August 8, 2022


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A messy room, some coffee and the direction of time

The article was written by Henrik Hogerod Carlsen

You may have been scolded because your room was messy. Naturally, you are cleaning your room. Then your things are placed in their fixed places, clothes are neatly hung in the wardrobe and the bed is made.

This is a possible case your room could be in, a small country.

Do you agree that there are endless ways to keep your room tidy? Yes, you can refurnish and move your photos from one wall to another. The room will still be in a ‘tidy’ state, so the word ‘tidy’ is called a macro status.

There are also many variations of your room in the case of the “messy” macro. Things and clothes can be thrown away, closet doors left open and the quilt in the middle of the floor.

entropy It is a measure of disorder in physics and chemistry. The entropy of a state depends on the number of possible microscopic states giving the same overall state. The more ways a large state can be created, the higher its entropy.

This tidy desk has low entropy. Photo: Henrik Haugerud Carlsen

Change is fun

Now imagine that your room is suddenly twice as big, but with the same things. The room is getting bigger, so if you are now redecorating, rearranging and cleaning, there are more possible combinations.

The entropy in a large space is greater than in a small space. There are several ways to place your items in big versus small.

When your room is tidy, your things will probably be more organized. Clothes are in cupboards and drawers, and your other things are probably tucked away in a corner on the desk. If we now imagine that everything you have contains some amount of energy, then we can say that energy is more concentrated. One explanation for the entropy is how the energy is dispersed in the system.

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If the energy is concentrated in an area then the entropy is low and if it is spread out over a larger area then the entropy becomes higher.

Henrik Haugerud Carlsen is a master’s student in condensed phase physics at the University of Oslo. This article was written as part of the MNKOM Communication Course.

Sprinkle milk in coffee

The entropy of the universe will always increase. It can never be smaller This is known as the second law of thermodynamics.

An example is pouring milk into a cup of coffee. The white milk will gradually mix completely with the coffee. It will be impossible to reverse the process and concentrate the milk again at the point where you poured it.

The same is true of the energy in the universe as well. It spreads outward until everything is evenly distributed, and it will not be able to gather again to the point where it all came from, the Big Bang.

Which way does time go?

In fact, it is this law, the second law of thermodynamics, that gives time a “direction”, also known as time arrow. All closed systems, think of your cup of coffee with milk in it, should go from the least likely cases, all the milk is in the center of the cup, to the more likely cases, the milk is mixed with the coffee.

In fact, there’s nothing directly stopping the milk in your coffee cup from ending up in an unlikely state, but it’s so improbable that it never will. Therefore, you must follow the second law of thermodynamics and move forward in the direction of time.


Schroeder: Introduction to Thermal Physics. Pearson, 2014.
Segal: Don’t Call Me Upset: The Naked Truth of EntropyCantor’s Paradise, 2021.
Wikipedia: Thermal death of the universe And the entropy (energy dispersion)

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