The Order of Time by Carlo Rovelli


Entropy and Time

  • all physics phenomena are reversible except those that involve heat transfer
  • heat cannot be transferred from a cold object to a hot object in forward time
  • heat transfer is integral to processes that are time constrained including things like growing old
  • causality also comes into being because of entropy

The idea of now is a myth

  • “Now” doesn’t exist.
  • The present is a myth we create in our heads to connect past and future
  • the world can be understood only as a sequence of Events and not just as States
  • there is no “absolute” reality. There is no absolute anything - time or space. even if such a thing exists it cannot be sensed by us.

Thought experiments

“Where” is relative and not absolute

  • Iterative Q&A
    • Where are you? I am in Mumbai.
    • Where is Mumbai? Mumbai is in India.
    • Where is India? India is a country on planet Earth
    • continuing this thread of reasoning we end with …
    • Where is the Universe? To which we can’t have a meaningful answer

“When” is relative and not absolute

  • A moment in time can be only defined as before or after another moment at THAT PARTICULAR PLACE
  • HB: this part is not clear or convincing

Entropy is relative to the OBSERVER and not absolute

  • take a pack of cards that is arranged by suit and by number
  • shuffle it
  • entropy as gone up would you say?
  • but that entropy increase is relative to your definition of attributes of each card
  • Well, you DEFINED the initial state of cards ordered in a certain manner important to YOU as being a state of low entropy
  • in the larger scheme of things how would you know entropy is going up and not down?


  • the world is a collection of events not ordered in time
  • the arrow of time is created by us to make sense of causal relationships between these events or increases in entropy
  • in order to understand this world we need to constantly connect prior causes to post effects
  • the prior and post requires us to define time in our heads
  • highest entropy state in the world is when everything is mixed up and spread equally everywhere - you look in any direction and see the same thing
  • low entropy states are when things are concentrated in a few places - for example 99.99999% of energy in the solar system is in the sun - this is a localized state of low entropy

What makes the world tick?

  • it is not the sun’s energy - instead it is the sun’s LOW ENTROPY
  • think about it - if the sun keeps giving earth energy the earth should get warmer and warmer and global warming should happen even without human intervention
  • this is not what happens - the earth is largely in steady state energy wise - it radiates nearly all of the sun’s back heat into space
  • not just the earth but you are also individually in a state of energy neutrality
  • if you are expending less energy than you consume you will gain weight and your body cannot indefinitely let you do that so it expends energy some other way like increasing metabolism and body heat expended or absorbing less nutrients from food ingested in the stomach or liver
  • instead what is happening is that the earth absorbs a few high energy photons from the sun and scatters many many many low energy photons in many directions.
  • for example a small sliver of the suns energy falls on earth but the earth appears blue-green from a large number of points in space - each of these points in space is receiving LOW ENERGY photons from earth
  • the important thing that is happening here is that the sun is sending a predictable stream of photons at the earth - this is a low entropy heat source
  • what this predictability allows for is evolution
  • the human body is able to take a variety of diverse low entropy food sources rice, beans, meat, vegetables - break these down and create low entropy feces & energy

What can cause lowering of entropy?

  • Gravity - if you think about a dispersed state of many molecules spread about vs gravitational collapse that is causing them all to fuse to a point - the latter has MUCH lower entropy
  • HB: CR says this is a theory and not confirmed

Causality, Time & Free Will

  • movement from lower to higher entropy leaves perceptible traces
  • these traces convey to us a sense of “fixedness” and “unchangeability” which we perceive as past
  • we do not see similar traces from the future which we take to imply free will
  • cause precedes effect
  • but this is a PERCEPTION
  • it is a thing we have cooked up to make sense of the world

Who am I?

  • how are we different from say an instance of a sufficiently complex software program that starts, runs and terminates? Which can be programmed to make copies of itself etc?

Three Ingredients of Human Identity

Point of View

  • each of us represents a unique view on the world
    • strawberries are tasty or bittergourd is bitter
    • god exists or doesn’t exist
    • my particular religion
    • my opinions and my morality

Empathetic Reflection

  • all animals need to be able to use their senses to process their environment.
  • they have some sense of where their bodies end and the external world begins.
  • Our minds are sufficiently developed to go one level higher and to understand how others perceive the world => how others perceive US.
  • Being able to contemplate how others perceive us leads to the reinforcement of Identity

String of Memories

  • we have a string of memories in our head both long term and short term and we need a story or a way of making sense of it all.
  • This evokes the question: Whose memories are these? What is the common thread behind these recordings in my brain?
  • Identity or self is a useful mental response to make sense of this.