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Still Waters

Time travel theory avoids grandfather paradox

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The possibility of going back in time only to kill your ancestors and prevent your own birth has posed a serious problem for potential time travelers, not even considering the technical details of building a time machine. But a new theory proposed by physicists at MIT suggests that this grandfather paradox could be avoided by using quantum teleportation and "post-selecting" what a time traveler could and could not do.So while murdering one’s relatives is unfortunately possible in the present time, such actions would be strictly forbidden if you were to try them during a trip to the past.

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Couldn't you also post-select to kill your grandfather?

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I still don't get what's so paradoxical about it. So what if you no longer exist after you kill your grandfather? If you get hit by a hammer, and the hammer then disappears, you've still been hit by a hammer.

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I still don't get what's so paradoxical about it. So what if you no longer exist after you kill your grandfather? If you get hit by a hammer, and the hammer then disappears, you've still been hit by a hammer.

I see your point.

I think the paradox is, after you go back in time and kill your grandfather, you were never born, so how could you have gone back in time and kill your grandfather? Say you were born in 1950 and you went back to 1910 and kill grampa, if it's the same timeline, as time goes by to 1950 you're now not alive to go back and kill the old man. In a sense though, so what if you're not now alive in 1950? The deed was done when you were alive when you went back and killed him.

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Who cares when or where the hammer was made? You still got hit.

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I still don't get what's so paradoxical about it. So what if you no longer exist after you kill your grandfather? If you get hit by a hammer, and the hammer then disappears, you've still been hit by a hammer.

how ever if you get hit by the hammer and then it disappears before it hit you, did you get hit by the hammer.

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If you hadn't got hit by the hammer, it wouldn't have hit you.

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If you hadn't got hit by the hammer, it wouldn't have hit you.

but if you get hit by the hammer and then go back two days and destroy that hammer how could the hammer have hit you. this is the pardox.

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Why would that matter? You got hit by a hammer in the past. If you go back to the past and remove the hammer, you did not get hit. If you go back to the past to get a cheeseburger, and incidentally happen to remove the hammer, you still did not get hit. Why you went back to the past is irrelevant to whether you got hit or not.

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Why would that matter? You got hit by a hammer in the past. If you go back to the past and remove the hammer, you did not get hit. If you go back to the past to get a cheeseburger, and incidentally happen to remove the hammer, you still did not get hit. Why you went back to the past is irrelevant to whether you got hit or not.

now the grandfather affect. you go back in time to before your father was conceived and kill your grandfather.

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You are now guilty of grandpatericide. Unless the prosecution can place you at 1936, or whenever the murder took place, you'll get off scot-free.

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I prefere the theory (hypothesis?) that because the time traveller and his machine travels outside of time, they are not affected by changes to the past (their current present) that would change things for the future self. Though their future self will be affected.

But that has the problem of being able to add an almost infinite amount of matter and energy to the universe. Since the time traveler goes back, and performs an action that makes his future self travel back in time again, but travels to the point next to where the present time traveller is in the past. Creating 2 time travellers. These 2 time travellers then perform an action that causes their future self to travel back in time again, but travels to a point next to where the present time travellers are in the past. Now there are 3. :lol:

Of course that goes against the laws of thermodynamics. But I'd doubt they'd stay relavent when it comes to timetravel.

(Oh and I have no clue whether or not "the traveller exists outside of time while traveling" makes any sense, it was just used to justify that alterations to his future self would neither affect him while in the past, nor if he returned to an altered future/present.)

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I like the theory of multiple realities to explain the grandfather paradox. If you travel back in time and kill your grandfather, then that consequence would lead to you not being born in an alternate reality. Therefore, you would still exist in the original reality that you first traveled back in time, yet not exist in the alternate reality that you created.

This is all purely theoretical, of course.

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I don't even see the need for separate timelines. After all, cause and effect only applies when you are dealing with linear time. If you are time traveling back and forth, you aren't dealing with linear time anymore, and cause and effect no longer apply.

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You are now guilty of grandpatericide. Unless the prosecution can place you at 1936, or whenever the murder took place, you'll get off scot-free.

sorry you never existed to go back to kill your granddad.

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sorry you never existed to go back to kill your granddad

You had to exist before you killed your grandad because you killed him, only after you killed him do you not exist.

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You had to exist before you killed your grandad because you killed him, only after you killed him do you not exist.

how could you have existed your dad was never born. if you never existed how could you go back in time to kill your granddad. so your granddad had your dad you were born and then went back in time to kill your granddad.

if you think that is bad, there is a pair of glasses in star trek that was never built.

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I hope this guy never builds a stupid time machine. Would save us alot of trouble thinking about it. How about someone who doesn't exist yet saves his granddad from stepping in front of a train, and the granddad lives and has another child who grows up to be this guy? It's the same stupid thing but kind of in reverse.

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how could you have existed your dad was never born. if you never existed how could you go back in time to kill your granddad. so your granddad had your dad you were born and then went back in time to kill your granddad.

Simple. Use a time machine.

Again, once you leave linear time, you need to forget about cause and effect. Cause and effect only matter in linear time.

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I don't even see the need for separate timelines. After all, cause and effect only applies when you are dealing with linear time. If you are time traveling back and forth, you aren't dealing with linear time anymore, and cause and effect no longer apply.

I understand the point you are trying to make.

However, we really don't know whether or not linear time would no longer apply to time travel. Timespace would have to "correct" itself somehow by doing something. Especially, if the person who killed their grandfather tried to go back to the future in which he came from. Someone cannot exist and non-exist at the same time on the same timeline. Either you would come back to a world that has no conception of who you are, since you never existed, or you could never go back to that time because, if you don't exist, you never entered the time machine in the first place. Sure, in the future from which you came you went through the time machine, but the act of you not being born negates that, unless there are alternate timelines. Perhaps, time can be "fragmented" to the point where the actions that you do while time traveling would have no effect on the future whatsoever.

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However, we really don't know whether or not linear time would no longer apply to time travel.

How could it? If it did, it would be a paradox. Ergo, it likely does not.

Timespace would have to "correct" itself somehow by doing something.

Why? It's a little bit like saying a river has to correct itself every time I jump in.

Especially, if the person who killed their grandfather tried to go back to the future in which he came from.

Again, why would time care where you came from? It doesn't care where the hammer came from.

Someone cannot exist and non-exist at the same time on the same timeline.

Of course not. If you are in the timeline, you exist. If you are not, you don't.

Either you would come back to a world that has no conception of who you are, since you never existed, or you could never go back to that time because, if you don't exist, you never entered the time machine in the first place. Sure, in the future from which you came you went through the time machine, but the act of you not being born negates that, unless there are alternate timelines.

I suspect that Time doesn't care a great deal about human paperwork.

Perhaps, time can be "fragmented" to the point where the actions that you do while time traveling would have no effect on the future whatsoever.

Now THAT is an interesting idea!

We tend to think of time as a fragile and delicate thing, like a house of cards. It may well be a bit more hardy than we give it credit for.

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Posted (edited)

How could it? If it did, it would be a paradox. Ergo, it likely does not.

You are right. Though stranger things could be possible.

Why? It's a little bit like saying a river has to correct itself every time I jump in.

This is a good analogy and it could be used to explain the multiple universe or the "fragmentation" theory I brought up. A river is, as you know, just a bunch of water molecules traveling together in the same direction. When they come against an object, like a person jumping in, the water doesn't go through or stop completely, but it goes around. The object divides the water that is traveling in a stream for a moment, then the water diverges again afterwards when it passes by. Going by this analogy, time could not be changed permanently because it will always go on the same path as before. This would kind of go with the theory of fragmentation more so than the alternate universe theory, since the effect on time would only be semi-permanent.

Edited by H.H. Holmes

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This is a very loose comparison, but it was an interesting thought, nonetheless.

From first-hand accounts from people who have used psychadelic drugs, such as LSD-25 or Psilocybin, they sometimes experienced time much like a movie reel. They could see their past, present, and future through each frame going in the order of time (past to future). This is interesting because each moment is it's own frame, which can exist by itself or as a whole with the rest of the frames to make a reel. A frame, by itself, is simply a picture in a long line of pictures that only make sense relative to eachother while in motion. IF time is fragmented like this, then altering one frame would only make a split-second alteration on reality, while the rest of the frames go on unchanged.

Just a thought.

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Simple. Use a time machine.

Again, once you leave linear time, you need to forget about cause and effect. Cause and effect only matter in linear time.

which is why most people think you enter a new time line.

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How could it? If it did, it would be a paradox. Ergo, it likely does not.

Why? It's a little bit like saying a river has to correct itself every time I jump in.

Again, why would time care where you came from? It doesn't care where the hammer came from.

Of course not. If you are in the timeline, you exist. If you are not, you don't.

I suspect that Time doesn't care a great deal about human paperwork.

Now THAT is an interesting idea!

We tend to think of time as a fragile and delicate thing, like a house of cards. It may well be a bit more hardy than we give it credit for.

you see you do understand the paradox. just because your trying to work your way around it.

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