• Question: is it true that the light that we see from the stars is the light that came of them millions of years ago and if so how dos that work !!!!!

    Asked by weekeenan to Anne, Carolyn, Joe, Mariana, Nick on 25 Mar 2010 in Categories: .
    • Photo: Nick Bradshaw

      Nick Bradshaw answered on 25 Mar 2010:

      It is true. It happens because light can only travel at a certain speed (around 300 million meters per second). Now this doesnt really affect us normally as everything we normally look at is so close that light gets to our eyes pretty much instantly. However stars are much much farther away. If a star was a million light years (a light year being literally the distance light travels in a year) away, then it would take light a million years to get from the star to our eyes. As a result we see stars as they were millions of years ago – for all we know they could all have exploded by now!

      For that matter, it actually takes light around 8 minutes to get to Earth from the Sun. So if aliens decided to blow it up, we would live in blissful ignorance for eight minutes before suddenly finding ourselvs inconveniently dead.

    • Photo: Carolyn McGettigan

      Carolyn McGettigan answered on 25 Mar 2010:

      Well, light travels at around 300 million metres per second. So, for light be reflected from the objects around us on Earth that are close by, that’s pretty much instantaneous. But, if you consider stars that are sufficiently far away, in our galaxy or other galaxies, then you’ve got to use the classic expression: Time = Distance/Speed. If the distance is big enough, then you could be seeing light coming from stars that was actually generated quite some time in the past.

    • Photo: Joseph Devlin

      Joseph Devlin answered on 25 Mar 2010:

      It is true, yes. Basically the light that we see was produced some time in the past and the specific time is determined by the distance between us and the object and by the speed of light. The SoL is fast so anything close (less than a million km away) appears to happen instantly (like turning on a light).

      But as you get further away, the distance becomes significant. For instance, the moon is roughly 300,000km away so it takes light roughly 1s to bounce off the moon and reach us on Earth. Light from the sun takes approximately 8 mins to reach us (we’re 150 million km from the sun) and light from the nearest star takes 4.2 years to reach us. Most stars aren’t that close…

      So when we look into the sky on a clear night, we are seeing the light from stars that started its journey often million of years ago.

      This is the main problem with the idea of contacting other species in space using radio telegraphs. To reach even the nearest star would take 4 years one-way. If intelligent life exists then it would have to be mighty close for us to speak to them because each message can only travel at the SoL. So assuming there is a species 100 light years away that wanted to chat with us. They would only just be getting light from our sun from 100yrs ago, long before we were even capable of sending signals to the stars. Bummer, eh?

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