by Matthew Cobb
I have a student who is writing a dissertation about the evolution of opsins – molecules that respond to light, which we use to see with. These molecules apparently have their origins deep in evolutionary time, long before there were animals, perhaps going back to 3.5 billion years, shortly after the appearance of life. While I was reading a draft, I wondered why organisms that use electromagnetic radiation for clocks and seeing (like us) and those that use it for getting energy (like plants, algae and cyanobacteria) all use pretty much the same part of the electromagnetic spectrum – the ‘visual’ spectrum. No organism can detect X-rays or radio waves (which are at opposite ends of the EM spectrum). Why not?
Unlike Jerry, I use Twitter, so I asked my tweeps why no organism can detect radio waves. Many of the answers fell into these three groups:
• What advantage would there be? Not much radio hitting the Earth.
• What emits radio waves that you would want to detect?
• Radio wavelength too long to provide differentiation at a cellular level?
The clearest answers came from two physicsts, @TommyOgden (a PhD student at Durham University), and from my friend and colleague @Tim_O_Brien, who is the Associate Director of Jodrell Bank radio telescope at the University of Manchester. Here’s my interpretation of what they tweeted. If they (or more likely, I) have made a mistake, chip in below.
Organisms use electromagnetic (EM) radiation to shift molecular energy levels, either for growth (photosynthesis) or in sensation (moving protons through cell membranes). Radio is too low-energy and has too long a wavelength to be able to move electrons from one energy state to another. At the other end of the spectrum, X-rays would be able to do this (they can excite the inner electrons), but there are not very many of them. Visible light is both highly energetic and is mainly what the sun produces.
So the simple answer seems to be – as you might expect – that life has been tinkering, making do with what it can find. The surface of the planet is covered with lots of this energy source, which is uniquely able to move electrons about, leading to the production of sugars in plants and sensory responses in animals (and other organisms).
Later on in evolution, other aspects of visible light – its directionality and its absorbability (?) by pigments led to the evolution of eyes, as areas of tissue shielded the detector from stimulation from all but a certain direction. If you were detecting X-rays, you’d have to ingest a lot of lead to be able to detect directionality.
One final point struck me, about quite how energy-poor radio waves are. Carl Sagan apparently said that all the radio waves detected by all the radio telescopes in the world were less than the energy released by a single snowflake touching the ground. Tim O’Brien was asked on the Jodcast (a podcast produced by Jodrell Bank) whether this was true. If you listen here at around 10:00, you can hear Tim work out the answer with some simple but occasionally mind-boggling sums.
If you don’t have the time, the summary of his answer is that the big dish of the Lovell telescope, detects 10-5 joules per year from Cygnus-A, one of the brightest radio-sources in the sky (800 million light years away). In comparison, a 50 watt light bulb produces 50 joules/second) Meanwhile, his guestimate of the potential energy released by a snowflake hitting the ground is 2x 10-6 joules. This is about five times less than the energy from Cygnus-A detected by Jodrell Bank in a year. So if any organism did have a reason to detect radio waves, it would have to be absolutely massive (much, much bigger than the Lovell telescope) to get any decent amount of energy out of it.
You can find an interesting discussion from a couple of years back of the evolution of transceivers, including references to various science fiction stories where people or animals could detect radio waves, here.
The bottom line of this story is probably the age-old evolutionary one of ‘if they could, they would; they don’t so they can’t’. On the other hand, at least I have a bit more of an idea of why – it’s all to do with physics, man.
h/t Tim O’Brien, Tommy Ogden and a Twitter cast of thousands (OK, six).