starship-design: Re: Paradox

[wharton@physics.ucla.edu (Ken Wharton)]

Isaac writes:

>(Note that it's insufficient to try and restrict universe splitting
>to "time travel" cases and not FTL.  FTL travel _is_ always time
>travel in at least some frames of reference, assuming the universe
>is flat/convex.)

True.  I wasn't thinking about FTL, just time travel.  But pure philosophy 
can get you a long way when talking about something as paradoxical as time 
travel, even without throwing in FTL physics.  But maybe by adding in FTL 
time-travel you can further restrict the possible time-travel philosophies.

>>In C) I did not mean that the universe would then proceed identically
>>the second time around; whoever travels back in time can survive and
>>change things around as reality unfolds again.
>
>But this is exactly the same case as in B, except that the old universe
>doesn't exist anymore.

True again; maybe they're identical.  Or maybe the division between B) and 
C) should have been whether the time travel creates the new universe, or 
whether it was there all along, waiting for you to come into it.  

>>As for the infinite-universe idea, there are some very good philisophical
>>reasons why it's very improbable.  After all, people we know behave in
>>predicable ways some of the time.  Why would this be if we're in one of
>>many universes?  Every past decision made by anyone would be random; 
there
>>would be no pattern to human behavior.  It's a strong argument against
>>the many-worlds idea.
>
>No it isn't.  Just because decisions are random doesn't mean there's
>no pattern.  Consider the act of randomly rolling a pair of dice.
>While the outcome is random, there are certain patterns--the result
>is always an integer between 2 and 12 (inclusive), sevens are most
>common, etc...

Do you really think that every pattern in the world is simply a matter of 
chance?  You can do an experiment of asking people how they're doing.  A 
large portion of them will actually respond by saying "fine" or whatever, 
even though the odds of everyone "randomly" behaving this way (and not 
saying random words, clawing your eyeballs out, etc.) are inconceivable.  
Every time you can even somewhat-accurately predict how someone will react 
to a given set of circumstances, you are helping to refute the infinite-
universe possibility.


>>Finally, I agree with Issac that there is a link between quantum 
mechanics
>>and time travel.  But I currently think that it is not quantum that 
allows
>>reverse-causality, but rather reverse-causality that creates quantum 
effects.
>
>Huh?  I don't think there's such a link.  Quantum mechanics does allow
>a mechanism by which what appears to be reverse-causality is possible,
>but OTOH it does so in a way which makes "normal" causality invalid
>(the idea that a cause will have one of the possible effects in one
>future universe, rather than _all_ of the possible effects in an
>infinite continuum of future universes).
>
>We currently think that this infinite continuum of possible futures
>eventually collapses into one, but this _is_ just an assumption--there's
>simply no way we can test it one way or another.  Even if the other
>possible futures exist, we can never "run into" them, so we can neither
>prove nor disprove their existence.  Thus, for all practical purposes
>the other futures don't exist.


Okay - I'm not only being speculative, but I'm so far off the main topic 
that I won't delve into it.  Suffice to say, I think that the accepted 
explanation of quantum mechanics is going to undergo a revolutionary change 
in the next decade or two, and it's going to link reverse-causality with 
quantum effects.  The idea is that when both causality and reverse-
causality happen together, the result is what we observe as quantum-type 
effects.  On the macroscopic scale, where reverse causality can't happen, 
you get the non-quantum world.  It replaces wave-particle duality with the 
idea that all matter is made of particles, but they seem to behave like 
waves because of backwards-causal interactions with their own future.

If this does become accepted some day it won't have a direct impact on 
starship design, other than to supply a fundamental explanation of why 
macroscopic time-travel is impossible, and therefore why FTL is impossible.

I'll leave it there for now.  But don't forget -- you heard it here first.

Ken