Intro: these lines are a mix of summary and notes from a couple of books from Carlo Rovelli on modern physics and time, Bertalanffy on General systems theory
We all experience the passage of time. We can’t touch time and we can’t see it, but it is evident that it is there: the cake will burn if it stays for too long in the oven; later today the sun will set and night will come and in time, we will grow old and our kids will grow up. Time always flows from the past to the present.
Time passes, that is a certainty. And yet, universal time is an illusion. There is robust evidence that time is not objective and universal, but rather subjective and conditional.
We have suffered many illusions in the past, and science has cast plenty of those away. Consider the daily illusion of the sunset: Today, as the sun sets, realize it is you who is moving, not the sun. The reality is that we are on board an organic spaceship turning backward, and as the sun hides behind us, we are shifting to an upside-down position, relative to the sun. The sun does not set, we do a backflip. Like the sunset illusion, universal time is also an illusion.
To understand time, we first need to realize a peculiarity of space: things fall, not because there is some force at the center of the earth pulling things down, but because space curves around big chunks of mass.
Mass is not so much attracted to bigger mass but rather it falls towards big mass because space has bent by its weight. Things that seem to fall into empty space are really picking up speed going down a slope.
There is one curiosity about bent space, that affects time: No matter if you are going super fast down the hill of bent space, or slower on the flatlands of the universe, the speed of light remains constant. No matter how fast you go, the speed of lights is and is perceived as the same. How is it possible that lights look the same regardless of whether we are stationary or going super fast?
That’s because time slows down where gravity is stronger. Running down a slope in space, you go faster, but time goes slower, so the speed of light stays constant. Experts call this gravitational time dilation.
Space affects time because both are intertwined in a mesh known as the space-time continuum. Let’s see how that plays out.
If time is slower around curved space, when you move away from a source of gravity, time speeds up. Clocks at the beach tic at a slower rate than the clocks at the top of Mount Everest and that clock tics slower than a clock on the moon. There are super high precision clocks that verify this.
This also applies if you are moving. the faster you move through space, the slower you move through time.
When we say Here, that is clearly a subjective statement. Here in California is different from Here in my hometown in Venezuela. They have Different lat-long coordinates. Now is also a subjective statement. Time is relative to where you are in space.
Past, present, and future are subjective and circumstantial.
If I see a light of a star that is millions of light-years away, the light I see Now is the light that emanated from its source many years in my past. By the time is see the lights, the light source is years into the future.
Another example: Astronauts age a bit slower than us on earth. If time near strong gravity goes faster than away from it, then an astronaut’s present can be my future, here on earth.
It is interesting to note that time is also relative to those sharing the same Here. For starters, not all animals experience time in the same way. There are slow-motion animals and fast motion animals. The Betta fish only recognizes the reproduced image of an opponent if presented in 30 frames per second. Show it slower, and it can’t see it.
A snail is a fast time animal: if a stick vibrated at a rate of 4 times per second, the stock would appear at rest to the snail. The perception of time depends on physiological conditions.
Even among humans sharing the same Here and the same _Now, _time is experienced differently. If two people are focused on the same event, but only one finds it exciting, the passage of time will be perceived differently.
One reason might be that while you are focused on the exciting event, you lose sight of all the other regular events that cue you in the pass of time. And then later, perhaps time feels like it took longer because there were tons of little details worth remembering, which gives you the sense of longer time passing.
There however one universal statement that can be made about time: it goes from past, to present.
Our universe came to being in a super-ordered and dense state, and since then everything has been moving slowly from there into more and more disorganized states. That disorganization of space is evidenced in the presence of heat.
Time is not a monolithic and universal arrow moving in one direction, experienced in the same way by everyone. Rather, it is a local experience, defined by the physics of the location, the physiological conditions of animal perception, and the psychological conditions on which all of this takes place.