Where does the difference between past and future come from? Why do we remember the past but not the future? Why does the entropy always increase in the universe? What exactly is The Arrow Of time? These are some the questions we are going to ponder upon today. So let’s begin!
You start your morning with a cup of tea but to your bad luck, you spill it all over the table and broke the cup into pieces on the floor. But you have very curious mind and instead of cleaning the mess you just made. You started thinking. Why can`t the cup get it self together? Like playback in movies.
The second law of thermodynamics is the answer to your question. The increase in disorder or entropy is what we call an ‘Arrow of time’. There are as such three arrows of time. Namely,
The thermodynamic arrow of time, the direction in which entropy increases. The psychological arrow of time, because of which we remember the things in past but not the future. And the Cosmological arrow of time, the direction in which the universe is expanding not contracting.
The Thermodynamic arrow of time
The second law of thermodynamics says that “The entropy or disorder of an isolated system tends to be maximum”. In layman`s language “Things get disordered with passage of time”. A very bad analogy I always draw is of your bedroom. You clean all your stuff, put everything in place but after few days things will get out of place by themselves. Because there is a lot of activity going on in your room. Our universe functions in a same way. At the time of big bang things were in an ordered state. However, there were some small fluctuations but nevertheless. As with the passage of time physical forces came into play and like your room. The disorderedness increased.
But why things happen in such a way? Why not go from being disordered to an ordered state? A simple answer: Because there are just more disordered states than ordered states. A jigsaw puzzle has only one ordered state which makes the picture perfect but many which don’t.
This thermodynamic arrow is responsible for our second in line. The psychological arrow of time.
The Psychological arrow of time
The psychological arrow of time is the reason we remember the things in past but not in future. We will draw the analogy from computer`s memory to our human brain. When a computer (or our brain) stores something in its memory. It goes from a state of disorder to order. To do this energy is required to the computer and energy is also dissipated in form of heat by cooling fans. Which increases the disorder of universe. The increase in disorder in such a process is greater than the increase in order in computer`s memory.
Hence, it follows that both the thermodynamic and psychological arrow points in same direction i.e. the forward direction of disorderedness. If that was not the case, we would remember things from the future but not the past. And it is surely not the case.
The Cosmological arrow of time
This is the direction of time in which the universe is expanding. If you want to know why universe is expanding or will keep expanding forever I suggest you to read my article on The fate of the universe. Moving forward, the universe we know is expanding and disorderedness increases with it. We can vouch for that from the Anthropic principle. We know our universe is expanding and not contracting because an intelligent life cannot not survive in contracting universe. Even if the universe starts contracting, the other arrows will not change their direction. A strong thermodynamic arrow is necessary for intelligent life to operate. Human beings consume food which is an ordered state of energy and convert it into heat. Thus the psychological arrow follows directly from the thermodynamic arrow. Hence, the thermodynamic arrow and the psychological arrow points in the same direction.
To summarize our discussion the Arrow of Time, which comprises three arrows: the thermodynamic arrow (increasing entropy), the psychological arrow (memory of the past but not the future), and the cosmological arrow (expanding universe). The second law of thermodynamics explains why things tend to become disordered with time. The psychological arrow relates to energy-consuming processes that create memories, increasing the universe’s disorder. The cosmological arrow signifies the universe’s expansion. These arrows all point in the same direction, from a more ordered past to a more disordered future, explaining why events cannot spontaneously reverse like playback in movies.