• Question: If we made a spaceship that travelled at the speed of light, and we shone a laser off the back, would the light move?

    Asked by jonnnnnny to Andrew, Ben, Beth, Heather, Louisa on 22 Jun 2010 in Categories: .
    • Photo: Ben Still

      Ben Still answered on 18 Jun 2010:

      Yes. Light always travels at the speed of light, when in open space, this is one of the key rules of Einstein’s theories of relativity. So both the people on the spaceship and observers watching from a distance will see the laser light moving at the speed of light.

      But we could never create a spaceship that travelled at the speed of light as it would require an infinite amount of energy.

      If there were a spaceship travelling near the speed of light, however, the wavelength (colour) of the light would look different to the people on the spaceship and those looking at it. To those on the space ship it would be the correct colour, the one the laser was expected to produce. To an observer however the laser light would be change extremely, it wavelength would be infinitely increased and would probably end up as microwaves, radio waves or worse.

    • Photo: Andrew McKinley

      Andrew McKinley answered on 18 Jun 2010:

      Good one! Einstein’s theory of relativity states that everything is relative, and the only constant is the speed of light. This means that whether we are moving towards or away from a light source, no matter how fast we are moving, we will still observe the speed of light to be the same. What would change however is the wavelength, or the colour of the light. This is something called the Doppler effect, and you will have heard the Doppler shift of sound waves – as an ambulance goes past you, the “nee-naw” gets lower in pitch. (if you google “doppler effect” you’ll find loads of interactive demos). Light does the same thing, but instead of getting lower in pitch, it does what we call “red shift” – this means that the light shifts towards the red – the longer wavelengths. A green laser shone off the back of a moving vehicle would look a different colour to a stationary observer, shifted towards the red end of the spectrum – depending how fast the ship is moving, the more red shifted the light will be.

    • Photo: Beth Dyson

      Beth Dyson answered on 21 Jun 2010:

      This question makes my head hurt! I think I’ll leave it to the physicists!!

    • Photo: Heather McKee

      Heather McKee answered on 22 Jun 2010:

      Great question this one really made me think, though i think they guys have pipped me to the post with the answer already, nice work!

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