How would an interstellar spaceship's speedometer work if everything else is moving?
A speedometer is a gauge or device used for measuring instantaneous speed of moving vehicle roughly speaking, so for example the reading shown on the speedometer of a car parked at road side displays zero implying it is at stationary position respective to the road or landmark etc as long the surrounding objects are at rest relatively to ground. I believe some of you already see where this is going...
Correction: This speedometer actually counts how many turns the wheel makes which then can be translated good indication of vehicle speed but not without flaw.
Another example is a speedometer or otherwise called pitometer log which is usually seen in boat or ship and I shall leave you to find out the working mechanism. (clue: differential pressure of water)
I know with GPS who is still using speedometer nowadays let alone in the future but I'm sure some of you are aware of the limitations.
- Please factor in time dilation when you approach closer to speed of light in vaccum. (e.g. Lorentz factor: <0.9)
- How would interstellar spaceship without FTL or wrap capability measures instantaneous speed accurately?
- If instantaneous speed is useless for space travel then what kind of measurement would be adopted instead? (e.g. light year is used instead of miles or kilometer etc.)