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I have no heart!!
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Discussion Starter · #1 ·
Can someone please explain the grandfather paradox and is there a way around it
 

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Basically, it's one of the contraddictions (=paradox) that emerge if you theoretically allow for time travel. That is, it would make time travel seem theoretically impossible, rather than physically impossible. It's one way of saying "even if there was magical or science-based machine that could allow you to travel back in time, once you're there you would be able to do something that contraddicts your own existence, hence such machine can't exist".

The paradox is summarized thus:

Suppose you build a machine that allows you to travel back in time. You use it, and for some reason intentional or not, you kill your own grandfather. Now that your grandfather is dead, you never existed... but if you never existed, who went back in time to kill your grandfather? No one. But if no one kills your grandfather, then you're born, and you go kill him. But if you kill him, you won't be able to back and kill him! See where I'm going with this?

Now, there are some ways to see past the contraddiction, but they're flimsy.
One would involve the creation of an alternate timeline, one where the world goes on without either you or your grandfather, and one where you travel back in time, and have no future after the present time you leave with your time machine.

Another approach would be more subtle and would involve a "stable" time loop, where you go back in time, kill your grandfather, and nothing happens. Puzzled, you take your time machine back to the future (I cannot believe I just typed that). And you notice that nothing has changed in the future, either! Eventually you discover that your time-travel adventure was always instrumental to creating the present you already knew, and the only thing that changed is your perception of it, that is to say, you learned about things you didn't know. Perhaps, that you had been adopted. Or that your grandma was already pregnant when you killed your grandfather. Or other details. Still, a bit flimsy. What if you go back in time and detonate a nuclear warhead? That's hardly something you could've missed...
Then again you could argue that maybe a necessary condition for you to be able to operate a time machine is that you have enough moral fiber not to completely devastate anything. Eh, it's complicated.
 

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Awesome. I was going to respond but I'm just way too tired. I actually just got back to the future and I'm tired as hell. By the way BTTF, my favorite movies.
 

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I have no heart!!
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Discussion Starter · #5 ·
Basically, it's one of the contraddictions (=paradox) that emerge if you theoretically allow for time travel. That is, it would make time travel seem theoretically impossible, rather than physically impossible. It's one way of saying "even if there was magical or science-based machine that could allow you to travel back in time, once you're there you would be able to do something that contraddicts your own existence, hence such machine can't exist".

The paradox is summarized thus:

Suppose you build a machine that allows you to travel back in time. You use it, and for some reason intentional or not, you kill your own grandfather. Now that your grandfather is dead, you never existed... but if you never existed, who went back in time to kill your grandfather? No one. But if no one kills your grandfather, then you're born, and you go kill him. But if you kill him, you won't be able to back and kill him! See where I'm going with this?

Now, there are some ways to see past the contraddiction, but they're flimsy.
One would involve the creation of an alternate timeline, one where the world goes on without either you or your grandfather, and one where you travel back in time, and have no future after the present time you leave with your time machine.

Another approach would be more subtle and would involve a "stable" time loop, where you go back in time, kill your grandfather, and nothing happens. Puzzled, you take your time machine back to the future (I cannot believe I just typed that). And you notice that nothing has changed in the future, either! Eventually you discover that your time-travel adventure was always instrumental to creating the present you already knew, and the only thing that changed is your perception of it, that is to say, you learned about things you didn't know. Perhaps, that you had been adopted. Or that your grandma was already pregnant when you killed your grandfather. Or other details. Still, a bit flimsy. What if you go back in time and detonate a nuclear warhead? That's hardly something you could've missed...
Then again you could argue that maybe a necessary condition for you to be able to operate a time machine is that you have enough moral fiber not to completely devastate anything. Eh, it's complicated.
Excellent!!!!!!!!
 

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FAILURE
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I remember this paradox I think Einstein was the one who said it. anyway, I first heard about it when I was in six grade I came to the conclusion that it was going to be a loop, since I have no idea about parallel universes were, but when I asked my brother he insisted that I was wrong and explained to myself they would both (grandfather and grandson) disappear at that instant.... long story short, so it takes me more than a decade before I could see that I a six grader had a stronger argument than his 10 year older brother.... yay.

I think a better paradox is "What will happen if you make a time machine that opened 15sec into the past at the same location. Then you take a gun and kill yourself 15sec in the past before you even turn on the machine.....Who killed you?"
 

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I have no heart!!
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Discussion Starter · #11 ·
I remember this paradox I believe Einstein was the one who said anyway I first heard about it when I was in six grade I came to the conclusion that it was going to be a loop, since I have no idea about parallel universes were, but when I asked my brother he insisted that I was wrong and explained to myself they would both (grandfather and grandson) disappear at that instant.... long story short, so it takes me more than a decade before I could see that I a six grader had a stronger argument than his 10 year older brother.... yay.

I think a better paradox is "What will happen if you make a time machine that opened 15sec into the past at the same location. Then you take a gun and kill yourself 15sec in the past before you even turn on the machine." Who killed you?
You killed yourself-- not only did you come from a differeent time but a paralell universe as well--- so the person you killed couldnt travel in time in that particular universe- and u will still exist because you come from a different paralell universe!
 

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FAILURE
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You killed yourself-- not only did you come from a differeent time but a paralell universe as well--- so the person you killed couldnt travel in time in that particular universe- and u will still exist because you come from a different paralell universe!
That's actually a pretty good answer, it makes sense.

But you know the theoretical answer for the paradox was (as I heard); the situation would never occur in the first place because the feedback from the natural radiation surrounding the portal made by the time machine would double itself exponentially (it loops on itself) thus destroying the portal in the instant it is activated. The situation then becomes an example of how the universe itself has made blocks to prevent paradoxes from happening as it is an unnatural part of the system.

This was sadly Stephen Hawking's argument as to why time travel is impossible... I died a little inside when I heard it :(
 

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Querdenker
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Interestingly, time is the only property of physics that can not have a negative value, or run backwards. As this thread shows, if it could have a negative value, the universe would be dysfunctional. Obviously the other properties having negative values does not make the universe is dysfunctional. So the only property that shouldn't have a negative value, doesn't.
 

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FAILURE
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Time travel is a possibility, were all standing on a huge time machine right now. The only problem is time travel only goes forward.
 

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is getting over herself
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buy an older gentleman some trousers that fit properly, and a belt that fits at normal, waist height. either that or a long coat.
 

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Let's say there was a galaxy 100 light years away (it takes light 100 years to travel here). If you had a really, really big telescope you could watch events happening 100 years ago on that other galaxy.

Let's say you came across a space ship that can do more than twice the speed of light.

If you travel at the speed of light for 100 years, you will end up at the other galaxy 100 years later.

If you travel at twice the speed of light for 50 years, you will end up at the other galaxy in 50 years, yet people on Earth will see you there 100 years ago in the other galaxy, yet 50 years would have gone by on Earth. If you turned around and traveled for another 50 years, 100 years would have gone by on Earth, and on the space ship.

If you travel at 4 times the speed of light for 25 years, you will end up at the other galaxy 75 years in the 'past'. If you get to that other far away galaxy and have the huge telescope, you can look at Earth and see it as it was 75 years ago. If you turn around and go 4 times the speed of light back for 25 years, 50 years will have gone past for the spaceship crew, but they would end up 50 years in the past...or would they? Would time go faster in one direction than in the other?.
 

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If you travel at 4 times the speed of light for 25 years, you will end up at the other galaxy 75 years in the 'past'. If you get to that other far away galaxy and have the huge telescope, you can look at Earth and see it as it was 75 years ago. If you turn around and go 4 times the speed of light back for 25 years, 50 years will have gone past for the spaceship crew, but they would end up 50 years in the past...or would they? Would time go faster in one direction than in the other?.
You're thinking of the twin paradox. It's family-related, but grandpa's not involved in time-dilation phoenomena.
 

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Twin paradox is usually for light-speed travel. Faster than light speed gets into time travel stuff*, and makes your head hurt.

*Possibly...it is theoretical physics for a reason.
 
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