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Message: <blockquote data-quote="freyar" data-source="post: 5699501" data-attributes="member: 40227">Thanks!  It's been a long road.True enough.  I'm trying to avoid being too technical also.  My point is that we can actually always think about time as just a normal line (in the rest frame of the "stuff" in the universe), and space is (averaged over long distances) flat also.  It's the expansion of space through time that makes the whole spacetime curved.  It is a bit subtle of a mathematical point, but it's a very important part of our understanding of cosmology today that the spatial slices of our universe are flat.I guess we're imagining that the light is moving in the x direction, then.  In that case, the x axis, y axis, and z axis all get stretched.  That stretches out the wavelength of the light, which is the extent of one cycle of the wave in the x direction, the direction it's moving.  The point is that the amplitude of the wave is not any kind of motion in y or z.  It's most emphatically not like a guitar string, where the wave is displacement of the string perpendicular to its average direction.  The amplitude of the wave is in the electric (and magnetic) field, which is not some kind of extent in space.  It's a separate variable, which is not expanding with the universe.  So that's why the amplitude doesn't get bigger.  That's actually what's true for a pulse of light with small extent, like a photon.  But if you have a big electromagnetic field, like a thermal bath of light filling the universe, the expansion of space spreads the photons out over time.  That decreases the strength of that bath of light; this is why the cosmic radiation was hot enough to break nuclei apart in the early universe but is under 3 degrees above absolute zero now.</blockquote>

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