What do physicists mean by "information"?

[koganbot]

What do physicists mean by information?

Every now and then I'll read a book by a scientist trying to explain a field or subfield or subproblem to laypeople like me (by Randall, Susskind, Greene, to name a few of the recent). I almost always like these books, but Sean Carroll's From Eternity To Here is the first that's really clicked for me. I wouldn't remotely claim to understand it. But I got enough while reading to follow what it was doing as it went circling and chasing a few basic questions. Fundamentally: our universe, or our part of it, seems to start with low entropy, the entropy increasing over time. (Other possibilities are explored.) Without this low-entropy start, time wouldn't have the direction it has. Why did we start with low entropy?

This actually encompasses a whole bunch of questions and contentions. I read the book several months ago, in spare time, needing all three library renewals and finishing on the last day, not thinking concentratedly enough to master one part before moving onto the next. I couldn't have mastered the parts anyway. At one point I copied down the following passage, writing underneath, "If I want to test whether I've understood the book so far, I could look at this passage and ask myself to explain it: to fill in the background, to describe how his argument got here. I don't think I can right now." I understand it even less a few months removed:

In that sense, the irreversibility that crops up when wave functions collapse appears to be directly analogous to the irreversibility of ordinary thermodynamics. The underlying laws are perfectly reversible, but in the messy real world we throw away a lot of information, and as a result we find apparently irreversible behavior on macroscopic scales. When we observe our cat's location, and our own state becomes entangled with hers, in order to reverse the process we would need to know the precise state of the outside world with which we are also entangled, but we've thrown that information away. It's exactly analogous to what happens when a spoonful of milk mixes into a cup of coffee; in principle we could reverse the process if we had kept track of the position and momentum of every single molecule in the mixture, but in practice we keep track of only the macroscopic variables, so irreversibility is lost.
--Sean Carroll, From Eternity To Here, p. 255

My question about information is this: We use the word "information" for a whole lot of fairly different stuff, but I think of it as including, e.g., "The test tomorrow is at 1:00 PM." But when a physicist like Leonard Susskind is talking about "conservation of information," I don't think he means that something like "The test tomorrow is at 1:00 PM" is preserved — e.g., would survive planets being absorbed into black holes and the black holes dissipating. But when he uses the phrase "conservation of information" he does indeed mean that all information is conserved after planets are absorbed and black holes dissipate; so I'm surmising that the sort(s) of information he's talking about differ from the type of info that would include "The test tomorrow is at 1:00 PM." But maybe the latter is included.* If so, how would that work?

*I read The Black Hole War and of course have forgotten most of what I read.

Comments

[dubdobdee.livejournal.com]

Had a quick look at the black hole/holograph links: what I *think* is being said is this

i: if something falls into a black whole it is destroyed (not simply transformed dispersed as if would be if it fell into a star), because right down to the smallest particles it passes irretrievably into a gravitational singularity
ii: in which case the "information" that is its basic waveform structure is destroyed
iii: except ii surely breaks the law of the conservation of information: the information that *is* the wave-form structure of the object has vanished irretrievably
iv: BUT black holes emit radiation [this was stephen hawking's proposal, i believe -- his great realisation and change of mind? it is a long time since i read "a brief history of time" tho it is in the other room
v: and moreover, they have this thing called (i think?) the light event horizon, where the light from objects that are falling into them remains held, unable to escape as a kind of image sphere AROUND a black hole
vi: and THIS is where the information that was the wave-form structure is forever held, as a permanent 2-dimensional holograph of the "last glimpse" as it were, of the destroyed 3-dimensional object
vii: which would be detailed enough in informational material, despite being "flat" as an image, for an observer with the correct tools and reading skills -- and presumably ability not to be dragged into the BH themselves -- to "read back" the 3-D wave-form detail from the this 2-D image (hence information is conserved)

[posts above has raised the issue that information in physics terms is not necessarily "communicable" -- as it must be for eg Claude Shannon -- bcz it doesn't require the presence of beings that can "read" the information <--- which is an interesting aspect of the problem, but not one that mathematicians and cosmologists spend much time on; they tend to operate as "under god's eye" communications theorists, even when militant aetheists; the information that is an object's waveform structure is deemed "objectively readable" even if no one is EVER going to be around who can actually read it] [for maths and physics this is a side-issue however: information in the social sense is very much NOT conversed, vast amounts is lost everyone someone dies -- the mathematical ideal is that, if the full picture of all waveforms could be retrieved, then ALL information, including the fleeting never-spoken thoughts of the long-dead, could be reconstituted, indeed the retrieval and the reconstitution would kind of be the same activity]

[dubdobdee.livejournal.com]

"conversed" = "conserved" where i wrote it

even tho i believe that's being claimed is the totality of cosmic conservation and the totality of the cosmic conversation are IDENTICAL hah!

[koganbot]

Made a new post continuing the conversation.

I wouldn't say Hawking had a "great realisation and change of mind" so much as he was convinced that Maldacena's and Witten's ideas regarding the equivalence of anti de Sitter space and a 4-D gravity-free description effectively won the argument for 't Hooft and Susskind's side and so defeated his own idea that information is destroyed when a black hole evaporates. Susskind thought that Hawking dawdled at admitting defeat, whereas some others thought he conceded too soon, and that the issue is still open.

[koganbot]

Not that I've actually read what Hawking himself wrote when he conceded; this would have been quite a while after Brief History Of Time, whereas the radiation stuff came earlier, maybe? but not tied to information, maybe? I should stop babbling, since I don't know what I'm talking about and don't even have time to look at Wikip.