Relativity is one of the most successful theories that Albert Einstein ever came up with. It shook the world by altering the way that we think of space and time.

One of the effects that come out of the theory of relativity is that different observers, traveling at different speeds, may take completely different measurements of the same event. However, all the measurements are technically correct. It's all relative. For example, a period of time for someone on Earth that lasts for hundreds of years may only be a couple of hours for someone zooming around in a rocket at close to the speed of light. One person may measure a stationary car to be one length, but when that same car starts racing along a track, its length appears shorter to a stationary person. These two effects are known as time dilation and length contraction.

You may be aware of the effects of relativity at insanely fast speeds: near the speed of light. It may surprise you to hear, then, that relativity is something that we experience every day. It's found in the most technical of places, and some places that may never even have occurred to you as being out of the ordinary. Since it's 100 years since Einstein published his paper on general relativity, it seems like the perfect occasion to find out how relativity affects us day-to-day.

GPS

Nearly anyone who has a smartphone has access to a global positioning system or GPS. Every time you try to plan a route from “my current location,” your phone needs to connect to a satellite to figure out exactly where “your current location” is. Satellites whoosh around the Earth at a pretty healthy speed: around 10,000 kilometers per hour (roughly 6213 miles per hour). This might sound fast, but it's only about a thousandth the speed of light, so you might not think that it's fast enough for relativistic effects to take place. But, even at a speed this much slower than the speed of light, the satellite still experiences time dilation: It gets “older” by roughly 4 microseconds every day. The satellite experiences the passage of time faster than people on Earth. Include the effects of gravity (which also causes time dilation) and this figure goes up to about 7 microseconds. 

You can barely even blink in 7 microseconds (0.000007 seconds) but if this effect were unaccounted for then your GPS would get you lost very quickly. After just a day, your location according to the GPS could be up to 8 kilometers (around 5 miles) away from your actual location. Fortunately, satellites are programmed to take these effects into consideration when planning your route. (Now, can it stop my GPS from directing me into rivers?)

Color of Gold

Gold has a characteristic, mellow, yellow color. Its beautiful sheen seems even more exotic when you find out that it's actually due to relativistic effects. Were you to calculate the frequency (color) of light that gold emits without taking relativity into consideration, you would predict it to have a silver sheen. However, the color gold actually leans further to the red end of the spectrum.

This discrepancy can be explained when examining how electrons in gold atoms move around in their shells. There is a total of 79 electrons zooming around a gold atom, and 79 protons in the nucleus. In the orbital closest to the nucleus (otherwise known as the 1s orbital), the electrons have to move at a shockingly fast speed. They move at roughly half the speed of light to avoid being dragged into the nucleus by the powerful positive charge from the protons in the nucleus, and that causes a lot of relativistic effects.

Because the electrons are moving so fast, the separate electron shells appear to be closer than they actually are. For an electron to jump to a higher energy level it needs to absorb a specific wavelength of light. In gold, the wavelengths that could be absorbed are usually in the ultraviolet range – beyond what we can see. However, when we account for the relativistic effects that appear to squeeze the shells closer together, we find that the gold actually starts to absorb light with a smaller frequency: blue light. 

The blue light is absorbed and only the red colors are reflected into our eyes. Hence, gold has a glamorous, yellowy sheen.

Electromagnets

Only some metals are naturally magnetic, like iron, for example. That being said, it is possible to create a magnet out of any metal by turning it into a coil of wire and running an electric current through it. These electrified metals have a strange property: they only magnetically affect objects that are moving and they don't have any effect on stationary objects. This is an electromagnet, and it is thanks to special relativity that this phenomenon is possible.

Electric current is the flow of free-moving electrons through a metal, surrounded by a grid of stationary protons. If a charged object sits still next to an electromagnet, then nothing happens to it. Even though the electrons are flowing, they occupy a similar amount of space to the protons so that over all the electrified metal has no effect on it.

However, if this charged object moves alongside the wire, then it starts to feel the effects of length contraction in the moving electrons. This means that the density of stationary protons becomes larger than the flowing electrons and the metal exhibits a positive charge, causing the object to be attracted or repelled.

 

 

Old TVs

Old televisions might be dying out, however the equipment inside them is still in common use today. Old TVs, before the invention of plasma screens, were kitted out with an instrument called a cathode ray tube. This device accelerates electrons and fires them behind a screen that has a coating that gives out light when hit by electrons. The result is that you could sit and enjoy a television broadcast. However, it isn't just as simple as firing a couple of electrons at a screen. The negatively-charged electrons are directed to the correct point on the screen using the positive charge of magnets so that viewers could watch a perfect image.

These electrons are moving at roughly a third of the speed of light. This means that engineers had to account for length contraction when designing the magnets that directed the electrons to form an image on the screen. Without accounting for these effects, the electron beam's aim would be off and create unintelligible images. 

So, forget reality TV, there's just as much entertainment in relativity TV! (Relatelevision?) 

^{Central Image, Gold: Pair of gold, Hellenistic earings. 3rd-2nd century B.C.E. WIkimedia Commons.}