Information is never lost, not even in a black hole (as per Hawking) seems to be the consensus among all or nearly all astro-physicists.
- Can someone knowledgeable please explain what they mean by “information?”
- If that proposition is true, does it mean that, in principle, It is possible to learn what happened at any moment in the past, by gathering all the bits of information, and analyzing how they fit together?
I really do hope to read responses to this question.
Physical information
I think you are referring to Physical Information:
https://science.jeksite.org/info1/pages/page2.htm#_Toc397846049
A quote from the above page: “The term physical information is used here for the concept that any physical non-uniformity, difference, or distinction is information. Physical information is conceptually related to thermodynamic entropy, which reflects the homogeneity of the distribution of energy and matter in a system (Avery, 2003; Brillouin, 1962; Lambert, 2005). A system with random, homogeneous distribution of energy and matter has high entropy and low physical information. On the other hand, a complex system with discrete elements and concentrated energies has lower entropy and higher physical information.”
So basically information is order. A completely random system has no order so it has no information. A highly ordered system like a diamond or a cat has a lot of information.
Information may never be destroyed, but that does not mean it is always easy to access. Information that goes into a black hole would require going into the black hole to recover it. A telescope looking at stars can see things that happened billions of years ago, so anything that ever happened and reflected light into space can be recovered in that sense, but you have to have a telescope recording the scene in the right place at the right time. If you wanted to see events that happened on Earth 1,000,000 years ago you would need a telescope placed a million light years away that was powerful enough to see details on Earth. It would take a million years (or more) to get a telescope there, so it’s not of much value for more immediate purposes. If we had the resources and time we could send out a telescope to say 10 light years away and once it gets there we could see events on Earth from 10 years ago. The light would take 10 years to reach the telescope and the images would take 10 years to get back to Earth though so the data would always be 20 years out of date. If you wanted to cover all of Earth you’d have to send out many such telescopes in all directions. It would be very expensive and difficult to do, but it might be interesting to try. What you could see would be limited by the capabilities of the telescope. You could not see inside buildings for example. The information related to what happened in a building would still exist, as perturbations of atoms and molecules on Earth. Trying to reconstruct what happened would require being able to identify all the atoms involved and backtrack all their interactions with other atoms from the present back to the time of interest, something that is likely to remain impossible for us for for a long long time. So the information will always exist, but for all practical purposes on a human scale of time and capability there is no way to recover most information. It exists in some ever increasing number of atoms zigging instead of zagging, but just the act of trying to measure all those zigs and zags would obscure the information being sought. You basically need omniscient knowledge of all the properties of all the atoms and photons in the universe to be able to reconstruct the information history of the universe.
A related definition of information can be found in cryptography in Shannon’s Information Theory. Here’s a long but fairly accessible explanation:
http://www.science4all.org/article/shannons-information-theory/
The Wikipedia explanation is somewhat more complex:
https://en.wikipedia.org/wiki/Information_theory
Going back and forward
There was also a belief for a while that if one knew all of the current states of everything, and knew their properties and how they worked, one could predict the future. Along the lines of – if we have all the information now, what happens next will be obvious.
Of course, it’s not quite that easy…