Einstein’s General Theory of Relativity, a 20th century gift

24 Nov 2015

Albert Einstein was the 20th century’s most famous physicist, revolutionising how we view gravity, light, time and, ultimately, everything. It’s almost 100 years to the day since he set the ball rolling with his masterpiece, the General Theory of Relativity.

The General Theory of Relativity sought to explain how time and gravity work together, describing a vastly different understanding to how we, along with planets and stars, operate.

On 25 November 1915, Einstein, speaking to the Prussian Academy of Science, explained his theory, which claimed that time is experienced differently by different actors, with size and speed key. Everything in the universe is moving around, relative to everything else.

For this to be true, space is wrapped around planets and stars. It is not a sheet, with an end point, and Earth does not orbit the sun because of a gravitational pull but, rather, because the sun has wrapped space around Earth.

Einstein Cross

What we consider gravity bends light, meaning that by the time the event reaches the viewer, they can, on occasion, see the same thing happen at different points. This was best exemplified last March when the first-ever supernova, Einstein Cross, was discovered.

An Einstein Cross is essentially our view of a space happening that has been bent around a galaxy in between us and it – the process is achieved by using a gravitational lens. The result is our telescopes picking up the event several times, in several places, in the same image.

A gravitational lens is a large galaxy or group of galaxies that bends or “lenses” light from a distant source as it travels towards us.

If the gravitational lens is directly in between the event and us, we get to see an Einstein Ring, with a perfect loop of light from the source wrapping around the lensing mass.

But if there is any misalignment along the way, we observe partial arcs or spots – in March we saw four spots, in the shape of a cross, thus an Einstein Cross.

Get Galileo on the case

All theories are there to be investigated, of course, and Einstein’s General Theory of Relativity is no different. In fact, the ESA is currently putting it to the test with a bunch of satellites that were knocked off course when sent up into orbit.

The fifth and sixth Galileo satellites were sent up to space last year, but their orbits were all wrong, rendering them pretty useless.

So ESA scientists spent the last year putting into place “a demanding set of manoeuvres” to align their erratic orbits into a more circular pattern, with the results making them the perfect foils for Einstein’s theory.

This is despite the fact that 5 and 6 are still not entirely ready for full operational use because they aren’t yet into a full circular route, climbing and falling 8,500km, twice a day.

This shifting to and from Earth is key to Einstein’s theory, in which he predicted that time would pass more slowly close to a massive object.

This was verified experimentally, most significantly in 1976 when a hydrogen maser atomic clock on Gravity Probe A was launched 10,000km into space, confirming the prediction to within 140 parts in a million – ESA’s latest test will refine that measurement even more.

Albert Einstein waxwork image via Lodimup/Shutterstock

Gordon Hunt was a journalist with Silicon Republic