The Riddle of the River by Catherine Shaw

alternating lines on the screen they set up to catch the rays. The apparatus functioned over several months, so that eventually by the earth’s motion, it had turned to a right angle from it original position. The light ray that had been along the earth’s circumference was now perpendicular and the perpendicular one was now in the direction where some resistance from the ether wind might be expected. If the two rays were travelling at slightly different speeds, it would have been reflected in the interference fringes, which would have been shifted. But in fact, they were exactly as before. So they concluded that there was no ether wind which could make any difference to the speed of light.’
    ‘It’s not hard to come up with ether theories that don’t contradict Michelson-Morley,’ said Ernest. ‘Professor Lodge thinks that the Earth’s atmosphere might drag the ether alongwith it, so that an experiment close to the Earth’s surface wouldn’t detect the wind.’
    ‘Why does one need ether at all?’ I asked. ‘Why can’t the universe just be empty for the most part, with stars and planets and things here and there?’
    Both men began to answer me at once, but Ernest’s voice was the louder.
    ‘Because we have understood that the behaviour of light is a wave-behaviour,’ he explained. ‘Light behaves just like waves you see propagating in the sea, or ripples in water when you drop in a stone. Physicists have been able to determine the speed and the wavelength now; the wavelengths, I should say, because each colour has its own. But what is a wave, Vanessa? Do you know?’
    ‘Ah,’ I said, ‘a movement in the water?’
    ‘Yes,’ he said. ‘But what many people don’t realise, is that the water only moves
up and down
when a wave passes through it. When you see a wave coming towards you on the beach, you may have the false impression that
water
from far off is coming towards you. But if you throw a rubber ball some distance away and watch its behaviour, you will see at once that it is lifted by the passage of the wave, and then lowered back to the place where you originally threw it. Actually, except at the very edge of the ocean, the particles of water are only temporarily displaced by the wave.’
    ‘Yes, well, all right,’ I said.
    ‘So the wave is just a force acting on the water, locally moving it up and down as it passes,’ he continued. ‘And that is how waves work in general. Sound, also, travels by waves – through the air. But our air is not emptiness, it is made up essentially of nitrogen, with some oxygen and a dash of otherelements. It is a kind of gas, with a certain thickness, which can be moved and shifted, as you can easily feel when you fan yourself. So it supports the motion of waves. Now, our air, or atmosphere, thins out to nothing as it goes up – virtually nothing is left after eighty miles or so. It’s just a kind of halo around the planet Earth. The question is: what lies outside it? Something must, for light reaches us through the space which is far beyond the atmosphere, from stars which are inconceivably distant. So, since light is a wave and a wave is a force acting on some material, we conclude that there must be some substance out there.’
    ‘Yet Michelson and Morley could not detect it, nor Sir Oliver, for that matter. I don’t say it isn’t a puzzle,’ said Arthur.
    ‘Oh, it’s there, all right,’ said Ernest. ‘The greatest minds believe in it, in fact the greatest minds invented it. Newton explains it in the
Opticks
, simply by the operation of removing air from a glass tube and leaving a vacuum behind. Light shone from the outside across the tube still goes through – ergo something is still in the tube, carrying it. He calls it the Medium – the ‘‘Etherial Medium”, and explains all refraction and diffraction of light by following its density, which will obviously be greater in the wide open spaces than within solid bodies. For that matter,