Original Link : https://medium.com/predict/is-time-an-illusion-2ee143dd653a

On the concept of time from three different perspectives: Physical, biological, and neurological

Time is one of my absolute favorite topics. We talk about time every day like it’s something we all know so well. And we do … but then we don’t. Because:

What is time really?

There is no final or objective answer to the question. It depends on the context, and even within a specific context such as physics, time is still not readily definable.

We can view time as a concept from three different perspectives:

  • Physical time
  • Biological time
  • Neurological time

Physical time ⏱

Even in the physical perspective, we can still find many definitions of time. There is the time that we measure with clocks, there is time as part of the fabric of the universe (spacetime) as described in Einstein’s relativity theory, and lastly, there is the time that governs the evolution of a quantum system.

And then there is the conflict of the role of time in the latter two: The relativity theory and quantum mechanics. Scientists are still working on trying to connect those two understandings of time.

The “objective” time that we can measure with clocks

There is a very precise definition for a period of time in physics:

One second is the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium 133 atom. —WhatIs.com

Okay, that sounds a little bit weird. Why do we have that definition?

Originally, one second was defined as 1/86,400 of the mean solar day so that there would be 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day; 60x60x24 = 86,400 seconds in a day. But then we needed time measurement to be more precise.

In 1967, the definition of a second was therefore redefined. The exact number of cycles as defined above was chosen to make the length of the defined second as close to the previously defined second based on the rotation of Earth. This frequency occurs when the Cesium-133 atom is at 0K — in reality, corrections are necessary. The atomic clock will only lose one second every 1.4 million years!

Time in relativity theory

In 1905 Einstein published a frame-breaking theory — the special theory of relativity. It combined space and time into one spacetime with three space-dimensions and one time-dimension. You cannot define a place in space, without defining the 4th time coordinate, and you cannot talk about a point on the timeline, without defining its three space coordinates. The fabric of the Universe consists of spacetime, where space and time will influence each other and one cannot be defined without the other.

In 1916 he published the general theory of relativity, where the theory was developed further and combined spacetime with the mass-energy equivalence.

Those theories proved that time — and how fast it flows — is not absolute, but relative to the observer.

For example, time is relatively slower in a higher gravity field, which means that time on Jupiter will pass slower as compared to time on Earth. Time on the surface on Earth also passes slower than time in the orbit of Earth (it’s further away from the center of gravity). This has been proven to be the case and is even a necessity to correct for in our GPS systems. Without the knowledge that Einstein has given us, Google Maps on your phone would not work as precisely as it does.

Those above-mentioned differences in time are only very subtle, not enough to really notice, but if we were near a gravity field like the one near a black hole horizon, the effects of time dilation would be more than noticeable. In a very high gravity field, minutes passed for you might mean years passed for people back on Earth.

Being in a gravity field is physically the same as accelerating, therefore the same kind of time dilation happens when you accelerate fast to near the speed of light.

Eternalism, presentism and the arrow of time

The view of time as part of spacetime often naturally leads to the view of the universe as a 4D ‘block-universe’, where every point in the timespace is equally valid and ‘now’ is physically nothing special. This point of view is called eternalism and is often seen in physics.

Presentism is the opposite view. In presentism, there is only now — the present — and nothing else. The past is some previous configuration of the universe that does no longer exist and the future is some yet-to-be-determined configuration. This view is more often seen in neuroscience.

There is nothing in the fundamental physics equations, that says anything about which way time flows. Is the flow of time then merely an illusion?

In reality, we don’t expect balloons to un-pop and eggs to be unscrambled, but physically it should not be impossible. The reason why ballons don’t un-pop and eggs don’t unscramble, is a statistical probability that it won’t happen. This statistical probability exists on the macrolevel, as the most probable configuration of each particle in the system.

Without any intervenience, the configuration of a system (that is anything that consists of a large enough number of particles) will always become more disordered in time and will require less information to describe. This is what we call entropy. Entropy is the level of disorder and will always tend to go towards higher levels. Entropy is what gives us the arrow of time on the macrolevel.

Time in quantum mechanics

The laws of entropy only make sense if we have a large enough amount of particles since it’s about statistical probabilities. Quantities like temperature, pressure, and entropy do not make sense on a microlevel: the particle (quantum) level.

And so it seems that the microscopic world is not irreversible as the macroscopic world. You cannot un-pop a balloon because of the statistical probability of the ballon-air-system not moving towards the lower entropy-levels with a whole balloon filled with air molecules inside. But if you zoom in, all the way down to the air-particles moving around and watch how they move, you would not be able to distinguish their individual movement from whether they are moving forward or backward in time.

“The microscopic laws are therefore said to be time reversal invariant, and the macroscopic laws, like the law of entropy increase, are not. “ — Pagels, Heinz R. The Cosmic Code: Quantum Physics as the Language of Nature

In the microscopic world, time is an illusion.

Biological time 🌍

As human beings living on Earth, our concept of time is very much influenced by the astronomical movements of Earth around our primal energy source: the Sun.

We live in specific time periods, such as days defined by the rotation of Earth around its axis and years defined by the rotation of Earth around the Sun. Even the Moon has an important meaning as each month originally was aligned with the Moon’s orbital period.

Our sense of time related to the day/night-phases is what defines our circadian clock. And this might be the key to why our brain has developed a sense of time at all. Time might be an illusion made up by our brain and memories, to differentiate between THEN and NOW.

“The concept of time is simply an illusion made up of human memories, everything that has ever been and ever will be is happening RIGHT NOW.” — express.co.uk

Once again, time might seem like an illusion — but also a smart and reasonable invention of the human brain. What a mess it would have been if everything was at once.

Neurological time 🧠

The final way we often talk about time is the subjective “feeling” of time. This is how the brain keeps track of time — time as seen from the neurological perspective.

Time sure flies when you are having fun. It seems like the boring meeting lasts forever.

Time tracking is a result of different areas of the brains and their functionalities. We have two different way of “keeping track” of time in our mind: The prospective and retrospective timing:

  • Prospective timing is your brain trying to count time on the fly. For example, if you try to count the seconds of a cold shower (I do that — 20 seconds x3 each morning) or if you are told to wait 5 minutes to call back and you try to keep track of the time without a clock.
  • Retrospective timing is trying to tell how much time has already passed, for example, if you are asked how long you showered yesterday or how long you had to wait before someone called you back.

An interesting thing about the subjective feeling of time is that there is a difference in how we perceive a time period depending on whether we use the prospective or retrospective timing.

When we say “time flies when you are having fun” or feel that the boring meeting never seems to end, we use the prospective timing. In the prospective timing, we feel time moving faster when we have interesting experiences while it drags when the brain is not entertained. For example, you feel the time of the delayed plane passing really slowly, while a day of checking various tourist attractions went by really fast.

The opposite seems to be true when we look back in retrospect. Remembering back on that holiday, the plane trip itself only occupies little space in our memory and might not seem like such a big time-chunk after all, while the exciting day of visiting pyramids will occupy a bigger time slot in the memory.

This has been tested in the laboratories, where studies show large differences in prospective and retrospective time estimates depending on the cognitive load.

“Studies have shown that people tend to overestimate the amount of time they wait in store lines, bank lines, or on hold on the phone by 25 to 100 percent. Indeed, companies subject us to elevator music while we are on hold waiting for customer service because some studies suggest that people report waiting less time if they are listening to music during their wait.” — Buonomano, Dean. Your Brain Is a Time Machine.

Our subjective sense of time is relative and depends on various factors, such as the context, our emotional state, stimuli, attention and more.

Another interesting fact about the brain and its relation to time is that our consciousness seems to only show us “summaries of short time periods”. We might not even have free will, according to some neuroscientific studies.

“What we consciously perceive as free will is presumably preceded by unconscious neural computations that are responsible for making decisions.” — Buonomano, Dean. Your Brain Is a Time Machine.

There is still a lot we don’t know about both our inner self and how the mind functions, and the outer, space and the universe and the laws of physics. Time seems to be a slippery concept, that isn’t easy to define.

Perhaps time is in the mind of the beholder.


  • Objective time can be measured with clocks.
  • Time not absolute and depends on the observer in relativity theory.
  • Time does not exist on the quantum level.
  • Time might be a consequence of how the brain perceives the world.
  • Free will might not exist.