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The Time-Cone Points Both Ways
(I actually wrote this up weeks ago, but never got around to posting it. A recent discussion on ![[livejournal.com profile]](https://www.dreamwidth.org/img/external/lj-userinfo.gif)
jducouer's LJ reminded me of it, so I'm digging it out now.)
Many of you are familiar with the Many Worlds Hypothesis. It's the idea that, in order to explain quantum indeterminacy, you assume that every possible outcome *does* happen - in some universe or other. Looking forward in time from the present moment, reality looks like a branching tree. Or, as Borges put it, a Garden of Forking Paths.
Why has no one tried looking the other direction? Just as the present moment may *lead* to a large number of possible futures, it may also have *originated* from a large number of possible pasts. The further into the past you look, the more uncertain you become about what it actually was.
Is this meaningfully true on a macroscopic (non-quantum) scale? Idunno. But it can at least provide an interesting metaphor. The Uncertainty Principle ("You cannot measure something without affecting it") isn't really true at the human scale, but it's a useful way of talking about certain situations.
For instance,gyzki recently quoted a discussion between two Soviet 'experts' about the Cuban Missile Crisis, which had the two of them in complete disagreement about whether there had actually been warheads in Cuba. Such a discussion could have been the result of a past in which there were warheads -- or a past in which there weren't. I don't have enough information to judge. It is even possible that that information no longer exists -- that that portion of the past has become indeterminate. I think it unlikely for something that recent, but not impossible.
This idea has been floating around my head for years now. One of the frequent thought experiments is tangentially related to the Cuban Missile Crisis: the assassination of JFK. There are dozens of different theories about it. Which is true? Are any of them definitively so? It seems likely to me that such a big event, within living memory, probably does have a reasonably singular truth behind it, which should in theory be discoverable.
But what happens when you go further back? What were the real circumstances around the death of Abraham Lincoln? Or Richard the Lionheart? Or Julius Caesar? Or Alexander the Great? How far back in time must one go before the circumstances around an important event become totally fluid?
Let's try another axis of investigation. Staying within 'living memory', but altering the scale of the events under consideration. Instead of the death of world leaders, let's consider entirely trivial events. What did I have for breakfast on this date 25 years ago? I don't know. I could make some guesses based on half-remembered habits, but could not answer with any confidence. I could have come to this point in my life from a past where I had a sandwich, or a past where I heated up some leftovers, or a past where I had some Spaghetti-Os, or any number of other possibilities. That breakfast wasn't important to my life. Having not stuck in my memory, or any obvious physical record, it remains uncertain.
Things may also become uncertain due to distance in space. The entire life of a typical Chinese peasant who was born in the same year I was probably had no noticable effect on my past. He may have died young, or lived to a ripe old age, stayed poor, or become prosperous.
This is not exactly solipsism, though it might be mistaken for it. It's the idea that those parts of reality which had no lasting impact on me remain, relative to me, undecided.
jducouer's LJ reminded me of it, so I'm digging it out now.)Many of you are familiar with the Many Worlds Hypothesis. It's the idea that, in order to explain quantum indeterminacy, you assume that every possible outcome *does* happen - in some universe or other. Looking forward in time from the present moment, reality looks like a branching tree. Or, as Borges put it, a Garden of Forking Paths.
Why has no one tried looking the other direction? Just as the present moment may *lead* to a large number of possible futures, it may also have *originated* from a large number of possible pasts. The further into the past you look, the more uncertain you become about what it actually was.
Is this meaningfully true on a macroscopic (non-quantum) scale? Idunno. But it can at least provide an interesting metaphor. The Uncertainty Principle ("You cannot measure something without affecting it") isn't really true at the human scale, but it's a useful way of talking about certain situations.
For instance,
gyzki recently quoted a discussion between two Soviet 'experts' about the Cuban Missile Crisis, which had the two of them in complete disagreement about whether there had actually been warheads in Cuba. Such a discussion could have been the result of a past in which there were warheads -- or a past in which there weren't. I don't have enough information to judge. It is even possible that that information no longer exists -- that that portion of the past has become indeterminate. I think it unlikely for something that recent, but not impossible.This idea has been floating around my head for years now. One of the frequent thought experiments is tangentially related to the Cuban Missile Crisis: the assassination of JFK. There are dozens of different theories about it. Which is true? Are any of them definitively so? It seems likely to me that such a big event, within living memory, probably does have a reasonably singular truth behind it, which should in theory be discoverable.
But what happens when you go further back? What were the real circumstances around the death of Abraham Lincoln? Or Richard the Lionheart? Or Julius Caesar? Or Alexander the Great? How far back in time must one go before the circumstances around an important event become totally fluid?
Let's try another axis of investigation. Staying within 'living memory', but altering the scale of the events under consideration. Instead of the death of world leaders, let's consider entirely trivial events. What did I have for breakfast on this date 25 years ago? I don't know. I could make some guesses based on half-remembered habits, but could not answer with any confidence. I could have come to this point in my life from a past where I had a sandwich, or a past where I heated up some leftovers, or a past where I had some Spaghetti-Os, or any number of other possibilities. That breakfast wasn't important to my life. Having not stuck in my memory, or any obvious physical record, it remains uncertain.
Things may also become uncertain due to distance in space. The entire life of a typical Chinese peasant who was born in the same year I was probably had no noticable effect on my past. He may have died young, or lived to a ripe old age, stayed poor, or become prosperous.
This is not exactly solipsism, though it might be mistaken for it. It's the idea that those parts of reality which had no lasting impact on me remain, relative to me, undecided.
no subject
... and why doesn't Lazarus Long, Hilda, Deety, etc, ever run into other analogues of themselves when they timeline-hop?
Meeting themselves
On the other hand, I think the convention at the end of the book mentions at least one classic character who's shown up in multiple versions. Not sure, though.
no subject
Hm. Bit of a bastardization of the normal terminology - in relativity, we talk about a "past light cone", but that's a rather different beast.
To answer your question - the reason this is not generally done is that the Universe is known to not be symmetric under time-reversal. Time's Arrow only points in one direction, which implies that how things unfold in the forward direction is not the same as in the backwards one.
Especially on the human scale, one must be very careful not to confuse what is plausible given only limited information, and what could, in reality, actually have resulted in our current state. We suppose that maybe it would not have mattered if there were warheads in Cuba, but that is merely a guess. It is also just as likely that this was one of the details that was pivotal in deciding how particular individuals behaved, thus being formative in the sequence of events.
Basically, there can be a very big difference between, "I don't know what happened," and, "It does not matter what happened." When considering the course of human events, we shoud not mistake the two.
no subject
My understanding had been that from a Newtonian/macroscopic scale, the only difference between forward in time and backward in time is that entropy increases.
no subject
Absolutely. This is a mistake that often occurs with people's understanding of how the (forward-looking) Many worlds hypothesis works.
entropy increases
One of these days, I have to take another crack at understanding entropy. The last time I attempted to wrap my head around it, all the definitions I could find seemed to have irreducible subjective elements. (Does any one reading this know of a good place to start?)
no subject
no subject
Yes. This is part of why the Many-Worlds hypothesis actually says that anything that is physically possible happens in some branch of the Universe. All that is not prohibited is mandatory.
My understanding had been that from a Newtonian/macroscopic scale, the only difference between forward in time and backward in time is that entropy increases.
You say that as if it is a small, fiddling detail we can ignore in most cases. It can be ignored in isolated, simple mechanical cases, but otherwise (say, in a complicated engine, or a biological life form, or economics) it is a cornerstone of how we operate.
no subject
I did not mean it that way.
I just don't see how increasing entropy by itself would be sufficient to make it any harder to predict the future from the present than to predict the past from the present. If anything, I'd think it would be easier to predict the future than to figure out the past: what will this piece of matter look like as it wears away? vs. how were these particles arranged before they wore down? Or... these two containers are about the same temperature right now... what temperature did they start at?
no subject
Of course. But I posit the possibility of "It is *impossible* to know what happened." I don't seriously believe that such a condition applies to something as recent as the Cuban Missile Crisis.
Hm. Lots of time travel stories that use the "time is invariant" model also use the notions of "event shadows", periods of time which, being unrecorded, might 'turn out'/'be manipulated' to contain events which work out to one's advantage. An interesting fictional tack might be to take the incredible amounts of obfuscation around, for example, the JFK assassination, as a deliberate attempt to *create* an event shadow...
no subject
In a practical sense, certainly. It may be, for example, that the only people present for a particular pivotal event died before reporting, and the information is lost. There are oodles of other such examples - no argument there.
My point is more that while we can come up with gedanken experiements like Schroedinger's Cat, the number of times when we actually have a situation where events did not take a particular form are limited. The general macro-scale case is that events really did take a particular form, but we don't know what that form was.
Mathematically, there is a very big difference between, "It happened a specific way, but I don't know it," and, "The events there are indeterminate, in a Quantum Mechanical sense."