We know that an electromagnetic wave is produced by periodically changing electric field (by an accelerating charged particle). We know that the electric field of a point charge varies inversely with square of distance from the charge. Therefore the amplitude of the electromagnetic wave such as light must go on decreasing and practically vanish on covering a finite distance. But , as we know, it doesn't happen and we see the light waves coming from stars very far away. Then what's happening there? Or am I wrong? Please explain.

  • $\begingroup$ In photon terms, sure for a single photon, maybe only a frog would see it, but obviously a star emits a large amount of photons. If no interaction occurs along the way, the wave/photon will keep going. van.physics.illinois.edu/qa/listing.php?id=84900 $\endgroup$ – user167453 Oct 9 '17 at 13:18
  • $\begingroup$ But why the amplitude doesn't decrease with time? $\endgroup$ – Gurbir Singh Oct 9 '17 at 13:21
  • $\begingroup$ Why should it? This is why we have the wave/photon duality, some things are easier to calculate or think about in momentum conservation terms. (Like a photon...tiny bullet), The energy density drops as the sphere expands, but the total flux remains constant. If the amplitude diminished, the total flux would also, but it doesn't. $\endgroup$ – user167453 Oct 9 '17 at 13:37
  • $\begingroup$ So it can't be explained by wave nature? $\endgroup$ – Gurbir Singh Oct 9 '17 at 16:35
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    $\begingroup$ It's (much) easier to use photons, at least for me it is. In the same way, explaining diffraction or interference using photons will give you a headache. But my argument is possibly wrong, if you take into account the fact that photons lose energy emerging from a large mass, such a neutron star. I need to look that up, (or rather, you do, sorry :). The model of an EM wave that the pictures show is not to be taken too far, it's the start of using math seriously, and forgetting about mental pictures, which are confusing. In other words, it's real physics...+1 $\endgroup$ – user167453 Oct 9 '17 at 16:57

The reason we can see stars from far away is because those stars are unimaginably bright up close. Stars do vary in brightness depending on the star type and distance, and there are certainly stars whose light we cannot see. Read about apparent magnitude and absolute magnitude.


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