• Question: Why does Light travel in straight Lines

    Asked by Bartosz to Daniel, Hannah, Maggie, Ry, Scott on 8 Nov 2017.
    • Photo: Ry Cutter

      Ry Cutter answered on 8 Nov 2017:

      We say light travels in the shortest possible distance. The shortest possible distance between two points is a straight line! That is unless you have a really heavy object. This curves space time, which changes the shortest distance into a bend! This is where general relativity comes in and where physics starts doing cool things 😀

      Great question,

      Ryan

    • Photo: Hannah Middleton

      Hannah Middleton answered on 8 Nov 2017:

      Light always takes the shortest path it can, but not always a straight line! Normally when we think about the shortest distance between two positions, we would draw a straight line. But all that changes if you change the shape of the space you are thinking about – like in the realms of general relativity!

      If you have two points on a piece of paper, the shortest path between them is a straight line. But if you think about two points on a ball or sphere, then to connect those two we actually draw a curve, following the shape of the ball (the shape of the “space”).

      This is just like what happens in general relativity! Massive objects change the shape of space around them (like putting a bowling ball on a trampoline) which means that light can end up traveling in a curve due to the curvature of space. This is something that we really see in space and is called gravitational lensing. Massive objects in space (like a cluster of galaxies) can bend light from far away galaxies, so that they appear as if they are warped or magnified by the lensing effect of gravity. Have a look at this video and the photos on this website!
      http://www.spacetelescope.org/science/gravitational_lensing/

    • Photo: Maggie Lieu

      Maggie Lieu answered on 8 Nov 2017:

      It doesn’t always! when you have very strong gravity, that gravity can bend light. This is known as gravitational lensing and sometimes we see galaxies in the sky appearing like arcs, this is due to the gravity of a foreground object pulling on the light of the galaxy.

      By measuring the bent shapes of galaxies we can find where things with a lot of gravity are in the Universe, things like dark matter.

    • Photo: Scott Melville

      Scott Melville answered on 8 Nov 2017:

      Hehe, allow me to give a mischievous answer. What do you mean by ‘straight’? Actually, one of the reasons Einstein is so famous is because he realized that we should define ‘straight lines’ to be ones which a light ray would follow. This makes sense – because like Ryan and Hannah say, light takes the shortest path, so why not call that the ‘straight’ path? This is super important because of how gravity affects light – as the others describe, heavy objects can bend light. This means that the ‘straight lines’ next to a star actually look bent compared to the ‘straight lines’ which would be there without the star. So it turns out that Newton’s First Law, that objects move in straight lines, is actually spot on – it’s just that large masses change what you mean by ‘straight’ 🙂 (don’t worry if that didn’t make sense – it took a lot of clever people several hundred years to come up with this, and here on Earth the gravity is so small that ‘straight’ lines will look exactly like the usual ones that you draw with your ruler)

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