> Michael Wiik wrote:
> >Randall Randall writes:
> >> OK, I can follow you so far: if you return to the past of
> >> yourself, you were always part of that past. However, if
> >> we introduce the idea of "Many Worlds", another possibility
> >> arises: that we may be able to return to a past exactly
> >> similar to our own, but with the addition of ourselves (the
> >> time travelers). The only difficulty I can see is that
> >> there appears to be no way to get back to the "present" you
> >> left.
> >Since it can't be tested, how would this be different from a
> disintergration device?
> Actually, it CAN be tested, interestingly enough, using reversible
Yes, but this isn't quite what I was speaking of.
> On a relevant note, tests have been done to see if one can travel to other
> histories, (ie testing for exact linearity in the waveform,) and it seems
> that we can't. Take a look at Q36-Q39 in the excellent "Many Worlds FAQ"
> at http://www.airtime.co.uk/users/station/m-worlds.faq
<snip quote from web>
This really has no bearing on my argument, since I am only speculating that one might be able to travel back to one's *own* timeline (except for the addition of oneself as the time traveler). That is, I don't see any reason that it would have to extend to crossing worlds "horizontally".
> 3) For some reason, time travel is limited in some way: perhaps it can't
> be used to travel further back than the date of the invention, for example.
> Again, this is another point which I can't honestly claim cannot be true.
> With my current understanding of physics, I can't currently imagine what
> would prevent time travel beyond a certain date, but then, I'm not out to
> invent a time machine. :)
Actually, AFAICS, *all* of the possibilities have this
feature. In general, time machines which make use of
relativity require large twists in spacetime (like
Tipler's cylinder), and so you can't travel to a time
when the artifact didn't yet exist.
I am not aware of anyone proposing time machines that
*don't* use this sort of method...
I am not aware of anyone proposing time machines that *don't* use this sort of method...
> However, I will make the argument that even this is unlikely. First, note
> that if I had an engine which could move me around faster-than-light (FTL)
> then I could travel back to any arbitrary point in time. This point has
> been pretty well hashed out; I think the FTL FAQ out there on the net
> discusses it in a way that makes sense to those who don't have much
> physics. Alternately, consider the scenario Haradon presents:
> Zeb Haradon said:
> >In "Hyperspace" by Michio Kaku, a scenerio is sketched out which seems like
> >it would work. You make a worm hole - he descibred how to do this, it
> >involves something with charging two sets of plates and relies on something
> >called the Casmir Effect, which has been observed in the lab on a small
> >scale, but he didn't go into much more detail about it - anyway, you make a
> >worm hole using the Casmir Effect, and you have the two ends of a wormhole
> >right next to eachother. You then put one on a ship, and accelerate it to
> >near-light speed, and fly it around for a while, then after 20 years or so,
> >return it to its original position. While this one end of a worm hole was
> >going very fast, time was compressed for it - say it was going fast enough
> >that only 10 seconds passed for it while 20 years passed for the other end
> >of the worm hole which did not travel. Now, you have a wormhole which has
> >one end at point A in space, and its other end only a mile or so away, but
> >one end is twenty years ahead of the other one. You pass through one end and
> >come out the other end twenty years into the future. Pass the other way, and
> >go twenty years into the past.
> FTL movement => arbitrary backwards time travel.
Not in this case. You can never go to a time when the wormhole did not yet exist.
BTW, I'm not sure why all my posts seem to get the subject replaced by the "To:" address...if anyone has a suggestion, I'm listening... :)