Time travel

[ProtoAvatar]

For example - let's say a time traveller wants to kill a person in the past. He knows that his victim originally lived beyond the past day he chose for his assasination attempt. He also has detailed knowledge about the victim's activities in that day and has concieved his assasination plan accordingly.
The time traveller will only be stopped:
if his weapon malfunctions - which is extremely improbable, because, let's say, he checked the weapon in detail prior to the time travel; he could also try to assasinate his victim multiple times with different weapons, until he accomplishes his goal;
if a victim's friend interferes - which did not happen in the original timeline;
if another time traveller interferes - which did not happen in the original timeline;
etc.

But see, that's the thing. According to the Novikov SCP, one of those seemingly improbable things *will* happen, if it's the only way to avert a paradox, and that seemingly improbable thing will have always been part of the timeline. If I understand the SCP correctly, the logic goes something like this:

- Quantum mechanics governs the outcome of events in a probabilistic way. Maybe the probability of a particular particle being observed with a clockwise spin is 50%, while the probability of it being observed with a counterclockwise spin is also 50%.

- If you add time travel into the mix, then certain outcomes will lead to paradoxes....a particular particle having a clockwise spin might cause that particle to collide with itself and prevent itself from time traveling in the first place.

- The universe disallows any of those states (that would lead to paradoxes) from existing. The quantum probabilities of things happening will be skewed in such a way so that only self-consistent timeloops can exist. And the thinking is that, for any set of initial conditions, there is at least *one* self-consistent solution. So no matter what you do, there will be perfect self-consistency, and no paradoxes will arise.

At least, this is my reading of the example with the billiard ball. The billiard ball hits itself at just the right angle, because that's the only way to avoid a paradox.

As I said - in the second edit of my previous post:
"In conclusion, in order to stop the well informed time traveller, the Novikov Consistency Principle has to break either causality (when events from the time traveller's past have changed without a cause) or probability (when extremly improbable events from the traveller's present or future are happening just to stop him)."
As you can see, I'm basically agrreing with you.

However, the Novikov Consistency Principle strikes me as doing exactly what it tries to prevent - breaking causality or probability (by causing ludicrously improbable events to occur) in order to preserve causality.

That's why, in my first post in this thread, I chose a modified version of the many-worlds theory (my modification, that is) as the means to avoid the paradoxes that arise from time travel:
"Personally, I beleive information, at least, can be sent into the past. What happens then?

Let's say a new timeline appears and the old one disappears.
But, in this case, where did the information that changed the past come from? From nowhere? In this case, the information disappears and the old timeline reappears.
But, if the old timeline exists again, then that information also exists. And if the information exists, then the new timeline appears again and the old timeline vanishes.
And so forth.
Essentially, I'm saying that both timelines exist, but consecutively not simultaneously."

 

[ProtoAvatar]

The more I think about the Novikov Consistency Principle vs the well informed time traveller, the more I realise that the Novikov SCP fails in at least one area - keeping the past exactly as it was.

In my example - the well informed time traveller knows that his weapon will malfunction and changes the weapon - the new weapon may not work, but that doesn't change the fact that the time traveller changed the weapon he took with him in the past;
Or the traveller knows that another time traveller will stop him and changes his modus operandi - he may be stopped again, but he changed his actions in the past.

When dealing with a well informed time traveller, the Novikov SCP seems to only be able to preserve the general outcome (preventing paradoxes), not the details of the past.

 

[FrankR]

Your well informed time traveler theory is very intriguing; and I do like it; but I think it needs some refinement, because I think it has an inherent flaw. It requires the TT to have accurate knowledge of everything and everyone the TT will encounter. And that data might not be available, or might be wrong.


-frank

 

[ProtoAvatar]

Your well informed time traveler theory is very intriguing; and I do like it; but I think it needs some refinement, because I think it has an inherent flaw. It requires the TT to have accurate knowledge of everything and everyone the TT will encounter. And that data might not be available, or might be wrong.


-frank

If the time traveller relies only on historical records, he could be lacking the necessary information.

But the time traveller could use his time machine to send in the past, let's say, nano spy devices (if he has the technology to build a time machine, he has the tech to make such undetectable devices) for information gathering purposes prior to his own journey into the past.
The well informed time traveller will travel back in time only after he has analysed the information gathered by the spy devices and knows in detail the events he intends to change.

Or, perhaps, the time machine itself allows him to witness the events of the past from his present.

In any case, gathering accurate information about the past shouldn't be a problem for someone with a time machine.

 

[chrisspringob]

The more I think about the Novikov Consistency Principle vs the well informed time traveller, the more I realise that the Novikov SCP fails in at least one area - keeping the past exactly as it was.

In my example - the well informed time traveller knows that his weapon will malfunction and changes the weapon - the new weapon may not work, but that doesn't change the fact that the time traveller changed the weapon he took with him in the past;
Or the traveller knows that another time traveller will stop him and changes his modus operandi - he may be stopped again, but he changed his actions in the past.

When dealing with a well informed time traveller, the Novikov SCP seems to only be able to preserve the general outcome (preventing paradoxes), not the details of the past.

No, see, you still seem to be describing things as if, when I go back in time to 2008, the universe "rewinds the tape" back to 2008, and then things play out in 2008 again, but that the SCP is making sure that everything plays out the same or similarly to how it did the first time around.

But there is no "first" or "second" time around. There is no rewinding of the tape. The idea that Novikov had was that 2008 would only play out once. Time would play out as normal, but then, say, on April 23rd, 2008, a future version of me would suddenly pop into existence, seemingly out of nowhere. This would happen because of the fact that, one year later, I would end up going back in time. The universe "knows" that this is going to happen, even though it hasn't happened yet.

After I arrive in 2008, there's nothing I can do to "change" anything in 2008, even in the minor details. Because the whole concept of "changing" anything makes no sense. There is no "original" version of 2008, from which to change things, so what does "changing" something even mean? There's only the one version of 2008, which includes whatever actions I take while I'm there.

But you're probably going to say "OK, you can't exactly change things in the strict sense, but, when you're in the past, you have information from the future. You can intentionally make sure that what happens doesn't match with the information you have, at least in terms of minor details, like how many times you loaded your gun." But the thing is, no you can't. Something would stop you. If you try to make things play out differently from the information you have from the future, either something would stop you from doing so, or it would turn out that your information from the future wasn't accurate (or meant something different from what you thought).

Now, one could imagine an extreme case, where you've set things up such that the very act of time traveling automatically creates a paradox. (Say, I send a bomb back in time to yesterday, to blow up my time machine before I can use it, and the bomb automatically goes off as soon as it arrives in the past, and there's no way for it to fail.) In that case, the time machine itself would fail. Something would go wrong to prevent the time travel from happening at all. At least, that's my reading of the Novikov SCP.

 

[Anticitizen]

As the past and future illusions are real when we let them we know the now that is ever textured and fictional frictions of surface and submarines that can make more of the ever lived life tenses and tones altered by blinking in and out of bubbling morphisms in tandem to the view that makes not much of a difference yet is different still.

"A casual stroll through the lunatic asylum shows that faith does not prove anything" - Friedrich Nietzsche

I believe faith is possible and that Friedrich was a lier but still a good quote who believes the mad men of societies casting ., speaking of proving something not that it really matters the proof is a blue green shade that shadows the lights even after they go out.

but why is it so beautiful why why why ., why not let it be beautiful ., so what.

You lost me.

 

[ProtoAvatar]

As the past and future illusions are real when we let them we know the now that is ever textured and fictional frictions of surface and submarines that can make more of the ever lived life tenses and tones altered by blinking in and out of bubbling morphisms in tandem to the view that makes not much of a difference yet is different still.

"A casual stroll through the lunatic asylum shows that faith does not prove anything" - Friedrich Nietzsche

I believe faith is possible and that Friedrich was a lier but still a good quote who believes the mad men of societies casting ., speaking of proving something not that it really matters the proof is a blue green shade that shadows the lights even after they go out.

but why is it so beautiful why why why ., why not let it be beautiful ., so what.

You lost me.

I half beleive that think has a typing monkey as a pet, and that monkey wrote this particular post.

 

[think]

As the past and future illusions are real when we let them we know the now that is ever textured and fictional frictions of surface and submarines that can make more of the ever lived life tenses and tones altered by blinking in and out of bubbling morphisms in tandem to the view that makes not much of a difference yet is different still.

"A casual stroll through the lunatic asylum shows that faith does not prove anything" - Friedrich Nietzsche

I believe faith is possible and that Friedrich lied about things but still a good quote who believes the mad men of societies casting ., speaking of proving something not that it really matters the proof is a blue green shade that shadows the lights even after they go out.

but why is it so beautiful why why why ., why not let it be beautiful ., so what.

You lost me.

I half beleive that think has a typing monkey as a pet, and that monkey wrote this particular post.

thanks proto and citizen if you smile that is all that matters yes the monkey was on my back I dare say It ate my mind whole and then indulged in yours as well but hey some jokes are more funny then others..

remember the day you thought the world was like brighter then usual ?? what ever but I was reassembling my registers and thought wave banks for the that moment and was not quite on top of things what ever the things that were are.

 

[ProtoAvatar]

The more I think about the Novikov Consistency Principle vs the well informed time traveller, the more I realise that the Novikov SCP fails in at least one area - keeping the past exactly as it was.

In my example - the well informed time traveller knows that his weapon will malfunction and changes the weapon - the new weapon may not work, but that doesn't change the fact that the time traveller changed the weapon he took with him in the past;
Or the traveller knows that another time traveller will stop him and changes his modus operandi - he may be stopped again, but he changed his actions in the past.

When dealing with a well informed time traveller, the Novikov SCP seems to only be able to preserve the general outcome (preventing paradoxes), not the details of the past.

After I arrive in 2008, there's nothing I can do to "change" anything in 2008, even in the minor details. Because the whole concept of "changing" anything makes no sense. There is no "original" version of 2008, from which to change things, so what does "changing" something even mean? There's only the one version of 2008, which includes whatever actions I take while I'm there.

But you're probably going to say "OK, you can't exactly change things in the strict sense, but, when you're in the past, you have information from the future. You can intentionally make sure that what happens doesn't match with the information you have, at least in terms of minor details, like how many times you loaded your gun." But the thing is, no you can't. Something would stop you. If you try to make things play out differently from the information you have from the future, either something would stop you from doing so, or it would turn out that your information from the future wasn't accurate (or meant something different from what you thought).

Now, one could imagine an extreme case, where you've set things up such that the very act of time traveling automatically creates a paradox. (Say, I send a bomb back in time to yesterday, to blow up my time machine before I can use it, and the bomb automatically goes off as soon as it arrives in the past, and there's no way for it to fail.) In that case, the time machine itself would fail. Something would go wrong to prevent the time travel from happening at all. At least, that's my reading of the Novikov SCP.

About gathering detailed information about the past:
"If the time traveller relies only on historical records, he could be lacking the necessary information.
But the time traveller could use his time machine to send in the past, let's say, nano spy devices (if he has the technology to build a time machine, he has the tech to make such undetectable devices) for information gathering purposes prior to his own journey into the past.
The well informed time traveller will travel back in time only after he has analysed the information gathered by the spy devices and knows in detail the events he intends to change.
Or, perhaps, the time machine itself allows him to witness the events of the past from his present.
In any case, gathering accurate information about the past shouldn't be a problem for someone with a time machine."

How can the Novikov Consistency Principle stop the well informed time traveler from changing the past?
As I've already mentioned (twice):
"In conclusion, in order to stop the well informed time traveller, the Novikov Consistency Principle has to break either causality (when events from the time traveller's past have changed without a cause) or probability (when extremly improbable events from the traveller's present or future are happening just to stop him)."

In order for the Novikov Consistency Principle to work, we have to assume that, for every attempt the time traveller makes to change history, there is a solution, a rearranging of events, that prevents him from changing the past.

When it comes to preventing paradoxes, I'm willing to accept this rather big assumption.

When it comes to keeping the past exactly as it was, no.
Why?
Because breaking causality and probability have their limits.

About breaking causality - this can't stop the well informed time traveller from, for example, bringing a different gun, because by the time the time traveller is in his past, he already has a different gun.

And breaking the laws of probability has its limits, too.
A quantum particle can have a clockwise spin, a counterclockwise spin or can be fuzzy. There is no forth option. The Novikov SCP would need a fourth, fifth and sixth option in order to keep the past exactly as it was when dealing with a well informed time traveller.
For example, how can the Novikov SCP stop a time traveller from changing his modus operandi? There is no option, no matter how improbable, that can stop a time traveller who knows exactly what he did form doing some details differently.

And if there is the first and second version of the past, there is a "rewinding of the tape".

 

[ProtoAvatar]

You lost me.

I half beleive that think has a typing monkey as a pet, and that monkey wrote this particular post.

thanks proto and citizen if you smile that is all that matters yes the monkey was on my back I dare say It ate my mind whole and then indulged in yours as well but hey some jokes are more funny then others..

remember the day you thought the world was like brighter then usual ?? what ever but I was reassembling my registers and thought wave banks for the that moment and was not quite on top of things what ever the things that were are.

Think, you have neither reason nor right to be upset. When you write posts such as yours, you must expect and accept responses like mine and Anticitizen's.

 

[chrisspringob]

And breaking the laws of probability has its limits, too.
A quantum particle can have a clockwise spin, a counterclockwise spin or can be fuzzy. There is no forth option. The Novikov SCP would need a fourth, fifth and sixth option in order to keep the past exactly as it was when dealing with a well informed time traveller.
For example, how can the Novikov SCP stop a time traveller from changing his modus operandi? There is no option, no matter how improbable, that can stop a time traveller who knows exactly what he did form doing some details differently.

And if there is the first and second version of the past, there is a "rewinding of the tape".

But that's the thing. The tape never gets rewound. Yesterday only happened once. It doesn't happen again. Therefore, the whole concept of yesterday "changing" makes no sense according to the Novikov SCP. "Changing" from what to what?

You say that there are limits to how much probability can be bent, but the advocates of the SCP say that no, there is always a self-consistent solution that involves no paradoxes, regardless of what initial conditions you set up. In the most extreme case, it would involve the subatomic particles in the time machine breaking the time machine apart (because of the particles spontaneously shifting to a certain state that causes that to happen) before you can use it. That's allowed under quantum mechanics. It's just extremely unlikely. But if that's the only way to avoid a paradox, then that's what would happen.

Look, I'm not saying that you can't believe that your own idea for how time travel would work is better. But ultimately, we have no idea what would happen, because we don't have a working time machine, and are unlikely to get one any time soon. But we can talk about which ideas have a logically consistent framework and which don't. The Novikov self-consistency principle seems logically consistent to me, and I don't see any holes in it. Of course, if you think a different logically consistent framework is better, that's fine.

 

[think]

I half beleive that think has a typing monkey as a pet, and that monkey wrote this particular post.

thanks proto and citizen if you smile that is all that matters yes the monkey was on my back I dare say It ate my mind whole and then indulged in yours as well but hey some jokes are more funny then others..

remember the day you thought the world was like brighter then usual ?? what ever but I was reassembling my registers and thought wave banks for the that moment and was not quite on top of things what ever the things that were are.

Think, you have neither reason nor right to be upset. When you write posts such as yours, you must expect and accept responses like mine and Anticitizen's.

that is the primary difference between in person conversations and these forums you read I was upset I put that Icon there as something that had its brain eaten by a monkey right I do apologize if you thought I was upset indeed you are right the icon is an upset one because its brain/skull is eaten thru : me I am in a good mood not upset but gland I could get a serious problem topic "time travel" looked at humorously :P

 

[ProtoAvatar]

thanks proto and citizen if you smile that is all that matters yes the monkey was on my back I dare say It ate my mind whole and then indulged in yours as well but hey some jokes are more funny then others..

remember the day you thought the world was like brighter then usual ?? what ever but I was reassembling my registers and thought wave banks for the that moment and was not quite on top of things what ever the things that were are.

Think, you have neither reason nor right to be upset. When you write posts such as yours, you must expect and accept responses like mine and Anticitizen's.

that is the primary difference between in person conversations and these forums you read I was upset I put that Icon there as something that had its brain eaten by a monkey right I do apologize if you thought I was upset indeed you are right the icon is an upset one because its brain/skull is eaten thru : me I am in a good mood not upset but gland I could get a serious problem topic "time travel" looked at humorously :P

Cool.

 

[ProtoAvatar]

But that's the thing. The tape never gets rewound. Yesterday only happened once. It doesn't happen again. Therefore, the whole concept of yesterday "changing" makes no sense according to the Novikov SCP. "Changing" from what to what?

You say that there are limits to how much probability can be bent, but the advocates of the SCP say that no, there is always a self-consistent solution that involves no paradoxes, regardless of what initial conditions you set up. In the most extreme case, it would involve the subatomic particles in the time machine breaking the time machine apart (because of the particles spontaneously shifting to a certain state that causes that to happen) before you can use it. That's allowed under quantum mechanics. It's just extremely unlikely. But if that's the only way to avoid a paradox, then that's what would happen.

Look, I'm not saying that you can't believe that your own idea for how time travel would work is better. But ultimately, we have no idea what would happen, because we don't have a working time machine, and are unlikely to get one any time soon. But we can talk about which ideas have a logically consistent framework and which don't. The Novikov self-consistency principle seems logically consistent to me, and I don't see any holes in it. Of course, if you think a different logically consistent framework is better, that's fine.

As I have proven earlier, there are limits to how much probability can be bent. For example, the subatomic particles in the time machine could break the machine apart by changing state only if there can be such a state, according to quantum mechanics. The machine may well be built so that this state cannot exist.

What you are saying is that, in order for the Novikov SCP to keep the past exactly as it was, it has to break not "only" causality and probability, but the laws of physics themselves - Entropy, QED, Relativity, etc. And I'm not talking about the laws of physics as humanity understands them - this understanding is incomplete/may be incorect - but as they exist and govern the universe.

The price of Novikov SCP's logical consistency is too high.

That's why I don't accept this theory - expecially considering that there are alternative theories as to how paradoxes via time travel can be avoided, theories that don't involve breaking every single law of physics, causality, and probability.

On the other hand, in my opinion, the past could be kept paradoxes-free by breaking "only" probability and causality - in this case, though, the details of the past could be changed (as long as no paradoxes arise). This variant of the Novikov SCP is much more acceptable - and is logically consistent.

 

[chrisspringob]

As I have proven earlier, there are limits to how much probability can be bent. For example, the subatomic particles in the time machine could break the machine apart by changing state only if there can be such a state, according to quantum mechanics. The machine may well be built so that this state cannot exist.

I'm not sure that that's true. Quantum mechanics allows for a lot of crazy things to happen. It's perfectly possible, say, for a cloud of virtual particles to pop into existence around my computer and annihilate it with matter-anti-matter reactions right now. It's really *really* improbable, but quantum mechanics allows that it *could* happen. I don't know that it's really possible to construct my computer in such a way that that can't happen.

But anyway, I didn't do this research myself. I'm just saying that advocates of the Novikov SCP published some papers in which they claimed to prove that *any* set of initial conditions has some kind of self-consistent solution. I'm not really qualified to render judgment on whether they're right or not. I'm just saying, it isn't obvious to me that they're wrong.

 

[ProtoAvatar]

As I have proven earlier, there are limits to how much probability can be bent. For example, the subatomic particles in the time machine could break the machine apart by changing state only if there can be such a state, according to quantum mechanics. The machine may well be built so that this state cannot exist.

I'm not sure that that's true. Quantum mechanics allows for a lot of crazy things to happen. It's perfectly possible, say, for a cloud of virtual particles to pop into existence around my computer and annihilate it with matter-anti-matter reactions right now. It's really *really* improbable, but quantum mechanics allows that it *could* happen. I don't know that it's really possible to construct my computer in such a way that that can't happen.

But anyway, I didn't do this research myself. I'm just saying that advocates of the Novikov SCP published some papers in which they claimed to prove that *any* set of initial conditions has some kind of self-consistent solution. I'm not really qualified to render judgment on whether they're right or not. I'm just saying, it isn't obvious to me that they're wrong.

"A lot of crazy things" does not mean an infinity of crazy things. There are limits.

The Novikov SCP would need to go way beyond those limits in order to keep the past exactly as it is, in every single detail, when a well informed time traveller tries to change history. In other words, as I have said:
"in order for the Novikov SCP to keep the past exactly as it was, it has to break not "only" causality and probability, but the laws of physics themselves - Entropy, QED, Relativity, etc. And I'm not talking about the laws of physics as humanity understands them - this understanding is incomplete/may be incorect - but as they exist and govern the universe."

For example, virtual particles exist around/in your PC and my PC and everywhere else at this very moment. But they are ephermal - they exist only for a moment before they dissapear - and they cannot destroy your PC. A virtual particle can become stable - can exist for a prolonged period of time -, at the event horizon of a singularity (according to Hawking), but, in this case, it extracts the necessary energy from the singularity. As you can see, even in quantum mechanics, there are rules, there are limitations.

Novikov SCP advocates proved that the SCP can stop paradoxes from occuring by bending probability - and perhaps causality? It's possible - I haven't read their proofs either, though.

Novikov SCP advocates proved that the SCP can keep the past completely unchanged, by bending "only" probability and causality, when a well informed time traveller tries to change the past? Doubtful - but you're welcome to prove me wrong.

 

[Noname Given]

The more I think about the Novikov Consistency Principle vs the well informed time traveller, the more I realise that the Novikov SCP fails in at least one area - keeping the past exactly as it was.

In my example - the well informed time traveller knows that his weapon will malfunction and changes the weapon - the new weapon may not work, but that doesn't change the fact that the time traveller changed the weapon he took with him in the past;
Or the traveller knows that another time traveller will stop him and changes his modus operandi - he may be stopped again, but he changed his actions in the past.

When dealing with a well informed time traveller, the Novikov SCP seems to only be able to preserve the general outcome (preventing paradoxes), not the details of the past.

No, see, you still seem to be describing things as if, when I go back in time to 2008, the universe "rewinds the tape" back to 2008, and then things play out in 2008 again, but that the SCP is making sure that everything plays out the same or similarly to how it did the first time around.

But there is no "first" or "second" time around. There is no rewinding of the tape. The idea that Novikov had was that 2008 would only play out once. Time would play out as normal, but then, say, on April 23rd, 2008, a future version of me would suddenly pop into existence, seemingly out of nowhere. This would happen because of the fact that, one year later, I would end up going back in time. The universe "knows" that this is going to happen, even though it hasn't happened yet.

After I arrive in 2008, there's nothing I can do to "change" anything in 2008, even in the minor details. Because the whole concept of "changing" anything makes no sense. There is no "original" version of 2008, from which to change things, so what does "changing" something even mean? There's only the one version of 2008, which includes whatever actions I take while I'm there.

But you're probably going to say "OK, you can't exactly change things in the strict sense, but, when you're in the past, you have information from the future. You can intentionally make sure that what happens doesn't match with the information you have, at least in terms of minor details, like how many times you loaded your gun." But the thing is, no you can't. Something would stop you. If you try to make things play out differently from the information you have from the future, either something would stop you from doing so, or it would turn out that your information from the future wasn't accurate (or meant something different from what you thought).

Now, one could imagine an extreme case, where you've set things up such that the very act of time traveling automatically creates a paradox. (Say, I send a bomb back in time to yesterday, to blow up my time machine before I can use it, and the bomb automatically goes off as soon as it arrives in the past, and there's no way for it to fail.) In that case, the time machine itself would fail. Something would go wrong to prevent the time travel from happening at all. At least, that's my reading of the Novikov SCP.

I think a simpler way to look at it is this (assuming one universaltimeline):
---------------------------------------------------------------------
Assuming you could travel back in time - it would mean you were ALREADY a part of what happened on the date you travelled back to. In the case of say, "I'm going to kill Hitler before he comes to power.."; it's NOT that the 'timeline knows and somehow prevents you'; it's that you were already there originally and couldn't get it done, becauase if you had done it successfully; history wouldn't have unfolded the way it in fact did. I hink IF time travel is possible, the reason things can't be changed is they unfolded the way they did with the respective time travcellers already involved with what occured.

 

[Ronald Held]

Your are talking of a closed time loop?

 

[chrisspringob]

Assuming you could travel back in time - it would mean you were ALREADY a part of what happened on the date you travelled back to. In the case of say, "I'm going to kill Hitler before he comes to power.."; it's NOT that the 'timeline knows and somehow prevents you'; it's that you were already there originally and couldn't get it done, becauase if you had done it successfully; history wouldn't have unfolded the way it in fact did. I hink IF time travel is possible, the reason things can't be changed is they unfolded the way they did with the respective time travcellers already involved with what occured.

That's right. You explained it more succinctly than I could. The point is that, if your time traveling to the past is already part of history, then the concept of "changing" the timeline is meaningless. Anything you did in the past has already happened, and is already a part of history, even before the actual act of going back in time takes place, so you wouldn't actually be changing anything by going back in time.

 

[ProtoAvatar]

Assuming you could travel back in time - it would mean you were ALREADY a part of what happened on the date you travelled back to. In the case of say, "I'm going to kill Hitler before he comes to power.."; it's NOT that the 'timeline knows and somehow prevents you'; it's that you were already there originally and couldn't get it done, becauase if you had done it successfully; history wouldn't have unfolded the way it in fact did. I hink IF time travel is possible, the reason things can't be changed is they unfolded the way they did with the respective time travcellers already involved with what occured.

That's right. You explained it more succinctly than I could. The point is that, if your time traveling to the past is already part of history, then the concept of "changing" the timeline is meaningless. Anything you did in the past has already happened, and is already a part of history, even before the actual act of going back in time takes place, so you wouldn't actually be changing anything by going back in time.

Yes - according to the Novikov SCP, a time traveller couldn't change the past because the past already "contains" his temporal intervention.

The problem is, a well informed time traveller can be stopped from "modifying" his temporal intervention only if probability, causality and the laws of physics are broken for this purpose.