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Comment Re:How can you detect the Doppler shift? (Score 1) 104

It comes from having a few other pieces of information. The main one is: Most of the universe is made of hydrogen.

We know some stars fall within a certain range of temperatures, we also know of a few events (such as hydrogen falling into a white dwarf or neutron star) that seem to be almost exactly the same, no matter where they happen, and can give an independent estimate of distance based on brightness.

So the key is, we look for some spectral lines (plural) that are a set fraction apart, or come from a known object. We use that to find the distance, then we can use that information to find other chemicals.

It's a bit hard to explain without a picture, but imagine you see a sequence of bright lines at 1, 2, 5, 7. Then another at 3, 6, 15, 21. (They're usually 335nm, vs 337nm etc, but the distinction is clearer with integers.

If you have enough other information/reasons to believe it to be consistent you can assume the second sequence is 3 times the first and thus is red-shifted by a factor of 3.

As you said/questioned this cannot be done with one frequency, but with many(or a single known source) you can figure out both distance and chemical makeup.

Comment Re:Is it possible to do automatic code migration? (Score 1) 357

Yes, there are both the 2to3 and the 3to2 (now somewhat redundant) packages. 2to3 will convert (most) python 2 code to python 3, but there are some limits on its abilities. Large libraries tend not to work very well, especially anything involving other languages (ie. c) 3to2 was for developers going the other way, maintaining python 2 compatibility whilst writing 3.0 code.

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