In science 'fact' can only mean 'confirmed to such a degree that it would be perverse to withhold provisional assent.' I suppose that apples might start to rise tomorrow but the possibility does not merit equal time in physics classrooms.
If an elderly but distinguished scientist says that something is possible he is almost certainly right but if he says that it is impossible he is very probably wrong.
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Whenever anyone says 'theoretically ' they really mean 'not really.'
