I'm flying from Stockholm to Copenhagen, back home after a day in the lovely capital city. A day of sunshine with the city and surrounding fields covered in fresh clean new snow. As we flew south the cloud cover began to increase and there was a rather spectacular wave pattern of the clouds. Almost perfect straight separated by clear sky of seemingly equal width. Why do the cloud align like this ? What are the forces that cause this coherence ? We see such patterns in sand and the same wave patterns exist in crystals, invisible to the naked eye of course. But clearly visible to neutrons which are able to observe such coherence in the atoms which make up the ordered crystals. They are called phonons since these waves have velocities similar to sound waves. There are also waves of magnetism in magnetic materials and these waves are called magnons. Again neutrons can observe them. But we are comparing waves in crystals which repeat every few Ångströms (one hundred millionth of a centimetre) with waves in sand of perhaps ten centimetres, and waves in the clouds of perhaps one kilometre. The underlying physics must be the same, but the forces are not. A ten trillion range in length scale ! These kind of phenomena are all around us. Must switch off, we're about to land.

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