Grandfather Paradox

[dmorgan]

I just found this on slashdot:

http://news.bbc.co.uk/2/hi/science/nature/4097258.stm

Now, from the books that I have read, it seems that you would have a chance (albeit incredably small) at killing your father/grandfather. Something along the multitude of 1 in a 1 [with a billion billion billion zeros after it]. This article seems to disprove what Stevn Hawkings (among several Theoretical Physists have stated.

Although there is a chance, it is impossible. Anyone who has read some recent theoretical physics ? Mine is kinda out of date

[Tiger Shark]

OK.... To take on Quantum physics.... WTF, I question everything....

QP states - in this article - that if daddy is at home sat on the sofa when you leave you can't kill him because you have already observed his "state" and he is still alive.

But the title of the thread is "Grandfather Paradox".....

It is therefore possible to go back in time to kill your grandfather between the time you were born and your grandfathers death if you never saw your grandfather, (tested his "state"). This, of course would work perfectly if your grandfather died prior to your birth because you would not have been able to observe his "state" - they cremated him and distributed his ashes..... Never observed, it is possible to kill him yourself by going back in time.

But in some ways this seems to be an over simplistic view of QP.... Since we claim to know that a change in the state of one entity affects another entity somewhere in the universe and the claim is that since the entity called daddy is alive when you leave you can't kill him why would you attack your daddy? You need to work out what other entity is his "complementary" and attack that. By altering the complimentary you will alter his state. Since his observed state was "alive" the result of altering his complimentary state would be to change his state.... that would be dead.....

I just killed daddy.....

[Aspman]

The beeb aticle seems to state that soemthing would stop you doing anything to cause a paradox.
But what if you were to go back further? I read a silly kids short story once but it used the butterfly moving a hurricane idea that if you go back far enough even the smallest actions could cause huge shifts in the furure. You go back 10,000,000 years and shoot some random animal, it dies somewhere it wouldn't have, something else gets extra food allowing it to breed when it would have died and so on....

The multiverse idea was esier to understand, you could go back in time an do anything you like but you'd create a new universe so that in one the timeline continues as you left it but where you just killed your father it would continue to a new outcome.

I don't like the idea of your actions suddenly becoming predetermined, smacks too much of destiny and that's a whole other argument.

[MURACU]

Or just take it like this .... You go back in time to kill your father who is alive in your present. lets say you kill your father before you are born. you come back to the present but your father has been dead for years and you dont exist. So if you dont exist you couldnt have went back and killed him. you didnt kill him so he is still alive and you are born so you go back to kill him ............... ect.
you could end up in a closed loop.

[rcgreen]

The plain boring fact of the matter is that you can't travel
backward in time, because your very presence there activates
the paradox, a logical closed loop of impossibilities, like an M C Escher
optical illusion. You can't continually walk up the stairs and,
every time you go around the courtyard, find yourself no further up.

In spite of Einstein's incoherent babble, time is a constant. An event
that has already happened in the past is forever fixed.
I am a big Dr. Who fan but we should try to remember why they
call it Science Fiction!

[RoadClosed]

In spite of Einstein's incoherent babble, time is a constant. An event
that has already happened in the past is forever fixed.

While an event in the past is fixed, time is not a constant. Pints of Guiness do not mysteriously undrink themselves. But even the beer gurgling event could be percieved as happening in two different instances by two different casual observers. And that is not even going into quantum, choas theories, distance perception on past/present etc . Gravity warps space. Space and time are related; therefore, time and space are not constant. The speed of light however is, and the time it takes light to reach us from say a distant star varies. There is no argument to this fact based on our cosmological knowledge.

//EDIT

Since we claim to know that a change in the state of one entity affects another entity

Tiger, we don't "claim" anything. Up until the 80s that was the case but evidence now proves that is the case.

[MURACU]

Pints of Guiness do not mysteriously undrink themselves.

True but they do go down at a rate faster than that at which you are drinking........
On a more serious note :

Gravity warps space. Space and time are related; therefore, time and space are not constant.

A simple experiment If I remember rightly is to get your hand on two perfectly synchronised Atomic clocks. Put one a on a table at your starting point and then take the other to your private jet and bring it up to your maximum altitude and fly around for a couple of hours. When you come back down they will be no longer in sync as time moves at different rates in a gravity well and outside it. Now if anyone would like to lend me their private jet with a well stocked bar I would be more than happy to carry out this experiment. :d

[Negative]

According to Peter Lynds (http://en.wikipedia.org/wiki/Peter_Lynds), time is an illusion: it has no indivisible unit, and the there is no such thing as "now", only sequences of events.

A simple experiment If I remember rightly is to get your hand on two perfectly synchronised Atomic clocks. Put one a on a table at your starting point and then take the other to your private jet and bring it up to your maximum altitude and fly around for a couple of hours. When you come back down they will be no longer in sync as time moves at different rates in a gravity well and outside it. Now if anyone would like to lend me their private jet with a well stocked bar I would be more than happy to carry out this experiment. :d

That will only show you that an atomic clock can be "altered" - it doesn't say anything about time, only about the perception you have about time. Following the logic in that experiment, if you have two watches, one of which shows the "correct" time and the other takes 1.3 seconds for every second, you are altering time... I don't think so. You're just altering the perception of time. Time doesn't move at different rates in a gravity well and outside it - the clocks do...

[dmorgan]

It is possible to move back in time (according to theory). Consider the fact the spacetime is just that... space AND time.
A good example is wormholes (although you need a naked singularity, which theoritically is possible, although not suitable for travel by humans, ie; we cant absourb billions of joules of gamma rays), since nothing can travel faster than light.. energy, matter, even information are bound by the speed limit, if you traveled through a wormhole, and you came out on the other end of the universe, you would have actually gone back in time (relative [thoery of relativity] to the time you left). Now say, you shot a beam of light at earth, then returned to earth and observed that beam of light. It would seemingly have come from the past...

Negative: You are bringing up the twins paradox, which means that time does have quantifiable units, they are just all relative to gravity, speed, and so on.

The bottom line is that the closer you become to the speed of light (or the deeper in a gravity well you are), the slower time goes for you. This means that if you observe a passing photon while you are traveling at near the speed of light, since time is slowed, to you it would seem like it [the photon] is traveling at its normal velocity. Same with gravity, since it bends spacetime.

Time travel IS possible (theortically, just not suitable for humans) although a naked singularity is incredibly hard to make, not to mention takes millions of years, it is possible... Much less a wormhole.

Then again, this is all far-flung theory. Not much practicality.

[Evil Moo]

Originally posted here by MURACU
.... lets say you kill your father before you are born. you come back to the present but your father has been dead for years and you dont exist. So if you dont exist you couldnt have went back and killed him. you didnt kill him so he is still alive and you are born so you go back to kill him ............... ect.

Exactly. All it does is give us something for the theorists to fry their brains over, lol.