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byrnema comments on Open Thread: February 2010 - Less Wrong
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What probability do you assign for it being possible to send information backwards in time, over any time scale?
We already know (theoretically) how to send logical information backwards in time, as in Gary Drescher. The other kinds of info-time-travel are probably conceptually inconsistent.
Is there a quick and easy way to understand "how to send logical information backwards in time" that doesn't involve watching a 30 min video?
Suppose that the universe is the deterministically evolving wavefunction, and that it makes sense to talk about causing a rock to be moved from here to there. Then you can cause a timeful universe-slice 100 years ago to be the sort of thing which will deterministically evolve until after 100 years the measure of a rock being moved from here to there is greater than it would have been had you not caused the rock to move.
If I'm not mistaken, in the Pearlian view of causality, if the universe is viewed as deterministic then it does not make sense to talk about causing a rock to be moved from here to there; an intervention or surgery has to happen from outside the system being modeled.
If there is a program P that as part contains yourself and everything you interact with, then the fact that in the future, you decide to do X (within P's execution), could be inferred from P in the past.
Thanks for the quick explanation. So that information was already there, and thus I wouldn't call that sending information back in time.
The problem is that all information is "already there", time itself is arguably how discovering implications of information that is already here feels from the inside. That is, when the world is viewed through deterministic laws, there is never any information that is present in the future, but "logically" absent from the past. The only difference between what is found in the past and what is found in the future is that it takes time to reach (=compute) more "distant" facts.
What do you mean by "here" - your brain? or a spacelike slice across the whole universe?
If a glass falls on the ground and shatters, is it "discovering implications of information", etc? If the answer is yes, does that mean it feels like something for the glass to shatter?
Sure.
No.
Why not?
Because a glass has no mind, naturally.
Yes. I was about to write,
"I do see a gray area that in a deterministic universe, any message that we would want to send from the future could be predicted now, so we in the future don't really need to send the message back -- we in the present just need to predict what the message is."
What you've written has clarified this even further -- depending upon the 'opacity' of the message, we might not be able to decipher the message any faster than just waiting for the future to evolve it.
I have a strange motive for these questions. I now understand that this message I'm worried about is 'already here' in some sense, and that is relevant. It might actually make my parent question moot. However, I think that that depends -- unexpectedly, for me -- on whether all information from the past is accessible to the future.
Information is not gained as you move forward in time. However, do you lose any information as you move forward in time?
In time-reversible deterministic world, information is gained from observation of stuff that wasn't in contact with you in the past, and logical information is also gained (new knowledge about facts following from the premises -- there is no logical transparency). Analogously, an action can be seen as "splitting", where you part with a prepared action, and action parts with you, so that you lose knowledge of that action. If you let info split away in this manner, you may never get it back.
You're a little over my head -- though I mostly follow.
My question was actually simpler. Is the world time-reversible? Do we know anything about that?
I'll contribute my thoughts on whether the world is time-reversible...
By time-reversible, I mean that information doesn't get "lost" as you move forward in time; that with unlimited information about the universe at time t you could deduce everything about the state of the universe at time t-ε.
Classical mechanics is reversible. If you have the velocity and positions of 3 billiard balls, you can deduce if and when they collided and what their original velocities were.
I think what we know about quantum mechanics is inconclusive; we don't know how to trace the wave-function backwards in a unique/deterministic way, but we don't know how to follow it forwards, either.
If you many-worlds, then all possible past universes make up the past universe, so you seem to have reversibility -- a reversibility that is no less determined and unique in the past direction as the future direction.
Being agnostic about many worlds, I would give a higher probability for reversibility over non-reversibility, just because of the reversibility of classical mechanics. However, 51% in favor of reversibility for a hand-waving intuition is pretty much just a random guess and I wonder if anyone has a tighter probability estimate, or other reasons?
0.1. But I suspect if I will run into problems if I try to cash in my probability assignment in, say, a prediction market.