Tuesday, December 1, 2009

ontological paradox...

Noticing today's RocketBoom on the Ontological Paradox made me start musing about one of the examples, specifically:
On his 30th birthday, a man who wishes to build a time machine is visited by a future version of himself. This future self explains to him that he should not worry about designing the time machine, as he has done it in the future. The man receives the schematics from his future self and starts building the time machine. Time passes until he finally completes the time machine. He then uses it to travel back in time to his 30th birthday, where he gives the schematics to his past self, closing the loop. Of course, the schematics must have come from somewhere.
There might be a simple way around the problem of where the information comes from if you accept the "many-worlds" interpretation of quantum mechanics:
  • Let M be the multiverse, or the set of all universes.
  • Let u be a universe in M where the man successfully develops the time machine on his own.
  • All he has to do is find any other universe u' in M such that the current time in u' is equal to a previous time in u (i.e. 30 years ago).
  • If, technically, he travels across universes from u to u' (as opposed to really traveling in time in his own universe) then the information (the plans) really come from u, not u', so there is no spontaneous generation of information.
  • The fact that the plans generate a "time" machine which allows the loop to "close" is completely accidental since "time" travel would in reality continually travel to another universe (e.g. once in u', he would need to find a u'', etc.)
  • Also, because he is traveling between different universes (as opposed to traveling backwards in time) he doesn't break any known laws of physics.
There are some problems with this way of thinking about things though...

If there are universes "in the past" and there are no limits to how far we can go "back", it implies that there is always at least one universe right now that is undergoing the very first instant of the big bang. I don't think "many worlds" allows such a juxtaposition. It's probably not a given that a universe u' can always be found.

You might say that this doesn't matter, because he's only going back 30 years... however, once the loop is "closed", it might easily execute more than n = (the age of the universe/30) times... if you imagine it takes n universes (u1...un) to perpetuate this cycle, then the last universe un in the chain must have been undergoing the first instant of the big-bang when he left his original universe u.
"why do we seem to only know about one universe"
There has always been another weird problem with the multiverse interpretation... supposedly, different event probabilities create different universes in which each probability actually happened. If that's true, then why do we seem to only know about one universe -- i.e. why does our consciousness seem always to travel with this particular branching path and not one of the others?

Then I realized this isn't such a problem after all: we might only remember this universe because at any given moment our memories are the sum total of our experience along only the path we are in. Our other "selves" would likely have different memories, but all similarly self-constrained to their paths.

There's no way to "peek around the quantum veil" as it were.

1 comment:

American Anti-theist said...

There is the other possibility, that time is not something through which we travel as such, that potentialities do not exist in another universe, and that this (now) is, indeed the only time and reality there is.

I'm aware of quantum experiments that seem to suggest that electrons can exist simultaneously everywhere they have the potential to exist, however I tend to think that this could be explained by things like cosmic strings and the possibility of electrons existing as multidimensional particles which simply extend in and out of our ability to detect them (sort of like thread sewn through a piece of paper).

Of course, I'm not a physicist. But I am curious about how much thought has gone into a continuous present model of time? I tend to think that time exists like this, as a continuous present. That what happened in the past, as such, modified the then world in such a way as to leave a physical impression. In the next instant, that past is gone. The only things left of it are the physical impressions made on the world in matter and memory.

The future always consists of potentialities for occurrence, but those occurrences don't exist yet. And as soon as choice or chance place their bets, the future is decided, but it is decided by the impression made in the now.

In every instant, the actions of the past and the decisions of the future are annihilated leaving only one reality. And even to use the word annihilated is to wax poetic, as the future has never really existed and never will. Just as the past no longer exists in any real sense. All that is left are the physical impressions that resulted in that time. But the time, itself, exists on no axis of experience.

I think when we talk of time, we talk about it metaphorically because we need to feel, biologically, a narrative connecting the events of our life. We have memories of these events and so it seems natural to believe that they still exist in some sense and that they can be revisited in some way.

The same with the future. We constantly project images of our future as this is a cognitive necessity for a living entity, to work as a probability engine and estimate its best chances for survival given a course of action in a given situation. This lends us to think of the future as something which exists, because it draws parallels with our memory. But I think this is a fallacy drawn from our biology as well.

In the end, time is a concept which helps us visualize the way things change in relation to each other in space. We need to visualize time as an axis to cognitively apprehend concepts like motion, acceleration, velocity, and force.

But I think that this visualization is ultimately a cognitive tool, much like zero-sum accounting. It captures an isolated snapshot of reality given certain assumptions but does not instruct us about the wider complexity of economics. It is easy to confuse the tool with the reality, I think.

But, also, like I said. I'm not a physicist. I'm as curious as the next guy to hear the latest reports on how things really work.