ELI5 Physicists Say That Information Is Never Lost but How the Heck Is It Stored Then and What Theoretical Possibility Could Recover It

Kent - December 11, 2023


The difference between a pile of ash and a piece of paper with a word written in ink is information. I get that part.

What I don’t get is why they say it is absolutely critical to physics that information can never truly be lost, thus the problem with black holes potentially destroying information.

And what I really don’t get is HOW that information is stored, and what possible theoretical mechanism could be employed to recover it.

Say we have a sheet of notebook paper in a glass box. Someone has written a word on that paper with a black Bic ballpoint pen. The box is sealed and the paper is set on fire burning it to a crisp. We then shake the box and the ashes are powdered and scattered.

Every atom involved is inside the box.

Where is the information and theoretically how we recover it and find out what the word was?

tl;dr: The word information in physics has a very specific mathematical meaning, it’s not what we generally think of as information.


In particular, information is a property of a closed physical system - so in your example, you can’t just consider the paper and ink, you also have to include the O2 molecules that took part in the burning, and the smoke that was produced afterwards, as well as everything else that physically interacts with those components of the system.

“In physics, the word information is closely related to microstates and probabilities. In some limited circumstances information is equal to entropy, but in most cases not. Information should never be confused with knowledge despite what natural language and the dictionary say.”

Words on a paper would in this case be knowledge not information. A better question would be “what does information mean in physics.”

I can not give a proper answer, just figured this might push you in the right direction if no one better gives an answer.

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