Hacker News Reader

Comment by SilasX | Hacker News Reader

IMHO, Julian Barbour and Gary Drescher have satisfactorily resolved this:

Memories[1] can only form in the increasing entropy direction.

Or, to be more precise, higher entropy states can contain memories of lower entropy states, but not vice versa. This remains true regardless of how the memory is represented.

So the arrow of time (and second law) are not facts about the universe, but about observers' ability to represent it.

Even if you wound back time past t=0 and kept the simulation going, you would see the same thing in the reverse direction, such that the negative time direction should now be considered futureward.

IOW, it's not that "time goes forward, entropy goes up, such coincidence". It's "if entropy were not higher, we could not regard it as the future."

[1] what Barbour calls "time capsules" and Drescher calls "[metaphorical] wakes"

View on HN

I don't know about this because you can create memories using only reversible circuits, which will not increase the entropy of the system. you're more resource-limited if forced to make it reversible (you have to store additional information to make it so) but you can still do interesting things. I'm not sure i buy that only irreversible processes constitute "the future".

The "write readably to a storage medium" part of a(n otherwise) reversible circuit will still be reversible.

Oops, that should be "will still be irreversible".

What powers the reversible circuits?

There are two conceptually different arrow of time problems.

1) observed, macroscopic arrow of time

2) microscopic, fundamental particle physics level arrow of time.

Thermodynamic solution is satisfactory solution only for macroscopic arrow of time.

Time reversal in general is not symmetric. There are particles that are violate CP and T-symmetry called kaons. That's completely different arrow of time.

Reading the chapter about time in "Good and Real" by Drescher blew my mind. See http://commonsenseatheism.com/?p=11068 for a short version.

This is neat. I think there's a way around it tho. What if you can division the futures high entropy states into k, lower entropy states, and subdivide those, and so on. Substructure. Information about some of those substates can flow back from the future and be detected by observers in the present, IMO. It's not easy, but it's possible and it happens. There's still signal. The bandwidth is not nearly as much as the flow from past to present, but it's still more than any single observer can possibly hope to ingest, so some details of the future can be reconstructed by some observers in the now.

Reply on news.ycombinator.com