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A comment on Milky Way’s Black Hole Just Flared, Growing 75 Times as Bright for a Few Hours. Parent comment

Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by roywiggins on Tuesday August 13, 2019 @11:31PM (#20692510)
The thing we are at rest (ish) relative to is the cosmic microwave background. That's why we aren't moving that much relative to the rest of the universe, on average.

Time dilation is relative between you and some other reference frame. All the "stuff" in the universe nearby is (fairly) static, so we can say we are not moving fast, but it's still relative to the stuff around is.

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Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by mirimir on Tuesday August 13, 2019 @11:35PM (#20692520)
Huh. So could one use that as a standard frame of reference?
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Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by philipov on Wednesday August 14, 2019 @12:55AM (#20692805)
Yes and no. The laws of physics do not pick out a preferred frame of reference, and they should be symmetrical under lorentz transformation, but the fact that we live in a cosmological universe starting with a big bang breaks that symmetry in practice.

It's similar to how the laws of physics are symmetrical under translation and rotation, but the presence of matter around us means the surface of the earth is not the same as the surface of the sun.

So, while we might use the CMB as a standard reference frame, the laws of physics should not.

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Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by roywiggins on Tuesday August 13, 2019 @11:54PM (#20692585)
Maybe, kinda. It's not obvious that it's more privileged than centering a reference frame on the Earth or the Sun. The CMB is maybe just an artifact of the Universe's history (like a Sun-centered one is an artifact of ours).

If we didn't have any CMB radiation floating around to measure, everything would probably work exactly the same. Maybe you could measure all the motion of all the matter in the visible universe and measure the motion of the "center" relative to us.

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