Does the sky ever end?
鈥� This is a very good question coming as it did from a 5-year-old, and the answer is the same however you look at it. If you look sideways, the sky seems to end at the horizon. But if you walk towards the horizon, you鈥檒l find the sky keeps on going until you鈥檝e walked right around the world. So there is no horizontal end to the sky.
If you look straight upwards on a cloudless night, you will see the stars. There are more distant stars that you can only see with a telescope, and more beyond that. So there seems to be no vertical end to the sky either.
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One day, scientists think, the sun will expand and perhaps swallow our planet. However, that鈥檚 so far off in the future that nobody need worry about it just yet.
鈥淚f we could travel through an infinite sky long enough, we would come to a planet exactly like Earth鈥�
The sky we know might seem doomed to end then, but it won鈥檛. It will still be there, just different.
David Muir, Edinburgh, UK
鈥� That depends on what you mean by 鈥渟ky鈥� and what you mean by 鈥渆nd鈥�. If, by sky, you mean the stuff above our heads that we see as blue/white/grey in the daytime, the answer is yes: it does end.
If you flew up in a rocket, you would pass through dust particles (which scatter sunlight, making the sky blue) and the white or grey clouds. And as you did, what鈥檚 above you would become darker and darker as the air becomes thinner and thinner, carrying less water vapour to form clouds and fewer dust particles to scatter light. You can see this beginning to happen if you are flying in a commercial airliner at 30,000 feet and you look upwards through a window.
At somewhere between 70,000 and 100,000 feet, what is above turns black. So the daytime sky that you see from the ground would effectively have ended.
But if you mean everything above our heads (relatively speaking) during both day and night, the answer is that we can鈥檛 be sure because we just can鈥檛 see that far.
Some people believe that the sky, also known as the universe, goes on and on forever, to infinity. If that is the case, then every possible combination of fundamental particles will occur, and recur. That means that if we could travel through the infinite sky for long enough, we would come to a planet exactly like Earth, with a country on it called South Africa, where a 5-year-old called Sabine is asking 鈥淒oes the sky ever end?鈥�
Alistair Scott, Gland, Switzerland
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This article appeared in print under the headline 鈥淒ying light鈥�