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## Article on Yahoo about time travel

I'm not sure where this belongs, but since I have a specific question, I'm putting it here. I saw this article about time travel in my Yahoo news feed. It seems to be an accurate article (except perhaps for the quote below) unlike many of the science articles that I've seen floating around in cyberspace. My one question comes from near the end of the article.
A couple of other domains offer hope for would-be time travelers. Moving faster than light — the universal reference point — would do the trick
.

I don't understand how superluminal speed results in moving back in time. As you approach c relative to an outsider, the observer sees your clock slowing down. However, if you were superluminal, then according to the Lorentz transforms, the clock does not run backward but instead time takes on a complex value. I have no idea how to interpret a two dimensional time value except to say that is why superluminal speed impossible.

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Most of the “explanations” I’ve seen are either frustratingly vague or even more frustratingly convoluted.
Here is one of the best...
http://sheol.org/throopw/tachyon-pistols.html

3. I point out that all the time. FTL throws you into imaginary time not back in time...but so many people have a hard time understanding imaginary numbers. But there is still the problem of getting to light speed so I consider the problem moot.

4. (packed into a DeLorean sports car for style points)
No; The stainless steel body is important for flux dispersal.

On a somewhat more serious note. I guess it's just a re-wording of what Wayne said... It would have been nice if they mentioned the energy levels required. But; not too bad for a layman's article.

I've always had a hard time with backward time >C. It's probably related to Wayne's comment about imaginary time, but to me it's conceptual. If we see C as a limit then what's going on between C and infinite speed. It's still going to take positive time to get somewhere even if your clock may run backwards (if that's what imaginary time implies).

5. Well, once time becomes imaginary, you can imagine you're going back in time.

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Originally Posted by Ara Pacis
Well, once time becomes imaginary, you can imagine you're going back in time.
Ara. Yep. You could imagine Paul Bunyan being 200 miles tall, and breathing with his toes, then instead of the Space Shuttle or heavy lift vehicles, he could just pick up satellites and throw them into orbit, too.

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Originally Posted by jfribrg
I don't understand how superluminal speed results in moving back in time. As you approach c relative to an outsider, the observer sees your clock slowing down. However, if you were superluminal, then according to the Lorentz transforms, the clock does not run backward but instead time takes on a complex value. I have no idea how to interpret a two dimensional time value except to say that is why superluminal speed impossible.
Superluminal travel means, to me, that the unit tangent of a trajectory has become space-like. It is possible to add two space-like 4-vectors to get a pure negative time 4-vector. Thus, if we travel out at superluminal speed, and return with the same speed, we would return before we left.

The energy consumed by our rockets would require an infinite amount of energy to get us to c, and an infinite amount of imaginary energy to get us from c to our cruising velocity.

Where to get an infinite supply of positive energy, or any supply of imaginary energy is beyond me.

I think that the final conclusion is that if faster-than-light travel occurs, then there are possible frames of reference in which causality is reversed (ie time travel occurs), if you consider all possible observers.

9. Originally Posted by NEOWatcher
I've always had a hard time with backward time >C. It's probably related to Wayne's comment about imaginary time, but to me it's conceptual. If we see C as a limit then what's going on between C and infinite speed. It's still going to take positive time to get somewhere even if your clock may run backwards (if that's what imaginary time implies).
For the person travelling at C the only travelling they experience is accelerating to C then decelerating from C. The moment they reach light speed distance and time have no meaning for them, that part of the journey would be instantaneous. For a relative observer they would view the journey to have taken x amount of time depending on the distance travelled. They would view the ship accelerating to C travelling at C for a while then decelerating. What Relativity implies is that if the traveller was able to travel FTL then they in theory would arrive before they started out according to their own clock. This creates a paradox and thus causality problems. So either FTL is impossible for the traveller resulting in no backward time travel, or time takes on a new meaning (as Wayne mentioned) where the observer records the traveller to be going at FTL but the traveller experiences imaginary time.

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Originally Posted by trinitree88
Ara. Yep. You could imagine Paul Bunyan being 200 miles tall, and breathing with his toes, then instead of the Space Shuttle or heavy lift vehicles, he could just pick up satellites and throw them into orbit, too.
"Eh-neeek-chock"

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If traveling back in time, where would the traveler end up, and how far back? If time only slows down, and 2 days pass for
the traveler, but how long passes at starting planet? 2 years, 20, 200?
If say, I take off from planet Earth on 2012Apr30, 9pm, and achieve FTL speed, and I fly towards nowhere for 2 weeks just to see
what would happen, then either A) I end up in the past... but when? 2 days before I departed? when the dinosaurs walked the Earth?
or
B) Upon return to Earth, the year is now 4012. Ifcourse, I am forgotten, no one speaks English, America no longer exists, people
are walking around Mars, contact with aliens has been made, people live to 200, and there is no more pizza... oh, the horror.

12. Originally Posted by cosmocrazy