Or perhaps radio waves don't travel at the speed of light forever. Maybe they slow down by 6 mph every 100 years.
That sort of thing might not be detected until you had a few probes a great distance from this planet.
On another note: isn't it wild that the U.S. has been sending probes into deep space for over thirty years?!
Three decades later most nations on this planet haven't even figured nuclear power out, much less how to send atomic-powered probes into the far reaches of space.
Or perhaps radio waves don't travel at the speed of light forever. Maybe they slow down by 6 mph every 100 years
I think that would make the probe appear to accelerate away from us as the radio waves would take longer to reach us and would therefore suggest that the probe was further away than it actually was (assuming you used the 'old' speed of light in your calculations).
That's known as the tired light theory. However, spectral emission and absorption lines from distant stars and nearby starts arrive on the same wavelenghts. If light slowed down with time, the arriving wavelengths would all be longer, and therefore all the spectral lines shifted for the more distant stars. There is no hint of that. Therefore light doesn't get tired or slow down with age.