In the realm of physics, fundamental laws such as Newton’s F=ma, the law of gravity, and Schrödinger’s equation describe the universe without specifying a direction for time. These laws connect the present with the past and future, treating both directions equally. However, when we look at the macroscopic world, there is one principle that introduces a direction to time: the second law of thermodynamics. This law states that in an isolated system, entropy, or disorder, tends to increase.
Entropy explains why certain processes, like mixing cold milk with hot coffee, result in a uniform lukewarm mixture that never spontaneously separates back into its original components. Once a system reaches its maximum disorder, or equilibrium, there is no further increase in entropy to guide the flow of time. The fact that we perceive time as moving forward indicates that we are not in equilibrium.
There are two main theories about why time flows in one direction. One possibility is that the universe is currently in a low-entropy state, with time moving forward and backward from this point, increasing entropy in both directions. The more widely accepted theory, however, is that the universe began with an even lower entropy state, and disorder has been increasing ever since. This initial low-entropy state is known as the Big Bang.
Approximately 13.8 billion years ago, the universe was hot, dense, smooth, and expanding rapidly. Although this might not seem like a low-entropy state, the high density of matter meant that gravitational forces were significant. In such a context, smoothness was not equilibrium but a delicately balanced low-entropy condition. Over time, matter clumped together to form stars, galaxies, and eventually black holes.
As the universe continues to expand and dilute, it is heading towards a high-entropy equilibrium state: empty space. Eventually, stars will burn out, black holes will evaporate, and the universe will be left with emptiness. At this point, the arrow of time will vanish, making life and consciousness impossible.
The presence of stars, galaxies, and life on Earth reflects our universe’s low-entropy beginnings. Although we do not know why the universe started in such an orderly state, it is this initial condition that allows for the flow of time as we experience it. The story of the universe, from the formation of stars to the emergence of life, is one of increasing entropy.
Time’s arrow is not a fundamental aspect of the laws of physics but rather a result of the specific initial conditions of our universe. Understanding this concept helps us appreciate the unique and dynamic nature of the cosmos.
For more insights into time and entropy, this article is based on a series of videos created in collaboration with physicist Sean Carroll, supported by Google’s Making and Science initiative. These videos are inspired by Carroll’s book, “The Big Picture: On the Origins of Life, Meaning, and the Universe Itself,” available online and in bookstores worldwide.
Conduct a simple experiment to observe entropy in action. Mix hot coffee with cold milk and observe the process of reaching thermal equilibrium. Document your observations and reflect on how this demonstrates the concept of increasing entropy.
Participate in a debate discussing the two theories of time’s direction: the low-entropy state of the universe and the Big Bang. Formulate arguments for each theory and engage with your peers to explore different perspectives.
Create a visual timeline of the universe’s evolution from the Big Bang to the present day. Highlight key events such as the formation of stars and galaxies, and discuss how these events relate to changes in entropy.
Identify and analyze examples of entropy in everyday life. Consider processes like cooking, cleaning, and natural phenomena. Discuss how these examples illustrate the concept of entropy and its impact on our perception of time.
Write a reflective essay on how understanding entropy and the arrow of time changes your perspective on the universe and your place within it. Consider the implications of a high-entropy future and the significance of our low-entropy beginnings.
Entropy – A measure of the disorder or randomness in a closed system, often associated with the second law of thermodynamics. – In thermodynamics, entropy tends to increase over time, leading to the eventual heat death of the universe.
Time – A dimension in which events occur in a linear sequence, from the past through the present to the future. – In the theory of relativity, time is intertwined with space to form the space-time continuum.
Universe – The totality of known or supposed objects and phenomena throughout space; the cosmos; everything that exists. – The observable universe is estimated to be about 93 billion light-years in diameter.
Gravity – A natural phenomenon by which all things with mass or energy are brought toward one another, including planets, stars, and galaxies. – Gravity is the force that keeps planets in orbit around stars and governs the motion of galaxies.
Thermodynamics – The branch of physical science that deals with the relations between heat and other forms of energy. – The laws of thermodynamics are fundamental principles that describe how energy is transferred and transformed.
Equilibrium – A state in which opposing forces or influences are balanced, often referring to a system where no net change is occurring. – In a closed system, chemical reactions will reach equilibrium when the rates of the forward and reverse reactions are equal.
Black Holes – Regions of space where the gravitational pull is so strong that nothing, not even light, can escape from it. – The event horizon of a black hole marks the boundary beyond which nothing can return.
Stars – Luminous spheres of plasma held together by gravity, undergoing nuclear fusion in their cores. – Stars are born in nebulae and spend most of their lives fusing hydrogen into helium in their cores.
Galaxies – Massive systems consisting of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. – The Milky Way is a spiral galaxy that contains our solar system.
Big Bang – The prevailing cosmological model explaining the origin of the universe, describing its expansion from a hot, dense initial state. – According to the Big Bang theory, the universe has been expanding for approximately 13.8 billion years.
