Exercises in temporal logic

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Mar 26, 2009, 2:08:08 PM3/26/09
If we want to understand physics, first of all we have to understand,
create a coherent picture of the nature of time.
So in this thread I would like to discuss about consequences of
hypothetical assumption that there is a possibility to influence the
past - does it have to lead to contradictions (time paradoxes)?
Observe that practically all fundamental equations of physics we
assume conserves CPT - so time arrow/2nd law of thermodynamics is not
a microscopic fundamental law, but only result of solution we are
living in - with boundary values with relatively small entropy (big

First of all, let's assume some interpretation of quantum
mechanics ... physics. Some people believes that physics isn't
deterministic (because of wavefunction collapses), some in many-worlds
interpretations (that such collapses splits worlds - creates
alternative realities).
In many world interpretation - time travels doesn't seem to be
forbidden - such travel would just create a new alternative timeline -
there is no problem with time paradoxes.

For now I'll assume the most probably (and physical) for me looking
point of view: full determinism - there is some 'wavefunction of the
universe' which evolves in an unique way. It's like in the general
relativity theory: there is some already created fourdimensional
spacetime and we 'travel' through its time direction - so the future
is already there (eternalism).
Observe that from our point of view there is no difference between
full determinism (the future is already determined) and eternalism
(the future is already there).

I'm afraid that full determinism/eternalism doesn't allow for perfect
time traveling, like made because of that our spacetime would be bent
so much that it would create stable 'wormholes' with endings in light
cones of each other - such stable time-loop would allow for paradoxes
physics couldn't handle with - like a machine which sends a beam to
itself iff it doesn't do it. It's one of reasons I don't like curved
manifolds view:

But let's think about a possibility of a channel in which we could
send information back in time on which we cannot fully rely -
sometimes it's easier for physics to make it lie.
Observe that quantum mechanics gives physics large freedom in choosing
effects below our recognition to stabilize such causal time-loops to
avoid time paradoxes: like choosing what spin will be measured, or
make some small influences on statistically looking behaviors ... or
eventually make that this backtime channel would lie.

Assume that there are allowed causal time-loops, but they have to be
So if there would be such 'device' to transmit information back in
time and I for example would send to me a picture of the girl who will
be my wife ... and just before sending it back in time I would change
my mind - this timeline would never appeared - there would be
completely different situation (without this knowledge).
So if I'd really get this picture from myself - I will have no choice
but to really send it.
Like in (a good...) SF movie - they go back in time to repair
something ... and finally it occurs that the situation is exactly as
it was ...

How to use such 'device'?
Imagine we take some real random number generator - for example
measuring spin of photon 45 degrees from its polarization.
Now the procedure is:
(1) make a choice according to this generator,
(2) if from future there is a message that it was a wrong choice -
take a different one
(3) wait for results of this choice
(4) finally if it was wrong choice - send this information back in
time to (2)

So if there was a satisfying choice - it has created stable time loop
- so in fact the possibility of using this device made that our random
number generator (quantum mechanics) has already chosen properly
before the loop.
It can be imagined that each of choice starts a different timeline -
in our language they are all entangled and physics will choose one of
them in what we call future, such that there for example are no time
paradoxes (like that on a string is allowed some discrete set of

What if there wasn't any satisfying choice? Would it create time
Not necessary - most probably the physics would destroy the weakest
link of such loop - make that this 'device' had lied.

Observe that even without this real random number generator, such
'device' could work without actually being used: if for example there
had to be a successful terrorist attack, there would be sent
information to prevent it ... and finally in a stable timeline this
attack would never happen (because of e.g. proper quantum choices
below our recognition).

It's great exercise to imagine the world with such devices.
If it would allow to send message only some very small time back, we
could use two such devices cyclically to increase this time as much as
we want.
It would have great use in science - choose a random amino acid
sequence/reaction parameters/... crypotokey ... and use above
procedure to ask if they are somehow optimal.
One would say - we could get technologies from the future ... but I
don't think it would be good choice for us in the future, because we
are not prepared for most of them ... like artificial intelligence -
for it 'we' in future would have to still want to send it, knowing the
I don't like a picture that only government/rich would have access
it ... but if it could be cheap, it would quickly spread:
Imagine our world with commonly available such 'devices'...

If someone would make a wrong choice - he could send this information
back to prevent it - so finally we wouldn't consciously make choices
which results wouldn't satisfy us! ... like choosing a school/job/...
politicians... there would be no random accidents ...
There would be also no hazard ... so what about economy? If some
papers would have to drop ... they would drop to zero...
So economy would have to completely change - that finally all is worth
as much as it is really worth ... there would no risk management ...
finally it should be extremely stable.
The same with persons - no one would longer rely on illusionary values
- we would have to work on our real values instead ... not depending
on luck, frauds ...

We could concentrate on studying/working/having fun/... building our
world without any worry...
Is such world without bad choices/risks perfect?
Could we in this way really send some information back in time?
That means it would make a stable timeline - that knowing this
information, after a few years we would really like to send it ...
reveling own future/destiny - taking away own free will ...
But people are not perfect ... one would from the beginning been told
that he has to do in life exactly this and this ... but I believe if
it's properly used - such timelines shouldn't be stable - never
happen ... for example even saying someone about his real future would
be considered a crime against his free will... in stable timeline
never happen (it would be prevented).
... I want to believe that in mature society it would be marginal
cases ... and the main use would be just existing of such possibility
- creating timeline without 'bad' choices ...

So in this picture we cannot send information back to change the past.
Possibility of sending information is already enough to make that to
prevent time paradoxes, physics would have to to prevent making 'bad'
choices: if we would make such choice - we would try to send this
information creating a paradox. So using a proper choice of what we
call uncertainty, physics should make that we make a 'good' choice.

What do You think about all of it?

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