Alice: Okay, it’s almost noon, and we’ve been sitting alone in this room for some hours now without seeing Carol, and this plan has been in place since last night. Bob, you wait until 6pm, and then check to see if the enemy has reached the pass yet. If they haven’t, come back and tell me. But if they have, stay when you are—and if I don’t hear from you at noon, I’ll go back to 6am and tell Carol.
And then Carol-at-6am has information about whether the enemy has reached the pass at 6pm?
Yeah. You can actually make arbitrarily long chains. You have each person go back in time and stop the next person if they’re not stopped. You “start” the chain at the end, and depending on when you do it, you can send back one bit. For example, you give Alice, Bob, Carol, and Daniel time turners. At midnight, Alice goes back to stop Daniel if Bob doesn’t stop her, at 6:00 AM, Bob goes back to stop Alice unless Carol stops him, etc. If the enemy attacks on the night of the 28th, Daniel stops Carol. If they don’t, he doesn’t. This means that if they attack, Daniel and Bob go back every day. If he does, Carol and Alice go back. You’d actually need a fifth person to make up for the fact that none of this is instantaneous, but if you have enough time-turners, you can send arbitrarily long messages arbitrarily far into the past.
You don’t actually know that Bob didn’t see the enemy at the pass, you only know that for some reason, Bob didn’t come back and tell you. Perhaps the reason he didn’t is because you would have sent that information back in time, and so he couldn’t.
Another possibility is that information loses “coherence” the further back it travels. (or forward, depending on which side your standing on) Think of it as a signal to noise problem—six hours isn’t the limit, it’s the limit of what we can correct for with the magic of the time turners. Prophecy seems to defeat the limit, but only by being nearly incomprehensible.
Or maybe it is possible, but insanely dangerous. There are hints that Atlantis was destroyed by something involving the time stream.
Perhaps the reason he didn’t is because you would have sent that information back in time, and so he couldn’t.
But every time someone uses a time turner, they send that information into the past. If it didn’t block them then, why would it block them now?
Another possibility is that information loses “coherence” the further back it travels.
There are ways of fixing that. For example, you could send people back in groups of three. Then you have them go back unless they’re stopped by at least two people.
Or maybe it is possible, but insanely dangerous.
That’s possible. The longer the time stream, the more likely that the closed time loop you end up with involves a hurricane or worse. I believe there was a book where the world ended because someone didn’t think about that. You could prevent it by allowing a “maybe”, so long as you make it likely enough that something you didn’t think of doesn’t become more likely.
Perhaps the reason he didn’t is because you would have sent that information back in time, and so he couldn’t.
But every time someone uses a time turner, they send that information into the past. If it didn’t block them then, why would it block them now?
Because you would have sent that information back in time. It didn’t block them “then” because they weren’t going to send the information further back. The effect could be more subtle—instead of preventing you from succeeding, it could prevent you from trying (don’t mess with time) or even make you not think of trying.
Another possibility is that information loses “coherence” the further back it travels.
There are ways of fixing that.
No, you can’t “fix” it, you can only reduce the effect. If a signal is weak, you can amplify it. But that only works up to a point. And apparently, that point is six hours, even with magical amplification and correction.
I believe there was a book where the world ended because …
I remember a short story by Larry Niven—Rotating cylinders and the possibility of global causality violation. The short story first appeared in Analog, was reprinted in CONVERGENT SERIES, and it contains the immortal line “I imagine the sun has gone nova”. Because the universe protects its cause-and-effect basis with humorless ferocity.
It didn’t block them “then” because they weren’t going to send the information further back.
They weren’t planning on it, but the information was sent nonetheless. P(Someone is going to go back and stop them from going back|They came back) < P(Someone is going to go back and stop them from going back|They did not came back)
But that only works up to a point.
Not really. The amount of time you can send back increases exponentially with the number of people sent back. If you only get it right a third of the time, sending one guy back only works a third of the time, but sending a hundred people back, you’d get about 67 +- 5 people sending the right bit, and you’d get it right about 99.98% of the time. If you have two hundred people, you’d get it right about 0.9999997% of the time.
They weren’t planning on it, but the information was sent nonetheless. P(Someone is going to go back and stop them from going back|They came back) < P(Someone is going to go back and stop them from going back|They did not came back)
That presupposes that P(Bob came back) is not affected by your decision to send the information further on.
I’m postulating that IF you would have sent the information further back, THEN P(Bob came back) = 0. Of course, it might not actually work that way, but if my supposition is correct, then Bob not coming back tells you nothing. The event only carries information if you aren’t going to make use of that information.
That presupposes that P(Bob came back) is not affected by your decision to send the information further on.
No. I gave an example in which it was not decided to send information back. It’s simply impossible to go back in time without proving that you weren’t killed by a time-travelling assassin.
So you’re thinking of something like this?
Alice: Okay, it’s almost noon, and we’ve been sitting alone in this room for some hours now without seeing Carol, and this plan has been in place since last night. Bob, you wait until 6pm, and then check to see if the enemy has reached the pass yet. If they haven’t, come back and tell me. But if they have, stay when you are—and if I don’t hear from you at noon, I’ll go back to 6am and tell Carol.
And then Carol-at-6am has information about whether the enemy has reached the pass at 6pm?
Yeah. You can actually make arbitrarily long chains. You have each person go back in time and stop the next person if they’re not stopped. You “start” the chain at the end, and depending on when you do it, you can send back one bit. For example, you give Alice, Bob, Carol, and Daniel time turners. At midnight, Alice goes back to stop Daniel if Bob doesn’t stop her, at 6:00 AM, Bob goes back to stop Alice unless Carol stops him, etc. If the enemy attacks on the night of the 28th, Daniel stops Carol. If they don’t, he doesn’t. This means that if they attack, Daniel and Bob go back every day. If he does, Carol and Alice go back. You’d actually need a fifth person to make up for the fact that none of this is instantaneous, but if you have enough time-turners, you can send arbitrarily long messages arbitrarily far into the past.
You don’t actually know that Bob didn’t see the enemy at the pass, you only know that for some reason, Bob didn’t come back and tell you. Perhaps the reason he didn’t is because you would have sent that information back in time, and so he couldn’t.
Another possibility is that information loses “coherence” the further back it travels. (or forward, depending on which side your standing on) Think of it as a signal to noise problem—six hours isn’t the limit, it’s the limit of what we can correct for with the magic of the time turners. Prophecy seems to defeat the limit, but only by being nearly incomprehensible.
Or maybe it is possible, but insanely dangerous. There are hints that Atlantis was destroyed by something involving the time stream.
But every time someone uses a time turner, they send that information into the past. If it didn’t block them then, why would it block them now?
There are ways of fixing that. For example, you could send people back in groups of three. Then you have them go back unless they’re stopped by at least two people.
That’s possible. The longer the time stream, the more likely that the closed time loop you end up with involves a hurricane or worse. I believe there was a book where the world ended because someone didn’t think about that. You could prevent it by allowing a “maybe”, so long as you make it likely enough that something you didn’t think of doesn’t become more likely.
Because you would have sent that information back in time. It didn’t block them “then” because they weren’t going to send the information further back. The effect could be more subtle—instead of preventing you from succeeding, it could prevent you from trying (don’t mess with time) or even make you not think of trying.
No, you can’t “fix” it, you can only reduce the effect. If a signal is weak, you can amplify it. But that only works up to a point. And apparently, that point is six hours, even with magical amplification and correction.
I remember a short story by Larry Niven—Rotating cylinders and the possibility of global causality violation. The short story first appeared in Analog, was reprinted in CONVERGENT SERIES, and it contains the immortal line “I imagine the sun has gone nova”. Because the universe protects its cause-and-effect basis with humorless ferocity.
They weren’t planning on it, but the information was sent nonetheless. P(Someone is going to go back and stop them from going back|They came back) < P(Someone is going to go back and stop them from going back|They did not came back)
Not really. The amount of time you can send back increases exponentially with the number of people sent back. If you only get it right a third of the time, sending one guy back only works a third of the time, but sending a hundred people back, you’d get about 67 +- 5 people sending the right bit, and you’d get it right about 99.98% of the time. If you have two hundred people, you’d get it right about 0.9999997% of the time.
That presupposes that P(Bob came back) is not affected by your decision to send the information further on. I’m postulating that IF you would have sent the information further back, THEN P(Bob came back) = 0. Of course, it might not actually work that way, but if my supposition is correct, then Bob not coming back tells you nothing. The event only carries information if you aren’t going to make use of that information.
No. I gave an example in which it was not decided to send information back. It’s simply impossible to go back in time without proving that you weren’t killed by a time-travelling assassin.