Entropy and the Arrow of Time

BriefHistoryTimeThree questions have plagued mankind for as long as we’ve had science fiction writers and theoretical physicists. They are: Is time travel possible? Why does entropy increase? And what the heck does 42 mean? There is no answer to the third question, but the answer to the first two are, rather surprisingly, one and the same. What is less surprising is the author of the solution–the brilliant and renowned physics Stephen Hawking.

Entropy is a short term for what is more formally known as the Second Law of Thermodynamics. While thermodynamics may sound like it’s only useful to those designing HVAC systems, the second law is actually applicable to everything from philosophy to science fiction to self-help guides. What the essence of the Second Law of Thermodynamics states is that everything in the universe, if left to its own devises, will follow a trend towards disorder. Cells die. Buildings decay. Stars go nova. Planets collide. The universe contracts. Death.

We observe this every day in the world around us, but it rarely occurs to us to question why. Never has a broken teacup leaped up onto the table and repaired itself. Never has a worthless pile of stones fallen into the shape of a perfectly formed wall. Every day we get a little older, a little more tired. Every day the laundry has to be done again, no matter how many times we do it. We commonly refer to this process as ‘time’ but it is more accurately known as the inexorable trend of order towards disorder–entropy.


The arrow of time, says Hawking, points only towards the future, because time is nothing more than a measurement of the decay of the universe. If the laws of entropy were ever to be violated, suspended, or run backwards, than time would no longer mean what we measure it to mean today. Time does not properly exist, no more than inches or metres are a substance that we can look at and observe. Time is only a clock, ticking away the seconds until the big crunch.

Why does entropy exist? Why does order tend towards disorder? These are questions better left to philosophers and theologians. Perhaps in other universes the laws of physics work differently, but in ours we are stuck with a reality that is insistent upon decay and death, and only the combined efforts of intelligence and energy can reverse the process to build, create, and live.

ad1d8f6b862c2d3fcf4e745514109f14What about time travel then? Is time travel possible? Suppose. Suppose you could find a way to reverse entropy. Suppose you could put back every broken teacup, repair every damaged cell. Suppose you could reverse the rotation of the earth around the sun, unburn the fuel the sun has burned, and untravel the distance the universe has expanded. Suppose you could rewind time. But it wouldn’t be localized, and it could hardly take place over great distances. If you could stop atoms from decaying you control all of space and time. With enough intelligence, and enough energy, it might even be possible.

But before you go rushing off to the garage to build that time machine bear in mind that when we say “enough energy” we mean “infinite energy.” So unless you have a power line running back to an infinite number of parallel universe, you’re going to be stuck in this reality; this time progression. But instead think of this. Every time you help restore an old building, or repair a hole in a road, or plant a tree, or mend a shirt, you are bringing a little bit of time travel into your world. Every time you wash your dishes, make your bed, or create something nobody else could create, you are creating order out of disorder in direct defiance of an entire universe of physical laws.


Katie Lynn Daniels is the author of Supervillain of the Day, and the mastermind behind Vaguely Circular. She blogs about science and things that are peripherally related to science. You can read all her posts here.


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