How does travelling faster than the speed of light mean time travel?

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JohnAdriaan

New member
Look up into the sky. Do you see the light from the Sun? That didn't get from the Sun to your eye instantly: it took 8 minutes. If you look at Alpha Centauri, that light didn't get here instantly either - it took more than four years to get here.
In other words, if Alpha Centauri was to explode tomorrow, you wouldn't know about it for four years, since the light from the explosion would take that long to get here.
But just say a traveller left Alpha Centauri a day after it exploded, on board a ship that could travel at twice the speed of light. He would arrive here in two years, and tell you "In two years Alpha Centauri will explode." Isn't that time travel?
 
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BigDaddy1

Member
First of all, it's not a necessary conclusion. If you're using physics (special relativity in this case) that *depends* on the vacuum speed of light to be a true barrier, then using those same physics to draw a conclusion from an impossibility is not necessarily valid.

The main argument for this relies on using special relativity to show that if you have faster-than-light transport in some inertial frame, then it is possible to find a different inertial frame where that same transport is backward in time.

If you've never worked with Minkowski diagrams, then this link might not mean much to you, but for those familiar with them you can see how the presence of a single faster-than-light transport path allows for time loops to appear.

http://www.theculture.org/rich/sharpblue/archives/000089.html
 
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