RE: Time travel is possible?

["Keith Thomas-Wurth" <aikeith@iinet.net.au>]

To reply to this message or start a new topic please email: BUPS-DIS@bups.org



You could say then, Newton said, motion changes with time, but Einstein
said, time changes with motion. 

Interestingly time travel has do with wormhole theory according to Prof
David Blair, he is published on the subject.  He is addressing a
philosophy group next month so that is going to be interesting. 

Keith


-----Original Message-----
From: owner-bups-dis@purplepancake.com
[mailto:owner-bups-dis@purplepancake.com] On Behalf Of Edward
Grefenstette
Sent: Tuesday, 25 October 2005 11:49 PM
To: Alice Evans
Cc: BUPS-DIS@bups.org
Subject: Re: Time travel is possible?

To reply to this message or start a new topic please email:
BUPS-DIS@bups.org


While it is true that by traveling at a speed close to c, the speed  
of light, you would indeed end up "later" in the future than you  
would have had you traveled the same duration by your watch at a  
lower speed. From an external observer's time frame, your journey  
takes the same time. Note however that (at least to my knowledge)  
this is a relativistic effect and not a directly proportional one,  
thus the inverse does not hold to be true. Indeed you do not travel  
slower through time by standing still, and you certainly have no way  
of traveling backwards in time according to this theory. Thus you  
would not meet your future self either as you would effectively just  
be traveling into the present anyway, and would have been gone during  
the duration of your journey from earth's point of view.

-- Edward.

On 25 Oct 2005, at 25/10/200513:32, Alice Evans wrote:

> To reply to this message or start a new topic please email: BUPS- 
> DIS@bups.org
>
>
> ** For Your Eyes Only **
>
>
>
> Arguably, time travel is possible. For example, an object traveling  
> at high speeds ages more slowly than a stationary object. This  
> means that if you were to travel into outer space and return,  
> moving close to light speed, you could travel thousands of years  
> into the Earth's future.
>
> Newton's most important contribution to science was his  
> mathematical definition of how motion changes with time. He showed  
> that the force causing apples to fall is the same force that drives  
> planetary motions and produces tides. However, Newton was puzzled  
> by the fact that gravity seemed to operate instantaneously at a  
> distance. He admitted he could only describe it without  
> understanding how it worked. Not until Einstein's general theory of  
> relativity was gravity changed from a "force" to the movement of  
> matter along the shortest space in a curved spacetime. The Sun  
> bends spacetime, and spacetime tells planets how to move. For  
> Newton, both space and time were absolute. Space was a fixed,  
> infinite, unmoving metric against which absolute motions could be  
> measured. Newton also believed the universe was pervaded by a  
> single absolute time that could be symbolized by an imaginary clock  
> off somewhere in space. Einstein changed all this with his  
> relativity theories, and once wrote, !
> "Newton, forgive me."
>
> Einstein's first major contribution to the study of time occurred  
> when he revolutionized physics with his "special theory of  
> relativity" by showing how time changes with motion. Today,  
> scientists do not see problems of time or motion as "absolute" with  
> a single correct answer. Because time is relative to the speed one  
> is traveling at, there can never be a clock at the center of the  
> universe to which everyone can set their watches. Your entire life  
> is the blink of an eye to an alien traveling close to the speed of  
> light. Today, Newtonian mechanics have become a special case within  
> Einstein's theory of relativity. Einstein's relativity will  
> eventually become a subset of a new science more comprehensive in  
> its description of the fabric of our universe. (The word  
> "relativity" derives from the fact that the appearance of the world  
> depends on our state of motion; it is "relative.")
>
> What remains of the killing-your-earlier-self paradox in general  
> relativistic time travel worlds is the fact that in some cases the  
> states on edgeless spacelike surfaces are 'overconstrained', so  
> that one has less than the usual freedom in specifying conditions  
> on such a surface, given the time-travel structure, and in some  
> cases such states are 'underconstrained', so that states on  
> edgeless space-like surfaces do not determine what happens  
> elsewhere in the way that they usually do, given the time travel  
> structure. There can also be mixtures of those two types of cases.  
> The extent to which states are overconstrained and/or  
> underconstrained in realistic models is as yet unclear, though it  
> would be very surprising if neither obtained. The extant literature  
> has primarily focused on the problem of overconstraint, since that,  
> often, either is regarded as a metaphysical obstacle to the  
> possibility time travel, or as an epistemological obstacle to the  
> plausibility of time travel in!
>  our world. While it is true that our world would be quite  
> different from the way we normally think it is if states were  
> overconstrained, underconstraint seems at least as bizarre as  
> overconstraint. Nonetheless, neither directly rules out the  
> possibility of time travel.
>
> If time travel entailed contradictions then the issue would be  
> settled. And indeed, most of the stories employing time travel in  
> popular culture are logically incoherent: one cannot "change" the  
> past to be different from what it was, since the past (like the  
> present and the future) only occurs once. But if the only  
> requirement demanded is logical coherence, then it seems all too  
> easy. A clever author can devise a coherent time-travel scenario in  
> which everything happens just once and in a consistent way. This is  
> just too cheap: logical coherence is a very weak condition, and  
> many things we take to be metaphysically impossible are logically  
> coherent. For example, it involves no logical contradiction to  
> suppose that water is not molecular, but if both chemistry and  
> Kripke are right it is a metaphysical impossibility. We have been  
> interested not in logical possibility but in physical possibility.  
> But even so, our conditions have been relatively weak: we have  
> asked only whether !
> time-travel is consistent with the universal validity of certain  
> fundamental physical laws and with the notion that the physical  
> state on a surface prior to the time travel region be  
> unconstrained. It is perfectly possible that the physical laws obey  
> this condition, but still that time travel is not metaphysically  
> possible because of the nature of time itself. Consider an analogy.  
> Aristotle believed that water is homoiomerous and infinitely  
> divisible: any bit of water could be subdivided, in principle, into  
> smaller bits of water. Aristotle's view contains no logical  
> contradiction. It was certainly consistent with Aristotle's  
> conception of water that it be homoiomerous, so this was, for him,  
> a conceptual possibility. But if chemistry is right, Aristotle was  
> wrong both about what water is like and what is possible for it. It  
> can't be infinitely divided, even though no logical or conceptual  
> analysis would reveal that.
>
> Similarly, even if all of our consistency conditions can be met, it  
> does not follow that time travel is physically possible, only that  
> some specific physical considerations cannot rule it out. The only  
> serious proof of the possibility of time travel would be a  
> demonstration of its actuality. For if we agree that there is no  
> actual time travel in our universe, the supposition that there  
> might have been involves postulating a substantial difference from  
> actuality, a difference unlike in kind from anything we could know  
> if firsthand. It is unclear to us exactly what the content of  
> possible would be if one were to either maintain or deny the  
> possibility of time travel in these circumstances, unless one  
> merely meant that the possibility is not ruled out by some  
> delineated set of constraints. As the example of Aristotle's theory  
> of water shows, conceptual and logical "possibility" do not entail  
> possibility in a full-blooded sense. What exactly such a full- 
> blooded sense would be in!
>  case of time travel, and whether one could have reason to believe  
> it to obtain, remain to us obscure.
>
>
>
> This message has been checked for viruses but the contents of an  
> attachment
> may still contain software viruses, which could damage your  
> computer system:
> you are advised to perform your own checks. Email communications  
> with the
> University of Nottingham may be monitored as permitted by UK  
> legislation.
>
>
>
> Browse or search the BUPS-DIS archives, or unsubscribe from the  
> mailing list at: http://www.bups.org/mailinglist.htm
>



Browse or search the BUPS-DIS archives, or unsubscribe from the mailing
list at: http://www.bups.org/mailinglist.htm

-- 
No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.1.361 / Virus Database: 267.12.5/149 - Release Date:
25/10/2005
 

-- 
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.361 / Virus Database: 267.12.5/149 - Release Date:
25/10/2005
 


Browse or search the BUPS-DIS archives, or unsubscribe from the mailing list at: http://www.bups.org/mailinglist.htm