consequently, besides Einstein’s relativity involving two or more observers, there is also a relativity of simultaneity at the cognitive level; one that results from the subjective perception of “now” for any given individual. In general terms, that is to say that:

Anything capable of detecting light has a relative present.

A statement that makes a lot of sense. Especially when we consider sentient matter, because it (and by extension us) is an island of perception. Just like the line where water and sky meet in every ocean (aka the horizon), the limit of how far we can see comprises all the phenomena that our brains compute as happening simultaneously, even when they are not. Ergo, our “sphere of the present” is established by the bounds of our perceptual horizons.

And if to this cognitive illusion we add the trickery behind the mathematics of time, it is easy to see how the present is nothing more than a useful hoax, sandwiched between past and future. Particularly because we represent the flow of time continuously, attaching a real number to every instant in order to track changes in time. But since said changes occur through cause (past) and effect (future), our definition of “now” must be set up at a time interval of zero duration. Thus, we render the present in time as nonexistent.

What does exist, however, is the recent memory of the immediate past and the expectation of the near future, which are only linked through the conceptual notion of a present.

Our present.

Or the delicate limit that delineates the fabric of subjective reality.

Time lapse of Earth’s orbit, as seen from Earth.



Alejandra Miranda

Spreading words with a sprinkle of fairy dust and, arguably, some science.