Determination of the speed of light: Four facts you might not know already

Light is no slouch – it is basically the fastest stuff there is. But it does have a top speed.

This was not always known. Until the 17^th century, it was thought light travelled instantaneously.

The man who first successfully demonstrated light’s limitations was Ole Roemer. Galileo had previously attempted to prove light moved at a particular speed in experiments involving lanterns being flashed back and forth from distant hilltops, but this small-scale investigation was not enough to prove his theory.

Roemer used a far grander stage for his demonstration.

In 1676 he timed the intervals between eclipses of Jupiter’s moons at different points on the planet’s orbit of the sun.

He discovered that the gap between eclipses of Jupiter’s moon Io rose by up to seven minutes when the earth was moving away from Jupiter, and came down when the planet approached Earth.

Through his measurements, Roemer was able to make a reasonably accurate estimate as to the speed of light.

His findings were initially controversial, and the director of the Royal Observatory, Giovanni Domenico Cassini, did not accept it. However, Roemer had convinced scientists including Isaac Newton, and his theory was proved two decades after his death by British astronomer James Bradley.

The 340^th anniversary of Roemer’s experiment has been celebrated in a Google Doodle.

The speed of light

The exact speed of light is 299,792,458 metres per second, and was only finally agreed in 1975 as physicists’ measurements became increasingly accurate.

A light-year is the distance light can travel in a single year. This is about 5.88 trillion miles, or 9.5 trillion km.

Most distant observable objects in the universe

We now know that if the source is far enough away, light can take billions of years to reach us.

The most distant object ever seen in the universe is a galaxy which has the unforgettable name MACS0647. The celestial wonder is a mind-bending 13.3 billion light-years away. The light started its journey to earth a long time ago, when the universe was still in its infancy.

Light sources

The Sun is a mere 94.5 million miles, or 149 million km from Earth. This means a beam of light takes a nippy eight minutes and 20 seconds to shoot over here.

The next closest star to us is Proxima Centauri, which is around 4.2 light-years away.

Is it possible to travel at the speed of light?

Albert Einstein was nothing if not a huge party-pooper for science fiction writers. His theories of relativity put paid to the idea we could somehow propel ourselves at the same speed as a light beam.

His theory proved that as you propel an object faster and faster, it’s relative mass, compared to when the object is at rest, increases.

As you near the speed of light, the energy required to keep the object moving faster eventually only serves to make the object’s mass larger and larger. Its mass eventually becomes infinite, and so does the energy required to move it. It is impossible. Oh dear.