# Thread: Exercises in temporal logic

1. Let's make some 'thought experiment' to exercise our intuition, think about the nature of physics - imagine we can make backtime information channel - let say for example 'send photon back in time' - such that it 'will hit' some detector not later but earlier (the same amount of time).
I'll show a funny thing - that we can use it ... not using it!
(in fact: using the possibility of using it)

First of all, let's assume some nature of time. 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).
For now I'll assume the most probably (and most 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.
I'll gladly discuss this choice, especially that I have much more arguments.

In this point of view (so called eternalism), there are allowed time-loops, but they have to be stable!
So if there would be such 'device' 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 it, not using it?
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 the next 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 has already chosen properly before the loop.
What if there wasn't any satisfying choice? It would create time paradox?
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, this 'device' could work without actually being used: if for example there had be a successful terrorist attack, there would be sent information to prevent it ... and finally in stable timeline this attack would never happen.

So 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 random protein 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.
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 control ... 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?
As in the example from the beginning - it looks that it would reduce our free will...
Not if we would use it in practice only for prevention - practically not using it ... using only its existence...
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).

What do You think about such hypothetical world?
It would be utopia or a horror...?

2.

3. Originally Posted by Jarek Duda
Let's make some 'thought experiment' to exercise our intuition, think about the nature of physics - imagine we can make backtime information channel - let say for example 'send photon back in time' - such that it 'will hit' some detector not later but earlier (the same amount of time).
I'll show a funny thing - that we can use it ... not using it!
(in fact: using the possibility of using it)

First of all, let's assume some nature of time. 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).
For now I'll assume the most probably (and most 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.
I'll gladly discuss this choice, especially that I have much more arguments.

In this point of view (so called eternalism), there are allowed time-loops, but they have to be stable!
So if there would be such 'device' 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 it, not using it?
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 the next 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 has already chosen properly before the loop.
What if there wasn't any satisfying choice? It would create time paradox?
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, this 'device' could work without actually being used: if for example there had be a successful terrorist attack, there would be sent information to prevent it ... and finally in stable timeline this attack would never happen.

So 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 random protein 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.
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 control ... 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?
As in the example from the beginning - it looks that it would reduce our free will...
Not if we would use it in practice only for prevention - practically not using it ... using only its existence...
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).

What do You think about such hypothetical world?
It would be utopia or a horror...?
I did not even need to read all of this spamillisheous, misleading post. A photon can never go back in time because it moves along a null path. Do you know what that means?

4. Have You heard about the phrase "thought experiment"?
You take some hypothetical situation (like a possibility of transmitting data into the past) and try to create some coherent picture of its consequences - it's kind of brain exercise of creating and analyzing abstract constructs, extremely important for (not only) science oriented persons.
If You don't wanna play - nobody makes You.

Along null path ... but in which direction? Or maybe it's just a path between two points in fourdimensional spacetime? The real question is if reason-result chains of relations can work in both time directions? For this thread let's assume that it can ... it's also suggested for example by CPT conservation.

5. Manynames, you've made very similar posts, so you shouldn't be criticizing this one.

6. Originally Posted by Manynames
I did not even need to read all of this spamillisheous, misleading post. A photon can never go back in time because it moves along a null path. Do you know what that means?
Heck I don't even know what that means. What's a "null path"?

7. Originally Posted by MagiMaster
Manynames, you've made very similar posts, so you shouldn't be criticizing this one.
Except, and usually it takes a good scientist at heart to distinguish the two, this person is using the term ''thought-experiment'' to reshape physics for their own good. I always remain at least within the lines of scientific rigor, and this person is not...

...If you don't mind me saying, but you should stay out of my conversation on physics if:

1)You are calling me a hypocrite

2)You don't even know how to distinguish a workable quantum theory to one that is pulling ideas straight out of the air.

8. Originally Posted by Numsgil
Originally Posted by Manynames
I did not even need to read all of this spamillisheous, misleading post. A photon can never go back in time because it moves along a null path. Do you know what that means?
Heck I don't even know what that means. What's a "null path"?
A null path is what all speed of light particles move along. It is part of the Minkowskian Geometry which explains that a particle does not even experience time, let alone moving backwards in time, as the OP is flouting about. His/her thought-experiment is highly flawed.

9. Originally Posted by Manynames
Originally Posted by MagiMaster
Manynames, you've made very similar posts, so you shouldn't be criticizing this one.
Except, and usually it takes a good scientist at heart to distinguish the two, this person is using the term ''thought-experiment'' to reshape physics for their own good. I always remain at least within the lines of scientific rigor, and this person is not...

...If you don't mind me saying, but you should stay out of my conversation on physics if:

1)You are calling me a hypocrite

2)You don't even know how to distinguish a workable quantum theory to one that is pulling ideas straight out of the air.

Anyway, I think you need to reread both the definition of thought experiment, the definition of assumption, the meaning of proof by contradiction and the post. I haven't read the post in depth, so I can't comment on the conclusions, but I can say that the basic method is fine. There's nothing wrong with assuming something like time travel is possible then working out what the consequences might be. In fact, if by making that assumption, you could rigorously construct a contradiction, you could prove that assumption was wrong. That is, you could prove time travel was impossible. (Note that the assumption doesn't actually violate any known physics. Time travel has not been proven impossible, or anything else for that matter.)

On the other hand, this thought experiment doesn't actually seem to be a thought experiment. I can see no assumptions that form the basis of a thought experiment, but I do see some built in assumptions. It seems like all of your arguments apply at least as well to your own threads. Ok, so this thread isn't perfect either, but at least the assumptions are spelled out in the beginning.

Originally Posted by Manynames
I did not even need to read all of this spamillisheous, misleading post. A photon can never go back in time because it moves along a null path. Do you know what that means?
So yes, I do mind you saying, since you're basically doing exactly what you accuse me of.

10. To prove that time travel is possible/impossible, we have to know the nature of time.
In many world interpretation - it's possible because such travel would just created new alternative timeline.

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 - it's one of reasons I don't like Einstein's view:
http://www.thescienceforum.com/viewtopic.php?t=15841

The other thing is sending information back in time through a channel on which we cannot fully rely.
Physics has full freedom of stabilizing such time-loops (maybe we could even say that it's what QM is mainly for): by using effects below our recognition, like choosing what spin we will measure using above generator, or make some small influences on statistically looking behaviors ... or eventually make that this backtime channel would lie.

So in this picture we cannot send information back to change the past.
Such possibility is already enough to make prevention against time paradoxes - 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.

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 ...
... 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 ...
????

 Bookmarks
##### Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement