On the science of iridescence in Hummingbirds, my bogey bird falls at last and the bizarre properties of the Photon

 My recent experiences with the hummingbirds of Costa Rica led me to ponder the why and how of the dazzling phenomena of Iridescence. As a recap, look at the two pictures of the Fiery-throated Hummingbird above.  From sideways on this Hummingbird appears emerald green but front on it is truly transformed into a veritable artists pallet of colours. To see the Fiery throat you have to be essentially face on, a few degrees either side and the fiery iridescence disappears. You would certainly be forgiven for thinking these were two entirely different birds.

 

I guess it’s fairly obvious that the colouration is not your run of the mill pigment colouration. In fact Hummingbirds are coloured by what’s known as structural colouration. This is a similar effect to how a prism splits light into its constituent colours. The Hummingbirds feathers have specialised pancake microscopic structures called melanosomes.  These contain little air bubbles that refract the light in a similar way to a prism creating intense iridescent  and shifting colours that change depending on the viewing angle. 

 

I can speculate on a number of reasons why this might have evolved. The male with the most eye-catching display will win the prize and the light show from the iridescence, particularly as the bird displays and changes angle, would be very hard to match with feather pigment colouration. Birds often moult out of breeding plumage to a duller winter plumage helping them to escape the attention of predators. The iridescence mechanism also has an energy advantage in that it does not require a full moult and also can only be seen from front on meaning that their visibility to predators is much reduced.

 

Most birders have a bogey bird, i.e one that they have dipped many times. Mine is a strange one, the American Ring-billed gull.  Its an odd one because while scarce there are usually two or three in the country, particularly over winter. I also find it troublesome that I’ve seen hundreds of these birds on my numerous work and pleasure trips to North America. On an autumnal Canadian holiday a few years back it was by far and away the most common gull. So, with their winter residency probably coming to an end, I decided to bite the bullet and stake out the long staying bird on the Hayle estuary in Cornwall. I hoped I would get it early enough that I could do some other pleasant spring birding. One thing birding is not, however, is predictable and sure enough it took most of the day and a few false starts before I had it nailed down curtesy of some local Gull experts.

 

Gulls are birders Marmite, you either like them or hate them. Personally, having spent hours scoping countless Herring and Common Gulls at Hayle, I wouldn’t care if I never saw one again in my life! I now have almost all the Gulls on my UK list I’m ever likely to see with the possible exception of the Ivory Gull where, at least the adult of this species, has the decency to stand out like a bird that’s been dipped in glossy white paint.

 

I did manage some other local birding, The highlight of which was a beautiful male Garganey but everything was pretty distant and didn’t trouble the camera.

 

Locally there are signs of spring at last. The Oystercatchers are back on the small island on the village lake where they breed and the Ravens are back on nest. We have lived in our Pirton house for pushing on for 6 years. It took two years to see a Red Kite from my garden but this spring they have been a regular feature – how wonderful!

  

Red Kite Over my garden

All this study of the mechanism of iridescence led me to ponder the nature of photons whose properties are truly bizarre when referenced to our everyday experience. So look away, again, now if Physics made you feel sick at school! The photon has no mass at all, it weighs absolutely zero. Think about that for a moment, how can an entity exist if it has no mass. It gets worse! Light from the sun takes about eight minutes from our frame of reference to reach earth. But from the photons point of view no time has passed at all! That’s right, as far as its concerned it arrived at earth at exactly the same time as it left the sun. So, not only do photons have no mass whatsoever they also do not experience time! This is all a consequence of Einstein’s famous two theories of relativity. I’m sure we are all familiar with faster than light spaceships in TV programmes and films like Star Trek allowing us to get to other stars in less than a lifetime but relativity absolutely forbids this! How much energy does it take to accelerate a golf ball to the speed of light? Relativity tells us that its infinite, i.e. more energy than is in the whole universe! The only reason a photon can travel at the speed of light is that it has no mass! 

 

Another bizarre fact about photons is, no matter how you measure it they always travel at the same speed in vacuum, an astonishing 3 x10⁸ meters per second. Let me try and explain why this is odd using a simple example from our everyday experience.  I’m on a platform and a train passes me at 60mph. A passenger on the train throws a ball out of the train at an additional 5miles an hour. How fast does the person on the platform measure the ball is moving relative to him? Well the velocities add so its 65 miles an hour right? Now replace the ball with a light beam. From the person on the platforms perspective the light beam should travel at 3 x10⁸ meters per second plus the speed of the train which is 60 miles per hour. Wrong! The person on the platform measures the speed as 3 x10⁸ meters per second. No matter how you try to measure the speed of light in vacuum from whatever reference point you choose its always the same!

 

The most famous experiment designed to measure how fast the Earth is moving through space using a light beam is the Michelson-Morley experiment, first performed in 1881. This is an absolutely key experiment in the history of physics that every physics student learns in their first year at university. Basically, by doing a similar experiment to the train one described above, i.e by measuring the speed of light in the direction that the earth is moving and then subtracting the measured speed of light in the opposite direction you should get how fast the earth is moving through space. Wrong! Why, well  because we now know that the speed of light in vacuum is always the same no matter how you measure it! 

 

The Michelson-Morley experiment is often described as the most famous "failed" experiment in history because it did not detect the expected motion, but this "null result" fundamentally changed our understanding of physics and led Einstein  to construct his theories of relativity.

 

Just a final factoid. Relativity expressly forbids faster than light travel as it would violate one of the most fundamental features of our reality, namely causality. If causality were to be violated an event could happen before its cause! Taking a simple example a snooker ball would move on its own volition before it was hit by the cue ball.

 

Footnote – my blogs are posted with sometimes rather imaginative spelling and grammar due to my extreme dyslexia!