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I just came across the old cliche that describes a star as a "little candle that shines its light so far," and am curious whether, barring obstruction and interference, one might be able to detect a candle at the distance of a star or more (obviously, using a keener instrument than the human eye).

I understand that we can think of the light from a candle as being a cloud of photons. I know that a photon can potentially travel the greatest distances available in the reachable universe, but it seems to me that from a weak source they must at some distance become so dispersed that even a single photon from the source would be unlikely to hit the earth. Is that right?

David
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  • Yes, thanks! So if I read you correctly, at some distance from any given source of light it becomes unlikely that we are going to detect any light from it in a given span of time? – David Apr 06 '16 at 05:47
  • Unlikely yes, but impossible no. Since photons are delocalised there is always a non-zero probability of detecting a photon from any source no matter how distant, though in reality that probability may be so low as to be zero in practice. – John Rennie Apr 06 '16 at 06:35

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