(adsbygoogle = window.adsbygoogle || []).push({}); Into "the past"?: Tricky SpaceTime Diagrams?

It's commonly stated that according to GR, travelling faster than light would mean going backwards in time (to "the past").

When I try to picture the situation with the aid of a spacetime (light-cone) diagram, I still don't get why should it lead to "the past".

If we start with 2 events separated by space but on the same horizontal line (let's say, the sun on the left side, and 4 light-years away to the right, the star alpha-centauri). Let's say the horizontal is sun's time 2004.

As the situation evolves in time, both objects go upwards in the diagram parallel to eachother. The light emmited by the sun is pictured as a diagonal line at 45º (light-cone), going upwards-right towards alpha-centauri.

The light-cone will reach the vertical line of alpha-centauri at a height of 4 years (sun's time) from the starting horizontal (sun's time 2008).

Now, if the sunlight would have travelled faster than light (the diagonal at more than 45º from the vertical), it would have arrived at alpha-centauri let's say in sun's time 2007, or 2005.

Isn't that still "the future" if the departure was in 2004? Sure it's "less far into the future" than at light speed, but still the future.

Shouldn't it get downwards in the diagram (below the starting horizontal) in order to be considered as going into "the past"? (before sun's time 2004)

I know I'm wrong but I'd like to understand where's the flaw. I guess there is something tricky about trying to visualize this with a spacetime diagram.

Or is it just that 2006 has to be considerd anyway as "the past" even the departure was in 2004?

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# Into the past ?: Tricky SpaceTime Diagrams?

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