The Electromagnetic Spectrum
Light, Sound and Waves

Talking about the speed of light

Teaching Guidance for 11-14 Supporting Physics Teaching

Links to the speed of light, including light years

Pupils are invariably fascinated by the links which can be made from the speed of light to distances across space, and travel times through space. It is really helpful to have some relevant facts and figures at the ready.

The light year is used in astronomy as a unit of distance that helps to deal with the vast scale of the universe.

1 light year is the distance travelled by light (through a vacuum) in one year and light travels 300 million metres in one second, so light travels (300 million)  ×  (365  ×  24  ×  60  ×  60) metre in one year.

Therefore 1 light year is 9.5 × 1015 metre.

For example, Supernova 1987a (a supernova is the explosion at the end of a massive star's life-time) occurred in a nearby galaxy called the Large Magellanic Cloud. Light from this supernova was observed on Earth in 1987, but the distance to the Large Magellanic Cloud is about 190 000 light years. Thus, we normally say that Supernova 1987a occurred in 1987, but it really happened about 190 000 years earlier. Only in 1987 did the light of the explosion reach the Earth! If we want to know what the Large Magellanic Cloud looks like now, we will have to wait 190 000 years.

In comparison, the Sun is only about 8 light-minutes away. That is, it takes 8 minutes for light to travel from the Sun to the Earth. So the light we see from the Sun represents what the Sun looked like 8 minutes ago, and we must wait another 8 minutes to see what it looks like now. It is an interesting fact that if the Sun went out we would not know about it until 8 minutes after the event! Reflected light from the Moon travels to the Earth in about 1 second.

  • The nearest star to the Earth (other than the Sun) is 4.3 light years away.
  • Our galaxy (the Milky Way) is about 100 000 light years in diameter.
  • The distance to the galaxy M87 in the Virgo cluster is 50 million light years.
  • The distance to the most distant object seen in the universe is about 12 billion light years (12 × 109 light year).

The most distant things that astronomers can see are about 12 000 000 000 light years away. Thus, the light that we presently see from these objects began its journey to us about 12 billion years ago. Since that is close to the estimated age of the universe, this light is a kind of fossil record of the universe not long after its birth! Thus the observation of very distant objects is in a very real sense equivalent to looking backwards in time.

And it works at all distances, even within the classroom. History is rushing in at you as fast as it can, 30 centimetre every nanosecond (0.000 000 001 second)!

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