Blue Sky Sprites

Have you ever looked at the sky on a sunny day and seen tiny white dots wiggling in your vision?
17 May 2016

Interview with 

Kate Storrs, The University of Cambridge


FameLab is a competition where scientists talk about an area of science that interests them for three minutes. Kate Storrs is the Cambridge FameLab champion for 2016 and she gave us her award winning talk... Cirrus Clouds

Kate - So, have you ever been looking up at the sky on a sunny day and noticed little specs drifting and wiggling and zipping around in your vision?

Chris - Many times.

Kate - Good. Well some of them are boring things. Some of them are just bits of gunk floating across your eye but others are something very special and strange and you can only see them against a bright blue light like the sky. So, next time it's sunny, look up and see if you can see tiny bright white dots. When you first spot them they look like sparks or pin pricks and then, as you watch for longer, you realise that each spark lives for about a second and then it wiggles along a little curvy path in that second and then pops out of existence.

Now, each one of those dots is an individual white blood cell moving through the veins in your eye. At the back of the eye you've got this tangled net of blood vessels that get infinitesimally narrow and the majority of things going down them are red blood cells, which are also tiny so they're fine. But, every once in a while a white blood cell comes along which is twice as big and it gets stuck for a moment in the narrow tubes and it has to squish and wiggle it's way free. So, if you happen to be looking at a bright light at that moment, you can see it getting stuck because the white blood cell is almost transparent. It lets the tiniest pin prick of light shine through and they're called blue sky sprites if you like whimsical names for things or Shearers blue field entoptic phenomenon, if you don't.

On top of the amazing fact that it means we can watch individual cells moving through our bodies, blue sky sprites have a couple of really surprising implications. So, first they show us that our eyes are back to front. That explanation I gave about white blood cells letting the light shine through only makes sense if the bit that detected the light was behind all the blood vessels but from a design perspective that seems crazy. It would mean that the light would have to pass through this whole tangle to get to the bit that mattered. But, crazy as it is, that's how the eye works or at least how it works in mammals and birds. If you were an octopus your eye would be the right way round and you wouldn't see blue sky sprites.

The other surprising implication is that we can watch our immune systems fighting disease. About ten years ago some scientists gave people a very small dose of e-coli bacteria. Just enough to trigger an immune response which means their bodies generated more white blood cells, and those people could see about 50% more blue sky sprites while they were fighting the e-coli than they could before.

For me though I love them because they're a reminder that science is not essentially high tech or complicated. At its heart science is about looking closely at things and even our most frivolous and personal experiences, gazing up at the sky on a sunny day can, if you really pay attention to them, turn out to contain whole worlds of surprising new knowledge.

Chris - Kate - why is this more apparent when you look at a nice bright blue sky than any other background?

Kate - Because your red blood cells are literally red, which means they absorb the blue light and so if you look at a blue light there's the largest spectral difference between what's being absorbed by the red blood cells and what's being let through by the white blood cells.

Chris - So it just makes the white ones much more apparent when you look at something that's a blue background?

Kate - Absolutely, yes!

Chris - Brilliant, thank you very much. And I can understand and now see why you did so well in FameLab. Thanks Kate.


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