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

by explodymatt (#26191943) Attached to: Water Detected At Record Distance From Earth
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:Triangulation (Score 1) 54

by explodymatt (#25774727) Attached to: Researchers Turn Tables and Walls Into "Scratch Input" Surfaces
I'm sure 3kHz is slow enough for interferometry to work. If we can do it with visible light sound is probably not an issue. Also most scratchy hissy sounds are around the 16kHz mark and I'm sure there are at least a few overtones in the ultrasonic range (still wavelength of 5-80cm). I'd imagine echoes from the borders of the table would be involved and other non-trivial bits, so I'd say a drawing surface is plausible (maybe not for detailed sketches but for something like a whiteboard).

Who goeth a-borrowing goeth a-sorrowing. -- Thomas Tusser

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