The runner and the time keeper are in different frames of reference.
ICANT appears to not understand relativity and either he or others may try to poo-poo it away (I've seen creationists try) without recognizing a practical application, the
Global Positioning System (GPS), which would not work without taking relativistic effects into account and compensating for them.
The last two decades of my career as an intelligent designer (AKA "engineer") was designing the embedded software for a line of products which incorporate
Global Positioning System (GPS) receivers, so I have some familiarity with how GPS works.
GPS provides position and time solutions for the receiver based on the positions
and ranges (called "pseudo-ranges" since they're derived from the signal's time-of-flight and the speed of light) of multiple satellites (at least three, though ideally four or more) relative to the receiver. The accuracy of those values determine the accuracy of the solutions (eg, position within 5 meters, time within less than 100 nanoseconds). Because the relativistic effects of gravity and motion would reduce that accuracy drastically, those effects must be compensated for in the position and time solutions. Our products depended on a highly accurate time solution.
From that Wikipedia article:
Wikipedia: GPS:
History
. . .
In 1955, Friedwardt Winterberg proposed a test of general relativity—detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predicted that the clocks on GPS satellites, as observed by those on Earth, run 38 microseconds faster per day than those on the Earth. The design of GPS corrects for this difference; because without doing so, GPS calculated positions would accumulate errors of up to 10 kilometers per day (6 mi/d).
. . .
Timekeeping
. . .
Accuracy
GPS time is theoretically accurate to about 14 nanoseconds, due to the clock drift relative to International Atomic Time that the atomic clocks in GPS transmitters experience. Most receivers lose some accuracy in their interpretation of the signals and are only accurate to about 100 nanoseconds.
Relativistic corrections
The GPS implements two major corrections to its time signals for relativistic effects: one for relative velocity of satellite and receiver, using the special theory of relativity, and one for the difference in gravitational potential between satellite and receiver, using general relativity. The acceleration of the satellite could also be computed independently as a correction, depending on purpose, but normally the effect is already dealt with in the first two corrections.
So relativity is very real and we use it every day, especially if we use a cell phone. Not only do our Map apps use our smartphone's integrated GPS receiver (or else gets that position and time information from the cell tower, which in turn gets that information from
its own GPS receiver (eg, the one incorporated in its frequency and time division subsystem, ie, our product)), but a GPS-based time solution is needed for the entire communications network to function.
Fun Trivia and Discovery:
While traveling by rail in Germany, I would use Maps on my smartphone to keep track of where we were. That worked fine in the first train which included Wifi, but not on trains without that service where the train car blocked the satellite signals (solutions depend on satellites being spread out in different directions, not all in one single direction like out of the window). However, I noticed that at most train stations our position would suddenly get updated. Later research revealed that wireless servers can provide that information to client devices. I had never thought of that.
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