Physics! Everyone’s favourite science … right?

Perhaps not, but regardless when we think of physics, a couple of things generally come to mind: Einstein and Relativity. We know Einstein was a genius, but really, relativity? Some pretty complex equations and Newtonian mechanics work well enough for NASA’s moon landings, so why? Why was Einstein a genius and why does it matter? Why should we care? What does it have to do with us today in every day real-life?

Well today I’d like to show one of the practical applications of Einstein’s General Theory of Relativity and next week some considerably less practical extensions!

So let’s start with the General Theory of Relativity. It’s a way of explaining gravity, space and time and how they’re related. Briefly, objects with greater mass (heavier things) make a larger indent in the ‘fabric’ of space-time (think a bowling ball vs a tennis ball on a trampoline) and so things ‘roll’ towards them more. It also indents time (don’t overthink that), so that objects in greater gravity have slower time. So if you lived on Jupiter instead of Earth then you might live up to a minute longer! Similarly, on a lower-mass object (like the moon) you’ll age quicker as compared to people on Earth. Here we’re taking Earth-time as our reference point and comparing the other times *relative *to this. Instead, if we take Jupiter-time as our reference then Earth-time is faster and moon-time even faster again.

However, the important thing is that exactly the same number of seconds would be in your life, regardless of where you lived and it would feel like the same time to you, just not to the observer on a different planet.

We’ve also got Special Relativity. This is pretty cool because, in brief, it’s talking about how time is relative to the speed an object is travelling at. I heard it once explained that everything (including us) travels at light-speed through time and space. Light doesn’t use any energy travelling through time, so it never ages. We, on the other hand, use our energy almost exclusively travelling through time; however, the faster we go, the less we travel through time. So a jet pilot would actually live longer (on scales of nanoseconds) because he continually flies at high speeds than if he had become a doctor who only drives in a car.

So a quick summary. Being close to heavy objects and travelling at high speeds both cause time to slow down as compared to the observer- you on Earth.

Now, our as to why *any *of this matters, have you ever resorted to using GPS to navigate? Been driving along when the helpful voice reminds you that you should have exited back there? And you’re just like, ‘oops, hope there’s another turn’. Well if we didn’t understand relativity, your GPS wouldn’t have a chance of navigating the correct exit.

The time-keeping of the GPS satellites is influenced by both special and general relativity, yet must have an accuracy of 20-30 nanoseconds to correctly determine positions of people on Earth. Special relativity acts to “slow down” the clocks as the satellites are moving at roughly 8000 metres per second, resulting in satellite clocks falling 7000 nanoseconds behind Earth clocks every day.

General relativity works in opposition, increasing the speed of time as the satellites are further from Earth’s gravity than clocks on the surface. In fact, enough that the satellite clocks have an ‘extra’ 45,000 nanoseconds every day compared to Earth clocks.

Doing a quick calculation (45,000 – 7,000 = 38,000), it is immediately obvious that the satellite clocks are far more out-of-sync than the required 20-30 nanosecond limit. So what do we do? The satellite engineers take the relativistic effects into account and build clocks that tick more slowly on purpose, determining exactly the required pace based on the height and speed of the satellite’s orbit.

So there we have it. Without Einstein, you’d struggle to navigate as easily and conveniently as we do today … and be sure to come back next week to explore some of the more theoretical and exciting aspects of Einstein’s genius!

*Note, the relativity calculations and information can be found in this article.*

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