in Science

It’s all relative

On my walk with the boys on Monday night we had a great ‘ramble chat’ that covered a vast range of topics. It was so lovely to hear them ask questions and respond to each other on how they interpreted the world. We got to talking about my work and I told them that my team had been tackling a problem of helping staff in different cities to have a faster connection to each other, but that there is a natural limit to how fast this can be. We talked about computer networks, the speed of light, and relativity. I gave them my understanding that time isn’t a thing that just exists on its own; it is related to space, and that time is perceived to be (or is?) slower for things that move faster. We watch a lot of Star Trek together and my youngest boy pointed out the connection where various fictional spaceships have ‘slingshotted’ around the Sun in order to gain enough speed to travel through time. I told them about the experiments where very accurate clocks were flown around in aircraft and got out of sync with the same clocks on the ground.

Back in the world of fact, it got me thinking about how much relativity really has an impact on everyday life. I wondered if going on lots of international business trips kept you younger, for example.

I had read before that GPS satellites have had to be designed to take relativity into account:

…the relativistic offset in the rates of the satellite clocks is so large that, if left uncompensated, it would cause navigational errors that accumulate faster than 10 km per day! GPS accounts for relativity by electronically adjusting the rates of the satellite clocks, and by building mathematical corrections into the computer chips which solve for the user’s location. Without the proper application of relativity, GPS would fail in its navigational functions within about 2 minutes.

So much for satellites that are moving at 14,000km/hr in orbits 20,000km above the Earth. What about people?

This article explains the impact on someone that travels around a lot at relatively high speed to the rest of us:

For this example we will look at an airline pilot. For simplicity let’s say that our pilot spends his or her whole career on the Atlantic route, flying (on average) 25 hours a week for 40 years at an average speed of 550 mph (880 km/h). This is undoubtedly a lot of “high” speed travelling but how much time will our pilot “save” due to time dilation?

… in a lifetime of flying our airline pilot saves a total of 0.000056 seconds as compared to an external observer.

Not much to be concerned about, but that number of 0.000056 seconds (56 microseconds) still seems big in that there are things in the real world that are that long. It’s roughly the same as the cycle time for the highest human-audible tone (20 kHz), or the read access latency for a modern solid-state computer drive which holds non-volatile computer data.

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