The Heat-Death of the Universe?

The heat-death of the Universe

The explanation about the heat-death of the Universe is a difficult story covering everything in time, the beginning and the very end of the universe. It even covers the deaths of galaxies, stars and life itself. What exactly is meant by the heat-death of the universe and how does it work?

A few terms and concepts are important in this tale, the arrow of time, entropy and the second law of thermodynamics.  The second law of thermodynamics gives the explanation of entropy. Before we can grasp the meaning of this law we need to know what entropy states.

What is entropy?

Entropy is a ‘measurement’ of disorder. You use the term entropy to describe a disordered and chaotic state of a system. For example if you look at ice on a molecular level it consists of water molecules that are neatly stacked and crystallised. If we were to place this ice in a room that is at room temperature, the entropy of the water molecules will increase. The neatly ordered crystallized structure of ice will fall apart and a liquid will form which is more chaotic in its nature. These molecules in liquid water are not neatly stacked but are floating around, bumping into each other. We call this liquid higher in entropy than the ice. Simply you can say entropy is in how many ways you can rearrange atoms and still make the appearance the same. If you take coffee and cream for example, you can rearrange a bit in the coffee or the cream and you won’t notice. When you mix the coffee and  the cream you would notice and this mixed state is a high entropy.

Why not from water to ice?

But what is keeping this from happening in reverse? Why doesn’t liquid water just crash into one another to form a block of ice?

This is because of the second law of thermodynamics. This states that the total entropy in an isolated system that undergoes a change can never decreases.  It moves towards a thermodynamic equilibrium which is also called maximum entropy. Things will not become more ordered at random, only more chaotic and disordered.

What is the arrow of time

This arrow of time is itself also strange concept when you look at it. If we are to imagine two balls on a pool table with nothing interacting with it and subsequently we shoot the balls against each other and film this event. If I were to play the tape of the balls bumping into each other, how do we know I wasn’t playing this tape in reverse? The thing is we cannot know. However, if I were to place a neat triangle of balls, as you do in pool, and shoot one ball against the stacked triangle, we know the triangle of balls would scatter across the table. If I were to play this tape in reverse, we would all instinctively know if it was reversed or not. We all know when I shoot an object against a stack of balls it will not order itself neatly. It will go from a low entropy to a high entropy, from ordered to disordered. The balls will not come flying from all the corners of the table and form a neat triangle. This is called the arrow of time. We can see irreversible and reversible processes.

As we stated that the second law of thermodynamics is “The total entropy of an isolated system that undergoes a change can never decrease” we could add; if the process is irreversible, then the total entropy of an isolated system always increases. In a reversible process, the total entropy of an isolated system remains constant.With a system that can interact with its surroundings we can say that the increase of entropy of one is greater than the decrease in entropy of another. Hence we can say that the change in entropy of the universe must be greater than zero for an irreversible process, and equal to zero for a reversible process. (Presuming the Universe is an isolated system with no ‘surroundings’, this has not been proven as of yet).

Using these rules we can get to the heart of the question; what is heat-death of the universe?

Heat-death of the universe

When we know all these fundamental rules we can also say that at some point in time, the universe will reach maximum entropy.  There is no more ‘free, workable’ energy left. Everything is neatly distributed, the universe will reach a state of uniform density and temperature. All  life and chemical reactions would stop because there is perfect disorder and like I said, no ‘workable’ energy left. No more energy left to create complex molecules or even to form bonds between atoms. This is the heat-death of the universe.

What kind of time span are we thinking of when we are imagining the heat-death of the Universe?

This has been predicted to be about 10^100-10^150  (10^100 = 1 googol) years after the Big Bang. Remember we are only 13.7 billion years away from the Big Bang at this point in time. In other words, it might take a …while.

How will this heat-death affect earth?

The earth will be long gone before maximum entropy will get to this lovely planet. As entropy also works on the Sun it will expand and transform during it’s lifetime. Turn into a red giant and afterwards a white dwarf. In our Sun’s lifetime it will probably consume our planet with everything on it in one fiery gulp. Millions of years of evolution, hard work, religious warfare and ideologies gone in only ‘a blink of an eye’, that’s relative to the universe it’s age of course.

(This piece is quite a bit longer than I intended it to be but I was asked to cover this subject. It’s quite a tough one in comparison to the other posts so far. Hope you understood it! If anyone else has a request, feel free to drop it in the comment section or to mail me.)

Extra links;

The brilliant Brain Cox presenting us the questions about the Arrow of Time.

Villanova University with the smart Dr. Sean Carroll talking about this very subject.


Serway, Beichner  1982-2000 Physics for Scientists and Engineers with Modern Physics 5th edition. Chapter 22; Heat Engines, Entropy and the Second Law of Thermodynamics

Nelson, Cox 2008  Lehninger Principles of Biochemistry 5th edition. Chapter 13.1 Bioenergetics and Thermodynamics

villanovauniversity The Origin of the Universe and the Arrow of Time

Sean Carroll, Ph.D.
Senior Research Associate in Physics
California Institute of Technology

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