Topics Physics and everyday science

What is entropy?

Entropy is a measure of how many microscopic arrangements of a system look the same from the outside. A tidy deck of cards is one arrangement among a staggering number of messy ones, so a well-shuffled deck almost certainly lands on a messy order. Physicists call that count, usually on a logarithmic scale, entropy. The more arrangements that fit what you see, the higher the entropy.

What makes it interesting is that it explains why some things happen and their reverse never does. Ice melts in a warm room, a drop of ink spreads through water, heat flows from a hot cup to the air. None of these would break the laws of motion if run backward, yet we never watch them undo themselves. The second law of thermodynamics says the total entropy of an isolated system does not decrease, and the reason is mostly statistics: there are simply far more messy states than tidy ones.

An episode on this topic would start with cards and gas, then build up to heat, engines, the arrow of time, and the common mistake of equating entropy with plain disorder. bre's hosts are AI and can be wrong, so you can press Talk and ask them to slow down or check a step.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Counting arrangementsStart with a deck of cards and a box of gas. Entropy begins as a count of the arrangements that look identical from the outside.
  2. Why a shuffle never unshufflesA thorough shuffle almost certainly lands on a messy order, and you will essentially never shuffle your way back to a sorted deck. The reason is odds, not a hidden force.
  3. Heat, temperature and energy spreadingHeat flows from hot to cold because energy has more ways to be spread out than to stay concentrated. That is the thermodynamic view of the same idea.
  4. Ice melting and a gas expandingMelting ice and a gas filling its container are the cleanest examples of entropy rising. Both are far easier to explain than trickier cases.
  5. Entropy and the arrow of timeThe laws of motion work the same forwards and backwards, yet we only see one direction. Why the universe began in a low-entropy state is still an open question.
  6. What entropy is notIt is not simply messiness, and it does not forbid order. Life and crystals can become more ordered locally as long as entropy is exported elsewhere, usually as heat.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Arlo. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Take a brand-new deck, sorted by suit and number. Shuffle it thoroughly and it almost certainly lands on a messy order. Shuffle forever and you will essentially never land back on the sorted one. Why is that, when every order is equally likely?

  2. ArloAI host

    Because there's one sorted order. And a mountain of messy ones.

  3. breAI host

    Right. Any single messy order is just as unlikely as the sorted one. There are just vastly more of them, so a messy result is what you get.

  4. ArloAI host

    So where does entropy come in?

  5. breAI host

    The count is one way to define it. Entropy measures how many arrangements fit what you can see. The sorted deck has one. A messy-looking deck has an enormous number.

  6. ArloAI host

    So it's a tally. Not a force.

  7. breAI host

    Yes, a tally, and the second law is the statistical tendency of systems to drift toward bigger tallies.

  8. ArloAI host

    Who counted that?

  9. breAI host

    Physicists did, using a logarithm so the numbers stay manageable. And a cup of coffee isn't a deck of cards, so that's where we go next: same idea, with energy spreading among countless molecules.

Questions people also ask

Is entropy the same as disorder?
Not exactly. Disorder is a handy picture, but entropy is really a count of the microscopic arrangements that match what you observe. Some systems can sometimes look tidier yet have higher entropy once every part is accounted for, so the word disorder can mislead.
What is the second law of thermodynamics?
It says the total entropy of an isolated system does not decrease over time. Heat flows from hot to cold, not the reverse, without outside work. The law is statistical: reversing such changes is not forbidden, just overwhelmingly improbable for large systems.
Does entropy explain why time only moves forward?
It is the leading explanation for why processes look one-directional, since entropy tends to rise toward the future. But it relies on the universe starting in a very low-entropy state, and why that was so remains an open question in physics.
Can entropy ever decrease?
Locally, yes. A freezer cools food, and living things build ordered structures, as long as entropy is exported elsewhere, usually as heat into the surroundings. The total entropy of the whole system plus its surroundings still does not go down.

Related topics

More: all 300 topics, physics and everyday science, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.