I can’t wrap my head around it. But maybe it might be a very silly question with a simple answer.
In: Physics
>Relative to each other one will seem stationary while the other will look like it’s going almost twice the speed of light.
You’re right that they’ll always appear stationary within their own reference frames.
Your guess at the speed they perceive the other ship to be moving would apply in basic (Galilean) relativity, but does not bear out in reality. We haven’t seen anything travelling faster than light. Special relativity can address this. A full discussion of special relativity goes well beyond the scope of the question and gets very mind-bendy.
Regarding what passengers see:
– they each perceive the other ship travelling clos_er_ to the speed of light, but still below the speed of light
– if they see a clock on the other ship, they’ll perceive it’s time to be passing more slowly than their own (yes both ships will perceive each other’s time to be slower than their own)
– if in identical ships they perceive the other ship to be shorter than their own (both will perceive each other’s ship to be shorter than their own)
– they’ll perceive the other ship to be blue-shifted as it approaches, and red-shifted after it passes
You might look at the claims above and assume there’s some stationary observational frame of reference from which we can measure the _true_ time, distance, velocity, etc. After all, the external observer, and observers on each of the ships are all measuring different times, lengths, and velocities than each other, and may even disagree on orders of events. Heres the real kicker… each of their frames of reference are equally valid.
It isn’t surprising that you can’t wrap your head around it. Most people are accustomed to Galilean relativity, and we don’t tend to have much experience observing relativistic objects.
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