What Is Time Dilation?
Fast spinning globe Slow spinning globe As explained in the Relativity page, objects of differing mass/energy/speed experience the passage of time at different rates, this difference is referred to as Time Dilation.

Time Dilation is typically used with reference to situations in which one party travels close to the speed of light and another party remains Earthbound. For both groups the subjective passing of time is identical, but the amount of layman's "Real Time" passing is different.

The Earthbound group will live and die in a fraction of the life of the faster moving group. Once you get used to the notion that time is dependent upon mass/energy then it all makes sense, but we all have a lifetime's experience telling us that there is such a thing as absolute Time.

Now a ghost is a very low speed/mass object, so if a ghost takes two seconds to walk across a room this will be such a short period of time in the faster time frame of the live human that the live human will be unable to see it.

Remember that with GPS a clock that is inaccurate by 1 microsecond will result in a reading that is off by about 300 meters. Relatively has been taken into account in the GPS system and the clocks have been tweaked to be different to Earth time by 38 microseconds, this is an adjustment that is equivalent to nearly 11 kilometres!

See Wikipedia’s entry.