4 ms·

Even ignoring galaxies, there's also the cosmic microwave background. It turns out that there is a rest frame in which this background looks the same in all di

Even ignoring galaxies, there's also the cosmic microwave background. It turns out that there is a rest frame in which this background looks the same in all directions. But if you keep accelerating in one direction, the CMB coming from there will keep blueshifting: from microwave to IR, to visible, and eventually hard gamma radiation.

eru 5y ago
> It turns out that there is a rest frame in which this background looks the same in all directions. I was really surprised when I heard this for the first time a while ago. Because it means that our universe does have a _preferred_ frame.
- chii 5y ago
  it's not really preferred in the sense that it's the correct one - just one which has some sentimental meaning to humans' interpretation.
  - eru 5y ago
    Well, 'preferred' as in 'different from all the others in a way even aliens would recognise' (ie in accordance with https://en.wikipedia.org/wiki/Copernican_principle https://en.wikipedia.org/wiki/Copernican_principle ).
    - TeMPOraL 5y ago
      Aka. a Schelling point. https://en.wikipedia.org/wiki/Focal_point_(game_theory) https://en.wikipedia.org/wiki/Focal_point_(game_theory)
      - eru 5y ago
        Well, it can serve as a Schelling point. But it's interesting, even without any game theory.
nabla9 5y ago
CMB rest frame is important and special unique frame to use as a reference. Relativity means that there are no special frames where the laws of physics are different.
- ithkuil 5y ago
  the CMB is a bit mind bending; it's related to the question "if the universe is expanding, in the past it was smaller and smaller; so where is the center?" Good read where you'll also learn about the "The Surface of Last Screaming": https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Linew...
  - cout 5y ago
    I understand how there can be no center if the universe has positive curvature. But if the universe is saddle-shaped and non-infinite, surely there has to be a center?
    - effie 5y ago
      I think not; consider pseudosphere, there is no obvious center. Its embedding in 3D space has infinite dimensions but the object itself has finite surface area. https://en.wikipedia.org/wiki/Pseudosphere https://en.wikipedia.org/wiki/Pseudosphere If you mean object with finite dimensions and negative curvature and no center, there seems to be such a thing, see Fig. n066200e: https://encyclopediaofmath.org/wiki/Negative_curvature,_surface_of https://encyclopediaofmath.org/wiki/Negative_curvature,_surf... EDIT Rozendorn's idea seems to be still an open question: https://mathoverflow.net/questions/52851/the-geometry-of-nadirashvilis-complete-bounded-negative-curvature-surface https://mathoverflow.net/questions/52851/the-geometry-of-nad...
    - feoren 5y ago
      Rather than think "surface of a balloon inflating", think "surface of an (infinite?) rubber sheet being uniformly stretched". It does intuitively seem like if there is 0 curvature and no "center", then the universe must be infinite though ...

eru 5y ago

> It turns out that there is a rest frame in which this background looks the same in all directions. I was really surprised when I heard this for the first time a while ago. Because it means that our universe does have a _preferred_ frame.

chii 5y ago
it's not really preferred in the sense that it's the correct one - just one which has some sentimental meaning to humans' interpretation.
- eru 5y ago
  Well, 'preferred' as in 'different from all the others in a way even aliens would recognise' (ie in accordance with https://en.wikipedia.org/wiki/Copernican_principle https://en.wikipedia.org/wiki/Copernican_principle ).
  - TeMPOraL 5y ago
    Aka. a Schelling point. https://en.wikipedia.org/wiki/Focal_point_(game_theory) https://en.wikipedia.org/wiki/Focal_point_(game_theory)
    - eru 5y ago
      Well, it can serve as a Schelling point. But it's interesting, even without any game theory.

nabla9 5y ago

CMB rest frame is important and special unique frame to use as a reference. Relativity means that there are no special frames where the laws of physics are different.

ithkuil 5y ago
the CMB is a bit mind bending; it's related to the question "if the universe is expanding, in the past it was smaller and smaller; so where is the center?" Good read where you'll also learn about the "The Surface of Last Screaming": https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Linew...
- cout 5y ago
  I understand how there can be no center if the universe has positive curvature. But if the universe is saddle-shaped and non-infinite, surely there has to be a center?
  - effie 5y ago
    I think not; consider pseudosphere, there is no obvious center. Its embedding in 3D space has infinite dimensions but the object itself has finite surface area. https://en.wikipedia.org/wiki/Pseudosphere https://en.wikipedia.org/wiki/Pseudosphere If you mean object with finite dimensions and negative curvature and no center, there seems to be such a thing, see Fig. n066200e: https://encyclopediaofmath.org/wiki/Negative_curvature,_surface_of https://encyclopediaofmath.org/wiki/Negative_curvature,_surf... EDIT Rozendorn's idea seems to be still an open question: https://mathoverflow.net/questions/52851/the-geometry-of-nadirashvilis-complete-bounded-negative-curvature-surface https://mathoverflow.net/questions/52851/the-geometry-of-nad...
  - feoren 5y ago
    Rather than think "surface of a balloon inflating", think "surface of an (infinite?) rubber sheet being uniformly stretched". It does intuitively seem like if there is 0 curvature and no "center", then the universe must be infinite though ...