Dove sono tutti quanti?

The universe is so huge that it is difficult to describe it using everyday measurements, so we can only try to ‘visualise’ it by using other means. In order to do this, we need to use a bit of imagination: let us ‘pretend’ that our planet is a basketball; if we then take a tennis ball and place it 7 steps away from the ball, this is the Moon. Now let's try to imagine these two balls floating in the dark, and separated by empty space. In order to be life-size, this model would have to be magnified 53 million times. On a cosmic scale, the distance between the Earth and the Moon is miniscule: if we wanted to add the Sun to our model, we would need another ball measuring roughly 26 metres in diameter, and place it almost 3 kilometres from the basketball, or Earth. Even if we reduce the size of the Sun to that of a speck of dust, which is about a hundredth of a millimetre, the centre of our galaxy, the Milky Way, would still be almost two thousand kilometres away, roughly the distance from Rome to Stockholm, as the crow flies. What’s more, the entire galaxy would look like a cloud, with a radius of thousands of kilometres, composed of hundreds of billions of tiny grains of dust, separated from each other by hundreds of metres of empty space. If we shrink the cloud by a million times, we have a more ‘manageable’ picture, in which the Milky Way is a disc measuring one metre in diameter and a couple of centimetres thick, but the Sun is a hundred times smaller than a hydrogen atom. As if that were not unfathomable enough, the M31 Andromeda galaxy, which is the closest to our own, is another cloud located some 20 metres away. The most distant galaxies are hundreds of kilometres away.