Why everything is solar powered (…or is it?)

Here’s an interesting game to play when you have an idle moment. It’s called ‘What Makes It Go?’ and the challenge is to find a moving object – any object, from a car to a kite to a cat – that didn’t ultimately get its energy from the sun.

Nobel physicist Richard Feynman used to play it with his dad when he was young. He recounts an example in his book Surely You’re Joking, Mr Feynman which begins with his father telling him that his wind-up toy car only moves because the sun is shining, with which Feynman disagrees:

‘No, the toy goes because the spring is wound up,’ I would say.

‘How did the spring get wound up?’ he would ask.

‘I wound it up.’

‘And how did you get moving?’

‘From eating.’

‘And food grows only because the sun is shining. So it’s because the sun is shining that all things are moving.’

Get the gist of the game? Let’s try another one: what makes a car go? It’s the engine that makes it go. But what makes the engine go? It runs on petrol. Where did the petrol come from? Fossilised plants… and the plants only grew because the sun was shining. Once again, we end up at the sun.

Apart from giving solar scientists a reason to annoy researchers in other energy fields (‘That wind power study you’re doing is really just a subset of solar science, you know,’*), this game is interesting because it reminds us that energy can never be created or destroyed, it just changes from one form into another. In the example above, we started with electromagnetic energy (sunlight) which was converted into chemical energy (stored in plants and petrol), then to thermal energy (in the engine) and finally kinetic or ‘motion’ energy (of the car). And for us on planet Earth, it’s sobering to think that we’re almost entirely reliant on a single energy source: our sun.

Just some more examples of things that are solar powered.

Notice that I said we are almost entirely reliant on the sun. I can think of at least three sources of energy that can’t be traced back to sunlight. Can you come up with any? And if you can, what are they traceable back to? I’ll be putting my answers in the comments section of this post in a week or so later today to give you a chance to think it over.


* This is an old but enduring game that scientists in different fields occasionally like to play. When I was a university undergraduate I remember chemists teasing biologists that biology was just a subset of chemistry; physicists taunting chemists that chemistry was nothing more than a subset of physics; and mathematicians gloating that physics was just a subset of maths. A recent episode of the TV show ‘The Big Bang Theory’ also had Sheldon (a physicist) and Amy (a neurobiologist) arguing over whether physics was ‘best’ because it explained neurobiology (because the brain obeys the laws of physics when it functions) or whether neurobiology was better because it subsumed physics (because it was Sheldon’s brain that let him understand how the universe worked).


6 Comments on “Why everything is solar powered (…or is it?)”

  1. Adrian James Edwards says:

    Gravitational energy, as in hydroelectric power, is indirectly related to the sun, via the water being raised by evaporation, but tidal power is related to the moon. How are geothermal and nuclear power caused by the sun?

  2. Keith Cohen says:

    Geothermal energy from radionucleide decay and retained heat from converting gravitational potential energy to heat during formation of the Earth.

    Nuclear fission of elements formed in the supernovae of *other* stars, which aren’t *our* Sun.

    Tidal energy created by gravitational effect of the moon on the earths oceans, originating from the kinetic energy of the moon, in turn created from the gravitational potential energy of the coalescing matter of our solar system. (moon as ejecta from a massive collision included).

    • Tania says:

      Those were my three exactly. Great, succinct explanations.

      Personally I still find it amazing to think that most of the heat inside the earth comes from the decay of elements (like uranium) that were formed in a previous generation of stars. It gives a pretty dizzying reminder of the timescale of the universe.

      These three (geothermal, nuclear and tidal energy) are all energy sources we can put to practical use. The game gets more interesting if you also include non-practical sources. General challenge: see what you all can come up with.

  3. Nice work, Keith.

    Would sub-sea hydrothermal vent ecosystems qualify as another? Sulphur consuming bacteria? (maybe partly geothermal in nature?)

  4. Keith Cohen says:

    Non-practical, but possibly YouTube material; Giant arrays of cats with buttered toast strapped to their backs. http://deepscience.com/justsilly/fun006.html

  5. You could always use gamma rays from, say, Eta Carinae to initiate the fusion of hydrogen into deuterium, producing about 0.42 MeV which could be absorbed by, say, a piece of bread. If you figure it takes about 0.6 megajoules to make a piece of toast, so:

    (0.6MJ/piece of toast)x(1,000,000J/1MJ)x(1MeV/1.6×10^-13MJ)x(1 fusion event/0.42MeV)
    =~ 9 x 10^18 fusion events/piece of toast

    Of course, you may be waiting around a bit for that, maybe keep the bread in a vacuum, and well shielded from the sun to make sure it’s radiation doesn’t contaminate your space toast).

    Of course, the bread would be of solar origin, unless it was bread made from those hideous tube things that live around geothermal vents on the bottom of the ocean…


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